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Visible-Light Communication Demonstrator:

System modeling and analogue distribution network

design

by

Amita Shrestha

A thesis for conferral of a Master of Science in Communications,

Systems and Electronics

Advisors:

Prof. Dr. Harald Haas Dr. Joachim W. Walewski

Cellular and Wireless Communications Group Dr. Sebastian Randel

Jacobs University Siemens AG, Corporate Technology,

Bremen, Germany Information and Communications

Munchen, Germany

Date of Submission: August 17, 2009.

School of Engineering and Science


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Declaration

I declare that I have prepared the master thesis

Visible-Light Communication Demonstrator:System modeling and analogue distribution network design

without illegal help. I also declare that contributions of other authors which are used

in the thesis or led to the ideas behind the thesis are properly referenced in written form.

I am aware that a master thesis, developed under guidance, is part of the examination and

may not be commercially used or transferred to a third party without written permission

from my supervisors. I declare that this thesis is not submitted elsewhere for conferral of

a degree.

Bremen, .

(Signature)

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Acknowledgements

No work is possible without the help of well-wishers, friends, teachers and many other

directly or indirectly related people around us. It gives me great pleasure to acknowledge

the help and guidance of each and every individual while working on the project.

Foremost, I would like to express my sincere gratitude to my advisors Prof. Harald

Haas, Dr. Joachim W. Walewski and Dr. Sebastian Randel for their continuous support,

motivation, enthusiasm, and immense knowledge. Their guidance helped me in all the time

of research and writing of this thesis.

My sincere gratitude goes to my colleagues Jeffrey, Florian and Beril for their help, and

valuable hints. Furthermore, my heartful thanks also goes to my brother Amit, and my

friend Saksham for their immense support, encouragement and help.

Last but not the least, I owe my deepest gratitude to my family and friends for their

unflagging love and support throughout my life.

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Abstract

Visible-light communications (VLC) is a technology for wireless communication using light

that can be perceived by the naked eye. VLC uses frequencies other than radio, and they

are unrestricted and licence free. In recent years, optical wireless communication (OWC)

for short ranges (up to 10s of meters) has experienced increasing interest amongst re-searchers. Currently, Siemens Corporate Technology (CT) is participating in Home Giga-

bit Access (OMEGA), an EU integrated platform within the seventh Frame Programme.

The technology to be demonstrated is visible-light communication using white LEDs with

a target data rate of 100 Mbits/s. To date, visible light communication using one LED has

been successfully implemented. However, in order to illuminate the VLC area of OMEGA

demonstration showroom, arrays of LEDs have to be placed on the ceiling. Modeling of the

placement of these LEDs needs to be designed such that the VLC area is homogenously

illuminated and leakage outside the area is minimized. In addition, analogue signal distri-

bution network has to be designed in order to distribute the signal to all the LEDs. This

thesis addresses these issues. The lighting levels within the VLC area was simulated for

different configurations of LED placement. Furthermore, various network topologies like

linear-bus, star, and tree were experimented. It was observed that star network uniformly

distributes the signal to all the LEDs and offers higher signal to interference ratio. Fol-

lowing the results of this thesis, star distribution network for the chosen LED placement

scenario will be implemented in the OMEGA demonstration showroom.


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Contents

1 Introduction 1

1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.3 OMEGA Project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.4 Organization of the Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2 Technical Background 8

2.1 Optical Wireless Communication . . . . . . . . . . . . . . . . . . . . . . . 8

2.2 White Light-Emitting Diodes (LED) . . . . . . . . . . . . . . . . . . . . . 10

2.3 Visible Light Communication based on White LEDs . . . . . . . . . . . . . 12

2.3.1 VLC Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.3.2 Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3 Network Architectures 17

3.1 Physical Networking Topologies . . . . . . . . . . . . . . . . . . . . . . . . 18

3.2 Ethernet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223.2.1 Ethernet Physical Layer Specification . . . . . . . . . . . . . . . . . 22

3.2.2 Ethernet Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

4 Coaxial Cables 27

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CONTENTS

4.1 Connectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

4.2 Equivalent Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

5 System Modeling 32

6 Inter-symbol Interference 39

6.1 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

6.2 Simulation of ISI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

6.2.1 ISI in Linear-Bus Network . . . . . . . . . . . . . . . . . . . . . . . 45

6.2.2 ISI in Star Network . . . . . . . . . . . . . . . . . . . . . . . . . . . 536.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

7 Measurement and simulation for different network topologies 60

7.1 Linear-Bus Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

7.2 Star Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

7.3 Tree Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

7.4 Summary of the Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

8 Final testing of the distribution network and analogue transmitters 71

8.1 Measurement of the signal for different analogue transmitters. . . . . . . . 73

8.2 Measurement of velocity of the signal propagating through the coaxial cable. 76

8.3 Measurement using power splitter . . . . . . . . . . . . . . . . . . . . . . . 77

8.4 Summary of the Measurement . . . . . . . . . . . . . . . . . . . . . . . . . 78

9 Conclusion and Outlook 80

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List of Figures

1.1 Footprint of OMEGA demonstration showroom showing VLC and IR region [1]. 2

1.2 Proposed block Diagram for VLC using multiple LEDs . . . . . . . . . . . 3

1.3 Footprint of the VLC area [1] . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.4 A block diagram of VLC transceiver . . . . . . . . . . . . . . . . . . . . . . 7

2.1 Two approaches for generating white emission from LEDs. [2] . . . . . . . 10

2.2 Block diagram overview of VLC PHY [3]. . . . . . . . . . . . . . . . . . . . 12

2.3 Different types of OSTAR Lightings [4]. . . . . . . . . . . . . . . . . . . . . 14

2.4 Typical LED driver circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.5 Picture of LED driving circuit . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.1 Bus Topology [5] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.2 Ring Topology [5] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.3 Star Topology [5] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.4 Tree Topology [5] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.5 A typical thinnet 10Base2 installation [6]. . . . . . . . . . . . . . . . . . . 23

3.6 A typical thicknet 10Base5 installation [6]. . . . . . . . . . . . . . . . . . . 24

3.7 A typical 10/100BaseT installation [6]. . . . . . . . . . . . . . . . . . . . . 25

4.1 An example of coaxial cable [7] . . . . . . . . . . . . . . . . . . . . . . . . 27

4.2 BNC and SMA connectors [7] . . . . . . . . . . . . . . . . . . . . . . . . . 29

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LIST OF FIGURES

4.3 Equivalent circuit diagram of Coaxial Cable . . . . . . . . . . . . . . . . . 30

4.4 Absolute cable impedance as a function of frequency [3]. . . . . . . . . . . 31

5.1 Model Room . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

5.2 LED placement in the OMEGA demonstration showroom. . . . . . . . . . 35

5.3 Illumination for configuration I. . . . . . . . . . . . . . . . . . . . . . . . . 36

5.4 Illumination for configuration II. . . . . . . . . . . . . . . . . . . . . . . . . 37

5.5 Illumination for configuration III. . . . . . . . . . . . . . . . . . . . . . . . 38

6.1 An optical communication scenario . . . . . . . . . . . . . . . . . . . . . . 406.2 Calculation of optical path difference [8]. . . . . . . . . . . . . . . . . . . . 41

