Two-stage opportunistic interference alignment for downlink MU-MIMO cellular systems

In this paper, a two-stage opportunistic interference alignment (OIA) algorithm is proposed for the downlink multi-cell multi-user multiple-input-multiple-output (MU-MIMO) system. Two precoders/postcoders are used at each transmitter/receiver respectively to combat interference. The inner precoder/postcoder pair are used to cancel out the inter-cell interference (ICI) using perfect IA approach, while the outer precoder/postcoder pair are used to minimize the intra-cell or the inter-user interference (IUI) using rank constrained rank minimization (RCRM) based IA approach. According to the limited spatial resources at base stations (BSs), in the second stage, users at which the sum interference span the lowest dimensions are selected by each BS. The proposed algorithm is compared with different downlink OIA algorithms from literature in terms of the average sum rate and the average achieved number of degrees of freedom (DoFs). Simulations show that the proposed algorithm achieves higher sum rate specially at high signal-to-noise ratios (SNRs). Moreover, the proposed algorithm achieves higher number of streams/user compared to other algorithms.