一种混沌伪随机序列复杂度分析法

分析了已有的序列线性复杂度分析方法，提出了用近似熵算法计算混沌运动的测度熵，作为衡量混沌伪随机序列复杂度的标准.理论研究表明，利用较短的观察序列，该方法能够准确地反映混沌系统和混沌伪随机序列复杂度的大小，可以作为判断利用混沌系统产生的伪随机序列的复杂度准则.实验结果表明该方法的有效性和理论结果的正确性. 关键词： 混沌 伪随机序列 熵     

正交多载波M元循环移位键控扩频水声通信

针对水声扩频通信中频带利用率低的问题, 提出了一种M元扩频技术和循环移位键控(CSK)技术相结合的多载波调制方式, 将M元扩频后选择的扩频码同时采用CSK调制, 并调制在多个正交的载波上, 显著提高了频带利用率. 在水声多途信道下, 采用相同的通信速率和带宽, m序列、Kasami小集合序列和Gold序列为扩频码时, 比较了系统的误码性能. 仿真结果和实验结果均表明, m序列为扩频码时性能最优, Gold序列性能最差. 由于m序列数量有限, 为了进一步提高通信速率, 并使其他序列的性能接近m序列性能, 提出了一种复合序列抑制载波间干扰的算法, 该算法能够很好抑制多途干扰, 使复合序列性能接近m序列的性能. 关键词： M元扩频')" href="#">M元扩频 循环移位键控 正交多载波 载波间干扰     

基于光子间隙随机分布的真随机数源

提出了利用模数转换器提高真随机数源速率的方案, 该方案基于衰减脉冲激光光子间隙随机分布, 可使随机数的产生速率提高十几倍.实验中将时间幅度转换仪与16位模数转换器相结合, 产生的各比特位随机数序列顺利通过了国际通用的随机数检测程序统计测试标准, 该方案实验装置简单, 增强了系统的抗干扰能力. 关键词： 衰减式单光子源 模数转换 延时符合 随机数检测     

切比雪夫光混沌发生器的优化

研究了利用马赫-曾德尔干涉仪来构造扩频序列的设计方法,并对以往提出的切比雪夫光混沌序列发生器的构造原理和方案进行了分析,针对其受到器件尺寸限制的不足,提出了利用余弦函数的性质对其改进的新方案,从而消除了原方案在设计上的局限性.同时通过外加电压对马赫-曾德尔干涉仪两臂折射率的调节增加了波长适应性功能,使得光混沌序列发生器可以在不同的入射波长下得到不同的切比雪夫光混沌序列. 关键词： 切比雪夫映射 混沌序列 Lyapunov指数 光混沌序列发生器     

一种混沌扩频序列的产生方法及其优选算法

提出一种基于组合映射模型产生混沌扩频序列的方法. 根据扩频序列的特性要求和多址干扰性能指标,给出了一种混沌扩频序列的优选算法;将得到的优选序列应用于直扩码分多址系统,在不同信道条件下进行仿真,并与优选的Logistic混沌扩频序列进行性能比较,结果表明本方法产生的混沌扩频序列具有和Logistic混沌扩频序列相近的良好性能,而且保密性更好. 关键词： 码分多址 优选算法 多径信道 误码率     

一种多通道混沌调相扩频方式及其在水声通信中的应用

直接序列扩频通信是一种低检测概率通信方式,在保密通信中得到了广泛的应用。但该通信方式仍存在缺陷,其采用伪随机序列作为扩频码,而伪随机序列具有周期性和二值性等较为明显的特征,在敌对环境下这些特征可能被利用,使得直扩信号重要的特征参数被估计出来,进而引发信息泄露。针对这个问题,本文提出采用混沌调相序列代替伪随机序列,混沌调相序列是恒包络的复数值序列,可用数量多,相关特性好,而且克服了伪随机序列周期性和二值性的不足,具有更高的保密性。为了提高通信速率,将混沌调相序列与多通道技术相结合,数值仿真结果显示了该通信方式优异的误码率性能,表明该通信方式在水声通信中应用的可行性。      

一种新的大容量的二维光正交码

构造了一种扩展的双曲线性同余码(EHLC)并分析了其码字性能.用该码作为时间扩频伪随机序列和以素数码(PC)作为波长跳频伪随机序列,再构成一种新的二维光正交码EHLC/PC.然后分析了EHLC/PC码字的性能,并与EQC/PC作了比较.相比之下,前者的互相关性能略有下降,但码字容量大大增加.因此,EHLC/PC适合于用户数较多的OCDMA系统.     

