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.
