Covert Communication In Congitive Radio Networks

The security of information transmission has become a major concern since the information is easily intercepted and interfered by unauthorized users due to the open wireless channel.Traditional encryption algorithms and physical layer security technologies,which rely on key encryption and the physical properties of wireless channels,respectively,guarantee the secure transmission of information.However,these two techniques can only ensure that the information cannot be fully decoded correctly by an unauthorized user and cannot protect the information transmission process itself.While wireless communication between users is the goal of covert communication technology,it also works to keep the transmission of information secret from unauthorized users.The issue of a lack of spectrum resources is becoming more and more evident due to the quick advancement of wireless communication technology,the rise in users,and the constant growth of application fields.By automatically acquiring the usable spectrum through spectrum sensing and it does not cause interference to authorized users.Cognitive radio(CR)can increase the efficiency with which spectrum resources are used.In this paper,covert communication technology and cognitive radio technology will be combined in-depth study.The main research contents are as follows:Firstly,aiming at the security of information transmission in cognitive radio networks.Two spectrum-sharing modes: overlay and underlay,and proposes a method of covert communication node selection.There are lots of secondary user transmitting nodes which want to send hidden information to secondary user destination nodes.The minimum detection error probability and the optimal decision threshold for illegal users are obtained by overlay mode.Next,aiming at maximizing the covert transmission rate under the condition that the minimum detection error probability satisfies certain constraints,the optimal selection method of secondary user transmitting nodes and jamming nodes,as well as the optimal transmission power range for secondary user transmission nodes and interference nodes are derived.Based on this,the closedform expression of the optimal covert transmission rate and its expectation are obtained.Unlike overlay mode,underlay mode requires that transmission power of the secondary user’s transmitting node and the interfering node’s interference to the primary user is less than a given threshold.After adding this constraint,the optimal selection method of secondary user transmitting nodes and interfering nodes under underlay mode are derived,the optimal transmitting power of the two-stage user transmitting node and the optimal transmitting power range of the interfering node are obtained,and the closed-form expression of the optimal covert transmission rate is obtained.The simulation results show that compared with existing methods,the proposed node selection methods in both two modes can effectively improve the covert transmission performance of secondary users.Secondly,based on the above research,a new covert transmission method based on user fairness is proposed by considering the fairness of transmission between secondary user transmitting nodes.The expression of the covert transmission rate is derived,and then the influence of instantaneous channel state information between nodes on the covert transmission rate is analyzed.In order to satisfy the fairness,the instantaneous value and expectation of the channel state information which affect the covert transmission rate are considered comprehensively.On this basis,the expressions of covert transmission rate in overlay and underlay are derived,and the Jain fairness index is used as the fairness criterion of this method.The simulation results show that the proposed fair transmission method improves fairness and covert transmission rate remarkably.