Research On Directional Modulation And Covert Communication In Wireless Secure Communication

The next generation of wireless communication technologies will be tasked to a diverse range of quality-of-service requirements in the application scenarios.The unremitting increase in the demand for mobile data capacity is anticipated to further explode in the coming years.A large amount of confidential information(e.g.,credit card information,physiological infor-mation for e-health)is transferred over the open wireless medium.This presents a significant challenge as the broadcast nature of wireless channels make it extremely vulnerable to secu-rity breaches.This dissertation focuses on wireless secure communication key technologies:directional modulation and covert communication,in which the transmission robustness and accuracy are emphatically studied for directional modulation.For covert communications,d-ifferent scenarios over fading channel are investigated.The main ideal and innovation of this dissertation can be expressed as follows:1)A novel robust directional modulation synthesis method is proposed for the uncertainty of desired direction angle.Specifically,we first derived the close form for a low-complexity dynamic DM synthesis method.Furthermore,in our robust solution,given a distribution of the estimation errors,we minimize distortion of the constellation points along the desired di-rection,based on the conditional minimum mean square error(MMSE)criterion.Simulation results show that our proposed robust DM method is capable of substantially improving the performance compared to state-of-the-art methods.2)In order to achieve more accurate wireless secure communication,a random frequen-cy diverse array based directional modulation with artificial noise(RFDA-DM-AN)scheme is proposed to enhance physical layer security of wireless communications.Specifically,we first design the RFDA-DM-AN scheme by randomly allocating frequencies to transmit antennas,thereby achieving two-dimensionally(i.e.,angle and range)secure transmissions,and outper-forming the state-of-the-art one-dimensional(i.e.,angle)phase array(PA)based DM scheme.Then we derive the closed-form expression of a lower bound on the ergodic secrecy capacity(ESC)of our RFDA-DM-AN scheme.Based on the theoretical lower bound derived,we further optimize the transmission power allocation between the useful signal and artificial noise(AN)in order to improve the ESC.Simulation results show that our RFDA-DM-AN scheme achieves a higher secrecy capacity than that of the PA based DM scheme and linear frequency diverse array(LFDA)based DM scheme.3)Covert wireless communication aims to hide the very existence of wireless transmis-sions in order to guarantee a strong security in wireless networks.In this work,we examine the possibility and achievable performance of covert communication in the relay networks.Specif-ically,the relay is greedy and opportunistically transmits its own information to the destination covertly on top of forwarding the source's message,while the source tries to detect this covert transmission to discover the illegitimate usage of the resource(e.g.,power,spectrum)allocated only for the purpose of forwarding the source's information.We propose two strategies for the relay to transmit its covert information,namely rate-control and power-control the transmis-sion schemes,for which the source's detection limits are analysed in terms of detection error probability and the achievable effective covert rates from the relay to destination are derived.Our examination determines the conditions under which the rate-control transmission scheme outperforms the power-control transmission scheme.As such,our conducted analysis enabled R to switch between these two strategies to achieve the maximum covert rate.Our investigation also demonstrated that covert communication in the considered relay networks is feasible and the effective covert rate achieved by R increases with its forwarding ability.4)This work examines the possibility,performance limits,and associated costs for a wireless-powered relay to transmit its own covert information to a destination on top of for-warding the source's information.Since the source provides energy to the relay for forwarding its information,the source does not allow the relay's covert transmission and is to detect it.Considering the time switching(TS)and power splitting(PS)schemes for energy harvesting,where all the harvested energy is used for transmission at the relay,we derive the minimum detection error probability at the source,based on which we determine the maximum effec-tive covert rate(ECR)subject to a given covertness constraint on the minimum detection error probability.Our analysis shows that the minimum detection error probability is the same for the TS and PS schemes,which leads to the fact that the cost of achieving the maximum ECR in both the two schemes in terms of the required increase in the energy conversion efficiency at the relay is the same.For example,the PS scheme outperforms the TS scheme in terms of achieving a higher maximum ECR when the transmit power at the source is relatively high.If the covertness constraint is tighter than a specific value,it is the covertness constraint that limits the maximum ECR,and otherwise it is upper bound on the energy conversion efficiency that limits the maximum ECR.5)To enhance the performance of point-to-point covert transmission,we propose a covert communication scheme where the transmitter attempts to hide its transmission to a full-duplex receiver,which is achieved by controlling the random transmit power of artificial noise at Bob to deliberately confuse the warden.Specifically,we first derive the false alarm and miss detection rate at the warden,based on which the optimal detection threshold is analytically determined and its expected detection error rate over wireless fading channels is achieved in a closed-form expression.Our analysis indicates that the artificial noise deliberately produced by the receiver with a random transmit power,although causes self-interference,offers the capability of achieving a positive effective covert rate for any transmit power subject to any given covertness requirement on the expected detection error rate.6)In order to hide both the wireless covert transmission and the transmitter itself,we adopt channel inversion power control(CIPC)to achieve covert communications aided by a full-duplex receiver.Specifically,the transmitter varies the power and phase of transmitted sig-nals as per the channel to the receiver,such that the receiver can decode these signals without knowing the channel state information.This eliminates the required feedback from the trans-mitter to the receiver,which aids hiding the transmitter from a warden.The truncated CIPC and conventional CIPC schemes are proposed and examined,where for truncated CIPC covert transmission ceases when the channel quality from the transmitter to the receiver is low,while for conventional CIPC covert transmission always occurs regardless of this channel quality.We examine their performance in terms of the achieved effective covert rate,which quantifies the amount of information that the transmitter can reliably convey to the receiver,subject to the constraint that the warden's detection error probability is no less than some specific value.Our examination shows that the truncated CIPC scheme can outperform the conventional CIPC scheme.