6.3 Optical power received from the transmitter . . . . . . . . . . . . . . . . . 42

6.4 Time delay of the signal arriving to the receiver, and power received . . . . 43

6.5 An example of linear bus setup with 16 LEDs. . . . . . . . . . . . . . . . . 45

6.6 LED placement in the OMEGA demonstration showroom . . . . . . . . . . 45

6.7 Case I: SIR for a bus network . . . . . . . . . . . . . . . . . . . . . . . . . 47

6.8 Case II: SIR for a bus network . . . . . . . . . . . . . . . . . . . . . . . . . 48

6.9 Case III: SIR for a bus network . . . . . . . . . . . . . . . . . . . . . . . . 49

6.10 Case I: SIR for linear bus network . . . . . . . . . . . . . . . . . . . . . . . 50

6.11 Case II: SIR for linear bus network . . . . . . . . . . . . . . . . . . . . . . 51

6.12 Case III: SIR for linear bus network . . . . . . . . . . . . . . . . . . . . . . 52

6.13 Star networking of 16 LEDs. . . . . . . . . . . . . . . . . . . . . . . . . . . 53

6.14 Case I: SIR for star network with unequal lengths of cable . . . . . . . . . 54

6.15 Case III: SIR for star network with unequal lengths of cable . . . . . . . . 55

6.16 Case III: SIR for star network with unequal lengths of cable . . . . . . . . 56

6.17 Case I: SIR for star network with equal lengths of cable . . . . . . . . . . . 57

6.18 Case III: SIR for star network with equal lengths of cable . . . . . . . . . . 58

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LIST OF FIGURES

7.1 Possible setup for VLC testbed in tapped bus topology . . . . . . . . . . . 61

7.2 Measured output voltage for tapped bus network using Coax Cable . . . . 62

7.3 Measured output voltage for tapped bus network using Coax Cable . . . . 63

7.4 Circuit diagram for simulation of linear bus network . . . . . . . . . . . . . 64

7.5 Simulated output voltage for tapped bus network using Coax Cable . . . . 64

7.6 Simulated output voltage for tapped bus network using Coax Cable . . . . 65

7.7 Same measurement as in Figure 7.2, but with a SMA-based network. . . . 65

7.8 Star topology using 1:16 power splitter . . . . . . . . . . . . . . . . . . . . 66

7.9 Tree cabling setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

7.10 Measured output voltage at each taps of one linear bus in tree network . . 69

8.1 Experimental setup for the measurement with driving circuit board . . . . 72

8.2 Picture of the experimental setup . . . . . . . . . . . . . . . . . . . . . . . 72

8.3 Photo-detector signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

8.4 Photo-detector signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

8.5 Signal propagation in different driver circuits . . . . . . . . . . . . . . . . . 768.6 Delay of the signal due to difference in the cable length . . . . . . . . . . . 77

8.7 Effect of using power-splitter to the signal received . . . . . . . . . . . . . 78

8.8 Effect of using power-splitter to the signal received . . . . . . . . . . . . . 79

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List of Tables

1.1 Specification of the analogue signal to be distributed. . . . . . . . . . . . . 5

2.1 Electro-optical specifications of OSTAR E3A [9]. . . . . . . . . . . . . . . . 13

3.1 Summary of different physical topologies. . . . . . . . . . . . . . . . . . . . 20

3.2 Ethernet physical layer specification [6]. . . . . . . . . . . . . . . . . . . . . 22

5.1 Values used for simulation of lighting levels at desk height . . . . . . . . . 34

5.2 Summary of the requirement and result for the simulation of illumination . 34

7.1 Components used for linear bus network measurement. . . . . . . . . . . . 61

7.2 Components required for the star network measurement. . . . . . . . . . . 66

7.3 Voltage available at the first port of the 16-way power-splitter . . . . . . . 67

7.4 Components required for the tree network measurement. . . . . . . . . . . 68

7.5 Voltage available at the output of 4-way power-splitter . . . . . . . . . . . 69

8.1 Summary of the modulation index for each analogue transmitters . . . . . 73

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Chapter 1

Introduction

1.1 Motivation

Wireless communications is the fastest growing segment of the communications industry.

From satellite transmission, radio, and television broadcasting to the ubiquitous mobile

telephone, wireless communication has revolutionized the way societies function [10]. The

thirst for higher data rate in wireless access network, wireless multimedia applications,

and wireless video is growing. To date, radio technology has been offering these services.However, due to the limited unlicensed bandwidth and increasing traffic radio spectrum is

becoming increasingly congested.

On the other hand, optical wireless communication provides a cost-effective, flexible so-

lution to the emerging challenges that system and service providers are facing [11]. Optical

wireless communication is primarily an indoor technology that has the potential to be used

as a medium for short-range high-speed wireless communications [12, 13]. Thus, OWC is an

attractive supplement for the existing radio technologies. Optical wireless communications

can be, for instance infra-red (IR) communications and/or visible-light communications

[12, 13]. IR communication for e.g. Infra-Red Data Association (IrDA), is widely spread

in applications like, in notebooks, cellphones, etc. Visible-light communication (VLC)

promises numerous applications. Room lights can broadcast alarms, smart-home applica-

tion messages, or transfer files. Billboards may transmit messages. Brake-lights of a car

may send warnings to the behind it in case of an emergency brake. In addition, VLC uses

frequencies other than radio frequency and they are licence-free, to the date. Thus, abun-

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CHAPTER 1. INTRODUCTION

dant unexploited spectrum is available for VLC [3]. The most appealing feature of VLC

is that the same sources can be simultaneously used for lighting, signalling, and display as

well as data communication.

This thesis is part of the OMEGA (Home Gigabit Access) project. OMEGAs goal is to

develop a technology, and eventually a global standard that enables people to set up ultra-

broadband home networks without having to install any new home wiring. LED-powered

VLC is one of the technologies to achieve this vision. The OMEGA project also aims

to provide a Gbit/s communications network using infra-red wavelengths and 100 Mbit/s

communication using VLC. Figure 1.1 shows the footprint of VLC area of the OMEGA

showroom and its dimensions. Arrays of LEDs will be placed on the ceiling of the VLC

area in such a way that it is homogeneously illuminated. In addition to illumination theseLEDs will be used for data communication.

Figure 1.1: Footprint of OMEGA demonstration showroom showing VLC and IR region[1].

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1.2 Objective

Figure 1.2: Proposed block Diagram for VLC using multiple LEDs

Figure 1.2 shows the one of the possible models for VLC broadcasting with an array of

LEDs. These array of LEDs are placed on the ceiling of a room and are used for lighting

as well as data communication. Major objectives of this thesis is listed below.

1. Find the proper LED placement, homogeneously illuminating the VLC area.

2. Design of analogue distribution network for the LEDs placed.

3. Analysis of inter-symbol interference for different LED placement scenario and net-

work topologies used.

Figure 1.3 shows the ceiling footprint of the OMEGA showroom demonstrator in more

detail. It explicitly shows the VLC area and its proposed dimensions. All LEDs placed on

the ceiling are used to broadcast the same information and need to be driven synchronously.

One of the objectives of this thesis is to model the placement of these LEDs on the ceiling

so that that the VLC area is homogeneously illuminated. In addition, there should be

minimum leakage outside the VLC in order to avoid interruption of TV viewing in IR

area. Also, the IR receiver is sensitive to the light. According to the standard [14],

practical range of illuminance level for office illumination at desk area is 200 to 1000 lx .

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Figure 1.3: Footprint of the VLC area [1]

Thus, 400 lx of illuminance level is required to illuminate the VLC area [3]. The target is

thus to place the LEDs in such a way that 400 lx of illumination level is maintained over

a maximum percentage of the VLC area.

In addition to the modeling of proper placement of LEDs on the ceiling, a network to

distribute analogue signal to these LEDs, needs to be designed. Nevertheless, it would be

desirable to transmit the baseband modulated signal through the distribution network, to

each of the LEDs. Modulator block and DAC could be placed at the input of each LEDs.

However, at the present situation, it is not feasible to install a modulator and DAC at

each LED. This is because the modulation block is currently implemented on an FPGA

testboard, at the transmitter. Placing such testboards at each LED increases the cost,

and requires extra space which is insufficient in our case. Table 1.1 lists the summary of

the requirements for the LED placement and analogue distribution network design. As

stated earlier the VLC area should be homogeneously illuminated with less leakage, the

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distribution network should be designed such that the cost is minimum and easy to install.

The issue of inter-symbol interference should also be considered (see Chapter 6).

(a) Qualitative specifications

Qualitative specifications

Illumination VLC area fully illuminated maximum area above threshold (i.e. 400 lx) minimum variation

Leakage No leakage outside VLC areaComponent Cost LeastType of cable Cheap

easy to installPre-Amplifier1 Ideally no amplifierNumber of LED Minimum possibleISI No ISI.

This implies that analogue signal has to reach each LED at the same time.Complexity Lowest

1Amplifier placed between DAC and the distribution network.

(b) Quantitative specifications

Quantitative specifications

Signal Bandwidth (Upper 3dB) 50 MHzSignal Bandwidth (Lower 3dB) 100 kHz

Nyquist symbol period (Ts) 10 nsMaximum time delay without ISI (half of Ts) 5 nsOutput of DAC 1 Vpp @ 50 Input signal to the driver circuit max 0.6 Vpp

Table 1.1: Specification of the analogue signal to be distributed.

Given that the bandwidth of the transmitted signal is limited to 50 MHz (LED mod-

ulation bandwidth), the Nyquist symbol period is limited to 10 ns, and ISI will occur if

transmitted data symbols experience delays larger than 5 ns [15]. The distribution network

needs to be designed in such a way that delay experienced by the data symbol transmitted

through LEDs and arriving at the receiver, is less than 5 ns. Looking into the design of

the distribution network different network topologies like bus, tree, and star and cables

like coaxial, optical fiber, are considered. Different types of networking techniques using

different cables are experimented and simulated (see Chapter 7).

