  The cloud radio access network (C-RAN) concept, in which densely deployed
access points (APs) are empowered by cloud computing to cooperatively support
mobile users (MUs), to improve mobile data rates, has been recently proposed.
However, the high density of active ("on") APs results in severe interference
and also inefficient energy consumption. Moreover, the growing popularity of
highly interactive applications with stringent uplink (UL) requirements, e.g.
network gaming and real-time broadcasting by wireless users, means that the UL
transmission is becoming more crucial and requires special attention. Therefore
in this paper, we propose a joint downlink (DL) and UL MU-AP association and
beamforming design to coordinate interference in the C-RAN for energy
minimization, a problem which is shown to be NP hard. Due to the new
consideration of UL transmission, it is shown that the two state-of-the-art
approaches for finding computationally efficient solutions of joint MU-AP
association and beamforming considering only the DL, i.e., group-sparse
optimization and relaxed-integer programming, cannot be modified in a
straightforward way to solve our problem. Leveraging on the celebrated UL-DL
duality result, we show that by establishing a virtual DL transmission for the
original UL transmission, the joint DL and UL optimization problem can be
converted to an equivalent DL problem in C-RAN with two inter-related
subproblems for the original and virtual DL transmissions, respectively. Based
on this transformation, two efficient algorithms for joint DL and UL MU-AP
association and beamforming design are proposed, whose performances are
evaluated and compared with other benchmarking schemes through extensive
simulations.
