Optimization Of The Pre-filters In Time Reversal Multi-user Security Systems

Employing the characteristics of the wireless channel to achieve secure transmission of information through physical layer technology is a way to realize security communications.The time reversal(TR)transmission has a natural anti-jamming and anti-eavesdropping capability due to its unique spatial and temporal focusing property.By using TR technology,good secure transmission performance can be obtained even when the transmitter is equipped with single transmitting antenna.This thesis studies the security scheme design and the optimization of the pre-processing filter to enhance the security performance of TR multi-user downlink transmission system.The main contents are as follows:1.The TR transmission technology is used to realize secrecy transmission in a twouser downlink system where the one user's information is kept secret from the other,or vice vesa.The impulse response of the two pre-processing filters are optimized to maximize the sum secrecy rate.The joint optimization problem of two filters is transformed into the two independent optimization sub-problems of each filter based on reciprocity principles.The problem is further converted into the problem of finding the largest eigenvalue and its corresponding eigenvector,which is solved by the iteration algorithm.The simulation results show that by optimizing the pre-filters,the system's sum secrecy rate is promoted and is obviously higher than that of the conventional TR pre-processing filter system and direct transmission system.2.The design and optimization of secure transmission schemes based on artificial noise(AN)under different eavesdropper's channel state information(ECSI)conditions in a TR multi-user downlink multiple access secure transmission system is studied.AN is placed in the nullspace of the legitimate channel when ECSI is unavailable.The optimization problem in this case is to minimize the total power used to send signals under each users' signal-to-interference-plus-noise ratio target constraints,so that the power used to send AN is maximized and the interception rate of the eavesdropper is minimized.The problem is solved by transforming it into a semidefinite program(SDP)using a semidefinite relaxation(SDR).The problem is jointly optimize the pre-processing filters and AN to maximize the sum secrecy rate under total transmit power constraints when ECSI is available.The problem is solved by transforming it into a convex iterative program using SDR and the first order Taylor approximation.Simulation results show that the sum secrecy rate of the optimized transmission scheme is higher than that of the TR systems adopting conventional pre-processing filters s with or without AN.