Robust Transmission for Massive MIMO Downlink with Imperfect CSI

In this paper, the design of robust linear precoders for the massive multi-input multi-output (MIMO) downlink with imperfect channel state information (CSI) is investigated, where each user equipment (UE) is equipped with multiple antennas. The imperfect CSI for each UE obtained at the BS is modeled as statistical CSI under a jointly correlated channel model with both channel mean and channel variance information, which includes the effects of channel estimation error, channel aging and spatial correlation. The design objective is to maximize the expected weighted sum-rate. By combining the minorize-maximize (MM) algorithm with the deterministic equivalent method, an algorithm for robust linear precoder design is derived. The proposed algorithm achieves a local optimum of the expected weighted sum-rate maximization problem. To reduce the computational complexity of the proposed algorithm, two low-complexity algorithms are then derived. One for the general case, and the other for the case when all the channel means are zeros. The optimality of the beam domain transmissions when all the channel means are zeros is also proved. Simulation results show that the proposed robust linear precoder designs apply to various mobile scenarios and achieve high spectral efficiency.