On cross-ambiguity properties of Welch-Costas arrays

We discuss cross-ambiguity properties of a specific family of Costas arrays called Welch-Costas (W-C) arrays. These properties are of interest in multiuser radar and sonar system, especially since Costas arrays are known to possess ideal auto-ambiguity functions. The theory of W-C arrays is reviewed. It is then proved that only pairs of W-C arrays can have at most two hits in their cross-ambiguity function (best possible case). The maximum number of hits in the cross-ambiguity functions of a family of W-C arrays is shown to be a function of the number of W-C arrays in the family. The upper bound on the number of hits in the cross ambiguity functions for a family of W-C arrays is also derived. Specific examples of how reducing the number of W-C arrays improves the cross-ambiguity properties are given for various types of prime numbers     

Periodic ambiguity functions of EQC-based TFHC

The time-frequency hop codes based upon extended quadratic congruences (TFHC-EQC) were developed previously (1988, 1991). The known results about the ambiguity functions of the TFHC-EQC are extended from aperiodic cases to periodic cases. Precisely, the following new results are proved: (1) The tight upper bound for the periodic auto-ambiguity function of the TFHC-EQC is 6; (2) the periodic cross-ambiguity upper bound of the TFHC-EQC is at least (N-1)/2; (3) there exists a subset TFHC-EQC of volume (N-1)/2 such that its periodic cross-ambiguity is tightly upper bounded by 8     

Time-frequency hop codes based upon extended quadratic congruences

Time-frequency hop codes are developed that can be used for coherent multiuser echolocation and asynchronous spread spectrum communication systems. They represent a compromise between Costas codes, which have nearly ideal autoambiguity but not so good cross-ambiguity properties, and linear congruential codes, which have nearly ideal cross-ambiguity but unattractive autoambiguity properties. Extended quadratic congruential code words are shown to have reasonably good autoambiguity and cross-ambiguity properties across the whole class of code sets considered. A uniform upper bound is placed on the entire cross-ambiguity function surface, and bounds are placed on the position and amplitude of spurious peaks in the autoambiguity function. These bounds depend on the time/bandwidth product and code length exclusively and lead naturally to a discussion of the design tradeoffs for these two parameters. Examples of typical autoambiguity and cross-ambiguity functions are given to illustrate the performance of the new codes     

Ambiguity properties of quadratic congruential coding

The ambiguity characteristics of multiple access frequency hop codes based on standard quadratic congruences are investigated in the light of results obtained for codes based on Costas arrays and extended quadratic congruences. While the autoambiguity properties are found to be very similar to those of Costa codes, i.e. nearly ideal, the cross-ambiguity properties of quadratic congruential codes are much better. These results are valid across the whole class of code sets considered, but they are obtained at some expense in the pulse compression characteristics of the codes. A uniform upper bound is placed on the entire cross-ambiguity function surface, and bounds are placed on the amplitude of spurious peaks in the autoambiguity function. These bounds depend on the time-bandwidth product and code length exclusively and lead naturally to a discussion of the design tradeoffs for these two parameters. Examples of typical autoambiguity and cross-ambiguity functions are given to illustrate the performance of quadratic congruential coding with respect to Costas coding     

Cochannel Interference Analysis between Spaceborne and Terrestrial Radars

An analysis of cochannel interference between spaceborne and terrestrial radars is presented. A generic modeling methodology is applied, which, it is felt, will represent the worst-case situation. In order to account for Doppler shifts, auto-and cross-ambiguity functions ions have been developed for determining matched filter outputs. The results support the ability of such radar systems to coexist relatively harmoniously utilizing currently accepted assignment procedures.     

Multiple Access Frequency Hopping Patterns with Low Ambiguity

A technique is discussed for the design of multiple access frequency hopping patterns which will allow a number of users to communicate reliably over a limited bandwidth in the presence of Doppler and timing asynchronies. The patterns are based on a subset of the Reed-Solomon codewords. They possess nearly flat ambiguity and cross-ambiguity functions and thus could be useful as discrete radar signals. In addition the structure of the patterns permits a straightforward Implementation.     

