CSNDSP 20081

Orthogonal Frequency Division Multiplexing for Indoor Optical Wireless

Communications using Visible Light LEDs

S. K. Hashemi1, Z. Ghassemlooy1, L. Chao2, and D. Benhaddou3

1Optical Communications Research Group, NCRLab,

Northumbria University, Newcastle upon Tyne, UK

Web site: http://soe.unn.ac.uk/ocr

2Department of Electronic and Information Engineering,

Hong Kong Polytechnic University3Engineering Technology Department, College of Technology,

University of Houston, USA

Contents

• Introduction• Optical Wireless Communication (OWC) Links• Combating Inter-symbol Interference • OFDM• Channel Estimation• Simulation results• Conclusions

CSNDSP 20082

CSNDSP 20083

Introduction

• Invented more than 40 years ago.• Has been adopted for several technologies:

– Asymmetric Digital Subscriber Line (ADSL) services.– IEEE 802.11a/g, IEEE 802.16a.– Digital Audio Broadcast (DAB).– Digital Terrestrial Television Broadcast: DVD in

Europe, ISDB in Japan– 4G, IEEE 802.11n, IEEE 802.16, and IEEE 802.20.

CSNDSP 2008

OWC Links

Non-LOS

Multipath Propagation Intersymbol interference (ISI) Difficult to achieve high data date due to

ISI

Non-LOS

Multipath Propagation Intersymbol interference (ISI) Difficult to achieve high data date due to

ISI

RxRxTxTx

LOSLOS

No multipath PropagationOnly noise is limiting factorPossibility of blockingTracking necessary to maintain LOS link

TxTx

RxRx

CSNDSP 20085

Techniques to Overcome ISI

• Guard slots - in digital modulation (PPM, PIM) • Spread spectrum - at the expense of reduced bandwidth

efficiency• Angle diversity - based on multibeam-narrow FOV

transceiver • Adaptive decision equalizer • USE OF multi-carriers transmission

– In this paper we investigate OFDM system where multipath induced ISI and the effects of background noise are reduced by taking the advantage of narrowband frequency interference, which affects only one of the frequency sub-bands when compare to the single-carrier modulation.

CSNDSP 20086

OFDM

• OFDM mainly used in RF based wireless communication schemes offering

– high data rates capability – high bandwidth efficiency – and capable of dealing with the multipath induced ISI [9]

• Has been proposed in optical wireless communication systems [10]

• Uses a large number of closely-spaced orthogonal sub-carriers, each modulated at a low symbol rate

CSNDSP 20087

OFDM - Types

Wideband-OFDM(W-OFDM) of Wi-LAN

www.wi-lan.com

Flash OFDMfrom Flarion

www.flarion.com

Vector OFDM(V-OFDM) of Cisco, Iospan,etc.

www.iospan.com

-- 2.4 GHz band-- 30-45Mbps in 40MHz-- large tone-width (for mobility, overlay)

-- Freq. Hopping for CCI reduction, reuse-- 1.25 to 5.0MHz BW -- mobility support

-- MIMO Technology-- non-LoS coverage, mainly for fixed access-- upto 20 Mbps in MMDS

•Wi-LAN leads the OFDM Forum -- many proposals submitted to IEEE 802.16 Wireless MAN•Cisco leads the Broadband Wireless Internet Forum (BWIF)

CSNDSP 20088

OFDM Signal

Ch.1

Ch.2 Ch.3 Ch.4 Ch.5 Ch.6 Ch.7 Ch.8 Ch.9 Ch.10

Ch.3 Ch.5 Ch.7 Ch.9Ch.2 Ch.4 Ch.6 Ch.8 Ch.10

Ch.1

Conventional multicarrier techniques

Orthogonal multicarrier techniques

50% bandwidth saving

Frequency

Frequency

CSNDSP 20089

OFDM - Spectrum

• The FFT (and its inverse, the IFFT) are used to create a multitude of orthogonal subcarriers using just a single TX.

• Subcarrier orthogonality must be preserved in the presence of– timing jitter– frequency offset– fading.

