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基于信道分段平滑的外辐射源雷达非平稳杂波抑制方法

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基于信道分段平滑的外辐射源雷达非平稳杂波抑制方法
万显荣* 刘玉琪 程 丰 易建新
(武汉大学电子信息学院 武汉 430072)
摘 要:复杂电磁环境下,外辐射源雷达中多径杂波可能具备非平稳的跳变特性。该文针对这种跳变型非平稳杂波,结合辐射源信号的正交频分复用(OFDM)调制特性,提出一种基于信道分段平滑的杂波抑制方法。首先建立了跳变杂波的时域信号模型,然后结合OFDM信号结构将其变换到子载波域,接着在子载波域对各OFDM符号进行信道估计与分段平滑,最后利用该信道平滑值和对应段的参考信号抑制非平稳杂波。仿真和实测数据表明,该文方法能够有效抑制跳变型的非平稳杂波。
关键词:外辐射源雷达;杂波抑制;信道分段平滑;非平稳杂波;正交频分复用(OFDM)
1 引言
外辐射源雷达,又称无源雷达,是一种利用第三方辐射源信号进行目标探测的双/多基地雷达系统,具有节约频谱资源、绿色环保、隐蔽性好、易于组网等诸多优势[1]。外辐射源雷达因发射信号未知,通常在接收端设置参考和监测两路通道,分别用来接收参考信号和目标散射信号。当前,随着数字广播、数字电视以及数字通信网络等在全球兴起,基于数字信号的外辐射源雷达逐步成为近年新体制雷达的研究热点[2–7]。相对于调频广播(FM)等模拟信号,新一代数字信号多采用正交频分复用(Orthogonal Frequency Division Multiplex, OFDM)调制技术。这是一种无线环境下的多载波传输技术,具有频谱利用率高、抗多径衰落能力强的优点[8]。



然而,第三方的数字信号并非专为雷达探测而设计,相干匹配后直达波和多径杂波的旁瓣掩盖目标回波,降低了目标探测性能,此时杂波抑制是提经ECA-C杂波抑制后,目标峰值被残留杂波旁瓣所掩盖,而本文算法能够有效抑制跳变型非平稳杂波,在目标距离维截面上,仅存在目标峰值,且无残留杂波。

图 4 信道响应幅值之和

图 5 信道响应幅值之和的微分

图 6 本文算法杂波抑制后的RD谱

图 7 杂波抑制后的目标距离维截面
进一步地,讨论不同 ∆c下本文算法的杂波抑制性能。其中杂波抑制性能由杂波抑制前后监测信号功率之比来评估,该评估标准称为杂波抑制比[20]。设置 ∆c是起点为0 dB,终点为2 dB,步进为0.2 dB的序列,图8给出了不同 ∆c值下算法杂波抑制比。图8中 ∆c=0表示监测信号无跳变杂波,此时所得杂波抑制比为其理论上限。当∆ c=0时,两种算法杂波抑制比相等,随着 ∆c值的增加,本文算法杂波抑制比保持不变,维持在理论上限附近,而ECA-C算法的杂波抑制比随之降低。这表明本文算法在不同 ∆c下均可有效抑制跳变杂波,而ECA-C算法的杂波抑制性能随跳变幅度 ∆c的增加而下降。当∆c=2 dB时,本文算法杂波抑制比比ECA-C算法高出约10 dB。
4.2 实测数据处理
为进一步验证所提算法的有效性,本节给出实测数据处理结果。武汉大学电波传播实验室利用自主研制的外辐射源雷达系统于2017年7月展开了外场实验,系统以数字电视信号作为机会照射源[21],并实验过程中持续探测到一类跳变型非平稳杂波。图9为某段实测数据的OFDM符号信道响应幅值之和,从图中可明显看出传播信道具备非平稳的跳变特性。

