Beamforming optimization with hybrid association in C-RANs under a limited backhaul

With the densification of heterogeneous networks in terms of both users and base stations (BS) being identified as an integral part of future communication systems, the problem of association needs to be reconsidered since conventional schemes which are simply based on the received signal strength lead to small cells being idle. Coordinated multipoint (CoMP) techniques show great potential in improving the network performance, however, the gain of such cooperative solutions is bottlenecked by the backhaul. We present an approach for determining the beamforming in the downlink (DL) for all users, while making use of a hybrid association strategy that depends on both power and distance. Moreover, we make use of CoMP transmission as a tool for interference management and performance improvement, this can be especially beneficial for cell edge users suffering from high interference which is common in heterogeneous networks. An optimization problem is formulated that provides flexibility in declaring individual transmit power and backhaul constraints per BS and maximizes the weighted minimum user rate in order to promote fairness. As the optimization problem is nonconvex we employ semi-definite-relaxation to obtain a sub-optimal solution. Using simulations we evaluate the performance of the proposed strategy and the gain achieved in comparison to conventional schemes. A suggestion is made for the optimal cooperation level based on multiple performance criteria.