用重要性抽样方法估计分组码同时纠错和检错时的性能

研究用重要性抽样仿真方法估计线性分组码同时纠错和检错时的不可检错误概率。从理论上得到了最佳偏置转移概率和仿真效率的表达式。对 (1 5,7) BCH码和 (2 3,12 ) Golay码的仿真表明,重要性抽样是一种非常有效的仿真方法。     

Detectability of Spread-Spectrum Signals

Methods of detecting spread-sprectrum signals without knowledge of the pseudorandom code used to generate the signal are described. Exact and approximate methods of calculating relationships among detection probability, false alarm rate, and signal-to-noise ratio are given for radiometers and for channelized pulse-detection systems. The detection performance of the radiometer is compared graphically with that of pulse-detection systems, for two different kinds of pulse detection decision rules. Detection performance as a function of certain signal parameters is shown to be very different for a pulse-detection system than for a radiometer, and this difference in behavior provides a basis for selecting signal parameters that minimize the probability of detection. The reasoning that underlies the selection process is explained, and the process is outlined for each of several signal parameters.     

Soldier Identification System utilizing low probability ofintercept (LPI) techniques

This paper documents the design of a laser/radio frequency (RF) Soldier Identification (ID) System developed by Dynetics, Inc., Harris, Corp., and the US Army Communications and Electronics Command (CECOM). The Soldier ID system includes an Interrogation Unit with a programmable activation code. The Interrogation Unit consists of a directive, eye-safe laser and a spread-spectrum RF transceiver. This allows for a low probability of intercept (LPI) interrogation, which is of interest during covert operations. A Responder Unit is worn, for example, by a soldier and transmits an LPI spread-spectrum RF response, only after receiving the proper interrogation codes. The basic subsystems for the identification system are a Laser Interrogation Unit, an RF Responder Unit, and an electronic Programming Unit. The operating principles for the subsystem are reviewed, and the design issues are discussed. In addition to the preliminary design performed under Phase I of the program, a breadboard system was developed to validate the proof-of-principle concept. Hardware implementation is reviewed and test results are presented. The brassboard development and engineering plans are also presented     

Biphase Sequence Generation with Low Sidelobe Autocorrelation Function

Since Barker-like sequences could not be found for sequence lengths greater than 13, radar system designers have been searching for biphase sequences with discrimination (ratio of the magnitude of the mainlobe to the magnitude of the peak sidelobe of the autocorrelation function of the sequence) greater than 13. We present new results on biphase Legendre sequences for all prime lengths. The results of the paper are as follows. 1) Two lemmas on the symmetric properties of the Legendre sequences are stated and using these lemmas and simple rules of modulo arithmetic, a method is suggested for generating long sequences which does not require multiplications and divisions. 2) From the properties of autocorrelation function of the cyclically shifted Legendre sequence (LS) for prime lengths, a computationally efficient method is evolved for carrying out systematic search for sequences with discrimination greater than 13. The proposed method reduces the computational load by a factor of approximately 16. Using these techniques, we generated codes up to length 1019. In each case we searched for the code which gives the maximum discrimination for that length.     

分组码在有记忆删除信道上的性能计算

 本文首先描述了一种有记忆删除信道模型,然后提出了分组码在该信道上的性能的迭代算法。最后以Golay(23,12,7)码为例给出了多种信道参数之下的性能曲线。     

Cross-Correlation Interference Effects in Multiaccess Optical Communications

The effects of the cross correlation between user codes in an opticalcode-division multiple-access communication system are investigated. The system model is a multiaccess satellite repeater in which the uplink and downlink channels are direct-detection, optical-polarization modulation links. The error probability is derived in terms of the cross correlation between the intended and interfering user codes. It is shown that the system error rate can be minimized by using code sequences in which the normalized second moment of the cross correlation between codes is small. The signalto- noise ratio (SNR) on the uplink is shown to be proportional to 1/K while the SNR on the downlink is proportional to 1/K1/2, where K is the number of users which are simultaneously accessing the system.     

