On the diversity-multiplexing tradeoff of unconstrained MIMO fading channels

In this work the optimal diversity-multiplexing tradeoff (DMT) is studied 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 the case N ≥ (K + 1)M - 1, i.e. user limited regime. On the other hand for the case N <; (K + 1) M - 1 it is shown that infinite constellations can not attain the optimal DMT. This is in contrast to the point-to-point case where infinite constellations are DMT optimal for any M and N. In general, this work shows that when the network is heavily loaded, i.e. K > max (1, N - M + 1/ M), taking into account the shaping region in the decoding process plays a crucial role in pursuing the optimal DMT.