Synchronization improvements using traceability in spread spectrumsignal design

Synchronization improvements using traceability information found in the narrowband auto-ambiguity function (NB AAF) are demonstrated. This new auto-ambiguity function property, traceability, is the key to defining a signal design approach for solving the synchronization problems in spread spectrum system such as communication systems, radar networks, and sonar systems. This work defines the property and introduces a new tracing synchronization scheme that significantly reduces initial synchronization time and the number of false synchronization events     

Ambiguity Functions for Quadratic Phase-Coded Pulse Trains

The matched filter ambiguity function is presented for a burst waveform composed of repeated subbursts, each one of which consists of N pulses in which the phase is varied quadratically from pulse to pulse. The resulting ambiguity function exhibits small residual ambiguities along the delay axis separated by the reciprocal of the pulse repetition frequency (PRF). A cross-ambiguity function is derived which reduces these ambiguities to zero amplitude. A third cross-ambiguity function is presented for a receiver matched to a generalized Hamming weighted repeated quadratic burst. The location in the delay/Doppler plane of the waveform ambiguities for these waveforms is compared with that of an uncoded pulse burst.     

Poly-phase codes and optimal filters for multiple user ranging

A technique is introduced to select poly-phase codes and optimal filters of a pulse compression system that have specific temporal and frequency characteristics. In the particular problem under study, multiple vehicles are assigned unique codes and receiver filters that have nearly orthogonal signatures. Narrowband users, that act as interference, are also present within the system. A code selection algorithm is used to select codes which have low autocorrelation sidelobes and low cross correlation peaks. Optimal mismatched filters are designed for these codes which minimize the peak values in the autocorrelation and the cross correlation functions. An adjustment to the filter design technique produces filters with nulls in their frequency response, in addition to having low correlation peaks. The method produces good codes and filters for a four-user system with length 34 four-phase codes. There is considerable improvement in cross and autocorrelation sidelobe levels over the matched filter case with only a slight decrease in the signal-to-noise ratio (SNR) of the system. The mismatched filter design also allows the design of frequency nulls at any frequency with arbitrary null attenuation, null width, and sidelobe level, at the cost of a slight decrease in processing gain     

Blind beamforming/geolocation for wideband-FFHs with unknownhop-sequences

An ESPRIT based adaptive geolocation and blind interference rejection scheme is herein proposed for multiple noncooperative wideband fast frequency-hop (FFH) signals of unknown hop sequences and unknown arrival angles using one electromagnetic vector-sensor. Beamforming weights need no adjustments to accommodate the frequency hops. The sources' polarizational diversity, angular spatial diversity, and temporal characteristics are all exploited. Applications include electronic surveillance and direction finding of noncooperative sources, plus blind cochannel interference rejection in frequency-hop code-division multiple-access (FH-CDMA) wireless communications     

Generalized Response of a Linear FM Pulse Compression Matched Filter

A pulse compression matched filter is analyzed so that the response may be computed when the pulse width, FM rate, and center frequency simultaneously differ from design conditions. Unilateral and bilateral time domain amplitude weighting for sidelobe reduction is included. A general cross-ambiguity function is defined to include these effects and some basic computed results are presented for the peak envelope response with various degrees of Hamming weighting. Computer evaluation of this cross-ambiguity function allows one to choose a combination of mismatches for signal design trade-off between resolution and detection performance. Since no restrictions are placed upon the mismatch parameters, this analysis may also be used to evaluate the filter discrimination against various interfering signals.     

