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Delay spread
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The different signal paths between a transmitter and a receiver correspond to different transmission times. For an identical signal pulse from the transmitter, multiple copies of signals are received at the receiver at different moments. The signals on shorter paths reach the receiver earlier than those on longer paths. The direct effect of these unsimultaneous arrivals of signal causes the spread of the original signal in time domain. This spread is called delay spread. The delay spread puts a constraint on the maximum transmission capacity on the wireless channel. Specifically, if the period of baseband data pulse is larger than that of delay spread, inter-symbol interference (ISI) will be generated at the receiver. That is, the data signals on two neighbouring pulse periods are received at the same time, which causes the receiver not to be able to distinguish them. Corresponding to the concept of delay spread, there is a term called coherence bandwidth used to measure the up-limit bandwidth that can be transmitted for a channel to be free of ISI. Coherence bandwidth is defined as 10% of the reciprocal of root mean square (rms) delay delay spread. If the bandwidth of a transmitter signal is less than the channel coherence bandwidth, the channel shows flat fading to be free of ISI. Otherwise, the channel shows frequency selective fading, and may surfer from ISI.
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Added: 26th October 2006 12:29:57 AM Modified: 26th October 2006 12:29:57 AM
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