WLC36-5: Rank-Reduction of Large MIMO Channels

Multiple-input-multiple-output (MIMO) systems are capable of delivering substantially higher throughput under favorable circumstances, at the cost of complexity in terms of the number of parameters that must be estimated and processed. Our results are directed toward lowering this computational complexity through reduction of channel information required at the transmitter. Specifically, we examine the case when only the number of degrees of freedom supporting a threshold SNR is known at the transmitter. We show capacity is maximized in this case with a rank-reduced MIMO channel and an equal power distribution over all available antennas. The number of independent sequences to transmit, i.e. the amount of rank-reduction, is computed by defining the problem such that the optimal number of active data sequences is a capacity maximization problem solvable at the receiver whose results are used at the transmitter. It is shown that in the absence of channel state information, the received SNR for each potential sub-channel is sufficient to determine the optimal number of independent data sequences to transmit.