Precoding for mmWave massive MIMO

Precoding plays an indispensable role in multiple-input multiple-output (MIMO) systems because it can compensate the path loss and eliminate the interference. While the fundamentals of precoding are the same regardless of carrier frequency, signal processing in millimeter-wave (mmWave) massive MIMO systems is subject to a set of nontrivial practical constraints, leading to different hardware architecture. This means that traditional precoding is no longer suitable, and we need to explore some new precoding schemes for mmWave massive MIMO systems.In this chapter, we first provide a brief introduction of the characteristics of the mmWave channel. After that, we review the traditional digital precoding and analog beamforming, and explain why they cannot be directly extended to mmWave massive MIMO systems. Then, we investigate a novel precoding scheme called hybrid analog and digital precoding. The key idea of hybrid precoding is to divide the conventional digital precoder into a small-size digital precoder (realized by a small number of radio frequency (RF) chains) to cancel interference and a large-size analog beamformer (realized by a large number of analog phase shifters) to increase the antenna array gain. Thanks to the low-rank characteristics of the mmWave channel in the spatial domain, a small-size digital precoder is enough to achieve the spatial multiplexing gain, making hybrid precoding enjoy the satisfying sum-rate performance with only a small number of RF chains. Finally, we summarize this chapter by comparing hybrid precoding with traditional digital precoding and analog beamforming, and highlight some other potential precoding schemes for mmWave massive MIMO systems.