  Millimeter-wave (mmWave) multiple-input multiple-out (MIMO) systems relying
on lens antenna arrays are capable of achieving a high antenna-gain at a
considerably reduced number of radio frequency (RF) chains via beam selection.
However, the traditional beam selection network suffers from significant
performance loss in wideband systems due to the effect of beam squint. In this
paper, we propose a phase shifter-aided beam selection network, which enables a
single RF chain to support multiple focused-energy beams, for mitigating the
beam squint in wideband mmWave MIMO systems. Based on this architecture, we
additionally design an efficient transmit precoder (TPC) for maximizing the
achievable sum-rate, which is composed of beam selection and beamspace
precoding. Specifically, we decouple the design problems of beamspace precoding
and beam selection by exploiting the fact that the beam selection matrix has a
limited number of candidates. For the beamspace precoding design, we propose a
successive interference cancellation (SIC)-based method, which decomposes the
associated optimization problem into a series of subproblems and solves them
successively. For the beam selection design, we propose an energy-max beam
selection method for avoiding the high complexity of exhaustive search, and
derive the number of required beams for striking an attractive trade-off
between the hardware cost and system performance. Our simulation results show
that the proposed beamspace precoding and beam selection methods achieve both a
higher sum-rate and a higher energy efficiency than its conventional
counterparts.
