The Gap Approximation for Gaussian Multiple Access Channels

The applicability of the gap approximation is investigated in the context of Gaussian multi-user channels where suboptimal transmission schemes are employed. In this case, the assumption of all users being subject to Gaussian interference may not always hold. Considering the two-user Multiple Access Channel as an example, and assuming uncoded Pulse Amplitude Modulation (PAM), it is shown that the gap approximation can be used, albeit in a modified fashion compared to the single-user Gaussian channel. It is also demonstrated that successive interference cancellation at the receiver does not cause performance degradation when PAM is used by all transmitters, similar to the case of capacity-achieving transmit waveforms.