Scalability of line-of-sight massive MIMO mesh networks for wireless backhaul

This paper considers an extended wireless network with multi-antenna nodes in line-of-sight (LoS) propagation environment. Assuming that the number of antennas at each node can be scaled as some arbitrary function of the number of nodes, we study the scalability of this network, i.e., its ability to deliver non-zero rate to each source-destination pair. Since the rank of the LoS multiple-input multiple-output (MIMO) channel starts collapsing with the increasing separation between the transmitter and the receiver, we consider two competing transmission strategies: (i) long hop: each source-destination pair minimizes the number of hops by sacrificing multiplexing gain and ideally achieving full power gain over each hop, and (ii) short hop: each source-destination pair communicates through a series of short hops each achieving full multiplexing gain. By characterizing the number of antennas required to achieve scalability in both the cases, we show that the antenna requirement is significantly less for the short hop case. These results have key applications in the design of wireless backhaul for cellular networks, where the possibility of having massive MIMO links is becoming a reality due to the increasing maturity of higher transmission frequencies, e.g., 28 and 38 GHz.