遗传算法在微弱GPS信号捕获方法中的应用

 研究了微弱GPS信号的捕获问题，以捕获时较低的虚警概率和较高的检测概率为目标，以累加时的粗捕获(C/A)码周期数和导航数据数，以及捕获门限等参数为决策变量，应用遗传算法(GA)来解决这一多目标优化问题。仿真结果表明，该方法能够有效地解决所提出的问题，所得解能够优化捕获时欲选取的参数，从而提高捕获性能。     

基于三类非线性函数的认证码的构造

为了将高度非线性函数应用于认证码构造理论．讨论了非线性函数的度量，给出了由三类非线性函数构造的Cartesian认证码，并且当编码规则按等概率分布选取时，分别计算了三类认证码的模仿攻击和欺骗攻击成功的概率。结果表明，第二类认证码模仿攻击成功的概率达到了下界。在n取值相同的条件下，第一类认证方案模仿攻击成功的概率比第三类的大。     

A new algorithm to optimize Barker code sidelobe suppressionfilters

The authors suggest a new algorithm for binary coding waveform sidelobe reduction after matched filtering and present a general method by which optimized sidelobe suppression filters for Barker codes can be obtained with a peak output sidelobe 2.62 dB lower than the results found in the literature (for 13-b Barker code). This optimization algorithm is also promising for other binary coding waveforms, such as truncated pseudonoise (PN) sequences and concatenated codes. This new approach can readily be applied to sidelobe-reduction filter design for other binary coding waveforms, such as truncated PN sequences, concatenated codes, etc., which often find their applications in radar systems and spread spectrum communication systems     

Codes cross-correlation analysis and data/pilot code pairs optimization for Galileo E1 OS and GPS L1C

The Galileo E1 open service (OS) and the global positioning system (GPS) L1C are intending to use the multiplexed binary offset carrier (MBOC) modulation in E1/L1 band, including both pilot and data components. The impact of data and pilot codes cross-correlation on the distortion of the discriminator function (i.e., the S-curve) is investigated, when only the pilot (or data) components of MBOC signals are tracked. It is shown that the modulation schemes and the receiver configuration (e.g., the correlator spacing) strongly affect the S-curve bias. In this paper, two methods are proposed to optimize the data/pilot code pairs of Galileo E1 OS and GPS L1C. The optimization goal is to obtain the minimum average S-curve bias when tracking only the pilot components a the specific correlator spacing. Figures of merit, such as S-curve bias, correlation loss and code tracking variance have been adopted for analyzing and comparing the un-optimized and optimized code pairs. Simulation results show that the optimized data/pilot code pairs could significantly mitigate the intra-channel codes cross-correlation, and then improve the code tracking performance of MBOC signals.     

Blind adaptive decision fusion for distributed detection

We consider the problem of decision fusion in a distributed detection system. In this system, each detector makes a binary decision based on its own observation, and then communicates its binary decision to a fusion center. The objective of the fusion center is to optimally fuse the local decisions in order to minimize the final error probability. To implement such an optimal fusion center, the performance parameters of each detector (i.e., its probabilities of false alarm and missed detection) as well as the a priori probabilities of the hypotheses must be known. However, in practical applications these statistics may be unknown or may vary with time. We develop a recursive algorithm that approximates these unknown values on-line. We then use these approximations to adapt the fusion center. Our algorithm is based on an explicit analytic relation between the unknown probabilities and the joint probabilities of the local decisions. Under the assumption that the local observations are conditionally independent, the estimates given by our algorithm are shown to be asymptotically unbiased and converge to their true values at the rate of O(1/k/sup 1/2/) in the rms error sense, where k is the number of iterations. Simulation results indicate that our algorithm is substantially more reliable than two existing (asymptotically biased) algorithms, and performs at least as well as those algorithms when they work.     

