Research On Nonlinear Precoding Techniques

Nowadays, people have more and more requirements on the transmission rate and reliability in wireless communication systems while the complicated environments and limited bandwidth restrict the development. Under this condition, MIMO and relay techniques have drawn more and more attention due to their potential to enhance spectral efficiency and link reliability. Recently, precoding technique has been introduced in MIMO and cooperative relay systems as a means for further performance enhancement. Precoding is an effective signal processing technique applied at the transmitter to eliminate spatial signal interference and reduce the complexity of signal detection at the receiver. The thesis focuses on the research of nonlinear precoding in MIMO and MIMO relay systems. We investigate the robust nonlinear transceiver design algorithms. Meanwhile, since security and energy efficiency have also become critical issues in wireless communication field, we further develop nonlinear precoding technique for secure communications to guarantee physical layer security and full-duplex (FD) wireless powered relay systems with self-energy recycling.Firstly, we introduce the fundamentals of MIMO precoding technique. We present the linear precoding algorithms based on zero-forcing (ZF), minimum-mean-square-error (MMSE) criteria and singular value decomposition (SVD) of the channels. Furthermore, block diagonalization (BD) algorithm for multi-user (MU) MIMO systems is also discussed. Compared with linear precoding, nonlinear precoding provides an alternative approach that offers the potential for performance im-provements. For nonlinear precoding, the basic concept of dirty paper coding (DPC) is presented as the theoretical basis. We then give a detailed introduction of the Tomlinson-Harashima precod-ing (THP) and vector perturbation precoding (VP), respectively. This thesis mainly focuses on the research of nonlinear precoding technique.Then, we study the nonlinear transceiver design. A robust nonlinear THP transceiver algo-rithm for MIMO amplify-and-forward (AF) relay systems in the presence of imperfect channel state information (CSI) is proposed. Specifically, we consider a novel successive-interference-cancellation (SIC) strategy for this system based on a structure with multiple interference cancel- lation branches. For each branch, the nonlinear transceiver design consists of a TH precoder at the source along with a linear precoder used at the relay and a linear MMSE receiver at the destination. We employ a diagonalization method along with some attributes of matrix-monotone functions to obtain the optimal relay and source precoders. The solution can be computed by using an iterative method via the Karush-Kuhn-Tucker (KKT) conditions. An appropriate selection rule is developed to choose the nonlinear transceiver corresponding to the best branch for data transmission. Sub-optimal ordering schemes are developed to select a subset from the optimal ordering scheme set in a low-complexity way. Furthermore, we extend the idea to the design of VP precoding using lattice reduction (LR) based on the multi-branch (MB) strategy for MU-MIMO systems. Simulation re-sults demonstrate that the proposed scheme outperforms existing transceiver designs with perfect and imperfect CSI.Next, we investigates the problem of physical layer security for MIMO systems operating in the presence of a passive eavesdropper. The THP design for secure communications in broadcast MIMO systems is proposed. We focus on optimizing the nonlinear transceiver to guarantee a certain Quality-of-Service (QoS) level for the intended receiver in terms of MSE. The scheme allocates the transmit power in order to achieve the target MSE for the intended receiver, and then uses the remaining available power to transmit artificial noise (AN) to degrade the eavesdropper’s channel. We then consider the robust nonlinear transceiver for secure communications with AN assisted MIMO Relay. The design problem can be reformulated as a two-level optimization, where the outer problem aims to optimize the source precoder as a function of the relay precoder, while the inner problem at the relay aims to jointly optimize the relay precoder as well as the power allocation between the information-bearing and the AN signals. For the inner problem, a bisection method is adopted to maximize the AN power level while satisfying the MSE requirement for the information-bearing signal. Regarding the outer problem, closed-form solutions for the precoders can be obtained by an iterative method based on KKT conditions. Simulation results are provided to illustrate the secrecy performance and advantages of the proposed nonlinear transceiver design with AN and THP.Finally, we address the issue of wireless powered communication networks (WPCN). We propose a robust nonlinear transceiver design in FD wireless powered relay systems with self-energy recycling. A novel two-phase transmission protocol is used for the wireless powered relay systems, in which the relay operates in FD mode with simultaneous information transmission and energy harvesting. We first consider the problem of minimizing the MSE of the system subject to the source power constraint and relay’s energy harvesting (EH) constraint. An iterative algorithm inspired by the alternating optimization is developed to solve the non-convex problem. Given the TH precoder and the relay precoder, we derive the closed-form optimum solution of the linear receiver. Given the TH precoder, the relay precoder can be obtained by convex optimization. Given the relay precoder, we can transform the non-convex TH precoder design problem into a difference of convex (DC) programming and propose a constrained concave convex procedure (CCCP) based iterative scheme to achieve a local optimum. We then consider the problem of minimizing the transmit power at the source subject to the MSE constraint and the relay’s EH constraint. Specifically, we propose an efficient initial feasibility search scheme. Simulation results are presented to demonstrate the effectiveness of the proposed robust design.