气象雷达超低副瓣失配滤波处理技术研究

在雷达信号处理中,通过匹配滤波进行脉冲压缩可以获得最大化的信噪比,有效地减小了雷达回波中噪声对信号的影响。然而,脉冲压缩的输出具有较高的距离旁瓣,在气象雷达探测中,由于空间分布的散射粒子之间反射强度相差较大,弱散射粒子的回波容易掩没在强散射粒子的旁瓣中,因此有超低旁瓣的需求。本文主要研究了能够降低旁瓣的非线性调频（NLFM）波形和最小积分旁瓣水平（ISL）失配滤波器,分析了多普勒频移对其性能的影响,并在最小ISL滤波器的基础上通过进一步对滤波器系数加权的方法,使得在回波具有多普勒频移的情况下也能达到超低旁瓣的性能。      

混沌二相码脉冲压缩性能分析

编码信号是多普勒敏感信号,回波多普勒频移的存在会导致编码信号的脉冲压缩性能下降。文中从积分旁瓣比以及信噪比损失两方面分析了多普勒频移对混沌二相码信号压缩性能的影响,并通过仿真验证了分析的正确性。     

四相Taylor码的旁瓣抑制加权技术

本文研究四相编码雷达信号的距离旁瓣抑制加权处理问题,将二相码的旁瓣加权技术推广到Taylor四相码,对相干、非相干和匹配滤波后加权、直接失配加权等处理方式进行了分析,并讨论了长、短码频率分集处理方式,给出了多种长度Taylor四相码的加权处理计算机模拟结果,最后讨论了技术实现时的问题.结果表明,结合雷达工作的具体要求,选择适当的处理方式,对子脉冲宽度为0.2μs的四相码信号进行旁瓣抑制加权处理,在63位及190位码长情况下,多普勒频率范围在±10KC时,主副比均能达到31dB以上,较好地满足了现代雷达系统对脉压主副比和多普勒性能的要求.     

相位编码信号的综合旁瓣能量改善技术

相位编码信号的综合旁瓣性能不佳是制约其在分布式目标观测应用中的主要因素。根据相位编码信号的均匀旁瓣结构有利于抑制旁瓣能量的性质,采用失配滤波技术对接收回波处理,获得了基本均匀的旁瓣输出,进一步通过接收回波提取出原始发射信号分量,通过一位时移、取复共轭形成了旁瓣对消器,经过旁瓣对消处理后极大地改善了相位编码信号的积分旁瓣比指标值,并且获得了类似线性调频信号的多普勒容忍性,本方法将使得相位编码信号在雷达对地观测等方面获得很好的应用前景。     

遗传算法在二相码优化搜索中的应用

旁瓣抑制一直是雷达信号处理中的关键过程,本文提出了一种利用遗传算法优化搜索二相码M序列的方法。以遗传算法搜索出的序列组作为M序列产生器的初始序列,可以得到非周期自相关特性较好的伪随机序列。利用此方法产生的M序列作为调制信号得到的二相编码信号不需要加权就可获得较高的主副比,从而避免了加权引起的失配损失。     

基于迫零算法的二相码旁瓣抑制滤波器设计

为了解决大时宽带宽积二相编码信号脉压旁瓣抑制滤波器设计过程中易出现局部最优问题,提出了基于迫零算法的失配滤波器设计方法.从时域波形人手分析了二相编码信号的脉压失配滤波,将旁瓣抑制滤波器的设计转换为迫零均衡问题,对迭代运算中的步长因子和误差向量进行了自适应修正,在提高了算法收敛速度的同时增加了稳健性.对不同码长m序列的仿真结果表明,基于迫零算法的失配滤波器设计方法,可以有效解决大时宽带宽积二相编码信号旁瓣抑制滤波器设计中局部最优的困扰,所设计的失配滤波器具有较好的峰值旁瓣电平性能.     

Taylor四相码频域旁瓣抑制数字滤波器的设计

本文分析了Taylor四相码信号的频谱结构,采用频域数字处理方法,研究了四相编码长码雷达信号距离旁瓣频域抑制滤波技术,提出一种适用于各种码长信号的旁瓣抑制滤波器(SSF)频域设计方法.并以127位Taylor四相码为例,对SSF处理的性能进行仿真分析和实验测试.当fd=0时,仿真与实测结果的主副比分别达到51dB和43dB.这表明采用这种方法所设计的T硝lor四相码信号SSF可以应用于现代雷达系统.     

