Research Of Time Reversal Based Physical Layer Security Techniques In Frequency Selective Channels

As a novel solution to enhancing wireless communication security, physical layer security aims at promoting the secure performance of communication systems by exploiting physical layer properties and has broad research and application prospects. Physical layer security techniques in frequency selective channels have captured reserchers' academic interests owing to the exponential development of broadband and ultra-wideband communications. Time Reversal(TR) not only mitigates the undesiable effects of multi-path propogation, but also fully harvests energy from the surrounding environment and adds them coherently in the intended receivers, thus focusing signals temperally and spatially and becoming one of the effective techniques to achieve physical layer security in frequency selective channels. In this thesis, TR based physical layer security techniques research in frequency selective channels is focused on. The main work is organized as follows:Firstly, TR based wireless communication transmission sytem model in both time and frequency domains are presented in detail. Four properties of TR are analyzed involving temporal compression, spatial focusing, channel hardening and multi-path diversity.Secondly, TR based physical layer security are investigated in single-carrier communication systems. When the channel state information(CSI) of eavesdroppers is unknown to transmitter, artificial noise is proposed to combine with the classical TR pre-filter in frequency selective channels and simulation results show that secure performance of the proposed scheme is superior to the classical TR pre-filter. When the CSI of eavesdroppers is known to transmitter, an optimal secrecy capacity achievable TR pre-filter is proposed and simulation resultes show that secure performance of the proposed scheme outperforms ‘Nulling Eve' pre-filter. Furthermore, an unequal protection wireless broadcast communication system is proposed by using aforementioned TR pre-filters with different secure ability.Finally, TR based physical layer security are explored in multi-carrier communication systems. System models of TR Orthogonal Frequency Division Multiplexing(OFDM) in both time and frequency domains are introduced and secure performance of TR precoding scheme in Multiple-input Single-output(MISO) OFDM syetems is analyzed compared with maximum ratio transmission precoding scheme. MISO TR OFDM systems with artificial noise are proposed at last. Simulation results show TR can improve secure performance in multi-carrier communication systems.