flat fading convolution
the behavior of the channel is generally specific to the radio environment. the frequency selectivity of the channel only depends on the system bandwidth. in other words, given the same channel (environment), it can be either frequency-selective (if the system bandwidth is large) or frequency-flat (if the system bandwidth is narrow).
Therefore, in order to simulate the frequency-selective channel, you just need to simulate multiple taps with the maximum delay larger than the sampling time.