多相脉压编码在旁瓣抑制下的多普勒性能

由频率调制导出的 P3、P4多相码是一类常见的脉冲压缩信号 ,已提出的几种旁瓣抑制技术可用于降低这类脉压信号的压缩输出旁瓣。本文主要研究在使用这几种旁瓣抑制技术时 ,多普勒频移对 P3、P4多相码压缩输出性能的影响 ,包括多普勒频移对输出峰值旁瓣电平 (PSL )以及信噪比 (SNR)损失的影响。文中给出了计算机仿真结果。     

基于二次曲线拟合的相位编码多普勒补偿研究

相位编码信号由于其脉冲间码型捷变、图钉状模糊函数和不存在距离-多普勒模糊等低截获性能,在当前雷达对抗领域获得广泛的关注和应用。由于目标运动会使相位编码回波受到多普勒频率调制,破坏了各个调相码之间的相位关系,相关处理会出现失配现象,输出主瓣降低,旁瓣升高,系统在探测高速目标会出现多普勒失配。根据m序列码脉冲压缩随着目标速度的失配变化和目标多普勒容限关系,本论文研究了较为实用的二次曲线拟合局部脉冲压缩曲线。根据三点多普勒补偿的脉压结果实现全局最优补偿的自动估算。所提算法具有简单的实现过程,易于工程实现。     

多相脉压缩码在旁瓣抑制下的多普勒性能

由频率调制导出的P3、P4多相码是一类常见的脉冲压缩信号，已提出的几种旁瓣抑制技术可用于降低这类脉压信号的压缩输出旁瓣。本文主要研究在使用这几种旁瓣抑制技术时，多普勒频移对P3、P4多相码压缩输出性能的影响，包括多普勒频移对输出峰值旁瓣电平（PSL）以及信噪比（SNR）损失的影响。文中给出了计算机仿真结果。     

采用方位向自适应滤波器提高SAR自聚焦的性能

机载合成孔径雷达的方位向降采样经常会降低原始图像的信噪比。自动聚焦是从雷达回波信号中提取多普勒参数,以获得高分辨率SAR图像。该文分析了信噪比对自聚焦的影响,提出利用自适应滤波器来做方位向预滤波和降采样,在降低采样率的同时,保持原始图像的信噪比,再经有效的自动聚焦算法,得到较准确的多普勒参数估计值,从而获取高分辨率SAR图像。通过对实际SAR回波数据的处理,证明了采用自适应滤波器后,图像信噪比能比采用普通滤波器时有较大提高,从而显著改善自动聚焦的收敛性,提高成像质量。     

基于二分搜索的宽带多普勒模型信号捕获方法

针对深空通信等超高动态低信噪比通信场景中,传统信号捕获方法存在着动态范围不足、精确度不够以及使用窄带多普勒模型导致对接收信号近似精确度不高的问题,提出一种基于二分搜索的接收端采样率调整方法。使用宽带多普勒模型对接收信号进行建模,并使用扫频余弦类信号进行多普勒频偏估计和定时估计,处理多普勒伸缩后的信号与本地信号采样率不匹配的问题。对所提方法进行同步性能仿真,仿真结果表明,所提方法采用正反扫频线性调频(UD-LFM)信号,能在信噪比为-49 dB、最大多普勒频偏为2 MHz的条件下,多普勒频偏估计误差小于400 Hz,定时估计误差不超过60 ns。相比不使用本文所提方法进行直接捕获,本文所提方法具有更高的捕获精确度以及更低的信噪比门限。     

一种快速消除多普勒频移对脉压影响的算法

针对多普勒频移带来脉压失配问题,为克服现有算法运算量大且占用大量存储空间的弊端,文中提出一种快速消除多普勒频移对数字脉压(DPC)影响的算法。该算法基于傅里叶变换性质,利用单向循环链表技术,对无多普勒频率时的匹配滤波系数表重新选址。仿真结果表明,新算法在大多普勒频移时信噪比损失小,且不会增加运算量和存储空间,具有很高的应用价值。     

Reducing ICI effect in OFDM system using low‐complexity Kalman filter based on comb‐type pilots arrangement

In OFDM systems, time‐varying channels destroy orthogonality between subcarriers causing Inter‐Carrier Interference. To reduce this effect, a Kalman filter, as a benchmark, is used for channel estimation, based on comb‐type pilot arrangements of the OFDM system. An advantage of comb‐type pilot arrangements in channel estimation is the ability to track the variation of the channel caused by Doppler frequency. Kalman method has been proposed to estimate the channel frequency response (CFR) at the pilot locations, then CFR, at data subchannels, is obtained by means of interpolation between estimates at pilot locations. The low‐complexity Kalman method is introduced to reduce the complexity of the system while achieving the same BER/SNR. Different types of interpolations have been also compared such as Low‐pass, Spline‐cubic and Linear interpolation methods. The BER/SNR performance of BPSK modulation schemes are considered for time varying Rayleigh fading channels. Our results has shown that the low‐complexity Kalman estimation, used with the pilot arrangement and a suitable interpolation method, gives almost the same performance as that of the Kalman method specially for low SNR values and hence the effect of Doppler shift effect is controlled. Copyright © 2010 John Wiley & Sons, Ltd.     

