Constant modulus hybrid beamforming for multi-user systems in the presence of blockers

International audienceHybrid beamforming (HBF) solutions, involving both analog and digital stages, are considered as key enablers for next generation of small cell networks wireless communications. They provide interference rejection and spatial multiplexing features for the base stations with fewer radio frequency (RF) chains and analog-to-digital converters (ADCs) compared to the optimal full digital beamforming. However jointly designing the digital beamforming matrix and the analog beamforming matrix with constant modulus entries makes the optimal signal-to-interference-plus-noise ratio (SINR) solution complex to find. In this paper we propose a simple solution and investigate its SINR loss as a function of angle of arrivals of blockers. The solution consists of setting the analog weights phases equal to those of a full digital beamformer and keeping the modulus of the weights constant, while the complementary digital part is designed with respect to the effective channel matrix. This algorithm is applied over different full digital algorithms, in a medium signal-to-noise ratio (SNR) and high blocker levels. We show that despite that the analog stage introduces losses, the digital stage is still able to compensate them almost to the performance of the optimal full digital beamforming, and so with minor difference between algorithms used in analog stage