## Phase distortion – the channel

#### The filtering effect of most wired channels will introduce some phase and hence group delay distortion into the signal, and here equalizers play the key role in measuring and compensating for much of this distortion. |

#### 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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