Robust Hybrid Analog/Digital Beamforming for Uplink Massive-MIMO with Imperfect CSI

In this paper, we study the design of hybrid analog/digital beamformers for uplink connection in massive multiple-input multiple-output (MIMO) systems under imperfect channel state information (CSI). The norm-bounded channel error model is used to capture characteristics of imperfect CSI in practical systems. The objective function is formulated based on the minimum mean squared error (MMSE) worst-case robustness. We consider both single user (SU) and multiuser (MU) reception modes of a millimeter-Wave (mmWave) massive-MIMO base station (BS). For the SU scenario, we study hierarchical beamformer optimization as well as joint precoder/combiner optimization for users with limited and extended computational capabilities, respectively. These optimization techniques are subsequently extended to the MU case where a new hybrid robust combiner design is proposed. Simulation results are presented confirming the superiority of our designs when compared to recent robust hybrid designs in the literature.