Energy-efficient hybrid beamforming in millimeter-wave communications using FDMA

This paper considers millimeter-wave access network systems using a hybrid beamforming structure, which is a combination of digital and analog beamformers. To multiplex mobile stations in the frequency domain along with the space and time domain improves a system capacity for the following reasons: it exploits frequency diversity, accommodates mobile stations that can only use a limited bandwidth in the same time slot, and increases uplink transmission power density. Therefore, we propose a hybrid beamforming method that provides high beamforming gain to more mobile stations multiplexed in the space and frequency domain than radio frequency (RF) chains of a base station by using multi-peak beams. Assuming a mobile station cannot receive a full bandwidth signal, we conduct downlink system-level simulations to which a millimeter-wave channel model proposed by MiWEBA is applied. The results demonstrate that the proposed method can provide almost the same throughput with 25% fewer RF chains.