  We study downlink of multiantenna cloud radio access networks (C-RANs) with
finite-capacity fronthaul links. The aim is to propose joint designs of
beamforming and remote radio head (RRH)-user association, subject to
constraints on users' quality-of-service, limited capacity of fronthaul links
and transmit power, to maximize the system energy efficiency. To cope with the
limited-capacity fronthaul we consider the problem of RRH-user association to
select a subset of users that can be served by each RRH. Moreover, different to
the conventional power consumption models, we take into account the dependence
of baseband signal processing power on the data rate, as well as the dynamics
of the efficiency of power amplifiers. The considered problem leads to a mixed
binary integer program (MBIP) which is difficult to solve. Our first
contribution is to derive a globally optimal solution for the considered
problem by customizing a discrete branch-reduce-and-bound (DBRB) approach.
Since the global optimization method requires a high computational effort, we
further propose two suboptimal solutions able to achieve the near optimal
performance but with much reduced complexity. To this end, we transform the
design problem into continuous (but inherently nonconvex) programs by two
approaches: penalty and \ell_{0}-approximation methods. These resulting
continuous nonconvex problems are then solved by the successive convex
approximation framework. Numerical results are provided to evaluate the
effectiveness of the proposed approaches.
