Abstract: Hybrid beamforming (HBF) technology becomes one of the key technologies in the millimeter wave (mmWave) mobile backhaul systems, for its lower complexity and low power consumption compared to full digital beamforming (DBF). Two structures of HBF exist in the mmWave mobile backhaul system, namely, the fully connected structures (FCS) and partially connected structures (PCS). However, the existing methods cannot be applied to both structures. Moreover, the ideal phase shifter is considered in some current HBF methods, which is not realistic. In this paper, a HBF algorithm for both structures based on the discrete phase shifters is proposed in the mmWave mobile backhaul systems. By using the principle of alternating minimization, the optimization problem of HBF is decomposed into a DBF optimization problem and an analog beamforming (ABF) optimization problem. Then the least square (LS) method is enabled to solve the optimization model of DBF. In addition, the achievable data rate for both structures with closed-form expression which can be used to convert the optimization model into a single-stream beamforming optimization model with per antenna power constraint is derived. Therefore, the ABF is easily solved. Simulation results show that the performance of the proposed HBF method can approach the full DBF by using a lower resolution phase shifter.

Key words: millimeter wave backhaul networks, fully connected structure, partially connected structure, hybrid beamforming, limited phase shifters resolution