Research On Physical Layer Security Technology Based On Direction Modulation In Multi-user Networks

With the explosive growth of wireless communications,the security of the transmit signals have gradually attracted wide attentions.Traditional communication methods based on the upper layer cryptographic techniques are no longer applicable,physical layer which is at the bottom of the network protocol enters people's sight,and people begin to study the issue of wireless physical layer secure transmission.Directional modulation,which is regarded as an effective physical layer secure transmission technology,has attracted people's interest.In this paper,we focus on the physical layer security based on directional modulation in multi-user networks.And with the assumption that the estimation error of direction angle toward each eavesdropper follows the Von Mises distribution,the robust beamforming matrices are designed to improve system security.The main contributions are as follows:(1)A Von Mises Distribution-Sum Secrecy Rate Maximization(VMD-SSRM)robust directional modulation method is proposed based on the assumption that the estimation error of direction angle toward each eavesdropper follows the Von Mises distribution.First,the expectation matrix of each estimated eavesdropping channel related coefficient is derived.Based on the derived expectation matrices,the system sum secrecy rate maximization problem subject to the total transmit power of the base station is formulated.Then semi-definite relaxation method and first-order approximation method based on Taylor expansion are employed to transform the non-convex optimization problem into a convex problem,and finally we solve this problem by Convex(CVX)optimization toolbox.(2)In order to optimize the system sum secrecy rate in the case of the maximum estimation error of direction angle toward each eavesdropper,we propose a Maximum Angle Estimation Error-Sum Secrecy Rate Maximization(MAEE-SSRM)directional modulation method.To design the robust transmit signal beamforming matrix and artificial noise beamforming matrix,first the upper and lower bounds of each estimated eavesdropping channel related coefficient are derived,then the system sum secrecy rate maximization problem based on the derived bounds is constructed.Next,we employ the variable transformation method to convert the non-convex optimization problem into a convex problem and finally solve it by CVX.(3)Considering the inadequacy of the proposed MAEE-SSRM method,a weighted MAEE-SSRM directional modulation method is proposed.It assumes that the maximum estimation error of direction angle toward each eavesdropper is known in advance,but only the positive and negative of the error is unclear.Then we introduce the weighting factor to describe this uncertainty in the optimization problem and employ the exponential variable substitution method to transform the non-convex optimization problem into a convex one,and finally solve it by CVX.(4)Considering the physical layer security based on directional modulation in multipath scenario,first we construct the multipath channel model.Based on the proposed channel model,then the system sum secrecy rate maximization problem under the constraint of the total transmit power of the base station is formulated.The same convex optimization method is used to convert the non-convex optimization problem into a convex problem and finally we solve it by CVX.