一种基于扩展素数码和单重合序列的二维光正交码EPC/OCS

以扩展素数码(EPC)作为时间扩频伪随机序列，单重合序列(OCS)作为波长跳频伪随机序列，构造了一种新的二维光正交码EPC/OCS，并分析了码字的互相关性能.与修正素数跳频码MPHC相比，EPC/OCS的波长数并不局限于素数，可以是任意整数，不仅构造灵活，而且可充分利用MW OCDMA系统的有效波长数.当系统的有效波长数大于某个素数时，EPC/OCS不仅码字容量大于MPHC，而且互相关性能也有所改善.理论分析表明，EPC/OCS可降低MW OCDMA系统误码率.     

基于线性反馈移位寄存器和混沌系统的伪随机序列生成方法

结合线性反馈移位寄存器（LFSR）和混沌理论各自的优点,采用循环迭代结构,给出一种将LFSR和混沌理论相结合的伪随机序列生成方法.首先根据LFSR的计算结果产生相应的选择函数,通过选择函数确定当前迭代计算使用的混沌系统,应用选择的混沌系统进行迭代计算产生相应的混沌序列;然后把生成的混沌序列进行数制转换,在将得到的二进制序列作为产生的伪随机序列输出的同时将其作为反馈值与LFSR的反馈值进行相应的运算,运算结果作为LFSR的最终反馈值,实现对LFSR生成序列的随机扰动.该方法既可生成二值伪随机序列,也可生成实值伪随机序列.通过实验对生成的伪随机序列进行了分析,结果表明,产生的序列具有良好的随机性和安全性. 关键词： 线性反馈移位寄存器 混沌系统 伪随机序列 随机性     

DM-UKF混沌拟合破译混沌直接序列扩频通信

针对已有混沌直接序列扩频通信系统的破译算法在低扩频因子和较大多径衰落下无法有效破译信息码的问题,提出了一种基于双模型无迹卡尔曼滤波混沌拟合的破译算法.所提算法联合多模型滤波原理,利用信息符号为有限集的特点,针对其不同取值分别建立对应的滤波模型.各模型下滤波器并行工作,利用广义同步系统同时拟合原混沌系统并估计混沌直扩信号,通过估计误差确定最佳匹配滤波模型,从而得到信息符号的估计.进一步通过引入误差控制因子,增大了不同模型下估计误差的距离,不仅有利于信息符号判定,并且减小了噪声和多径衰落对破译结果的影响.理论推导和仿真结果均证明提出的算法优于已有破译算法.     

Comparison between direct-sequence and multicarrier spread-spectrum acoustic communications in time-varying channels

Underwater communication experiments have been conducted in the Norwegian Oslofjord. Two modulation schemes are compared in a 7-kHz frequency band on a 14-kHz center frequency. The first scheme is direct-sequence spread spectrum (DSSS), using a 7-chip spreading code to achieve a raw data rate of 1000 bps on a single carrier. The second scheme is multicarrier spread spectrum (MCSS) and accomplishes spreading by using seven subbands. The DSSS receiver equalizes on the chips prior to explicit symbol despreading, whereas MCSS features joint multiband equalization and despreading. Four channels are examined, from nearly static to overspread. In slowly varying channels, MCSS offers the best performance. DSSS has the best tracking potential for rapidly varying channels, where the challenge is to obtain reliable chip decisions before symbol despreading. The tracking potential can be realized to some extent by hypothesis-feedback equalization. It is further shown that adaptive equalizers are capable of code conversion, i.e., the DSSS receiver can demodulate the MCSS waveform, and vice versa. Neither receiver requires knowledge of the spreading code in order to despread the data.     

A spread spectrum technique for the study of outdoor noise propagation

This paper describes field measurements to assess innovative correlation techniques for the study of meteorological and topographical effects on sound propagation. To take advantage of the properties of coded signals in a time-varying system, the correlation signal is produced by the modulation of a code sequence onto an acoustic carrier. An established method of increasing signal-to-noise ratio is to use correlation techniques with maximum length sequences. However, this standard method is restricted in its use outdoors because of the time-variant nature of the atmosphere. On the other hand, the correlation properties of a directly carrier-modulated code sequence modulation signal may be exploited in a time-varying environment. An experiment is described in which the correlation properties of the spread spectrum signal are demonstrated and are used to calculate accurate times of flight that compare well with sonic anemometer measurements of speed of sound. The results illustrate that an acoustical spread spectrum system can provide significantly improved ways of measuring sound propagation outdoors.     

