UNIVERSITY OF OULU P .O. Box 8000 F I -90014 UNIVERSITY OF OULU FINLAND

A C T A U N I V E R S I T A T I S O U L U E N S I S

Professor Esa Hohtola

University Lecturer Santeri Palviainen

Postdoctoral research fellow Sanna Taskila

Professor Olli Vuolteenaho

University Lecturer Veli-Matti Ulvinen

Director Sinikka Eskelinen

Professor Jari Juga

University Lecturer Anu Soikkeli

Professor Olli Vuolteenaho

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ISBN 978-952-62-1361-3 (Paperback)ISBN 978-952-62-1362-0 (PDF)ISSN 0355-3213 (Print)ISSN 1796-2226 (Online)

U N I V E R S I TAT I S O U L U E N S I SACTAC

TECHNICA

U N I V E R S I TAT I S O U L U E N S I SACTAC

TECHNICA

OULU 2016

C 585

Helal Chowdhury

DATA DOWNLOAD ONTHE MOVE IN VISIBLE LIGHT COMMUNICATIONS:DESIGN AND ANALYSIS

UNIVERSITY OF OULU GRADUATE SCHOOL;UNIVERSITY OF OULU,FACULTY OF TECHNOLOGY

C 585

ACTA

Helal C

howdhury

C585etukansi.kesken.fm Page 1 Tuesday, October 11, 2016 3:13 PM

A C T A U N I V E R S I T A T I S O U L U E N S I SC Te c h n i c a 5 8 5

HELAL CHOWDHURY

DATA DOWNLOAD ON THE MOVE IN VISIBLE LIGHT COMMUNICATIONS: DESIGN AND ANALYSIS

Academic dissertation to be presented with the assent ofthe Doctoral Training Committee of Technology andNatural Sciences of the University of Oulu for publicdefence in the OP auditorium (L10), Linnanmaa, on 2December 2016, at 12 noon

UNIVERSITY OF OULU, OULU 2016

Copyright © 2016Acta Univ. Oul. C 585, 2016

Supervised byProfessor Marcos Katz

Reviewed byProfessor Dominic O’BrienProfessor Thomas D.C. Little

ISBN 978-952-62-1361-3 (Paperback)ISBN 978-952-62-1362-0 (PDF)

ISSN 0355-3213 (Printed)ISSN 1796-2226 (Online)

Cover DesignRaimo Ahonen

JUVENES PRINTTAMPERE 2016

Chowdhury, Helal, Data download on the move in visible light communications:Design and analysis. University of Oulu Graduate School; University of Oulu, Faculty of TechnologyActa Univ. Oul. C 585, 2016University of Oulu, P.O. Box 8000, FI-90014 University of Oulu, Finland

Abstract

In visible light communication (VLC), light emitting diodes (LEDs) are used as transmitters; theair is the transmission medium and the photodiodes are used for receivers. This is often referredto as light fidelity (Li-Fi). In this thesis, we provide the methodology to evaluate the performanceof VLC hotspot networks in the context of data downloading on the move scenarios by usingthroughput-distance relationship models. In this context, first we study the different properties ofoptical transceiver elements, noise sources, characterization and modelling of artificial lightinterference, different link topologies and then we introduce the throughput-distance relationshipmodel.

Secondly, the analytically based throughput-distance relationship has been developed forevaluating the performance of VLC hotspot networks in indoor environment in both day and nightconditions. Simulation results reveal that background noise has a significant impact on theperformance of VLC hotspots. As expected, in both indoor and outdoor environments the VLChotspot performs better at night than during day. The performance of VLC hotspot networks is alsoquantified in terms of received file size at different bit error rate requirements and velocities of themobile user.

Thirdly, we study the performance of hybrid (Radio-Optical) WLAN-VLC hotspot andcompare its performance with stand-alone VLC-only or WLAN-only hotspot cases. In this case,we also consider the data download on the move scenarios in an indoor environment for a single-user as well as for multi-user cases. In this hybrid WLAN-VLC hotspot, both the WLAN and theVLC are characterized by their throughput and communication range. Simulations have beenperformed to evaluate the performance of such network for data downloading on the movescenario by taking into account performance metrics such as filesize, average connectivity andsystem throughput. Simulation results reveal that the considered hybrid WLAN-VLC performsalways better than stand-alone VLC-only or WLAN-only hotspot both for a single and multi-usercases.

Finally, this thesis analyses the feasibility and potential benefits of using hybrid radio-opticalwireless systems. In this respect, cooperative communication using optical relays are alsointroduced in order to increase the coverage and energy efficiency of the battery operated device.Potential benefits are identified as service connectivity and energy efficiency of battery operateddevice in an indoor environment. Simulation results reveal that user connectivity and energyefficiency depend on user density, coverage range ratio between single-hop and multi-hop, relayprobabilities and mobility of the user.

Keywords: optical hotspots, optical wireless communications, throughput-distancerelationship model, visible light communications

Chowdhury, Helal, Tiedon lataus liikkeessä käyttäen näkyvään valoonpohjautuvaa tiedonsiirtoa: suunnittelu ja analyysi. Oulun yliopiston tutkijakoulu; Oulun yliopisto, Teknillinen tiedekuntaActa Univ. Oul. C 585, 2016Oulun yliopisto, PL 8000, 90014 Oulun yliopisto

Tiivistelmä

Näkyvään valoon pohjautuvassa tiedonsiirrossa (VLC) valodiodeja (LED) käytetään lähettiminä,ilma on siirtokanava ja valoilmaisimia käytetään vastaanottimina. Tätä kutsutaan usein nimellälight fidelity (Li-Fi). Tässä työssä tarjoamme menetelmiä VLC ”hotspot” verkkojen suoritusky-vyn arviointiin tiedonsiirtonopeus-etäisyysmalleilla skenaarioissa, jossa tietoa ladataan liikkees-sä. Tässä kontekstissa tutkimme ensin optisen lähettimen komponenttien eri ominaisuuksia,kohinan lähteitä, keinovalon häiriömalleja ja tiedonsiirtolinkkien topologioita, jonka jälkeenesittelemme tiedonsiirtonopeuden ja etäisyyden välisen mallin.

Toiseksi kehitetyn analyyttisen tiedonsiirto-etäisyys mallia käytetään arvioitaessa VLC hots-pot verkkojen suorituskykyä sisäympäristössä sekä päivä että yö olosuhteissa. Simulointientulokset osoittavat, että taustakohinalla on suuri vaikutus VLC verkkojen suorituskykyyn. Kutenodotettua, sisä- ja ulkotiloissa VLC hotspot toimii paremmin yöllä kuin päivällä. VLC hotspotverkkojen suorituskyky arvioidaan myös vastaanotetun tiedoston koon, eri bittivirhesuhteen vaa-timuksilla ja liikkuvan käyttäjän nopeuden suhteen.

Kolmanneksi tutkimme hybridi WLAN-VLC hotspot verkon suorituskykyä ja vertaamme sensuorituskykyä pelkän VLC- tai WLAN hotspot tapauksessa. Käsittelemme myös skenaarioitajossa tiedoston lataus tapahtuu liikkeessä sisätilassa yhden käyttäjän sekä monen käyttäjän tapa-uksissa. Tässä hybridi WLAN-VLC hotspot, sekä erilliset WLAN- ja VLC verkot ovat määritel-ty niiden tiedonsiirtonopeuden ja kantaman perusteella. Näiden verkkojen suorituskykyä arvioi-taessa on tehty joukko tietokonesimulointeja verkossa tapahtuvasta tietojen lataamisesta liik-keessä ottamalla huomioon suorituskyvyn mittarit kuten tiedoston koko, keskimääräinen yhtey-den kesto ja saavutettu läpäisy. Simuloinnin tulokset paljastavat, että hybridi WLAN-VLC toimiiaina paremmin kuin pelkkä VLC tai WLAN hotspot sekä yhden että monen käyttäjän tapaukses-sa.

Lopuksi työssä analysoidaan ehdotetun järjestelmän toteutettavuus ja mahdolliset edut käy-tettäessä hybridejä radio-optisia langattomia järjestelmiä. Tältä osin esitellään myös kooperatii-viseen viestintään perustuvat optiset releet parantamaan verkon kattavuutta ja energiatehokkuut-ta akkukäyttöisissä laitteissa. Mahdolliset hyödyt tunnistetaan palvelun konnektiivisuudessa jaenergiatehokkuudessa akkukäyttöisissä laitteissa sisätiloissa. Simulointien tulokset osoittavat,että käyttäjien konnektiivisuus ja energiatehokkuus riippuvat käyttäjätiheydestä, kantaman jaetäisyyden välisestä suhteesta yhden hypyn ja monen hypyn välillä, releointi todennäköisyydes-tä ja käyttäjien mobiliteetista.

Asiasanat: optinen kuormittajat, optisen langattoman viestinnän, suoritusteho matkansuhde malli, valaistusdatansiirto

To my father

&

Suha, Basil and Fougia

8

Preface

I would first like to express my sincere gratitude to my thesis advisor Prof. MarcosKatz for his continuous guidance and support in my pursuit of Ph.D studies. I wouldalso like to express my deepest gratitude to Prof. Kaveh Pahlavan who showed me thepath of research and make me understand the karma of researcher. In addition, I wantto thank Adj. Prof. Sastri Kota for his valuable suggestions and comments regardingthe research in satellite communications.

I want to thank Prof. Pentti Leppänen, Prof. Matti Latva-aho, Prof. Jari Iinatti forgiving me the opportunity to pursue my doctoral studies in the department of commu-nication engineering, University of Oulu, Finland..

I want also to thank the reviewers of this thesis, Prof. Dominico O’Brien fromUniversity of Oxford, England, and Professor Thomas Little from Boston University,Boston, USA for their valuable comments that helped to improve the final version ofthe thesis.

It is my pleaser to thank many of my colleagues and co-author at CWC, who havebeen involved in the related work resulting with this thesis. Specially, I want to thankProf. Ari Pouttu, Ijaz Ahmed, Kari Kärkkäinen, Babar Shahzad Chaudary, HamidrezaBagheri, Dr. Janne Lehtomäki, Dr. Juha Pekka Mäkelä, Dr. Jaakko Huusko, Dr. Ani-mesh Yadav, and Dr. Pradeep Kumar. I would like to thank all administrative personnelof CWC. Specially, Jari Sillanpaa, Kirsi Ojutkangas, and Eija Pajunen for computer andofficial related problems.

Finally, I would like to thank my family and friends for their continuous supportand encouragement when needed. I would like to dedicate this thesis to my family -my father Aminul Haque Chowdhury, my daughter Suha, my son Basil, and to my wifeFougia Hoque for their patience and endless love.

9

10

List of abbreviations and symbols

4G 4th generation of mobile communication technology

5G 5th generation of mobile communication technology

AIr advanced infrared

ANSI American national standards institute

AoA angle of arrival

APD avalanche photodiode

AWGN additive white gaussian noise

BER bit error rate

BS base station

CENELEC committee for electrotechnical standardization

CIE international commission on illumination

CPC compound parabolic concentrator

CSK color shift keying

COST committee on science and technology

dB decibel

DC direct current

DCO-OFDM DC biased optical orthogonal frequency division frequency

DD direct detection

DH-PIM dual header pulse interval modulation

DMT discrete multi-tone

DPIM digital pulse internal modulation

DPPM differential pulse position modulation

D2D device-to-device

DSD dynamic spot-diffusing

EMI electro-magnetic interference

EMO European mobile observatory

EU European Union

EPA environmental protection agency

FOV field of view

FSO free-space optical

GHz giga Hertz

11

GaN gallium nitride

HD high-definition

HAN home access networks

HetNet heterogeneous networks

ICT information and communication technology

IEC international electrotechnical commission

IEEE institute of electrical and electronics engineers

IFFT inverse fast fourier transform

IM/DD intensity modulation/direct detection

IR infra-red

IRC infrared communication

IrDA infrared data association

ISI inter symbol interference

ISM industrial, scientific and medical

IoT internet of things

JEITA Japan electronics and information technology industries association

Li-Fi light fidelity

LCD liquid crystal display

LD laser diode

LED light-emitting diode

LOS line-of-sight

LSD light shaping diffusers

MAC medium access control

MAI multiple access interface

MIMO multi-input-multi-output

MSD multi-spot diffusing

MT mobile terminal

M2M machine to machine

NLOS non line-of-sight

NRZ non-return-to-zero

OFDM orthogonal frequency division multiplexing

OMEGA home gigabit access

OLED organic light emitting diode

OOK on-off keying

OW optical wireless

12

OWC optical wireless communication

PAM pulse amplitude modulation

PCM pulse coded modulation

PHY physical layer

PARR peak-to-average power ratio

PDF probability density function

PLC power line communication

PSK phase shift keying

PoE power-over-ethernet

PoF plastic optical fiber

PIN p-type-insulator-n-type

PN pseudo-noise

PPM pulse position modulation

PPP public private partnership

PSD power spectral density

PTM pulse time modulation

PWM pulse width modulation

QAM quadrature amplitude modulation

QoS quality of service

RAT radio access technology

RF radio frequency

RZ return-to-zero

SDR software define radio

Si APD silicon avalanche photodiode

Si PIN-PD silicon p-type-insulator-n-type photodiode

SINR signal-to-interference-plus-noise ratio

SISO single-input-single-output

SNR signal-to-noise ratio

SSL solid-state lighting

TIA transimpedance amplifier

TOV turn-on voltage

TTA telecommunication technology association

UV ultra-violet

US United States

UWB ultra wide band

13

VLAN visible local area network

VLC visible light communication

VLCA visible light communication association

VLCC visible light communication consortium

VPPM variable pulse position modulation

WDM wavelength division multiplexing

WLAN wireless local area network

WLED white light emitting diode

WPAN wireless personal area network

WWRF wireless world research forum

α1 entrance angle

α2 exit angle

λ wavelength

λscale scaling factor

Ω spatial angle

Φ luminous flux

Φmax full angle of LED

ϕ angle of irradiance

ϕen energy flux

Ψ 12

half power semi angle

θ travelling angle

Ad area of photodetector

A f effective collection area of the PD

B receiver bandwidth

D distance between transmitter and receiver

Dt travelling distance

dhor horizontal separation between transmitter and receiver

dvlc total travelling distance in VLC coverage

dwlan total travelling distance in WLAN coverage

g(Ψ) concentrator gain

H0 channel DC gain

HB average steady background irradiance

Hreflec gain from reflected path

h vertical separation between transmitter and receiver

iLED current through the LED

14

imax maximum current

iD diode current

io saturation current

IB total DC photocurrent

Inatural DC current produce by natural light

Iartificial DC current produce by artificial light

I luminous intensity

I(0) luminous intensity at center

Ihybrid file size in WLAN-VLC

KB Boltzmann’s constant

Kmax maximum visibility

k knee factor

kr refractive index

Km maximum visibility

Kscale scaling factor

L minimum distance

L0 pathloss at the first meter

m Lambertian order

mopt optimal Lambertian order

N0 Gaussian noise

nd diode ideality factor

nfloor number of floors

N number of users

n(t) signal independent shot noise

Pmax maximum power

Pn average power of ambient light

Pr received optical power

Pt transmitted power

Ptotal total received power

q electron charge

Q(x) Q function

r horizontal distance

R radius of VLC coverage

rsr radius of single-hop coverage

Rmr radius of multi-hop coverage

15

Rb data rate

Ri radiant intensity

Rr photodiode responsivity

R0(ϕ) Lambertian pattern

RF feedback resistance

S(r) spatial throughput

Svlc VLC throughput

Swlan WLAN throughput

Tk( f ) absolute temparature

Tsg signal transmission of the filter

tt residence time in VLC and WLAN coverage

v velocity of the mobile user

V (λ ) standard luminosity curve

VT ( f ) thermal voltage

vin power amplifier input voltage

vout power amplifier output voltage

vmax maximum output voltage

vLED voltage through LED

xi(t) instantaneous optical power

16

Contents

AbstractTiivistelmäPreface 9List of abbreviations and symbols 11Contents 171 Introduction 21

1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

1.2 Visible light communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

1.3 State-of-the-art and related work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

1.3.1 Data rate improvement in VLC testbed . . . . . . . . . . . . . . . . . . . . . . . . . . 27

1.3.2 Hybrid radio-optical wireless communications . . . . . . . . . . . . . . . . . . . 29

1.3.3 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

1.3.4 Thesis contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

1.3.5 Author’s contributions and thesis outline . . . . . . . . . . . . . . . . . . . . . . . . . 32

2 Optical transceiver, noise and interference 352.1 Optical transmitters in VLC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

2.1.1 Generation of white light with LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

2.1.2 Radiometry and photometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

2.1.3 LED I-V characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

2.2 Wireless optical receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

2.2.1 Elements of a photodetector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

2.3 Noise sources in wireless optical communications . . . . . . . . . . . . . . . . . . . . . . . 45

2.3.1 Shot noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

2.3.2 Thermal noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

2.3.3 Measurement-based noise characterisation . . . . . . . . . . . . . . . . . . . . . . . 46

2.4 Basics building blocks of optical transceiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

2.5 Modulations in VLC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

3 Link characterisation in VLC 533.1 Basics link types of VLC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

3.1.1 Directed LOS link design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

3.1.2 Non-directed link design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

17

3.2 Measurement based link design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

4 Data downloading on VLC coverage 634.1 Motivation and related work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

4.2 Geometry of VLC hotspot coverage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .65

4.3 Transformation of inclined path to horizontal path . . . . . . . . . . . . . . . . . . . . . . . 66

4.4 Mathematical framework for a data downloading scenario . . . . . . . . . . . . . . . . 67

4.5 VLC hotspot design parameters and their relationships . . . . . . . . . . . . . . . . . . .70

4.6 Theoretical and empirical throughput-distance models . . . . . . . . . . . . . . . . . . . 71

4.6.1 Empirical polynomial based throughput-distance model . . . . . . . . . . . 72

4.6.2 Throughput vs. distance at daytime. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .72

4.6.3 Throughput vs. distance at nighttime . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

4.6.4 Calculation of average throughput and file size . . . . . . . . . . . . . . . . . . . 73

4.7 Numerical results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

4.8 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

5 Data downloading in hybrid WLAN-VLC networks 795.1 Hybrid WLAN-VLC networks in indoor environment . . . . . . . . . . . . . . . . . . . 79

5.1.1 Coverage and data rate of VLC small cell . . . . . . . . . . . . . . . . . . . . . . . . 80

5.1.2 Coverage and data rate of WLAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

5.2 Hybrid WLAN-VLC: single-user case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

5.3 Hybrid WLAN-VLC: multi-user case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

5.4 Performance evaluation of hybrid WLAN-VLC . . . . . . . . . . . . . . . . . . . . . . . . . 88

5.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

6 Co-operative relays in VLC and hybrid WLAN-VLC networks 976.1 Overview and background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

6.2 Scenario description. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .99

6.2.1 Relay selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

6.3 Performance evaluation of co-operative VLC . . . . . . . . . . . . . . . . . . . . . . . . . . 101

6.3.1 Co-operative hybrid WLAN-VLC networks . . . . . . . . . . . . . . . . . . . . . 103

6.3.2 Energy consumption analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

6.4 Performance evaluation of hybrid WLAN-VLC networks . . . . . . . . . . . . . . . 106

6.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

7 Summary and future directions 1117.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

7.2 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

18

References 115

19

20

1 Introduction

It is difficult to make predictions,

especially about the future.

Attributed to Niels Bohr

1.1 Background

Double-digit annual growth rates of network traffic in all network segments is expectedto increase remarkably over the coming years and beyond. Recently, we have also wit-nessed a remarkable increase in the use of smart phones. The wireless world researchforum (WWRF) predicts that 7 trillion wireless devices will serve 7 billion people by2020. This prediction reveals that the number of network-connected wireless deviceswill reach 1000 times the world’s population by 2020 [1]. The paradigm of communica-tions will also expand from ’human-human’ to ’human-thing’, and ’thing-thing’ (alsocalled machine to machine (M2M)) [2]. The Internet will become the Internet of Things(IoT) [3]. In IoT, every object or thing will be connected virtually and the Internet willbe the basic infrastructure for supporting connections of these interconnected objects.Therefore, IoT will bring a massive surge of smart connected devices enabling newservices and business across industries. As more and more devices go wireless, thesubstantial growth of tele-traffic and therefore the need for more spectrum usage is alsoincreased tremendously. Hence, this high volume of tele-traffic is clearly leading to agreater thirst for spectrum to support wireless broadband [4]. Radio frequency (RF) inthe range of (1-2) giga Hertz (GHz) proven for best propagation conditions is alreadycongested. Therefore, this spectrum scarcity referred to as the spectrum crunch has tobe tackled by appropriate countermeasures in future wireless communication systems[1].