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Figure 1.4: A block diagram of VLC transceiver to be developed within OMEGA (EU, FP7-1).Project Partners: FT: France Telecom, HHI: Fraunhofer-Heinrich Hertz Institute, UoA:University of Athens, Siemens CT MM 6 (Packaging & Assembly) and Siemens CT IC 2(Network & Multimedia Communications).

its equivalent circuits are discussed in detail. The simulation for the Illumination and ISI

is presented in chapter 5 and 6 respectively. The measurement of different network topolo-

gies and different cables, and its result are are discussed in chapter 7. The final test of the

analogue transmitters and the chosen distribution network are presented in chapter 8. The

outcomes are discussed together with possible directions for future research in chapter 9.

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Chapter 2

Technical Background

2.1 Optical Wireless Communication

Optical wireless communication (OWC) refers to the use of free-space propagation of opti-

cal waves. Although wireless conventionally is synchronous to radio technology, the next

generation of wireless communication systems (4G) will be based on several complemen-

tary access technologies, one of which could be OWC [20, 21, 12]. The availability of a

huge and unregulated bandwidth, without electromagnetic interference (EMI) with radiowaves, make OWC a viable candidate to supplement the existing spectrum-starved radio

communication. Furthermore, OWC signals can be confined in space since they do not

penetrate through walls. If each room is considered a cell then there is thus no inter-cell

interference. This enables a simple design of high-capacity wireless local-area networks as

the same operating frequencies can be used in adjacent cells. Since, for the same reasons,

OWC offers high degree of privacy and security against eavesdropping, it can also be used

for the transmission of content-sensitive data.

Nevertheless, OWC also comes with some disadvantages. For example, in many indoor

environments there exists intense ambient optical noise arising from sunlight, incandescent

lighting and fluorescent lighting, which induce noise in a visible-light receiver [22]. Because

visible light can not penetrate walls, communication from one room to another requires the

installation of VLC access points that are interconnected via a wired backbone. However,

transmitter power may be limited by concerns of power consumption and eye safety [23].

Visible-light signals can represent a hazard to the human eye when transmitted power

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CHAPTER 2. TECHNICAL BACKGROUND

exceeds a certain threshold [24].

OWC can be classified as Infrared optical wireless or visible-light Optical Wireless de-

pending upon the region of spectrum used as a medium for data transmission. The majority

of installed systems operate in the near infrared (IR) at wavelengths either around 850 nm

or in the range of 1550 nm (mainly due to existing optical sources, receiver technology, and

radiation-safety regulations) [25]. Infrared technologies have been widely researched and

have lead to numerous point-to-point applications such as short-range low-speed links ad-

hering to the Infra-Red Data Association (IrDA) standard [25]. Recently, communication

via visible-light has gained attention in research, driven by progress in visible light LED

technology [20].

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2.2 White Light-Emitting Diodes (LED)

Figure 2.1: Two approaches for generating white emission from LEDs. [2]

LEDs are semiconductor devices that emit light when biased in the forward direction of

the p-n junction. LEDs present many advantages over traditional light sources, including

lower energy consumption, longer lifetime, improved robustness and smaller size [26]. One

interesting characteristics of LEDs is that they are capable of switching on and off faster

than than what human eye can distinguish. The power of the light emitted by LED can

be readily modulated by altering the driving current applied to the device. Therefore,

besides becoming popular for illumination purpose, LEDs can also be used in wireless datatransmission. LEDs are also used for architectural lighting due to the inherent ease of

dimming and color rendering [27]. In automotive applications, LEDs are extensively used

for tail, brake and indicator lights. Traffic signals also use LEDs for reasons of reliability

and lifetime [28].

Two approaches are generally used to generate white-light with LEDs. The first ap-

proach is to combine light from, e.g., red, green and blue (RGB) LEDs [ 2]. Typically, these

triplet devices consist of a single package with three emitters and combining optics, and

they are often used in application where variable color emission is required. These devices

are attractive for VLC as they offer the opportunity for transmitting different data on each

LED. The other technique is to use a single blue LED which is coated with, or sometimes

embedded in, a layer phosphor that emits red-shifted light upon absorbing a portion of

blue light emitted by the LED. The red-shifted emission mixes additively with the non-

absorbed blue component to create the required white color (see Figure 2.1). At present,

the later approach is often favored due to the lower complexity and cost [28]. Nevertheless,

in single-chip devices the phosphor typically limits the speed of overall optical response.

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However, as observed by Grubor et al in [15], their disadvantageously small modulation

index can be increased from 3 to 20 MHz when detecting only the blue part of the emitted

spectrum [19].

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2.3 Visible Light Communication based on White LEDs

A blockdiagram overview of simple VLC Physical Layer is shown in Figure 2.2. The

Figure 2.2: Block diagram overview of VLC PHY [3].

VLC PHY consists of mainly digital and analogue transmitter, as well as the analogue

and digital receiver. The digital transmitter consist of data source, baseband modula-

tor, and digital-to-analogue converter (DAC). Similarly, the digital receiver contains a

analogue-to-digital converter (ADC), baseband demodulator, and data sink. The analogue

transmitter includes a LED driving circuit (trans-conductance amplifier, TCA [3]) and

visible-light source, viz. the LED. The receiver includes imaging optics, a photo diode,

a trans-impedance amplifier (TIA), and a band-pass filter. The digital PHY delivers an

AC baseband signal (UAC) to a driving circuit that linearly amplifies the AC signal and

transforms it into a current. This current is then then added onto the DC bias current by

aid of, e.g., a bias tee. Since LEDs works in a linear region with unipolar driving currents,

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the absolute driving current (DC+AC) has to be larger than zero. The total current ILED is

fed to the LED, which, in turn, emits a modulated optical power Popt. The received power

(Popt) impinges onto an optical concentrator (lens), is directed through an optical filter,

and converted into a current IPD in a photo diode. The AC component of the current is

then trans-impedance amplified (UPD) and band-pass filtered, (UPD,filter). The output from

the band-pass filter is converted to digital signal by aid of an ADC. Finally, the digital

signal is demodulated.

2.3.1 VLC Components

OSTAR Lighting

In order to illuminate the OMEGA showroom (13 m2 footprint) with manageable number of

LEDs, OSTAR lighting modules were chosen for this project, since they provide noticeable

higher illuminance flux than comparable high-power LED [4, 3]. OSTAR Lighting source

was developed with an emphasis on lighting, e.g., room lighting, architectural lighting,

industrial lighting, radiator as well as spot lighting and flashlights. In general, there

are four variants of the OSTAR Lighting, which differ only slightly from each other (see

Figure 2.3). The first two are based on a module with 4 semiconductor chips (E2); one

variant is constructed without a lens, the other with a lens (ExA and ExB respectively).The other two modules are based on a construction with 6 semiconductor chips (E3). In

order to keep the overall number of LEDs low, the 6-chip OSTAR version is chosen, viz.

E3A [3]. Table 2.1 states the technical parameter of 6-chip OSTAR modules.

OSTAR Type E3ANo. of LED chips 6Typical bias voltage (V) 21Corresponding typical bias current (A) 0.7Corresponding luminous flux (lm) at 0.7 driving current 300

Corresponding typical illuminance (cd) 95Maximum Dc bias current (mA) 1Full viewing angle at half illuminance 130

Table 2.1: Electro-optical specifications of OSTAR E3A [9].

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(a) Modules of the OSTAR-Lighting, with

and without primary optics

(b) OSTAR-Lighting with lens and 6 chips

(LEW E3B)

Figure 2.3: Different types of OSTAR Lightings [4].

DC Driving Circuit

Figure 2.5 shows the typical LED driver circuit. As mentioned in Section 2.2 the power

of the light emitted by LED can be readily modulated by altering the driving current

applied to the device. For small-package LEDs typical DC driving currents amount to10s of mA and for lighting white LEDs the driving currents can exceed 1 A [16]. It was

measured that for E3B LED, the normalised optical power for LED driving current beyond

900 mA were influenced by excess heat dissipation from the LED and these values were

hence dropped when fitting the measured data [3]. The driving currents of several hundred

Figure 2.4: Typical LED driver circuit for modulating the optical output form a (white)LED [16].

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milliamps at several volts for OSTAR are supplied by commercial driver ICs and units.

These devices typically create electrical noise in the kHz or MHz region [16]. This noise

does not affect the illumination purpose much as it is a problem in data communication.

However, this electrical noise from the DC source decreases the signal to interference ratio

of the transmitted data signal. This case is of particular concern when increasing the data

rate beyond the bandwidth limit by the use of spectrally efficient modulation [15]. The

picture of the driving circuit used in the project is shown in Figure 2.5.

