Lock detection in Costas loops

Previous analyses of lock detector algorithms for Costas loops have ignored the effects of the inherent correlation between samples of the phase error process. In this work, analysis and simulations are used to quantify the effects of phase correlation on lock detection for the `square law' and `absolute value' type detectors. Results which depict the lock detection probability as a function of loop signal-to-noise ratio for a given false alarm rate are obtained. It is shown that the square law detector experiences less degradation due to phase jitter than the absolute value detector and that the degradation in detector signal-to-noise ratio is more pronounced for squarewave than for sinewave signals     

False-Lock Performance Improvement in Costas Loops

A technique is described to reduce the probability of false-lock in Costas loops. This is where a frequency offset arises at a multiple of half the incoming data rate due to finite bandwidth of the arm filters. A modification is described to process components of this offset so as to augment the feedback control voltage. The performance is analyzed and it is shown how certain false-lock positions may be avoided.     

Tracking Performance of Costas Loops with Hard-Limited In-Phase Channel

It is becoming increasingly popular in the design of suppressed carrier receivers, which employ Costas loops for earrier reconstruction, to hard-limit the output of the in-phase channel. Doing so allows replacement of the analog multiplier, which forms the loop error signal, with a chopper-type device which typically exhibits much less dc offset. The false lock behavior of such a hard-limited loop was recently investigated and shown to be quite different from that of the conventional Costas loop without the hard limiter. This paper presents the companion, analysis of the tracking performance of the hard-limited loop and assesses the penalty, if indeed it is a penalty rather than an improvement, in this performance relative to the conventional Costas loop with an analog third multiplier. In particular, for the case ofRCarm filters and NRZ data, the squaring loss (or equivalently the linear loop tracking jitter) is evaluated and illustrated as a function of the ratio of arm filter bandwidth to data rate and data signal-to-noise ratio. Superimposed on these numerical results will be the corresponding ones for the conventional Costas loop. As a finale, the equivalence in operation of the Costas loop with hard-limited in-phase channel and a baseband modulation carrier reconstruction loop referred to as a demod/ remod loop is discussed.     

On the Calculation of Squaring Loss in Costas Loops with Arbitrary Arm Filters

The calculation of the optimum performance of suppressed carrier receivers with Costas loop tracking is directly related to evaluating the loop's so-called squaring loss. Recent work by the author and others presented specific numerical results for this loss when the input data were biphase-L(Manchester coded) and the Costas loop arm filters were of then-pole Butterworth type. These results were largely obtained by numerical integration on a digital computer. This paper presents a partial fraction expansion technique for arriving at closed form expressions for squaring loss for Costas loops with arbitary arm filters and NRZ as well as Manchester coded data. Specific closed form results are given for one and two pole Butterworth filters as examples.     

Phase Error Statistics in Costas Loops Due to Channel Distortion and Noise

This paper treats the problem of phase recovery loop performance loss due to a nonideal RF channel. The linear channel under consideration is assumed to suffer from a variety of impairments such as bandwidth limitation, hardware imperfections, and asymmetry of its transfer function. The phase error probability density function of a first-order Costas loop is presented. It has been determined that the phase error is biased in general, and the condition for eliminating this bias is pointed out. To avoid computational complexity and elude computer programming, a simple method is presented for hand computation of the ISI intensity in the loop. Channel bandwidth reduction also introduces phase jitter which can be controlled by proper signal spectrum shaping. For example, it is possible to communicate with a spectral efficiency of 1 bit/s/Hz without degrading the loop performance. Examples have been presented to examine a few practical problems.     

Discrete Time Analyses of Nonuniform Sampling First- and Second-Order Digital Phase Lock Loops

The present paper considers discrete time analyses of firstand second-order digital phase lock loops. These loops are characterized by the fact that they track the zero crossings of the incoming signal; consequently, the sampling intervals are nonuniform. The firstorder loop is analyzed for phase step and frequency step inputs; mean time to skip cycle is also considered. For phase step input, approximate expressions are obtained for the steady-state phase error probability density and phase error variance, the second of which leads directly to a theoretical prediction of threshold. The second-order loop is analyzed for frequency step input. Approximate expressions for the steady-state phase error probability density, phase error variance, and a theoretical prediction of threshold are obtained. The analyses are confirmed by numerical results and simulation.     

