For practical communication systems, we calculate the signal-to-noise ratio (SNR) per channel like this,
SNR = Ωd / Ωn
where, Ωd = (P' P)
where P is the channel impulse response. For example, P = [p(0) p(1) p(2) ... p(M-1)]
If the signal is being received at the receiver through multiple channels, then the average SNR per channel is calculated as,
Average SNR = [SNR(1) + SNR(2) + SNR(3) + ... + SNR(n)] / n