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Enhancement of Wi-Fi Communication Systems

through Symbol Shaping and

Interference Mitigation

Presented byTanim M. Taher

Date: Monday, November 26th, 07

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ACKNOWLEDGEMENTS

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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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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

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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:

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Sinusoidal Symbol Shape:

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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.

10010110111010

Use Correlator to obtain timing information

Use Correlator to decode the received bits.

10010110101010

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Simulation Results

Pulse Shape Used

Filter Order

Bit Error Rate at SNR levels:

–11.5 dB –11 dB –10 dB

Rectangular 5 3.70E-03 2.74E-03 9.00E-04

Logarithmic 3 2.48E-03 1.40E-03 5.60E-04

Sinusoidal 2 2.62E-03 1.36E-03 3.80E-04

Sinc-function 2 2.80E-03 1.98E-03 3.80E-04

Table: Simulated BER measurements.

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The Comblock receiver.

Table: Experimental BER measurements at receiver-to-transmitter distance of 1 meter.

Pulse Shape Used Experimental BER

Rectangular 9.99E-03

Logarithmic 6.22E-03

Sinusoidal 3.71E-03

Sinc-function 5.84E-03

The Comblock transmitter

Oscilloscope plot of Experimental Data Waveform

Experimental Wi-Fi with Symbol Shaping

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Line Coding with Buffering to prevent discontinuities

Pulse Shape Used

Bit Error Rate at SNR levels:

–4.5 dB –4 dB –3 dB

Rectangular 0.40E-04 0.00E-04 0.00E-04

Logarithmic 2.80E-04 2.20E-04 0.20E-04

Sinusoidal 2.96E-03 1.58E-03 4.40E-04

Sinc-function 2.46E-03 1.38E-03 3.60E-04

0 0.5 1 1.5 2 2.5 3

x 107

-110

-100

-90

-80

-70

-60

-50

-40

-302 bit buffer signal PSD

Frequency in Hz

Pow

er

in d

Bm

0 0.5 1

x 10-6

-2

0

2Plot of bit +1; state 1

Time in s0 0.5 1

x 10-6

-2

0

2Plot of bit +1; state 2

Time in s

0 0.5 1

x 10-6

-2

0

2Plot of bit -1; state 3

Time in s0 0.5 1

x 10-6

-2

0

2Plot of bit -1; state 4;

Time in s

1

0

--- + + +-+ +- +

+-+ +-+ + +---

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Line code with 3 bits buffered11

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01

00

0 0.5 1

x 10-6

-2

0

2Plot of bit +1; state 1

Time in s0 0.5 1

x 10-6

-2

0

2Plot of bit +1; state 2

Time in s

0 0.5 1

x 10-6

-2

0

2Plot of bit +1; state 3

Time in s0 0.5 1

x 10-6

-2

0

2Plot of bit +1; state 4

Time in s

0 0.5 1

x 10-6

-2

0

2Plot of bit -1; state 5

Time in s0 0.5 1

x 10-6

-2

0

2Plot of bit -1; state 6

Time in s

0 0.5 1

x 10-6

-2

0

2Plot of bit -1; state 7

Time in s0 0.5 1

x 10-6

-2

0

2Plot of bit -1; state 8

Time in s

--- + - + +-+ + +

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• 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

( ) ( , )cos 2 ( , )sin 2c c cv t x n k f t y n k f t ( , ) cos , sin ,I Qx n k a w n k a w n k ( , ) sin , cos ,I Qy n k a w n k a w n k

Chip # 1 2 3 4 5 6 7 8

vector 1 –1 1 –1 –1 –1 1 1 1

vector 2 1 –1 1 1 –1 1 1 1

vector 3 1 1 1 –1 1 1 –1 1

vector 4 1 1 –1 –1 –1 1 1 –1

0 2 4 6

x 10-7

-1

0

1

Plot of vector 1

Time in s0 2 4 6

x 10-7

-1

0

1

Plot of vector 2

Time in s

0 2 4 6

x 10-7

-1

0

1

Plot of vector 3

Time in s0 2 4 6

x 10-7

-1

0

1

Plot of vector 4

Time in s

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Symbol shapes Used

0 2 4 6

x 10-7

-1

0

1

Plot of vector 1

Time in s0 2 4 6

x 10-7

-1

0

1

Plot of vector 2

Time in s

0 2 4 6

x 10-7

-1

0

1

Plot of vector 3

Time in s0 2 4 6

x 10-7

-1

0

1

Plot of vector 4

Time in s

0 2 4 6

x 10-7

-1

0

1

Plot of vector 1

Time in s0 2 4 6

x 10-7

-1

0

1

Plot of vector 2

Time in s

0 2 4 6

x 10-7

-1

0

1

Plot of vector 3

Time in s0 2 4 6

x 10-7

-1

0

1

Plot of vector 4

Time in s

Sincm pulse shapes Sinusoidal pulse shapes

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CCK Pulse Shaping: RESULTS

PSD plots (experimental) Simulated BER graph (1 dB improvement)

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Motivation for MWO study

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

MWO PSD spans ISM band

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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.

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Frequency (GHz)

Tim

e (m

s)

2.4 2.41 2.42 2.43 2.44 2.45 2.46 2.470

2

4

6

8

10

12

14

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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

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• 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

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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 SpectrogramFrequency (GHz)

Tim

e (

ms)

2.4 2.41 2.42 2.43 2.44 2.45 2.46 2.470

2

4

6

8

10

12

14

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Spectrograms

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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.

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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:

2.4 2.42 2.44 2.46 2.48 2.5

x 109

0

0.2

0.4

0.6

0.8

1

Frequency (GHz)

Nor

mal

ized

Am

plitu

de

Transient Power vs frequency in model

Frequency (GHz)

Tim

e (

ms)

2.4 2.41 2.42 2.43 2.44 2.45 2.46 2.470

2

4

6

8

10

12

14

16

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Model #2 for the MWO Signal

• Mathematical Representation of model MWO signal:

( ) ( )n

v t c t nT

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

1

1

( ) cos 2

cos 2

( ), ( 1)

N

n d nn

N

n d nn

N

c t E f p t t f t

E f p t t f t

s t and N b f f

( ) sinc ( + ) , 0.5 ,n pp t b t t T

2

2

( )

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( ),

N n

h

f f

fN nn O

h

f fE f E e

f

h N pkf f f where

( ) ( ) cos 2 sin(2 ) , 0.5 ,c ac ss t A x t F t f t t T ( ) cos(2 )acx t f twhere

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Model #2 Results (PSD)

Simulated PSD

Experimental PSD

Emulated PSD

Experimental PSD

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Model #2 Results (Spectrograms)

Emulated Spectrogram

Simulated Spectrogram Experimental Spectrogram

Experimental Spectrogram

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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.

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Interference Mitigation Circuit Theory• Interference Mitigation theory:

Baseband Converter

Threshold Detector

Transient Detector

Transmit Controller

(50 / 100 %)

60 Hz AC Line Reference

yT (t)• Circuit Block Diagram:

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Interference Mitigation Results

MWO Case BER

1 NO MITIGATION 11.29%

2 NO MITIGATION 1.661%

3 NO MITIGATION 0.7315%

1 MITIGATION 0.0000%

2 MITIGATION 0.0000%

3 MITIGATION 0.0000%

Baseband digital logic circuit and Wi-Fi transmitter

Table: Experimental BER Measurements

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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.

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Thank you!

Questions?