LEO小卫星突发数据通信系统中载波恢复环路的误捕获分析

本文对LEO小卫星突发数据通信系统中载波恢复环路的误捕获现象进行了分析与计算机仿真,并根据分析与领导具的结果提出了一种实用的抗误捕获设计方法。本文的分析结果在系统测量中得到了验证,对于类似系统的抗误捕获设计与分析有一定的实际意义。     

子孔径NCS算法中虚假目标产生的机理与消除方法研究

子孔径结构的引入是实现Ultra Wide Band SAR(UWB SAR)实时信号处理的关键。将子孔径结构与Nonlinear Chirp Scaling(NCS)算法相结合的子孔径NCS算法可以较好的实现UWB SAR实时信号处理,但子孔径结构的引入使得成像结果中存在虚假目标的影响。针对子孔径NCS算法中存在虚假目标的现象,该文从理论上分析了虚假目标产生的机理,并提出了对距离弯曲校正前的子孔径回波两端补零的改进子孔径NCS算法消除虚假目标,最后通过仿真和实测的UWB SAR回波数据验证了该文理论分析以及所提方法的正确性。     

基于图论理论解决关系数据库死锁方法的研究

根据关系数据库的基本理论，关系型数据库对数据的存储使用了标准的共享锁、修改锁、独占锁等机制，提供数据库、表、页面、记录等多种不同的资源粒度以及锁升级机制。因此所有的关系型数据库，都存在着并发访问的锁资源争夺风险，如何能在确保数据一致性的前提下避免因锁资源争夺而导致数据库系统性能下降甚至崩溃，一直是数据库系统及其相关应用等领域内关注的焦点。文章将基于图论理论的基础，分析和探讨解决关系数据库死锁的思路和方法，通过构造一些基础的数学模型来分解和本质化死锁现象，提出一些解决方法和理论思路。     

Constructions and properties of Costas arrays

A Costas array is an n × n array of dots and blanks with exactly one dot in each row and column, and with distinct vector differences between all pairs of dots. As a frequency-hop pattern for radar or sonar, a Costas array has an optimum ambiguity function, since any translation of the array parallel to the coordinate axes produces at most one out-of-phase coincidence. We conjecture that n × n Costas arrays exist for every positive integer n. Using various constructions due to L. Welch, A. Lempel, and the authors, Costas arrays are shown to exist when n = p - 1, n = q - 2, n = q - 3, and sometimes when n = q - 4 and n = q - 5, where p is a prime number, and q is any power of a prime number. All known Costas array constructions are listed for 271 values of n up to 360. The first eight gaps in this table occur at n = 32, 33, 43, 48, 49, 53, 54, 63. (The examples for n = 19 and n = 31 were obtained by augmenting Welch's construction.) Let C(n) denote the total number of n × n Costas arrays. Costas calculated C(n) for n ≤ 12. Recently, John Robbins found C(13) = 12828. We exhibit all the arrays for n ≤ 8. From Welch's construction, C(n) ≥ 2n for infinitely many n. Some Costas arrays can be sheared into "honeycomb arrays." All known honeycomb arrays are exhibited, corresponding to n = 1, 3, 7, 9, 15, 21, 27, 45. Ten unsolved problems are listed.     

Improving Frequency Acquisition of a Costas Loop

A Costas loop for tracking a BPSK signal does not acquire when the initial frequency error is comparable to the loop bandwidth. This paper describes methods for deriving control proportional to frequency error so as to improve the frequency acquisition capability by an AFC augmentation. A composite AFC/Costas loop is realized by combining the individual loops. Pull-in from a frequency error much greater than the Costas loop bandwidth is now feasible. The increase in phase error due to the AFC capability is evaluated by a linearized analysis. If the bandwidth of the AFC portion is sufficiently narrow, the degradation is negligible. Improved frequency acquisition can also be realized by a simple modification of the Costas loop to remove the low-pass filter on the quadrature phase detector channel, with negligible degradation to phase tracking.     