针对雷达LPI波形验证的目标RCS估算

根据某型雷达低截获概率波形设计的需要,分析了雷达探测目标的高频散射机理,并运用物理光学法、等效电磁流法计算了约定目标在不同姿态下和不同电磁波频率照射下的RCS值,从而为验证雷达波形在能探测到目标情况下的LPI性能提供了支撑。实验结果表明,基于电磁场理论的目标RCS估算方法是研究雷达LPI波形的一种有效工具,方法也适合于其他领域RCS估算的需要。     

线性分组码不可检错误概率新的下限

线性分组码的不可检错误概率是评价自动请求重传(ARQ)系统的重要参数。不可检错误概率的计算涉及码的重量分布,而大部分码的重量分布还不知道,因此,研究不可检错误概率的限显得非常重要。本文得到了GF()上(n,k)线性分组码不可检错误概率的两个新的下限,在一定程度上,改进了Wolf等人的结果。由于新的限与码的重量结构无关,因此可以方便地估计任何线性分组码的不可检错误概率。     

Biphase Barker-Coded Data Transmission Recommended Techniques for CAS

The use of biphase modulation and Barker data coding within the 200-us range/Doppler transmission proposed for collision avoidance systems (CAS) is described. Following a general discussion of requirements for compatibility between important CAS threat-evaluation measurements and less essential message communications, emphasis is placed on consideration of the binary Barker pulse sequences and the recommended biphase modulation techniques for data transmission.     

Random signal radar - a winner in both the military and civilian operating environments

Besides some general requirements, modern military radars should possess excellent LPI (low probability of intercept) properties. And civilian radars should satisfy the rigorous requirement of EMC (electro-magnetic compatibility) performance. The special features of random signal radar are analyzed here based on the different requirements for military and civilian use. Results show that random signal radar has excellent LPI property and EMC performance simultaneously, which can satisfy both military and civilian requirements.     

A Delay-Lock Loop for Tracking Pulsed-Envelope Signals

A sampled-data delay-lock loop (SDDLL) which tracks the arrival time of a biphase modulated, pulsed-envelope RF signal is described. A pseudo-noise (PN) code generator is used as the modulation source as in conventional delay-lock loop tracking devices. Time base shifting of the loop's local clock is performed by digital means. The technique permits the use of a stable-frequency clock to maintain accurate timing between timing correction instants. An expression is derived for the standard deviation of the timing error due to additive Gaussian noise at the SDDLL input. Experimental results are then given which include the effects of control-system quantization error on timing jitter. Assuming that the errors due to input noise and quantized timing corrections are independent and additive, the theoretical and experimental results agree to within approximately one decibel.     

Limitations of radiometer performance in spherically invariantnoise

The radiometer is a common method for detection of unknown signals in noise. Most analyses of radiometer performance are based on assumptions of stationary Gaussian noise with known marginal statistics. In this note, we use a spherically invariant noise model to derive simple expressions for radiometer performance degradation in noise variance uncertainty. Numerical examples are provided to show that channel uncertainty imposes a substantial penalty in detection performance     

Detection of a Distributed Target

The influence of increasing range resolution on the detectability of targets with dimensions greater than the resolution cell is studied. An N-cell target model is assumed, which contains k reflecting cells, each reflecting independently according to the same Rayleigh amplitude distribution. It will be referred to as the (N,k) target. Detection based on one transmitted pulse is performed against a background of white normal noise. Detection in stationary clutter is also considered. The optimum detector is obtained but, in view of its complexity, the performance of a simpler detector, the square-law envelope detector with linear integrator (SLEDLI), is analyzed, and a formula for the probability of detection is obtained. Graphs are presented which show the probability of detection as a function of signal-to-noise ratio (SNR) for various values of N k, and false alarm probability. For N/k not too large it is shown that the SLEDLI is near optimum.     

Quantization of PMT Signals for Meteorological Satellites

Quantization rules are examined for a satellite radiometer in the visual range. Effects of photomultiplier tube (PMT) noise on the detection of cloud brightness levels are of particular interest because the expected radiance levels are high. Effects of area integration are considered to improve the probability of detection at high radiance levels.