On the optimum processing gain of a block-coded direct-sequence spread-spectrum system

The optimization between the processing gain and the coding gain of a constant-bandwidth direct-sequence spread-spectrum communication system operating in the presence of either a tone jammer or a Gaussian jammer such that the bit-error rate (BER) is minimized is presented. Both bipolar phase-shift keyed pseudo-noise (PN) modulation and quaternary phase-shift keyed PN modulation are considered. Binary BCH codes are used to illustrate the techniques for optimization. The exact BER and its Gaussian approximation are derived and compared. During the optimization, two different approaches are used to obtain the indicator functions of the optimum code. These indicators are tested and shown to yield good results.<>     

Coherent Detection of Frequency-Hopped Quadrature Modulations in the Presence of Jamming--Part I: QPSK and QASK Modulations

This paper examines the performance of coherent QPSK and QASK systems combined with FH or FH/PN spread spectrum techniques in the presence of partial-band multitone or noise jamming. The worst-case jammer and worst-case performance are determined as functions of the signal-to-background noise ratio (SNR) and signal-to-jammer power ratio (SJR). Asymptotic results for high SNR are shown to have a linear dependence between the jammer's optimal power allocation and the system error probability performance.     

多频幅度调制已调信号的构建与仿真

相关文献对各种单频幅度调制已调信号已经作了描绘仿真,但是对于多频幅度调制已调信号的描述,由于计算量巨大,准确描绘仿真困难,现文献中未见介绍.文章利用方波信号的有限项构建多频调制信号,对模拟幅度调制的几种形式进行了数学描述,用MATLAB实现了信号仿真,将单频幅度调制信号描绘扩展到多频幅度调制信号的构建与仿真,得到了预期结果.     

Improving FH and FH/PN spread spectrum communications through PN M-ary phase sequences and signal editing

Pseudo-noise (PN) or direct sequence (DS) modulation, frequency hopping (FH) and their combination (i.e. the hybrid FH/PN approach) are common spread spectrum (SS) communications techniques used to achieve anti-jamming (AJ) and/or low probability of intercept (LPI). This study investigates the following FH and FH/PN related issues: (i) derive and apply M-ary phases of PN sequences to increase the AJ and transmission efficiency, (ii) use of erasure channel to reduce received errors and unreliable data, (iii) employ error correction coding and signal editing (SE) to improve communication surviv-ability, and (iv) adopt surface acoustic wave (SAW) devices to simplify the synchronization procedure in PN correlation. A special example is also given to illustrate the potential AJ improvement when an SS communications system employs both concatenated coding and concatenated SE techniques.     

Antijam capability analysis of RS-coded slow frequency-hoppedsystems

The application of Reed-Solomon codes in slow frequency-hopped systems has been extensively studied. Earlier investigations assumed an infinite interleaving length and considered partial-band noise jammers only. This paper extends previous efforts by analyzing the effect of finite interleaving length and the impact of band multitone jammers. We also explain why two-threshold (2T) erasure-insertion methods (EIM) are needed and examine their performance. Numerical results are presented to compare the effectiveness of the EIM and jammer types and to study the relationships among the hop rate, the interleaver size, and the code rate. The use of 2T-EIM necessitates the estimation of several additional channel and signal parameters. Simple and effective estimation algorithms are provided as well     

Optimization of a hybrid SFH/DS MFSK link in the presence of worstcase multitone jamming

The performance of a hybrid slow frequency hopped direct sequence multiple frequency shift keyed (SFH/DS MFSK) communication link in the presence of partial band multitone jamming is considered. The communication signal is characterized by a DS-spread MFSK signal bandwidth that may exceed the FH slot spacing, and the signal consists of multiple M-ary symbols per DS code period. An equation for the bit error rate (BER) is developed for the signal in the presence of multitone jamming. The equation is flexible enough to allow examination of optimization of jamming strategy. It is shown that the jamming strategy may be optimized by varying the number of tones, their placement, or both. It is shown that in the presence of optimized jamming, overlapping systems are more resistant to jamming than nonoverlapping systems with equal parameters (except for the number of FH slots). It is also demonstrated that, in some cases, the addition of DS spreading to an FH system, without increasing the total system bandwidth, can significantly improve the system's resistance to partial band multitone jamming     

Efficient Multitone Detection [DSP Tips & Tricks]

This article presents the digital signal processing (DSP) tricks employed to build a computationally efficient multitone detection system, implemented without multiplications and with minimal data memory requirements. More specifically, the article describes the detection of incoming dial tones, validity checking to differentiate valid tones from noise signals, and the efficient implementation of the detection system. While the discussion focuses on dual-tone multifrequency (DTMF) telephone dial tone detection, the processing tricks presented may be employed in other multitone detection systems     

