1. IASTED- WOC- Canada 07 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. IASTED- WOC- Canada 07 2 Contents  Introduction to optical wireless  Modulation schemes  Digital PIM  Coded DPIM  Results + comments

3. IASTED- WOC- Canada 07 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. IASTED- WOC- Canada 07 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. IASTED- WOC- Canada 07 5 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 Rx Tx  LOS LOS  No multipath Propagation  Only noise is limiting factor  Possibility of blocking  Tracking necessary to maintain LOS link LOS  No multipath Propagation  Only noise is limiting factor  Possibility of blocking  Tracking necessary to maintain LOS link Tx Rx

6. IASTED- WOC- Canada 07 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. IASTED- WOC- Canada 07 7 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

8. IASTED- WOC- Canada 07 8 Digital Pulse Interval Modulation  Variable symbol length Where T b is input bit rate and T s is DPIM slot duration  A symbols starts with pulse followed by k empty slots. 1≤ k≤ L and L = 2 M  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)T s

9. IASTED- WOC- Canada 07 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. IASTED- WOC- Canada 07 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. IASTED- WOC- Canada 07 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. IASTED- WOC- Canada 07 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. IASTED- WOC- Canada 07 13 DPIM - Decoder  Viterbi ‘Hard ‘ decision decoding  The Chernoff upper bond on the error probability is: where P se 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. IASTED- WOC- Canada 07 14 DPIM - Block Diagram Convolutional Encoder Optical TxPhotodetector Sampler Viterbi Decoder Matched Filter DPIM I k Input I k DPIM estimate AWGNR

15. IASTED- WOC- Canada 07 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. IASTED- WOC- Canada 07 16 Results – Slot Error Rates With/Without Guard Slots Code gain of 4.8 dB at P se 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. IASTED- WOC- Canada 07 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. IASTED- WOC- Canada 07 18 Packet Error Rates PER against the electrical SNR for coded and un-coded 8,16,32 – DPIM(1GS) at 100 Mbps. 10 - -8 -6 -4 -12

19. IASTED- WOC- Canada 07 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. IASTED- WOC- Canada 07 20 Thank You!