Research On Precoding Techniques In Wireless Cooperative/Relay Communication Systems

The wireless cooperative/relay technique introduces a new type of spatial diversity in communication systems. In wirelss cooperative/relay communication systems, terminals share system resource with others to enhance the system reliability and capacity. The so-called "virtual antenna array" is constructed, so that the virtual multi-input-multi-output (MIMO) system model can be formulated, which can achieve high spectral efficiency. As one of the promoting link adaptation techniques, precoding attracts much attention recently in MIMO systems and has been widely investigated. Due to the similarity between cooperative systems and MIMO systems, precoding techniques can also be used in cooperative systems. The subject of this dissertation is the investigation of precoding techniques in wireless cooperative/relay systems. Precoding designs for the system reliability and the spectral efficiency are considered. The implementation complexity and the channel state information (CSI) requirement are also considered in the research.The existing precoding techniques in wireless cooperative/relay systems are firstly reviewed and discussed in this dissertation. According to the construction of antenna arrays, precoding in cooperative/relay systems can be summarized as three categories:1. the local spatial precoding with single relay node; 2. the distributed spatial precoding through multiple nodes; 3. the temporal precoding through successive transmitted signals at the source node. Each category can be further divided into several sub-categories according to the implementation scenarios. The three categories are all investigated in this dissertation.The second part of this dissertation deals with the local spatial precoding. Firstly, in the linear relaying systems with single relay node, we focus on the zero-forcing (ZF) relaying system. A spatial channel pairing or mapping matrix is proposed between the backward ZF filter and the forward ZF filter to reduce the relaying noise. The spatial channel pairing matrix is suitable for systems with single source node, while the spatial channel mapping matrix is suitable for systems with single destination node. Since the spatial channel pairing or mapping matrix improves the received signal-to-interference-and-noise ratio of each data stream, the system reliability and the spectral efficiency are both enhanced. Secondly, a unified framework is presented, including all kinds of linear relaying systems with single-source single-relay single-destination. Based on the unified framework, the complete design of spatial channel mapping matrix is presented, i.e. the spatial channel mapping matrix is presented at the relay node between the backward filter and the forward filter to reduce the relaying noise or the local noise at the destination node. Analysis and simulation results in several systems with single relay node demonstrate the advantage of the spatial channel mapping matrix design.The distributed spatial precoding through multiple nodes is investigated in the third part of this dissertation. According to the transmitting signal of the source node, there are two modes of precoding in multi-relay systems, i.e. the broadcasting mode and the unicasting mode. Firstly, precoding schemes in the broadcasting mode are studied, and the QR decomposition (QRD) based scheme is chosen since it performs well on the system capacity. Two methods for improving the QRD based scheme are presented:1) permutation optimization; 2) distributed spatial precding (power allocation among relay nodes). The first method utilizes the different decomposition results of matrix QRD with column permutation. A modified QRD algorithm is presented to decompose a matrix with column permutation optimization. The permutation optimization with the modified QRD algorithm obviously improves the system capacity with slight complexity increase. The second method utilizes distributed precoding through multiple relay nodes. With the assumption that the powers of all relay nodes are constrained as a whole, the distributed precoder design is equivalent to an optimization problem on a spherical surface. A novel particle swarm optimization (PSO) algorithm on the spherical surface is presented to enhance the system capacity with acceptable complexity. Additionally, the unicasting mode is studied. Several unicasting schemes are proposed and analyzed. Application scenarios and capacity performance of the broadcasting and the unicasting modes are compared. With a typical unicasting scheme, the exact expression of the received signal-to-noise ratio (SNR) is derived. A relay node selection strategy based on the unicasting mode is presented, where the CSI feedback is simplified.The last part of this dissertation discusses the temporal precoding in cooperative/relay systems. Temporal successive transmitted symbols (vectors) at the source node in the broadcasting phase and multi-accessing phase are treated as a virtual antenna array. In multi-relay systems with single-antenna nodes, the temporal precoding at the source node and the distributed spatial precoding through relay nodes are jointly designed to maximize the system capacity. In cooperative/relay systems with multi-antenna nodes, the CSI of all links in the system are required at the source node. A CSI compression and feedback scheme basing on the ZF relaying protocol is presented to support the temporal precoding at the source node, where the equivalent channel and the relaying noise information related to each relay node are compressed into two positive real coefficients. According to the analysis of the singular values of the normalized equivalent channel matrix, the upper bound of the number of data streams supported by the temporal precoding is derived. Various factors affecting the performance of the temporal precoding are also analyzed.