Optimum binary spreading sequences of Markov chains

In the presence of multipath, even if it is supposed that the received signal is the input to the correlation receiver matched to the PN code signal of some user, the multiple-access interference from the other channels still depends on even or odd autocorrelation values. This is the most significant difference between the asynchronous direct sequence code division multiple access (DS-CDMA) systems with multipath and those without multipath. Here, two-state Markov chains are considered and the optimum binary spreading sequences generated by such Markov chains are given, as far as bit error probabilities of DS-CDMA systems in multipath environments are concerned     

窄带干扰环境下直扩信号伪码的盲估计方法研究

为了解决窄带干扰条件下低信噪比直扩信号扩频码的盲估计问题,该文提出了一种直扩信号的相关矩阵累加平均结合矩阵特征分解的方法。该方法首先按照已获知的直扩信号扩频码周期、码速率等二次统计特性参数对接收信号进行取样处理以获得一个观察向量集,接着用所提出的方法对该集合进行处理,其结果可以估计出信号所夹杂的窄带干扰(以多音干扰为例)特征波形,同时还可以进一步估计出信号的扩频码序列。利用前者,可以进一步消除信号所夹杂的窄带干扰,而利用后者则可以对接收信号实行盲解扩处理。理论分析和数值结果都表明了该方法非常有效,并能工作在较低的输入信噪比和很低的信号干扰比环境下。     

Performance analysis for impulse radio and direct-sequence impulse radio in narrowband interference

The impact of narrowband interference (NBI) on two ultra wideband (UWB) systems is analyzed. The two systems are impulse radio (IR) and a variation of it, termed direct-sequence IR (DS-IR). The signal-to-noise ratio (SNR) at the decision device of a correlation receiver is computed for both systems, assuming that the NBI is wide sense stationary and that the channel is frequency-selective. The SNR is expressed by means of a simple equation involving the signal and the interference spectrum. Next, a statistical model for the interference is introduced, considering the interference as the sum of a given number of sinusoidal signals with random powers and frequencies. The bit-error rate of IR and DS-IR is derived. The results are then specialized and compared with simulations in three case studies of practical interest, where the NBI is a single jammer with deterministic power and frequency, a multitone signal with random frequencies, or a grid of interfering signals with random powers.     

Direct-sequence spread-spectrum using directionally constrainedadaptive beam forming to null interference

A direct-sequence spread-spectrum communication system using an adaptive antenna array to null interference is examined. The system model is a modification of Compton's (1978) system, using a digital implementation and a directional constraint. Computer simulations are used to compare the behavior of the directionally constrained model with Compton's model with emphasis on spread-spectrum code synchronization. In the directed system, nulling of a tone jammer occurs without respect to direct-sequence code acquisition, and nulling of an off-beam desired signal occurs very slowly. Theoretical results are obtained by using the system covariance matrix, and incorporating the effects of spread-spectrum filtering to compute the system response as a function of time. The simulated results indicate that incomplete directional knowledge may be exploited to enhance signal quality and speed up code acquisition     

基于矩阵填充和三阶相关的长短码DS-CDMA信号多伪码盲估计

由于长短码直扩码分多址(LSC-DS-CDMA)信号包含了多个用户的长码和短码,已有的直扩码分多址信号的盲伪码估计方法不再适用。为此该文提出一种基于矩阵填充和三阶相关的伪码估计方法。首先从理论上将结构复杂的LSC-DS-CDMA信号构建为多用户短码扩频的缺失矩阵模型,将复合码矩阵估计建模为盲源信号分离问题;然后将矩阵填充理论应用于复合码矩阵估计,提出基于奇异值阈值算法和快速独立成分分析算法的各用户复合码序列估计方法;最后利用m序列的移位相加性特性,提出延迟三阶相关算法,从各用户复合码序列中估计其包含的长短伪码序列。仿真表明,当信噪比高于-2 dB时,该文算法的长短伪码估计平均误码率低于0.1%。     

多音干扰信号对直扩通信系统伪码捕获过程的干扰效果分析

传统的对直扩通信系统干扰样式分析一般认为多音干扰能量分散,干扰效果不如单音干扰理想。从组成多音干扰的每一个单音干扰分量入手,分析其干扰效果,进而得到多音干扰的总体干扰效果。仿真结果表明:在有用信号频率无法准确侦察的情况下,多音干扰能够有效提高干扰范围,干扰效果较好;在确定多音干扰频率分布时,应该充分考虑有用信号频率的分布,做到在其频率范围内的任意有用信号都有一个或者多个单音干扰分量能够对其有效干扰;在确定多音干扰的功率分配时,应该充分考虑有用信号频率的概率分布,对于干扰有用信号频率出现概率较大区域的单音干扰分量,其功率也应该较大。     

