Research On Low Probability Of Intercept Communication Technology With Multiple Antennas

The rapid development of wireless communication technology has made the Internet of Things a reality,with increasing demand for wireless communication and everexpanding mobile internet services.However,the inherent broadcasting nature of wireless communication poses serious challenges to information security.As a significant supplement to traditional information encryption,physical layer low probability of intercept(LPI)communication technology has received considerable attention,aiming to ensure wireless communication security through signal concealment,feature hiding,and information obfuscation.By employing beamforming technology,multi-antenna communication can concentrate signals for transmission in specific directions,thus enhancing the resistance to interception of communication signals.Although a significant amount of research has been conducted on multi-antenna LPI technology,existing research still suffers from four major shortcomings: 1)low system covert rate? 2)high probability of co-directional interception?3)lack of near-field broadband secure communication schemes? 4)absence of multi-user covert access schemes.Addressing these challenges,this thesis focuses on multi-antenna LPI communication technology and primarily investigates the following four aspects from the perspectives of signal concealment and information obfuscation:(1)The thesis studies multi-antenna covert communication technology based on fasterthan-Nyquist(FTN)transmission to improve the system’s covert rate.Firstly,an analysis framework for detection performance in single-input single-output(SISO)scenarios is established for FTN transmission,and an optimal precoding scheme maximizing covert rate is proposed.Theoretical analysis and numerical simulation results demonstrate that precoded FTN transmission outperforms traditional FTN and Nyquist transmission in signal concealment performance.Subsequently,the thesis further investigates multi-antenna FTN covert communication technology,proposing a low-complexity linear precoding and equalization method.By decoupling the multiple-input multiple-output(MIMO)FTN channel into independent sub-channels,the maximization of covert rate problem is simplified into a power allocation problem,and an optimal power allocation scheme is proposed.Numerical results verify that the proposed FTN transmission scheme effectively enhances the covert rate of MIMO systems.(2)The thesis investigates two-dimensional covert communication technology assisted by frequency diverse array(FDA)to address the problem of co-directional interception in traditional phased array multi-antenna communication systems.By analyzing the geometric characteristics of covertness constraint under FDA-assisted maximal ratio transmission,two performance metrics are firstly proposed: the feasibility of two-dimensional covertness and the covert rate under a given area with no eavesdroppers,to evaluate the covert communication performance of FDA.Subsequently,a two-dimensional beamforming method for FDA is proposed to jointly optimize the beamformer and carrier frequency increment.For the perfect channel state information(CSI)scenario,closed-form beamformer for maximizing the covert rate is provided,along with a frequency increment optimization scheme based on block successive upper-bound minimization(BSUM)framework.For partially known CSI scenarios,an optimization problem maximizing the worstcase secrecy rate is constructed,and the beamformer and frequency increment are optimized separately using convex hull and BSUM over sample space(BOSS)techniques,respectively.Simulation results demonstrate that FDA achieves not only signal concealment in direction and distance dimensions but also significantly improves the covert rate compared to traditional phased array systems.(3)The thesis studies secure analog beamfocusing technology under near-field broadband effects to enhance system secrecy rate.Addressing the near-field effects and broadband beamsplitting phenomenon,based on cascaded true-time delayer(TTD)and phase shifter(PS),an analog structure is proposed to maximize the secrecy rate of near-field broadband transmission through subcarrier power allocation and analog beamfocusing.However,the constant modulus constraint of the analog structure leads to non-convex optimization problems.To address this,a two-stage approach is proposed.Firstly,a semi-digital surrogate problem for the original problem is constructed,and the subcarrier power and semi-digital beamformer are optimized using the alternating optimization(AO)framework.Then,by alternately optimizing the delay of TTD and the phase of PS,the analog beamformer is approximated to the semi-digital beamformer.Additionally,by utilizing the geometric properties of near-field broadband propagation,a low-complexity beamfocusing method is proposed to configure TTD and PS according to the beamsplitting trajectory equation.The obtained values can serve as robust initial values for optimizationbased methods.Numerical results confirm the superiority of the proposed method over traditional methods,significantly enhancing the secrecy rate of near-field broadband transmission at high energy efficiency.(4)The thesis investigates multi-user covert communication technology in multiantenna systems to address the challenge of multi-user covert access.Firstly,the thesis adopts rate-splitting multiple access(RSMA)to simultaneously serve multiple conventional and covert users.To balance the performance of covert and conventional communication,an optimization problem maximizing the minimum covert rate while meeting the requirements of conventional rate and covertness constraints is constructed,and an AO method is proposed to solve this non-convex problem.Numerical results confirm the advantage of RSMA over other multiple access schemes in improving multi-user covert rate.Subsequently,the thesis considers an integrated sensing and communication(ISAC)system comprising multiple radar targets and conventional and covert users,considering deterministic and probabilistic CSI error models.By jointly optimizing the transceiver beamformers and the covariance of the dedicated radar waveform,robust ISAC transceivers are designed under the worst-case and outage-constrained scenarios,balancing the performance of radar,conventional communication,and covert communication.Due to CSI errors,the optimization problems are non-convex.By comprehensively utilizing a series of techniques including S-procedure,Bernstein-type inequalities,and matrix lifting,an alternating double check(ADC)optimization framework efficiently solves the problem.Numerical results demonstrate the effectiveness of the proposed method in balancing the performance of radar,conventional communication,and covert communication.