Optimal Path Construction with Decode and Forward Relays in mmWave Backhaul Networks

In this paper, we consider the problem of constructing paths using decode and forward (DF) relays for millimeter wave (mmWave) backhaul communications in urban environments. Due to the large number of obstacles in urban environments, line-of-sight (LoS) wireless links, which are necessary for backhaul communication, often do not exist between small-cell base stations. To address this, some earlier works proposed creating multi-hop paths that use mmWave relay nodes with LoS communication between every pair of consecutive nodes to form logical links between base stations. We present algorithms, based on a novel widest-path formulation of the problem, for selecting decode and forward relay node locations in such paths. Our main algorithm is the first polynomial-time algorithm that constructs a relay path with a throughput that is proven to be the maximum possible. We also present variations of this algorithm for constrained problems in which: 1) each possible relay location can host only one relay node, and 2) minimizing the number of hops in the relay path is also an objective. For all of the proposed algorithms, the achievable throughput and numbers of relays are evaluated through simulation based on a 3-D model of a section of downtown Atlanta. The results show that, over a large number of random cases, our algorithm can always find paths with very high throughput using a small number of relays. We also compare and contrast the results with our earlier work that studied the use of amplify-and-forward (AF) relays for the same scenario.