Recently, the green communications campaign has gained momentum in a globalconsensus to reduce the temperature increase due to carbon dioxide (CO2) emissions.The European Commission (EC) has proposed to cut CO2 emissions by 40% by 2030.As the volume of data traffic increases, the carbon footprint of using mobile networks isalso increased [5]. In 2011, 22% of the carbon footprint represented by communication

21

networks is reported and expected to almost double by 2020 unless underlying networktechnologies are significantly improved [5–7].

It is envisioned that future high data rate wireless services such as 5thgeneration(5G) will be based on exploiting multiple wireless access technologies [8]. Therefore,5G enabled terminals are also expected to be multi-standard supporting wireless de-vices. These multi-standard wireless devices will be more complex and power-hungrysignal processing hardware. This will raise a significant threat to using mobile devicesfor continuous service connectivity due to the requirement of high power consumptionof transmission, reception and other processing hardware [9]. In this case, the batterypower of mobile devices will drain rapidly and mobile users will be relentlessly search-ing for power outlets rather than network access, which will bind the mobile user to asingle location. This is known as the "energy trap" [4, 10].

The world mobile and wireless infrastructure community have taken initiatives tomeet the above-mentioned grand challenges. As a result, many important technical,regulatory, economical, and social issues are considered in designing future 5G wire-less communication systems [9, 11–14]. The standardisation of 5G networks is ex-pected to be finalised around 2018 and the technology will start to be deployed around2020 [7]. Massive multi-input-multi-output (MIMO), ultra dense networks, moving net-works, device-to-device (D2D), ultra reliable, and massive machine communicationsare considered to be the key components of 5G wireless networks. It is envisioned that5G networks will achieve 1000 times the system capacity, 10 times the spectral andenergy efficiency. 5G wireless networks are also expected to provide peak data rate of10 giga bits per second (Gbps) for low mobility and peak data rate of 1 Gbps for highmobility [11, 15].

Lighting is a major source of electric energy consumption. It is estimated that 19%of all electricity is used for lighting [16]. Hence, the development of more energyefficient lighting sources is important. Future light emitting didode (LED) lightingnot only achieves significant energy savings, but also carbon footprint reductions [5].As a result, a significant activity toward the development of solid state sources such aswhite LEDs (WLEDs) have already been started to replace incandescent and fluorescentlights.

In Europe, most countries have already started to ban the use of incandescent andfluorescent lights [17]. The phase-out will deliver considerable savings to the environ-ment and the economy. In this respect, energy efficient solid state based lighting suchas LED will bring revolutionary advances in the use of light for illumination. It is also

22

expected that LED lighting will surpass the current lighting options in the near futurein terms of both cost and efficiency. A ten dollar 60 Watt LED light is already on themarket and the expected price of this LED light will be halved by 2020 [17, 18]. Thus,it is expected that the reduction of cost and the improvement of performance of LED interms of lumen per Watt will also continue in near future. It is predicted that the lumenper Watt of LEDs will reach 200 around 2025 [17].

In addition to lighting capabilities, WLEDs can also be used to provide wirelesscommunications by modulating the light sources with data, at a rate much faster than theresponse time of the human eye [19, 20]. Thus, it is foreseen that light transceivers willbe built in LED light fixtures. LEDs can easily serve as sources for even very high datarate applications. In this case, multiple wired and wireless backbone technologies suchas the broadband power line communication (PLC) (IEEE 1901, ITU-T G.9960/61),60 GHz millimeter (mm) wave and the recently proposed low-cost plastic optical fibre(POF)-based backbone technologies can be used to build VLC network architecture[21–24]. This will extend the indoor range from a single room (radius 3-10 m) to alarge building with hundreds of rooms. As a result, illumination and communicationcan be achieved with a single platform.

The dual use of LEDs for illumination and communication promises a sustainableand energy-efficient approach and has the potential to revolutionise how we use light atpresent. VLC can be possibly used in a wide range of applications including wirelesslocal area networks (WLAN), wireless personal area networks (WPAN) and vehicularnetworks among others [25–29]. Wireless communication using the visible light spec-trum is known as visible light communication (VLC). Optical wireless communication(OWC) enables wireless connectivity using infra-red (IR), visible or ultraviolet bands.VLC is a subset of OWC [30, 31].

1.2 Visible light communications

In VLC, the visible part of the spectrum is used for communication purposes. Thevisible light spectrum is unlimited and 10,000 times larger than the range of radio fre-quencies between 0 Hz to 30 GHz as shown in Figure 1 [32]. In VLC, the WLEDssource are used as transmitters; the air is the transmission medium, and the positiveintrinsic-negative (PIN) or more sensitive avalanche photodiode (APD) are used for re-ceivers. Communication network builds upon WLEDs and photodiode is often referredas light fidelity (‘Li-Fi’) [33]. VLC provides access to several hundred tera Hertz (THz)

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of unlicensed spectrum. One of the key merits of optical wireless systems is the rela-tively low transceiver complexity and low energy consumption of LEDs [5]. As a result,VLC may facilitate the low energy-per-bit required for data transmission in comparisonto RF systems. The visible light spectrum extends from 380 nm to 780 nm in wave-length. The VLC spectrum creates no electromagnetic interference to RF systems andvice versa.

RadioMicro-

waves

Infra-

red

Ultra-

violetX-rays Gamma

Rays

Frequency [Hz]

0 10x103 11x103 14x104 14x109.7 16x103 19x103

X ~ 10,000

Fig. 1. The electromagnetic spectrum.

There are already several standards associated with visible light communications[34–36]. In 2003, the visible light communication consortium (VLCC) established inJapan. In 2007, Japan electronics and information technology industries association(JEITA) established standards and VLCC introduced specification standards in 2008[37]. CP-1221 and CP-1222 are the standards for visible light ID and visible lightbeacon systems respectively [34, 35]. The visible light ID system can be used for ap-plications such as location-based services and digital signage [17]. Another activity isunder the umbrella of the telecommunications technology association (TTA) that sup-ports the standardization of VLC in Korea and worldwide. The VLC working groupin TTA started in May 2007. In 2011, VLC was standardized and published as theIEEE 802.15.7 standard [36]. The IEEE 802.15.7 standard promises to be a very at-tractive candidate as a future high data rate and power-efficient technology in 5G. Re-cently, revision of the IEEE 802.15.7-2011 standard for VLC is prepared and denoted as802.15.7r1. European committee on science and technology (COST) 1101 research net-work OPTICWISE group was active and contributing to this standard. OPTICWISE is

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an associate member of the European 5G public private partnership (PPP). The revisionaddresses low data rate communication based on imaging sensors [38]

In the IEEE 802.15.7 standard, both physical (PHY) and medium access layer(MAC) are defined for short-range optical wireless communications using visible lightspectrum. The standard is capable of delivering 5G compatible data rates which issufficient to support audio and video multimedia services and also considers mobil-ity of the visible link [36]. The concept of physical and logical mobility is discussedin [36]. This standard supports the device discovery mechanism through which shortrange co-operation can also be possible within homogeneous networks with three differ-ent classes of devices: infrastructure, mobile, and vehicle [37]. Recently, the successorto the VLCC another visible light communication association (VLCA) established inMay 2014 in Japan to further research, develop, plan and standardise advanced visiblelight communications systems [39]. There may arise multiple applications of such tech-nology in future. It belongs to the green technologies category when used for lightingpurposes, becoming even more environmentally friendly as it supports communicationfunctionalities compared to RF alternatives. VLC can be used both for indoor as wellas outdoor communications [40].

The VLC market categorizes both for slow and high data rate applications [41–44].However, it does not imply that VLC is the universal replacement for short-range basedRF technologies such as wireless local area networks (WLAN), Zigbee, ultra wide band(UWB), 6LoWPAN and Bluetooth. The application of optical wireless (OW) systemsis limited when considering coverage area and user mobility where RF technologiesprove invaluable. Moreover, communication with VLC is challenging due to occlusion,which can occur if severe misalignments and the presence of physical obstructions be-tween sources and detectors are present in indoor as well as outdoor environments. Vis-ible light waves predominantly follow line-of-sight (LOS) propagation. Hence, VLCis more vulnerable than RF in terms of reliable connectivity, especially when mobileusers move. However, among the key merits of VLC are that it exhibits several ap-pealing attributes when compared to RF [6]. The comparison between radio and op-tical wireless communications (VLC is a subset of OWC ) is summarised in Table 1.Co-operation between radio frequency and VLC systems may also be envisioned tobuild hybrid radio-optical communication systems for higher throughput, better cover-age, and higher energy efficiency in future heterogeneous networks (HetNets) solutions[4].

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Table 1. Comparison between RF and optical wireless based systems.

Characteristics Radio Optical

Spectral availability Limited AbundantSpectral regulation Highly regulated No regulationElectromagnetic interference created Possible NoneSensitive electromagnetic interference Possible NoneDominant noise source Other users Natural and artificial lightQuality of service Best effort Best effort

1.3 State-of-the-art and related work

The first optical wireless communication was developed and named as photophone byAlexander Graham Bell in 1880. In this photophone experiment, he modulated sun-light with voice signal and transmitted it over a distance of around 200 m. A vibratingmirror for sending and a parabolic mirror with a selenium cell at its focal point wereused to receive the signal. The photophone did not work very well due to severe inter-ference and the use of ordinary receiver. However, in 1960s the discovery of opticalsources such as the laser diode (LD) changed the fortune of OWC. Short-range opticalIR based wireless communication is discussed in [45, 46]. The first IR-based local-areanetworks using diffuse links and their advantages and drawbacks are compared in [45].An experimental pulse coded modulation (PCM) link operating at 125 kbps and phaseshift keying (PSK) operating at 64 kbps were established in this work. Communica-tion range were realized up to 50 m. Performance of an experimental 50 mega bits persecond (Mbps) on-off-keyed diffuse IR link is also described in [46].

In July 1990, the IEEE 802.11 standard project was started for the specification ofWLAN for different technologies including radio and IR [47]. Due to the lack of interestfrom potential vendors, IR-based WLAN could not penetrate into the market. However,after the invention of high brightness gallium nitride (GaN) LED by Nakamura in 1993,LED technology is maturing into the low-cost, energy-efficient lighting of the future.LEDs make them also attractive candidates for use in short to medium range data linkssupporting Mbps rates.

Pioneer work to transmit data using LEDs in the visible light spectrum began in2003 at the Nakagawa laboratory in Keio University, Japan [48–50]. In [48], the basicproperties of LED lights, illumination using LED lighting and design of WLED lightare discussed. Several research papers on VLC [51–53], which are related to [48–50]have been published also to investigate the performance of VLC. The research work in

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[51] propose a simple method to equalise the WLED. Simulation results indicate thata simple equaliser can double the maximum data transmission rate compared with theunequalised channel. This paper has shown that the maximum 32 Mbps with non-returnzero on-off-keying (NRZ-OOK) at a bit error rate (BER) of 10−6 is achievable. In [52],several challenges and possibilities in VLC are discussed. Complex modulation and par-allel communication (optical MIMO) are suggested to increase the data rate in [54, 55].Provision of uplink and regulatory challenges are also discussed in these papers. Sev-eral hardware demonstrations work on data rate improvement and hybrid radio-opticalwireless communication system are discussed in the following two subsections.

1.3.1 Data rate improvement in VLC testbed

Numerous papers have been published on increasing the data rate in VLC systems espe-cially for point-to-point link configuration [56–63]. In most of these works, hardwaredemonstrations were carried out utilizing different modulation schemes, blue filteringas well as pre and post equalizers to improve the data rates. For example, in [56], an ex-perimental demonstration had been conducted building technology-independent MAClayer and PLC for services and connectivity to any number of devices in any room ofa house and apartment. This work was part of the European union (EU) funded homegigabit access (OMEGA) project to investigate optical-wireless communications [64].Data rate were achieved up to 73 Mbps in their demonstration using VLC. The pro-totype consists of two parts: digital signal processing and an analogue part. In theirexperimental setup, digital signal processing was implemented on a Vertex-5 field pro-gramming gate array (FPGA) board where serial input was mapped to a 16-quadratureamplitude modulation (16-QAM) symbol stream. On the analogue front end, the trans-mitter consisted of driving circuit, trans-conductance amplifier, and commercial Osram(OSTAR E3B) high-power LEDs. On the receiver side, imaging optics, a colour filter,a photodiode, a two stage trans-impedance amplifier, band-pass filter were used.

The data rate of 80 Mbps using pre-equalized white LED was demonstrated in [57].In this work, a pre-equlisation technique had been applied to extend the modulationbandwidth. The equalised bandwidth was 45 MHz. A single Luxeon white LED wasused as VLC transmitter. The signal was pre-equalised by a driver and combined withthe direct current (DC) signal via a bias-tee. The DC current was set to 200 mA. Onthe receiver side, a blue filter, concentration lens, PIN type photodetector and low noisetransimpedance amplifier were used. In [58], 125 Mbps over 5 m distance in an indoor

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environment by using on-off keying (OOK) was reported. In the experimental setup,a lamp with six chips that provides luminous flux of about 400 lm was considered.Moreover, a blue filter was applied to suppress the phosphorescent component of thewhite light. A large area silicon PIN diode with effective area 100 mm2 and polymerlens with 70o field of view (FOV) were used to receive the signal.

In [59], OOK modulation and without equalization, 230 Mbps and 125 Mbps datarate were achieved using APD and a PIN diode respectively. A typical commerciallyavailable LED module consists of four chips providing luminous flux of 250 lm wasused as transmitter. A blue filter with a cut-off wavelength of 500 nm was used in frontof the photodiode. The diameter of 3 mm of APD and effective area of PIN photodi-ode were considered in the receiver side. With a PIN based receiver, 125 Mbps wasachieved at 1000 lux. On the other hand, a data rate of 230 Mbps was achieved at 1000lux due to the enhanced sensitivity of the APD. The authors of [59] further claimed513 Mbps point-to-point visible light communication link and reported in [60] usingdiscrete multitone modulation (DMT) modulation. Further improvement in data rateswas continued by the same group and achieved 803 Mbps and reported in [61]. In thisdemonstration, wavelength division multiplexing (WDM) VLC link with DMT modu-lation was used for red-green-blue (RGB) LEDs. On the receiver side, commerciallyavailable APD of 3 mm diameter combined with a glass lens of 8 mm focal length wasused for detection. The gross transmission rate was about 293.7 Mbps, 223.4 Mbps and286 Mbps for the red, green and blue channels respectively which led to an aggregatedata rate of about 803 Mbps.

Data rate records in Gbps for VLC had reported in [62, 63, 65]. In [62], the firstGbps VLC link based on RGB LEDs had demonstrated. In this work, QAM on DMTwas used in the WDM link. DMT signals consisted of 128 subcarriers within the base-band bandwidth of 100 MHz. Large area silicon APD of 3 mm diameter combinedwith a glass lens of 20 mm diameter and 20 mm focal length were used for detection.The detector was followed by a low-impedance amplifier to amplify the signal level upto the operation range. The transmission performance measurements were performedwith and without the presence of crosstalk. In the case of without crosstalk, the grosstransmission rate were about 444 Mbps, 518 Mbps and 589 Mbps for the red, greenand blue channels respectively, which led to an aggregate data rate of about 1.55 Gbps.On the other hand, in the case of with crosstalk the gross transmission rate were about376 Mbps, 439 Mbps and 430 Mbps for the red, green and blue channel respectively,which led to aggregate data rate of about 1.25 Gbps. 1 Gbps for single channel and

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2.1 Gbps WDM transmission at usual illumination levels were also reported in [63]. Acellular structure using an angle-diversity configurations had been implemented offer-ing reasonably wide coverage for indoor communications was reported in [19, 66, 67].Their work was designed for home access networks (HAN). HAN consists of VLCto deliver data in downlink and infra-red communication (IRC) for uplink to provideGbps bi-directional communications. Recently, 100 Gbps with wide FOV optical wire-less communications has been reported in [68]. In this work, the authors described anindoor optical bi-directional wireless link with an aggregate capacity over 224 Gbpsoperating at around 3 m range with a FOV of 60o. This type of wide FOV will offerpractical room-scale coverage for wireless communication.

1.3.2 Hybrid radio-optical wireless communications

Numerous studies also have discussed the use of VLC and the RF systems as comple-mentary technology [6, 69–75]. The main goal of those studies was to investigate andexploit the heterogeneous nature of such technologies and find the benefit from thembeing used in combination. In [69], the authors developed a seamless communicationmethod between radio and optical communication systems where the hybrid (Wi-Fi)-IRcommunication system continued to use RF transmission when the communication pathof the optical wireless transmission was obstructed and switched back to optical wire-less transmission as soon as the optical wireless connection had become stable again.The purpose of using this hybrid radio-optical architecture was to use high speed andsecure communication via optical communication whenever it was available.

In [70], different types vertical handover (VHO) schemes between radio and opticalcommunications are proposed: immediate VHO (I-VHO), dwell VHO (D-VHO) anda fuzzy logic-based vertical handover for an integrated (Wi-Fi)-IR system. In theseschemes, VHO decision-making algorithm determines whether a VHO should be per-formed and when. The purpose of the work was to trigger an optimal handover decisionbetween radio and optical communication systems for providing a better quality of ser-vice (QoS) to the users.

In [71], authors had studied and compared the radio-optical based sensor networklife time with the RF-only based wireless sensor network. Results showed that theradio-optical based sensor network lifetime lasts at least twice as long as its RF-onlycounterpart because of low energy consumption by optical transmission in hybrid radio-optical systems.

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Studies in [72] introduced a hybrid scheme in VLC where spotlighting hotspot cellsfor high data rate and uniform lighting for moderate data rate and relatively wide cov-erage. They found that spotlighting VLC had several benefits over uniform lightingimplementations in terms of the achievable high data rate. The authors in [73] inves-tigated the criteria for vertical handover in a hybrid WLAN-VLC system which wascomprised of broadcast VLC hotspots and Wi-Fi connectivity with a goal to avoid ser-vice disconnections and to optimally distribute available resources among users.

In [6, 74], authors analysed the feasibility and potential benefits of using hybridradio-optical wireless systems where co-operative communication using optical relayswere also introduced in order to increase the coverage and energy efficiency of batteryoperated devices. In [75], turbo coded data transmission over a hybrid channel consist-ing of parallel free space optical and radio frequency links were considered.

Recently, the demonstration of practical indoor hybrid WLAN-VLC Internet accesssystems have been proposed in [76]. In their demonstration, VLC is used as a sup-plementary downlink channel along with conventional Wi-Fi connectivity. The maincomponent of this demonstration were: GNU Radio software-defined radio toolkit withuniversal software radio peripheral (USRP) developed by Ettus research and its parentcompany [77]. Osram semiconductor LEDs (LUW CN5M) and a commercial photo-diode with transimpedence amplifier (PDA36A) were used as transmitter and receiverrespectively. They compared the performance between Wi-Fi and hybrid VLC in termsof Website loading time and average download data rate. They had noticed that as thenumber of users increased, the hybrid WLAN-VLC performance was better than Wi-Fionly.

1.3.3 Motivation

Recently, research in VLC is gaining momentum in many directions. Most of the re-search works focus on data rate improvement in the testbed system as discussed in Sec-tion 1.3.1. Optical MIMO and OFDM technologies have also recently been introducedin VLC to improve data rate and coverage [78–80]. Research on hybrid architecture ofWLAN-VLC has also been proposed and reported as discussed in Section 1.3.2. How-ever, very little has been published so far about the deployment of VLC wireless infras-tructures [4, 10]. Although it is very early to speculate on the infrastructure deploymentprocedure of VLC, it is important to target a specific network topology and evaluate theperformance in terms of achievable data rates in some typical usage scenarios.

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As it is mentioned, illumination and communication can be provided within a singleplatform. Therefore, the deployment of such VLC systems has to be jointly optimisedwith illumination [81]. However, to get the benefits of both illumination and communi-cation in a single platform, it leads to a lot of challenges in validating the applicability ofsuch a communication system [82]. For example, VLC performance depends on manynatural events such as day and night events, where natural as well as artificial lightsources produce noise and interference. In addition to that, VLC is challenging due toocclusion, which can occur due to severe misalignments and the presence of physicalobstructions between the transmitter and receiver. Therefore, convincing conclusionsfor deployment can only be drawn by evaluating the performance of such networks. Theobtained results and proposed methodologies of this feasibility study will then providefundamental information to design and deployment related issues of VLC hotspots. Inthis respect, this thesis will focus on the coverage and data rate design of future VLCand hybrid WLAN-VLC networks considering many important essential design param-eters related to VLC and WLAN hotspots in day and night environments.

