Energy Efficiency Optimization in Reconfigurable Intelligent Surface Aided Hybrid Multiuser mmWave MIMO Systems

The energy efficiency (EE) of the reconfigurable intelligent surface (RIS) aided multiuser (MU) millimeterwave (mmWave) multi-input multi-output (MIMO) downlink is maximized by jointly optimizing the transmit power and number of active radio frequency (RF) chains. The base band (BB) transmit precoder (TPC) of this system is derived first by using the zero-forcing (ZF) technique for a given RF TPC and combiner at the base station (BS) as well as the phase shift matrix at the RIS. Furthermore, the ergodic sum-rate of the system is derived assuming a large number of antennas at the BS and users, followed by formulating the EE maximization problem subject to specific constraints imposed on the transmit power and on the number of RF chains. Then a low complexity sequential method is proposed for solving the above optimization problem, thus determining both the optimal transmit power and the number of active RF chains. Finally, simulation results are presented for demonstrating the improved EE performance of an RIS-aided mmWave MIMO system for a limited number of RF chains and transmit power at the BS.