What is the coherence bandwidth of the channel?
If you have a multipath channel with impulse response

$$h(t)=\sum_{k=0}^{m-1}a_k\delta(t-\tau_k)\tag{1}$$

the corresponding frequency response is

$$H(j\omega)=\sum_{k=0}^{m-1}a_ke^{-j\omega\tau_k}=e^{-j\omega\tau_0}\sum_{k=0}^{m-1}a_ke^{-j\omega(\tau_k-\tau_0)}\tag{2}$$

Assume that the delays are ordered such that $\tau_0&lt;\tau_1&lt;\ldots &lt;\tau_{m-1}$. Note from $(2)$ that the periods of the complex exponentials are given by $1/(\tau_k-\tau_0)$, and the smallest period is $1/(\tau_{m-1}-\tau_0)$, which is simply the inverse of the largest delay difference. This largest delay difference is called the channel's delay spread. Its inverse, i.e., the smallest period of the complex exponentials in the frequency domain is referred to as the coherence bandwidth.

The importance of the coherence bandwidth is the following: in a frequency range which is considerably smaller than the coherence bandwidth, the channel's frequency response can be considered flat (i.e., it doesn't change much). So if your signal's bandwidth is small compared to the channel's coherence bandwidth, it will not be distorted very much. If the signal bandwidth is larger than the coherence bandwidth, the signal will experience frequency-selective fading.