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Enhancement of Wi-Fi Communication Systems through Symbol Shaping and Interference Mitigation

Enhancement of Wi-Fi Communication Systems through Symbol Shaping and Interference Mitigation. Presented by Tanim M. Taher Date: Monday, November 26 th , 07. ACKNOWLEDGEMENTS. Presentation Outline. Barker Symbol Shaping Symbol Shaping and Line coding for Barker spread Wi-Fi

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Enhancement of Wi-Fi Communication Systems through Symbol Shaping and Interference Mitigation

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  1. Enhancement of Wi-Fi Communication Systems through Symbol Shaping andInterference Mitigation Presented by Tanim M. Taher Date: Monday, November 26th, 07

  2. ACKNOWLEDGEMENTS

  3. Presentation Outline • Barker Symbol Shaping • Symbol Shaping and Line coding for Barker spread Wi-Fi • Symbol shaping for CCK spread Wi-Fi • Experimental study of MicroWave Oven (MWO) emissions • Analytical Model #1 for MWO signal • Analytical Model #2 for MWO signal • MWO Interference Mitigation for Wi-Fi Communications • Conclusions

  4. Symbol Shaping Study

  5. Achieving FCC Spectral Mask: Pulse Shaping or Filters? • All IEEE 802.11 systems use filters to meet FCC spectral mask • Filters introduce Inter-Symbol-Interference (ISI) • Symbol shaping lowers out-of-band interference power without ISI

  6. The Barker Spread sequence • The Barker chip sequence used in the 1 Mbps 802.11 standard is: B = [+1,−1,+1,+1,−1,+1,+1,+1,−1,−1,−1] • For transmitting bit 1, transmit chip sequence +B • For transmitting bit 0, transmit chip sequence –B • Spectral mask unmet:

  7. Sinusoidal Symbol Shape:

  8. Generate random bit sequence and spread each bit by pulse shape to obtain data waveform. Upload the data waveform to the Comblock transmitter. Design Pulse Shape adhering to Barker Sequence in MATLAB. Transmit over the Air. 10010110111010 Comblock receiver captures the received data waveform for computer download. Examine Bit Error Rate Use Correlator to decode the received bits. Use Correlator to obtain timing information 10010110101010 System Performance Test

  9. Simulation Results Table: Simulated BER measurements.

  10. The Comblock receiver. The Comblock transmitter Oscilloscope plot of Experimental Data Waveform Experimental Wi-Fi with Symbol Shaping Table: Experimental BER measurements at receiver-to-transmitter distance of 1 meter.

  11. 1 ---+++-++-+ Barker Sequence 0 +-++-+++--- Reversed Sequence Line Coding with Buffering to prevent discontinuities

  12. 11 Plot of bit +1; state 1 Plot of bit +1; state 2 2 2 10 0 0 -2 -2 0 0.5 1 0 0.5 1 Time in s Time in s -6 -6 x 10 x 10 Plot of bit +1; state 3 Plot of bit +1; state 4 2 2 0 0 -2 -2 0 0.5 1 0 0.5 1 01 Time in s Time in s -6 -6 x 10 x 10 Plot of bit -1; state 5 00 Plot of bit -1; state 6 2 2 0 0 -2 -2 0 0.5 1 0 0.5 1 Time in s Time in s -6 -6 x 10 x 10 Plot of bit -1; state 7 Plot of bit -1; state 8 2 2 0 0 -2 -2 0 0.5 1 0 0.5 1 Time in s Time in s -6 -6 x 10 x 10 Line code with 3 bits buffered ---+-++-+++

  13. Used to transmit data at 5.5 Mbps and 11 Mbps. Equations: The 5.5 Mbps signal has 4 unique vector sequences for x(n,k) and y(n,k) that can be symbol shaped: CCK symbol shaping

  14. Symbol shapes Used Sincm pulse shapes Sinusoidal pulse shapes

  15. CCK Pulse Shaping: RESULTS Simulated BER graph (1 dB improvement) PSD plots (experimental)

  16. Microwave Oven (MWO) Studies

  17. Why can I never connect to the internet during lunch time everyday? Motivation for MWO study MWO PSD spans ISM band

  18. Time domain MWO signal • The Residential MWO signal is synchronized with the 60 Hz AC line cycle, and it radiates for less than half a cycle. • Zero-span measurement at 2.455 GHz. Note the changing amplitude in the middle. • Transients are observable before and after the AM-FM signal.

  19. Spectrogram Analysis of MWO Signal • Spectrogram shows AM-FM nature of MWO signal. • The frequency sweeping is roughly sinusoidal in nature. • Observe the high transient energy concentrated in frequencies near FM signal. Transients AM-FM Signal

  20. MWO Model #1 features • Following time domain characteristic: • AM-FM signal • Transients represented by sinc pulses: • Large bandwidth lower power sinc pulse • Narrower Bandwidth high power sinc pulse modulated near AM-FM signal.

  21. Simulated with 100 KHz carrier Simulated with 1 MHz carrier Experimental PSD Spectrograms Simulated with 100 KHz carrier Simulated with 1 MHz carrier Experimental Spectrogram Simulation Results Power Spectral Densities

  22. Problem with Model #1 • For a bandwidth of 50 MHz, the transient durations come out to be in the order of nanoseconds as opposed to milliseconds. • The FM carrier frequency of an MWO is not constant but varies: • The transient power PSD is not flat, but follows a curve similar to the bell curve, but with a short tail on the high frequency curve.

  23. New Model • The carrier frequency Fc was made random. • The transients were formulated as a sum of sinc pulses modulated at uniformly spaced frequencies, where the sinc pulse power was a function of the frequency following a modified Rayleigh distribution plot:

  24. , where T = 1/fac and fac = 60 Hz. where where Model #2 for the MWO Signal • Mathematical Representation of model MWO signal:

  25. Experimental PSD Experimental PSD Emulated PSD Model #2 Results (PSD) Simulated PSD

  26. Model #2 Results (Spectrograms) Experimental Spectrogram Emulated Spectrogram Experimental Spectrogram Simulated Spectrogram

  27. MWO Interference and Mitigation • Complete experimental Wi-Fi system was setup. • The effect of MWO interference on BER was measured for this Wi-Fi setup. • Interference was mitigated by cognitive radio circuit.

  28. Circuit Block Diagram: yT (t) Baseband Converter Threshold Detector Transient Detector 60 Hz AC Line Reference Transmit Controller (50 / 100 %) Interference Mitigation Circuit Theory • Interference Mitigation theory:

  29. Interference Mitigation Results Table: Experimental BER Measurements Baseband digital logic circuit and Wi-Fi transmitter

  30. Conclusions • Complete Wi-Fi system was implemented. • Pulse Shaping was thoroughly applied to IEEE 802.11 Barker Spread Signal and Wi-Fi performance was improved. • Pulse shaping was applied to 5.5 Mbps CCK spread signal. • MWO signal was examined meticulously. • Good analytical model was developed and verified by emulation and simulation. Model is useful in network simulation studies. • An interference mitigation technique was developed for Wi-Fi system that eliminates MWO interference. This technique significantly enhances Wi-Fi system performance in interference environments.

  31. Thank you!Questions?

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