Since the magnitude response of such a wireless channel also varies with frequency, the range of frequencies over which this magnitude can be assumed approximately constant is termed as Coherence Bandwidth $B_{\text{coh}}$ of the channel. Clearly, when the delays $\tau_i$ are small, the rotation periods of frequency domain complex sinusoids in Eq (\ref{equation-time-shift-property}) are large (because the period in frequency domain is given by $1/t_0$ in that equation, just like the period in a time domain sinusoid is $1/F_0$). Similar to frequency flat and frequency selective channels, there are also time flat (slow) fading and time selective (fast) fading wireless channels for which specific techniques need to be employed.