Anatomy of Radio LAN

1
Anatomy of Radio LAN

• Onno W. Purbo
• Onno_at_indo.net.id

2
Reference

• http//www.hpl.hp.com/personal/Jean_Tourrilhes/Lin
  ux/Linux.Wireless.wlan.html

3
Functional Diagram
4
Spread Spectrum Techniques

• Direct Sequence
• Frequency Hopping

5
Direct Sequence
6
Frequency Hopping
7
Frequency Hopping
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Comparisons

• DS modem more complicated, MAC in DS much
  simpler.
• FH appears to be cheaper.
• BW sharing, DS tends to have lower overhead on
  the air.
• DS vs. FH religion war ?

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Diversity

• Diversity
• Redudancy in the system to overcome noise and to
  increase the reliability of the system.
• Antenna Diversity

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Antenna

• Directional Antenna
• Sectoral Antenna
• Beam forming Antenna

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Range Issues

• Propagation
• Attenuation
• Reflection
• Fading

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Transmitter Power

• Higher power
• Drain battery quickly
• Frequency reuse drawback

13
Sensitivity

• Defined as
• Point at 3 packet loss (400 byte packet in
  Gaussian channel).
• 7 dB worse if we use Raleigh channel antenna
  diversity.
• Normally 80 dBm, better 90 dBm.

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AttenuationPropagation Range
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SNR

• Signal-to-Noise Ratio
• Difference of power in the receiver between a
  valid signal and noise.

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Modulation
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Multi-rate system

• Better throughput ?more bandwidth.
• The higher the signalling rate, the shorter the
  distance.

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2FSK

• B base frequency
• d deviation
• symbol
• 0 B d
• 1 B d

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Frequency Modulation 2FSK
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4FSK

• B base frequency
• d deviation
• Symbol
• 00 B 1/2d
• 01 B 1/2d
• 10 B 3/2d
• 11 B 3/2d

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IEEE 802.11 (11Mbps)

• CCK Complementary Code Keying
• 11 Mbps
• 5.5Mbps

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IEEE 802.11a (OFDM)

• Orthogonal Frequency Division Multiplex (OFDM).
• Using equalization in post processing, overcome
  delay spread.
• OFDM is pre-processing technique.
• Use in 5Ghz UNII Band.

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Interference Noise
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Interference Noise

• Fading
• Variation in time.
• Microwave oven other interferer.
• Forward Error Correction (FEC)
• Not effective, better retransmit the packet.
• Delay Spread

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Multipath Delay Spread