Low RF-Complexity Technologies for 5G Millimeter-Wave MIMO Systems with Large Antenna Arrays

Millimeter-wave (mmWave) MIMO with large antenna array has attracted considerable interests from acad emic and industry, as it can simultaneously provide wider bandwi dth and higher spectrum efficiency. However, with hundreds of antennas employed at mmWave frequencies, the number of radi o frequency (RF) chains required by mmWave MIMO is also huge, leading to unaffordable hardware cost and energy consumpti on in practice. In this paper, we review low RF-complexity technologies to solve this bottleneck in mmWave MIMO systems. We first describe the evolution of low RF-complexity technologies f rom cellular frequencies to mmWave frequencies. Then, we discu ss two attractive low RF-complexity technologies for mmWave MIMO systems in detail, i.e., hybrid precoding and beamspac e MIMO, including their fundamental principles, applicatio n advantages, and design challenges. We compare the performanc e of these two low RF-complexity technologies to draw some insig hts about how they should be employed in practical mmWave MIMO systems. Finally, we conclude this paper by highlighting th e potential opportunities in this emerging research directi on.