Study On Relay Schemes And Channel Security For Two-Way Cooperative Communication Systems

The open nature of wireless channel makes the study on security of wireless communication is more challenging. In1949, Shannon illustrated the basic concept of communication and provided the model of communication system, which laid the groundwork for secure communications, afterwards Wyner proposed the concept of secrecy capacity on wiretap channel. Although the wireless network technology has been mature, it is difficult to study the security of wireless channel due to the broadcast nature. Unlike the conventional security researches based on the upper layer protocols, the discussions in this article focuse on the channel security with the secrecy rate as the performance index over three-node two-way cooperative communication systems based on information theory.The power allocation for secure AF (Amplify and Forward) two-way cooperative systems is studied. Two secure models are discussed,1) For a two-way cooperative communication system with untrusted relay, the algorithm of average power allocation is introduced first, and a relationship between the secrecy rate and channel coefficients is obtained in the high SNR regime. Then an optimal power allocation scheme is introduced based on channel stated information, and the corresponding closed-form solution for allocation factor is derived by applying the Lagrange multiplier.2) In most literature, an independent jammer is used to enhance the security of system, and it is not easy to control the corresponding power allocation and the position of jammers. Under the node power constraint, a security strategy is proposed, where each node intelligently allocates a part of energy to transmit message signal and the rest to create jamming signal in order to confuse the eavesdropper. Based on this method, an optimal power allocation scheme and a suboptimal power allocation scheme are introduced under two channel constraints which correspond to global instantaneous knowledge of all links and average knowledge of eavesdropper links, respectively, and the optimal power allocation scheme is always superior to the suboptimal power allocation scheme via performance analysis and computer simulation. In view of the negative effect on legitimate nodes, an optimal power allocation scheme with "known" jamming is proposed to enhance the security and overcome the drawback of mutual interference, where the artificial jamming can be decoded and filtered at three legitimate nodes but not at the eavesdropper node. Simulation results and analysis confirm the optimal power allocation scheme with "known" jamming provides higher secrecy rate and robustness than other schemes. The study on relay selection technology for secure DF (Decoded and Forward) two-way cooperative communication networks is developed. Without the jamming strategy, the conventional relay selection scheme, random relay selection scheme and optimal relay selection scheme are introduced, and the respective selection algorithms are also given. Analysis and computer simulation show that the optimal relay selection scheme performs better over both conventional relay selection scheme and random relay selection scheme. Based on the jamming strategy, an optimal relay selection scheme with jamming is proposed. As the jamming has the side-effect on legitimate channels in some system configurations, and then a novel intelligent switching between the optimal relay selection scheme and the optimal relay selection scheme with jamming is further proposed to mitigate the negative impact of jamming, and the optimal switching scheme is demonstrated to provide the highest secrecy rate among the proposed schemes via simulation results.