Low Complexity Hybrid Precoding Design for Sub-Connected Massive MIMO Systems

Hybrid analog and digital precoding architectures facilitate the practical implementation of millimeter wave (mmW) massive multiple-input multiple-output (mmW-mMIMO) systems by reducing the number of employed radio frequency chains. The spectral efficiency (SE) optimization problems of these systems are non-convex and NP-hard due to the joint optimization between the analog and digital precoding and the constant modulus constraints required by the analog phase shifters. To address this problem, we propose a decoupled two-stage design where in the first stage, a closed approximation of the effective channel is proposed, hence the SE maximization problem is recast as a effective channel gain maximization problem, then the analog precoding matrices are determined, which are taken into account in the second stage to design the digital precoding matrix to maximize the system's SE. Simulation results are provided and validate the effectiveness of our proposed hybrid precoding schemes.