However, many research questions have to be left unanswered in order to keep thiswork at a manageable size. For example, connection set-up time, vertical handover al-gorithm development in hybrid WLAN-VLC are not considered in this thesis. In thisrespect, we sufficiently narrowed the goals for this thesis and focused on designingand evaluating the performance of different VLC and hybrid WLAN-VLC hotspot net-works with some valid assumptions. We summarize the thesis contribution in a conciseproblem formulation in the next section.

1.3.4 Thesis contributions

The main contribution of this thesis is to develop the framework for designing and thecomparative performance analysis of VLC and hybrid WLAN-VLC hotspot networks.In this respect, we consider three different types of hotspots network topologies: stan-dalone VLC, hybrid WLAN-VLC, and co-operative relay-assisted hybrid WLAN-VLChotspot networks. In all cases, data download on the move scenario is considered in per-formance evaluation both in indoor and outdoor environments. Another contribution ofthe thesis is to present a mathematical framework to calculate the average throughput ondata download on the move scenario. In this mathematical framework, first we developthe theoretical throughput-distance relationship for VLC and then this relationship is ap-proximated to two empirical throughput-distance models to calculate the downloaded

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file size while crossing the VLC or hybrid WLAN-VLC hotspot coverage. In case ofWLAN, measurement based empirical throughput-distance relationship is used.

In this thesis, we provide two types of mobility models. In the first mobility model,the main assumption is that the single mobile user passes through the coverage of theVLC hotspot through a straight inclined path. In this respect, we introduce a simplebut novel rotational technique where equivalent straight horizontal paths can be derivedusing the entry and exit angles of the mobile user. The purpose of this rotation of theinclined path to horizontal is to simplify the calculation of average throughput and filesize. Secondly, the random waypoint mobility model is used in hybrid WLAN-VLCand co-operative hybrid WLAN-VLC scenarios in case of multi-user cases. The ob-tained results and proposed methodologies provide fundamental information to designand deployment related issues of standalone VLC and hybrid VLC-WLAN hotspot net-works.

1.3.5 Author’s contributions and thesis outline

This thesis is based on three journal papers [10, 83, 84] and seven conference papers [4,6, 9, 74, 85–87]. The author of the thesis has had the main responsibility in developingthe original ideas, developing the mathematical framework, analysing the performanceand writing all of these papers. Other co-authors provided constructive criticism duringthe writing process of these papers. The research work has been performed under thesupervision of Prof. Marcos Katz who also provided invaluable comments and supportthroughout the writing of this thesis. In addition, the author of the thesis has publisheda number of supplementary papers which is related to this thesis [88, 89]. The author ofthe thesis is also the main author of all these papers. Research work [88, 89] has beencarried out under the supervision of Prof. Kaveh Pahlavan. Research work in [10] isperformed under the supervision of Adj. Prof. Sastri Kota. This thesis is composed offive technical chapters. The main contributions of this thesis are addressed in Chapter4 to 6. In the following, the content of the chapters is outlined briefly.

In Chapter 2, the elements of solid state lighting (SSL) optical transceiver devicessuch as LD, LED, PD and their properties, noise and interference characteristics in VLCnetworks are discussed. The generation of white light with LEDs, conversion of radioto optical properties and vice versa are addressed in this chapter. There are variousnoise sources present in the wireless optical links. A brief introduction to all thesenoise sources is addressed in this chapter. Finally, the characterization and modelling

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of artificial light interference, basic and advanced modulation techniques used in VLCare also discussed in this chapter.

In Chapter 3, four basic link topologies are described. Classification of these linkcharacteristics are done mainly according to the degree of directionality and the pres-ence of uninterrupted LOS between the transmitter and receiver. The basic characteris-tics of these link topologies are described in detail. Different types of diffuse links suchas the multi-spot diffuse link and dynamic multi-spot diffuse links are also discussedalong with four basic link topologies. Finally, link design of LOS and non-line-of-sight(NLOS) are also derived and discussed in details in this chapter.

In Chapter 4, LOS link and noise characteristics of optical wireless system in anindoor and outdoor environment are discussed. Theoretical and empirical throughput-distance relationship development are presented in details in this chapter. The math-ematical frame work for data downloading scenario is also presented in this chapter.Simulation results and discussion are reported in Section 4.7. Finally, conclusions aredrawn in Section 4.8.

In Chapter 5, we study the performance of the hybrid (radio-optical) WLAN-VLChotspot and compare its performance with stand-alone VLC-only or WLAN-only hotspotcases. We consider the data download on the move scenario in an indoor environmentfor the single-user as well as for the multi-user cases. Throughput-distance relation-ships are developed both for WLAN and VLC to evaluate the performance of an indoorenvironment by taking into account the radio and optical channel characteristics. Simu-lation results and discussion are reported in Section 5.4. Finally, conclusions are drawnin Section 5.5.

In Chapter 6, the scenario description of hybrid WLAN-VLC networks where relayselection, mobility and energy consumption are discussed. Numerical examples basedon chosen system model parameters of WLAN-VLC are also presented in this chap-ter. Finally, in Chapter 7, we summarize and conclude the dissertation and some openproblems for future research are addressed.

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2 Optical transceiver, noise and interference

The aim of this chapter is to introduce the fundamental building blocks of VLC transceiversystems. Various types of noise and interference present in VLC receivers and opticalchannels are also discussed in this chapter. It is predicted that LED will be used in futurelighting systems because of its energy efficiency and reliability compared to existinglighting solutions [90–92]. In VLC, white and coloured LEDs are used as transmitters;the air is the optical transmission medium, and the appropriate photodiodes or imagesensors are used as signal receiving components. Apart from LEDs, other SSL devicessuch as LD and organic LED (OLED) can be used as sources of data transmission [93–95]. Generation of white light with LEDs, conversion of radio to optical propertiesand vice versa are addressed in this chapter. Different noise sources and their impacton the performance on VLC networks are also addressed. Finally, basic and advancedmodulation techniques used in VLC are discussed in this chapter.

2.1 Optical transmitters in VLC

Many existing conventional lighting sources such as incandescent, fluorescent lightsand future SSL sources LD, LED, OLED can be used as optical sources for transmis-sion of data in VLC. The intensity of these optical sources can be modulated to transmitdata [26]. SSL devices such as LD/LED can be flickered faster than incandescent andfluorescent lights. In this respect, SSL based lighting sources are better than otherconventional lighting sources in higher data rate applications. For example, using flu-orescent light, data can be transmitted up to tens of kilo bits per second. On the otherhand, SSL devices such as LD/LED can be flickered fast enough to achieve a data rateup to several Gbps [19, 96, 97]. Moreover, reliability, stability, low power consumption,low cost and easy controllability of SSL sources such as LD/LED put them well aheadof conventional lighting sources and they are envisioned as future transmission sourcefor VLC [98]. LEDs/LD can be fabricated to emit light across a wide range of wave-lengths from the visible to the infra-red parts of the electromagnetic spectrum [31]. InVLC, the role of all these optical transmitters is to convert an electrical input signal intothe corresponding optical signal [31].

In many cases, LDs are preferable over LEDs because of their high data rate trans-mission capabilities and higher optical power outputs [31]. Data rate in the order of

35

Table 2. Comparison between LED and LD.

Characteristics LED LD

Optical output power Low power High powerOptical spectral width (25−100) nm (0.01−5) nmModulation bandwidth Tens of kHz to hundreds of MHz Tens of kHz to tens of GHzE/O conversion efficiency (10−20)% (30−70)%Eye safety Considered eye safe Must be rendered eye safeDirectionality Beam is broader and spreading Beam is directionalReliability High ModerateSource type Lambertian Point

Gbps can be achieved using high powered LDs as transmission source [99]. However,high powered LDs are potentially dangerous to eye safety especially in indoor envi-ronment, which may deliver very high power within a small area on the retina therebyresulting in permanent blindness. There are a few international standard bodies such asinternational electrotechnical commission (IEC), American national standards institute(ANSI), and committee for electrotechnical standardization international (CENELEC)which provide safety guidelines for both LD and LED emission level in terms of maxi-mum permissible exposer (MPE) values [99]. The MPE is the highest power or energydensity in W/cm2 or J/cm2 of a light source that is considered safe and represents negli-gible probability for creating damage to the eye. Moreover, LD is not suitable for usingas lighting source for illumination. Therefore, LD is not preferable for wide coverageof hotspot networks as so-called visible local area network (VLAN).

In indoor VLAN types of network, wide-angle radiation patterns of white and col-ored LEDs may be used as transmitters. On the other hand, LEDs are the preferred, asa light source for most future indoor as well as outdoor lighting applications. TheseLEDs are available in both visible and IR wavelengths. IR LED wavelengths range be-tween (830-940) nm. On the other hand, visible LED wavelengths fall into the regionbetween (400-700) nm, where a variety of colors including red, yellow, orange, amber,green, blue are available [100]. However, the closer the wavelength is to the visible partof the spectrum, the safer that wavelength is for the eyes. The comparison between thecharacteristics of LD and LED is summarized and shown in Table 2.

Recently usage of another SSL device such as OLED is gaining momentum in softlighting and display applications [94]. Great picture quality, brilliant colors, lowerpower consumption; faster refresh rate, better contrast, and greater brightness of OLEDput it well ahead of LCD [94, 95]. As such, they are expected to serve in the nextgeneration of full color displays and flat panel lighting [94]. An OLED is made from

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carbon-based organic materials that emit light when electricity is applied. It can alsobe easily be adapted for lighting and data communication in a number of applications[41–43, 90]. However, OLED offers much lower bandwidth than non-organic LEDs.The frequency response of OLED is much slower than conventional LED [94]. Thebandwidth of the OLEDs depends also on the other factors such as manufacturing pro-cess, material and size of the device. According to the recent development of OLED,a moderate data rate can be achieved using a small photoactive area of OLED. A datarate of 2.5 Mbps is achieved using an OLED with low bandwidth reported in [94]. TheLuminous efficacy of typical OLED is (40-60) lm/W. However, new materials researchin organic devices has already started to increase the high luminous efficacy up to 120lm/W, which will really satisfy the future need of illumination and moderate data rateapplications for VLC communication systems [101–103].

Aluminium Gallium Nitride (AlGaN) based micro-light emitting diode arrays havealso been developed for display panels. The range of each pixel of AlGaN is in therange of (14-84) µm [104]. A high data rate in the order of Gbps can be achievedincorporating parallel communications of Micro-LEDs. Micro-LED design and perfor-mance are discussed in detail in [105]. There are also other higher bandwidth LED suchas resonant-cavity LED (RCLEDs) and edge-emitting LED. RCLEDs provide data rateup-to 1 Gbps. On the other hand, edge-emitting LED provides data rate over hundredof Mbps. However, the edge-emitting LED has better output intensity than the RCLED[106].

2.1.1 Generation of white light with LEDs

The general purpose of white LEDs is to provide white lighting for illumination, whilewhite light emitting diodes are the most common optical sources proposed for transmis-sion of data in VLC. There are mainly two approaches for generating white light usingLEDs.

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White Light White Light

Red Blue Green Yellow phosphor Blue LED

(a) (b)

Fig. 2. White light: (a) Trichromatic based white LED. (b) Phosphor based white LED.

In the first approach, three primary colors such as red, green, blue are mixed to pro-duce white light, where the wavelength of red, green, and blue are 625 nm, 525 nm, and470 nm respectively. In this case, these three color emitters consist of a single packagewith combining optics [32]. These three primary colors can also produce other arbitrarycolors such as yellow, orange, amber by modulating the drive currents in each individ-ual LED of trichromatic LEDs. Hence, this kind of LED enables easy color renderingby adjusting each color independently. The production of white light using trichromaticLEDs is shown in Figure 2(a). Trichromatic LEDs are preferred over phosphorescentLEDs due to their faster rise-time and each color can be modulated independently whichcontributes to tripling the total throughput [62, 87].

In the second approach, white light is generated by the use of phosphor togetherwith a short-wavelength of blue or ultra violet (UV) light. This second approach iscalled phosphor-converted white light [80]. In this case, phosphor material used inLEDs is illuminated by blue light. Some of the blue light will be converted to yellowlight by the phosphor. The remaining blue light, when mixed with the yellow light,will produce white light. Commercial phosphor-based LEDs have limited bandwidthwhich is typically (2-3) MHz [107]. This is due to the slow response of the phosphor.However, bandwidth can be increased up to 20 MHz by using blue filter to suppress theslow phosphorescent components at the receiver. The production of phosphor-convertedwhite light using phosphor material is shown in Figure 2(b).

2.1.2 Radiometry and photometry

Electromagnetic radiation within the frequency range of (3×1011−3×1016) Hz is iden-tified as optical radiation [108]. Radiometry involves the measurement of this optical

38

Table 3. Radiometry parameters.

Quantity Terminology Units

Power radiant power flux Watt (W)Power per unit area Irradiance W/m2

Power per unit solid angle radiant intensity W/srPower per area per solid angle radiance W/(sr.m2)

Table 4. Photometry parameters.

Quantity Terminology Units

Power luminous flux lmPower per unit area illuminance W/m2

Power per unit solid angle luminous intensity candela (cd) (= lm/sr)Power per area per solid angle luminance cd/m2

radiation. The wavelength of the ultraviolet, the visible and the infra-red fall into thisfrequency range. However, in radiometry, this measurement does not take into accountthe specific sensitivity of the human eye, rather it represents the light transmitting capa-bility of a light source for the wide spectrum. Typical radiometric units include Watt,irradiance, radiant intensity and radiance as shown in Table 3. On the other hand, pho-tometry examines only the radiation that humans can see. Thus, photometric parameterstake only account into the visible band with different weights for various wavelengths[100]. Typical photometric units include lumen, illuminance, luminous intensity andluminance as shown in Table 4.

VLC is an opto-electrical wireless communication system where conversion be-tween radiometry to photometry or vice versa is necessary. In order to convert radiomet-ric values into photometric values, and vice versa, the relative visibility of the light ofthe particular wavelength should be taken into account, which is represented as the eyesensitivity curve [31]. The eye sensitivity curve is also called the luminous efficiencycurve and it is the ratio of any photometric unit to its radiometric equivalent unit.

The most common unit in radiometry is the Watt, which measures radiant flux(power), while the most common unit in photometry is the lumen, which measuresluminous flux. The relationship between radiometric and photometric can be expressedas

Photometricunit[lx] = radiometricunit[Watt]×683(

lmW

)×V (λ ), (1)

where V (λ ) is the standard luminosity curve. Hence, for monochromatic light of 555nm, radiometric 1 Watt is equivalent to photometric 683 lumens. For light at other

39

−4−2

02

4

−4

−2

0

2

4650

700

750

800

X[m]Y[m]

Illu

min

ance

[lu

x]

660

670

680

690

700

710

720

730

740

750

760

Fig. 3. Illuminance distribution.

wavelengths, the conversion between Watts and lumens is slightly different, becausethe human eye responds differently to different wavelengths.

Two basic properties of an LED are luminous intensity and transmitted opticalpower. Luminous intensity is used to express the brightness of LED. On the otherhand, transmitted optical power is used to indicates the total energy radiated from anLED.

Mathematically, luminous intensity can be expressed as luminous flux per solidangle and is given as [31]

I =dΦdΩ

, (2)

where Φ is the luminous flux and Ω is the spatial angle. Φ can be calculated from theenergy flux ϕef as

Φ = Kmax

∫ 780

380V (λ )ϕef (λ )dλ , (3)

where Kmax is the maximum visibility, which is about 683 lm/W at 555 nm wavelength.The integral of the energy flux ϕef in all directions is the transmitted optical power Pt

given as [31]

Pt = Km

∫ Λmax

Λmin

∫ 2π

0ϕef dθdλ , (4)

where Λmin and Λmax are determined from the photodiode sensitivity curve. For anLED lighting with a Lambertian radiation pattern, the radiation intensity at the receiving

40

plane is given asI(ϕ) = I(0)cosm(ϕ), (5)

where ϕ is the angle of irradiance and I(0) is the center of luminous intensity. m is theorder of Lambertian emission. The illuminance distribution with a semi-angle 60o isshown in Figure 3. According to the international lighting standard organization, fullillumination with a mean level of 350 lux is needed in both office and home environ-ments.

2.1.3 LED I-V characteristics

In VLC, the real value of baseband signal is modulated onto the instantaneous power ofthe optical carrier. In this case, a modulation signal is linearly encoded using a currentsource driver. However, one of the disadvantageous properties of LEDs is their non-linear relationship between current and voltage. The Shockley Equation models thediode current iD as a function of the diode voltage VD as

iD = i0(eVD

ndifVT −1), (6)

where iD, VD, and i0 are the diode current, diode voltage, and saturation current respec-tively. ndif is known as the diode ideality factor and VT is the thermal voltage. Therelationship between LED I-V characteristics is shown in Figure 4.

In every LED, minimum current is required for photon emission to occur. This isknown as conduction region and there is a certain threshold value of voltage at whichit starts to show the linear relationship between diode voltage and diode current. Thisthreshold value is known as the turn-on voltage (TOV) [80]. However, above of TOV,current and voltage of diode will not follow the linearity in LED I-V characteristics.Therefore, the range between minimum and maximum allowed current where the rela-tionship between current and light output is linear can be considered as the operatingregion for signal modulation. Currently, the question of combating non-linearity isone of the biggest challenges for VLC systems specially for OFDM signal [109–111].Therefore, one approach to combat non-linearity is to operate the LED in a small range(between minimum and maximum of TOV) where its output characteristic is linearenough [111].

41

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90

0.002

0.004

0.006

0.008

0.01

0.012

Forward voltage [V]

For

war

d cu

rren

t [A

]Operating region

Fig. 4. LED I-V characteristics.

In [80, 109], measured LED I-V is approximated with the 6th degree polynomialwhere the top and bottom of the curve are saturated in a so-called S-shaped curve. Intheir work, they had performed analysis of OFDM signal using the model of 6th degreefitted polynomial function of LED I-V curve. In a radio frequency systems, a commonlyused model to describe the non-linearity behaviour of power amplifier (PA) is Rappsmodel. The Rapps model can be described as [112]

Vout =Vin(

1+( VinVmax

)2k)(1/2k)

, (7)

where Vout is the PA output voltage, Vin is the PA input voltage, Vmax is the maximumoutput voltage and k is called the knee factor that controls the smoothness of the transi-tion from the linear to the saturation region.

In [80, 109], Rapps model has been modified to describe the increase the linearity ofI-V characteristics of LED. The modified LED model behavior is described as follows

iLED =

hvLED i f vLED ≥ 0

0 i f vLED < 0,(8)

where iLED and VLED are the current through the LED and the voltage across the LEDrespectively. The transfer function hvLED describes the dependence of the emitted optical

42

0 0.2 0.4 0.6 0.8 10

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

LED(V)

LED

(A)

k=40k=3k=2

Fig. 5. The non-linear modeling.

power on the driving current. It is seen in Figure 5 that the saturation part of LED I-Vcharacteristics is controlled by knee factor k and it is also seen that as the value of k

increases, the linearity of I-V characteristics also increases. The modified Rapps modelfor LED can be written as

Vout =f (vLED)(

1+( f (vLED)imax

)2k)1/2k , (9)

where imax is the maximum permissible current.

2.2 Wireless optical receiver

In a wireless optical receiver, a photodiode is used to detect light and convert it intoelectrical signal. Two main components of photo detector circuits are the photo diodeand resistor. There are mainly two types of basic detectors: the PIN and the APD. ThePIN photodetector performs better at longer optical wavelengths, while APD performsbetter at shorter wavelengths [113].

The image sensor used in a cell phone camera or digital camera can also be used asan optical receiver. The image sensor used in these devices is basically an array of pho-todiodes. Some advantages of an image sensor receiving system over the conventionalphoto diode receiving system are parallel receptors and robustness against interferinglight sources. Image sensors used for digital cameras or video cameras usually have a

43

Fig. 6. Basic elements of optical receiver.

frame rate of tens of frames per second. The automotive applications using image sensorfor vehicle-to-infrastructure-VLC (V2I-VLC) and vehicle-to-vehicle-VLC (V2V-VLC)are presented in [114].

2.2.1 Elements of a photodetector

The performance of most OW systems is determined by the receiver rather than transmit-ter. The VLC receiver is composed of optical collection elements such as a concentrator,optical filter, photodiode and amplifier, as shown in Figure 6.