Figure 2.5: Picture of the LED driving circuit used in the project.

2.3.2 Modulation

Various schemes have been investigated. Non-return-to-zero on-off-keying (NRZ-OOK)

has been used for several demonstrations [29, 15, 28], and this scheme has the advantage

of simplicity and good immunity to LED non-linearity. The high channel signal to noiseratio (SNR) makes multilevel modulation seems attractive, and discrete multitone (DMT)

has been investigated in this purpose [28, 29, 30, 31]. In OMEGA project, the data link

is bandwidth-limited at the transmitter side to 10s of MHz, which poses a hurdle for the

OMEGA target data rate of 100 Mbit/s. This bandwidth limitation can, for instance, be

overcome by spatial multiplexing equalisation, and multi-level modulation. The OMEGA

VLC prototype will either rely on the latter or a combination of equalisation and multi-

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level modulation [28]. Grubor et al. demonstrated VLC data transmission in excess of

100 Mbit/s for a 3-dB bandwidth of 20 MHz. This result was achieved by the use ofquadrature amplitude modulation on discrete multitones [15].

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Chapter 3

Network Architectures

Center of the tiles on the ceiling of the VLC area are possible grid points where we can put

the lamps, and the light is produced by LEDs to illuminate the VLC area in the demon-

stration showroom (see Figure 1.3). All these lamps are used for wide-area broadcast and

the same information needs to be transmitted from each of them in a synchronised matter.

Each of the lamps is modulated by the aid of an LED driver circuit (see Section 2.3.1). The

information to be transmitted by the LED is modulated by the use of a digital baseband

modulator and the light is converted in to an analogue signal. This analogue signal needs

to be distributed to all LED driver circuits in order to broadcast the data through the

lamps. The output of the DAC as specified in Table 1.1, is maximum 1 Vpp at 50 , and

the same signal amplitude is required at the input of each driving circuits. Thus, the task

at hand is to design an analogue distribution network satisfying the specifications.

Similar to the physical network topologies used in LAN (typically used for networking

computers and other types of terminals), the analogue transmitter in our system can be

connected using linear bus, star, or tree networks. The properties of these networks is

discussed hereafter. Unlike computer networking in LAN, it is not necessary for us to

follow the Ethernet physical layer specifications as we are designing analogue distributionnetwork. However, it can be advantageous for us to follow these specifications (see table

3.2). The remainder of this chapter is organized as follows. Different possible physical

networking topologies are explained in Section 3.1, and in Section 3.2 we provide a short

introduction to the Ethernet.

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CHAPTER 3. NETWORK ARCHITECTURES

3.1 Physical Networking Topologies

Network topology is the study of the arrangement of the elements (links, or nodes etc.)of a network. The physical topology of a network describes the layout of the cables and

workstations, and the logical order and location of all network components. The four most

widely used topologies are the following

Bus Topology: A linear bus topology consists of a main run of a cable with aterminator at each end (see Figure 3.1), and it uses the main run to connect all the

devices. The terminator shown in Figure 3.1 is replaced with 50 coax terminatorand coaxial cable is used for the cabling. The cable functions as a shared communi-

cation medium, that devices attach or tap into, by aid of an interface connector. A

device willing to communicate with another device on the network sends a message

onto the wire that all devices receive, but only the intended recipient accepts and

processes the information. Bus networks are relatively easy to install and require less

cables compared to the other alternatives [5]. However, failure in the main cable will

disable the whole network.

Ring Topology: This is a network topology where each device has exactly twoneighboring devices for communication (see Figure 3.2). All messages travel through

a ring in the same direction (clockwise or anticlockwise). However, if there is a failure

at any point the device can send the information in the opposite direction.

Star Topology: Star topologies are used in many home networks. A star networkfeatures a central connection point called a hub, which may be a hub, a switch, or

a router (see Figure 3.3). Typically unshielded-twisted-pair (UTP) cables are used

for connecting devices to the hub. Compared to the bus network, it requires more

cables but a failure in one cable does not affect the other lines.

Tree Topology: This topology is known as a hybrid of bus and star topology.It integrates several star topologies together into one bus as shown in Figure 3.4.

Typically hubs are connected to the main bus and devices are connected to the hub

as in a star.

Table 3.1 depicts the summary of different topologies used in our experiment and their

ranking based on their feasibility of implementation in our project, where 1 represents the

best candidate.

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Figure 3.1: Bus Topology [5]. In the figure computers stand for the devices to be connected.

Figure 3.2: Ring Topology [5]. In the figure computers stand for the end-devices to beconnected in the network.

Figure 3.3: Star Topology [5]. In the figure, computers stand for the end-devices to beconnected in the network.

Comparing the characteristics of different topologies with the requirements specified

in table 1.1, star topology seems to be the best networking topology for the project.

Various experiments were conducted using different topologies. Detail description of the

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Figure 3.4: Tree Topology [5]. In the figure computers stand for the end-devices to beconnected in the network.

Characterstics Linear Bus Star TreeCables used Coaxial & SMA Coaxial & SMA Coaxial & SMAAttenuation higher than star and tree lowest higher than star

Output at end-devices non-uniform amplitude 1 uniform non-uniformISI higher than star and tree lowest lower than bus

Pre-Amplifier 2 very high-powered medium-powered high-poweredInstallation Complexity highest least higher than bus

Ranking (1-3) 3 1 (best) 21

non-uniform amplitude of the signal implies that the amplitude of the signal at each tap is not equal. This is the case in

linear-bus and tree network.2

Pre-Amplifier is needed in between DAC and the power-splitter to amplify the signal before dividing the signal among all

the analogue transmitters.

Table 3.1: Summary of different physical topologies.

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experiments and results are presented in Chapter 7. They indicate that the star topology

was the most suitable topology for our project.

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3.2 Ethernet

Ethernet was originally based on the idea of computers communicating over a shared coaxialcable acting as a broadcast transmission medium. Ethernet LANs consist of network nodes

and interconnecting media. The network nodes fall into two major classes:

Data Terminal Equipments (DTE): DTEs are either the destination or sourceof data frames. Some of the typical DTEs are PCs, workstations, print servers etc.

Data Communication Equipments (DCE): DCEs are the network devices thatreceive and forward frames across the networks. Repeaters, network switches, routers,

interface cards, modems are some of the typical examples of DCE.

3.2.1 Ethernet Physical Layer Specification

Table 3.2 provides a summary of various physical layer specifications defined for 10Mbps

to 100Mb/s Ethernet. The first version of Ethernet, which was introduced in the 1980s,

supported a maximum data rate of 10Mb/s. Later fast Ethernet standards increased this

maximum data rate to 100 Mb/s. Today, Gigabit Ethernet technology further extends

peak performance up to 10 Gb/s.

Standard Cables used Maximum speed Maximum length Topology10Base2 Thin Coaxial 10 Mbps 185 meters Bus10Base5 Thick Coaxial 10 Mbps 500 meters Bus10BaseT Unshielded Twisted Pair 100 Mbps 100 meters Star

100BaseTX Unshielded Twisted Pair 100 Mbps 100 meters Star10BaseF Optical Fiber 10 Mbps 2000 meters Star or Tree

Table 3.2: Ethernet physical layer specification [6].

3.2.2 Ethernet Types

Different cables like coaxial, Twisted pair and 0ptical fibers are used in the Ethernet

standards.

1. Ethernet Coax

In the beginning, coax was the most common cable used for connecting workstations

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in a small office or home networks. Depending upon the thickness of the cable, they

are used in the 10Base2 or 10Base5 Ethernet standards. A detailed description of

the 10Base2/5 standards is provided below.

10Base2 Ethernet: It is also called cheapernet or thinnet, as it uses thecoaxial cable which is thinner and cheaper compared to the one used in 10Base5

Ethernet. It basically uses RG-58A\U, 50- thin coax cable. It supports atapped bus topology as shown in Figure 3.5. The thinnet coax is routed from

one device to another in a daisy-chain fashion. At each device, a T connector

is used to tap the coax. At each end of the cable, a 50- terminator (grounded

at one end) is placed to minimize reflections of the LAN signal [6]. Connectors

that are used with thinnet are BNC connectors. The maximum allowable length

of the thinnet is 185 m, while the minimum separation between two devices is

0.45 m.

Figure 3.5: A typical thinnet 10Base2 installation [6].

