Massive-MIMO Enabled FDD Wireless Backhaul Small-Cell Relay Networks: AF Protocol Based Designs With Low Channel Estimation and Feedback Complexity

To combat the explosive growth of mobile data traffic, massive-multi-input multi-output (MIMO)-enabled wireless backhaul small-cell (SC) network (WBSN) has been investigated in recent years. One of the key challenges for the WBSN is to reduce the backhaul processing delay. However, a large portion of current research has mostly been focused on the decode-and-forward (DF) protocol that inherently contains a large transmission latency due to complicated decoding process at each SC base station (SBS). In this paper, we investigate the amplify-and-forward (AF)-based WBSN. Unlike the DF schemes, the AF-WBSN is more attractive due to the cost effectiveness and lower computational complexity. In frequency division duplex (I-DD) systems, however, such advantages come at the expense of high channel estimation complexity, because the macro-cell base station (MBS) in the AF-WBSN is required to know the global channel state information, which causes a significant feedback delay as well as overhead. The channel estimation becomes even more challenging when it comes to the massive MIMO systems where the MBS is equipped with a large excess of antennas. To tackle the problem, we propose a novel transceiver design for the FDD massive-MIMO enabled AF-WBSN whose channel estimation and feedback complexity reduce to the level of its DF counterpart with a low latency by leveraging the antenna correlation at the MBS and the mean squared error decomposition properties at the SBSs. Finally, numerical results verify the efficiency of our proposed designs.