With NOMA, all users spread their signal across the entire bandwidth and send signals simultaneously, much like in the CDMA system. However, rather than decoding every user treating the interference from other users as noise, a successive interference cancellation (SIC) receiver is needed to achieve capacity. That is, after one user is decoded, its signal is stripped away from the aggregate received signal before the next user is decoded. With SIC, the set of all rates 􏷪$(R_1, R_2)$􏷫 satisfies the three constraints: $$R_1 < \log_{2}\left(1+\frac{P_1}{N_0}\right)$$ $$R_2 < \log_{2}\left(1+\frac{P_2}{N_0}\right)$$ $$R_1+R_2 < \log_{2}\left(1+\frac{P_1+P_2}{N_0}\right)$$ The first two say that the rate of the individual user cannot exceed the capacity of the point-to-point link with the other user absent from the system (these are called single-user bounds). The third says that the total throughput cannot exceed the capacity of a point-to-point AWGN channel with the sum of the received powers of the two users.
It is obvious that the range of capacity region is related to SNR. The relationship between them will be shown in slide 6. In the next demo slides, $N_0$ = $0$ dBm, namely $1$ milliwatt, is hold for simplicity.