Frequency selective propagation effects on spread-spectrum receiver tracking

This paper presents results of investigations of frequency selective scintillation effects, due to transionosphetic propagation, on the performance of spread-spectrum receivers. Emphasis is placed on pseudonoise (PN) direct sequence systems. Pertinent features of the disturbed ionospheric propagation channel are described together with the characteristics of resulting satellite signal scintillations. Effects of frequency selective scintillations on PN spread-spectrum code correlation and tracking are illustrated in some detail. The design of the receiver PN code tracking loop is an important factor in determining the significance of these effects. Of the designs that have been investigated, a dedicated (non-time-shared) noncoherent code tracking loop appears to be least susceptible to losing code lock in the presence of frequency selective scintillation. Possible design modifications to further enhance the robustness of PN code tracking are discussed.     

Error rate for optimal follower tone-jamming

The performance of a countermeasure technique in the presence of the optimal follower multitone jammer is evaluated for frequency-hopped spread spectrum (FHSS) communications. It is shown that, with a certain probability, the optimal jammer will have dual tones in a frequency channel     

m序列复合码的一种快速相关算法

在直扩系统中,PN码的快速捕获十分重要。由于m序列的优选对有限,故在实际应用中采用复合码作为PN 码。常用的复合码主要有Gold序列和Kasami小集合序列。本文提出了一种利用快速沃尔什变换(FWT)实现对复合码进行快速相关的算法。这种算法适用于由两个m序列模2加形成的复合码,并原则上可以推广到Kasami大集合序列的捕获上。分析表明,我们提出的算法与传统相关算法相比,降低了算法的时间复杂度。     

A closed-loop coherent PN acquisition system with a pre-loopestimator

We propose a closed-loop system with a pre-loop code phase estimator (PLE) for acquisition of the pseudonoise (PN) signal in direct-sequence spread-spectrum systems. It consists of two major parts: a PLE and a closed-loop acquisition subsystem. The system uses an auxiliary signal, as opposed to the PN signal itself, to correlate with the incoming signal. The PLE uses the correlation result to obtain an estimate of the incoming code phase. Then, starting with this initial estimate, the closed-loop subsystem will acquire the code phase of the incoming PN signal. With coherent carrier demodulation, mean and variance of the acquisition time of the proposed scheme are obtained and compared to those of the conventional serial-search acquisition receiver and the closed-loop system without the pre-loop estimator. Results show that, with proper choices of parameters, the proposed system acquires the PN phase significantly faster than the other two systems     

多激光制导目标时激光角度欺骗干扰问题研究

针对多机编队空对地激光制导精确打击时激光角度欺骗干扰问题,对多目标时指示激光信号的方位和编码特征进行了理论研究。结果表明:当指示激光信号时间差小于任一信号的重复周期且信号的重复周期为整数倍关系,或者指示激光信号时间差大于激光欺骗干扰设备解码时间时,激光欺骗干扰设备可以对组合信号进行解码,并分析了相应干扰方法。上述两种情况之外则会组合为伪随机序列码信号,激光欺骗干扰设备不能正确解码及实施干扰。     

多径环境下异步长码直接序列码分多址信号伪码序列及信息序列盲估计

针对低信噪比 (SNR)下存在多径效应的传统单通道异步长码直接序列码分多址(DS-CDMA)信号伪码序列(PN)及信息序列难估计问题,该文提出一种基于平行因子的多通道盲估计方法。该方法先将接收到的多径信号建模为多通道模型,然后将长码DS-CDMA信号建模成短码DS-CDMA信号的缺失数据模型,形成观测缺失数据矩阵,并将其等效为缺失平行因子模型,最后利用正则交替最小二乘法(ALS)对缺失平行因子进行低秩分解,实现多径环境下长码DS-CDMA信号各用户伪码序列及信息序列的盲估计。仿真结果表明,序列的估计性能与多径环境密切相关,且在莱斯因子为10,多径路数为3,通道数为4,用户数为6,信噪比大于–10 dB的条件下,伪码序列及信息序列的估计错误率均低于1%。     

同/异步短码DS-CDMA信号伪码序列及信息序列盲估计

针对传统单通道异步直接序列码分多址(DS-CDMA)信号信息序列及伪码序列(PN)难以估计的问题,该文提出一种基于平行因子(PARAFAC)的多通道盲估计方法。该方法首先将信号建模为多通道接收模型,然后将观测数据矩阵等效为平行因子模型,最后使用迭代最小二乘算法对平行因子进行低秩分解,进一步完成对DS-CDMA信号各用户的信息序列及伪码序列进行估计。仿真实验表明,该方法不仅能有效地对同步、异步短码DS-CDMA信号的伪码序列及信息序列进行估计,而且能在通道数为6、信噪比(SNR)为–10 dB的条件下,实现10个用户伪码序列的有效估计。