  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 (DF) and compress-and-forward (CF) 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 optimizing 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
(CGPs). 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 (GP), 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.
