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

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5 Achieving FCC Spectral Mask: Pulse Shaping or Filters? All IEEE 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 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 System Performance Test Design Pulse Shape adhering to Barker Sequence in MATLAB. Transmit over the Air. Upload the data waveform to the Comblock transmitter. Examine Bit Error Rate Comblock receiver captures the received data waveform for computer download. Generate random bit sequence and spread each bit by pulse shape to obtain data waveform Use Correlator to obtain timing information Use Correlator to decode the received bits

9 Simulation Results Pulse Shape Used Filter Order Bit Error Rate at SNR levels: –11.5 dB–11 dB–10 dB Rectangular5 3.70E E E-04 Logarithmic3 2.48E E E-04 Sinusoidal2 2.62E E E-04 Sinc-function2 2.80E E E-04 Table: Simulated BER measurements.

10 The Comblock receiver. Table: Experimental BER measurements at receiver-to-transmitter distance of 1 meter. Pulse Shape UsedExperimental BER Rectangular9.99E-03 Logarithmic6.22E-03 Sinusoidal3.71E-03 Sinc-function5.84E-03 The Comblock transmitter Oscilloscope plot of Experimental Data Waveform Experimental Wi-Fi with Symbol Shaping

11 Line Coding with Buffering to prevent discontinuities Pulse Shape Used Bit Error Rate at SNR levels: –4.5 dB–4 dB–3 dB Rectangular0.40E E-04 Logarithmic2.80E E E-04 Sinusoidal2.96E E E-04 Sinc-function2.46E E E

12 Line code with 3 bits buffered x Plot of bit +1; state 1 Time in s x Plot of bit +1; state 2 Time in s x Plot of bit +1; state 3 Time in s x Plot of bit +1; state 4 Time in s x Plot of bit -1; state 5 Time in s x Plot of bit -1; state 6 Time in s x Plot of bit -1; state 7 Time in s x Plot of bit -1; state 8 Time in s

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 Chip # vector 1– vector 21– vector 3111–111 1 vector 411–1 11

14 Symbol shapes Used Sinc m pulse shapes Sinusoidal pulse shapes

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

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17 Motivation for MWO study Why can I never connect to the internet during lunch time everyday? 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 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. AM-FM Signal Transients

20 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. MWO Model #1 features

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

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

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

26 Model #2 Results (Spectrograms) Emulated Spectrogram Simulated Spectrogram Experimental 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 Interference Mitigation Circuit Theory Interference Mitigation theory: Baseband Converter Threshold Detector Transient Detector Transmit Controller (50 / 100 %) 60 Hz AC Line Reference y T (t) Circuit Block Diagram:

29 Interference Mitigation Results MWOCaseBER 1 NO MITIGATION 11.29% 2 NO MITIGATION 1.661% 3 NO MITIGATION % 1MITIGATION0.0000% 2MITIGATION0.0000% 3MITIGATION0.0000% Baseband digital logic circuit and Wi-Fi transmitter Table: Experimental BER Measurements

30 Conclusions Complete Wi-Fi system was implemented. Pulse Shaping was thoroughly applied to IEEE 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?

32 Symbol Shaping Example