Joint interference and phase alignment in multiuser MIMO interference channels

It has been shown that interference alignment (IA) is able to achieve the capacity of wireless interference networks. However each node requires at least local channel state information (CSI) to align all interferences. After that, zero-forcing is needed to recover the d degrees of freedom related to each transceiver pair. In this paper we present a novel technique consisting of both interference and phase alignment in K-user multi-input multi-output (MIMO) interference systems employing M-ary phase-shift keying (M-PSK) modulation. We first utilize standard interference alignment (IA) to eliminate the interference caused by each transmitter to the other K - 1 receivers. Then instead of performing the zero-forcing (ZF) projections to extract the signal subspace, we utilize the availability of local CSI at each transmitter to align the phases of transmitted symbols such that they add up coherently at the intended receive node and therefore achieve maximum ratio combining (MRC) diversity gain of order d for each degree of freedom. We show that the proposed joint interference and phase alignment (JIPA) technique achieves better symbol error performance than ZF-based IA (ZFIA) and attains more instantaneous output SNR without the need to increase the transmitted power.