convolutional coded dpim for indoor non diffuse optical wireless link
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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. Contents. Introduction to optical wireless Modulation schemes

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convolutional coded dpim for indoor non diffuse optical wireless link
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

contents
Contents
  • Introduction to optical wireless
  • Modulation schemes
  • Digital PIM
  • Coded DPIM
  • Results + comments
optical wireless communication introduction
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
owc challenges
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
owc links
OWC - Links

Rx

Tx

  • LOS
  • Non-LOS
  • Multipath Propagation
  • Intersymbol interference (ISI)
  • Difficult to achieve high data date due to ISI
  • LOS
  • No multipath Propagation
  • Only noise is limiting factor
  • Possibility of blocking
  • Tracking necessary to maintain LOS link

Rx

Tx

digital modulation schemes
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)
digital modulation schemes1

Frame 2

0 1 0

Frame 3

1 1 0

Frame 4

1 1 1

Frame 1

0 0 0

Information

Digital Modulation Schemes

DPIM

digital pulse interval modulation
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

dpim what does it offer
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
dpim convolutional coding
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.

dpim convolutional coding1
DPIM - Convolutional Coding
  • (3,1,2) convolutional
  • encoder .
  • ½ code rate and
  • constraint length = 3
  • Generator function
  • g0= [100], g1 = [111] and g2 = [101]
dpim convolutional coding2
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.
dpim decoder
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.

  • It is also possible not use Viterbi algorithm instead one can use a simple look-up table.
dpim block diagram
DPIM - Block Diagram

AWGN

R

Convolutional

Encoder

Optical Tx

Photodetector

DPIM

Input Ik

Viterbi

Decoder

Matched

Filter

Sampler

DPIM

estimate

results slot error rates upper bounds
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
results slot error rates with without guard slots
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
results slot error rates with without guard slots1
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.
packet error rates

8

,

16

,

32

-

DPIM with one guard band

@

R

=

100

Mbps

Uncoded

8

-

DPIM

R

Coded Upper

E

Bound

8

-

DPIM

P

,

Uncoded

32

-

DPIM

r

o

r

r

Coded Upper

e

t

Bound

32

-

DPIM

e

k

c

Uncoded

a

P

16

-

DPIM

f

o

Coded Upper

y

t

i

Bound

16

-

DPIM

l

i

b

a

b

o

r

P

-

2

-

1

0

1

2

3

4

5

6

7

8

Electrical SNR

(

dB

)

Packet Error Rates

-

4

10

-

6

10

-

8

10

-

10

10

-

12

10

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

final comments
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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