Multi-User Cooperative Base Station Systems With Joint Precoding and Beamforming

Interference among multiple base stations that coexist in the same location limits the capacity of wireless networks. In this paper, we propose a method to design a single/multi-stream multi-user multiple-input multiple-output (MIMO) cooperative downlink transmission scheme employing precoding and beamforming under both the per base station (BS) and total BS power constraints. The per BS and total BS power constraints are the constraints where the power for each BS and all BSs is limited to a particular value, respectively. The algorithm eliminates the interference and achieves symbol-error rate (SER) fairness among different users. To eliminate the interference, Tomlinson-Harashima precoding (THP) is used to cancel part of the interference while the transmit-receive antenna weights cancel the remaining part. An iterative method based on the uplink-downlink duality principle is used to generate the transmit-receive antenna weights. The algorithm provides an equal signal-to-interference-plus-noise-ratio (SINR) across all users and reduces its computational complexity by trading off the complexity for a slight performance degradation. The proposed methods are extended to the scenario when the receiver does not have complete channel state informations (CSIs). Lastly, we propose a new method of selecting the user precoding order, which has a much lower complexity than VBLAST ordering but with almost the same performance. The simulation results show that the proposed schemes considerably outperform existing nonlinear and linear cooperative transmission schemes in terms of SER performance and approach an interference free performance.