CONVOLUTIONAL CODED DPIM FOR INDOOR NON-DIFFUSE OPTICAL WIRELESS LINK

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IASTED- WOC- Canada 07

CONVOLUTIONAL CODED DPIM FOR INDOOR NON-DIFFUSE OPTICAL

WIRELESS LINK

S. Rajbhandari, Z. Ghassemlooy, N. M. Adibbiat, M. Amiri and W. O. Popoola

Optical Communications Research Group,Northumbria University,

Newcastle, UK

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Contents

Introduction to optical wireless Modulation schemes Digital PIM Coded DPIM Results + comments

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Optical Wireless Communication - Introduction

Uses light (visible or Infrared (IR )) as a carrier. The medium is free-space (outdoor and Indoor) Line-of-sight (LOS) or diffuse or hybrid License free with abundance bandwidth, and high data

rate No multipath fading but Protocol transparent High security

Free from electromagnetic interference Compatible with optical fibre (last mile bottleneck?) Low cost of deployment

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OWC - Challenges

Power limitation: due to eye and skin safety Intersymbol interference due to multipath

propagations Intense ambient light noise Limited user mobility Large area photo-detectors - limits the data

rate

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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 Propagation Only noise is limiting factor Possibility of blocking Tracking necessary to maintain

LOS link

TxTx

RxRx

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Digital Modulation Schemes

On-off Keying (OOK) Pulse position modulation (PPM) Subcarrier modulation Digital pulse interval modulation (DPIM) Dual-header pulse interval modulation

(DH-PIM)

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Digital Modulation Schemes

Information

Frame 4

1 1 1

Frame 3

1 1 0

Frame 2

0 1 0

Frame 1

0 0 0

DPIM

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Digital Pulse Interval Modulation

Variable symbol length

Where Tb is input bit rate and Ts is DPIM slot duration A symbols starts with pulse followed by k empty

slots. 1≤ k≤ L and L = 2M

Guard slot(s): Added after the pulse to provides immunity to ISI arising from multipath propagation. With g guard slots the minimum and maximum symbol

durations are * gTs and (L+g)Ts

s

bavg T

LogLTL

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DPIM- What does it offer?

Bandwidth efficient compared to PPM.

Built-in slot and symbols synchronisation.

Higher through put compared to PPM.

Better performance in diffused environment compared with PPM

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DPIM - Convolutional Coding

Has not been done before

Linear block codes like Hamming code, Turbo code and Trellis coding are difficult (if not impossible ) to apply in PIM because of variable symbol length.

Hence, Convolutional code is employed- since the acts on the serial input data rather

than the block.

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• (3,1,2) convolutional encoder .• ½ code rate and constraint length = 3•Generator function g0 = [100], g1 = [111] and g2 = [101]

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2 empty slots for all the symbols to ensure that memory is cleared after each symbol.

Trellis path is limited to 2.

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DPIM - Decoder

Viterbi ‘Hard ‘ decision decoding The Chernoff upper bond on the error

probability is:

where Pse is the slot error probability of uncoded DPIM.

)1(4,1

),(

sese ppDII

IDTPb

It is also possible not use Viterbi algorithm instead one can use a simple look-up table.

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DPIM - Block Diagram

Convolutional Encoder

Optical Tx Photodetector

SamplerViterbi

DecoderMatched

Filter

DPIM Input IIkk

DPIMestimate

AWGN R

kI

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Results – Slot Error Rates Upper Bounds

• Difficult to ascertain exact hamming distance•Union bound is utilised to evaluate the performance.•A close match at upper bound, less than 0.5 dB gap•The DPIM(2GS) gives the best performance

2

1

2

1

00)(,,

NQ

L

L

N

EQ

LP

avg

avg

avgNGBDPIMslote

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Results – Slot Error Rates With/Without Guard Slots

• Code gain of 4.8 dB at Pse of 10-4 for all cases.

• Increasing number of guard slot improves the performance at the cost of bandwidth.

• 0.5 dB improvement in SNR requirement for each increment in number of Guard slot for M=4

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Results - Slot Error Rates With/Without Guard Slots

• Higher bit resolution provides better performance ( at the expense of bandwidth)

• The code gain is 0.6 higher for bit resolution of 5 compared to 3.

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Packet Error Rates

PER against the electrical SNR for coded and un-coded 8,16,32 – DPIM(1GS) at 100 Mbps.

-2 -1 0 1 2 3 4 5 6 7 8Electrical SNR (dB)

Pro

babi

lity

of P

ack e

t err

or,

PE

R8,16,32-DPIM with one guard band @ R=100Mbps

Uncoded8-DPIM

Coded UpperBound 8-DPIMUncoded 32-DPIM

Coded UpperBound 32-DPIM

Uncoded16-DPIMCoded UpperBound 16-DPIM

10-10

10-8

10-6

10-4

10-12

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Final Comments

Applying Convolutional coding has resulted in improved PER performance for DPIM scheme.

Higher SNR can be achieved at the cost of lower throughput.

Inclusion of one guard slot marginally reduces the probability of an error.

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Thank You!

CONVOLUTIONAL CODED DPIM FOR INDOOR NON-DIFFUSE OPTICAL WIRELESS LINK

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