Performance Analysis Of Physical Layer Security In Multiuser Communications

Physical layer (PHY) security is potential to provide "perfect secrecy" independent on the computational complexity of devices, thus has drawn increasing attention as one of the most important security technologies for future wireless communications. It has been extensively studied and well developed for the single user system. However, most practical systems have more than one users. Due to the existence of both inter-user interferences and information leakage, research of PHY security in multiuser systems are much more complicated. And many critical problems still need to be solved, such as the capacity analysis, the secure channel coding, the analysis and design of secure transmissions based on PHY security, and so on.This thesis mainly focuses on the secure transmission problem in multiuser systems, and aims to utilize artificial noise (AN), precoding, user selection to enhance the secrecy performance. Impacts of the CSI accuracy, the network load, the power allocation (PA), the eavesdropping side information and other key system parameters on the secrecy performance are also analyzed. The derived analytical results can provide simple and useful insights into the design of secure transmissions in practice.Main contributions of this thesis are summarized as follows.(1)In the AN assisted multiuser system with external eavesdroppers, the closed-form ergodic secrecy sum-rate is derived based on random matrix theory and large system analysis approach. The availability of closed-form expression greatly reduces the complexity of system design and analysis, and allows analytical insights to be obtained. Based on the closed-form expression, the optimal PA between information signals and the AN can be easily obtained numerically. A simple analytical result of the optimal PA is also derived in the high-SNR regime. Moreover, impacts of the CSI accuracy on the secrecy performance and the optimal PA are also evaluated. And the scale law of CSI feedback to maintain an upper-bounded secrecy rate loss is obtained.(2) In the multiuser downlink with independent confidential messages and limited CSI feedback, the closed-form ergodic secrecy sum-rate is derived based on RVQ theory and quantization cell approximation (QCA) approach. It is shown that the achievable secrecy rate in a limited feedback channel is upper bounded, and even reduces to zero at high SNR. To maintain a constant secrecy rate loss, the feedback bits should increase almost linearly with the SNR and the number of transmit antennas. The corresponding scale law is obtained in a closed form. Moreover, the minimum CSI feedback required for a positive secrecy rate, and the optimal transmit SNR and network load maximizing the secrecy rate are also studied. All these results provide useful insights into practical system design.(3) The secrecy rate balancing problem is first considered in the multiuser downlink with independent confidential messages and individual secrecy constraints of users. The purpose is to maximize the secrecy throughput or to minimize the total transmit power, on the premise that all minimum secrecy rates of users are simultaneously satisfied. Corresponding PA optimization models are first established, and the necessary conditions for their global optimum solutions are then derived. Then based on these necessary conditions, the global optimum solutions for ZF precoder are derived in closed forms, and simple iterative PA algorithms are designed for general precoders. The proposed algorithms can meet the individual secrecy requirements of users, and at the same time enhance the secrecy performance of the whole network, which is significant for both users and network operators.(4) For the single input and single output multiuser system with several curious users, the closed-form ergodic secrecy outage probability of the optimal user selection strategy is first derived based on order statistics theory. Then based on the closed-form expression, analytical insights are obtained to show the impacts of eavesdropping side information on the secrecy performance and the design of selection strategy. For the multi-input and single output multiuser system, the closed-form ergodic secrecy sum-rates for various numbers of selected users are derived based on RVQ theory and QCA method. These results can be used to calculate the optimal number of selected users for given system parameters. Based on this, a low-complexity AN assisted multi-mode transmission strategy is proposed to improve the average secrecy rate.In conclusion, this thesis studies secure transmissions based on PHY security in multiuser systems. Important technologies like AN, precoding, power control, and user selection are adopted to enhance the secrecy performance. And useful analytical insights are obtained by deriving closed-form expressions of the secrecy sum-rate or outage probability, and also by evaluating the impacts of PA, CSI accuracy, eavesdropping side information, network load and other key parameters on the secrecy performance. These results can provide low-complexity guidance on the design of channel estimation, resource allocation, user scheduling, etc., in the area of PHY security.