Research On High Performance Covert Frequency-hopping Sequence Based On Cognitive Radio

With the growing requirement of information security and privacy,information secure transmission of communication systems that can provide a gratifying transmission rate and good quality of service has been received increasing attention.As a widely applied communication technique,wireless communication techniques have been rapidly developed in recent years.However,due to the open nature of wireless communication,the secure issue of the wireless communication is always concerned.Moreover,with the significant improvement of computers,especially the emerging quantum computing,the time for attacking the conventional encryption algorithms is increasingly short.In this case,the methods for guaranteeing information secure transmission that rely on the conventional cryptography theories have been severely threatened.Thus,it is important to design communication systems with high security under the condition that the transmission rate is guaranteed.Since signal detection is the precondition of all signal analysis and processing progresses(e.g,decoding,parameter estimation),if the confidential signal is enabled to be difficultly detected or continuously analyze when it is detected,it is difficult for the eavesdroppers to process the received signal even when the information is transmitted without any encryption.Frequency hopping systems that can flexibly hop the operational frequency to achieve diverse purposes are widely used in military and civilian communications due to their excellent characteristics,such as the non-stationary and agility.Moreover,it is difficult to detect or real-time analyze frequency hopping sequences when the frequency hopping sequences are unknown compared with the common signal with constant frequency.Thus,by exploiting the characteristics of frequency hopping systems,this paper is focused on the design of covert frequency hopping sequences.The main contributions of this paper can be summarized as follows.1.A covert frequency hopping communication model is designed.Based on the typical eavesdropping channel model,this paper constructs a single-user and multi-user frequency hopping signal interception model,and analyzes the conditions of covert frequency hopping communication under multi-user conditions from a theoretical perspective.The conditions of covert frequency hopping communication are respectively deduced under two conditions:the conditions for known the number of users and the conditions for unknown number of users.The derivation results are verified by simulation.2.Methods of designing covert frequency hopping sequences for wideband and narrowband receivers are proposed respectively.When generating the frequency hopping sequences,the methods map each element in a probability vector to the probability of selecting a corresponding frequency slot.The design of the probability vector is the key to the covert frequency hopping sequence.When it is generated,it first obtains the channel state information through the cognition.Then the design of the probability vector of the frequency hopping sequence is modeled as an optimization problem,and the final probability vector is obtained by solving this optimization problem.This optimal probability vector can achieve the lowest probability of detection while maintains several constraints.Since the system bit error rate and Hamming correlation are important,these two metrics are also considered in the optimization problem to ensure the communication performance and network capability of the frequency hopping sequence.Since this kind of frequency hopping system must maintain the same probability vector both at the transmitter and receiver during communications,it will cost greater synchronization overhead than other frequency hopping systems.Therefore,to address this problem,this paper also proposes and tests two 8-bit floating-point quantization methods based on the characteristics of probability vectors.Moreover,in order to make the proposed frequency hopping sequence have a wider range of applications,this paper also expands the design scenario from point-to-point or single-user scenarios to broadcast and multi-detector scenarios.This is done by extending the dimensions of the corresponding vectors in the optimization problem.The simulation results not only verify the proposed covert hopping sequences but also verify the proposed 8-bit based quantization methods.It can be clearly seen that the proposed frequency hopping sequences and their corresponding quantization methods has better performance than other references.3.A message-driven frequency hopping system with low hardware cost is designed from a system level.The basic idea of the frequency hopping system is to make use of the dwell time of a frequency hopping signal to transmit a part of the information,which results in the dwell time of each hop of the system is a true random number.This can make it difficult for the illegitimate users to estimate the parameters of the system.This dissertation introduces the structure of the transmitter and receiver of the system,and analyzes the lower bound on the bit error rate of the system in the case of additive white Gaussian noise channel.In order to improve the system transmission rate,this paper also proposes a multi-carrier or multi-user extension method based on Latin square.Finally,the performance of the related frequency hopping system is compared with the hardware complexity and the maximum transmission rate.Simulation results reveal that the proposed frequency hopping system has better performance.