Performance Analysis of Zero-Forcing Beamforming Strategies for the Uplink of an MU-MIMO System with Multi-Antenna Users

We conduct a comprehensive evaluation of the performance of the uplink of OFDMA-based MU-MIMO systems with multi-antenna users, for three Zero-Forcing (ZF) Beamforming (BF) strategies: Coordinated-Transmit-Receive-1 (CTR1), where only the strongest data stream is enabled per scheduled user; Block Diagonalization (BD), where all possible streams are enabled per scheduled user; Coordinated-Transmit-Receive-Flexible (CTRF), which allows a flexible stream allocation per user. The Radio Resource Management (RRM) of the uplink of all OFDMA-based systems must be done over an entire Time-Slot (TS) due to power management, making it challenging. To enable this study, we propose an efficient heuristic based on greedy-up searches for stream-sets that provides feasible solutions. It operates over the TS and considers fairness, practical Modulation and Coding Schemes and all RRM processes. The results show that, for Rural Macro scenarios, BD (resp. CTR1) could replace the more complex CTRF if the number of users is small (resp. large), while for Urban Macro scenarios, CTR1 emerges as an alternative to CTRF due to its similar performance. We also show that the system parameters can substantially impact the performance of the ZF strategies and that BD performance is more impaired with a simpler power management scheme than CTR1 and CTRF.