Hybrid precoding for millimeter wave massive MIMO with analog combining

The utilization of millimeter wave (mmWave) frequency has shown great potential in developing the next-generation wireless systems. Conventional fully digital precoding which needs one radio frequency (RF) chain per antenna is impractical in large-scale mmWave systems because of the high hardware complexity, power consumption and cost. In this paper, we propose a scheme with hybrid precoding at the base station (BS) and analog combining at each user under the limitation of RF chains. The analog precoding matrix and analog combining vectors match with the transmit and receive array response vectors of each user's strongest path, respectively. Then a low-dimensional baseband zero-forcing (ZF) precoding is preformed based on the equivalent channel seen from baseband to eliminate inter-user interference. This scheme requires only two scalar symbol feedback instead of explicit channel information for each user during one coherent period. We derive closed-form expression representing spectral efficiency of the proposed scheme and the conventional fully digital ZF precoding for comparison. It is concluded that under certain conditions, the proposed scheme can reach the performance of fully digital ZF precoding, which is considered as a virtually optimal linear precoding scheme in massive multiple-input multiple-output (MIMO) systems. Simulation results are provided to validate analytical results.