Research On Covert Communication Technique And Strategy For Cognitive Radio

With the rapid development of information technology,wireless communication has brought great convenience to people’s lives.However,due to the openness of wireless channels,data transmission is facing more and more security risks.There are possibly various attacks,such as information disclosure of mobile users,eavesdropping and network criminals,etc.,which impose serious threats to the future wireless communications.With the continuous improvement of users’ security requirements,users are more concerned about whether the communication behavior and transmitted information can effectively avoid being detected and intercepted by unauthorized users.Hence,secure and reliable communication transmissions have attracted extensive attention and research in recent years.In view of the increasing improvement of existing attack methods and theories,the available covert communication techniques have been unable to completely guarantee the security and reliable of information transmissions,and the threat of communication security is imminent.In order to realize the highly secure and reliable information transmissions,it is necessary to explore a new theoretical system of covert communication in wider area and deeper level.This thesis studies the covert communication techniques and strategies for cognitive radio(CR).Under the background of multi-domain non-stationary control mechanism theory,from the perspective of frequency domain,based on cryptographic algorithms,the frequency-domain non-stationary random control techniques are proposed.From the perspective of energy domain,based on the interference noise,the uncertainty of channel and local noise at eavesdropper(Eve),the non-stationary transmission schemes are presented,and a large number of numerical results are given to verify the performance of the proposed algorithms and schemes.The main research work and contributions of this thesis are described as follows:1.From the perspective of frequency domain,under the background of frequency hopping(FH)technique,the non-stationary random control generation scheme based on the Government Standard(GOST)algorithm is investigated,which effectively achieves the balance between security and efficiency.Meanwhile,it proves that the sequence generated by the proposed scheme is unpredictable and therefore has high security.Moreover,taking the impossible differential attack as an example,the security of the GOST algorithm is analyzed.Compared with the data encryption standard(DES)algorithm,the adopted GOST algorithm is more resistant to the impossible differential attack.Numerical results show that the proposed scheme has excellent comprehensive performance,which is mainly reflected in uniformity,randomness and complexity,and is superior to the existing DES algorithm and chaotic mapping.2.From the perspective of frequency domain,under the background of differential frequency hopping(DFH)technique,the non-stationary random control generation scheme based on hybrid encryption algorithm is investigated.First,it proves the equivalence between the cryptographic algorithm and the G function algorithm of DFH.Secondly,based on a multi-user DFH communication network,the novel G function schemes based on hybrid encryption algorithm are proposed,that is,the G function assisted non-stationary control generation algorithm and the higher-order G function assisted non-stationary control generation algorithm.The advantages of symmetric cryptographic algorithm and asymmetric cryptographic algorithm are fully utilized by the proposed algorithms,and the balance between efficiency and security is also considered.In addition,the security and ergodicity of the proposed algorithms are analyzed in detail.Finally,the numerical results show that the proposed algorithms are significantly better than the reversible hash algorithm and affine transformation in terms of uniformity,randomness,complexity and security.3.From the perspective of energy domain,the non-stationary transmission scheme for CR network is investigated.First,the system model of multiple cognitive users is considered,where multiple secondary transmitters(STs)opportunistically send confidential information to a secondary receiver(SR)under the protection of another carefully-selected ST who can emit interference to the eavesdropper(Eve).Assuming channel uncertainty of the wiretap link,it analytically finds that,compared with channel uncertainty,jamming signal power dominates the Eve’s detection error probability when it is sufficient large.Meanwhile,to address efficiency and fairness performance,three joint user-jammer scheduling schemes are proposed,namely,rate-oriented secondary user scheduling(R-SUS),link-oriented secondary user scheduling(L-SUS),and fairness oriented secondary user scheduling(F-SUS).Our results have shown that the presence of channel uncertainty at Eve and a friendly jammer allow ST to achieve a remarkable amount of covertness.4.From the perspective of energy domain,the non-stationary transmission scheme for cooperative CR network is investigated.First,based on a given system model,primary transmitters(PTs)seek the help of ST to forward information in order to economize power,in return,ST is able to transmit its own information by exploiting the PT’s spectrum.Under the premise of assuming channel uncertainty of the wiretap link and the local noise uncertainty at Eve,it analytically finds that,compared with channel uncertainty,noise uncertainty dominates the Eve’s detection error probability.Moreover,for the CR network of multiple cognitive users,a transmission strategy for the best channel gain is proposed in order to maximize the covert throughput of the entire system.Meanwhile,the average detection error probability,the average covert rate and the average covert outage probability are analyzed.Numerical results show that the presence of channel uncertainty and noise uncertainty at Eve allow ST to achieve a remarkable amount of covertness.