Light enters the receiver through the concentrator. The optical concentrator is usedto compensate for high spatial attenuation due to the beam divergence from the LEDs toilluminate a large area. By using the appropriate concentrator, the effective collectionarea can be increased. For the optical collection element, the aim is to maximize boththe FOV as well as the collection area.

In VLC, both natural daylight and artificial illumination sources act as noise sources,most of the energy from the sun is in the visible and infra-red spectrum [46, 115]. There-fore, it is important to employ both electrical as well as appropriate optical filter to rejectunwanted DC noise components in the recovered data signal. Scattered sunlight, which

44

concentrates around zero Hz, can be easily removed by an electrical filter [108, 116].On the other hand, optical filters are used to reduce the background interference from ar-tificial illumination. The optical filter is usually used to pass only the relatively narrowband radiation of the transmitter, while rejecting the (optically) broadband of unwantedlight falling outside the useful band.

Photodetectors convert the optical power to a current proportional to it [31]. Thiscurrent is usually converted to a voltage with the aid of a transimpedance amplifier, andthen further amplified before data recovery. The photodiodes with good responsivityto visible light are Si PIN-PD (silicon p-type-insulator-n-type photodiode) and Si APD(silicon avalanche photodiode). The silicon material photodiode operates from (400-1200) nm. There are also many other photodiodes whose bandwidths are over 200MHz and is much wider than the VLC LED transmitter [31].

2.3 Noise sources in wireless optical communications

There are various noise sources present in the wireless optical links. Sources of thisnoise are in the channel as well as generated locally in the receiver. The noise gen-erated in VLC are shot noise, optical excess noise, photodetector dark current noise,photodetector excess noise and thermal noise [117]. However, in this work we onlyconsider shot noise produced by natural and artificial noise sources such as sun, fluores-cent and incandescent light sources. The basic definitions of shot and thermal noise aregiven below:

2.3.1 Shot noise

Shot noise is considered to be the dominant noise sources in wireless optical commu-nications [46]. The origin of shot noise is due to the presence of both ambient lightand transmitted signal. Shot noise is modeled as a Poisson distribution with a whitepower spectral density due to the discrete random nature of energy and charge in thephotodiode. Mathematically, it can be expressed as [31, 46]

σ2shot = 2qRrPn, (10)

where q is the electron charge. Rr and Pn are the photodiode responsivity and the aver-age power of ambient light respectively.

45

2.3.2 Thermal noise

In conducting materials, noise is generated due to the random motion of electrons inresistive and active devices. This random motion of electrons which gives rise to noisevoltage is called thermal noise [31]. A large number of free electrons and ions areresponsible for generating this noise, which is bounded by molecular forces in the con-ductor. Hence, there is a continuous transfer of energy between the ions and electrons.This is the source of resistance in the conductor. Thermal noise is also a function oftemperature. Thermal noise is generated independently of the received signal and canbe modeled as

σ2thermal =

4kBTk

RFI2Rb +

16π2kBTk

gmΓ(Cd +Cg

)2I3R3B. (11)

The first term represents thermal noise generated in feedback resistor. kB and Tk are theBoltzmann’s constant and absolute temperature respectively. RF is the feedback resis-tance. Second term describes the thermal noise from the field-effect transistor (FET)channel resistance where Γ is the FET channel noise factor, gm is the FET transin-ductance, Cd is the capacitance of a detector. Cg is the FET gate capacitance, andI3 = 0.0868.

Sum of contributions from the shot and the thermal noises can be written as

σ2total = σ2

shot +σ2thermal. (12)

2.3.3 Measurement-based noise characterisation

In [115, 118], the shot noise produced by natural and artificial light sources is charac-terised as DC current. The characterisation of noise and interference was done throughextensive measurements. Measurements had been performed: with and without a filter.In the case of a filter, a long-pass absorption optical filter with a cut-off wavelength 800nm was used. In those measurements, sunlight and artificial lights had been consideredas sources of noise and interference. For example, direct and indirect sunlight usedas ambient noise source in the receiver. On the other hand, incandescent lamps withtungsten filament, fluorescent lamps with conventional and electronic ballasts used asartificial noise sources. Irradiance produced by sunlight is steady and has a slow varia-tion of intensity in comparison to the irradiance produced by artificial lights.

46

Table 5. Background current in day and night conditions.

Source Without optical filter (µA) With optical filter (µA)

Direct sunlight 5100 1000Indirect sunlight 740 190Incandescent light 84 56Fluorescent light 40 2

Shot noise power is directly proportional to that current [115, 118]. This inducedcurrent can be easily included in the system models to account for the shot noise pro-duced by the background light. Induced noise current also depends on the effective areaand type of photodiode used in the optical receiver. In [115, 118], the experiment wasperformed by measuring the current induced on a 0.85 cm2 Silicon PIN photodiode.The measured shot noise in day and night conditions is shown in Table 5 [115, 118].Background current Ib can be expressed as [115, 118]

Ib = Inatural + Iartificial, (13)

where Inatural and Iartificial are the DC currents produced by natural and artificial lightsources respectively. Background noise generated by different noise sources, are listedin Table 5 [115, 118]. The shot noise is independent of the signal and can be modeledas a white Gaussian N0 given by [119]

N0 = 2qIb. (14)

2.4 Basics building blocks of optical transceiver

The equivalent baseband model of an IM/DD optical wireless link can be summarizedby the following

y(t) = Rr xi(t)⊗

h(t)+n(t), (15)

where⊗

is the convolution operation. xi(t), h(t), and n(t) represent the instantaneousoptical power, baseband channel impulse response, and signal independent shot noiserespectively. In optical wireless communications, the instantaneous optical power xi(t)

is proportional to the generated electrical current, hence xi(t) represents the power than

47

)(tx )(thR

)(tn

)(ty

Fig. 7. Equivalent optical wireless baseband model.

the amplitude of the signal. This imposes two constraints such as the xi(t) should be non-negative and maximum optical transmit power should not exceed eye safety requirementlimit. According to the first constraint, xi(t) should be nonnegative can be written as

xi(t)≥ 0. (16)

Secondly, according to the second constraint, the average value of xi(t) must not exceeda specified maximum power value Pmax, that is given by

Pmax = limT→∞

12T

∫ T

−Txi(t)dt. (17)

The impulse response h(t) is generally used to analyze effects of multipath disper-sion in indoor optical wireless channels. The channel impulse response can be modeledas [31]

h(t) =

2t0

t3 sin2(FOV )t0 ≤ t ≤ t0

cos(FOV )

0 elsewhere,(18)

where t0 is the minimum delay.The channel DC gain can be expressed as [31]

H(0) =∫ ∞

−∞h(t)dt. (19)

In general, signal-to-noise ratio (SNR) is used to express the quality of performance incommunication systems. In such case, the performance of wireless optical link at thebit rate Rb can be related to SNR as [46]

SNR =R2

r H2(0)P2t

RbN0. (20)

48

We assume that the transmitter transmits the signal using an OOK modulation technique.The BER of the OOK can be expressed as [46]

BER = Q(√

SNR), (21)

and the function Q(x) is defined as [46]

Q(x) =1

√2π

∫ ∞

xe(

−y22 )dy. (22)

2.5 Modulations in VLC

In VLC, the information is carried by the intensity (power) of the light. As a result,the information-carrying signal has to be real valued and strictly positive [80]. Thereare also other issues such as dimming support and flickering that have to taken intoaccount when designing modulation schemes in VLC [37]. Dimming support is neededfor power saving and energy efficiency. The purpose of dimming support is to main-tain communication while the user arbitrarily dims the light source. On the other hand,flickers refer to the fluctuations of the brightness of light which can cause noticeablenegative and harmful physiological changes in humans. To avoid flicker, the changesin brightness must fall within the maximum flickering time period. Optimal flickerfrequency greater than 200 Hz is generally considered safe. Therefore, designing mod-ulation techniques for VLC dimming support and flickering issues should also be takeninto account in such way that both illumination and communication are well supported[120–125].

There are number of methods which can be used to modulate the data over thevisible light spectrum. The most common methods are: OOK, pulse width modulation(PWM), pulse position modulation (PPM), variable pulse position modulation (VPPM),color shift keying (CSK), and orthogonal frequency division multiplexing (OFDM) [36,37, 126–130].

In OOK modulation, LEDs are turned on or off dependent on the data bits being ”1”or ”0”. A bit ”1” is simply represented by an optical pulse that occupies the entire or partof the bit duration while a bit ”0” is represented by the absence of an optical pulse. Boththe return-to-zero (RZ) and non-return-to-zero (NRZ) schemes can be applied in VLC.In the NRZ schemes, a pulse with a duration equal to the bit duration is transmitted to

49

represent ”1” while in the RZ schemes the pulse occupies only the partial duration ofbit. PWM is a way of encoding analog signal levels into a pulsing signal digitally. Insuch case, the information is encoded into the duration of pulses. In PWM, more thanone bit of data can be conveyed in longer pulse than for OOK. In PPM, information isrepresented by the position of the pulses within the fixed time frames. In comparison toOOK, PPM modulation technique has a higher signal bandwidth and power efficiency.It has the advantage of containing the same amount of optical energy within each frame.However, in the presence of multipath echoes, PPM does not perform well due to findthe correct pulse position. Color-shift keying (CSK) is a visible light communicationintensity modulation scheme where the data can be carried by the color itself by red,green, and blue light emitting diodes [128].

Variable PPM (VPPM) is a modulation scheme that is compatible with a dimmingcontrol that varies the duty cycle or pulse width to achieve dimming, as opposed to am-plitude. VPPM combines 2-PPM with PWM for a dimming control. Bits ”1” and ”0”in VPPM are distinguished by the position of a pulse, whereas the width of the pulse isdetermined by the dimming ratio [36]. In pulse interval modulation (PIM), informationis encoded by inserting empty slots between two pulses. The PIM offers a reduced com-plexity compared to PPM due to its built-in-symbol synchronization. There are alsoother modulation schemes based on the PPM and PIM have been suggested and inves-tigated in [31, 131]. The suggested modulation schemes either improve throughput orreduce power requirements by adopting a pattern of complex symbols or by adoptingmultilevel amplitude. Digital (PPM), digital (DPIM) and dual header PIM (DH-PIM)are examples of these three new modulation methods based on PPM and PIM for opticalwireless communication, these new modulation techniques can be used as substitutionfor PPM and PIM for their better performance in terms of power efficiency and band-width efficiency. Theoretical analysis and simulation results show that DPPM, DPIMand (DH-PIM) are more applicable for future optical wireless communication systemspecially in VLC [31, 131].

IEEE 802.15.7 VLC has defined three PHY and their respective modulations tech-niques according to the applications it used. PHY I is intended for outdoor use withlow data rate applications. This mode uses OOK and variable pulse position modula-tion (VPPM) modulations with data rates in the tens to hundreds of Kbps. PHY II isintended for indoor use with moderate data rate applications. This mode also uses OOKand VPPM with data rates in the tens of Mbps. In PHY III CSK modulation methodis used to transmit data through the light’s color property of a multi-color light source.

50

Table 6. Comparison between different modulation schemes.

Modulation Dimming support Susceptibility toLED non-linearity

Spectral efficiency Flicker

OFDM No Hi 3−4 MediumSM No Low 2 HiVPPM Yes Low <1 MediumMPPM Yes Low <1 LowEPPM Yes Low <1 LowMEPPM Yes Low 2−3 Very Low

It provides data rates in the range (12-96) Mbps. Reed Solomon codes can be used forforward error correction in these modulation schemes.

Recently, many advanced modulation techniques such as expurgated PPM (EPPM),and orthogonal frequency-division multiplexing (OFDM) have been proposed to in-crease the data rate to multi-Gbps in VLC. EPPM is suitable for peak power limitedcommunication systems [54, 132]. To increase spectral efficiency, multilevel forms ofEPPM have also been proposed in [55]. Both EPPM and multilevel EPPM (MEPPM)are able to support a wide range of optical peak to average power ratios (PAPRs) andtransmit high speed data even in highly dimmed scenarios. A comparison of differentmodulation schemes is shown in Table 6.

OFDM is a special version of subcarrier modulation where all the subcarrier fre-quencies are orthogonal [133]. In this modulation technique, intensity is modulated viathe time variant OFDM signal to achieve wireless access. There are mainly two tech-niques to generate unipolar OFDM: DC-biased optical OFDM (DCO-OFDM), asym-metrically clipped optical OFDM (ACO-OFDM) [134, 135]. In DCO-OFDM, a DCbias is put into the signal where constellation size determines the optimum bias. Util-ising a proper DC operating point, the optical carrier intensity is modulated in bipolartime domain results. On the other hand, ACO-OFDM clips the OFDM signal at thezero level where data is carried in only odd subcarriers. ACO-OFDM is far more powerefficient than DC biased OFDM although which are coming at the cost of losing half ofthe available bandwidth. There is also another technique to generate unipolar OFDMsignal named itself unipolar OFDM (U-OFDM [136]. In this technique, OFDM sam-ples are rearranged and sent them in separate positive and negative blocks. It has thesame spectral efficiency as ACO-OFDM, but from the power efficiency point of view itis better than ACO-OFDM.

51

52

3 Link characterisation in VLC

In VLC, link topologies can be configured in many ways. However, they are typicallyclassified into four different basic links. Classification of these topologies are donemainly according to the degree of directionality and the presence of uninterrupted LOSbetween the transmitter and receiver. The basic characteristics of these link topologiesare described in detail. Different types of diffuse links such as multi-spot diffuse links,dynamic multi-spot diffuse links are also discussed along with four basic link topolo-gies. Link design of LOS and NLOS are also derived and discussed in this chapter.Finally, measurement-based link design and their performance are discussed.

3.1 Basics link types of VLC

Four different basic link characteristics in an indoor environment are shown in Figure 8.These four link characteristics are classified as: directed-LOS, non-directed-LOS, non-directed-NLOS and tracked [31, 45, 46]. There are mainly two criteria used to classifythese link topologies. The first is the degree of directionality and the other is the pres-ence of uninterrupted LOS between transmitter and receiver. In directed directionality,both transmitter and receiver have narrow FOV. On the other hand, in non-directed di-rectionality, both transmitter and receiver have wide FOV. If the LOS path is present indirected directionality, then this link topology is defined as directed-LOS, otherwise linktopologies can be classified as directed-NLOS and non-directed-NLOS. Non-directed-NLOS is also known as diffuse link [31, 46].

Directed LOS is typically used in point-to-point communication links in indoor andoutdoor environments as shown in Figure 8(a). This type of link topology experienceslower path loss and less impact of ambient light noise. As a result, hundreds of Mbpsdata rates can be achieved in directed LOS indoor as well as outdoor OW links. How-ever, since this link topology requires strict alignment for LOS between transmitter andreceiver, mobility is an issue to work with this link topology.

In non-directed-LOS, a wide beam transmitter and wide FOV receivers are usedto achieve a broader coverage area as shown in Figure 8(b). Non-directed-LOS can beused in point to multi point communications. In this type of link topology, in addition tothe LOS link there are also other multiple signal reflections from the walls and objectsof the room. As seen in Figure 8(c), the link without LOS and completely depending on

53

Tx

Rx

Tx

Rx Rx

Tx

Rx Rx

(a) (b)

(c) (d)

Fig. 8. Link topologies: a) directed-LOS, b) non-directed-LOS, c) non-directed-NLOS, d)tracked.

reflections from the walls and ceiling is known as a non-directed-NLOS or diffuse link.Diffuse links are relatively immune to blockage and pointing errors and permit a greatdegree of mobility for the receivers in an indoor environment. However, the receivedsignal is corrupted by multipath dispersion. As a result, large number of collectedreflections received at the receiver limit to achieve a high data rate due to inter-symbolinterference [31, 46].

Multibeam transmitters together with a multiple element narrow FOV angle diver-sity receiver is also suggested in [137, 138]. This type of link configuration is alsoknown as quasi multi-spot beam. In this link architecture, a combination of point-to-point links with the mobility is achieved through diffuse links. In this case, each dif-fusing spot would be considered as a source of LOS link to a narrow FOV of receiver.Although diffuse and multi-spot diffuse links depend on the reflections but there arenoticeable difference between these two link configurations. In the diffuse link, lightemits over a large divergence angle, on the other hand, in multi-spot diffuse link a se-ries of narrow divergence beams directed to the ceiling which results into far smaller

54

Table 7. Comparison of three link models.

Characteristics Point-to-point Diffuse Quasi-diffuse

Range Long Moderate ModerateRate High Low ModerateLOS Yes No NoMobility No Yes YesImplementation cost Low Moderate High

path loss in multi spot diffuse systems than in diffuse systems. The multi-spot diffusereceiver consists of a series of narrow FOV elements directed to the ceiling which re-sults into less ambient noise in the receiver in comparison to wide FOV used in diffuselinks which allows large ambient noise. Therefore, multi-spot diffuse links provide bet-ter communication links and mobility in comparison to diffuse links at the expense ofcomplex transceiver architecture. A comparison of these three link topologies (point-to-point, diffuse, quasi-diffuse) is shown in Table 7.

A special case of the general spot diffusing model is proposed as dynamic spotdiffusing (DSD) and reported in [99], where the transmitter consists of one or a smallnumber of spots, which are translated across the ceiling in a closed path. The detectoris composed of multi-element imaging receivers. Data is received only whenever thespot is in the FOV of the receiver. In DSD, the channel is time-varying because of thespot motion. In comparison to the conventional spot-diffusing techniques, DSD has farfewer spots and hence has a lower multipath distortion. Tracked directed links are adifferent link architecture than others as mentioned earlier. In this case, the base station(BS) is mounted on the ceiling and the mobile terminal (MT) is placed at the table heightas shown in Figure 8(d). In case of both for uplink and downlink, the beam from thetransmitter is concentrated onto the receiver. In this architecture, diffuse link, directedlink, position detection, and tracking can be realized with one and the same transceiverhardware.

In a tracked system, both diffuse and tracked directed links are used for high-speedcommunications. In this case, diffuse link is used for connectivity and tracked directedlinks are used for high-speed communications. The transmitter in tracked directed linkarchitecture is composed of a laser diode array in combination with multiple-beam form-ing optics. On the other hand, the receiver is composed of a wide angle lens, and anarray of APDs. Because of the narrow beam of transmitter and narrow FOV of thereceiver, high bandwidth becomes available due to minimum multipath signals. More-over, interference from the sun is also reduced because of pointing a directive receiver

55

LED

Photodetector

Half-power angle

Concentrator

ψcΨ

φ D

Fig. 9. Link configuration in LOS case.

selectively toward the transmitter. A manually tracked directed IR link with 155 Mbpsover a distance of 2 m is reported in [139].

3.1.1 Directed LOS link design

In OWC, the radiant intensity Ri for LOS propagation model, as shown in Figure 9 canbe expressed as [46]:

Ri = PtR0(ϕ), (23)

where Pt is the transmitted power and R0(ϕ) is the Lambertian pattern. The Lambertianpattern at the incidence angle ϕ can be expressed as:

R0(ϕ) =(m+1)

2πcosm(ϕ), (24)

where m is the order of the Lambertian radiation pattern and related to the LED semi-angle at half-power Ψ1/2 given by

m =ln2

ln(cos(Ψ1/2)). (25)

It should be noted that R0(ϕ) is a function of two angles: angle of incidence ϕ and halfpower semi angle Ψ1/2.

56

10

10

10

10

10

10

10

10

20

20

20

20

2020

20

2030

3030

30

3030

30

40

40

40

4040

40

50

50

5050

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60

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60

60

60

70

70

7070

80

8080

90

90

90 100

100 110

110

X[m]

Y[m

]

Data Rate [Mbps]

−2 −1 0 1 2−2.5

−2

−1.5

−1

−0.5

0

0.5

1

1.5

2

2.5

10

20

30

40

50

60

70

80

90

100

110

Fig. 10. Data rate in LOS link.

Without considering reflections, DC gain H(0) in LOS channel can be expressed as

H(0) =

(m+1)Ad

2πD2 cosm(ϕ)Ts(ψ)g(ψ)cos(ψ) 0 ≤ ψ ≤ Ψc

0 ψ > Ψc,

(26)

where D is the distance between LED and photo detector (PD), Ad is the detector area,Ts(ψ) is the signal transmission of the filter, g(ψ) is the concentrator gain and Ψc isthe concentrator FOV that can be represented as a semi-angle. The relationship amongreceived power Pr, Pt and H(0) can be expressed as

Pr = H(0)Pt . (27)

The concentrator is made of transparent materials such as glass or plastic, which can beused to increase the physical area. The concentrator gain g(ψ) of an ideal non-imagingconcentrator with refractive index kr can be expressed as

g(ψ) =

k2

r

sin2(Ψc)0 ≤ ψ ≤ Ψc

0 ψ > Ψc.

