Energy-Efficient Multi-Tap Single Carrier Hybrid Beamforming in Uplink Wideband Millimeter Wave Systems

Millimeter wave systems are expected to provide several Gbps data rate with large bandwidth. To exploit wideband channels, mitigating inter-symbol interference (ISI) is a challenging problem. Most of the existing approaches employ multi-carrier modulation, in which a baseband beamformer is used per subcarrier, resulting in a significant number of computations. To make an energy-efficient system with a low computational complexity, we propose a single carrier hybrid beamforming scheme equipped with multi-tap analog beamformer that can mitiagte ISI in analog domain so that a simple baseband beamformer can be used. Our design aims to maximize the achievable sum rate of multi-user uplink system, and optimizes the beamforming matrix with a complex circle manifold-based conjugate gradient algorithm with a proposed initialization allowing fast convergence. Considering the baseband and circuit power consumption, the proposed scheme achieves a higher energy efficiency than conventional beamforming schemes in practical transmit power region. In addition, we find a sufficient condition for the numbers of antennas and analog beamformer taps that can completely eliminate both inter-symbol and inter-stream interferences.