线性分组码限距离译码器的性能分析

为了提高自动重传请求(ARQ)系统的通过率,在混合ARQ系统中,使用限距离译码器改善系统的性能。在限距离译码时,线性分组码纠正重量t的错误图样。纯纠错译码器和纯检错译码器都是限距离译码器的特例。本文首先介绍了Krautchouk多项式的几个重要性质,然后,用线性分组码及其对偶码的重量分布,分别导出了两种形式的错误译码概率的简化计算公式。由于生成函数方法的使用,成功地得到了重复码、汉明码和扩展汉明码的错误译码概率的解析公式,最后,讨论了Golay码和某些本原BCH码错误译码概率的计算方法。     

Efficient wideband signal parameter estimation using a radon-ambiguity transform slice

A novel efficient technique based on a single slice Radon-ambiguity transform (RAT) for time-delay and time-scale estimation is proposed. The proposed approach combines the narrowband cross-ambiguity function (NBCAF), the wideband cross-ambiguity function (WBCAF), and a single slice RAT to estimate multiple target parameters in noisy environments. The square modulus of Gaussian-enveloped linear frequency modulated (GLFM) signals has high-energy centrality in the ambiguity plane. Its peaks in the NBCAF fall along nearly straight lines whose slopes depend on the Doppler rates of the moving targets. These lines could be effectively detected by computing the entire Radon transform of the NBCAF for all possible angles; however, it is a computationally intensive procedure. It is shown that without calculating the entire RAT, it is possible to estimate target parameters using only a single slice of the RAT, i.e., using an appropriate projection of the NBCAF. It is demonstrated that the proposed method can successfully separate overlapping targets efficiently. The efficiency is achieved due to fast Fourier transform (FFT)-bascd processing, use of a single slice of RAT, and the use of only one-dimensional (1-D) searches.     

Sets of One and Higher Dimensional Welti Codes and Complementary Codes

A Welti code is a binary sequence with an impulse-like autocorrelation function. A set of such codes may possess vanishing cross-correlation functions. The elements of Welti codes must be members of a set of at least two orthogonal vectors or subcodes. First, methods for synthesizing sets of one-dimensional Welti codes with vanishing cross-correlation functions, and conditions upon their existence are discussed. Then, construction methods of sets of two and higher dimensional Welti codes are presented. Based on these constructions, further sets of mutually orthogonal complementary codes in one or more dimensions can be derived. The use of such signals relates to various topics such as communication, radar and navigation systems, measuring and identification in one or higher dimensional systems, synchronization and spatial alignment, or coded aperture imaging.     

Group-Complementary Array Coding for Radar Clutter Rejection

A radar waveform design technique which utilizes Lagrange's method of multipliers to control temporal sidelobes and to reduce Doppler sidelobes is described. This classical method of constrained optimization is applied to the problem of synthesizing a radar wave-form where mismatch loss is the objective function to be minimized. The associated constraints are taken from expressions for the composite temporal sidelobes of the cross-correlation response and the peak correlation response where sets of code words are used to modulate a series of radar pulses. The resulting code sets and receiver reference sets are called group-complementary and produce a trench parallel to or on the range axis of the cross-ambiguity surface.     

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。     

Quasi-Complementary Sequences

A method of constructing digital phase reversal codes which have few nonzero sidelobes and which are available in frequent lengths is given. This is particularly important in the area of longer codes. The codes are a form of combined sequence and, hence, have the desirable property of bandwidth reduction for the outer code. However, only two member codes are required. A major usage of these codes may be in addressing.     

Turbo乘积码编译码算法研究

介绍了基于Chase算法[1,3]的Turbo乘积码软入软出(SISO)迭代译码算法。对Turbo乘积码在加性高斯白噪声(AWGN)信道下的性能进行了仿真和分析,结果表明:Turbo乘积码译码复杂度较低,在编码效率较高时,仍具很好的译码性能。     

机翼设计软件的完善及商用化研究

在对原有的机翼设计软件进一步完善的基础上,开发了基于Windows操作系统的设计软件工程包;通过图形用户界面实现了设计过程的菜单式选择和设计变量的模块化输入;开发了设计过程中目标压力分布的可视化修改模块,设计过程的实时显示模块,及与其它图形绘制软件的接口;提高了软件的可视化程度,使原有的机翼设计软件的商用性得到了很大的提高,探讨了国产CFD软件商用化的可行性。     

基于非线性函数认证码的新构造

基于非线性函数构造了一类新的认证码,计算出了该认证码替换攻击成功的概率和模仿攻击成功的概率,并与前人所做的认证码进行了比较,分析了所构造认证码的优缺点。