(28)

57

LED

Photodetector

Fir

st

re

fle

cto

r α

β

1φ

φ D

cΨψ2d

1d

Fig. 11. Link configuration in N-LOS case.

The effective collection area of the PD can be represented as [46]

A f =

Ad cos(ϕ), |ϕ |< FOV

0, |ϕ | ≥ FOV.(29)

Data rate and coverage for the LOS link case is shown in Figure 10. In this case, weconsider a general indoor scenario where 5×5×3 m3 room is chosen for LOS commu-nication. In this model, the placement of the receiver is 0.85 m on top of the ground.The numbers of LEDs in the transmitter are 12. The half-power angle of each transmit-ter is 85o. The transmit power of each LED is 12 mW. The ambient noise is modelledas N = 10−22. The transmission and concentrator gain of the photodetector are chosento be 1. FOV and responsivity of the receiver are 50o and 0.85 cm2 respectively. Themaximum achievable data rate is 110 Mbps.

3.1.2 Non-directed link design

In non-directed LOS and non-directed NLOS (diffuse) links, optical path loss dependson many factors such as room dimensions, ceiling reflectivity, walls and objects withinthe room. Moreover, the position and orientation of the transmitter and receiver, win-dow size and place and also affects on the performance. The total received power innon-directed link is composed of direct path and the first order reflected path. It can be

58

2020

20

20

2020

20

2040

40

40

40

40

40

60

60

6060

60

80

80

80

80

100

100

120

X[m]

Y[m

]Data Rate [Mbps]

−2 −1 0 1 2−2.5

−2

−1.5

−1

−0.5

0

0.5

1

1.5

2

2.5

20

30

40

50

60

70

80

90

100

110

120

Fig. 12. Data rate and coverage in LOSlink.

0.05 0.05 0.05

0.05

0.05

0.050.050.05

0.050.05

0.05

0.05

0.05

0.1 0.1

0.1

0.1

0.1

0.10.1

0.10.1

0.1

0.10.1

0.1

0.15 0.15 0.15

0.15

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0.150.150.15

0.15

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0.15 0.15

0.150.15

0.2 0.2 0.2

0.2

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0.20.20.2

0.2

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0.35

X[m]

Y[m

]

Data Rate [Mbps]

−2 −1 0 1 2−2.5

−2

−1.5

−1

−0.5

0

0.5

1

1.5

2

2.5

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Fig. 13. Data rate and coverage in NLOSlink.

expressed as

Ptotal =(Hlos(0)+Hnlos(0)Pt

)=

(Hlos(0)+ ∑

#reflecHreflec(0)

)Pt , (30)

where Hreflec represents the channel DC gain on reflection points and can be expressedas

Hreflec(0) =(m+1)Ad ρ dA

2π2d21d2

2cosm(ϕ)Ts(ψ)cos(β )cos(α)g(ψ)cos(ψ) 0 ≤ ψ ≤ Ψc

0 ψ > Ψc,

(31)

where d1 and d2 are the distance between an LED and a reflective point and the distancebetween a reflective point and a receiver respectively. ρ is the reflectance factor. dA isa reflective area of small region. α is the angle of incidence to a reflective point andβ is the angle of irradiance to the receiver. Data rate and coverage for non-directedNLOS link design case is shown in Figure 13. In non-directed cases, we keep boththe transmitter and receiver specification the same as in the LOS case. The reflectioncoefficient of the walls of the room is 0.8.

Figure 13 shows the data rate of the diffuse link. In this case, we only consider thedata rate is achieved only from the reflected paths of the walls. We keep the same valueof the parameters of the transmitter, receiver and room size as we consider in case ofLOS and non-directed LOS.

59

Data

sourceLEDModulation

Pre-

equalization

LED

driver

DetectorData

sinkDemodulation

Post-

equalizationAmplifier

Op

tical

Ch

an

ne

l

Fig. 14. Block diagram of VLC transceiver systems.

3.2 Measurement based link design

In this section, a simple GNU radio based testbed has been implemented to evaluatethe performance of the VLC link. GNU radio is a free software development toolkitfor runtime signal processing that has the capability of interacting with the externalhardware typically used for transmitting RF signals [119]. However, in this work, thishardware namely the universal software radio peripheral (USRP) is interfaced with anoptical front end to create a point to point VLC link.The purpose of using USRP in thetestbed for providing a versatile and cost efficient platform for the system designing.Moreover, the system designs can be modified and duplicated easily, for example, theused parameters like frequencies, amplification and filtering are simply controllable inreal time with the software used in USRP. The systematic block diagram of the VLCsystem implemented in the testbed is shown in Figure 14.

In the testbed, both the transmitter and receiver have a signal processing part and ananalogue part. For example, in the transmitter side, the data source, modulation and pre-equalization are considered to be the signal processing part. The characteristics of thesesignal processing modules are implemented in the USRP device. However, the pre-equalization can also be implemented with external circuits. On the other hand, such asthe LED driver, the LED and the transmitter optics are considered as the analogue part.Similarly in the receiver side post-equalization, the demodulation and data sink can beconsidered as the signal processing part and the detector and amplification belong tothe analogue part.

The raw data from the data source generated in USRP is used to feed the modulationmodule. The USRP provides many different modulation schemes to modulate incomingdata from the data source. Modulation schemes such as OOK, PPM, pulse-amplitude

60

Fig. 15. Visible light communication testbed implemented in University of Oulu.

modulation (PAM), PWM are available for selection in USRP. In this testbed, we useGMSK modulation to modulate the incoming data that is already coded in the GNU Ra-dio [140]. Several measurements were conducted during the implementation to verifythe adoption of the GMSK in the VLC testbed. Verification is done by calculating anerror vector magnitude (EVM) using Agilent E4446A PSA Series spectrum analyzerequipment and Agilent 89600 VSA 16.0. The result of this verification is discussed in[140].

The experimental setup of the VLC prototype is shown in Figure 15. GNU radiosoftware is used to interact between two N210 software radio peripherals (USRP2).Two types of LEDs, such as phosphor coated LED and trichromatic LEDs (RGB) areused as transmitters in VLC link measurement. A commercial RGB LEDs (LED EN-GIN, 03MC00, 40 W, 81 mm footprint) LEDs, and a blue-phosphor LED (LUXEON)were used in this testbed. On the other hand, a Thorlabs photodetector (PDA36A) isused as receiver. This particular photodiode has a built-in transimpedance amplifier thatis set at the default value of 10 dB. Data rate and light intensity are recorded by varying

61

the distance between the transmitter and receiver both for phosphorus and trichromaticLEDs as shown in Figure 16 and Figure 17 respectively. It is obvious as it is seen alsoin both of these Figures that as the distance between transmitter and receiver increases,the data rate and light intensity also decreases. The maximum data rates realized were6.25 Mbps and 12.5 Mbps for the blue-phosphor and RGB LEDs, respectively. Theo-retically, in case of RGB LEDs, all three colours could be modulated simultaneouslyto achieve bitrate up to 37.5 Mbps. Lower data rate achieved for blue phosphor due toslow response rates of the phosphor required to produce white. The blue LED emis-sion maximum at 450 nm, corresponds with a 0.3 A/W responsivity in the photodiodeused in this testbed, whereas at longer wavelengths the responsivity is up to 0.9 A/W.However, the obtained results are still significantly lower than VLC links with customsoftware and hardware that have achieved several Gbps.

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

2

4

6

8

10

12

14

Distance [m]

Dat

a ra

te [

Mb

ps]

RGB LEDsBlue LED

Fig. 16. Data rate vs. distance.

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

100

200

300

400

500

600

700

800

Distance [m]

Lig

ht

inte

nsi

ty[l

ux]

Fig. 17. Light intensity vs. distance.

62

4 Data downloading on VLC coverage

In this chapter, we provide the methodology to evaluate the performance of VLC hotspotnetworks in the context of data download on the move by using throughput-distance re-lationship models. In this regard, the motivation and related work of data download onthe move in sparse-based coverage are described in Section 4.1. The concept of sparse-based infostation coverage is introduced in this section. Secondly, the geometry of VLChotspot coverage is described in Section 4.2, where many optical design parameterssuch as FOV of the receiver, semi-angle at half-power of the transmitter are consideredin modeling. Mobility related parameters such as travelling path, velocity of the userare also taken into account in the analytical model. The transformation of travellinginclined path into horizontal path is introduced in Section 4.3. Detailed descriptionsof the mathematical framework and VLC hotspot design parameters and their relation-ships are given in Section 4.4 and 4.5 respectively. Theoretical throughput vs. distancerelationship and its approximation to the so-called empirical throughput-distance rela-tionship model is introduced and derived analytically by taking into account the opticaldesign parameters such as FOV of the receiver, semi-angle at half-power of transmitter,ambient and artificial shot noise and so on in Section 4.6. The purpose of this empiricalthroughput-distance relationship model is to calculate the average throughput analyti-cally in the receiving plane.

Performance analysis of such VLC hotspot networks in the context of data down-load on the move scenarios is given in Section 4.7. Performance of VLC hotspot net-works is quantified in terms of received file size. Simulation results reveal that back-ground noise has a significant impact on the performance of VLC hotspots. As expected,the VLC hotspot performs better at night than in the daytime. Finally, conclusions aredrawn in Section 4.8.

4.1 Motivation and related work

Data download on the move is seen as a promising application for the next generationwireless communication systems. An example of such a paradigm of wireless com-munication systems for delivering information services to a mobile user is infostation[141]. The coverage area of infostation is sparse and, as a result, as long as the mobileuser is in the coverage area of the infostation he/she may download information to the

63

AP

Moving-in Moving-out

Entry point Exit point

x

y

-x

-y

θ

Travelling angle

Fig. 18. Data download on the move.

mobile terminal storage for later usage. The retrieved downloaded information can be avideo file, a piece of music, a weather report, and so forth. The concept of infostation isnot new: there exists of such models for information distribution [88, 89], drive-thru In-ternet [142], and roadside infrastructures [27, 143], all of which are examples of sparsecoverage-based wireless networks. The naming varies, but in terms of functionalitiesand characteristics, they share major commonalities with the infostation. The overallcoverage of such wireless communication systems is considered to be sparse [10]. Theperformance of IEEE802.11-based sparse coverage WLAN hotspot is studied in [142].In [144, 145], measurement-based performance analysis of IEEE802.11b/g-based sys-tems for static and nomadic users in case of outdoor drive-through scenarios also havebeen carried out extensively and reported in [88, 89]. However, very few papers ad-dressed the issue of performance analysis of VLC-based systems especially in the datadownload on the move scenario [86, 146]. Unless, otherwise mentioned further, theconcept of infostation will be termed as hotspot throughout this chapter.

The data download on the move scenario is shown in Figure 18. In this scenario,the concept of moving-in and moving-out is introduced. In the moving-in case, thereceived throughput by the mobile user gradually increases from the entry point to thepoint when the distance between the moving user and access point (AP) is at its mini-mum. On the other hand, in the moving-out scenario, the received throughput starts to

64

LED LED

h

Receiving plane

Optical

receiver

Lighting equipment

Horizontal distance

Ve

rtic

al

dis

tan

ce

cΨψ

φD

),( yxr

Fig. 19. VLC hotspot coverage.

decreases from the maximum throughput to minimum at the coverage end as shown inFigure 18. The angle created by the entry and exit points at the center of coverage iscalled the travelling angle. In Figure 18, the segments "ab" and "bc" show the travellingdistances at a certain travelling angle of a mobile user in moving-in and moving-outscenarios, respectively. The details of travelling path of the mobile user and its math-ematical framework for calculation of average throughput in data download in on themove scenario are discussed in Section 4.3 and 4.4 respectively.

4.2 Geometry of VLC hotspot coverage

In any optical transmitter, the half-power angle is defined as the highest angle that thetransmitter can illuminate. On the other hand, FOV is the highest angle within which thereceiver can receive signal rays for any optical receiver. In this work, we will considerthe worst case alignment link geometry of the optical hotspot [147]. In the worst casealignment, the divergence angle and acceptance angle are the same. The divergenceangle varies between 0o to the maximum half-power angle ϕmax. On the the other hand,the acceptance angle belongs to FOV. These angular representations provide the simplelink geometry in terms of hypotenuse and the vertical and horizontal distance betweentransmitter and receiver as shown in Figure 19. As a result, the relationship between

65

divergence angle and acceptance angle in the worst-case alignment can be written as

ϕi = ψi = arccos( h

Di

), (32)

where h and Di are the vertical separation and instantaneous hypotenuse distance be-tween transmitter and receiver. According to (32), (26) can be written as

H(0) =

(m+1)Ad

2πD2 cos(m+1)(ϕ)Ts(ψ)g(ψ) 0 ≤ ψ ≤ Ψc

0 ψ > Ψc.

(33)

Equation (33) can be written as a function of horizontal and vertical distances as follows

H(0) =

(m+1)Ad

2π(r2 +h2)cos(m+1)(ϕ)Ts(ψ)g(ψ) 0 ≤ ψ ≤ Ψc

0 ψ > Ψc,

(34)

where r is a function of (x,y) coordinates in the receiving plane. If the coverage ofhotspot is considered as circular, then the maximum horizontal separation betweentransmitter (WLED) and receiver (photodetector) will represent the radius R of VLChotspot. The relation among horizontal separation r, D, and vertical separation h can beexpressed as

r =√

D2 −h2 0 < r ≤ R. (35)

4.3 Transformation of inclined path to horizontal path

The inclined and the transformation of the inclined path to the horizontal path are shownin Figure 20(a) and Figure 20(b), respectively. The mobile user enters the coverage atpoint A′, known as entering point and exits the coverage at point B′ Aknown as exitpoint, as shown in Figure 20(a). For the inclined path, the entrance angle is denotedwith α1, and the exit angle is denoted with α2. The angle between entrance point andexit point is called the travelling angle, which is denoted as θ . It should be noticedthat the maximum range of the travelling angle can be limited to the range of (0o-180o) instead of (0o-360o) for the calculation of average throughput over the simplifiedhorizontal path. Moreover, the entrance and exit angle of the inclined path can betransformed as α1 = αs and α2 = αs. After transformation, the average throughput over

66

'A

'B

1α

2αθ

y

x

y

sα sαθ

B

L

tDA B

(a) (b)

Fig. 20. Transformation from inclined to horizontal path: a) inclined path b) transformedhorizontal path.

the inclined path A′B′ will be the same as the calculation of the average throughputover the horizontal path AB. Depending on the difference between the entrance and exitangle of inclined path, after rotation the travelling angle θ of the horizontal path can beexpressed as:

θ =

|α1 −α2| |α1 −α2| ≤ 180o

360o −|α1 −α2| |α1 −α2|> 180o.(36)

It should be noticed that the maximum range of the travelling angle can be limited tothe range of (1o-180o) instead of (1o-360o) for calculation of average throughput overthe simplified horizontal path.

4.4 Mathematical framework for a data downloading scenario

The footprint of the coverage of LED source is shown in Figure 21. We assume that thecoverage of the hotspot is circular. The entrance and exit points of the inclined path arecoordinated as A′ (xa,ya) and B′ (xb,yb) respectively.

Assuming constant velocity, the path that the mobile user travels can be simplyexpressed with

(x(t) ,y(t)) = ((xb − xa) t + xa,(yb − ya) t + ya) , (37)

67

O O O O

R

L r

( )yx ,( )yx ,−A B

( )0,R− ( )0,Rθ

Horizontal

path

A’

B’

( )aa yx −− ,

( )bb yx −,

'θ

tD

Fig. 21. Footprint of VLC coverage.

where the parameter t varies between zero (corresponding to the entrance point) to one(the exit point). To characterise the throughput as a function of the distance, i.e., thethroughput at distance r is S (r). The calculation of instantaneous distance from thecenter of coverage to the mobile user can be written as

r =√[(xb − xa) t + xa]

2 +[(yb − ya) t + ya]2. (38)

Now, calculation of average throughput of the mobile user (while it passes through thecoverage) can be written as

Sav(r) =1∫

t=0S(√

[(xb − xa) t + xa]2 +[(yb − ya) t + ya]

2)

dt, (39)

where S(√·) represents spatial throughput-distance relationship function. This function

can be either measurement- or analytical-based. However, the calculation of (39) forarbitrary entrance and exit points is cumbersome. Hence, in order to simplify the calcu-lation of the average throughput, we rotate the inclined path to an equivalent horizontalpath as discussed in Section 4.3 and shown in Figure 21. In this case, entrance and exitcoordinates points of inclined and horizontal path is related as xa = x, xb =−x, ya = y,yb = y. The entrance and exit points of the transformed horizontal path is coordinatedas (due to the symmetry of entrance and exit angle) A′ (−xa,yb) and B′ (xa,yb) respec-

68

tively as shown in Figure 21. The calculation of average throughput over the simplifiedhorizontal path is given below:

After rotation of the inclined path to the horizontal path, (38) is simplified andwritten as

r =√[x(2t −1)]2 + y2. (40)

In Figure 21, it is also shown that the minimum distance between mobile user and thecenter of coverage is L, which for a particular travelling angle θ is L = Rcos

(θ/

2),

where R is the coverage radius. The travelling distance Dt = 2Rsin(θ/

2). However,

it should be noted that the travelling distance is equal before and after rotation (e.gDt = A′B′ = AB) that is the travelling angle of horizontal path θ is not affected bythe rotation, that is θ will remain same as it was for inclined path. Finally, averagethroughput over the simplified horizontal path can be expressed as

Sav(r) =1∫

t=0

S(√

[x(2t −1)]2 + y2

)dt, (41)

where x = Dt2 = Rsin

(θ/

2)

and y = L. It is easily seen that (41) leads to much simpli-fied integration in comparison to (39). By taking velocity v into account, the dwellingtime of the mobile user in VLC hotspot can be found with tdwell = Dt

/v = 2x

/v. Now

by simply multiplying the dwell time with the average throughput, the size of the trans-ferred information or file size can be written as

It(θ) = Sav · tdwell. (42)

Using (40), transferred information or file size can be written as follows:

It(θ) =2xv

1∫t=0

S(√

[x(2t −1)]2 + y2

)dt. (43)

Before deriving relationship between throughput and distance. Let’s represent (33) as

H(0) =Cc

D2 , (44)

whereCc =

(m+1)Ad

2πcos(m+1)(ϕ)Ts(ψ)g(ψ). (45)

69

In this work, the indoor environment is considered into two classes: daytime and night-time. Note that the effect of background noise will vary depending on its particularclass. For specific value of BER and N0 and using the relationship of data rate andhypotenuse distance D can be expressed as [4]

Rb =R2

rC2c P2

t

(Q−1(BER))2N0D4 . (46)

Moreover, the relationship between Rb and distance D can be represented as

Rb = K ·D−4, (47)

where K is a scaling factor and is given as

K =R2

rC2c P2

t

(Q−1(BER))2N0. (48)

The value of scaling factor K depends on FOV, BER, transmit power, noise, detectorarea, concentration gain of the concentrator and others.

4.5 VLC hotspot design parameters and their relationships

Prior to deploying any wireless networks, a number of scenarios and the relationshipof key design parameters are targeted to evaluate performance in terms of capacityand reliability. Some examples of these relationships are SNR vs. throughput, SNR vs.distance, BER vs. data rate and so on. These relationships are used as tools in analyticalmodels for analyzing as well as designing wireless networks before being deployed inpractice.

Figure 22 shows the FOV changes due to the horizontal separation between trans-mitter and receiver with step changes of 0.1m. As mentioned earlier in the case of worstcase alignment, the irradiance and reception angle will be the same. It is seen in Fig-ure 22 that as the step value increases the alignment between irradiance and receptionangle also increases. In this particular case the half-power at semi-angle is chosen as85o and FOV of the receiver is chosen as 60o.

Figure 23 shows the variation of received power with Lambertian order m. It is alsoseen that for a fixed Lambertian order the narrow semi-angle at half power provides thehigher received power. This received power difference is more noticeable in higher

70

0 0.5 1 1.5 2 2.5 30

10

20

30

40

50

60

Horizontal distance [m]

Inci

denc

e an

gle

[deg

ree]

Fig. 22. FOV vs. horizontal distance.