Counterexample of truncated Costas optical orthogonal codes

The following results are proven in this paper: 1) neither periodic autonor cross-correlation of the truncated Costas optical orthogonal code (TC OOC) is upper bounded by 1; 2) TC OOC is a class of (ω(2p-3), ω,2,2) optical orthogonal codes     

Hangup in Phase-Lock Loops

A phase-lock loop occasionally will take a long time to settle to equilibrium. Phase dwells at a large error for a prolonged interval. This phenomenon has been dubbed "hangup." The periodic nature of phase detectors is responsible for hangup, which occurs near the reverse-slope, unstable null. Restoring force is small in the vicinity of the reverse null, and noise causes the loop to equivocate about the null. Hangup is very troublesome when fast acquistion is needed with high reliability. One example is synchronization of digital communications. Hangups can be avoided if a large restoring force is applied for large phase errors and if equivocation is prevented. An implementation of an antihangup circuit is proposed.     

数字Costas锁相环的设计与仿真

接收机本地载波跟踪与恢复是基于软件无线电原理进行数字信号相干解调的前提,本文从Costas锁相环原理出发,提出了数字域实现载波跟踪与恢复的设计方法,给出了详细的设计,并根据理论分析进行了系统仿真。     

数字Costas锁相环的改进及应用

介绍了Costas锁相环的基本原理,然后提出了一种适合电视信号色度副载波恢复的改进数字Costas锁相环的基本原理.并且详细介绍了其积分清零器、相位检测器、数字环路滤波器、NCO等各个基本部件的设计,最后在Matlab上给出了该算法的仿真结果并且做了分析.     

A remark on the definition of Costas arrays

A weaker definition of Costas arrays is shown to be equivalent to the standard one. A Costas array is a diagram whose corresponding frequency-hopping pattern has good range-doppler ambiguity properties. Our result implies that even when we demand ostensibly less restrictive ambiguities in the doppler direction, the resulting waveform becomes a Costas waveform.     

基于粗糙集-支持向量机理论的过滤误报警方法

为过滤入侵检测系统报警数据中的误报警,根据报警的根源性和时间性总结出了区分真报警和误报警的19个相关属性,并提出了一种基于粗糙集-支持向量机理论的过滤误报警的方法。该方法首先采用粗糙集理论去除相关属性中的冗余属性,然后将具有约简后的10个属性的报警数据集上的误报警过滤问题转化为分类问题,采用支持向量机理论构造分类器以过滤误报警。实验采用由网络入侵检测器Snort监控美国国防部高级研究计划局1999年入侵评测数据(DARPA99)产生的报警数据,结果表明提出的方法在漏报警约增加1.6%的代价下,可过滤掉约98%的误报警。该结果优于文献中使用相同数据、相同入侵检测系统的其它方法的结果。     

时序电路的自锁与自解锁

本文讨论了在不考虑冗余状态设计时序电路时，可能会导致“自锁”现象，并提出了设计自解锁功能异步时序电路应当遵循的一种规范化方法。     

Golomb Costas序列的结构及其在OFDM系统中的应用

研究了用循环移位法获得的Golomb Costas序列族的特性,建立了含有一个间隙行和一个间隙列的Golomb Costas序列的结构理论,深入研究了含有一个间隙行和一个间隙列的Golomb Costas序列的代数结构、构造方法和自（互）相关特性,并证明了相关的定理。探索了用含有一个间隙行和一个间隙列的Golomb Costas序列设计OFDM 系统中跳频图样的方法,举例说明了如何设计跳频码和怎样将跳频码分配给 OFDM 系统中的用户。用含有一个间隙行和一个间隙列的Golomb Costas序列设计跳频码能获得理想的自相关特性,并且当无线通信系统中多普勒频移受限时能获得极佳的互相关性能。     

一种基于Costas环的BPSK解调设计

设计了一种基于软件无线电思想的中频数字化接收机系统,该系统由数据采集模块、数字下变频（DDC）模块和数字解调模块构成。文中重点研究了基于Costas环的BPSK相干解调,并介绍了载波同步及BPSK解调的工作流程图,最后通过Simulink对BPSK解调原理进行了仿真验证。