10Base5 Ethernet: Original implementation of Ethernet used 50- thick coaxcable like RG8, which is now referred as thicknet. It supports the signalling rate

of 10 Mb/s over maximum length of 500 m. It allows larger length than thinnet

but is more complex to install. It consists of a thicknet backbone cable that

is tapped with a series of transceivers or media attachment unit (MAU). Eachdevice is connected to a single transceiver with a transceiver cable (often referred

to as AUI cable). A typical 10Base5 ethernet installation is shown in Figure 3.6.

Similar to thinnet, two ends of the network should be terminated using 50-

terminators. The connectors used in thicknet are N connectors.

2. Ethernet Twisted pair

The introduction of twisted pair wiring into standard Ethernet networking ushered a

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Figure 3.6: A typical thicknet 10Base5 installation [6].

new age of network connectivity. 10BaseT (and 100BaseT) Ethernet uses unshielded-

twisted-pair (UTP) cables and it supports active star topology, unlike the tapped

bus topology in 10Base2 and 10Base5. In the beginning, wiring in existing telephone

system was the main goal of 10BaseT. However, now it is extensively used in LAN

connections. According to the Ethernet standard, the maximum allowed cable length

is merely less than 100 m (90 m for horizontal cable and 10 m for other cords at each

end) [6]. Data transmitted by a device first goes through the hub, which repeats the

signal to all other connected devices. Thus, if a hub is used, the cable length of each

terminal is independent of the length of rest of the networks.

3. Ethernet Optical Fibre

Recently, Ethernet over optical fiber is widely used, specially for networking in big

buildings. It supports everything from fast Ethernet to gigabit Ethernet. Optical

fiber network can also be relevant in our project because it supports high data-date,

and is less bulky compared to the coaxial cables. Ethernet over optical fiber has been

standardized as explained below.

FOIRL: This was the original standard for ethernet over fiber . In this standard,optical fiber cable is used only as a inter-repeater link. The original FOIRL

specification described a link segment of up to 1,000 meters to be used between

repeaters only [32].

10BaseFL: It is an updated version of FOIRL. 10BASE-FL signaling equipmentis designed to inter-operate with existing FOIRL-based equipment. 10BASE-FL

provides a fiber-optic link segment that may be up to 2000 meters long, provided

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Figure 3.7: A typical 10/100BaseT installation [6].

that the segment only uses 10BASE-FL devices [32].

10BaseFB: This system allows many Ethernet repeaters to be linked in series,exceeding the usual limit on the total number of repeaters that may be used in

a given 10 Mbps Ethernet system. 10BaseFP: The fiber passive (FP) standard provides a set of specifications for

a passive fiber optic mixing segment. It is based on a non-powered device that

acts as a fiber-optic signal coupler, linking multiple devices (e.g. computers) on

a fiber optic media system. According to the standard, 10BASE-FP segments

may be up to 500 meters long; a single 10BASE-FP fiber optic passive signal

coupler may link up to 33 devices [32].

3.3 Summary

Various experiments and study were done for all the topologies. For linear bus network,

measurements were done using thinnet coaxial cable and SMA cable. For star network,

coaxial cable (SMA and BNC) were used as it is easy to install and has a higher bandwidth

compared to the twisted pair cable. However, BNC cable is preferred to SMA cable because

of its wide availability and low cost, and that of its related components. Star networking

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using optical fiber could also be a good candidate for designing the analogue distribution

network. However, the experiment for optical fiber star network was not performed due

to high complexity in design of the required transceiver, and limited time-frame. Detail

description of the experiments, simulations and results are explained in Chapter 7.

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Chapter 4

Coaxial Cables

Coaxial cable, or coax, is an electrical cable with an inner conductor surrounded by a

tubular insulating layer, typically of a flexible material with a high dielectric constant (see

Figure 4.1). The insulating layer is surrounded by a conductive layer (metallic shield),

and finally covered with a thin insulating layer on the outside. The inner conductor and

the outer insulating layer have a common geometrical axis. That is why it is called a

coaxial cable. Coaxial cable is used as a transmission line for radio-frequency signals,

in applications such as connecting radio transmitters and receivers with their antennas,

computer network connections, and in distributing cable television signals. Coaxial cable

confines the electromagnetic field within the space between the inner and outer conductors.

Thus, it protects the signal from electromagnetic interference.

(a) Coaxial cable cutaway (b) RG59 flexible coaxial cable. A: outerplastic sheath B: copper screen C: inner di-electric insulator D: copper core

Figure 4.1: An example of coaxial cable [7]

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CHAPTER 4. COAXIAL CABLES

Similar to an electrical power cord, coaxial cable also conducts AC signals between

locations, but the cable is designed to carry radio-frequency currents (typically few GHz).

Current travels from the source in one of the conductors and returns in the other. Coaxial

lines can be bent and moderately twisted without detrimental impact on its performance.

In radio-frequency applications up to a few gigahertz, the wave propagates primarily in

the transverse electric magnetic (TEM) mode, which means that the electric and magnetic

fields are both perpendicular to the direction of propagation. However, above a certain

cutoff frequency, transverse electric (TE) and/or transverse magnetic (TM) modes can also

propagate, as they do in a waveguide. It is usually undesirable to transmit signals above

the cutoff frequency (typically few GHz), since it may cause multiple modes with different

phase velocities to propagate, interfering with each other. The outer diameter is roughly

inversely proportional to the cutoff frequency.

Coaxial cable come with BNC (BNC cable) connectors as well as SMA connectors (SMA

cable) which is described hereafter.

4.1 Connectors

From the signal point of view, a connector can be viewed as a short, rigid cable. The

connector is designed to have the same impedance as the attached cable in order to avoidthe reflection. Connectors are often plated with high-conductivity metals such as silver or

gold, while some connectors use nickel or tin plating. Silver is used due to its excellent

conductivity. Although silver oxidizes quickly, the silver oxide that is produced is still

conductive. This may pose a cosmetic issue, but it does not degrade performance. Par-

ticularly, in our project we will be using SMA and BNC connectors, which are described

below:

BNC Connector:

The BNC (Bayonet Neill-Concelman) connector is a very common type of RF con-

nector used for terminating coaxial cable. It is used for RF- signal connections, for

analog and serial digital interface video signals, amateur radio antenna connections,

aviation electronics (avionics) and many other types of electronic test equipments.

BNC connector were commonly used in 10base2 thin Ethernet networks, both on

cable interconnections and network cards. BNC connector that are found commer-

cially, feature impedances of 50 and 75 . 75- BNC connectors are primarily used

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for video and DS3 Telco central office applications, whereas 50- BNC connectors

are used for data and RF communication [7]. Figure 4.2(a) shows some of the BNC

connectors.

SMA Connector:SMA (SubMiniature version A) connectors are coaxial RF connectors developed in

1960s as a minimal connector interface for coaxial cable with a screw type coupling

mechanism. The connector has a 50- impedance. It offers excellent electrical per-

formance from DC to 18 GHz [7]. Some examples of SMA connectors are shown in

Figure 4.2(b).

(a) BNC Connector (b) SMA Connector

Figure 4.2: BNC and SMA connectors [7]

4.2 Equivalent Circuit

A transmission line can be considered to consist of a network of very large number of

cascaded T-sections, each of very small length l. The Figure 4.3 depicts one of the

T-sections. The parameters associated with transmission lines as shown in the figure are

resistance per unit length, R [/m], inductance per unit length, L [H/m], conductance per

unit length, G [S/m], and capacitance per unit length, C [F/m]. The value of inductance

and capacitance per unit length can be determined using following equations [ 3].

C =2

ln(Dd ), and (4.1)

L =

2ln

D

d

, (4.2)

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Figure 4.3: Equivalent circuit diagram of Coaxial Cable. The introduced symbols areexplained in the text body [3].

where and are the dielectric constant and magnetic permeability of the insulator,

respectively. They are mathematically defined as,

= 0r, and (4.3)

= 0r (4.4)

where, 0 and r is dielectric constant of free space and relative dielectric constant, re-spectively. Similarly, 0 and r is the permeability of free space and relative permeability

respectively. D is the inside diameter of the shield, and d is the outside diameter of the

inner conductor [7]. The series resistance per unit length R is the resistance of the inner

conductor and the shield at low frequency. At higher frequencies, the skin effect increases

the effective resistance by confining the conduction to a thin layer of each conductor. The

conductance per unit length G is usually very small because insulators with good dielectric

properties are used. At high frequencies, a dielectric can have a significant resistive loss

[7].

