I. Introduction
Multi-user multiple-input multiple-output (MU-MIMO) has been well recognized as an effective means to increase capacity in the downlink [1]–[3]. However, challenges arise in practical cellular systems where inter-cell interference remains the bottleneck limiting the achievable performance. Therefore, base station (BS) cooperation was recently proposed as a way to alleviate the issue, e.g., [4]–[10], which is greatly motivated by the fact that BSs may be connected via high-speed backhaul links and the channel state information (CSI) and/or data and/or precoding matrices can be shared among the BSs for coordinated transmission. Such BS cooperation in the downlink can improve sum-rates and reduce outage as compared to the conventional or single-cell signal processing where the interference (often from other cells) is treated as noise.