Physical Layer Security Improvement in Cognitive Radio Networks

Abstract

Wireless communication created a revolution in our lives. The exponential increase in wireless devices and their usage led to the problem of growing spectrum scarcity. Cognitive radio (CR) technology provided a promising solution for the spectrum scarcity issues in wireless networks by allowing an unlicensed user called a secondary user (SU) to access the radio spectrum when it was not occupied by a licensed user called primary user (PU) without interfering with the primary user. To ensure that the PU was not kept interfering from SU, the underlay paradigm was used. On the other hand, security issues took of an increasingly important role for future 5G systems, and CR was no exception. Due to the open and dynamic nature of cognitive radio networks (CRNs), physical layer security paradigm was presented to secure wireless communication systems, but a limited attention was paid to securing CRNs in the physical layer. The main objective of this thesis is to analyze and improve the physical layer security in the CRNs considering both passive and active eavesdroppers. Two main approaches were proposed to achieve this goal. First, new relay selection schemes are proposed to improve the physical layer security in the CRNs. A trusted decode-and-forward (DF) relay is selected to help the SU transmission and improve the secrecy rate in the presence of multi eavesdroppers and multi-PUs. The secrecy rate of the SU is characterized under both its own transmit power constraint as well as a set of interference power constrained each imposed at one PU, in order to preserve its quality of service (QoS) constraints. Second, the problem of passive eavesdropper is considered where the channel state information (CSI) is unknown at the transmitter side. To secure communication over MISO CRNs in the presence of passive eavesdropper, three different beamforming schemes are proposed; namely, scaled beamforming (SB), projected beamforming (PB), and cognitive beamforming (CB). The performance of the proposed schemes is analyzed in terms of the achievable secrecy rate and the intercept probability and is simulated using MATLAB. The performance analysis and simulation results reveal that the proposed schemes improve secrecy rate greatly and provide a very low intercept probability.