基于幅度加权的预失真线性调频超声编码激励

为了提高医学超声成像的轴向分辨力和确保对比度,该文提出一种基于幅度加权的预失真线性调频编码新方法.该方法将线性调频发射信号幅度加权技术和回波信号旁瓣抑制技术相结合,一方面补偿超声探头对发射信号的影响,使得回波信号的带宽不局限于探头,提高轴向分辨力;另一方面消除发射信号幅频特性的菲涅耳波纹,提高发射信号的带宽并采用失配滤波器进行脉冲压缩,实现旁瓣抑制,确保成像对比度.仿真结果表明:相对恒包络线性调频编码,预失真线性调频编码方法不仅提高了轴向分辨力,而且最大旁瓣幅度减小至-48 dB以下,满足医学成像对比度要求.FieldII仿真B超图像结果表明:恒包络线性调频和预失真线性调频编码方法的轴向分辨力分别是0.35 mm和0.25 mm.     

Resolution function of nonsinusoidal radar signals. II.Range-velocity resolution with pulse compression techniques

For pt.I see ibid., vol.32, no.2, p.153-60 (May 1990). The thumbtack range-velocity resolution function of nonsinusoidal radar signals has been realised with signals having a large number of characters with triangular correlation functions. In reality, a radar signal rarely consists of one pulse; rather, it consists of thousands of pulses structured by various coding techniques. Pulse compression is a technique for obtaining high range resolution with long coded signals. The thumbtack range-velocity resolution function based on the pulse compression principle is realized by coding the transmitted signal using complementary codes. Increasing the code length will increase the pulse compression ratio and the time-sidelobe-free region, Doppler processing of correlation functions based on the pulse compression technique can realize the thumbtack range-velocity resolution function. The range-velocity resolution function is obtained by computer simulation and represented by a three-dimensional surface as a function of target range and velocity for signals with a different number of characters. Plots of range-velocity resolution functions for characters with different code lengths are presented     

基于压缩感知的二维雷达成像算法

压缩感知理论能够有效地降低高分辨率雷达成像系统的数据率。该文通过对复基带雷达回波信号模型的稀疏性分析,提出了一种具有保相性的压缩感知距离压缩算法。在此基础上建立了距离向采用压缩感知距离压缩算法,方位向采用传统的雷达成像算法处理的雷达2维成像方案。通过对仿真和实测逆合成孔径雷达数据的成像处理验证了方案的有效性。     

The weighted diagnostic distortion (WDD) measure for ECG signal compression

In this paper, a new distortion measure for electrocardiogram (ECG) signal compression, called weighted diagnostic distortion (WDD) is introduced. The WDD measure is designed for comparing the distortion between original ECG signal and reconstructed ECG signal (after compression). The WDD is based on PQRST complex diagnostic features (such as P wave duration, QT interval, T shape, ST elevation) of the original ECG signal and the reconstructed one. Unlike other conventional distortion measures [e.g. percentage root mean square (rms) difference, or PRD], the WDD contains direct diagnostic information and thus is more meaningful and useful. Four compression algorithms were implemented (AZTEC, SAPA2, LTP, ASEC) in order to evaluate the WDD. A mean opinion score (MOS) test was applied to test the quality of the reconstructed signals and to compare the quality measure (MOSerror) with the proposed WDD measure and the popular PRD measure. The evaluators in the MOS test were three independent expert cardiologists, who studied the reconstructed ECG signals in a blind and a semiblind tests. The correlation between the proposed WDD measure and the MOS test measure (MOSerror) was found superior to the correlation between the popular PRD measure and the MOSerror.     

Nonsinusoidal radar signal design for stealth targets

The detection of stealth point targets challenges the design of conventional radars using sinusoidal carriers since the objective of stealth technology is to reduce the radar cross section (RCS) of targets to a level where the radar receiver cannot detect the target. While there are a number of techniques employing different technologies to reduce the RCS of targets, shaping and coating the target with absorbing material are the most useful ones. The analysis and design of nonsinusoidal radar signals is based on modeling stealth point targets by a two-layer structure consisting of a metal surface covered with a coat of absorbing material. The design is presented for two classes of signals: uncoded signals and pulse compression signals using Barker codes. The relationship between target response, absorbing material time delay, time variation of transmitted pulses and coding features are determined and analyzed. While sliding correlators are used for detection and selection of various target responses, their output autocorrelation functions are determined analytically in terms of transmitted signal autocorrelation functions. Thumbtack range-velocity resolution functions are obtained for transmitted signal characters with a single pulse and characters with coded waveforms, for different pulse duration. It is shown that the range resolution can be improved by the proper choice of the transmitted signal duration relative to absorbing material time delay. Thumbtack range-velocity resolution functions similar to those of conventional point targets can also be realized     

