Exploiting Spatial Interference Alignment and Opportunistic Scheduling in the Downlink of Interference-Limited Systems

In this paper, we analyze the performance of single-stream and multistream spatial multiplexing (SM) systems employing opportunistic scheduling in the presence of interference. In the proposed downlink framework, every active user reports the postprocessing signal-to-interference-plus-noise power ratio (post-SINR) or the receiver-specific mutual information (MI) to its own transmitter using a feedback channel. The combination of scheduling and multiantenna receiver processing leads to substantial interference suppression gain. Specifically, we show that opportunistic scheduling exploits the spatial interference alignment (SIA) property inherent to a multiuser system for effective interference mitigation. We obtain bounds for the outage probability and the sum outage capacity for single-stream and multistream SM employing real or complex encoding for a symmetric interference channel (SIC) model. The techniques considered in this paper are optimal in different operating regimes. We show that the sum outage capacity can be maximized by reducing the SM rate to a value less than the maximum allowed value. The optimal SM rate depends on the number of interferers and the number of available active users. In particular, we show that the generalized multiuser SM (MU SM) method employing real-valued encoding provides a performance that is either comparable or significantly higher than that of MU SM employing complex encoding. A combination of analysis and simulation is used to describe the tradeoff between the multiplexing rate and the sum outage capacity for different antenna configurations.