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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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IASTED- WOC- Canada 07
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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DPIM - Convolutional Coding
• (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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DPIM - Convolutional Coding
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!