| Cooperative communication has become an important technique in wireless communication due to its advantages,such as ubiquitous coverage and high capacity with low cost in infrastructure and maintenance.However,because of the open and broadcast nature of radio signal propagation,highly standardized transmission schemes,and the requirement of relays' help,cooperative communication systems are vulnerable to many kinds of attacks,for example,eavesdropping attacks,jamming attacks,tampering attacks,and black-hole attacks.Different from the key-based cryptographic methods,the physical layer security is a powerful method for secure communications by exploiting the property of the wireless channels,which has lots of advantages,such as no need of centralized trusted third party and a large size of key pool,low computational requirement,low latency,and small energy consumption.Due to the rapid development of artificial intelligence and communication techniques,the attackers are becoming more and more intelligent,thus they are able to cause more damages to wireless communications.This thesis focuses on proposing physical layer security techniques to defend against the passive eavesdropping attacks,half-duplex active eavesdropping attacks,full-duplex active eavesdropping attacks,and the eavesdropping attacks and link attacks caused by malicious relays in the cooperative communication system,thereafter to improve the reliability and security of the communication system.Since the cooperative communication technique can improve the secrecy capacity through increasing the capacity of networks,it is well-suited for improving the physical layer security.The studies on how to improve the physical layer security by using cooperative communication technique subjects to the above security problems therefore have theoretical significance.Furthermore,with the dramatically increasing number of devices in the wireless networks,the relays become selfish.They have their own objectives,e.g.,competing for the limited resources instead of considering the system security objective.Therefore,the relays are not necessary to join in cooperative communication-based physical layer security technique.The problem then comes to select the relays and stimulate the relays to join in the cooperative communication.Game theory is a mathematical model of conflict and cooperation behaviors among the intelligent rational decision-makers,which has been widely applied in biological science,economics,and some other fields.Therefore,it is applied in this thesis to model the behaviors among different nodes(e.g.,the transmitter,the relays,and the attackers)and find the optimal strateragy to situmilate the relays to join in the cooperative communication and improve the secrecy capacity of cooperative communication system.Firstly,a coordinated multiple-relays based physical layer security technique is proposed to defend against multiple eavesdroppers in the cooperative communication system.In order to benefit the selfish relays in forwarding the signals and improve the secrecy capacity,the interactions and competitions between the transmitter and the multiple relays are modeled as a Single-Leader Multiple-Follower Stackelberg(SLMFS)game.An optimal price allocation algorithm and an approximate optimal strategy(i.e.,phase coordinated vector and power allocation)algorithm are proposed to achieve the Stackelberg equilibrium.The influences of different cooperative transmission protocols,i.e.,decode-and-forward,amplify-and-forward,one relay-based,and multiple relays-based cooperative transmission protocol,on the security performance are discussed.Numerical studies demonstrate that the proposed scheme can greatly improve the utilities of both the transmitter and multiple relays over the results of the Nash equilibrium scheme and random power control scheme,which means that the relays are more willing to participate in the cooperative communication,and the transmitter can achieve better secure transmission based on the proposed scheme.It is also shown that the security performance of the SLMFS model-based physical layer security technique can be improved by increasing the number of relays.Compared with the cooperative jamming scheme and random phase scheme,the proposed scheme performs better in defending against the passive eavesdropping attacks.Secondly,this thesis studies how to cope with the half-duplex active eavesdropping attacks.The half-duplex active eavesdropper switches the attacks between the eavesdropping and jamming attacks to improve the ability in intercepting the cooperative communication,therefore causes more damages.A Coping with Half-duplex Active Eavesdropping attacks based on Zero-sum game(CHAEZ)model is proposed in this thesis.The CHAEZ model studies the interactions and competitions between defenders(transmitter and relays)and attacker(half-duplex active eavesdropper)in the worst case scenario,i.e.,the half-duplex active eavesdropper can obtain the information of transceiver's communications.Based on the analysis of CHAEZ model,a strategy selection algorithm is proposed to achieve the Nash equilibrium.Through the CHAEZ model,defenders can improve the security performance through studying the behaviors of the attacker and adjusting its strategy properly.Our findings indicate that the secrecy capacity of cooperative communication system can be improved through increasing the number of channels in the CHAEZ model-based physical layer security technique.Compared with