Research On WFRFT-based Physical Layer Security Methodologies For Wireless Communications

With the proliferation of wireless communication services,human society has been extremely facilitated over the past decade.However,with ubiquitous connected devices monitoring,recording and analyzing on our daily activities,wireless communications also give rise to a commensurate increase in security concerns.Traditionally,information security is build upon the cryptography mechanisms which has been proved to be ill suited to the openness,the mobility as well as the limited computing capabilities of the wireless systems.In this context,physical layer(PHY)authentication,which is agnostic to the underlying network infrastructures and radio access technologies,have been proposed as alternative solutions for wireless communication security.As a complement,the emergence of PHY-security fix up the lackage of secure functionality at lower layers.By exploring some underneath operations,such as channel coding,modulation and feature extraction,the PHY-security can employ intrinsic features associated with wireless signal for the sake of enhancing the protection of wireless communications.In this context,PHY-security is a promising paradigm for neutralizing the vulnerability of wireless communication systems.Recently,various research efforts have been presented on PHY-security,however,customized hardware designs are required for the majority of existing strategies,which prohibits their implementations in real and practical security systems.To this end,Weighted fractional Fourier transform(WFRFT),a novel time-frequency signal processing tool,is introduced into PHY-security by this thesis.WFRFT-based theoretical secure transmission and authentication paradigms have been presented for wireless communication security.The reasons for employing WFRFT for security objective can be attributable to the following features: 1)being a generalization of Fourier transform,signal conversions in WFRFT domain is mathematically and physically interpretable;2)the procedure of WFRFT is highly compatible with current communication schemes,which makes WFRFT more feasible to be implemented;3)the inherent and controllable variability in statistical characteristics reveals WFRFT potentially to be a practicable PHY security approach.Information theoretic transmission and PHY-authentication are two major applications of PHY-security,which would also be the research emphasis of this thesis.For PHY-authentication,this thesis starts with the intrinsic PHY features of the WFRFT signal.Two WFRFT-based authentication schemes,WFRFT-based Tag Feature Forging(WFRFTTFF)and WFRFT-based Gaussian Tag Embedding(WFRFT-GTE),have been proposed to deal with different senecious.To be specific,WFRFT-TFF can hide and forge the modulation paradigm to mislead attackers in signal demodulation.Meanwhile,WFRFT-GTE is designed to hide the presence of authentication tag against the adversaries by superimposing a low-power Gaussian WFRFT signal onto the message signal.Tradeoff analysis has been provided to balance the robustness of user authentication,the reliability of the message delivery and the stealthy of the embedded signal.Literally,instead of exploiting the channel or hardware characteristics that are out of control,the WFRFT-based PHY-authentication schemes reduce the complex signal feature extraction and analysis procedure,which would be feasibly integrated into most of the current wireless systems.Due to the rapid development in signal detection and analysis,direct sequence spread spectrum(DSSS)systems,which used to be an effective PHY-secure scheme with low detective possibility(LPD),are facing critical challenges recently.To this end,this thesis provides an extension investigate on DSSS systems within WFRFT domain.The impact on the correlated characters of spread sequences cased by the implementation of WFRFT has been investigated.Both theoretical analysis and numerical simulation results show that the implication of WFRFT significantly enriches the freedom of DSSS system design.In this context,a WFRFT-based parallel combinatory spreading(PCS-WFRFT)system aiming to guarantee physical layer(PHY)security is proposed.The proposed scheme inherits the LPD feature of DSSS system and the time-frequency coexistence of WFRFT system simultaneously.Numerical simulations are conducted to evaluate the performance of the proposed system in terms of average bit error rate(BER)and the equivalent signalto-noise-ratio(SNR)lose.Simulation results show that the PCS-WFRFT scheme leads to a flexible implementations and an improvement in secrecy performance with a tolerable BER lose at low SNR domain.For information theoretic transmission,the PHY secure mechanism of WFRFT is firstly investigated by examining the attainable secrecy capacity and the BER of the eavesdroppers under a classical three-point wiretap channel.It is proved that the secrecy superiority of WFRFT relies on the bit energy redistribution.Specifically,the implementation of WFRFT achieves an identical ”Artificial noise ” effect at modulation domain,which neutralizes the channel dependence of existing strategies.Moreover,this thesis also shows the equivalence between M-WFRFT and 4-WFRFT which provides an unified decoding strategy for M-WFRFT.Therefore,no matter how does the transmitter structure change,the receiver can remain unchanged.Thereby,this thesis further proposes a WFRFT-based cooperation scheme to enhance PHY-layer security in an energy efficient way.Simulation results demonstrate that the WFRFT-based user based user cooperation scheme leads to a significant performance advantage,in terms of secrecy ergodic capacity,compared with the conventional security-oriented user cooperation schemes,such as relay-jamming and cluster-beamforming.