  Millimeter-wave (mmWave) MIMO with large antenna array has attracted
considerable interests from academic and industry communities, as it can
provide larger bandwidth and higher spectrum efficiency. However, with hundreds
of antennas, the number of radio frequency (RF) chains required by mmWave MIMO
is also huge, leading to unaffordable hardware cost and power consumption in
practice. In this paper, we investigate low RF-complexity technologies to solve
this bottleneck. We first review the evolution of low RF-complexity
technologies from microwave frequencies to mmWave frequencies. Then, we discuss
two promising low RF-complexity technologies for mmWave MIMO systems in detail,
i.e., phased array based hybrid precoding (PAHP) and lens array based hybrid
precoding (LAHP), including their principles, advantages, challenges, and
recent results. We compare the performance of these two technologies to draw
some insights about how they can be deployed in practice. Finally, we conclude
this paper and point out some future research directions in this area.