the cooperative jamming scheme,the CHAEZ scheme performs better in defending against the half-duplex active eavesdropping attacks.Thirdly,this thesis proposes a novel cooperative communication-based physical layer security technique for defending against the full-duplex active eavesdropping attacks.A full-duplex active eavesdropper acts as both a jammer and a classical eavesdropper in a full-duplex mode,and aims at maximizing the eavesdropping ability.A Coping with Full-duplex active Eavesdropping attacks based on Three-stage Stackelberg game(CFETS)model is designed to study the competitions among the defenders and attackers,and thereafter to achieve the cooperative communication to improve the secrecy capacity and defend against the full-duplex active eavesdropping attacks in the worst case scenario,i.e.,the full-duplex active eavesdropper starts attacks first and can predic the defenders5 corresponding strategies.Moreover,a transmit power allocation and relay selection algorithm is proposed to achieve the Stackelberg equilibrium of the CFETS model.Through the CFETS model,the relays can be motivated to join in the cooperative communication and the security performance can be improved through studying the attacker's behaviors.Numerical studies indicate that with an increasing number of channels,the CFETS model-based physical layer security technique can significantly reduce the intercept probability;therefore,improve the security performance.It is also shown that the proposed scheme outperforms other existing schemes,e.g.,Nash equilibrium scheme,random power control scheme,power control with smart jamming game in a single-channel model/multiple-channel model.The cooperative communication-based physical layer security techniques proposed above are developed under the asumption that the relays can be trusted,which may not be the real case because the nodes can join in and leave the networks dynamically and some relays may be attacked by malicious nodes.Therefore,some of the relays in the cooperative communication system may be untrusted.In order to defend against the eavesdropping attacks caused by the untrusted relays(which are more difficult to detect),two physical layer security techniques,i.e.,Two-Way Relaying-based Physical Layer Security(TWRPLS)and Cooperative Jamming-based Physical Layer Security(CJPLS)are proposed.Through modeling the security problem as a secrecy capacity optimization problem,the optimal strategies of both TWRPLS and CJPLS techniques are obtained,which can increase the capacity of main channel and decrease the capacity of wiretap channel,respectively.Therefore,the security performance of cooperative communication can be improved.The theoretical and numerical studies demonstrate that the performance of TWRPLS technique can be improved by increasing the number of relays;and the CJPLS technique can achieve a better performance through increasing the number of friendly jammers.Finally,considering that the malicious relays can cause both eavesdropping attacks and link attacks,which will obviously result in more security threats.In this thesis,the transmitter will separate the messages into several parts and motivate each relay to forward one part of messages to increase the secrecy degree.Besides,a dynamic trust management is proposed for the transmitter to evaluate and timely update the trust value of each relay.In order to defend against both eavesdropping attacks and link attacks caused by malicious relays at the same time,a Coping with Malicious Relays based on Trust Repeated Game(CMRTRG)model is proposed.In the CMRTRG model,the relays can be motivated to join in a long-term cooperation.Then an optimal relay selection algorithm is developed for maximizing the secrecy degree with the guarantee of a reliable cooperative communication.Our findings indicate that there is a trade-off between the reliability and security of cooperative communications.In details,if the transmitter chooses more relays for cooperation,the secrecy degree will be improved,while the reliability of cooperative communication will be decreased.On the contrary,if less relays are chosen for cooperation,the reliability of transmission will be increased,while the secrecy degree of cooperative communication system will be reduced.Numerical results show that the proposed CMRTRG model-based physical layer security technique can achive a long-term cooperation between the relays and the transmitter,and maximize the secrecy degree with the constraint of reliability in the cooperative communication system.This thesis studies the security problems,i.e.,defend against the passive eavesdropping attacks,half-duplex active eavesdropping attacks,full-duplex active eavesdropping attacks,and the eavesdropping attacks and link attacks caused by malicious relays,in the cooperative communication system.Several cooperative communication-based physical layer security techniques are proposed for improving the reliability and security of cooper:ative communication system.In order to fully study the behaviors of attackers and stimulate relays to join in the cooperative communication for improving the security performance,this thesis applies game theory to model the competetions among users,i.e.,attackers,transmitter and relays.In this way,the defenders can choose their best strategies for defending against the attacks,therefore,the physical layer security of cooperative communication system can be improved. |
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