In optical fibre links, distortion is usually attributed to time
dispersion in the fibre, which gives rise to an upper limit on the data rate that can be
supported. The time dispersion, which is caused by rays passing through the fibre with
different numbers of reflections (called multi-mode propagation),
is most effectively dealt with by using a single or mono-mode fibre
and corresponding laser source, although these are more costly to manufacture.
In the wireless channel, phase distortion is not a problem for signals travelling along
any individual path between source and receiver. In applications where the signal travels
by multiple paths, however, each of which may have a different path length and hence
relative time delay, the phases of the components making up the composite received signal
will all be different. This can create significant problems for data signals as discussed
in Section 4.5.
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