0 2 4 6 8 10 12 14 16 18 20−52

−50

−48

−46

−44

−42

−40

−38

Lambertian order of LED

Rec

eive

d po

wer

[dB

m]

Fig. 23. Lambertian order vs. power.

order Lambertian order. For achieving a higher transmission bandwidth the half-powersemi-angle of the LED can be tuned to its optimal value. The optimal Lambertian andtransmitter semi-angle at half-power is calculated as [148, 149]

mopt =−1

ln(

cos(ϕmax)) −1. (49)

ϕ 12 opt = arccos

(exp( − ln(2)

−1ln(cos(ϕmax))−1

))0 < ϕ 1

2 opt < 90o. (50)

4.6 Theoretical and empirical throughput-distance models

Signal coverage and data rate are the two important design parameters in any wirelesscommunication system. In this work, the relationship between these two design param-eters and their approximated representative models are termed as throughput-distancemodels. Determination of signal coverage and data rate are influenced by a varietyof factors, most prominently the frequency of operation and the terrain [150]. It mayalso be influenced by the choice of several design parameters, e.g., modulation andcoding, constellation size, power level, multiple access scheme, and many others. Theeffect of all these parameters to the throughput may be described as the throughput-distance model. Several throughput-distance models for hotspots have been discussedin [10, 88, 89]. Throughput performance dependent on the distance based data rate ofVLC hotspots has investigated in [86]. In this chapter, we have developed a polyno-mial based empirical throughput-distance model. This empirical throughput-distancemodel has been approximated from an analytically derived throughput-distance rela-

71

tionship. The purpose of using this empirical throughput-distance model is to simplifythe problem. This simplification will provide a tractable solution for calculating aver-age throughput and file size of a mobile user when he/she is downloading data on themove.

4.6.1 Empirical polynomial based throughput-distance model

The second order empirical polynomial model is used to approximate the theoreticalthroughput-distance model. In this case, we ran a MATLAB basic fitting tool to fit thetheoretical throughput-distance to polynomial throughput distance model. This basic fit-ting tool uses the least square (LS) method. The approximated second order polynomialthroughput-distance relationship model can be written as

S(r) =

ar2 +br+ c, 0 < r < R

0, otherwise(51)

where the a and b represent the first and second term coefficients of second order poly-nomial respectively, and c represents the constant term.

4.6.2 Throughput vs. distance at daytime

In the daytime, direct and indirect sunlight causes interference to the performance ofthe VLC hotspot. By taking into account direct and indirect sunlight and other light-ing noise sources the relation between the data rate and distance is shown in Figure 24.Figure 24 shows the theoretical throughput-distance models in case of with and withoutusing filter. Both of these theoretical models are then approximated using the polyno-mial as below

Td−wf = 0.69d2hor −3.9dhor +5.6. (52)

Td−wof = 3.282d2hor −18.58dhor +26.74. (53)

It is noticed that the throughput vs. distance relationship are approximated to polyno-mial functions. In this case we have assumed that coverage of a VLC hotspot is circular.Any point from the center of coverage is a function of both horizontal as well as verti-cal distance. However, for further analysis we represent this data rate versus distancerelationship into two dimensional coordinates, where dhor can be represented by (x,y)

coordinates.

72

0 0.5 1 1.5 2 2.5 30

5

10

15

20

25

30

35

Distance [m]

Thr

ough

put [

Mbp

s]

Daytime with filterApproximated daytime with filterApproximated daytime without filter Daytime without filter

Fig. 24. Polynomial fitting to the theo-retical throughput-distance model at daytime.

0 0.5 1 1.5 2 2.5 30

100

200

300

400

500

600

700

Distance [m]

Thr

ough

put [

Mbp

s]

Nighttime with filterApproximated nighttime with filterApproximated nighttime without filterNighttime without filter

Fig. 25. Polynomial fitting to the theoret-ical throughput-distance model at night-time.

4.6.3 Throughput vs. distance at nighttime

At nighttime, artificial light sources such as incandescent lamps with tungsten filaments,halogen and mercury lamps, fluorescent lamps with different colours and fluorescentlamps are considered to be the major noise sources. By taking into account with filteredand not filtered noise sources, the throughput-distance model can be represented asshown in Figure 25. The approximated throughput-distance model can be expressed as

Tn−wf = 33.03d2hor −187dhor + 269.1. (54)

Tn−wof = 70.62d2hor −3.99.7dhor + 575.4. (55)

4.6.4 Calculation of average throughput and file size

In this section, we will calculate the theoretical average throughput and file size substi-tuting respective throughput-distance models (52, 53, 54, 55) into (41) and performingthe integration, the average throughput for each throughput-distance model can be cal-culated as

Sav(r) = a(

x2

3+ y2

)+

b4x

(2Rx− y2 log

(−x+R)(x+R)

)+ c, (56)

73

Equation (56) can be expressed in terms of the travelling angle θ by using x=Rsin(θ/

2)

and y = Rcos(θ/

2). For the average throughput we get

Sav (θ) = aR2(

1− 23

sin2(θ2)

)+

bR

(12+

cos2( θ

2

)4sin

( θ2

))× log

(1+ sin

( θ2

)1− sin

( θ2

))+ c.(57)

Similarly, using the transferred information in terms of file size can be expressed as

It (θ) =2aR3

vsin(

θ2

)+

bR2

2v

(2sin

(θ2

)+ cos2

(θ2

)log

(1+ sin

( θ2

)1− sin

( θ2

))+2Rsin

( θ2

)v

c

).

(58)

4.7 Numerical results

In this section, we evaluate the performance of VLC hotspots, for indoor as well as foroutdoor environments using throughput-distance models. The parameters used in thesimulation environment are given in Table 8. Numerical results are provided and theperformances between day and nighttime cases are compared. The received file size bya mobile user serves as a performance metric.

Table 8. Simulation model parameters.

Simulation Parameters Value

Transmit power 20 mWPhotodiode responsivity 0.27Semi-angle at half power 60o

FOV at the receiver 30o

Detector physical area of PD 28 mm2

Coefficient of optical filter 1.0Mobile user velocity [0.4−1] m/sBER [10−9]

It is seen in Figure 26 that as the travelling angle increases, the data download interms of file size also increases. When the travelling angle increases, the mobile userdwell more time on the coverage of hotspot and download more data on its digitalstorage. For example, in all cases (with filter and without filter), at relatively higher

74

0 20 40 60 80 100 120 140 160 1800

0.5

1

1.5

2

2.5

3

3.5

4

4.5x 10

5

File

size

[Kby

te]

Travelling angle [deg]

v= 0.5 m/sec without filterv= 1 m/sec without filterv= 0.5 m/sec with filterv= 1 m/sec with filter

Fig. 26. File size vs. travelling angle in an indoor environment at nighttime.

travelling angles such as at 120o, the mobile user can download more in comparison toa relatively lower travelling angle such as 80o. Figure 26 also shows the improvement inperformance whenever the noise and interference cancellation filter is used. For exam-ple, at 179o travelling angle with using filter, an almost double file size data downloadin comparison to without using filter at velocity 0.5m/sec is achieved.

Figure 27 shows the received file size of low mobility mobile user in the daytimein an indoor environment. Two different velocities are considered for evaluating theperformance of the VLC hotspot. It is clearly seen that the performance decreasesdrastically in the daytime in comparison to the nighttime. At nighttime, there will bezero solar radiation and only artificial light sources such as fluorescent and tungstenlight bulbs will contribute to background noise. At relatively higher travelling angles,for example, between (140o −180o) the received throughput at v = 0.5m/sec with andwithout filter will be around between (140-270) and (30-60) respectively.

Figure 28 shows the received file size and dwelling time in the coverage of VLChotspots at velocity v = 0.5 m/sec in day and night conditions. It is seen that the max-imum dwelling time at v = 0.5 m/sec is 12 sec. At 9.89 sec, when the travelling angleis at 100.6o, the download file size at nighttime will be 200 Mbytes. However, at thesame dwelling time the download file size at daytime will be 6.1 Mbytes.

75

0 20 40 60 80 100 120 140 160 1800

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2x 10

4

File

size

[Kby

te]

Travelling angle [deg]

v=1 m/sec without filterv=0.5 m/sec with filterv=1 m/sec without filterv=0.5 m/sec with filter

Fig. 27. Filesize vs. travelling angle in an indoor environment at daytime.

0 20 40 60 80 100 120 140 160 1800

200

400

X: 110.6Y: 6.181

Travelling angle [deg]

File

size

[Mby

te]

X: 110.6Y: 140.9

0 20 40 60 80 100 120 140 160 1800

10

20

X: 110.6Y: 9.89

Dw

ellin

g T

ime

[Sec

]

Dwelling timeReceived Filesize at night timeReceived Filesize at day time

Fig. 28. Filesize and Dwelling time vs. travelling angle.

4.8 Conclusions

In this chapter, we studied the performance of optical wireless hotspots (a.k.a. VLChotspots), where the visible light spectrum is considered in local access points. In con-

76

trast to radio frequency based wireless networks, VLC is more sensitive to many factorssuch as alignment between transmitter and receiver, FOV of the receiver, semi-angle oftransmitter, state of the optical channel and different background noise at in the day andat nighttime. To evaluate the performance of such wireless communication networks,the impact of the above mentioned optical designing factors needs to be considered andmodelled. In this chapter, we provide a methodology to evaluate the performance ofvisible hotspot networks using throughput-distance relationship models. First we de-rived the theoretical average throughput-distance model and then we approximated thisto the empirical throughput-distance model to calculate the file size that can be down-loaded while mobile users move through the VLC hotspots. Simulation results showthat there is a large impact of background noise on the performance of a VLC hotspot.As expected, the VLC performance in both indoor and outdoor environments is betterat night than in the daytime. Performance of VLC hotspot networks are also quantifiedin terms of received file size. Numerical examples show that in indoor environmentsa mobile user can download more data at nighttime than at daytime. Finally, we ar-gue that the presented framework can be used as a mathematical tool for evaluating theperformance of a VLC hotspot before it is deployed in practice.

77

78

5 Data downloading in hybrid WLAN-VLCnetworks

In this chapter, we study the performance of hybrid (radio-optical) WLAN-VLC hotspots.In this respect, firstly we determine the coverage and data rate of VLC and WLAN. VLCcoverage and data rate are described in Subsection 5.1.1. Very short-range VLC smallcells (limited to 1 m coverage range) are considered for higher data rate regions. Unlikein Chapter 4, the data rate of VLC is considered to be constant in this chapter. Thecoverage and data rate of WLAN are described in Section 5.1.2. The measurement-based throughput-distance model in indoor environment is considered for WLAN. Thescenario of hybrid WLAN-VLC for single-user cases is described in Section 5.2. Multi-user hybrid WLAN-VLC is discussed in Section 5.3. Finally, simulation results arediscussed in Section 5.4. Simulation results reveal that the considered hybrid WLAN-VLC always performs better than the stand-alone VLC-only or WLAN-only.

5.1 Hybrid WLAN-VLC networks in indoor environment

In 5G, the possible integration of VLC technology with legacy radio access technologies(RATs) has been envisioned to built the hybrid network in future wireless networks [13].In hybrid wireless networks, a mix of two or multiple wireless technologies, frequen-cies, cell sizes and different network architectures are used to optimally respond accord-ing to the demand of mobile users as well as multi-vendor, multi-service providers. Al-though the concept of hybrid wireless networks is not new, very few research work hasbeen conducted, especially in the area of hybrid WLAN-VLC networks [4, 73, 76, 86].Detailed descriptions of research work related to hybrid WLAN-VLC has already beendiscussed in Section 1.3.4 of Chapter 1. In hybrid WLAN-VLC networks, it is assumedthat a mobile device is equipped with both optical (VLC) and radio (WLAN) air in-terfaces that enable connecting to either radio and optical networks. The decision onwhether to be connected to an optical or radio network is purely based on the availabilityor quality of the channel. In special cases, the user may connect both radio-optical net-works concurrently for better coverage and higher throughput, and increased reliability.

79

−10−5

05

10

−10

−5

0

5

10−30

−29.5

−29

−28.5

−28

−27.5

X[m]Y[m]

Rec

eive

d p

ow

er [

dB

m]

Fig. 29. Spotlighting coverage.

5.1.1 Coverage and data rate of VLC small cell

In this chapter, a small VLC cell and the comparatively larger coverage of a WLANcell are considered in an indoor hybrid WLAN-VLC network. In [151], the small cellof VLC known as spotlighting had been studied for illumination and communication.In such a scheme, a number of small cells of VLC was used for providing a higher datarate as shown in Figure 29. Authors in [151] also introduced a hybrid scheme wherethe small cell VLC was responsible for providing both uniform lighting and communi-cation. In [152], the concept of cell zooming was introduced to improve traffic distri-bution in multi-user environments and handover at cell edges. In their work, the cellcoverage regions were dynamically altered according to the need of required signal-to-interference-plus-noise ratio (SINR) distribution in an indoor environment while main-taining constant illumination. However, unlike in [151, 152], in this work we willconsider WLAN for providing wide coverage and VLC small cell for providing higherdata rates.

In case of VLC small cells, the required light irradiance Pr of circular lighting LEDsource can be determined from light field radius Rs and its transmit power Pt regardlessof the distance parameter and can be expressed as [151]

Pt =Pr

πR2s. (59)

80

BER=10^−3 BER=10^−5 BER=10^−60

1

2

3

4

5

6

7

8

9

BER

Thr

ough

put [

Mbp

s]

Night TimeDay Time

Fig. 30. Data rate at three different BER requirement in indoor environment.

In (59), note that the received power is inversely proportional to the square of radiusRs. Therefore, for a constant radius of VLC spotlighting the received power Pr will beproportional to the transmit power Pt with a scaling factor π . However, in this work wemodel the throughput and coverage of VLC small cell according to the two piece-wisethroughput distance model studied in [85], where the first piece represents the constantdata rate till one meter of coverage range. The constant throughputs of small singleLED VLC cell at three different BERs at day and night condition are shown in Figure30.

5.1.2 Coverage and data rate of WLAN

WLAN is a multi-rate system meaning that when a mobile user moves inside the cov-erage area of the cell, the received SNR changes according to the distance betweenmobile user and access point (AP). In such a multi-rate system, SNR is a random vari-able and therefore the mobile user can operate at one of the multi-rate choices of datarates according to the value of its received SNR. In a single mobile user environment,the achieved average throughput is the average of all data rates at which it operateswhile moving in the area. In a multiple user environment, the average throughput peruser is also a function of the MAC technique employed and the number of users in thearea. Therefore, the average of the data rates observed by a user located randomly in

81

the coverage area of multi-rate system can be expressed as [150]

Rav = ΣNraten=1 PnRn, (60)

where Rav is the average spatial data rate, Rn is one of the available multi-rate, and Pn isthe probability of occurrence of that data rate. If a terminal is located randomly in thecoverage area, the probability of being in each area is given by the ratio of the area forthe specific data rate to the total coverage area as

Pn =Ai

πD2max

, (61)

where Ai and Dmax are the specific data rate area and the maximum coverage rangerespectively.

In general, channel models include several geometry and physical phenomena, suchas refraction, diffraction, propagation medium (dry or moist air), the distance betweenthe transmitter and the receiver and so on. The simplest way to represent the relationshipbetween the effects of all these factors vs. distance is by using the path-loss model. Anindoor path-loss model in multi-storey buildings can be expressed as [150]

Lp = L0 +nF +10log(Ltr), (62)

where F represents the signal attenuation provided by each floor, L0 is the path lossat the first meter. Ltr is the distance between the transmitter and receiver in meter, n

is the number of floors through which the signal passes. In fact, no specific channelmodel is valid for all environments. Thus, a proper model of coverage for any wirelesstechnology can only be established by making a new measurement and calculations forthe transmission in that specific environment. In this work, instead of a pathloss modelwe have used the throughput-distance relationship model to characterise the channelpropagation with respect to distance for the indoor WLAN channel model.

The simplest model for the throughput-distance relationship is a linear model. In-deed, the experimental data collected in [150] shows the linear throughput-distancerelationship of the IEEE 802.11b in a typical office building. In the linear throughput-distance relationship model, the throughput decreases linearly as distance between themobile user and AP increases. The coverage and throughput-distance relationshipmodel for WLAN are shown in Figure 31 and Figure 32 respectively. Therefore, in

82

−40 −30 −20 −10 0 10 20 30 40−40

−30

−20

−10

0

10

20

30

40

X[m]

Y[m

]

5.4 Mbps

4.8 Mbps

2.4 Mbps

Fig. 31. Indoor WLAN coverage. ( [83]c⃝ 2014 John Wiley& Sons).

0 5 10 15 20 25 30 350

1

2

3

4

5

6

Distance [m]

Thr

ough

put [

Mbp

s]

(5.4=Higher rate<=−3.34)

(0<=Lower rate<1.338)

(1.338<Middle rate<=3.34)

Fig. 32. Indoor WLAN throughput. ( [83]c⃝2014 John Wiley& Sons).

the case of WLAN, the relationship between throughput S(rw) and distance is given by:

S(rw) =

amaxrw +bmax, 0 < rw < Rmax

0, otherwise.(63)

The parameters amax and bmax are used to calculate the maximum coverage distanceand maximum provided data rate of AP. At rw = 0, the maximum data rate receivedby mobile user is S(0) = bmax. The parameter amax is related to the maximum rangedistance of AP which follows as amax =− bmax

Rmax.

5.2 Hybrid WLAN-VLC: single-user case

The scenario for data downloading in hybrid WLAN-VLC hotspots is shown in Fig-ure 33. In this scenario, limited width and a relative long corridor in the office buildingis considered as the service area of hybrid WLAN-VLC hotspot networks where adja-cent walls of the corridor are assumed to be transparent. Therefore, indirect sunlightis assumed to be present at the photo detector in the daytime. A series of optical LEDlight sources are aligned in a row in the ceiling of the corridor for illumination as wellas for the communication purposes. Coverage of each spotlighting cell can be consid-ered as a VLC hotspot. Therefore, depending on the placement, the number of opticallight sources will represent the number of hotspots in the area of corridor. We haveconsidered multiple VLC hotspots within a wider range of WLAN coverage. Data ratesof WLAN is channel adaptive. As a result, according to the placement of WLAN ac-cess point, different data rates such as higher data rate, a moderate data rate as well as

83

−6 −4 −2 0 2 4 6−5

−4

−3

−2

−1

0

1

2

3

4

5

(−4,0) (0,0) (4,0)

Length [m]

Wid

th [

m]

↑ Spotlighting Coverage

dv: travelling distance in spotlighting cell

dw

: travelling distance in WLAN cell

dv

dw

Segmented corridor

← WLAN Coverage→

AB C

Fig. 33. An example scenario of data downloading in a hybrid WLAN-VLC hotspot coverage.( [83] c⃝2014 John Wiley& Sons).

lower data rate will be provided to the mobile user in the coverage of WLAN [88]. Thedistance between two consecutive VLC hotspots may vary depending on the placementof LED light sources.

It is also assumed that the a mobile user terminal is equipped with both optical andradio transceivers. In the proposed scenario a mobile a user will always have ubiquitouscoverage of WLAN meaning that when the mobile user is in or out of the coverage ofthe VLC hotspot, he/she may have a connection with the WLAN link. However, inthis work we have assumed that when the user is in VLC spotlighting coverage he willbe connected with the VLC link. In this case, we assume the transmitter and receiverare aligned to each other in the VLC hotspot by using an electronic tracking system.We have also assumed that initial and final connectivity of the mobile user during datadownloading will be started and ended at the VLC coverage, as shown in Figure 33.Seamless vertical handover is considered between the WLAN and VLC hotspot. Theamount of information download to the user digital storage device depends also onmany optical as well as radio design parameters, such as FOV of LED and PD, physicalarea of detector, dwelling time in the coverage of WLAN or VLC hotspot, mobile uservelocity and environment (e.g. day and nighttime).

84

B

( )0,R− ( )0,R

θ1α 2α

( )0,0

( )bb yx ,

),( cc yxC

R

rA( )aa yx ,

Fig. 34. Circular coverage area of radius R for a VLC hotspot. ( [83] c⃝ 2014 John Wiley&Sons).

