How calculate channel gains in NOMA?
You won't know the exact channel, instead you'd use the channel estimates.
The basic idea behind Non Orthogonal Multiple Access (NOMA) is that you are receiving multiple overlapping signals, decoding the strongest signal, subtracting it, decode the next strongest signal, and so on. With that being said, NOMA and the associated successive interference cancellation (SIC) receiver requires a high enough SNR so as to be able to reliably decode all of the signals. For this high SNR case, it is reasonable to assume that the channel estimates you have will be good enough for the processing.