Spread Spectrum and 802.11b

1
Spread Spectrumand 802.11b

• Chris Heegard
• Alantro Communications
• 141 Stony Circle
• [phone]
• May 8, 2000

2
IEEE 802.11b isnot Spread Spectrum

• This is a scientific statement
• Not political (e.g., the FCC rules)
• The blocklength (8) of CCK does not relate to
  gain (6 dB ???)
• The scrambling period (256) of PBCC does not
  relate to gain (24 dB ???)

for rates above 2Mbps
3
Definitions

• W - Bandwidth (Hz)
• R - Data Rate (bps bitsHz)
• Spectral efficiency (bits)
• Spreading Ratio
• Relates the of dimensions used by a waveform
  set to the bandwidth occupancy of the set
• Waveform complexity (Hz)

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What is Spread Spectrum?

• No
• Yes
• IEEE802.11b
• 1-Barker 2-Barker
• 5.5-CCK 11-CCK
• 5.5-PBCC 11-PBCC
• 22-AC

Note The value of r assumes no excess bandwidth
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Why not High Rate and Spread Spectrum

• Shannon Theory tells us why

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Shannon Theory with Spreading
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Spreading at Low Rate
8
Spreading at High Rate