Investigation Of Physical Layer Security In Untrusted Relay Networks

The rapid development of wireless communications makes it possible for the data trans-mission with ultra-high traffic,ultra-high density of users and ultra-high mobility.However,due to the broadcast nature of wireless medium,it is a challenging work to protect the pri-vacy and ensure the secure transmission in the multiuser systems.To make things worse,due to the intelligentizing and enhancing computing capability of the wireless nodes,tradi-tional encryption-based security mechanisms have to face the challenge of the brute-force attack.Physical layer security is based on the information theoretic security,which aims to safeguard data transmissions by exploiting the randomness of wireless channels.This tech-nique is becoming increasingly recognized as one of the promising methods to achieve the secrecy communications in wireless systems which has been attracted enormous academic and industrial attention.As a general application in wireless communications,cooperative relaying is also wide-ly introduced in physical layer security.However,in some public or financial networks,the relay may not always be trusted since the different security clearances of each node and the different levels of access to the information.Therefore,how to enjoy the connectivity of the cooperation as well as keeping confidential information from being intercepted by the untrusted relay is our main focus of the thesis.The existing work in the multiuser un-trusted relay networks has some limitations in the following aspects.Firstly,the existing literature is limited in the“single-source and multiple-destination”or“multiple-source and single-destination”scenario,however,in the multiple“source-destination”pairs scenario,e.g.,D2D,P2P,how to enhance the security by taking the advantages of multiuser diver-sity is deserved to be further studied.Secondly,most of the existing work mainly focuses on the orthogonal multiple access,as for the non-orthogonal multiple access?NOMA?,it is great challenging to promise security since it is necessary to prevent the multiple infor-mation signals from being intercepted by the untrusted relay simultaneously.Meanwhile,when utilizing cooperative jamming to safeguard transmission,it is necessary to reduce the impact of the jamming signal at all NOMA users.Therefore,it is very meaningful to design the secrecy transmission scheme in NOMA untrusted networks.Lastly,using cooperative jamming to prevent the confidential message from leaking also brings extra energy con-sumption.Meanwhile,users in many wireless applications may not have a sustained energy supplement rather than a powerful base station,thus limits the cooperative jamming.Based on the aforementioned observations,this thesis investigates the physical layer security in some typical communication scenarios for the untrusted relay networks,in which cooperative jamming and multiuser diversity can be used as the key factors to improve the security.In particular,the main contributions of this thesis are summarized as follows.1.Physical Layer Security in Multiuser Untrusted Relay NetworksTo improve the secrecy performance for the multiple source-destination pairs scenario,we propose three cooperative jamming based user-pair selection schemes,i.e.,1.opportunis-tic user-pair selection?OUS?;2.greedy user-pair selection?GUS?;3.genie-aided user-pair selection?GAUS?.Comprehensive performance analysis is studied for the proposed the OUS scheme and the GUS scheme,both the secrecy outage probability?SOP?and the ergodic se-crecy rate?ESR?have been derived in closed-form expressions.secrecy diversity order is also presented to reflect the asymptotic performance in the high signal-to-noise ratio?SNR?regime.Simulation results and theoretical analysis show that,using multiuser diversity is a better way to improve the security in the untrusted relay networks compared to the cooper-ative diversity.Meanwhile,after the full understanding of the topology for the considered networks,we extend our work to the multiple destinations and single source with multiple antenna scenario and the optimal joint antenna and user selection scheme has been proposed.The impacts of both the number of antennas and the number of users on SOP is studied and the secrecy diversity order of the considered networks is proved to be min^N_S,₂M.2.Physical Layer Security in NOMA Untrusted Relay NetworksWe consider a NOMA untrusted relay networks with a single far user and multiple near users scenario.We design the far user based cooperative jamming strategy to achieve secure communications,and proposed a user selection scheme to maximize the received SNR at the near user.We also prove that the proposed selection scheme can achieve the optimal secrecy rate at both near user and far user.Then,we obtain the lower bound of ESR and the asymptotic expressions for both near user and far user under the proposed scheme under Rayleigh fading channel.The simulation results and theoretical analysis show that,the ESR of the near user increases with the increased number of users at the high SNR regime,while the ESR of the near user will converge to a constant.3.Physical Layer Security in EH Multicast Untrusted Relay NetworksFirstly,an energy harvesting?EH?destination aided cooperative jamming framework is proposed to achieve a non-zero secrecy rate.Then,we proposed 3 jammer selection schemes to further improve physical layer security.In addition,for the proposed maximum interfer-ence jammer selection scheme,we first obtained the steady-state distribution of the number of available EH destination users,and then use the distribution to derive the achievable ESR in both lower bound and approximation expressions.the effect of both the energy arrival rate and the number of destination nodes on ESR has been shown via numerical simulations.It can be revealed that as the number of destinations increases,the ESR is determined with two common behaviors regarding the multicast nature and energy arrival rate:1.the proba-bility that a jammer exists increases with the increasing number of destinations but limited to one.2.multicast nature forces the secrecy rate to decrease with the increasing number of destinations.