Frequency

Magnitude

T0

CSNDSP 200810

OFDM System Architecture

Pilot signals uniformly inserted into all symbols (sub-carriers) for channel estimationfor diffuse optical links only.

)}1(,),1(),0({ ppppP NXXX

CSNDSP 200811

OFDM

• IFFT converts X(k) of length N into a complex time-domain OFDM signal.

• In order for the IFFT/FFT to create an ISI-free channel, the channel must appear to provide a circular convolution.

• To mitigate the effects of multipath induced ISI, a guard interval of G-sample (or cyclic prefix (CP)) is inserted between symbols.

• The length of CP depends on the channel delay spread and is normally considered to be grater than or equal to the channel length (impulse response time) and less than symbol duration

))}(({)( nkXIFFTnx N

CSNDSP 200812

Circular Convolution & DFT/IDFT

• Circular convolution:

• Detection of X (knowing H):

(note: ISI free! Just a scaling by H)

• Circular convolution allows DFT!

CSNDSP 200813

OFDM - Cyclic Prefix (CP)

• OFDM signal with CP is x[n]L, and so y[n] = x[n] * h[n].

First v samples of ycp interference from preceding OFDM symbol => discarded. The last v samples disperse into the subsequent OFDM symbol => discarded. This leaves exactly L samples at output y, to recover the L data symbols embedded in x.

CSNDSP 200814

OFDM

• The received OFDM signal propagated through the channel h(n) is given by:

• where w[n] is the additive white Gaussian noise, is the photodetector responsivity, and denotes the circular convolution.

Channel models:• Recursive algorithm to take into account higher order reflections [11], • MIMO method for speedy calculation of the impulse response [12]. • Mont Carlo ray-tracing algorithm and its modified version uses both the

Lambert’s diffuse and the Phong’s models to approximate surfaces with a strong specular component [13-14].

• The Sphere model in [15] • Fast iterative model capable of calculating high-order reflections [17].

][][][.][ nwnhnsnyr

CSNDSP 200815

OFDM - Channel Estimation

• With no knowledge of the channel, channel estimation is required for equalization and decoding provided

• In one dimensional channel estimation schemes pilot insertion is done in

Block-type - all sub-carriers is used as pilot in a specific period. It uses the least square (LS) or the minimum mean-square error in a slow fading channel [18])

Comb-type - part of the sub-carriers are always reserved as pilot for each symbol, and it uses LS with interpolation and the maximum likelihood in a rapidly changing channel [19]) - Adopted

Tim

eCarriers

Tim

e

Carriers

Block type

Come type

CSNDSP 200816

OFDM – Interpolation Schemes

• Pilot symbols uniformly inserted into symbols according to

• Information on the exact locations of the pilot and data symbols as well as the values of the pilot symbols are already available at the receiver.

• • The estimated channel at pilot subcarriers for the LS estimation (no need of prior

knowledge of noise variance) is given as:•

• Interpolation schemes– linear interpolation, – second-order interpolation, – spline cubic interpolation – time domain interpolation which are used in the one dimensional model to extract channel

information on the data sub-carriers Hd [18]. In [21, 23] detailed information on the number of pilots used and how to located them as optimal pilot design techniques are given.

1,,1,0)(

)()(ˆ p

p

pp Nm

mX

mYmH

})),/((),1(),1)/(()0(),0({ pNdNFloordXpXpNdNFloordXdXpX

CSNDSP 200817

-60 -40 -20 0 20 40 60-50

-40

-30

-20

-10

0

10

f [MHz]

pow

er s

pect

rum

mag

nitu

de [

dB] OFDM spectrum for N

FFT = 128, N

w in = 12, N

guard = 24, oversampling = 1

0 20 40 60 80 100 120 140 160 180 200-0.2

-0.1

0

0.1

0.2time domain signal (baseband)

sample nr.

imaginaryreal

OFDM - Simulation

OFDM modulation

(IFFT)

Channel coding /

interleaving

Guard interval

I/Q I/QSymbol mapping

(modulation)

Transmitter

N symbols

OFDM demod. (FFT)

Decoding / deinter-leaving

Guard interval removal

Time sync.

