I. Introduction
MASSIVE MIMO is a promising technology to meet the future capacity demand in wireless cellular networks. Equipped with a large number of antennas, the system has a sufficient number of degrees of freedom (DoF) to exploit the spatial multiplexing gains for intra-cell users and to mitigate the inter-cell interference. However, the corresponding beamforming (precoder) designs for such multiuser MIMO (MU-MIMO) interference networks are challenging even in traditional MIMO systems with a small number of antennas. In [1], the beamformers are jointly optimized among BSs, where the uplink–downlink duality is used to obtain the global CSI in a time-division duplex (TDD) system. Using alternative optimization techniques, WMMSE algorithm is proposed in [2] with the objective to maximize the weighted sum rate for multi-cell systems. Moreover, interference alignment (IA) approaches were used in [3], [4] for downlink interference cellular networks.