Joint Collaborative Beamforming Design And Multiple Relays Physical Layer Security

With the develop of wireless communication technology and the popularization of mobile phone, we have the requirements of higher quality about communication service and data transmission. This demand driven the development of the wireless communication. In the recent 4G network, MIMO(Multiple Input Multiple Out-put) technology have been applied because of the unique advantages. The MIMO technology can improve the spatial diversity gain and multiplexing gain, reduce the effect of signal fading, improve information transmission rate and the system quality of service. However, MIMO will bring higher cost because the equipment is not easy to move.Security and privacy have long been the main research topics in communica-tions, especially for wireless communications due to the broadcasting nature of wire-less media. Among all the security and privacy request, confidentiality is one of the most important problems. Conventionally, it is realized by using encryption method at the application layer. Due to the dynamic topology, the lack of infrastructure and the key distribution issues, etc., physical layer secured wireless communications are greatly desired and have been an active research topic for several decades. Physical layer security is the no leakage transmission in the physical layer of TCP/IP model through a serious of techniques. Wyner has introduced the noisy wiretap channel in 1975, where the eavesdropper gets a degraded version of the legitimate. Wyner determined the secrecy capacity defined as the supremum of communication rates to the legitimate receiver.In cooperative relay networks, using multiple mobile terminals as relays, through mutual cooperation to forward signal will form a similar MIMO system. The cooper-ative communication have the advantage of MIMO technology and easy portability. In a multi-relay network, using beamforming technique can realize the directional signal transmission and improve the receiver Signal To Noise Ratio. The multi relay cooperative communication system can improve the safety of the system capacity, increase the security degree of freedom in the transmission take advantage of some technology such as beamforming, Cooperative jamming and interference alignment. The main contribution of our thesis are listed as follows:We study two approaches to optimize the problems between capacity and power in a three-hop multirelay network, which constructed by two separated relay clusters. In the first approach, two beamforming weight vectors are designed to maximize the capacity under the power constraints of relay clusters. While in the second approach, we minimize the power of total relay nodes as well as meet the minimal capacity demand. In both of design schemes, we turn into two beamformer vectors to only one though a series of mathematical manipulation, which contribute to reducing the computation complexity in the implementation of genetic algorithm. Simulation results show that our proposed approaches significantly outperform the previous methods conducted.We consider the physical layer security problem in the relay networks with each node having one antenna. The relay nodes are employed not only to aid the information transmission, but are also used to improve the network security in the physical layer by transmitting Gaussian artificial noise to jam a potential malicious eavesdropper. By allowing shared randomness between the jamming nodes, we show that the maximum secure degrees of freedom (DoF) of the concerned network is 1/2 almost surely. The necessary and sufficient conditions to attain the optimal DoF are established and the condition for achieving the maximum DoF is given explicitly which serves as a great guidance for the general system design. Moreover, following such conditions, a simple DoF-optimal beamforming algorithm is developed, and it works very well in the high SNR regime as expected, which is verified by computer simulations.