ZF-Based Downlink Hybrid Precoding and Combining for Rate Balancing in mmWave Multiuser MIMO Systems

This paper proposes a new design strategy for hybrid precoding and combining in the downlink of millimeter wave (mmWave) multiuser multiple-input multiple-output (MU-MIMO) channels. When channel state information is available at the transmitter, the proposed scheme designs the analog precoder and combiners by iteratively factorizing the matrices for fully digital precoding and combining, respectively, using the alternating optimization technique. Then, the digital precoder and combiners are obtained through block diagonalization of effective MU-MIMO channels composed of the analog precoder, MU-MIMO channels, and analog combiners, in order to eliminate inter-user interferences. Moreover, focusing on rate balancing among users, we derive a new power allocation algorithm that exploits a modified gradient descent method. The proposed method iteratively adjusts the power allocated to each user in terms of maximizing the minimum user rate. Through numerical simulations, we verify the convergence of the proposed design procedure for hybrid precoding and combining. Moreover, it is shown that the proposed method outperforms the conventional hybrid precoding and combining methods for rate balancing as well as achieves the minimum user rate close to the performance upper bound.