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

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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.


Convolutional coded dpim for indoor non diffuse optical wireless link

Thank You!

20


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