【作者】 张春海；

【导师】 张尔扬；

【作者基本信息】 国防科学技术大学 ， 信息与通信工程， 2006， 博士

【摘要】 直接序列扩频(DSSS)技术具有良好的保密性、灵活的信道分配能力以及较强的抗多径、多址干扰能力,在个人通信网、无线局域网、第三代移动通信、卫星通信以及军事战术通信等领域得到广泛应用。直接序列扩频系统本身固有一定的抗干扰能力,研究表明,不增加扩频增益的条件下,在接收机解扩之前借助信号处理的方式对接收信号进行预处理可以显著增加系统的抗干扰能力。本文结合中频数字化抗干扰接收机的后续发展需求,主要对DSSS系统抗干扰技术进行深入地探讨。给出简化的接收机信号模型和常用的窄带干扰模型(音调干扰、AR干扰、低速数字干扰等),分析了DSSS技术对阻塞高斯干扰、单音干扰、部分频带干扰等的抑制性能。由不同的干扰模型可以得到不同的时域处理技术。总结和比较了基于AR干扰模型的各种干扰估计抵消算法性能;从音调干扰模型出发,提出一种新的基于子空间跟踪的时域干扰抑制技术;讨论了基于低速数字干扰等效“虚拟”多用户干扰模型的最小误码率接收机的变换域归一化最小均方自适应实现结构及其稳态性能。变换域处理技术对干扰模型不敏感,是目前最有潜力的处理方法。文中首先给出基于多速率信号处理和滤波器组理论的变换域抗干扰处理的统一理论框架,离散傅立叶变换、重叠变换、余弦调制滤波器组等都可以看作是滤波器组的特例:其次研究了基于重叠加窗DFT变换的窄带干扰抑制技术,分析了加窗DFT变换对扩频信号的信噪比损失,并比较了重叠相加法、重叠选择法等的性能,在变换基的长度足够大的条件下,基于中心极限定理得到了变换后扩频信号和噪声之和为窄带高斯过程的结论,在此基础上提出一种基于谱线幅度的平方服从指数分布假设的自适应门限窄带干扰抑制算法;最后研究了基于余弦调制滤波器组的干扰抑制技术,讨论了二次约束最小二乘法、罚函数法等原型低通滤波器设计方法,并提出一种基于子带功率估计结合最小Akaike信息准则的修正的K谱线干扰抑制算法。讨论了基于时频分布干扰瞬间频率估计结合IIR滤波器设计的慢时变线性扫频干扰抑制技术。提出利用IIR滤波器取代FIR滤波器进行干扰抑制,以牺牲部分相频特性为代价,换取干扰抑制滤波器更好的幅频响应,并推导了Kuwan-Martin结构IIR滤波器信干噪比改善因子的闭合表达式,结果表明,IIR滤波器对DSSS信号的失真远小于FIR滤波器,信干噪比改善因子与干扰瞬时频率及系统扩展比无关,性能优于时变FIR滤波结构。更多还原

【Abstract】 Direct sequence spread spectrum (DSSS) communication systems have many advantages, including low probability of intercept (LPI) transmission, multiple accessing, multipath suppression and narrowband interference suppression. It has been widely used in personal communication network (PCN), wireless LAN (WLAN), 3G mobile communication and military tactical communication. As is well known, the inherent processing gain of spread spectrum systems will, in many cases, provide the system with interference rejection capability. But if the level of interference is too high, the interference immunity can be improved significantly by using signal processing techniques that complement the spread spectrum modulation.The research of this dissertation is based on the demands of DSSS narrowband interference suppression intermediate frequency digitalized receiver. A simplified signal model of digitalized receiver and the signal model of interference, such as tone interference, autoregressive interference and digital interference, are provided. And it is analyzed in detail the capability of DSSS communication systems in suppressing broadband Gaussian interference, single tone interference and partial band interference.Different time domain interference suppression techniques can be established for different interference models. Firstly, the performances of linear and nonlinear algorithms based on interference estimator/subtracter are compared. Secondly, a novel interference suppression algorithm based on subspace tracking is proposed for the tone interference model. Finally, the stability performance of the transform domain normalized least mean square (TDNLMS) adaptively implementing structure of minimum bit error ratio (MBER) receiver is analyzed.Filter bank is the multirate signal processing structure, which decomposes a digital signal into a bank of frequency bands. The discrete Fourier transform (DFT), lapped orthogonal transform (LOT) and cosine modulated filter banks (CMFB) are all special cases of this general structure. Firstly, the signal-to-noise ratio degradation of windowed discrete Fourier transform (WDFT) is studied and the improved methods, namely overlap-add and overlap-select, are proposed. In the case that only the direct sequence spread spectrum signal and additive Gaussian noise are present at the receiver, the square of amplitude of the received signal after WDFT is assumed to be exponential distributed. An improved narrowband interference detection and suppression algorithm is proposed based on this assumption. Cosine-modulated filter banks have been studied extensively due to their virtues of easily design and efficient implementation. The design of prototype filter satisfying perfect reconstruction property has been formulated as a quadratic-constrained least-squares (QCLS) optimization problem, whose constrained matrices are symmetric and positive definite. A near perfect reconstruction prototype filter design technique is employed in this dissertation. And a novel algorithm for narrowband interference suppression based on subchannel power estimation and modified K-bins is introduced.The interference characterized by its instantaneous frequency can be effectively mitigated in DSSS communications using open-loop adaptive excision filters. On condition of the accurate estimation of instantaneous frequency of the interference, the closed-form expression of signal to interference and noise ratio (SINR) improvement of Kwan-Martin IIR filter is derived. The IIR filter introduces less distortion to DSSS signal compared with the five-coefficients FIR filter and the improvement of SINR is independent of the instantaneous frequency of interference and the spreading gain.更多还原