Energy-Efficient Hybrid Precoding with Switches and Inverters for Millimeter-Wave MIMO Systems

This paper investigates low-complexity hybrid precoding for a millimeter-wave multi-user massive multiple-input-multiple-output (MIMO) downlink, where a base station simultaneously serves multiple single-antenna users over frequency-flat narrowband channels. The design seeks to improve the trade-off between achievable sum rate, power consumption, and computational cost using a switch-inverter (SI) architecture that replaces high-resolution phase shifters (PSs) with switches and inverters. Digital zero-forcing (ZF) precoding addresses inter-user interference through the effective channel, while a dynamic programming (DP)-based sequential procedure selects SI states for local analog beamforming. The procedure reuses partial complex sums and performs exact amplitude comparisons within a restricted candidate set, without a guarantee of global optimality. Energy efficiency (EE) is evaluated as the bandwidth-normalized achievable sum rate divided by the total consumed power. To improve this metric, the revised DP procedure is combined with antenna deactivation under prescribed analog beamforming-gain constraints. This gain-based design does not directly maximize the multi-user sum rate or EE. For the evaluated 128-antenna SI array, antenna deactivation yields an observed EE improvement of approximately 15% relative to the corresponding all-active DP configuration. Coupling between active and inactive antenna elements is considered, and a corresponding feeding-voltage adjustment is derived to maintain the beam pattern. Applicability to wideband transmission is discussed through a conditional analysis of local analog beamforming gain.