图 8 算法杂波抑制性能对比

图 9 实测数据信道响应幅值之和
为便于分析,以该段实测数据为例,在监测通道中注入一信噪比约为–32 dB的动目标信号,其时延和多普勒频率分别为38 ms和30 Hz。图10为杂波抑制前的RD谱。由于多径杂波的存在,仿真目标被掩盖,且多径杂波的跳变使其旁瓣在RD谱上展宽。图11为ECA-C杂波抑制后的RD谱,ECA-C算法能够抑制零多普勒杂波,但对于跳变型的非平稳杂波,抑制后存在大量杂波残留,仿真目标被该残留杂波所掩盖。
图12为该段实测数据监测信道响应幅值之和的微分。如图所示,微分操作在传播信道跳变的起始和终止时刻形成明显峰值,通过检测峰值位置即可对信道的跳变段和非跳变段进行识别与分段。图13是本文所提算法杂波抑制后的RD谱,对比图11,该算法能够有效抑制跳变型非平稳杂波,经杂波抑制后,时延3.8 ms,多普勒频率为30 Hz的仿真目标凸显。图中以50 Hz为周期的残留杂波是由接收机内部结构造成,对该类杂波的抑制是后续工作之一。图14对比了实测数据经ECA-C算法和本文算法抑制后的目标距离维截面。相比于ECA-C算法,本文算法对跳变型的非平稳杂波更有效,杂波抑制后的目标回波不会被残留杂波的旁瓣所掩盖。

图 10 实测数据杂波抑制前RD谱

图 11 ECA-C算法杂波抑制后的实测数据RD谱

图 12 实测数据信道响应幅值之和的微分

图 13 本文算法杂波抑制后的实测数据RD谱

图 14 实测数据经杂波抑制后的目标距离维截面
5 结束语
本文利用发射源信号的CP-OFDM调制特性,提出了一种基于信道分段平滑的外辐射源雷达非平稳杂波抑制方法。针对外场实验探测到的跳变型非平稳杂波,文章推导了其子载波域信号模型。在此基础上,针对杂波的跳变特性,提出了基于信道分段平滑的杂波抑制方法。相对于现有杂波抑制方法,本文方法利用了发射信号的调制特性,通过分段平滑抑制,有效缓解了残留杂波对目标回波的掩盖。仿真和实测数据均验证了所提算法的有效性。本文当前工作侧重于跳变型非平稳杂波的抑制,对该类杂波形成机理的探究是后续工作的重点。
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Nonstationary Clutter Suppression Method for Passive Radar Based on Channel Segmentation and Smoothing
WAN Xianrong LIU Yuqi CHENG Feng YI Jianxin
(School of Electronic Information, Wuhan University, Wuhan 430072, China)
Abstract: In the complex electromagnetic environment, multipath clutter in passive radar may be nonstationary and has jump characteristics. In order to suppress this kind of non-stationary clutter, a clutter suppression method is proposed based on channel segmentation and smoothing, which combines the Orthogonal Frequency Division Multiplexing (OFDM) modulation of the transmitting signal. First, the temporal domain signal model of the jumping clutter is established. Then it is transformed into subcarrier-domain by using the OFDM structure. After channel estimation of each OFDM symbol and smoothing the segmented channel estimation,the non-stationary clutter can be suppressed by the smoothed channel estimation and reference signal in each segment. Simulation and experiment data show that the proposed method can effectively suppress the nonstationary clutter with jumping characteristic.
Key words: Passive radar; Clutter suppression; Channel segmentation and smoothing; Non-stationary clutter;Orthogonal Frequency Division Multiplexing (OFDM)
中图分类号:TN958.97
文献标识码:A
文章编号:1009-5896(2020)01-0132-08
DOI: 10.11999/JEIT190754
收稿日期:2019-09-29;改回日期:2019-11-12;网络出版:2019-11-30
*通信作者: 万显荣 xrwan@whu.edu.cn
基金项目:国家自然科学基金(61931015, 61701350),国家重点研发计划(2016YFB0502403),博士后创新人才支持计划(BX201600117),湖北省自然科学基金(2016CFA061),湖北省技术创新重大专项(2019AAA061)
Foundation Items: The National Natural Science Foundation of China (61931015, 61701350), The National Key Research and Development Program (2016YFB0502403), The Postdoctoral Innovation Talent Support Program (BX201600117), The Natural Science Foundation of Hubei Province (2016CFA061),The Major Technical Innovation Projects of Hubei Province(2019AAA061)
万显荣:男,1975年生,教授,博士生导师,研究方向为新体制雷达设计,如外辐射源雷达,高频雷达系统及信号处理.
刘玉琪:男,1990年生,博士生,研究方向为雷达信号处理、实时信号处理.
程 丰:男,1975年生,副教授,硕士生导师,研究方向为阵列信号处理、雷达信号处理.
易建新:男,1989年生,博士,研究方向为雷达信号处理、目标跟踪和信息融合.
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