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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. Introduction to optical wireless Modulation schemes

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CONVOLUTIONAL CODED DPIM FOR INDOOR NON-DIFFUSE OPTICAL WIRELESS LINK

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  1. 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

  2. Contents • Introduction to optical wireless • Modulation schemes • Digital PIM • Coded DPIM • Results + comments

  3. 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

  4. 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

  5. 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

  6. 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)

  7. 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

  8. 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

  9. 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

  10. 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.

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

  12. 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.

  13. 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.

  14. DPIM - Block Diagram AWGN R Convolutional Encoder Optical Tx Photodetector DPIM Input Ik Viterbi Decoder Matched Filter Sampler DPIM estimate

  15. 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

  16. 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

  17. 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.

  18. 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.

  19. 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.

  20. Thank You! 20

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