  The cloud-radio access network (CRAN) is expected to be the core network
architecture for next generation mobile radio systems. In this paper, we
consider the downlink of a CRAN formed of one central processor (the cloud) and
several base-station (BS), where each BS is connected to the cloud via either a
wireless or capacity-limited wireline backhaul link. The paper addresses the
joint design of the hybrid backhaul links (i.e., designing the wireline and
wireless backhaul connections from the cloud to the BSs) and the access links
(i.e., determining the sparse beamforming solution from the BSs to the users).
The paper formulates the hybrid backhaul and access link design problem by
minimizing the total network power consumption. The paper solves the problem
using a two-stage heuristic algorithm. At one stage, the sparse beamforming
solution is found using a weighted mixed `1=`2 norm minimization approach; the
correlation matrix of the quantization noise of the wireline backhaul links is
computed using the classical rate-distortion theory. At the second stage, the
transmit powers of the wireless backhaul links are found by solving a power
minimization problem subject to quality-of-service constraints, based on the
principle of conservation of rate by utilizing the rates found in the first
stage. Simulation results suggest that the performance of the proposed
algorithm approaches the global optimum solution, especially at high
signal-to-interference-plus-noise ratio (SINR).
