Anatomy of Radio LAN

Onno W. PurboOnno@indo.net.id

Reference http://www.hpl.hp.com/personal/

Jean_Tourrilhes/Linux/Linux.Wireless.wlan.html

Functional Diagram

Spread Spectrum Techniques Direct Sequence Frequency Hopping

Direct Sequence

Frequency Hopping

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 …

Diversity Diversity

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

Antenna Diversity

Antenna Directional Antenna Sectoral Antenna Beam forming Antenna

Range Issues Propagation

Attenuation Reflection Fading

Transmitter Power Higher power:

Drain battery quickly Frequency reuse drawback

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.

Attenuation:Propagation & Range

SNR Signal-to-Noise Ratio

Difference of power in the receiver between a valid signal and noise.

Modulation

Multi-rate system Better throughput more

bandwidth. The higher the signalling rate, the

shorter the distance.

2FSK B = base frequency d = deviation

symbol 0 B + d 1 B – d

Frequency Modulation 2FSK

4FSK B = base frequency d = deviation

Symbol 00 B+ 1/2d 01 B – 1/2d 10 B + 3/2d 11 B – 3/2d

IEEE 802.11 (11Mbps) CCK – Complementary Code

Keying 11 Mbps 5.5Mbps

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.

Interference & Noise

Interference & Noise Fading

Variation in time. Microwave oven & other interferer. Forward Error Correction (FEC)

Not effective, better retransmit the packet.

Delay Spread

Multipath & Delay Spread