Cooperative Relaying for Multi-User MIMO Wireless Backhaul Networks

With the increasing deployment of more short-range small cells using mmWave technologies, backhaul becomes a bottleneck and wireless backhaul is relatively attractive compared to wired backhaul due to lower cost and efficient operation. As such, cooperative relaying for multi-user multiple-input multiple-output (MU-MIMO) wireless backhaul networks is studied, in which the source sends independent messages to K destinations with the help of N relays where the relays operate in full- or half-duplex mode. Herein, we focus on a line-of-sight dominant environment in which each of the channel matrices between nodes becomes rank-deficient due to the lack of multi-paths. In such cases, the multiplexing gain from using multiple antennas is severely limited, and the capacity cannot be improved with the number of antennas. To overcome the rank deficiency, we develop a novel cooperative relaying scheme that utilizes relays as active reflectors. In the proposed scheme, the source distributes the streams over the relays as uniformly as possible, and the relays extract the streams of each destination based on their received signals and forward them to their corresponding destinations, while any inter-user and inter-relay interference is also properly nulled out. We analyze the achievable sum degrees of freedoms (DoFs) and sum rates with full-duplex and half-duplex relays, and derive an upper bound on the sum DoF, which is tight when the relays operate in full-duplex mode. The results demonstrate that our scheme offers significant improvement to system throughput in comparison with direct transmission without cooperative relaying.