The mobility in VLC for single-user cases is shown in Figure 34, where the mobileuser enters the coverage at point A, with coordinates (xa,ya) known as the entering pointand exits the coverage at point B, with coordinates (xb,yb) known as the exit point, asshown in Figure 34. For such an inclined path, the entrance angle is denoted by α1,and the exit angle is denoted by α2. The angle between entrance point and exit point iscalled the travelling angle which is denoted by θ . Assuming constant velocity, the paththat the user travels can be simply expressed as

(x(t),y(t)) = ((xb − xa)tp + xa,(yb − ya)tp + ya), (64)

where the parameters tp varies from zero (corresponding to the entrance point) to one(the exit point). Let us characterize the throughput as a function of the distance, i.e., thethroughput at distance r is S(r). The instantaneous distance from the center of coverageto the mobile user can be calculated as

r =√

[(xb − xa)tp + xa]2 +[(yb − ya)tp + ya]2. (65)

For single-user cases, a special case of a random walk mobility model with constantspeed and limited movement direction was adopted to predict the mobile user move-ments throughout the hybrid WLAN-VLC hotspot coverage. Two assumptions havebeen made in dealing with the mobility of the user in WLAN and VLC hotspots cov-erage as shown in Figure 34. The first assumption is that the travelling path within the

85

coverage of VLC hotspot or in the WLAN will be a straight, inclined or horizontal path.Secondly, we have assumed that the mobile user will only turn at the transition pointbetween the VLC hotspot and WLAN. This transition point constitutes the exit point ofthe present hotspot and the entry point of the next VLC hotspot. For example, if A andB are the entry and exit points of the VLC hotspot then C is the entry point of next VLChotspot. In that case, the the travelling distance in VLC hotspot will be |A−B|2 and thetravelling distance in WLAN hotspot will be |B−C|2, where C is the transition pointsas shown in Figure 34. | · |2 is a Euclidean norm. It should be noted that the generationof random entry and exit point is done using the concept of travelling angles which isassumed to be uniformly distributed [88].

The size of the transferred information from the VLC hotspot during the dwellingtime at velocity v and for travelling distance dvlc and with average throughput Svlc isIvlc = (Svlc.dvlc)/v. Similarly, the size of the transferred information from WLAN fortravelling distance dwlan and with average throughput Swlan is Iwlan = (Swlan.dwlan)/v.However, the calculation of average throughput in hybrid coverage of WLAN-VLCwill be as follows:

Shybrid =

(Svlcdvlc

dvlc + dwlan+

Swlandwlan

dvlc + dwlan

), (66)

where dvlc and dwlan are the total distance traveled by the mobile user in the VLC andWLAN coverage, respectively. Finally, the received file size in hybrid coverage ofWLAN-VLC can be expressed as

Ihybrid = Shybrid tt, (67)

where tt is the sum of residence time spent by a mobile user in the VLC spot cell andthe WLAN coverage.

5.3 Hybrid WLAN-VLC: multi-user case

The scenario of data download in hybrid WLAN-VLC coverage for the multi-user caseis shown in Figure 35. In this case, ten mobile users are shown for five mobility slotsin the hybrid coverage of WLAN and VLC. Mobility slots represent the varying condi-tions, such as position, and direction at certain time steps. The inner small circles withradius 1 m represents the VLC coverage. On the other hand, the large circle representsthe coverage of WLAN with radius 10 m. The mobile users are uniformly distributed

86

−10 −5 0 5 10−10

−8

−6

−4

−2

0

2

4

6

8

10

X[m]

Y[m

] Spotlights

Mobile moves for 5 mobility slots

← WLAN coverage →

Fig. 35. Random walk mobility for five mobility slots in multi-user case. ( [83] c⃝ 2014 JohnWiley& Sons).

around the region of both WLAN and VLC. When the mobile users are inside the cov-erage of the VLC hotspot then they will communicate with the optical VLC interfaceotherwise they will communicate with radio WLAN interface of the mobile device.

For a single-user case, we have considered constant velocity and unidirectionalmovement of the mobile user as discussed in Section 5.2. However, in practice, a mo-bility model should attempt to mimic the movement of a mobile user as realistically aspossible. The most commonly used mobility model in the literature is the random way-point model where users move randomly: the destination, speed and direction are allchosen randomly in the course of its movement. In random mobility, a velocity vectorv = (v,θdir) is associated itself with two parameters, the first parameter is the magnitudeof velocity v of the mobile user itself and the second parameters θdir is the direction ofangle. The direction of angle is a random variable. The positions of the mobile user aredefined by two dimensional coordinates (x,y). At every ∆t time step, both the velocityand the position of the mobile user are updated. Updated velocity and its direction canbe expressed as [4]

x(t +t) = x(t)+ v(t) · cos(θdir(t)). (68)

y(t +t) = y(t)+ v(t) · sin(θdir(t)), (69)

87

Table 9. Simulation model parameters. ( [83] c⃝2014 John Wiley& Sons).

Simulation Parameters Value

Transmit power 20 mWPhotodiode responsivity 0.27Semi-angle at half power 60o

FOV at the receiver 30o

Detector physical area of PD 28 mm2

Mobile user velocity [0.4−1] m/sNo. of spotlights [4,6,8,9,18,27]BER [10−3,10−5,10−6]Rmax 32 mR 1 m

where v is the velocity assumed to be uniformly distributed where it varies betweenminimum velocity vmin and maximum velocity vmax. A mobile node at each momenthas five different possible directions to move with each direction. In this work, we haveconsidered that each mobile has a higher probability in moving in the same direction asthe previous move. The assigned values of probabilities to each direction of angle areas follows: p0 = 0.7, p1 = 0.1, p2 = 0.05, so that p0 +2 · p2 +2 · p2 = 0.05.

5.4 Performance evaluation of hybrid WLAN-VLC

In this section, we evaluate the performance of the hybrid WLAN-VLC communicationsystem for indoor environment at day and nighttime for the single-user as well as multi-user cases. Numerical results are achieved by using analytically and measurement-based throughput-distance relationship models for the VLC and WLAN, respectively.Monte-Carlo simulation results are also carried out for 1000 iterations for random mo-bility for the mobile user along with other system design parameters, as given in Table9. The received file size of moving user serves as a performance metric for the single-user case and average connectivity and system throughput performance metrics for themulti-user case throughout the whole study of this work.

Figure 36 shows the received file size in hybrid WLAN-VLC and VLC-only cover-age for the indoor environment at nighttime at different BER requirements for the case.In this particular simulation environment, eight spotlights are considered. It is seen thatthe received file size decreases as the BER requirement gets higher both for the hybridWLAN-VLC and VLC-only cases. It should be noted that the performance differencebetween the hybrid WLAN-VLC and VLC-only is not so significant at all BER require-

88

BER=10^−(3) BER=10^−(5) BER=10^−(6)0

1

2

3

4

5

6

7

8

9

Ave

rage

File

size

[Mby

te]

Night Time

HybridVLC

Fig. 36. Received Average Filesize for single-user case, [ v = 1m/sec, No. of spotlights = 8 ].( [83] c⃝ 2014 John Wiley& Sons).

ments. This is due to the arrangement of VLC spotlights and mobility model for thesingle-user case (see Figure 33). The difference between two adjacent spotlights is 2 m.In such a case, the residence time in WLAN coverage will be much less in comparisonto VLC spotlights coverage. As a result, the contribution of data downloaded file sizefrom the WLAN will be much less in hybrid WLAN-VLC. Then, it can be said thatin the case of a narrow corridor (for example 2 m width), VLC-only network can pro-vide reasonable downloaded file size in comparison to the hybrid WLAN-VLC, if VLCspotlight hotspots are placed closely enough.

Figure 37 shows the file size in hybrid WLAN-VLC and VLC-only coverage forthe indoor environment during day at different BER requirements for a single-user case.It is clearly seen that the performance of hybrid WLAN-VLC and VLC-only decreasedrastically at all BER levels in comparison to the indoor environment performance atnight. This performance decrement happens in the daytime because of the impact ofindirect sunlight on the photodetector.

Figure 38 shows the three sets of CDF plots of received information file size at nightfor three different sets of spotlights deployed in a corridor or big hall for the single-usercase. In this simulation, we mainly focus on studying the impact of the number ofspotlights deployment in a certain length of corridor or room for hybrid WLAN-VLC

89

BER=10^−(3) BER=10^−(5) BER=10^−(6)0

0.5

1

1.5

2

2.5

Day Time

Ave

rage

File

size

[Mby

te]

HybridVLC

Fig. 37. Received Average Filesize for single-user case, [v = 1m/sec, No. of spotlights = 8 ]. ([83] c⃝ 2014 John Wiley& Sons).

2 3 4 5 6 7 8 9 10 110

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Filesize [Mbyte]

CD

F

↑ # of spotlight=8

↑ # of spotlight=6

# of spotlight=4 →

Fig. 38. CDF vs.Filesize at nighttime [ BER=10−3, v = 1m/sec ]. ( [83] c⃝ 2014 John Wiley&Sons).

90

0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.60

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Filesize [Mbyte]

CD

F

↑ # of spotlight=6

↑ # of spotlight=8

↑ # of spotlight=4

Fig. 39. CDF vs. Filesize at daytime [ BER=10−3, v = 1m/sec ]. ( [83] c⃝ 2014 John Wiley&Sons).

networks. The purpose of this is to evaluate the performance of hybrid WLAN-VLCnetworks in terms of received file size while the mobile user moves through a corridor.It is obvious, as reflected in the simulation results, that when the number of spotlightsincreases, the received file size also increases. This is due to the fact that when thenumber of spotlights increase, the high data rate space of VLC and residence time of amobile user in VLC are also increased in the combined coverage of WLAN-VLC. Asa result, received file size is also increased. It is easily seen that with 50% probabilitythe amount of information will be less than or equal to 4 Mbyte, 6 Mbyte and 8 Mbytewhen the number of VLC spotlight cells will be 4, 6 , and 8 respectively

Figure 39 shows the three sets of CDF plots of received information file size inthe daytime for three different sets of spotlights deployed in a corridor. At daytimecondition, the received file size will be considerably smaller than at night. With 50%probability, the amount of information will be less than or equal to 1 Mbyte, 1.6 Mbyteand 2 Mbyte when the number of VLC spotlight cells is 4, 6, and 8, respectively. Itis also seen that at a certain probability (e.g., 50%) the downloaded received file sizedoubles as the number of VLC spotlights double.

Figure 40 shows the CDF plots of received file size in case of the hybrid WLAN-VLC and WLAN-only at different velocities. The main purpose of this simulation is to

91

6 8 10 12 14 16 18 20 220

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Filesize [Mbyte]

CD

F

WO−WLAN:v=0.75 m/secWO−WLAN: v=0.5 m/secW−WLAN: v=0.5 m/secW−WLAN: v=0.75 m/sec

Fig. 40. CDF vs. Filesize at different velocities at nighttime [ # of spotlights = 8,BER=10−3 ]. ([83] c⃝ 2014 John Wiley& Sons).

assess the impact of velocity on the performance of hybrid WLAN-VLC only. It is seenthat the performance of both hybrid WLAN-VLC and WLAN-only decreases rapidlywhen velocity increases. For example, in the case of hybrid WLAN-VLC, it is seenthat with 50% probability the amount of information will be less than or equal to 16.1Mbyte and 10.7 Mbyte at v = 0.5 m/sec and v = 0.75 m/sec, respectively. However, atthe same velocity, the performance difference is not much between the hybrid WLAN-VLC and WLAN-only. It is also seen in the simulation results that with 50% probabilitythe amount of information will be less than or equal to 16.1 Mbyte, 15.4 Mbyte forhybrid WLAN-VLC and WLAN-only, respectively at v = 0.5 m/sec. This is due to theplacement of number of VLC hotspots.

Figure 41 shows the CDF plots of received file size in case of hybrid VLC-WLANand WLAN only at daytime for different velocities. The same conclusion can be madeas it is done for Figure 40. In both cases (Figure 40, and Figure 41), it can be concludedthat when the velocity of the mobile user increases, the residence time either in hybridWLAN-VLC or WLAN also decreases. As a result, data downloaded file size will beless when velocity increases.

Figure 42 shows the performance of hybrid VLC-WLAN in terms of average con-nectivity for multuser and random walk mobility cases. Simulation has been performedfor three different number of spotlight sets 9, 18 and 27 and the coverage range of the

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1.5 2 2.5 3 3.5 4 4.5 5 5.50

0.1

0.2

0.3

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1

Filesize [Mbyte]

CD

F

W−WLAN:v=0.5 m/secW−WLAN:v=0.75 m/secWO−WLAN:v=0.5 m/secWO−WLAN: v=0.75 m/sec

Fig. 41. CDF vs. Filesize at different velocities at day time [ # of spotlights = 8, BER=10−3 ]. ([83] c⃝ 2014 John Wiley& Sons).

# of spotlights=27 # of spotlights=18 # of spotlights=90

10

20

30

40

50

60

70

80

# of Users N=10

Ave

rage

Con

nect

ivity

(%

)

VLCWLAN

Fig. 42. Average connectivity in multi-user case. ( [83] c⃝ 2014 John Wiley& Sons).

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BER=10^−3 BER=10^−5 BER=10^−60

0.2

0.4

0.6

0.8

1

1.2

1.4

Number of Spotlights= 27

Sys

tem

Thr

ough

put [

Gbp

s]

VLCWLANHybrid

Fig. 43. System Throughput in multi-user case. ( [83] c⃝ 2014 John Wiley& Sons).

WLAN is 10 m. It can be easily be seen that as the number of spotlights increases, theaverage connectivity in VLC coverage also increases. However, the average connec-tivity in WLAN always outperforms average connectivity in VLC. This is due to thelarge coverage area covered by the WLAN than the coverage area covered by numberof spotlights.

Figure 43 shows the system throughput in VLC, WLAN and hybrid WLAN-VLCcoverage at different BER requirements. Simulation has been performed for a numberof users N = 10 and number of spotlights = 27. It shows that even with limited connec-tion, the system throughput with VLC connectivity is better than WLAN connectivity.It can also be easily seen that the system throughput in hybrid VLC-WLAN coverageis far better than in individual VLC or WLAN coverage. System throughput in hybridVLC-WLAN coverage is the summation of throughput in VLC and WLAN. This is dueto the utilisation of both connectivity of VLC and WLAN by mobile users.

5.5 Conclusions

We have studied the performance of hybrid WLAN-VLC hotspot network in a datadownload on the move scenario. The performance of the hybrid WLAN-VLC hotspot

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network is characterised by taking into account the throughput of radio and optical tech-nology. Apart from the throughput parameter other key parameters such as velocity ofthe mobile user, travelling angle and FOV of the LED source are also taken into account.Finally, simulations have been performed to evaluate the performance of such networkfor the data downloading on the move scenario by taking into account performance met-rics such as information file size. Simulation results helped to understand that there is asignificant impact on the performance due to the velocity of the mobile user. Moreover,simulation results reveal that as the number of spotlight cells placement in the corridorincreases, the performance of the hybrid WLAN-VLC hotspot system increases. Hence,it is worthwhile to further investigate data download on the move issues in the hybridWLAN-VLC hotspot scenario by taking into account detailed physical layer models,different network loads and various application scenarios.

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6 Co-operative relays in VLC and hybridWLAN-VLC networks

In this chapter, we evaluate the performance of relay-assisted VLC and relay-assistedhybrid WLAN-VLC hotspot networks. In relay-assisted VLC networks, we have devel-oped an optical relay based multi-hop communication simulation environment. In thisrespect, an overview and background of multi-hop relaying techniques are introducedin Section 6.1. The scenario of co-operative relays in multi-hop VLC communicationis described in Section 6.2. The motivation for this work is to use co-operative relays toreduce the on-demand recovery of connection. A relay selection algorithm is also intro-duced in this work and described in Section 6.2.1. Numerical results for relay-assistedVLC networks are shown in Section 6.3. The simulation environment of relay-assistedVLC hotspot networks has been further extended to relay-assisted hybrid WLAN-VLChotspot networks. The purpose of this extension is to exploit the benefits of each tech-nology such as reliable connectivity of WLAN and higher throughput and improvedenergy efficiency of VLC. The scenario of co-operative hybrid WLAN-VLC networksis described in Section 6.3.1. Energy consumption analysis is described in Section 6.3.2.Numerical results for relay-assisted hybrid WLAN-VLC networks are given in Section6.4. Finally, conclusions are drawn in Section 6.5.

6.1 Overview and background

In a multi-hop relaying technique, if a MT is unable to connect to the AP directlythen it can connect with the AP through relay station (RS). The first benefit of multi-hop relaying comes from the reduction in the overall path loss between the AP andthe MT. Another benefit of multi-hop relaying is the path diversity gain that can beachieved by selecting the most favorable, multi-hop path in the shadowed environment[153, 154]. The performance of such multi-hop relay based networks depends on manyfactors, such as the number of RSs in the cell, transmission range of MT, mobility of therelaying node, traffic volume in the network and others. The targeted scenario of multi-hop communication is shown in Figure 44. The description of the scenario is given inSection 6.2. Relay schemes are not considered in the current IEEE 802.15.7 standard,but this standard supports device discovery mechanisms, through which short-range co-

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Multihop region

Single-hop

region

LOS

LOS

LOS

Cooperative

communication

NLOS

LED

Source

LOS

Idle MT

Fig. 44. Multi-hop communication.

operation can also be possible within homogeneous networks [36]. In this regard, manyresearch works have proposed the future adoption of relay schemes in LED identifica-tion (LED-ID) systems [155, 156]. The LED-ID system typically uses the LOS channelto achieve high data rates and bright illumination. In the LED-ID system, the visiblespectrum is used for downlink and the IR band is used for uplink communication. In[155], three scenarios have been presented: i) Direct communication, ii) LED-ID co-operative communication and iii) LED-ID multi-hop communications. The motivationfor that work is to use co-operative relays and multi-hop communications to reduce la-tency and on-demand recovery. In [156], first a scenario on the shadow region problemis presented and then a co-operative optical relay for LED-ID systems is proposed toextend the coverage range of LED-ID networks. In both [155, 156], co-operative MACprotocols were proposed using co-operative relay schemes for downlink communica-tions in LED-ID systems. In [119], two novel MAC protocols and a novel scheme arestudied and designed for multi-hop multi-access in VLC communications.

In [6, 146], the main goal of the work is to expand the limited coverage of VLC.In this respect, the connectivity performance of MTs has been performed using varyingnumber of simulation parameters. In [156], the authors develop a VLC based multi-hopaudio data transmission system prototype using RS. The demonstration of their proto-type has been shown to transmit a high quality of audio signal at a long distance. In their

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proposed multi-hop transmission system, audio data is encoded using the Sony/PhilipsDigital Interface Format (S/PDF) standard. The multi-hop system consists of two RSs.At each relay, the received digital audio signal is improved and amplified before be-ing transmitted to the next RS. At the receiver, the encoded audio signal is decoded,amplified and converted into an audio signal.

6.2 Scenario description

The scenario of co-operative relays in multi-hop VLC communications is shown inFigure 45. In this figure, the coverage area of a VLC source is shown in the segmentedarea of a typical conference room. The inner circle represents the coverage area of VLCwhich is denoted as AI, known as the single-hop region (SR). On the other hand, theouter circle represents the coverage area of multi-hop region (MR) which is denoted asAo. The coverage range of the inner and outer circles are rsr and Rmr respectively. Themobile terminals are uniformly distributed around the region of both inner and outercircles. We assume that mobile terminals in the inner circle will have LOS communi-cations to the AP and transmit directly to AP. The mobile terminals in the outer circlewill connect via relays situated in the inner circle. However, all the MTs that are out-side of the inner region will not be able to connect unless any relay presents an activetransmitter transmission range. It should be noted that each MT will be considered asan active transmitter when it is in transmission mode, otherwise MTs will act as relays.For simplicity, we consider the coverage range dmin of MT/relay to be circular. At anyinstance, MT may act as a relay and during that time it may receive a packet from aparticular active transmitter resides in AI. The details of varying conditions of MT interms of position and operation mode are given below:

As seen from Figure 45, there are ten MTs in the coverage of both SR and MR. Theposition of each MT is shown for three mobility slots. A mobility slot represents thevarying conditions such as position, direction and operation mode of an MT at a certaintime step. Active transmitters in the SR region are marked as green circles. On the otherhand, active transmitters in the MR are marked as red circle. It is also seen that relaysare marked as ’+’ sign. Connected with active transmitters and disconnected relays arerepresented as red + and with blue + respectively. Inactive transmitters are marked ascircle (o) with black colour. The operation mode of MTs is also seen in Figure 45. Itshould be noted that each MT can change its operation mode from relay (+) to activenode (o) or vice versa.

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−4 −2 0 2 4−4

−3

−2

−1

0

1

2

3

4

X[m]

Y[m

]

← MTs in AI

MTs in AO

→

← Relays

AO

AI

Rmr

rsr

Fig. 45. VLC hotspot (spotlighting cell) coverage. ( [74] c⃝ 2013 IEEE).

