On the Diversity-Multiplexing Tradeoff of Unconstrained Multiple-Access Channels.

In this paper, the optimal diversity-multiplexing tradeoff (DMT) is investigated for the multiple-input multiple-output fading multiple-access channel with no power constraints (infinite constellations). For $K$ users ( K>1$ ), M$ transmit antennas for each user, and N$ receive antennas, infinite constellations in general and lattices in particular are shown to attain the optimal DMT of finite constellations for N\ge (K+1) M-1 , it is shown that infinite constellations cannot attain the optimal DMT. This is in contrast to the point-to-point case in which the infinite constellations are DMT optimal for any M$ and N$ . In general, this paper shows that when the network is heavily loaded, i.e., , considering the shaping region in the decoding process plays a crucial role in pursuing the optimal DMT. By investigating the cases in which the infinite constellations are optimal and suboptimal, this paper also gives a geometrical interpretation to the DMT of infinite constellations in multiple-access channels. [ABSTRACT FROM AUTHOR]