Hybrid Beamforming in Frequency Selective Massive MIMO Systems: A Single-Carrier or a Multicarrier Problem?

In this paper, we propose novel hybrid analog and digital beamforming techniques for the uplink of massive multiple-input multiple output (MIMO) systems under frequency-selective and rich scattering propagation conditions. Considering an L-tap wireless channel, we first derive the closed-forms for single-carrier L-tap and 1-tap RF beamformers. Then, we demonstrate that the proposed beamformers significantly reduce the root mean square (RMS) delay spread of the effective channel observed at the baseband. We also provide the analytical expressions for the asymptotic sum-rate and illustrate that the performance of the proposed beamformers are comparable to that of digital matched-filtering (MF). Moreover, we evaluate a hybrid beamforming technique using our proposed single-carrier beamforming methods at the RF along with a multicarrier technique using zero-forcing (ZF) at the baseband. Furthermore, we examine the performance of this method for a subconnected architecture, and with the use of digital phase shifters. Our simulation results and analysis indicate that single-carrier RF beamforming with multicarrier baseband techniques is indeed a promising solution for frequency-selective, rich scattering massive MIMO scenarios. Index Terms Hybrid beamforming, massive MIMO, frequency-selective channels.