Convolutional Coded DPIM for Indoor Optical Wireless Links

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 LCS2006

2. 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 . LCS2006

3. Optical 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. LCS2006

4. Practical 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. LCS2006

5. Digital 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) LCS2006

6. Digital Modulation Techniquesfor OWC LCS2006

7. DPIM • 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 . LCS2006

8. DPIM – 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). LCS2006

9. 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) LCS2006

10. DPIM- 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 LCS2006

11. Why use Error Control Coding? • Improves the reliability of system. • Improves the Signal to Noise ratio (SNR) required to achieve the same error probability. • Efficient utilizationof available bandwidth and power. LCS2006

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. LCS2006

13. The convolutional CodingState diagram • (3,1,2) convolutional • encoder . • ½ code rate and • constraint length = 3 • Generator function • g1 = [111] and g2 = [101] LCS2006

14. 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. LCS2006

15. 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. LCS2006

16. 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! LCS2006

17. System Block Diagram h(t) PIM Modulator Convolutional Encoder Optical Transmitter + Shot Noise n(t) Input Bits PIM Demodulator Viterbi Decoder Sampler Output Bits Threshold Detector Matched Filter Optical Receiver LCS2006

18. CC-DPIM : Upper Error bound • Difficult to ascertain exact Hamming distance of an • convolutional 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. LCS2006

19. Performance comparison of CC-DPIM with different guard slots • A code gain of 4.8 dB achieved at slot error rate of 10-4. • DPIM(2GS) offers an improvement • of 0.5 dB and 1dB • in SNR compared • to DPIM(1GS) • and DPIM(0GS). LCS2006

20. Performance 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. LCS2006

21. 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 LCS2006

22. 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 LCS2006

23. Thank you! LCS2006