Multi-layer transmission and hybrid relaying for relay channels with multiple out-of-band relays

In this work, a relay channel is studied in which a source encoder communicates with a destination decoder through a number of out-of-band relays that are connected to the decoder through capacity-constrained digital backhaul links. This model is motivated by the uplink of cloud radio access networks. In this scenario, a novel transmission and relaying strategies are proposed in which multi-layer transmission is used, on the one hand, to adaptively leverage the different decoding capabilities of the relays and, on the other hand, to enable hybrid decode-and-forward and compress-and-forward relaying. The hybrid relaying strategy allows each relay to forward part of the decoded messages and a compressed version of the received signal to the decoder. The problem of optimising the power allocation across the layers and the compression test channels is formulated. Albeit non-convex, the derived problem is found to belong to the class of so-called complementary geometric programs. Using this observation, an iterative algorithm based on the homotopy method is proposed that achieves a stationary point of the original problem by solving a sequence of geometric programming, and thus convex, problems. Numerical results are provided that show the effectiveness of the proposed multi-layer hybrid scheme in achieving performance close to a theoretical (cutset) upper bound.