Research On Physical Layer Security Technology Of Intelligent Reflecting Surface Assisted Communication Systems

The physical layer security,which is based on information theory,realizes the confidential transmission of information by using the time-varying and random nature of wireless channels.Depending on the the spatial gain,the multiple input multiple output(MIMO)technology can significantly improve the communication capacity of the communication system.At the same time,the multi-antenna technology can make the transmitting beam more concentrated and have a certain direction by using beamforming,which provides a new idea for the physical layer secure transmission.Intelligent Reflecting Surfaces(IRS),as a new type of metamaterial planar array composed of a large number of low-cost and energy-saving passive reflecting elements,can change the electromagnetic propagation environment to enable the construction of reflected signals and direct link signals.The superposition can not only effectively improve the spectrum efficiency of the system,but also the IRS can control the reflection direction of the incident signal,which will help improve the security of information transmission.In the current analysis of physical layer security,the main focus is on the situation where the transmitting end is multi-antenna and the receiving end is single or multi-antenna.The methods to deal with the problem mainly include semi-definite relaxation methods and majorization-minimization methods,although the above two methods can improve the performance of the physical layer security,but the complexity is relatively high.Therefore,this thesis focuses on the security issues of IRS-assisted communication systems.The main contents are as follows:1、The security design of the physical layer of the IRS-assisted multiple input single output(MISO)system Under the condition that the sender knows all the channel state information,the expression of the average security and speed of the system is derived for the IRS-assisted MISO communication system.Since this expression is a coupling form of the beamforming vector and the IRS phase shift matrix,an alternate iteration method is proposed to optimize the design of the beamforming vector and the IRS phase shift matrix alternately.Firstly,fixed the phase shift matrix of the IRS,using the method of generalized rayleigh entropy to optimize the beamforming vector,while the phase shift matrix of the IRS is fixed.Then,fixed the beamforming vector,the dinkelbach algorithm is used to convert the fractional problem into a non-fractional problem,and then the conjugate gradient descent algorithm based on riemannian manifolds is used to optimize the IRS phase shift matrix.Simulation results show that the proposed scheme can effectively improve the average security rate of the system.2、The security design of the physical layer of the IRS and artificial noiseassisted MIMO system By mixing the artificial noise into the transmission signal of the system,the physical layer security performance is further improved for the IRS-assisted MIMO communication system.In the case where the channel state information is known to the sender,an optimization problem aiming at maximizing the security rate is constructed,and the optimization problem is equivalent to a three-variable coupling optimization problem,which based on the mean square error criterion.The method of alternating iteration is also used to optimize the objective function.Firstly,fixed the IRS phase shift matrix,using the interior point method in convex optimization to optimize the beamforming matrix and the artificial noise covariance matrix,and then using the conjugate gradient descent algorithm based on riemannian manifold to optimize the IRS phase shift matrix.The simulation proves that the combination of IRS technology and artificial noise can effectively improve the securecy sum rate of the system.