基于成像光通信的时间-空间扩频混合光码分多址编码方案

为了在成像光通信中充分利用时间和空间两种频带资源,以成像光通信为基础,提出一种利用二维光正交Pattern码结合时域扩频实现光码分多址(OCDMA)编码的新方案。分析在该方案中存在的多址干扰噪声,并给出了噪声的概率密度函数,然后讨论由多址噪声引起的误码率与判决门限、用户数、码长积和码重积之间的关系。在相同的码容量下与时域扩频光码分多址编码和空间扩频光码分多址编码方案相比,该方案能降低对相关器件性能(时域码片数量和空间像素规模)的要求,而且还可获得更小的误码率。最后给出编码解码的光学实现方案。     

Fourier频率域随机谱隐秘信息加载与增量补偿系统

提出一种基于光学原理的Fourier频率域随机谱隐秘信息加载与增量补偿系统,并给出一种隐秘信息的加载方式,可实现光学图像信息隐藏.宿主图像经过随机相位调制后进行Fourier变换得到频率域的随机谱,在此随机谱上加载隐秘信息,同时,系统对由隐秘信息的加入所产生的影响进行补偿.利用该系统所得到的载体图像未经正确的密钥-随机相位-调制时,隐秘信息将弥散在载体图像的Fourier频率域空间,具有较好的不可检测性.此外,即使攻击者知道隐秘信息的存在甚至知道隐藏方法,但在不知道密钥的情况下,依然无法破解隐秘信息的内容     

Chaotic direct sequence spread spectrum for secure underwater acoustic communication

Direct sequence spread spectrum (DSSS) has been widely used in underwater acoustic (UWA) communications. In DSSS communications, spreading sequence’s period and chip rate are important characteristic parameters. However, these characteristic parameters may be detected in hostile environments due to the periodic nature of spreading sequence and their well-known construction process. In this paper, a novel chaotic direct sequence spread spectrum (CD3S) method is proposed for secure UWA communications. In the CD3S communications, chaotic sequences acting as encryption keys are used to encrypt the phases of transmitted signals. Consequently, the application of chaotic sequences disguises these obvious characteristic parameters of spread spectrum (SS) signals. It is difficult for unauthorized users to detect or intercept CD3S signal without knowledge of corresponding chaotic sequences. Hence, CD3S signal has a lower probability of detection and interception. Moreover, CD3S can achieve similar bit error ratio (BER) performance compared with DSSS in actual UWA communications. Also, the receivers that are suitable for DSSS communication can also be applied to CD3S communication. Numerical simulations demonstrate its excellent performance and potential applications in confidential UWA communications.     

光纤中高斯型伪随机序列码偏振度的理论分析和实验研究

推导了准单色光波情况下高斯脉冲伪随机序列偏振度的简洁数学表达式.理论分析结果表明,偏振度与差分群延迟之间的关系与线路啁啾和光纤色散无关,并且此关系可以由光信号频谱半宽度Δω和分光比γ唯一确定.用10 Gbit/s 7级m序列归零码进行的实验表明:实验结果与理论推导基本一致,从而验证了推导公式的正确性.     

Optical security system employing shifted phase-encoded joint transform correlation

A new optical security system is proposed using a shifted phase-encoded joint transform correlation (JTC) architecture. In the proposed technique, at first, the address code is fed into two channels where one channel is shifted by 180°. The output signals from both the channels are phase-masked and then added with the input image to be encrypted. The joint power spectrum (JPS) obtained from one channel is subtracted from the JPS of the other channel, and the modified JPS is inverse Fourier transformed to yield the encrypted image. For decryption, the received signal is Fourier transformed and multiplied by the phase mask and the address code, which is then inverse Fourier transformed to generate the output signal. The proposed technique does not require complex conjugate of the address code otherwise required in the classical double random phase encryption. Also the decryption result is much more enhanced when compared to the output generated by alternate JTC techniques. Computer simulation results verify that the encryption and decryption are very much secure and efficient in both noise-free and noisy conditions.     

Breaking a chaotic secure communication scheme

The chaotic direct sequence spread spectrum (CD3S) has been extensively studied and accepted as an approach of chaotic secure communication. However, we find that CD3S is insecure and present the results of breaking CD3S. A modified unscented-Kalman-filter chaos fitting method is proposed. With the proposed method, an intruder can use a different chaotic system to fit the transmitter's chaotic system and break the CD3S scheme without any knowledge of the chaotic transmitter's structure, parameters or initial value, even under the following conditions: (A) the chaotic spreading sequence is complex and generated by two chaotic maps; (B) there exists noise in the CD3S signal; (C) the CD3S signal is constructed in frames. Simulation results verify the method.