Transmission Techniques and Resource Allocation in Coordinated Multi-Point Systems

This thesis is focused on the design of transmission schemes and resource allocation algorithms for both downlink and uplink of coordinated multi-point (CoMP) systems. In current cellular systems, which employ a full frequency reuse of the spectral resource, interference has been recognized as one of the most limiting factor: in particular, user equipments (UEs) close to the cell edge typically suffer strong inter-cell interference (ICI). This interference, which is due to the transmission of neighbouring base stations (BSs) in the downlink and UEs in neighbouring cells in the uplink, limits the fairness across the UEs and, in general, the overall system performance. It has been shown that BS coordination is able to dramatically improve the system performance with respect to non-cooperative schemes by limiting the impact of ICI. Coordination is obtained by sharing among the BSs channel state information (CSI), data to be sent to the UEs in the downlink and signals received by the BSs in the uplink. However, BS coordination also poses several new challenges. First of all, the backhaul may not be able to support the sharing among the BSs of all the data to be sent to the UEs or signals received by BSs. Moreover, obtaining reliable CSI at the BSs for all the UEs in the network may be an issue because of a) limited bandwidth available for the feedback channel in frequency division duplex (FDD) systems and b) noise on channel estimation in time division duplex (TDD) systems. Thereby, it is crucial to develop practical algorithms that deal efficiently with both of these issues in order to achieve, in a realistic scenario, the performance gain enabled by cooperation. This thesis is basically divided into two parts. In the first part we focus on downlink CoMP systems and we provide four main contributions. First, we propose a novel transmission scheme based on linear precoding and quantization of quadrature amplitude modulation (QAM) constellations. With the aim of reducing the backhaul occupation, a set of cooperative BSs transmit toward the UE a quantized version of the QAM symbol by using a joint precoder, while the serving BS transmits a quantization error symbol. In order to avoid modifications of the receiver, we impose that the combination of the two symbols through the channels yields at the UE the original QAM symbol. Numerical results show that this approach ensures a network spectral efficiency close to a theoretical limit obtained by using the more complex Slepian-Wolf encoding. Secondly, we develop a greedy dynamic algorithm to jointly organize BSs in clusters and to schedule UEs in each cluster. A considerable gain is achieved by the proposed approach with respect to both non-cooperative schemes and static clustering. Thirdly, we focus on the feedback design in FDD systems and we consider a practical strategy where each UE quantizes the CSI by using codebooks designed for a single-cell scenario. We propose two algorithms which allow the UE to a) select a subset of preferred BSs and b) optimize the number of feedback bits allocated to the quantization of each CSI. In particular, more feedback bits are sent to BSs with stronger signals. Finally, we adapt our scheme based on QAM quantization to a different scenario where a set of relays assists the transmission of a BS toward a UE. In such a case, the proposed joint precoding is employed by the relays to serve the UE, whereas the links connecting the relays to the BS represent the backhaul infrastructure. In the second part of the thesis, we consider the uplink of CoMP systems by assuming that UEs transmit toward BSs by using single carrier frequency division multiple access (SC-FDMA), which is the modulation employed in the uplink of Long Term Evolution (LTE). In this scenario, we propose a new scheduler of the signals exchanged among BSs on the backhaul that allows each BS to share only a subset of the subcarriers of each SC-FDMA block.