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Anatomy of Radio LAN Onno W. Purbo Onno@indo.net.id
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Reference http://www.hpl.hp.com/personal/Jean_ Tourrilhes/Linux/Linux.Wireless.wlan.ht ml
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Functional Diagram
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Spread Spectrum Techniques Direct Sequence Frequency Hopping
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Direct Sequence
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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 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
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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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Attenuation: Propagation & 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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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
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