6.2.1 Relay selection

In this work, the scheduling process is mainly controlled by two parameters: relayprobability p and transmission range dmin of MTs. In the relay selection scheme asshown in Figure 46, we will follow the following procedure: Let S denotes the set ofMTs in AI as relay candidates and the distance from each relay to the AP is li wherei ∈ S. For an active transmitter, j in AO, li j(t) denotes the distance between relay i andactive transmitter j at time ti. In this respect, we choose the relay node as follows:

argmini

= minλscale li +(1−λscale) li j, i ∈ S (70)

where λscale ∈ [0,1] is a scaling factor. The relationship between the number N of MTs,radius of the multi-hop region Rmr and radius of transmission range dmin of MT/relaycan be expressed as [157]

µ =N ·dmin

2

R2mr

. (71)

It can be seen that as the radius of multi-hop region increases, the value of node densitydecreases [157, 158].

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Is VLC AP within

the range

Is there any relays

within the range

Is any relay closer to the

VLC AP than the mobile

itself

Stay idleTransmit to the

best relay

Transmit directly to

the VLC AP

NO

YES

NO

NO

YES

YES

Fig. 46. Relay selection algorithm.

6.3 Performance evaluation of co-operative VLC

In this section, numerical results are given on the performance of user connectivity.In this respect, Monte-Carlo simulation results are obtained for 100,000 iterations byusing the system design parameters as given in Table 10 and other required parametersdescribed in Section 6.2.

Figure 47 shows the CDF vs. average percentage of connectivity of MTs with theAP. In this simulation, we investigate the impact of numbers of MTs on connectivityby considering the single-hop coverage range (2 m), multi-hop coverage range (4 m),and relay probability (p = 0.2). The velocity v is uniformly distributed. It is seen thatas the number of MTs increases, the connectivity performance of MTs also increases.This is due to the fact that when the number of MTs increases, the user density also in-creases, hence the probability of getting a higher number of relays also increases within

Table 10. Simulation model parameters.

Simulation parameters Value

No. of nodes [5,10,15]Transmission range of MT 2 mRelay probability [0.2,0.4,0.6]Single hop cell radius 2 mMulti-hop cell radius [4,6,8] mVelocity [0.1,0.2,0.9] m/s

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Average Connectivity (%)

CD

F

N=10 (wor)N=15 (wor)N=20 (wor)N=10 (wr)N=15 (wr)N=20 (wr)

WOR

WR

Fig. 47. CDF vs. average percentage of connectivity, [ Rmr = 4 m, p = 0.2, dmin = 2 m]. ( [74]c⃝ 2013 IEEE).

the transmission range of dmin of MT in AO. As a result, with relay (WR) case, the aver-age percentage of connectivity will increase. On the other hand, in the case of withoutrelay (WOR), an active transmitter in MR will not be able to connect with relays, whichresults in no user connectivity, hence the performance of average percentage of con-nectivity will decrease. For example, with 50% of probability for N = 15, the averagenumber of MTs are connected is less than or equal to 37 with relay case compared toless than or equal to 18 for without relay.

Figure 48 shows the CDF vs. average percentage of user connectivity for WR andWOR for different coverage ranges of MR. Relay probability and velocity of MTs arethe same as considered for the simulation results for Figure 47. In this work, we haveconsidered two-hop communications. According to our assumptions, only those trans-mitters that are in MR (AO) will communicate to AP via with only those relays thosewho are in SR (AI). So, when the coverage range of MR increases, the distance be-tween the active transmitter and relays goes beyond the transmission range of the activetransmitter and it will unable to connect to the AP via the relay. Hence, the more thecoverage range of MR increases, the more the number of active transmitters in AI willbe unable to connect to the AP. This phenomenon is true for both WR and WOR. For

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Average Connectivity (%)

CD

F

WR R= 4 mWR R= 6 mWR R= 8 mWOR R= 4 mWOR R= 6 mWOP R = 8 m

WR & WOR R=6 m

WR & WOR R=4 m

WR & WOR R=8 m

Fig. 48. CDF vs. average percentage of connectivity, [ N = 10, p = 0.2, rsr = 2 m, dmin = 2 m]. ([74] c⃝ 2013 IEEE).

example, in the case of WR, there is a 50% probability the average number of MTs con-nected will be equal to or or less than 37% when Rmr = 4 m. With the same probabilitythe average percentage of MTs will be connected will be equal to less than 10% and 5%for Rmr = 6 m and for Rmr = 8 m respectively.

Figure 49 shows the CDF vs. average percentage of connectivity of users with theAP. In this simulation, three different constants velocities are chosen unlike the othersimulations results (Figure 47, and 48), where the considered velocity for each userwas chosen as randomly distributed. Again, these simulations demonstrate that theperformance of user connectivity increases with WR in comparison to WOR. However,it is seen that the considered velocities in the range [0.1− 0.9] m/s almost have thesame impact on the performance of user connectivity both for WR and WOR. There isno significant difference among the considered velocities.

6.3.1 Co-operative hybrid WLAN-VLC networks

The scenario of multi-hop communication (Section 6.2) has been extended to hybridWLAN-VLC systems where the coverage of WLAN is assumed to be provided. Itis assumed that MT is equipped with both radio and optical wireless interfaces. The

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Average Connectivity (%)

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F

v= 0.1 m/secv= 0.2 m/secv= 0.9 m/secv= 0.1 m/secv= 0.2 m/secv= 0.9 m/sec

WithRelay

WithoutRelay

Fig. 49. CDF vs. average percentage of connectivity [ N = 10, rsr = 2 m, dmin = 2m]. ( [74]c⃝ 2013 IEEE).

coverage range of VLC is assumed to be 2 m. The transmission range of the VLCinterface is assumed to be 1 m. Furthermore, we have assumed that VLC will be theprimary system, meaning the MT will communicate with the AP either through single-hop or multi-hop communication using an optical wireless interface, or then it willcommunicate with the WLAN radio interface. If the MT in the multi-hop region anddoes not find any relay to communicate with, the AP will then communicates withthe WLAN. Active transmitters (e.g., that are in AI) that are connected directly areconsidered as VLC users. On the other hand, the active transmitters that are in multi-hopregion that connected via a relay are considered as multi-hop VLC users, and those whowill connect via the WALN interface are considered as WLAN users. Both VLC andmulti-hop VLC users can also be considered as optical interface users. The definitionof other concepts such as single-hop and multi-hop regions, and the relay selectionalgorithm are considered to be same as described in Section 6.2.

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3 4

5

6

7

8

X[m]

Y[m

]

WLAN Coverage

WLAN Coverage WLAN Coverage

Multihop Coverage

VLC Coverage

Fig. 50. Layout of hybrid coverages.

6.3.2 Energy consumption analysis

Energy consumption during the transmission of a data packet by MT in the case of asingle-hop can be expressed as [8]

ESh = TtxPtx = PtxLp

Rr, (72)

where Ttx is the transmission time, Ptx is the transmitted power, Lp is packet length andRr is the transmission rate. It is not straight forward to compare energy consumption forthe transmission of data of different wireless systems. However, energy consumptionfor transmission of data for IEEE 802.11g and optical link have been compared and areshown in Table 11 [159].

In the multi-hop case, the total energy consumption is the sum of energy consump-tion at the MT and the relay, which can be expressed as

ETh = EMT−RS +ERS−AP, (73)

where EMT−RS and ERS−AP are the energy consumption by MT and relay respectively.In this work, the data rate of both relay and active transmitter for VLC interface are

considered as same. In that case, energy consumption for multi-hop can be represented

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Table 11. Energy consumption.

Standard Power consumption (W)

IEEE 802.11 (g) 1.25Optical Link 0.3

as [160, 161]

ETh = (Rn +1)PtxLp

rvl, (74)

where Rn represents the number of hops. Energy consumption in the hybrid systemwhere energy consumption has been calculated according to the technology specifictransmission power and data rate which can be represented as

EH = Nw

T

∑m=1

Pw

rw+Nsh

T

∑m=1

Pv

rv+Nmh

T

∑m=1

Pvl

rvl, (75)

where Nw, Nsh, Nmh represent the number of MTs in WLAN, single-hop and multi-hopcoverage respectively. Similarly, Pw, Pv, Pvl are the transmission powers of WLAN andVLC interface respectively. T is the number of mobility slots. In all cases, the lengthof transmitted packet Lp is assumed to be 1.

6.4 Performance evaluation of hybrid WLAN-VLC networks

In this section, the connectivity and energy consumption performance of hybrid WLAN-VLC will be evaluated using system design parameters as given in Table 12.

Table 12. Simulation model parameters.

Simulation Parameters Value

No. of Nodes [5,10,15,20, 25]Relay probability [0.2,0.4,0.6]Single hop cell radius [2 m]Multi-hop cell radius [4,5,6,7,8,9,10] mVelocity [0.5, 0.7,0.9] mVLC data rate 10 MbpsWLAN data rate 5 Mbps

Figure 51 shows the three sets of plots of average connectivity in hybrid WLAN-VLC coverage. In this particular simulation, results of 20 MTs are involved with relayprobability p = 0.2. In this figure, the blue plot shows user connectivity via WLAN.

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2 2.5 3 3.5 4 4.5 50

20

40

60

80

100

Coverage Ratio

Ave

rage

Con

nect

ivity

(%

)

VLCMultihop VLCWLAN

Fig. 51. Coverage range ratio vs. average connectivity. ( [6] c⃝ 2013 IEEE).

The red plot shows single-hop connectivity via VLC. Finally, the green plot shows userconnectivity in a two-hop case. It is seen that as the coverage range ratio increases alarge number of active transmitters will be connected via WLAN. This is because whenthe coverage ratio increases, a large number of users will be positioned at the outer areaof the boundary. Such a large number of active transmitters can not connect to the APvia relay due to the short transmission range dmin of MT. As a result, MTs will connectto the AP via WLAN.

Figure 52 shows the simulation results of average percentage connectivity vs. num-ber of MTs. In this simulation, we investigate the impact of numbers of MTs on con-nectivity performance by considering single-hop coverage range (2 m), multi-hop cov-erage range (4 m), and relay probability p = 0.2. It is seen that as the number of MTsincreases, the connectivity performance of MTs is also increased for multi-hop VLCand WLAN interfaces. This is due to the fact that when the number of MTs increases,the node density also increases. Hence, the probability of getting a higher number ofrelays also increases within the transmission range dmin of MT in Ao. However, manyof the MTs in Ao will be disconnected because of not finding the relays; in such casesthe MTs will be connected via WLAN interface. As a result, when the number of users

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5 10 15 20 250

10

20

30

40

50

60

Number of Users (N)

Ave

rage

Con

nect

ivity

(%

)

VLCMultihop VLCWLAN

Fig. 52. Number of users vs. average connectivity. ( [6] c⃝ 2013 IEEE).

increases, the co-operative connectivity as well as connectivity using complementarytechnology WLAN will increase to extend the short coverage range of VLC.

Figure 53 shows the CDF vs. total energy consumption for transmission of activetransmitters for ten mobility slots. The red plot shows the energy consumption fortransmission when all MTs use radio (WLAN). On the other hand, the blue plot showsthe energy consumption for transmission when both radio (WLAN) and optical (VLC)technologies are used for transmission. It is seen that when the active transmitters useshybrid system for transmission, then the total energy consumption is less than whenthey use only radio (WLAN) system. For example, at 50% probability total averageenergy consumption by hybrid system is less than 23 µJ in comparison to less than 36µJ for the WLAN system. Almost 1.58 times less energy is consumed by hybrid users(VLC, VLC multi-hop, WLAN) users in comparison to only WLAN interface users.

6.5 Conclusions

In this chapter, we have evaluated the performance of relay-assisted VLC and relayassisted hybrid WLAN-VLC hotspot networks. In relay-assisted VLC networks, we de-veloped an optical relay based multi-hop communication simulation environment. Re-lay selection algorithm is also introduced in this work. The motivation of this work is

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15 20 25 30 35 40 450

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Total Energy Consumption (µJ)

CD

F

Hybrid WLAN

Fig. 53. CDF vs. Average aggregated energy consumption. ( [6] c⃝ 2013 IEEE).

to use co-operative relays to reduce on-demand recovery. In this respect, the percent-age of connectivity is chosen as the performance metric to evaluate the performanceof such co-operative relay-assisted networks. The simulation environment of the relay-assisted VLC hotspot network is further extended to the relay-assisted hybrid WLAN-VLC hotspot network. The purpose of this extension is to exploit the benefits of eachtechnology such as the reliable connectivity of WLAN and the higher throughput andimproved energy efficiency of VLC. Simulations have been performed in order to eval-uate the connectivity and energy efficiency performance of such homogeneous and hy-brid networks. Simulation results reveal that user connectivity and energy efficiencydepend on user density, coverage range ratio between single-hop and multi-hop, relayprobabilities and mobility of the user. Finally, it can be concluded that hybrid radio-optical wireless systems have a positive impact on the performance of user connectivityand energy consumption of the mobile device with the expense of providing complexityin the communication architecture.

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7 Summary and future directions

The imagination must feed on the past

in creating the future.

Bulletin of the Atomic Scientists

(Vol.15, No.2)

In this chapter, we will summarise and conclude the research work presented in thisthesis. The main findings are highlighted and conclusions are drawn. Finally, futurework related to VLC and hybrid WLAN-VLC are presented.

7.1 Summary

The purpose of this thesis is to study the performance of stand-alone VLC and hybridWLAN-VLC based networks. Performance analysis of such networks is needed beforethey deployed in practice. In this regard, first we created three scenarios namely a)stand-alone VLC hotspot b) hybrid WLAN-VLC hotspot, c) co-operative optical relay-based networks. Secondly, we studied the link design parameters for both VLC andWLAN. Thirdly, we developed the mathematical framework to study the performanceof such networks in data download on the move scenarios. In this regard, first we high-lighted the three grand challenges such as spectrum scarcity, reduction of (CO2) emis-sions for green communication, and energy efficiency of mobile devices in Chapter 1.VLC may play an important role in mitigating some of these challenges, being a key net-works for 5G technology and beyond. An extensive literature review, especially on datarate improvement in hardware testbed and hybrid WLAN-VLC networks, was also car-ried out in Section 1.3.1 and Section 1.3.2 respectively in Chapter 1. The fundamentalsof VLC transceiver systems, such as different types of LEDs, LED I-V characteristicsand characteristics of photodetectors were studied in Chapter 2. The presence of dif-ferent types of noise and interference in the optical channel were also discussed in thischapter. Finally, different modulation techniques in VLC were discussed in Chapter 2.

In practice, many of the design parameters such as FOV of the receiver, divergenceangle of LED, degree of directionality between transmitter and receiver as well as am-bient noise were considered in VLC link designing. We described these four basiclink topologies. These four link characteristics were classified as: directed-LOS, non-

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directed-LOS, non-directed-NLOS and tracked. The coverage and data rate of such linkdesigns were also provided in Chapter 3. Many other advanced link topologies such asa multi-spot diffuse link, DSD were also discussed in Chapter 3. In this thesis, non-directed LOS with tracking link topology was assumed exist between transmitter andreceiver.

In Chapter 4, we investigated the first scenario (data download on the move in VLCcoverage). In this respect, first we introduced the data download on the move scenariowith related concepts such as moving-in, moving-out, travelling angle in Section 4.1.A mathematical framework to calculate the average throughput and file size were alsointroduced in Section 4.3. Theoretical and polynomial-based throughput-distance rela-tionship models were also developed for the day and night conditions. Transformationof straight inclined travelling paths to equivalent horizontal travelling paths using ro-tational technique was used in this chapter. It was showed that this rotation (inclinedpath to horizontal path) simplifies the calculation of average throughput. Measurementbased shot noise was also included in the mathematical framework. Analytical solu-tions for calculating average throughput and file size for single-user case were provided.Simulations were also performed to evaluate the performance of such hotspot networkin the context of data download on move scenario. The simulation results revealed thatthere was a large impact of background noise on the performance of VLC hotspots. Asexpected, in both indoor and outdoor environments, VLC hotspot performed better atnight than during the day. The performance of VLC hotspot networks was also quanti-fied in terms of received file size at different bit error rate requirements and velocitiesof the mobile user.

In Chapter 5, we studied the performance of second scenario (data downloading inhybrid WLAN-VLC networks). We considered data download on the move scenarioin an indoor environment for single-user as well as for multi-user cases. In this hy-brid WLAN-VLC hotspot, both the WLAN and the VLC were characterized by theirthroughput and communication range. Short-range VLC small cells were consideredfor high data rate region. A constant data rate was assumed for the VLC small cell,in this respect three sets of data rates at three different BER requirements were pro-vided in Section 5.1.1. The coverage range of the VLC small cell was considered tobe one meter. On the other hand, an integrated complementary WLAN hotspot wasused for providing reliable connectivity and moderate data rate support. In this re-spect, measurement-based coverage range and data rate were provided for WLAN asdiscussed in Section 5.1.2. Descriptions of environments for both single as well as

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multi-users for two different mobility models were created and discussed in Sections5.2 and 5.3 respectively. The impact of the velocity of mobile user(s) and the numberof VLC small cells present in the considered service area were also considered on theperformance of hybrid WLAN-VLC small cells. Simulations had been performed toevaluate the performance of such network for data downloading on the move scenarioby taking into account performance metrics such as file size, average connectivity andsystem throughput. Simulation results revealed that the considered hybrid WLAN-VLCperforms always better than standalone VLC-only or WLAN-only hotspot both for sin-gle and multi-user cases.

Finally, in Chapter 6, we studied the performance of a third scenario (co-operativerelays in VLC and hybrid WLAN-VLC networks). In relay-assisted VLC networks, wedeveloped an optical relay based multi-hop communication simulation environment. Arelay selection algorithm was also introduced in this work as discussed in Section 6.2.1.The motivation of this work was to use co-operative relays to setup the communicationbetween AP and transmitter if direct communication is not available. The percentage ofconnectivity was chosen as a performance metric to evaluate the performance of suchco-operative relay-assisted networks. The simulation environment of the relay-assistedVLC hotspot network was further extended to the relay-assisted hybrid WLAN-VLChotspot network as discussed in Section 6.4. The purpose of this extension was to ex-ploit the benefits of each technology, such as the reliable connectivity of WLAN and thehigher throughput and improved energy efficiency of VLC. Simulations had been per-formed in order to evaluate the connectivity and energy efficiency performance of suchhomogeneous and hybrid networks. Simulation results revealed that user connectivityand energy efficiency depend on user density, coverage range ratio between single-hopand multi-hop, relay probabilities and mobility of the user. Finally, it can be concludedthat hybrid radio-optical wireless systems had a positive impact on the performance ofuser connectivity and energy consumption of the mobile device.

The framework of this work may be applied in future deployment of VLC com-munication system in indoor and and outdoor environments. It is also noticed that theperformance of such a network degrades rapidly in the presence of natural as well asartificial noise. Therefore, suitable filtering methods, for example, optical and electri-cal filtering method can be used to mitigate these problems. Thesis also dealt with thehybrid communication systems and provide also the benefits of using it.

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7.2 Future work

This thesis essentially focuses on the performance analysis of future VLC and hybridWLAN-VLC network systems. In doing so, we had few assumptions to simplify ouranalytical models. For example, connection set-up time, link discovery mechanism, fastlink recovery algorithm, switching algorithm between VLC and WLAN or vice versawere not taken into account in the analytical model. Existing mathematical frameworkof data downloading on move scenario can be extended by considering any of theseassumptions.

This thesis deals also with data download on move scenario in multi-user environ-ments. However, multi-user interference is not consider in this thesis. So, existingsimulation environment can be further extended by considering multiple access inter-ference and resource allocation mechanism to avoid the multi-user interference. Cellzooming technique studied in [152], can be one of the possible solutions to mitigate theinterference produced in multi-user VLC communication environment.

We have also shown that hybrid WLAN-VLC performs better than the standaloneVLC and WLAN in terms of connectivity and throughput. In this respect, developmentof both theoretical and practical analysis of PHY and MAC designs for hybrid WLAN-VLC will be interesting topic to explore. Therefore, future objective will be to designand analyze suitable unified PHY techniques including channel coding and modulationtechniques for energy and spectrally efficient hybrid WLAN-VLC systems. Moreover,in this thesis seamless handover has been assumed to select the best available VLCor WLAN wireless networks. In practice, a suitable handover technique is needed toswitch between these heterogeneous networks. However, all-radio vertical handover(VHO) approaches cannot be applied to the hybrid radio and LOS optical circumstances,since the channels have different properties. Hence, characteristics of radio and opticalchannel models have to be revisited and modified. Feasibility study of integrating VLCin 5G HetNet solution in emerging software defined networks (SDN) will be interestingtopic to explore as well.

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