Performance Optimization For IRS-assisted Covert Communications With Eavesdropper's Channel And Noise Information Uncertainties

Rapidly developing wireless communication networks bring convenience to our life,but at the same time raise a series of security issues.A large amount of private information is transmitted through open wireless channels,which may easily lead to information leakage.Covert communication technology can fundamentally solve the security problem of wireless communication transmission by hiding the private information into the ambient noise,so that eavesdroppers cannot detect the existence of information transmission,thus ensuring communication security.However,wireless channels are usually fading channels and the fading of these channels is an uncontrollable factor.Besides,although the 5G physical layer technology can adapt to the time-varying wireless environment,it is still not possible to actively configure the wireless propagation channels due to the random nature of signal propagation.These factors increase the difficulty of achieving covert communication.In recent years,intelligent reflecting surface(IRS)has received more and more attention as an emerging technology.The IRS-assisted covert communication system improves the covertness of information transmission by actively configuring the wireless channel.In covert communication systems,the eavesdropper usually employs a radiometer to detect the presence of legitimate transmissions,which leads to its noise information uncertainty.In addition,since the eavesdropper is an illegal node in a legitimate communication system,it usually cannot fully estimate the channel state information(CSI)of the channels between it and the legitimate nodes,i.e.,it has channel information uncertainty on these channels.In practice,the simultaneous existence of noise information uncertainty and channel information uncertainty at the eavesdropper is almost inevitable,while the research work of performance optimization for IRS-assisted covert communication systems in this case is still missing.This thesis investigates this topic in the following aspects.(1)An IRS-assisted single-user covert communication system is studied,where an IRS is deployed for covert transmission from a legitimate transmitter(Alice)to a legitimate receiver(Bob)with the presence of an eavesdropper(Willie).Since Willie is not a legitimate communication node and cannot control the reflection of the IRS,it has uncertainty about the channel information of the Alice-IRS and IRS-Willie links.Also,Willie has uncertainty about its noise power.Having such channel and noise information uncertainties,Willie finds an optimal power detection threshold to minimize its false detection probability.Under this system setup,we investigate maximizing the covert rate of the legitimate communication from Alice to Bob,by jointly optimizing Alice's transmit power and the IRS's reflecting phase shifts.We have proposed two efficient joint optimization algorithms for the continuous and discrete IRS reflecting phase shift cases,respectively.The proposed algorithm for the previous case is based on the semidefinite relaxation technique,and that for the latter case is based on the alternating optimization technique.Simulation results have demonstrated the superiority and necessity of jointly optimizing the transmit power and IRS reflecting phase shifts on improving the covert rate performance,as compared to other benchmark schemes.(2)An IRS-assisted multi-user covert communication system is studied,where Alice uses time division multiple access(TDMA)to transmit covert information to users in separate time slots and wants to ensure that each user's information is not eavesdropped.The reflection coefficients of the IRS and the transmit power of Alice in different time slots are jointly optimized to maximize the minimum reception rate of all users.Considering the continuous and discrete IRS reflection phase shift cases separately,the SDR technique and alternating optimization are used to solve the non-convex original problem and obtain an approximate solution.The simulation results show that the algorithm proposed in this thesis can effectively improve the covert rate of users in the IRS-assisted multi-user covert communication system.