自编码扩频系统的信号同步和检测技术研究

【作者】 林丹；

【导师】 李仲令；

【作者基本信息】 电子科技大学 ， 通信与信息系统， 2006， 硕士

【摘要】 扩频技术由于其具有抗干扰、隐蔽性好等优点,广泛应用于军事和民用通信系统中。然而,现在使用的扩频序列都是基本固定的伪随机序列,它的周期性和确定性与信息论对信号波形的要求相抵触,而且可能被窃听者复制,使信息安全性受到威胁。为了提高扩频通信系统的抗干扰性、降低信号被截获的概率,我们希望扩频码是随机的、动态变化的。自编码扩频通信是一种较佳的安全通信方式,它利用传输的数据信息产生扩频码,避免了使用m序列、混沌序列等伪随机序列发生器,保证了扩频序列的随机性、动态可变性和信号的低截获率。扩频信号的解扩需要在接收端产生本地扩频序列。自编码扩频系统的扩频序列是接收端事先未知的,不能用事先设计的本地扩频序列产生器产生,需要由恢复的数据获得。因此,迅速、可靠地建立同步是自编码扩频通信最主要的环节之一。扩频序列的同步包括序列捕获和序列跟踪两个阶段。本文首先概述了自编码扩频通信的原理、自编码扩频序列的产生方法及其特性和扩频通信系统编码同步的理论,然后针对自编码扩频通信系统提出了扩频序列捕获方案:周期性地加入混沌序列同步码,并采用恒虚警率匹配滤波器捕获法;在加性白高斯噪声信道、瑞利衰落信道和多址干扰情况下进行了仿真,分析了各种捕获性能:在选择性能最优的混沌序列、适当的序列长度、虚警概率及门限值的情况下,可以获得较短的捕获时间和较大的捕获概率。捕获阶段完成之后进入跟踪阶段,本文采用自动调节系统惯用的方法,利用锁相环原理来完成扩频系统的码同步跟踪过程。针对锁相环的重要组成部分-鉴相器和环路滤波器做了具体分析,提出了将卡尔曼滤波器用做环路滤波器以改善环路的跟踪性能。由卡尔曼滤波器实现的具有时变增益的数字锁相环可以使环路在捕获时具有较大的增益,而锁定以后增益减小,从而同时实现快速捕获和可靠跟踪;并可以避免假锁问题。接收数据的最终目的是为了检测出数据,首先是对扩频信号进行解扩,其次是对剩下的载有信息的信号进行解调。扩频信号的解扩是通过与一个本地参考信更多还原

【Abstract】 Spread spectrum technique has been widely used in military and civilian communications for its advantages, such as anti-jamming (AJ) and low probability of detection (LPD). However, the traditional spreading codes are usually deterministic pseudo-noise (PN) codes, which can be duplicated, potentially compromising the transmission security. Then the time varying and random spreading codes are expected. In this thesis, we present a novel spread spectrum communication system, Self-Encoded Spread Spectrum (SESS) system, which does not use PN codes, and is unique in that traditional transmit and receive code generators based on maximal-length sequences or chaotic signals are not needed. Then the spreading codes are stochastic and variable.The data recovery at the receiver requires the spreading codes. In SESS system, the receiver doesn’t know which spreading code will be used, so first it must de-spread the received signal correctly to regenerate the spreading code. Therefore, fast and reliable synchronization algorithm is one of key technologies in SESS system. The process of synchronization is ordinarily accomplished in two stages, PN acquisition and PN tracking.In this thesis, first, we present the theory of SESS system, the generation of the SESS spreading code and its characteristics and the acquisition theory of conventional spread spectrum communication system. An efficient acquisition scheme based on periodically transmitting the synchronization head, which is composed of binary chaotic codes, using the matched filter and automatic decision threshold-level control based on a so-called constant false alarm criterion for SESS system is present. The acquisition model of SESS system is built and simulated in the AWGN channel, the Raleigh fading channel and imulti-address interfere condition. If choosing the chaotic code with best performance and proper length, proper probability of false alarm and threshold, small acquisition time and large acquisition probability will be achieved.更多还原