  Recent studies on cloud-radio access networks assume either signal-level or
scheduling-level coordination. This paper considers a hybrid coordinated scheme
as a means to benefit from both policies. Consider the downlink of a
multi-cloud radio access network, where each cloud is connected to several
base-stations (BSs) via high capacity links, and, therefore, allows for joint
signal processing within the cloud transmission. Across the multiple clouds,
however, only scheduling-level coordination is permitted, as low levels of
backhaul communication are feasible. The frame structure of every BS is
composed of various time/frequency blocks, called power-zones (PZs), which are
maintained at a fixed power level. The paper addresses the problem of
maximizing a network-wide utility by associating users to clouds and scheduling
them to the PZs, under the practical constraints that each user is scheduled to
a single cloud at most, but possibly to many BSs within the cloud, and can be
served by one or more distinct PZs within the BSs' frame. The paper solves the
problem using graph theory techniques by constructing the conflict graph. The
considered scheduling problem is, then, shown to be equivalent to a
maximum-weight independent set problem in the constructed graph, which can be
solved using efficient techniques. The paper then proposes solving the problem
using both optimal and heuristic algorithms that can be implemented in a
distributed fashion across the network. The proposed distributed algorithms
rely on the well-chosen structure of the constructed conflict graph utilized to
solve the maximum-weight independent set problem. Simulation results suggest
that the proposed optimal and heuristic hybrid scheduling strategies provide
appreciable gain as compared to the scheduling-level coordinated networks, with
a negligible degradation to signal-level coordination.
