Flexible backhaul design with cooperative transmission in cellular interference networks

Interference is an important factor that limits the rates that can be achieved by mobile users in a cellular network. Interference management through cooperation has emerged as a major consideration for next-generation cellular networks. In this thesis, we focus on the downlink of a sectored hexagonal cellular network, under the assumption of local interference i.e., the interference at each user is only due to transmitters in neighboring sectors. We explore the potential degrees of freedom (DoF) gain in this network under constraints on the cooperation between base-stations. The constraints that we consider are the cooperation order M, and the average backhaul load B, which denote the maximum and the average number of transmitters, respectively, that jointly transmit any message. We first study the DoF gains in a scenario where mobile receivers can be associated to any neighboring cell but no cooperative transmission is allowed, and derive bounds on the maximum achievable per user DoF for orthogonal schemes. We then show that by combining cooperative transmission with flexible message assignment to the transmitters, it is possible to achieve a per user DoF strictly greater than that without cooperation. The proposed cooperative transmission scheme does not require extra backhaul capacity, as it uses a smart assignment of messages to transmitters to meet an average backhaul load constraint of one message per transmitter. The schemes presented are simple zero-forcing beamforming schemes that require linear precoding over a single time/frequency slot (one-shot). Similar schemes are proposed which achieve a per user DoF greater than half with a minimal increase in the backhaul load. These results are derived for networks with intra-cell interference and networks without intra-cell interference