I/Q I/Q

symbol de-mapping

(detection)

Channel est.

ReceiverFFT-part

time

1 OFDM symbol

Channel impulse response:

0101010010110

CSNDSP 200818

Parameters Values

No. of Les 4Transmitted optical power Ps 20.0 mWCentre luminance intensity of LED 730 mcdField of view (FOV) 60o Photodetector surface area Ar 1 cm2

Lambert mode 0.6461Concentrator refractive index nr 1.5Photodetector responsivity 0.53 (A/W)FFT size 256Number of data subcarriers 128Number of pilots 16, 32, 64Cyclic prefix (CP) ratio 1/4The radiation pattern is assumed to be Lambertian

Simulation Parameters

01

23

45

0

1

2

3

4

50

1

2

3

Hei

ght[

m]

Length[m]Width[m]

2.5m

(3.5,3.5)m

(1,1)m

(3.5,1)m

(1,3.5)m

LEDs interval: 1cm

CSNDSP 200819

Horizontal Illuminance Distribution

01

23

45

0

1

2

3

4

5200

400

600

800

1000

1200

1400

x[m]

Distribution of horizantal illuminance [lx]

y[m]

Illum

inan

ce[lx

]

Number of LEDs60 x 60 (4 set)

CSNDSP 200820

Results - Error Performance for LOS optical wireless OFDM link employing BPSK, QPSK and 16-QAM

0 5 10 15 202010

-4

10-3

10-2

10-1

100

Eb/N

o(dB)

SE

ROFDM over AWGN Channel

BPSK SimulationBPSK TheoryQPSK SimulationQPSK Theory16-QAM Simulation16-QAM Theory

CSNDSP 200821

Results – Error performance for diffuse OW OFDM link with QPSK & LS channel estimation for a number of pilot symbols

0 5 10 15 20 2510

-3

10-2

10-1

100

Eb/N

0(dB)

SE

RNpilot=16

Npilot=32Npilot=64

Conclusions

• Visible light LED’s was proposed for both lightening and communication system

• OFDM technique employed to combat ISI due to the multipath reflections

• Due to the variation of the optical channel impulse response in OW links, pilot signals added and the channel is estimated.

• We simulated the illumination to meet the ISO for lighting and found the number of LEDs for the simulation parameters.

• Performance of the system for both LOS and Diffuse link for BPSK,QPSK and M-QAM was simulated.

CSNDSP 200822

CSNDSP 200823

Thank you!

CSNDSP 200824

Inter-Symbol-Interference (ISI) due to Multi-Path Reflections

Transmitted signal:

Received Signals:Line-of-sight:

Reflected:

The symbols add up on the channel

Distortion!Delays

CSNDSP 200825

OFDM Symbols

Group L data symbols into a block known as an OFDM symbol. An OFDM symbol lasts for a duration of T seconds, where T = LTs. Guard period > delay spread OFDM transmissions allow ISI within an OFDM symbol, but by including a

sufficiently large guard band, it is possible to guarantee that there is no interference between subsequent OFDM symbols.

The next task is to attempt to remove the ISI within each OFDM symbol

CSNDSP 200826

Received LOS Power

01

23

45

0

1

2

3

4

5-6

-4

-2

0

2

x[m]

Distribution of Received Power [dBm]

y[m]

Rec

eive

d P

ower

[dB

m]

Maximum_power = 1.2270 [dBm]

Minimum_power = -5.2891 [dBm]

CSNDSP 200827

OFDM Systems

System FFT Size

NumberCarriers

ChannelSpacing

kHz

BandwidthMHz

SampleRateMHz

SymbolDuration

sec

Data

Rate

Mbits/s

HyperLAN/2 64 524

312.5 16.25 20 3.20.8

6-54

802.11a 64 524

312.5 16.56 20 3.2 0.8

6-54

DVB-T 20481024

1712842

4.464 7.643 9.174 224 0.68-14.92

DAB 20488192

1536 1.00 1.536 2.048 24/48/96msec

3.072

ADSL 256 (down)64 (up)

36-1277-28

4.3125 1.104 1.104 231.9 0.64-8.192