Doppler optimised mismatched filters

The problem of designing radar detection filters that minimise sidelobe levels in both Doppler and range co-ordinates is considered. The new algorithm for a Doppler optimised mismatch filter is constructed. The generalisation enables sidelobe suppression for the polyphase and complex multilevel sequences. For minimax filter coefficients calculation, the iterative reweighted least-square procedure is successfully applied. Favourable experimental results are obtained, especially for polyphase codes.<>     

On the robustness of decision-feedback detection of DPSK and differential unitary space-time modulation in Rayleigh-fading channels

Decision-feedback differential detection (DFDD) of differential phase-shift keying (DPSK) and differential unitary space-time modulation (DUST) in Rayleigh-fading channels exhibits significant performance improvement over standard single-symbol maximum-likelihood detection. However, knowledge of channel fading correlation and signal-to-noise ratio (SNR) is required at the receiver to compute the feedback coefficients used in DFDD. In this letter, we investigate the robustness of the DFDD to imperfect knowledge of the feedback coefficients by modeling the mismatch between estimated feedback coefficients and ideal coefficients in terms of mismatch between the estimated values of fading correlation and SNR and the true values. Under the assumption of a block-fading channel when nondiagonal DUST constellations are used and a continuous fading channel otherwise, we derive exact and Chernoff bound expressions for pair-wise word-error probability and then use them to approximate the bit-error rate (BER), finding close agreement with simulation results. The relationships between BER performance and various system parameters, e.g., DFDD length and Doppler mismatch, are also explored. Furthermore, the existence of an error floor in the BER-vs-SNR curve is investigated for the infinite-length DFDD. For the special case of Jakes' fading model, it is shown that the error floor can be removed completely even when the Doppler spread is over-estimated.     

An adaptive filtering technique for pilot-aided transmissionsystems

One of the main design tradeoffs of pilot-based techniques of combating the effects of multipath fading is the pilot filter bandwidth. The approach presented is to store a precalculated bank of pilot filters and automatically select the best one as the signal-to-noise, (SNR) or vehicle speed change. The authors address the issue of adjusting the filter bandwidth as speed changes. They call this technique the filter switching algorithm. This technique can provide a 1.0 dB improvement for a normalized Doppler frequency of 2%; up to 2.0 dB can be gained when operating at a lower Doppler frequency. The algorithm is found to be very robust, in the sense that self-noise and decision direction have little effect on its operation and the resulting bit error rate     

滤波与正交基分解在磁性目标检测中的应用

通过对磁性目标磁异常信号时域和频域特性的分析可知,磁异常信号属于低频信号,且在实际磁异常探测中磁性目标信号被噪声淹没,信噪比较低。针对这种情况提出了基于低通滤波与正交基分解的检测方法,先分析数字滤波器特性,设计约束最小二乘FIR滤波器滤除高频噪声,再利用正交基分解检测算法,大幅提升了磁性目标信号的信噪比,从而实现了磁性目标检测。仿真试验表明,该方法可滤除高频噪声,提升信噪比,增强对磁性目标的检测能力     

Design of Irregular LDPC Codes for BIAWGN Channels with SNR Mismatch

Belief propagation (BP) algorithm for decoding lowdensity parity-check (LDPC) codes over a binary input additive white Gaussian noise (BIAWGN) channel requires the knowledge of the signal-to-noise ratio (SNR) at the receiver to achieve its ultimate performance. An erroneous estimation or the absence of a perfect knowledge of the SNR at the decoder is referred to as ?SNR mismatch?. SNR mismatch can significantly degrade the performance of LDPC codes decoded by the BP algorithm. In this paper, using extrinsic information transfer (EXIT) charts, we design irregular LDPC codes that perform better (have a lower SNR threshold) in the presence of mismatch compared to the conventionally designed irregular LDPC codes that are optimized for zero mismatch. Considering that min-sum (MS) algorithm is the limit of BP with infinite SNR over-estimation, the EXIT functions generated in this work can also be used for the efficient analysis and design of LDPC codes under the MS algorithm.     

一种基于散焦偏移差的全带宽机载SAR动目标检测方法

该文分析了斜视下的动目标特性,在深入研究目标偏移量与多普勒参数间相互关系的基础上,提出了一种基于散焦偏移差的全带宽单通道机载合成孔径雷达动目标检测方法,该方法利用动/静目标存在多普勒中心频率差时,调频率偏差会引起动/静目标产生不同偏移量的特点,分离动/静目标.与传统的单通道检测方法相比,该方法具有更强的动目标检测能力、不受斜视角限制的特点.