Noise radar using random phase and frequency modulation

Pulse compression radar is used in a great number of applications. Excellent range resolution and high electronic counter-countermeasures performance is achieved by wideband long pulses, which spread out the transmitted energy in frequency and time. By using a random noise waveform, the range ambiguity is suppressed as well. In most applications, the random signal is transmitted directly from a noise-generating microwave source. A sine wave, which is phase or frequency modulated by random noise, is an alternative, and in this paper, the ambiguity function and the statistical characteristics of the correlation output for the latter configuration are further analyzed. Range resolution is then improved because the noise bandwidth of the modulated carrier is wider than that of the modulating signal, and the range sidelobes are also further suppressed. Random biphase modulation gives a 4-dB (/spl pi//sup 2//4) improvement, but much higher sidelobe suppression could be achieved using continuous phase/frequency modulation. Due to the randomness of the waveform, the output correlation integral is accompanied by a noise floor, which limits the possible sidelobe suppression as determined by the time-bandwidth product. In synthetic aperture radar (SAR) applications with distributed targets, this product should be large compared with the number of resolution elements inside the antenna main beam. The advantages of low range sidelobes and enhanced range resolution make frequency/phase-modulated noise radar attractive for many applications, including SAR mapping, surveillance, altimetry, and scatterometry. Computer algorithms for reference signal delay and compression are discussed as replacements for the classical delay line implementation.     

超宽带搜救雷达发射信号及径向分辨率研究

研究给出了城市灾害救助与生命搜索用超宽带搜救雷达的发射信号模型、目标信息模型、回波模型，在此基础上对雷达径向分辨率与发射信号脉冲宽度、脉冲间隔、脉冲串长度的关系进行了分析探讨。并对模型在Matlab上做了仿真。得出结论，随着雷达脉冲信号宽度增加，信号的UWB特性逐渐消失，目标对信号的多散射中心个数逐渐减少，直至成为点散射，雷达径向分辨能力逐渐变差，雷达一维时间图像逐渐变得模糊。     

PERFORMANCE ANALYSIS OF PULSE COMPRESSION USING PHASE-CODED SIGNALS FOR SPARSE-ARRAY SYNTHETIC IMPULSE AND APERTURE RADAR

Sparse-array Synthetic Impulse and Aperture Radar (SIAR) can isotropically radiate by employing multiple frequencies (synthetic pulse) and multiple antennas (synthetic antenna). According to Ambiguity Function(AF), its range resolution depends only on bandwidth of transmitted signals, however, the distance grating lobes emerge when increasing the time-bandwidth product of transmitted signals. The performance of pulse compression is analyzed with the transmitted signals modulated by phase-coded sequences. It is seen that greater ratio of pulse compression and suppression of range sidelobe in SIAR can be obtained, and its effective range and range resolution is increased as well.     

基于相关运算的声弹性法螺栓预紧力测量

为了在低压激励条件下,利用声弹性准确测量螺栓预紧力,提出采用基于相关运算的信号处理方法。该方法通过比较发射波在接收波中最相似位置获得超声传播时间,从而测量预紧力;相关运算对噪声具有抑制能力,可以实现低信噪比下的传输时间测量,结合内插可提高时间测量分辨力;选择线性调频脉冲作为发射波,开展不同电压激励下的声弹性螺栓预紧力测试。确定了超声传播时间的最大误差不超过±2.9%。测试结果表明：该方法能够实现低压激励条件下超声传播时间的准确测量,结合内插法可以提高时间测量的分辨力。     

超高分辨率机载SAR 宽带激励源设计与实现

针对分辨率优于0.1 m 的机载合成孔径雷达(SAR)系统,该文设计实现了中心频率14.8 GHz,带宽3.2 GHz的宽带线性调频(LFM)激励信号源。详细介绍了技术方案的选择,关键技术的实现,并对产生的宽带调频信号进行了详细的测试与分析。该信号源作为机载SAR 系统中子系统的一部分,完成了飞行试验,并获得了分辨率优于0.1m 的雷达图像,验证了该方案设计和技术实现的有效性。      

