Research On Multi-user Assisted Covert Communication

Nowadays,the security of communication is becoming a promising problem,as the eavesdropping of data might lead to privacy leakage and deteriorate the value of the information.Covert communication is the key technology for information secrecy,which protects the communication link from being detection by the eavesdropper.In this case,the signal sent through wireless channels is unable to be detected at the first place,thus unable to be decrypted.This thesis introduces the covert communication model with the assistance of other users,and proposes schemes for achieving positive covert rate,parameters are optimized to improve the performance of the system.Models developed in this thesis are shown as below:1)multiple access channel model with random located source nodes,2)NOMA based broadcast model,3)mobility assisted Ad-hoc networks.The main results are summarized as follows:(1)Derive a close-form expression of upper bound of ergodic capacity.Covert communications in the distributed antenna system are considered,where multiple random located sources nodes aim to communicate with a destination node without being detected by a random located eavesdropper.The sum of probabilities of detection errors is taken as the covertness metric.To exploit the space diversity,the equal gain transmission strategy is employed.Then,the detection performance is analyzed and a tight upper bound of ergodic capacity is provided.It is found that a positive covert rate is achievable with increasing source nodes.(2)Analyze the power allocation scheme for multi-user covert communication.Consider a broadcast model consists of two users whose transmissions are based on NOMA protocol.A jammer is utilized to deteriorates the performance of detection,and the closed-form expression of average error probability is derived.Then,the transmission rate for each user is analyzed and the maximum minimum covert rate is optimized for fairness,based on which the transmission power and allocation scheme are given.The result indicates the power allocation scheme is unrelated to the covertness metric.(3)Analyze the node function for multi-user network.Discuss a wireless ad-hoc network with mobile nodes,each node can be either a user or a jammer,the key point is to determine the appropriate ratio of two groups to achieve the maximum covert throughout.Relative entropy is used as the covertness metric,and the network throughout is analyzed based on multi-user transmission strategy.The ratio and transmission power are optimized and the result shows the throughout increases with more jammer nodes under the condition of low signal to noise ratio.