Symbol shaping for barker spread wi fi communications
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Symbol Shaping for Barker Spread Wi-Fi Communications. Tanim M. Taher Matthew J. Misurac Donald R. Ucci Joseph L. LoCicero. Presented by Tanim M. Taher. Outline. Background Motivation Simulation and Experimental Methodology Pulse Shapes Used and Results Performance Results

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Symbol shaping for barker spread wi fi communications l.jpg

Symbol Shaping for Barker Spread Wi-Fi Communications

Tanim M. Taher

Matthew J. Misurac

Donald R. Ucci

Joseph L. LoCicero

Presented by

Tanim M. Taher


Outline l.jpg
Outline

  • Background

  • Motivation

  • Simulation and Experimental Methodology

  • Pulse Shapes Used and Results

  • Performance Results

  • Line Coding

  • Conclusions


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Spectral Mask Background

  • The Industrial, Scientific and Medical (ISM) bands are overcrowded.

  • The Federal Communications Commission (FCC) limits the output power to 1 watt.

  • FCC regulates out-of-band Power in Wi-Fi systems using a rigid Spectral Mask.

  • Most modulation schemes require high order filters to achieve spectral mask.



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Why Pulse Shaping?

  • Filters add to hardware cost, and introduce Inter-Symbol-Interference (ISI) that lowers the Bit Error Rate (BER) vs Signal to Noise Ratio (SNR) performance.

  • Shaping the transmitted symbols as opposed to filtering prevents ISI, while lowering out-of-band interference power.


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The Barker spread 1 Mbps 802.11 signal

  • Access Points and laptops use a spreading code called the 11-chip Barker to expand the bandwidth of 1 Mbps data signals.

  • The spread spectrum system more robust to noise, multi-path fading, and narrowband interference.

  • This 1 Mbps communication system is used for transmitting all the Packet headers and Physical Layer Convergence Protocols.

  • At higher noise levels, this system is used for transferring all data.


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More about the Barker Sequence

  • The Barker chip sequence used in the 802.11 standard is:

    B = [+1,−1,+1,+1,−1,+1,+1,+1,−1,−1,−1]

    where rectangular pulses are used to represent each chip (polarities varied according to B) in the sequence.

  • The Barker sequence (B) has very good auto-correlation properties and this is what minimizes multipath effects.


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Motivation for Project

  • The problem is that the Barker spread data waveform does not adhere to the spectral mask.

  • The rectangular Barker waveform was modified by pulse shaping to achieve better spectral performance in relation to the spectral mask.

  • The resulting modulation system was studied by simulation and experimentation. The PSD and BER performance were examined.

Simulated PSD of rectangular unfiltered rectangular pulse Barker waveform.


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MATLAB Simulation Methodology for each Pulse Shape

Generate random bit sequence and spread each bit by pulse shape to obtain data waveform.

1) Design Pulse Shape adhering to Barker Sequence.

2) Examine its Auto-correlation properties.

Obtain the PSD of data waveform using the Welch method.

Add Additive White Gaussian Noise (AWGN).

10010110111010

Use Correlator to obtain timing information from the “received signal”

Examine Bit Error Rate

Use Correlator to decode the received bits.

10010110101010


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Experimental Emulation

  • Comblock Devices were used to transmit and receive the waveforms experimentally. MATLAB software was used to do the coding and decoding in a workstation.

The Comblock transmitter

The Comblock receiver.


Experimentation methodology for each pulse shape l.jpg
Experimentation Methodology for each Pulse 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


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Logarithmic Symbol Shape:

  • Practical devices to inexpensively generate these symbols can be manufactured using discrete-time analog memory devices (like Pulse Amplitude Modulation, PAM, chips)




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More of the Sinusoidal Symbol Shape:

Simulated PSD

Experimental PSD


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More Sinusoidal Material:

Oscilloscope plot of Experimental Data Waveform

Time Auto-correlation


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Performance Results

  • Bit Error Rate (BER) dropped as the filter order dropped.

Table 2. Experimental BER measurements at receiver-to-transmitter distance of 1 meter.

Table. 1. Simulated BER measurements.


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Line Coding and Pulse Shaping

  • A modified Barker spread system was examined that buffered 2 bits.

  • A line coding involving a total of 8 pulse shapes was developed and tested.

  • The idea was to eliminate discontinuities that arises when a bit transition occurs from 1 to 0 or vice versa.

  • The system buffers 3 bits in order to eliminate discontinuities and by selecting the appropriate pulse shape (from pool of 8) to transmit.


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Plots of 3 bit buffer system.

+1 to +1 bit transition

-1 to +1 to -1 bit transition

+1 = bit 1

-1 = bit 0

  • However, the results showed no significant spectral improvement.

  • The unbuffered sinusoidal system gives best BER performance.


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Conclusions

  • Pulse Shaping was thoroughly applied to 802.11 Barker Spread Signal.

  • Complete simulation and experimental studies were performed to examine performance. Analytical study was performed for Sinusoidal pulse shape.

  • The spectral performance was improved and BER reduced.

  • Future Work will look at 802.11 CCK signals.


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Thank you!Questions?Tanim Taher ([email protected])


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