Phase-controlled null-space expansion for analog beamforming massive MIMO in high-mobility scenarios

Abstract This paper proposes a novel null-space expansion (NSE) scheme based on phase-only control to exploit the degrees of freedom of massive antenna elements even in analog beamformers. Downlink multi-user Multiple-Input Multiple-Output (MIMO) spatial multiplexing is performed through precoding in the baseband digital signal processing unit. In higher frequency bands, beamforming using array antennas is essential to compensate for distance attenuation. To balance implementation costs and MIMO spatial multiplexing, a hybrid architecture combining analog and digital processing has become mainstream. Meanwhile, in massive MIMO, which offers a large spatial degree of freedom, NSE has been proposed as an effective solution to mitigate the degradation of spatial multiplexing performance caused by propagation channel variations due to user mobility. However, its application has so far been limited to fully digital configurations. We newly apply NSE to the phase-only adaptive nulling (POAN) framework and clarify its effectiveness through computer simulations. Furthermore, since commercially available phase shifters are digitally controlled, we evaluate the impact of phase quantization error to confirm the practical feasibility of the proposed scheme.