Coding Schemes With Constant Sphere-Decoding Complexity and High DMT Performance for MIMO Multiple Access Channels With Low-Rate Feedback

Abstract

In a MIMO MAC where the base station has fewer receive antennas than the transmit antennas of all users, sphere (lattice) decoding for the existing MIMO-MAC codes requires an exhaustive search of exponentially large size before processing the root of a sphere-decoding tree. In this paper, two coding schemes are proposed and are shown to yield a constant sphere-decoding complexity, independent of the numbers of users and transmit antennas. The schemes require a channel feedback, but only at an extremely low rate. The first scheme is based on user selection, and the second scheme selects jointly users and transmit antennas, using a fast antenna selection algorithm recently proposed by Jiang and Varanasi. It also involves a design of rate-assignments that maximizes the overall DMT performance. It is shown that both schemes yield DMT performances far superior to the optimal MIMO-MAC DMT without channel feedback in certain multiplexing gain regime. Simulation results confirm that in some cases the second proposed scheme can provide an astonishing SNR gain of 14.5 dB at outage probability 10(-6) compared to the optimal coding schemes without feedback.