Distributed beamforming in cognitive multi-cell wireless systems by fast interference coordination

We consider the energy-efficient design of transmit beamforming in the downlink of a cognitive multi-cell wireless system. In this system, multiple secondary cells minimize their total transmit power while satisfying given SINR requirements for the secondary users. At the same time, the secondary base stations are not allowed to exceed aggregate interference temperature limits to the users of a primary system. We propose a distributed optimization framework to decompose this multi-cell problem into parallel single-cell beamforming subproblems. As a result, the cognitive cells independently compute their beamformers using only local channel information. Moreover, they coordinate their inter-cell interference as well as the interference temperature to the primary users by exchanging some signaling information through a backhaul. This coordination is achieved through a consensus-based optimization. In contrast to classical decomposition methods, our approach is based on the alternating direction method of multipliers. Therefore, it leverages the augmented Lagrangian technique to accelerate the interference coordination between the secondary cells, thanks to the quadratic penalty terms that are added in the objective function with the standard pricing terms. Numerical results are provided to verify the effectiveness of this distributed algorithm.