Hybrid analog and digital beamforming is a promising candidate for
large-scale mmWave MIMO systems because of its ability to significantly reduce
the hardware complexity of the conventional fully-digital beamforming schemes
while being capable of approaching the performance of fully-digital schemes.
Most of the prior work on hybrid beamforming considers narrowband channels.
However, broadband systems such as mmWave systems are frequency-selective. In
broadband systems, it is desirable to design common analog beamformer for the
entire band while employing different digital beamformers in different
frequency sub-bands. This paper considers hybrid beamforming design for systems
with OFDM modulation. First, for a SU-MIMO system where the hybrid beamforming
architecture is employed at both transmitter and receiver, we show that hybrid
beamforming with a small number of RF chains can asymptotically approach the
performance of fully-digital beamforming for a sufficiently large number of
transceiver antennas due to the sparse nature of the mmWave channels. For
systems with a practical number of antennas, we then propose a unified
heuristic design for two different hybrid beamforming structures, the
fully-connected and the partially-connected structures, to maximize the overall
spectral efficiency of a mmWave MIMO system. Numerical results are provided to
show that the proposed algorithm outperforms the existing hybrid beamforming
methods and for the fully-connected architecture the proposed algorithm can
achieve spectral efficiency very close to that of the optimal fully-digital
beamforming but with much fewer RF chains. Second, for the MU-MISO case, we
propose a heuristic hybrid percoding design to maximize the weighted sum rate
in the downlink and show numerically that the proposed algorithm with practical
number of RF chains can already approach the performance of fully-digital
beamforming.

本文研究毫米波系统中具有 OFDM 调制的杂合波束成形技术，提出一种全新的并行设计算法，旨在实现具有最大的频谱效率，同时减小硬件复杂度并接近全数字波束成形方案的性能。