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Chapter: Wireless Propagation Channels
Section: Multipath Fading, Fade Durations

Appropriate Choice of Packet Length

Multipath fading affects the throughput of data packets on a wireless Rayleigh-fading link:
If the rate of fading is fast, relative to the packet length, it is likely that each received packet contains burst errors.

Example 1: Noise-Limited Rayleigh-Fading Link


Figure: Probability that a packet is received without fade in a (frequency nonselective) Rayleigh fading channel versus fade margin. Orange: single Rayleigh fading interferer, no noise. Violet: Many weak interferers or Gaussian Noise. Various data block or "packet" durations T, relative to Doppler spread f.


Figure: Effective Throughput of fade-free packets in a (non-selective) Rayleigh fading channel, as a function of number of bits (48 header bits not included) in a packet for bit rates of 1200; 16k and 256 kbit/s. Doppler spread 60 Hz. Fade margin 10 dB.

Example 2: ALOHA Random Access


Figure: Probability of successful transmission in wireless data network with ALOHA random access, versus terminal location.

How to select a bit rate?

Packet length and rate of fading
Slow versus fast fading. Role of error correction

Fast bit rates ensure good throughput: Most packets are successful, but some are lost in fade. With slow transmission rates on the other hand, all packets are likely to contain one or more bursts of errors, so virtually all packets may be lost.

Note however, that with increasing bit rates, channel dispersion becomes a problem.


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