Sujan Rajbhandari LCS2006 1 Convolutional Coded DPIM for Indoor Optical Wireless Links S....

Sujan Rajbhandari LCS2006

1

Convolutional Coded DPIM for Indoor Optical Wireless Links

S. Rajbhandari, N. M. Aldibbiat and Z. Ghassemlooy Optical Communications Research Group,

School of Computing, Engineering and Information Sciences, The University of Northumbria,

Newcastle upon Tyne, U.K.Web site: http://soe.unn.ac.uk/ocr


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Optical Wireless Communication

Definition : a telecommunication technology that uses light propagating in free space to transmit data between two points . [http://en.wikipedia.org/wiki/Free_Space_Optics.]

Also popularly known as free space optics (FSO) or Free Space Photonics (FSP) or open-air photonics .


3Optical Wireless – Advantages

Unregulated bandwidth, free for commercial and personal use.

200 THz bandwidth in the 700-1500 nm range. No multipath fading. Availability of low cost optical transmitter and

receiver. Small cell size. Can not penetrate through wall- same frequency can be

utilized in adjacent rooms.


4Practical Implementations - Issues

Intense ambient noise.

Average transmitted power is limited due to eye safety.

Do not penetrate wall, thus a need for infrared access point.

Large area photo-detectors – limiting the bandwidth.


5Digital Modulation Techniquesfor OWC

Modulation scheme adopted should have one or two of the following characteristics: power efficient – Since the maximum power that can be

transmitted is limited because of eye safety. bandwidth efficient – particularly in non-line of sight

configurations

Types On-Off Keying (OOK), Pulse Position Modulation (PPM) ,

Digital Pulse Interval Modulation (DPIM), Dual Header Pulse Position Modulation (DH-PIM), Differential Amplitude Pulse-Position Modulation (DAPPM)


6Digital Modulation Techniquesfor OWC


7DPIM

DPIM is an anisochronous pulse time modulation technique.

A symbols starts with a pulse followed by k empty slots. 1≤ k≤ L and L = 2M .

Guard slots can be added to provide resistance to ISI arising from multipath propagation .


8DPIM – contd.

For DPIM with a guard band of g guard slots DPIM(gGS) the minimum and maximum symbol durations are gTs and (L+g)Ts, respectively, where Ts is the slot duration

where Tb is the bit duration and Lavg is the mean symbol length (no. of slots).

avg

bs L

LogLTT


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Error Performance of DPIM

The slot error rate for DPIM with no guard slot, Pse(0GS)

The slot error rate with 1 guard slot, Pse(1GS)

21

122

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000)1( N

QL

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avg

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avgavg

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LNQ

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LNQ

LL 1

2

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21

2

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00)0( N

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10DPIM- Comparison with other modulation schemes

Bandwidth efficient compared to PPM.

Built-in slot and symbols synchronisation.

Higher transmission capacity compared to PPM.

Resistance to effect of multipath propagation compared to PPM


11Why use Error Control Coding?

Improves the reliability of system.

Improves the Signal to Noise ratio (SNR) required to achieve the same error probability.

Efficient utilization of available bandwidth and power.


12 Convolutional Coded DPIM

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.

So either convolutional code or modification of convolutional codes are only alternatives because convolutional encode act on serial input data rather than block.


13The convolutional CodingState diagram

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


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Error performance

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 ppDIIIDTPb


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CC-DPIM(2GS) Speciality

2 empty slots at in all the symbols so that memory is cleared after each symbol.

Trellis path is limited to 2. No need to use Viterbi algorithm instead we

can use simple look-up table.


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Look-up Table

Consider received sequence to be {00 00 10 11 00} The closest match to the sequence in the look-up

table is {00 11 10 11 00} i.e. correct decision!


17 System Block Diagram

PIMDemodulator

PIM Modulator

Convolutional Encoder

Optical Transmitter

Matched Filter

ThresholdDetector

+ Shot Noise n(t)

Output Bits

Input Bits

ViterbiDecoder

Sampler OpticalReceiver

h(t)


18CC-DPIM : Upper Error bound

•Difficult to ascertain exact Hamming distance of anconvolutional encoder.

•Union bound is utilised to evaluate the performance.

•The simulation result is expected to be less than but close match to the error bound.


19Performance comparison of CC-DPIM with different guard slots

•DPIM(2GS) offers an improvement of 0.5 dB and 1dB in SNR compared to DPIM(1GS) and DPIM(0GS).

•A code gain of 4.8 dB achieved at slot error rate of 10-4.


20Performance of DPIM for different bit resolution

A code gain of ~4.9 dB , 4.8 dB and 4.5 dB for M= 5, 4 and 3, respectively at Pse of 10-4.

•Code gain increases as Pse decreases.


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Comparisons with other modulations

• The performance of CC-DPIM(2GS) close to CC-DH-PIM1

with formal requiring 1 dB more SNR..

•CC-DPIM performances better than uncoded PPM


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Conclusions

Convolutional coded DPIM offered an improvement of 4.5dB compared to uncoded DPIM.

CC-DPIM(2GS) performed better than CC-PIM(1GS) and DPIM(0GS) .

Performance of CC-DPIM is very close to performance of CC-DH-PIM1

Simple implementation when using 2 Guard slots instead of 1 or no guard slot in DPIM, since no need for Viterbi decoding algorithm


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Thank you!

Sujan Rajbhandari LCS2006 1 Convolutional Coded DPIM for Indoor Optical Wireless Links S....

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