Two-Step Hybrid Multiuser Equalizer for Sub-Connected mmWave Massive MIMO SC-FDMA Systems

Most of the works consider hybrid fully connected architectures to overcome the constraints of millimeter wave massive MIMO systems. However, these schemes require a one-to-one connection between the RF chains and antennas. In this paper we propose a two-step broadband multiuser equalizer for hybrid sub-connected architectures, which is a more realistic approach for practical systems. The low-complexity user-terminals employ only analog precoders computed from the knowledge of the average angle of departure of each cluster, which is constant over the bandwidth. At the receiver side, we design a hybrid multi-user equalizer by minimizing the average bit-error-rate. A two-step approach is considered, where the analog part is constant over the iterations due to hardware constraints and the digital part is iterative. The analog part is also constant over all subcarriers while the digital part is computed on a per subcarrier basis. The proposed sub-connected based equalizer is compared with the fully connected counterpart. The results show that the performance of the proposed scheme is close to the fully connected one after just a few iterations performed at the digital domain.