The most important characteristic of coaxial cable is the characteristic impedance,

denoted by Z0. It is defined as the ratio of voltage to current and is given by,

Z0 = (

R +jwL

G +jwC. (4.5)

where, w is the angular frequency in radian. Due to cable insulation, G can be neglected

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when compared to wC. Therefore, the equation for characteristic impedance reduces to,

Z0 = (

R

+jwL

jwC. (4.6)

At higher frequencies R can be neglected compared to wL. The equation can thus be

further reduced to

Z0 =

L

C. (4.7)

Eqn. (4.7) implies that the characteristic impedance is independent of the frequency at

higher frequency. This claim is corroborated by help of Figure 4.4. Coaxial cable must

Figure 4.4: Absolute cable impedance as a function of frequency [3].

always be connected to a matched load otherwise the transmitted signal will be reflected

back to the source. Thus, for signal distribution, impedance matching is a must. Commer-

cial coaxial cables have characteristic impedance of 50, 52, 75 and 93 . The RF industry

uses standard type-names for coaxial cables. RG-6 with characteristic impedance of 75

is the most commonly-used coaxial cable for home use, and the majority of connections

outside Europe are by F connectors. However, in our project BNC and SMA cable with

characteristic impedance of 50 were used because of the standards followed and wide

availability of related components. The performance of both of the cables were found to be

similar. However, coaxial cable is favored because of easy availability and low cost of BNC

cables and other components with BNC connector, compared to the SMA (see Chapter 3).

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Chapter 5

System Modeling

One of the objectives of the project is to model the number of LEDs and their placement

on the ceiling such that the VLC area (see Figure 1.3) is fully illuminated, satisfying the

specifications listed in Table 1.1. For proper lighting, a certain brightness of the illuminated

surface is required, and for a reliable high-speed data transmission, sufficient optical power

is needed. Both of these conditions need to be considered in the system design.

Since general lighting can be considered as the primary purpose for an LED source

(with data transmission as its secondary function), we need to ensure sufficient horizontal

brightness at the desktop surface. As mentioned earlier in Section 1.2, we regard 400

lx as a minimal brightness at the desktop height within the VLC area in the OMEGA

demonstration showroom (see Figure 1.3), and aim for 400 lx over the area [15].The Figure 5.1 shows a model room of desk-top height 1.95 m introducing the illu-

minance and other parameters explained hereafter. According to Gfeller and Bapst [2],

illuminance is defined as luminous flux per unit area

E = /A = I()/r2, (5.1)

and depends on the source luminous intensity I() [cd] in the direction [rad]. The

luminous intensity of a generalised Lambertian source with the lambert index mTx, as

assumed in this thesis [13, 33] is given by [2]

I() = I0 cosmTx(). (5.2)

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CHAPTER 5. SYSTEM MODELING

E accounts for the radiation pattern of the light source and the distance to the illuminated

surface, r. The horizontal illuminance is the projection of the illumination E onto the

horizontal plane, viz.

Eh = E cos = I0 cosmTx() cos/r2. (5.3)

where I0 = I(=0) = (mTx + 1)/(2) is the maximum luminous intensity of an LED [2],

is the angle of irradiance, is the angle of incidence, and r is the distance between an

LED and a detectors surface (see Figure 5.1). The order of Lambertian emission mTx is

given by the semi-angle max at half illuminance of an LED 1/2:

mTx = 1/ log2(cos max). (5.4)

Simulations for calculating the horizontal illuminance over the VLC area were done using

Figure 5.1: Model Room. Definition of the shown parameters are introduced in this chap-ter.

the parameters listed in Table 5.1.

In order to investigate the behavior of each configuration of LED placement, some

parameters like standard deviation of illuminance within the VLC area, and percentage of

the VLC area featuring an illuminance above 400 lx, and leakage percentage which is the

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Parameters ValuesLED modulation bandwidth 50 MHz

Maximum allowed delay without ISI 5 ns1

Field of View (half-angle) 30

Half Power angle (half-angle, max) 19

Room height (measured from the floor) 2.75 mDesk height 0.8 m

Room height (measured from the table) 1.95 mMinimum separation of LEDs 0.55 m

1For each submitted data symbol, all signals arriving at the receiver with a delay of more than half of the symbol period

after the first signal, contribute to ISI [15].

Table 5.1: Values used for simulation of lighting levels at desk height, and ISI analysis inChapter 6. (see Figure 5.1 and 1.3.)

percentage of illumination outside the VLC area with respect to the total illumination due

to the LEDs. Figure 5.2(a) shows the VLC area and possible locations where LEDs can be

placed. The ceiling of the VLC demonstrator has square tiles and LEDs have to be placed

at the center of these tiles (see Figure 1.3). Different configurations were simulated and

some of the possible configurations are shown in Figure 5.2(b), 5.2(c) and 5.2(d). Results

of the lighting calculations are shown in Figure 5.3, 5.4 and 5.5 respectively. Table 5.2 lists

the summary of the simulation result and the requirement.

Parameter RequirementConfig 1 Config 2 Config 3Fig:5.2(b) Fig:5.2(c) Fig:5.2(d)

Illuminance above 400 lx (in %) maximum 96% 98% 98%No. of LEDs minimum 17 18 18

Std. deviation of E within the VLC area minimum 12% 9% 11%Leakage (in %) minimum 10% 10% 10%

Table 5.2: Summary of the requirement and result for the simulation of lighting levels atdesk height.

Comparing the result of various configurations with the requirement of the project (see

Table 5.2), it can be seen that the first configuration is the best with high illumination

within the area, lowest standard deviation and minimum number of LEDs satisfying the

lighting requirement. Configuration I is chosen for final implementation of the OMEGA

project because it uses only 17 number of LEDs ie. one less than other configurations and

it has competitive values for other parameters.

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0 1 2 3 4 5 60

0.5

1

1.5

2

2.5

3

3.5

4

VLC area

Room dimension in xdirection [m]

Roomd

imensioniny

direction[m]

VLC area

Possile placement of LEDs on the ceiling of the showroom: Grid

Grid

(a) Possible positions for LED placement onthe ceiling of the showroom

0 1 2 3 4 5 60

0.5

1

1.5

2

2.5

3

3.5

4


Roomd

imensioniny

direction[m]

LED Placement

Grid

LED

(b) Configuration I

0 1 2 3 4 5 60

0.5

1

1.5

2

2.5

3

3.5

4

Room dimension in xdirection

Roomd

imensioniny

direction

LED Placement

Grid

LED

(c) Configuration II

0 1 2 3 4 5 60

0.5

1

1.5

2

2.5

3

3.5

4

Room dimension in xdirection

Roomd

imensioniny

direction

LED Placement

Grid

LED

(d) Configuration III

Figure 5.2: LED placement in the OMEGA demonstration showroom.

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100

100

100

200

200

20

020

0

200

300

300

300

300

300

300

400

400

400

400

400

500

500

500

500

500

500

500

600

600

400

E

=12.8 %Leakage =10.7 % Mean=514 lxAbove 400 lx = 96.6 %

position of Rx in x direction [m]

pos

itionofRxinydirection[m]

1 2 3 4

0.5

1

1.5

2

2.5

3

3.5Illuminance

Grid

LED

Figure 5.3: Illumination for configuration I.

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100

100

100

200

200

200

200

200

200

300

300

300

300

300

300

400 4

00

400

400

400

500

500

500

500

500

500 6

00 6

00

600

600

600

600

600

E

=11.2 %Leakage =10.2 % Mean=565 lxAbove 400 lx = 98.2 %

position of Rx in x direction [m]

po

sitionofRxinydirection[m]

1 2 3 4

0.5

1

1.5

2

2.5

3

3.5Illuminance

Grid

LED

Figure 5.5: Illumination for configuration III.

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Chapter 6

Inter-symbol Interference

6.1 Theory

Intersymbol interference (ISI) is a form of distortion of a signal in which one symbol

transmitted at certain time overlaps with the subsequent symbols. This is an unwanted

phenomenon as the previous delayed signal when combined with the present signal, act

as interference, thus making the communication less reliable and limiting the transmission

speed. ISI stems from multi-path propagation of the emitted signals (through cables aswell as air). Thus, in any communication, one of the important tasks is to limit ISI. In

VLC, the amount of ISI depends on the chosen transmission scenario (like room properties,

distribution of emitters at the ceiling) and the transmitter itself (directionality of emission,

transmitted power, modulation bandwidth). For ISI analysis, it is assumed that for each

submitted symbol all signals arriving at the receiver with a delay of more than half of the

symbol period after the first signal, contribute to ISI [15]. Therefore, the received optical

signal power is,

PR,sig =i

PR (ti Ts/2) (6.1)

PR,ISI =i

PR (ti > Ts/2) (6.2)

Given that the bandwidth of the transmitted signal is limited to 50 MHz (see Table 1.1 in

page 5), the nyquist symbol period is limited to 10 ns, and ISI will occur if transmitted

data symbols experience delays larger than 5 ns. Important properties to be considered in

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CHAPTER 6. INTER-SYMBOL INTERFERENCE

the ISI analysis are the half-power (HP) angle of the transmitter, and field-of-view (FOV)

for the receiver. The half power angle is the angle between the points where radiance

has decreased to half of the maximum values and FOV is the angle between the points

from where the receiver is receiving half of the maximum signal received. Mathematical

definitions of these parameters are given below.

