Adaptive Hybrid Beamforming in Millimeter Wave MIMO System

In millimeter-wave (mmWave) MIMO systems, a hybrid beamformer is widely utilized to improve spectral efficiency. In this paper, we propose a hybrid beamforming strategy in downlink without knowing the user's DOA/DOD angles. Because of the digital precoder's orthogonal feature, we exploit the phases of an equivalent precoder defined by the digital precoder and the optimal digital precoder to calculate the analog precoding matrix without the requirement of estimation of direction-of-arrival/departure (DOA/DOD). Furthermore, analog and digital precoders are updated by alternating minimization methods to boost the spectral efficiency reaching asymptotically fully digital precoders. Simulation results demonstrate that the proposed algorithm performs better than the traditional hybrid precoding algorithm.