【作者】 赵文；

【导师】 马洪；

【作者基本信息】 华中科技大学 ， 电磁场与微波技术， 2019， 博士

【摘要】 通信技术的飞速发展极大地改变了人们的日常生活方式,对社会发展产生深远影响。加密技术可保证通信系统的信息安全,保护合法用户的隐私,因此研究安全通信技术具有重要意义。基于密码学的网络层加密技术虽然可保证传输信息的保密性,但非合作方可通过开放的无线信道截获信息,并凭借处理能力不断提升的后端处理系统破解加密信息。相比之下,物理层加密技术可提供更可靠的保密性能,但目前通信系统的安全技术依然会受到诸多因素的制约,如:系统同步性能、加密参数初值敏感性、加密过程引起的系统功率损失以及安全通信距离等。为此,本文提出基于信道非线性变换的物理层加密技术,利用非线性变换改变信道传输特性,可有效降低非合作用户的信号接收质量,提升截获信号的破解难度。同时本文还对上述加密方法的非合作方解密策略进行研究,提出一种基于最优延时搜索的非线性盲辨识补偿算法,通过分析补偿前后加密信号解调效果的改善度,评估加密技术的安全性能。本文主要研究内容和成果包括以下几个方面:(1)针对物理层安全问题,讨论基于信道非线性变换实现物理层加密的基本方法:简述射频通信系统的非理想传输特性所引起的信号失真,分析各类失真的产生机制、系统模型以及模型辨识方法,通过理论分析和仿真实验,明确以信道群延时函数和传递函数非线性变换为核心的两种信道加密思路。此外,本文还对上述两种加密技术的非合作方解密策略进行分析,并针对加密系统的非线性失真和群延时失真级联效应,提出基于最优延时搜索的非线性盲辨识补偿思路。(2)针对物理层安全问题,提出一种群延时函数非线性变换(GDNT:Group Delay function Nonlinear Transformation)加密技术,其核心原理为利用群延时函数设计非线性变换对,使加密前后的信号相关性最小,从而满足完美保密性(PS:Perfect Secrecy)条件。受到严重码间串扰的影响,加密信号解调星座点散乱分布,无法经抽样判决译码为正确信息,且由PS条件知,此时非合作方只能通过随机猜测的方式推测原始信息内容。GDNT以加密前后信号相关性的频域形式为目标函数,以相关性最小为准则推导群延时函数加密参数的非线性方程组,并以傅里叶级数的改进模型表示群延时函数,利用加权非线性最小二乘算法(WNLS:Weighted Nonlinear Least Square algorithm)推导加密参数的迭代公式。经加密实验和非合作方解密实验验证,GDNT具有良好的加密效果和安全性能。(3)针对物理层安全问题,提出一种传递函数非线性变换加密技术,其核心原理为利用记忆非线性变换对(MNT:Memory Nonlinear Transform pair)改变传输信道的幅度、相位传输特性,在原始信号带内生成非线性频谱增生隐藏原始信号,降低其信号干扰比,恶化非合作方的信号接收质量。MNT基于信号三阶互调差频分量的幂级数模型构建非线性变换对,并对加密后信号的发射频谱进行优化,再利用相位滤波引入记忆效应,以振荡波函数和伪随机序列表示群延时,在频域设计相位加密滤波器。最后基于MNT加密系统进行信号加密效果测试和合作方信噪比损失分析,以此评估加密系统性能。(4)针对MNT加密系统研究非合作解密策略,通过分析加密系统的群延时失真和非线性失真的联合效应,提出一种基于最优延时搜索的非线性系统参数盲辨识补偿(GSIC:best Group delay Searching based nonlinear system blind Identification and Compensation)技术作为非合作解密方法。GSIC以强功率信号经非线性行为模型的输出信号与弱功率信号的瞬时功率残差作为目标函数,基于瞬时功率残差最小的辨识准则,推导求解非线性行为模型核系数的线性方程组,并利用加权迭代改善法(WII:Weighted Iterative Improvement algorithm)解决病态线性方程组求解问题,经仿真信号以及接收前端实采信号的非线性补偿实验验证,GSIC对接收系统弱非线性具有显著抑制效果。最后利用GSIC对MNT加密系统进行非合作解密测试,评估GSIC的解密性能,验证MNT系统频谱优化策略以及引入复杂记忆效应的必要性。更多还原

