Analog-only beamforming for near-field multiuser MIMO communications

For near-field multiuser communications based on hybrid beamforming (HBF) architectures, high-quality effective channel estimation is required to obtain the channel state information (CSI) for the design of the digital beamformer. To simplify the system reconfiguration and eliminate the pilot overhead required by effective channel estimation, we consider an analog-only beamforming (AoBF) architecture in this study. AoBF is designed to maximize the sum rate, it is transformed into a problem maximizing the power transmitted to the target user equipment (UE) and meanwhile minimizing the power leaked to the other UEs. To solve this problem, we use beam focusing and beam nulling and propose two AoBF schemes based on the majorization–minimization algorithm. First, the AoBF scheme based on perfect CSI is proposed, with the focus on beamforming performance and regardless of CSI acquisition. Then, the AoBF scheme based on imperfect CSI is proposed, where low-dimensional imperfect CSI is obtained by beam sweeping based on a near-field codebook. Simulation results demonstrate that the two AoBF schemes can approach HBF schemes in terms of the sum rate and outperform HBF schemes in terms of energy efficiency.