Uplink Two-Phase Multi-User Detection for C-RAN with Massive MIMO Antenna Systems

Multi-User reception over massive Multiple-Input Multiple-Output (MU-MIMO) antenna systems is one of the key 5G technologies as it significantly improves 5G network spectral efficiency and capacity by supporting simultaneous transmission from multiple users on the same time and frequency resources in the uplink. Multi-user interference in this case is suppressed by using advanced beamforming techniques at the base station (BS) receiver which rely on channel state information (CSI). Such CSI is typically estimated with the help of orthogonal Demodulation Reference Signals (DM-RS) transmitted along with the data channel. As the number of simultaneously served uplink users increases, the number of orthogonal DM-RS signals (antenna ports) and, therefore, the DM-RS overhead also increase linearly. In this paper we propose a two-phase MIMO signal processing technique that reduces the DM-RS overhead by using non-orthogonal DM-RS uplink transmission. The proposed technique can be efficiently realized in a centralized radio access network (C-RAN) architecture with the first phase of signal processing implemented in the remote radio unit (RRU) and the second phase of signal processing in the centralized base-band unit (BBU) connected with each other by a fronthaul (FH) network.