| In this thesis, the security issues in the physical layer of wireless communication systems are investigated. One of the most challenging problems in securing wireless communications is that traditional cryptography based authentication information is always computable regardless of the encryption key size. The broadcast nature of wireless communication further exposes the messages to eavesdroppers. To address these security issues, the inherent characteristics of the wireless multi-path channels are explored. The randomness of wireless channel signatures is exploited to differentiate the wireless terminals. The reciprocity feature of the wireless channel enables legitimate users to transmit encrypted information through the wireless medium without exchanging the encryption key. In the proposed method, three different features are employed to discriminate wireless users. By combining the phase and amplitude information, a superior authentication performance can be achieved. The diversity technology was introduced to further improve the authentication schemes. The RAKE receiver is a typical technology applying time diversity to capture and estimate signatures of wireless channels. The poor correlation of the spreading code will minimize the impact resulting from noise and user interference. By incorporating RAKE receiver technology, the performance of proposed authentication method is further improved under low transmission power situations. To extend the application of the proposed method, the coherence of time-variant channels for the continuous symbols is exploited to achieve authentications between mobile terminals. Furthermore, in a multi-hop wireless communication system, a novel network coding method is introduced to forward secret keys across multiple relay nodes. Under this protocol, cooperative relay nodes cannot decipher the key due to the lack of complete information.;For wireless cooperative relay networks, a theoretical analysis is conducted on the outage probability at a target secrecy capacity in a Rayleigh fading environment. According to this study, with the presence of channel diversity, an intended receiver can always achieve a relatively high secrecy capacity even at a low signal to noise ratio. A maximal ratio combining (MRC) of channel diversity is proposed to enhance the security of the wireless communication system in normal operating scenarios. Mathematical analysis and numerous simulations demonstrate the high effectiveness of the proposed scheme. |
