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802.11 802.11a 802.11b 802.11g 802.11n Adaptive antenna system (AAS) Adaptive Antenna System (AAS) Delay spread Doppler spread Fading in wireless communications Frequency reuse Handoff protocols Intercell and intracell handover ISM bands OFDM Paging system Simplex, half-duplex and full-duplex Types of cellular networks Types of spectrum sharing WiMax service classes
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Doppler spread
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Due to Doppler effect, if a transmitter is moving away from a receiver, the frequency of the received signal is lower than the one sent out from the transmitter; otherwise, the frequency is increased. In wireless communications, there are many factors that can cause relative movement between a transmitter and a receiver. It can be the movement of a mobile such as a cell phone; it can be the movement of some background objectives, which causes the change of path length between the transmitter and the receiver. The lengths of signal path are often different, which correspond to different movement speeds of transmitter signals, and in turn different frequency shifts on the signal paths. As a result, a frequency spread is caused in the signal spectrum.
Corresponding to Doppler spectrum spread, there is a concept called coherence time, which is related to the reciprocal of the maximum Doppler shit. Coherence time is used to measure a time interval, in which a smaller amount of fading is occurred. Specifically, if the baseband signal varies faster than the coherence time, the distortion from Doppler spread fading is negligible. Such a situation is called slow fading. Otherwise, if the baseband signal varies more slowly than the coherence time, the distortion from Doppler spread fading may be significant. This situation is called fast fading.
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Added: 26th October 2006 12:29:30 AM Modified: 26th October 2006 12:29:30 AM
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