Achievable rates for transmission of discrete constellations over the   Gaussian MAC channe
  In this paper we consider the achievable rate region of the Gaussian Multiple Access Channel (MAC) when suboptimal transmission schemes are employed. Focusing on the two-user MAC and assuming uncoded Pulse Amplitude Modulation (PAM), we derive a rate region that is a pentagon, and propose a strategy with which it can be achieved. We also compare the region with outer bounds and with orthogonal transmission. 
Rizwan Ghaffar, Dimitris Toumpakaris and Jungwon Lee
cs.IT math.IT