Measurement of data compression in advanced group 4 facsimile systems

The CCITT (Consultative Committee for International Telephone and Telegraph) has defined Group 4 facsimile apparatus as that which transmits an ISO A4 (210 × 297 mm) document over public data networks including Packet Switched Data Networks (PSDN) and Circuit Switched Data Networks (CSDN). Data compression is employed in Group 4 systems to minimize the number of bits required for transmission of a message. CCITT Recommendation T.6 defines the basic Group 4 coding scheme to be a variation of the modified READ code (MRC II) which is the Group 3 coding option. This paper measures the compression of the MRC II algorithm for three different documents at resolutions of 200, 240, 300, 400, and 480 picture elements per inch. The compression measurement was accomplished by means of computer simulation. It was determined that the bits per page measure increases linearly with resolution. Results are presented and conclusions drawn. The Group 4 CCITT Recommendations include a mode of operation known as Mixed Mode where a page is divided into two parts: character-coded information and facsimile-coded information. This paper describes four alternative mixed-mode algorithms and measures the compression of each coding technique for CCITT document number 1. The compression measurements were performed by means of computer simulation. Conclusions are drawn regarding the relative advantages of the four algorithms.     

基于LASSO的雷达脉压压缩方法

雷达抗干扰性能是衡量一部雷达优劣的重要指标,直接决定着雷达作战效能的发挥。常见的抗干扰措施有副瓣对消、脉冲压缩、动目标检测、恒虚警处理等。文中结合LASSO回归算法,提出了一种基于LASSO的脉冲压缩抗干扰措施。该方法利用LASSO回归具有稀疏性的特质,结合雷达回波信号设置匹配字典,使用冗余预测变量构造数据集,并使用交叉验证构造LASSO模型,拟合识别预测变量,实现了目标检测。将该方法与脉冲压缩方法对比,仿真实验证明采用LASSO算法不仅不需要考虑旁瓣的影响,还获得了更好的目标分辨能力,并且在较小信噪比条件下目标检测效果也比较好。     

Doppler-shift correction by the DFT,for a QPSK satellite modem

This paper describes a novel technique for the measurement and correction of the Doppler shift (frequency offset) in a received QPSK signal that has been transmitted over a satellite link. The satellite is here assumed to be in a non-geostationary orbit, and correction of the Doppler shift is required to be achieved as soon as the satellite rises above the horizon, when the Doppler shift may be very high and the signal/noise ratio very low. The receiver now has no prior knowledge of the transmitted data, so that a non-data-aided system must be used, capable of operating under seriously adverse conditions. The selected technique measures the DFTs of successive blocks of samples of the received and demodulated signal, and from the DFTs it derives a control signal for the VCO, which generates the in-phase and quadrature reference carriers for the coherent demodulator. The frequency of the latter is adjusted to reduce the residual Doppler shift in the demodulated signal to a sufficiently small value, so that correct phase synchronization of the VCO on to the received signal carrier can then be achieved by means of an appropriate conventional system (not considered here). The paper describes the new technique of Doppler shift correction, and presents the results of computer-simulation tests to measure the time taken to correct an initial Doppler shift of 5 kHz in the received QPSK signal, at very low signal/noise ratios.     

基于循环相关的LFM脉冲雷达宽带回波Doppler和多径时延的联合估计

基于循环相关特性,该文提出了针对线性调频(LFM)脉冲雷达宽带回波Doppler和多径时延联合估计的方法。该方法首先利用雷达回波的循环相关变换,通过多径分量的能量累积估计Doppler频移尺度参数,然后结合发射信号的先验信息构造降阶函数,将回波信号降为多个单频信号,最后利用零频率截面循环谱能够将时间上有重叠的多径回波信号在该截面上分开且分辨率加倍的特性,估计出各多径分量的时延值。该方法只需要一个脉冲信号即可完成Doppler和多径时延的联合估计,对低信噪比噪声有较强的容忍性,能抑制各径相关交叉项的影响,并且具有较高的多径时延估计分辨率。文中给出了计算机仿真实验,仿真结果表明了该算法的有效性。     

新型双频相干脉冲压缩测速测距激光雷达

激光雷达具有抗干扰能力强、分辨率高、隐蔽性好等优点,已被广泛应用于精密测量、侦察监视、火控、制导等领域。针对远程激光测速测距中回波信号微弱难以检测的现实情况,提出了一种新的双频激光测速测距方法,采用脉冲压缩技术实现信号检测。通过实验,本文对该方法的原理进行了分析与验证。结果表明,该方法可以实现对运动目标的测速与测距。     

