Hybrid Beamforming for Relay-Aided mmWave Backhaul links

Massive multiple-input multiple-output (MIMO) and millimeter-wave (mmWave) technologies have emerged as a promising solution to enhance the backhaul wireless link in 5G Heterogeneous networks (HetNets). These mmWave backhaul links, however, are very susceptible to a significant path loss due to the blockage of the line-ofsight and massive antenna arrays may not be sufficient to alleviate such losses. To this end, relays are usually deployed to provide alternative routes that help boost links with high path loss. In this paper, therefore, we consider using relay base stations (RBS) in mmWave backhaul links between small cell base stations (SBS) and a macro-cell base station (MBS). It is assumed that the SBSs, the RBSs, and the MBS are all equipped with massive antenna arrays employing hybrid analog and digital beamforming. The analog beamformers are based on the selection of fixed multi-beams using a constrained eigenbeamforming scheme while the digital beamformers are based on the maximum ratio transmission and maximum ratio combining (MRT/MRC) schemes that maximize the transmit and receive SINRs of the effective channels created by the actual channel and the analog beamformer. The performance evaluation in terms of the beampatterns and the ergodic channel capacity shows that the proposed HBF scheme achieves near-optimal performance with only 4 RF chains and requires considerably less computational complexity