Research On RIS Assisted Wireless Location Technology In MIMO Communication Systems

In the civil domain,wireless location techniques usually rely on communication devices to achieve target localization by processing the signals transmitted between users and base stations.The fifth generation(5G)mobile communication technology,with its advantages of high positioning accuracy and low latency,can provide good positioning services for virtual reality,smart factories,intelligent driving and so on.Compared with the previous generations of mobile communication technology,5G develops towards higher frequencies,bringing faster communication rates.In addition,the application of massive multi-input multi-output(MIMO)technology greatly improves the spatial resolution of the system.However,higher frequency electromagnetic signals(such as millimeter wave)usually have the disadvantages of short wavelength,short propagation distance and poor diffraction ability,which will lead to smaller signal coverage of 5G base stations.For network connectivity and stability,more 5G base stations are often needed to cover the same large area,which will greatly increase the demand for base stations in 5G networks.On the other hand,signals are easily occluded by obstacles or fall into signal blind spots during the propagation process,which greatly reduces the reliability of system positioning.In order to solve these problems,this paper introduces Reconfigurable Intelligent Surface(RIS)technology.The RIS could be easily deployed in the wireless transmission environment.On the one hand,by rebuilding the signal transmission path,the positioning ability of the system could be restored and the signal coverage could be expanded.On the other hand,the positioning accuracy of the system could be improved by leveraging more path information.For the wireless localization problem of RIS-assisted MIMO communication system,the main work of this paper is summarized as follows.(1)This paper studies the wireless localization problem of RIS-assisted MIMO communication system in the ideal environment without multipath.The conclusion is drawn by analyzing the effects of two RIS phase configurations,time-invariant phase and time-varying phase,on the system.When the time-invariant RIS phases are used,the only channel parameters that can be estimated by the system are the Time of Arrival(TOA)and the Angle of Departure(AOD)of the signal,and no Angle information related to the RIS can be estimated.The role of the RIS in the system at this time is similar to that of an ordinary scatterer in space.At this time,when the direct path between the user and the base station is blocked,only one RIS cannot be used to locate the user in 3D space including the array rotation Angle.When the time-varying RIS phases are used,the system can estimate all the channel parameters and realize the three-dimensional positioning of the user,including the estimation of the user’s array rotation Angle.(2)This paper studies the wireless localization problem of RIS-assisted MIMO communication system in multipath environment.The non-line-of-sight(NLOS)path signals caused by scatterers are no longer regarded as noise or interference,but these multipath information are effectively used for localization by extracting useful information related to position coordinates and orientation angles.In this paper,the RIS is modeled as a uniform planar array(UPA),and the base station and user are modeled as uniform linear arrays(ULAs).Considering the 3D localization problem as well as the actual scattering scenario,all available channel parameters are used for localization.Compared with the existing research results,the system model and localization scenario considered are more general and practical.(3)Aiming at the wireless localization problem of RIS-assisted MIMO communication system in multipath environment,a parameter estimation algorithm based on two-stage hybrid RIS phases is proposed according to the analysis results of RIS phase configuration.The algorithm divides the estimation process of channel parameters into time-invariant stage and time-varying stage,and uses space alternating generalized expectation-maximization(SAGE)algorithm in each stage.Firstly,the joint estimation problem of multiple path parameters is transformed into the estimation problem of multiple continuous single path parameters,and then the multi-dimensional parameter search problem of a single path is transformed into multiple one-dimensional parameter search problems.When the number of scatterers in the path is large,the algorithm can greatly reduce the number of parameters in each estimation and reduce the estimation complexity.The simulation results show that the root mean squared error(RMSE)of the proposed algorithm can approach the Cramer-Rao lower bound(CRLB),and it has good estimation performance.(4)Aiming at the problem of using intermediate parameters to estimate the target position in the two-step localization technique,two localization algorithms based on intermediate measurements are proposed: the localization algorithm based on LM and the localization algorithm based on Taylor series.The simulation results show that under the same conditions,the RMSE of the localization results based on Taylor series is closer to CRLB,which has good localization performance.