Hybrid beamforming has drawn attention with the increasing concern on high frequency band communication and the appearance of flexible structures of base stations. This paper considers the practical transmitter structure that each antenna is only connected to a unique RF chain and optimizes the analog and digital beamforming matrices to maximize the achievable rate with transmit power constraint. Following alternating optimization principle, closed-form relationships between analog and digital precoders are obtained when both amplitude and phase or only phase can be adjusted to form analog beams. Simulation results demonstrate the advantage of our proposed methods over the existing beam steering method in terms of achievable rate with different scale antenna array. This reveals that the method can be applied to both low and high frequency band communications. When increasing the number of propagation paths, contrast to the traditional beam steering method, the performance of proposed methods becomes better while the complexity almost keeps at an acceptable constant level.
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