Coherence time (communications systems)

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In communications systems, a communication channel may change with time. Such a change is much more significant in wireless communications systems, due to doppler effects.

In a simple model, a signal x(t-1) transmitted at time t-1 will be received as

y(t − 1) = x(t − 1) * h(t − 1),

where h(t-1) is a channel impulse response (CIR). A signal transmitted at time t-2 will be received as

y(t − 2) = x(t − 2) * h(t − 2).

Now, if h(t − 1) − h(t − 2) is relatively small, the channel may be considered constant within the interval t-1 to t-2.

Coherence time (Tc) will therefore be given by

Tc = t − 2 − t − 1.