FOV = tan1r1

h

(6.3)

HP = tan1r2

h

(6.4)

where r1, r2 and h are exemplified in Figure 6.1. The optical path differences between the

Figure 6.1: An optical communication scenario elucidating FOV and HP (see Eqs (6.3)and (6.4)) [8].

signal can cause delay spread. These path differences are also due to line-of sight (LoS)

signals from different sources arriving at the same receiver. If more than one transmitters

fall within the FOV of the receiver, the receiver will capture strong signals from different

transmitters with certain time delays. For the scenario shown in Figure 6.2, the delay

between the signals is given by Eq. (6.5). In case the signals depart at the same time from

all the transmitters.

tair =(w x)

2 + h2

x2 + h2

c , (6.5)

where c is the vacuum speed of light and w, x, h are defined in Figure 6.2. If all the

transmitters are driven in lock step, which is the case in our scenario, a difference in

cabling length from a common distribution point to each of the LEDs can also lead to ISI.

Therefore, the total delay between the signals arriving from different transmitters is the

sum of delay due to electrical and optical path difference. This total delay must be less

than 5 ns in order to avoid ISI. The time delay due to electrical path difference in the

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Figure 6.2: Calculation of optical path difference [8].

one-dimensional case of Figure 6.2 is given by,

tcable =lcable

Vg=

w

0.66c. (6.6)

where Vg = 0.66c is the group velocity in [34]. And the total delay can be calculated as,

ttotal = tair + tcable. (6.7)

For a source with Lambertian radiation characteristic, the optical power PR,opt of signal

coming from each LEDs, and incident on the photosensitive area A can be calculated with

the help of Figure 6.3 and eqn. (6.8).

dPR,opt =1

r2I() T() dA

=1

r2I() T() cos dARx

=I0r2

mTx + 1

2cosmTx()

mRx + 1

2cosmRx() cos dARx

=I0

r2

(mTx + 1)(mRx + 1)

42

cosmTx+mRx+1() dARx , (6.8)

Here, we assume that the solid angle (d) under which the transmitter appears at the

receiver is much smaller than one [33]. This is because ARx which is the photosensitive

area of the receiver, is very less compared to r2 (ARx


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Figure 6.3: Optical power received from the transmitter. Definition of the shown parame-ters are explained in Chapter 5 and 6.

by Eq. (5.4) in page 33 and,

mRx = 1/ log2(cos FOV), (6.9)

The photodiode (receiver) converts optical power into a current that is proportional to

the overall received power. Therefore, the following applies:

iR (PR,opt) (6.10)

Furthermore, the electrical power can be calculated with the help of Eq. (6.1),

PR,sig =

i

iR (ti 5 ns)2

, (6.11)

PR,ISI =i

i2R (ti > 5 ns) . (6.12)

Now, in order to calculate the difference in time delay (ti), the first reference signal needs

to be determined. The first signal arriving to the receiver might have negligible power.

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Thus in our simulation, the first signal received by the receiver with power greater than

or equal to the 5 % of the maximum power received, is considered as the reference signal.

Lets consider a scenario when LED (lets say LED I), which is very near to the receiver is

connected to the signal source by the aid of very long cable and the LED (lets say LED

II), which is far from the receiver is connected by aid of smallest possible length of the

cable. In this case the signal from the LED II will arrive at the receiver earlier but with

negligible signal power. If the the signal from the LED I (nearest LED) arrives 5 ns later

than that from the LED II then LED I is the candidate for ISI. Nevertheless, the power

of the signal received from LED I which is the candidate for ISI, is very high compared to

the LED 2. In order to avoid such conditions, defining the threshold for the signal power

received was necessary to estimate the signal-to- interference ratio (SIR). Figure 6.4 shows

an example of the received signals and their time delay. Thus, the signal to interference

Figure 6.4: Time delay of the signal arriving to the receiver and power received. tref is thedelay of the reference signal and ti is the time delay of the ith signal. Here, Pmax is the

maximum power received by the photodetector.

ratio, SIR, can be inferred by calculating,

SIR =PR,sigPR,ISI

. (6.13)

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According to the paper by Grubor et al. [29], the optical signal-to-noise ratio (SNR) were

greater than 20 dB (SNR >> 20 dB). This implies the target electrical SNR to be 40 dB,

as electrical current is proportional to the optical power. However, we are calculating SIR

assuming zero noise power. The signal to noise and interference ratio (SNIR) is the ratio

of signal power to the sum of noise power and the interference power which can be inferred

as,

SNIR =PR,sig

Pnoise + PR,ISI. (6.14)

In our simulation we have considered Pnoise = 0. Addition of noise power will even decrease

the SNIR. Thus, the target SIR is considered to be 40 dB. The configuration for which the

SIR is greater than 40 dB is only favorable for the final implementation.

6.2 Simulation of ISI

Simulations were carried out in order to assess the effect of ISI in our system, for various

placements of LEDs. Value of parameters used in the simulation are listed in Table 5.1 on

page 34.

Since these simulations are somewhat involved, especially when it comes to interpreting

their outcome, we discuss one case in detail.Let us consider 16 LEDs that are placed on the ceiling of the room for illumination and

they are connected through a linear bus network. For simplicity, it is assumed that the

receiver is placed directly under the LED 1 (see Figure 6.5). Depending upon the FOV of

the receiver, it can receive appreciable signals from one or more transmitters. Figure 6.5

shows one of the possible linear bus configurations for the setup using 16 LEDs. However,

the number of LEDs and separation between them depends upon the LED placement

design explained in Chapter 5. Let us assume, LED 2 and LED 8 fall within the field of

view of the receiver (see Figure 6.5). Thus, the delay of the signal coming from LED 2 and

LED 8 should be less than or equal to 5 ns, in order to avoid ISI. In this way, calculating

the power received by the receiver from LEDs within its field of view, the SIR can be

calculated. Such analysis of ISI was conducted for linear bus and star networks, and the

results are presented hereafter.

In our simulation, the contribution from all the LEDs outside the FOV of the receiver

is also calculated, however their contribution to PR,ISI is rather negligible.

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Figure 6.5: An example of linear bus setup with 16 LEDs. Dotted circle shows the regionwithin which the receiver can receive signal, when placed directly under the first LED.

6.2.1 ISI in Linear-Bus Network

0 1 2 3 4 5 60

0.5

1

1.5

2

2.5

3

3.5

4

VLC area


Roomd

imen

sioniny

direction[m]

Possile placement of LEDs on the ceiling of the showroom: Grid

Grid

(a) Possible positions for LED placement onthe ceiling of the showroom

0 1 2 3 4 5 60

0.5

1

1.5

2

2.5

3

3.5

4


Roomd

imen

sioniny

direction[m]

VLC area

LED Placement

Grid

LED

(b) One of the possible configurations for LEDplacement

Figure 6.6: LED placement in the OMEGA demonstration showroom

Figure 6.6(a) shows the VLC area and possible locations where LEDs can be placed.

LEDs should be placed in such a way that the room is homogenously illuminated with

minimum leakage (see Chapter 5). For an example in the simulation, LEDs are placed on

the ceiling as shown in Figure 6.6(b). Moreover, these arrays of LEDs can be connected in

various ways.

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Configuration I:In this configuration, the LEDs are connected as shown in Figure 6.7. Depending

upon the position of the receiver, three cases are simulated and results are presented

here. Initially 16 LEDs were proposed and this configuration was chosen. However,

it was found that this configuration is the worst possible cabling in SIR point of

view. Figure 6.7 shows that when receiver is placed under first LED, the ti for the

signal from all the LEDs except the neighboring LED, is greater than 5 ns [see Eq.

(6.1)]. Thus, all other LEDs contribute to ISI. The signal to interference ratio (SIR)

is calculated to be 7 dB. Figure 6.9 shows the values of SIR when the receiver isplaced at the grid points. It can be seen that for some positions of the receiver the SIR

is even negative (see Figure 6.8). This is because of the worst cabling configuration.

The signal from the nearest LED is arriving to the receiver with long delay thus,

increasing the ISI power.

