Interference management in cooperative multi-cell networks
In multi-cell networks where resources are aggressively reused, eliminating interference is the key factor to reduce the system energy consumption. This thesis proposes interference management techniques based on beamforming with different levels of cooperation amongst base stations (BSs). First, a multi-cell beamforming (MBF) technique is introduced to design beamformers as if geographically distributed BSs were a single BS. The aim is to minimise the total transmit power across the network while maintaining the required signal-to-interference-plus-noise ratio (SINR) for every user. An iterative algorithm is proposed to solve the optimisation problem of MBF. Since the MBF scheme requires the circulation of all users’ data amongst coordinating BSs, a user position aware (UPA) algorithm is developed for MBF to reduce the backhaul overhead by allocating each user to nearby BSs only. To completely avoid user data circulation, a semi definite programming (SDP) algorithm, named as coordinated beamforming (CBF), is introduced to jointly calculate beamformers for all coordinating BSs in a manner that each BS transmits to its local users only. Taking into account errors in channel estimations, robust beamforming designs are developed for CBF. Next, fast wireless backhaul protocols, i.e., Star and Ring, are proposed using network coding to enable links amongst coordinating BSs. The maximum achievable throughput of each protocol is analysed. The power consumption of the Ring protocol is characterised and used to compare and evaluate the performance of the proposed beamforming schemes. The deployments of MBF, UPA-MBF and CBF schemes require a central unit for a group of coordinating BSs as well as backhaul links amongst them. In fact, a central unit may not always be available, e.g., in femtocell and self-organising networks, while backhaul links may be limited. Hence, distributed beamforming (DBF) is proposed to independently design beamformers for the local users of each BS. In DBF, the combination of each BS’s total transmit power and its resulting interference power toward other BSs’ users is minimised while the required SINRs for its local users are maintained. SDP and iterative algorithms are introduced to solve the optimisation problem of DBF.
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