Key Theory And Technique For Ambient Backscatter Based Covert Communications

Ambient backscatter communications(AmBC)are one of the core technologies for future Io T systems.With AmBC,Io T devices can harvest energy from ambient RF signals e.g.,broadcast TV s,cellular network,or WiFi signals,to activate the circuit,and transmit signals via reflecting the ambient signals with different the antenna impedance.Compared with traditional active communication technologies,AmBC do not require high power consumption to activate the RF module,thus can meet the demand of low power consumption and high endurance of Io T devices.On the other hand,since it does not require dedicated RF resource and spectrum resource,AmBC can also share the frequency band with traditional active communication,thus improving the spectral efficiency.Thus,AmBC are promising approaches to improve spectrum utilization and endurance of Io T devices.Unlike previous studies focusing onthe communication performance,this thesis investigates AmBC from the perspective of communication security.Specially,we aim to explore the penitential to realize covert data transmission via exploiting the existing direct link interference.s.The main work includes the following three parts.In the first part,we investigate the covert communication performance of singleantenna AmBC systems.We first demonstrate that the single-antenna AmBC system cannot achieve constant-rate steganographic communication in scenarios without any uncertainty.Then,the necessary conditions for implementing constant-rate steganographic communication are analyzed for three typical situations respectively,such as ambient signal power uncertainty,incomplete Channel State Information(CSI),and eavesdropping party noise uncertainty.At last,a method to maximize the covert communication capacity of AmBC is proposed.In the second part,we investigate the covert communication performance of the multi-antenna AmBC system.First,it is shown that the multi-antenna AmBC system cannot achieve a constant rate of covert communication without system uncertainty.Then,a semi-normal relaxation method is used to optimize the covert communication capacity of the system,and simulation results demonstrate that the usage of multiple antenna techniques can give an effective enhancement of the covert communication capacity of the system.The third part investigates the covert communication capacity of the AmBC system under multi-antenna eavesdropping users.It is demonstrated that the AmBC system can achieve constant-rate covert communication in the scenario where the eavesdropper does not have complete CSI,and the conditions for implementing constant-rate covert communication are also given.