Precoding on the broadcast MIMO channel with delayed CSIT: The finite SNR case

Recent information theoretic results suggest that precoding on the multi-user downlink MIMO channel with delayed channel state information at the transmitter (CSIT) could lead to data rates much beyond the ones obtained without any CSIT, even in extreme situations when the delayed channel feedback is made totally obsolete by a feedback delay exceeding the channel coherence time. This surprising result is based on the ideas of interference forwarding and alignment which allow the receivers to reconstruct an information allowing them to cancel out the interference completely, making it an optimal scheme in the infinite SNR regime. In this paper, we formulate a similar problem, yet at finite SNR. We propose a new construction for the precoder which matches the previous results at infinite SNR yet reaches a useful tradeoff between interference alignment and signal enhancement at finite SNR, allowing for significant performance improvements in practical settings.