Compression of transmitted pulses in plasmas

The compression of frequency-modulated RF pulses is discussed when the dispersive property of a lossless, isotropic, and homogeneous plasma is used to enhance the resolution of transmitted pulses at the receiver. Using the basic concept of group velocity, an expression is obtained for the carrier phase variation of a rectangular envelope pulse so that the pulse collapses on itself at the receiver. Unlike the chirp radar pulse, the frequency variation is not linear and the optimum detection of transmitted pulses depends on both the plasma frequency and the distance between the transmitter and the receiver. The method of convolution was used to obtain numerical results, which indicate that the formulation given results in compression for short propagation distances.     

FMICW信号在岸-舰双/多基地地波超视距雷达中的应用

为了解决探测距离与分辨率的矛盾，并在后续信号处理中避开直达波的影响，岸-舰双／多基地地波超视距雷达适合采用线性调频中断连续波（FMICW）信号。文中结合该雷达的发射信号模型，分析了回波信号所包含的目标距离信息和多普勒信息，介绍了对接收信号的处理方法，对回波信号进行数字解调等处理后，得出了目标的距离信息和多普勒信息。实测数据的处理结果证明，用FMICW信号作为该雷达的发射信号可以有效地检测出运动目标。     

探地雷达回波信号小波压缩方法研究

介绍了多分辨分析和小波变换的基本理论以及常用小波变换压缩数据的三种方法：只保留近似信号；全部保留近似信号及细节信号中的较大值；保留近似信号及细节信号中的较大值。将紧支集小波（Daubechies)和正交三次B-样条小波压缩探地雷达数据进行了对比，计算表明正交三次B-样条小波变换方法效果较好，而在全部保留近似信号及只保留细节信号中数值较大的系数时，压缩比大而重建雷达数据与原始雷达数据间的均方差较小。     

Discrete cosine harmonic wavelet transform and its application to signal compression and subband spectral estimation using modified group delay

This paper proposes a new harmonic wavelet transform (HWT) based on discrete cosine transform (DCTHWT) and its application for signal or image compression and subband spectral estimation using modified group delay (MGD). Further, the existing DFTHWT has also been explored for image compression. The DCTHWT provides better quality decomposed decimated signals, which enable improved compression and MGD processing. For signal/image compression, compared to the HWT based on DFT (DFTHWT), the DCTHWT reduces the reconstruction error. Compared to DFTHWT for the speech signal considered for a compression factor of 0.62, the DCTWHT provides a 30% reduction in reconstruction error. For an image, the DCTHWT algorithm due to its real nature, is computationally simple and more accurate than the DFTHWT. Further compared to Cohen–Daubechies–Feauveau 9/7 biorthogonal symmetric wavelet, the DCTHWT, with its computational advantage, gives a better or comparable performance. For an image with 6.25% coefficients, the reconstructed image by DFTHWT is significantly inferior in appearance to that by DCTHWT which is reflected in the error index as its values are 3.0 and 2.65%, respectively. For spectral estimation, DCTHWT reduces the bias both in frequency (frequency resolution) and spectral magnitude. The reduction in magnitude bias in turn improves the signal detectability. In DCTHWT, the improvement in frequency resolution and the signal detectability is not only due to good quality DCT subband signals but also due to their stretching (decimation) in the wavelet transform. The MGD reduces the variance while preserving the frequency resolution achieved by DCT and decimation. In view of these, the new spectral estimator facilitates a significant improvement both in magnitude and frequency bias, variance and signal detection ability; compared to those of MGD processing of both DFT and DCT fullband and DFT subband signals.     

Locally Optimal Run-Length Compression Applied to CT Images

Run-length data compression techniques are described that preserve image content. After decompression, images are restored to their original state without loss in image gray scale or resolution. The first technique introduces terminology and illustrates a simple method for efficient picture archiving. It demonstrates the principle of run-length techniques. A second more general approach encodes picture information in a manner that adapts to local variation in pixel standard deviation. Among several options of compression formats, the one that delivers the best local compression is selected. Results of our compression techniques are given for several hundred computed tomography (CT) pictures with comparison to image entropy measures. A general solution to the optimal run-length compression of digital data is outlined. Routine application of the locally optimal method is also described.