This configuration was indeed found to be a worst case compared to the other con-

figurations presented in this chapter. This is because the path difference due to the

difference in cable length between two neighboring LEDs is large enough to qualify

them as ISI. Since the signal received from neighboring LEDs is appreciable (see Fig-

ure 6.7), this results in a low SIR [see Eq. (6.13)]. Since, SIR for this configuration

is very less compared to the target SIR i.e. 40 dB, this configuration is not favorable

for implementation (see Figure 6.9).

Configuration II:LEDs can also be connected as shown in Figure 6.10. In this case, SIR is calculated

to be 7.3 dB and 5.6 dB for the receiver positions shown in Figure 6.10 andFigure 6.11, respectively. SIR for case II (see Figure 6.11), is less than for case I.

This is because in case II the receiver is near to more LEDs that are contributing

to ISI, thus increasing PR,ISI. SIR of the signal for this cabling configuration, for

different positions of the receiver, is shown in Figure 6.12.

SIR for this configuration is slightly higher than compared to the configuration I. It

can be observed comparing the values of SIR for different positions of the receiver

in Figure 6.9 and 6.12 respectively. This is because in configuration I the cabling

separation between neighboring LEDs are longer than in case of configuration II.

Thus, neighboring LEDs become candidates for ISI, leading to high PR,ISI and low

SIR. However, this configuration is also not favorable, as SIR is very less than 40 dB

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0.5 1 1.5 2 2.50

0.5

1

1.5

2

2.5

3

3.5

0

1.91e008

2.23e008

4.25e008

4.62e008

6.7e008

3.03e009

2.53e008

6.43e008

6.45e009

6.2e008

1.02e008

1.28e008

3.26e008

3.59e008

5.62e008

6e008

ti [s]

0.15

0.269

0.15

0.000413

2.6e010

1.46e017

0.0376

0.0376

2.01e018

2.61e005

8.53e021

1.73e011

2.6e010

1.73e011

9.82e015

3.01e019

3.58e024

PR

[W]

SNR (in dB)=1.32 Cabling Type =Linearbus network, configuration I


Roomd

imensioniny

direction[m]

VLC area

Grid

LED

Rx Position

Ref LED

Figure 6.8: Configuration I, Case II: SIR for a bus network. (When receiver is placeddirectly under the neighboring LED). Same cabling configuration and signal propagationas in Figure 6.7. All the quantities in this figure are introduced in Figure 6.7. Here, tifor all LEDs except the neighboring LED is > 5 ns. Thus, SIR is lower.

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0.5 1 1.5 2 2.5

0.5

1

1.5

2

2.5

3

3.5

7.349

1.322

4.351

0.6501

6.391

4.033

6.382

0.4705

0.3985

2.6

1.735

11.58

6.593

1.15

9.321

7.565

9.773

0.9492

7.554

0.8727

3.929

7.398

4.402

7.349

SNR (in dB)

Cabling Type = Linearbus network, configuration I

Rx position in xdirection [m]

Rxpositioniny

direction[m]

SNR vs position of Rx

Grid

LED

Rx Position

Power Splitter

Figure 6.9: Configuration I, Case III: SIR for linear bus network. (When receiver is placedat the indicated grid positions). Same cabling configuration and signal propagation in thecable as in Figure 6.7. Here, SIR at every positions of the receiver is very much less than40 dB.

49


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0.5 1 1.5 2 2.50

0.5

1

1.5

2

2.5

3

3.5

0

3.03e009

6.5e009

1.03e008

1.44e008

1.86e008

4.2e008

4.84e008

2.16e008

3.99e008

2.47e008

3.81e008

3.55e008

3.34e008

3.14e008

2.97e008

2.82e008

ti

[s]

0.269

0.15

0.000413

2.6e010

1.46e017

3.58e024

0.15

2.61e005

8.87e025

0.000413

1.77e026

2.6e010

1.73e011

9.82e015

3.01e019

3.58e024

5.47e029

PR

[W]

SNR (in dB)=7.35 Cabling Type =Linearbus network, configuration II


Roomd

imensioniny

direction[m]

VLC area

Grid

LED

Rx Position

Ref LED

Figure 6.10: Configuration II, Case I: SIR for linear bus network. (When receiver is placeddirectly under the first LED). Dotted arrows show the cabling configuration and the signalpropagation in the cable. All the quantities in this figure are introduced in Figure 6.7.Here, ti for all LEDs except the one above the receiver is > 5 ns. Thus, SIR is lower.

50


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0.5 1 1.5 2 2.50

0.5

1

1.5

2

2.5

3

3.5

0

2.53e009

5.55e009

9.05e009

1.28e008

1.7e008

4.2e008

4.75e008

2e008

3.99e008

2.32e008

3.81e008

3.51e008

3.26e008

3.04e008

2.84e008

2.67e008

ti

[s]

0.15

0.269

0.15

0.000413

2.6e010

1.46e017

0.0376

0.0376

2.01e018

2.61e005

8.53e021

1.73e011

2.6e010

1.73e011

9.82e015

3.01e019

3.58e024

PR

[W]

SNR (in dB)=5.6 Cabling Type =Linearbus network, configuration II


Roomd

imensioniny

direction[m]

VLC area

Grid

LED

Rx Position

Ref LED

Figure 6.11: Configuration II, Case II: SIR for linear bus network. (When receiver is placeddirectly under the neighboring LED). Same cabling configuration and signal propagationas in Figure 6.10. Here, ti for all LEDs except the one above the receiver is > 5 ns. Thus,SIR is lower.

51


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0.5 1 1.5 2 2.5

0.5

1

1.5

2

2.5

3

3.5

7.349

5.6

4.549

6.585

6.594

7.554

0.9534

0.4705

0.475

2.6

1.811

0.8727

6.391

1.073

1.681

7.795

1.819

6.642

4.033

11.58

7.542

7.553

11.58

4.032

SNR (in dB)

Cabling Type = Linearbus network, configuration II

Rx position in xdirection [m]

Rxpositioniny

direction[m]

SNR vs position of Rx

Grid

LED

Rx Position

Power Splitter

Figure 6.12: Configuration II, Case III: SIR for linear bus network. (When receiver isplaced at the grid position). Same cabling configuration and signal propagation as inFigure 6.10. In this configuration also SIR for every positions of the reciever is very muchless than 40 dB.

52


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6.2.2 ISI in Star Network

Figure 6.13: Star networking of 16 LEDs.

Figure 6.13 shows an example of cabling in a star fashion. In our case, power splitter

is used as a central hub and all LEDs are connected to the power splitter with the aid of

coaxial cable of equal, or unequal lengths as discussed below.

ISI in Star Network with unequal lengths of cableSimulations were done for different positions of the receiver, when all LEDs are

connected to the power splitter with different lengths of the coaxial cable. Lets

consider an example as shown in Figure 6.14, power-splitter is placed near the first

LED. All the LEDs are connected to the output ports of the splitter through coaxial

cables. The length of the cables matches that of the distance between power-splitter

and the LEDs. Results of the simulations are shown in Figure 6.14 and 6.15, for

different positions of the receiver.

The simulation was also done by varying the position of the power splitter. It can

be seen in the Figure 6.16 that SIR can be improved if the splitter is placed at the

center of the VLC area. This is because the cabling distance between most of the

LEDs and the splitter is less in comparison to the earlier case (compare Figure 6.15

and 6.16).

In star network, SIR is higher than in a linear bus networks because the cabling

length for every LED is less than that in the case of linear bus network. Thus, for

the particular position of the receiver, the difference in delay of the signal ti is

smaller thus decreasing the number of signals contributing to ISI. This configuration

can be implemented for final demonstration when power splitter is placed at the

53


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center of the VLC area, as the SIR is calculated to be higher than 40 dB for most of

the positions of the receiver (see Figure 6.16).

0.5 1 1.5 2 2.50

0.5

1

1.5

2

2.5

3

3.5

0

3.03e009

6.5e009

1.03e008

1.44e008

1.86e008

3.03e009

7.4e009

1.91e008

6.5e009

2.03e008

1.03e008

1.1e008

1.28e008

1.54e008

1.86e008

2.22e008

ti[s]

0.269

0.15

0.000413

2.6e010

1.46e017

3.58e024

0.15

2.61e005

8.87e025

0.000413

1.77e026

2.6e010

1.73e011

9.82e015

3.01e019

3.58e024

5.47e029

PR

[W]

SNR (in dB)=32.9 Cabling Type =Star Network with unequal lengths of cable


Roomd

imensioniny

direction[m]

VLC area

Grid

LED

ash rest ha

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