Fronthaul Functional Split of IRC-Based Beamforming for Massive MIMO Systems

In centralized radio access network (C-RAN) architecture, the baseband unit (BBU) is connected to the remote radio unit (RRU) via a fronthaul (FH) interface. For massive MIMO systems in the 5G era, the traditional FH interface (e.g. CPRI) becomes a network bottleneck since the required FH capacity increases proportional to the number of antennas. New functional split between RRU and BBU is thereby considered where RRU carries out more processing than RF-related operation to make the FH traffic only scale based on the number of MIMO layers. This paper provides one alternative of such functional split in the uplink direction implementing interference rejection combining (IRC). The proposed approach reformulates IRC into a zero-forcing (ZF) process and decomposes the latter into two parts. The first part performs maximum ratio combining (MRC) which is much simpler than the full IRC and is therefore implemented in RRU. The second part handles the remaining calculations (such as matrix inversion) with high complexity and is implemented in BBU. The results show that the proposed functional split can deliver performances with both intra- and inter-cell interferences effectively mitigated, while guaranteeing low-complexity RRU implementation and moderate FH-traffic demands, which is proportional to the number of MIMO layers plus interferers.