Multiuser gaussianity and constellation randomization for the MIMO multiple access channel: a unified view

In recent analyses of multiuser MIMO systems and their capacity achieving transmit strategies, Gaussian codebooks are usually used. For the case in which the input signals are constrained to belong to a finite constellation, we study and explain two effects, namely multiuser Gaussianity (MuG) and constellation randomization (CoRa). MuG means that the more users transmit simultaneously, the higher is the similarity of the received density to the optimal Gaussian one. Hence, the instantaneous mutual information is increased by MuG. This leads to a tradeoff between multiuser diversity and MuG. On the other hand, CoRa is done by linear precoding by random unitary matrices. Based on the observations from MuG, we explain why the outage capacity is increased by CoRa. The effects are illustrated by various numerical simulations.