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

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

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

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Enhancement of Wi-Fi Communication Systems through Symbol Shaping andInterference Mitigation

Presented by

Tanim M. Taher

Date: Monday, November 26th, 07

- 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

Symbol Shaping

Study

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

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:

Sinusoidal Symbol Shape:

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

Table: Simulated BER measurements.

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.

1

---+++-++-+

Barker Sequence

0

+-++-+++---

Reversed Sequence

Line Coding with Buffering to prevent discontinuities

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

---+-++-+++

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

Symbol shapes Used

Sincm pulse shapes

Sinusoidal pulse shapes

CCK Pulse Shaping: RESULTS

Simulated BER graph (1 dB improvement)

PSD plots (experimental)

Microwave Oven (MWO)

Studies

Why can I never connect to the internet during lunch time everyday?

MWO PSD spans ISM band

- 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.

- 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

- 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.

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

Power Spectral Densities

- 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.

- 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:

, where T = 1/fac and fac = 60 Hz.

where

where

- Mathematical Representation of model MWO signal:

Experimental PSD

Experimental PSD

Emulated PSD

Simulated PSD

Experimental Spectrogram

Emulated Spectrogram

Experimental Spectrogram

Simulated Spectrogram

- 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.

- Circuit Block Diagram:

yT (t)

Baseband Converter

Threshold Detector

Transient Detector

60 Hz AC Line Reference

Transmit Controller (50 / 100 %)

- Interference Mitigation theory:

Table: Experimental BER Measurements

Baseband digital logic circuit and Wi-Fi transmitter

- 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.