Research On Theory And Approaches Of Physical Layer Security In Satellite-terrestrial Communications

Space-air-ground integrated network(SAGIN)enables ubiquitous network coverage and flexible resource management,which can improve network capacity and enhance network performance,provides seamless connectivity.It shows promising application prospects in industrial field,such as Internet of things(Io T),cloud computing,and vehicleto-vehicle networks,etc.With accelerating the construction of low earth orbit(LEO)constellations and new infrastructures for the fifth generation(5G)networks,SAGIN urges an organic integration of satellite networks,aircraft,and terrestrial networks,which enables the cooperation crossing the dimensions and networks for improving the utilization of network resource and wireless access ability,to meet escalate service requirements in multiple dimensions,such as terrestrial,sea,aerial,and space.Due to the open connectivity and dynamic topology,SAGIN is facing with severe security challenges in access authentications,secure routing,and secure transmissions.For secure transmissions,different from encryption algorithms in the upper layer which are computationally intractable and relies on key management,physical layer security(PLS)performs lightweight and key-free,which has been widely investigated in terrestrial communication networks.PLS is an information theory secure approach based on the random differences of wireless channel and the security capacity/rate is generally used for the security metric.Due to the inherent broadcasting nature of wireless signals and the openness of wireless channels,satellite-terrestrial communications are well known to be vulnerable to eavesdropping threats.Especially for the satellite-terrestrial downlink communications with large communication coverage,the transmitted signals are easier to be eavesdropped.Particularly,since the distance difference between different satelliteterrestrial links is negligible,the channels between satellite and users are generally similar,which poses great challenges to implement PLS in satellite-terrestrial communications by the weak random difference between channels.Moreover,considering the limited resource at satellite(such as power resource and data processing ability),and satellite spot beam is hard to distinguish with adjacent users,traditional PLS approaches are difficult to be applied in satellite-terrestrial communications effectively.Thus,it is urgent to conduct a research on theory and approaches of PLS in satellite-terrestrial communications.To achieve secure transmissions of satellite-terrestrial downlink communications with considering channel similarity,in this thesis,we conduct a research on the theory and approaches of PLS in satellite-terrestrial communications,where the main research contents are summarized as follows.First,focusing on the secure transmissions in satellite-terrestrial multi-carrier communications,a secure waveform design based on secure truncating orthogonal frequency division multiplexing(STOFDM)is proposed by using the concept of secure encoding.STOFDM utilizes the self-induced inter-carrier interference(ICI)to degrade the eavesdropper(Eve).By using the secure truncating information,an approach termed quasiorthogonal compensation is further proposed to cancel the impact of ICI on the legitimate user,and then the secrecy rate performance is improved.In addition,the closed-form expression of secrecy rate of STOFDM is derived,which is verified by numerical analysis results.Second,focusing on the secure transmissions of multi-user in satellite-terrestrial downlink,a PLS communication approach is proposed based on frequency-domain nonorthogonal multiplexing(FD-NOM),which induces inherent inter-user interference(IUI)to suppress the signal-to-interference-plus-noise ratio(SINR)of Eve.Further,a FD-NOM multi-user cooperation scheme is proposed to enhance the SINR of legitimate users,and then the secrecy rate performance is improved.In addition,the lower bound of secrecy rate is analyzed and the tightness of this bound is verified by simulations.Third,focusing on the secure transmissions in multi-beam satellite-terrestrial downlink with relaying cooperation,the secure model of relaying cooperation for satelliteterrestrial downlink communications is first conducted,where the co-channel interference caused by frequency reuse among multi-beam can degrade Eve.Meanwhile,the signal quality in main channel is enhanced by relaying cooperation and meanwhile artificial noise(AN)is generated by the relay to degrade Eve deliberately.Finally,an approach for jointly optimizing satellite beamforming and relaying power allocation is proposed to maximize the secrecy rate of satellite-terrestrial link,and the efficiency of our proposed approach is verified by simulations.