Introduction
Hybrid precoding architectures in millimeter wave (mmWave) systems can be classified into the partially-connected and the fully-connected structures. In the fully connected structure, each RF chain is connected to all antennas, so it provides full beamforming gain [1], [2], [3] –[5]. In [6], it focused on single-user precoding designs and adopted partially-connected structure to reduce the power consumption of phase shifters. In [7], a multiuser case with an imperfect CSI scenario was considered. It first estimated the channels, and then used the estimations to design hybrid precoders and combiners, but a high complexity is required. Particularly, in [2], an orthogonal matching pursuit (OMP)based algorithm was proposed for hybrid precoding and combining design.