【Abstract】 The remarkable advancement in communication technology has greatly changed the way people live their lives and contributes to a profound impact on social progress.Since encryption methods are the most effective way to achieve data security and protect privacy of legitimate user,it is significant to study on security communication technologies.Although network layer encryption methods basing on cryptography can guarantee information confidentiality,illegal users may intercept the signals through open wireless channel and crack information using increasingly powerful backend system.By contrast,physical layer encryption method could provide more reliable secrecy performance.However,the existing communication security technologies are affected by many factors,such as synchronization performance,sensitivity of initial value,power loss caused by encryption processing and secury communication distance.For this reason,this paper propose the physical layer encryption methods based on channel state information nonlinear transformation.By nonlinear transformation,channel transmission characteristic is changed so that the signal reception quality of illegal users is effectively deteriotated and meanwhile the difficulty of decoding the intercepted signals is increased.On the other hand,illegal user’s decryption strategies for the above encryption methods are studied.A blind nonlinear identification and compensation method based on optimal group delay searching has been presented in this paper.The proposed encryption system’s security performance is evaluated by comparison of the demodulation effect before and after nonlinearity compensation.The main researches of this thesis can be concluded as follows:(1)For the physical layer security issue,the basic ideas of carrying out nonlinear transformation on channel state information to realize physical layer encryption are discussed: the non-ideal transmission characteristics of communication system will bring in signal distortions.Their generation mechanism,models and identification methods are discussed.After theoretical analysis and simulation,the encryption ideas of implement nonlinear transformation on channel’s group delay function and transmission function are established.Besides that,illegal user decryption strategies for the proposed encryption methods above are also discussed and a blind nonlinear identification and compensation method basing on best group delay searching is designed so that it can deal with the joint effect of nonlinear distortion and group delay distortion of encryption system.(2)For the physical layer security issue,a group delay function nonlinear transformation based encryption method(GDNT)is proposed.Its core principle is taking advantage of group delay function to generate nonlinear transformation pair so that the correlation of signals before and after encryption can reach its minimum value to meet the perfect secrecy(PS)condition.Suffered from inter-symbol interference,encrypted signal’s constellation will be scattered and neither can become correct information through sampling judgement and coding.According to PS condition,the only way for illegal user to obtain original information is randomly guessing.GDNT makes use of signal correlation in frequency domain to form objective and derives nonlinear equations of encryption parameters in group delay function when objective reaches its minimum.Then group delay function is expressed by improved Fourier series model and iterative formula of its encryption parameters is given basing on weighted nonlinear least square algorithm(WNLS).Finally,the results of encryption and illegal user decryption experiments show that GDNT has a satisfactory encryption effect and reliable security.(3)For the physical layer security issue,a transmission function nonlinear transformation based encryption method is proposed.Its core principle is taking advantage of memory nonlinear transform pair(MNT)to change channel’s magnitude-phase transmission characteristics so that nonlinear spectral regrowth is generated within original signal band.These nonlinear distortions will reduce original signal to interference ratio and deteriorate illegal user’s signal quality.MNT creates nonlinear transformation pair with power series model of difference frequency component of third order intermodulation.Then optimizing strategy for control band of encrypted signal is provided.In addition,memory effect is brought into MNT basing on phase filtering,where group delay is represented by oscillating wave function and pseudo random sequence respectively.The design steps of phase encryption filter in frequency domain are given.At last,the encryption effect and legal user’s loss of signal to noise ratio for MNT encryption system are shown to evaluate its performance.(4)For the sake of study illegal user decryption strategies for MNT encryption system,this paper analyzes its joint effect of group delay distortion and nonlinear distortion and provides a best group delay searching based nonlinear system blind identification and compensation method(GSIC)as illegal user decryption method.GSIC takes the instantaneous power residual error between weak signals and output signals when strong signals going through nonlinear behavior model as objective function.The linear equations of kernels of nonlinear behavior model are derived when objective reaches its minimum value.After that,weighted iterative improvement algorithm(WII)is used to solve the ill-conditioned equations.Through verification experiments of compensating nonlinearities of simulation signals and signals from real world front-end,GSIC’s nonlinear distortion suppression performance is satisfactory.Finally,GSIC is used as an illegal user decryption method for MNT encryption system to evaluate GSIC’s decryption performance and at the same time,the necessities of MNT system spectrum optimization strategy and deep memory effect are verified.更多还原