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基于微波光子倍频与去斜接收的雷达收发单元具有宽带工作能力,并且去斜信号的采样率低、数据量小,用于构建阵列雷达时,可以在数字域完成真延时补偿与波束扫描。图4所示为基于微波光子倍频与去斜接收构建的1发N收相控阵雷达结构示意图[27]。该系统利用DPMZM进行微波光子四倍频调制,得到的光信号经过掺铒光纤放大器(Erbium-doped optical fiber amplifier, EDFA)与光分路器(Optical splitter)后得到N+1路信号,其中1路经光电转换后得到雷达发射信号,其余N路作为参考信号,分别对接收的N路回波信号进行去斜处理,得到的多个去斜信号经低通滤波、采样后完成数字信号处理。在数字信号处理模块中,首先将多路去斜信号进行快速傅里叶变换得到对应的一维距离像,然后以其中一个接收天线对应的一维距离像为参考计算每个天线之间的延时差并进行真延时补偿,得到补偿后的信号为[27]:
图 4 基于微波光子倍频与去斜接收的相控阵雷达结构示意图
Figure 4. Setup of the phased array radar based on microwave photonic frequency multiplication and de-chirp receiving
$$\begin{split} \\ R{'_i}(r) = {\left\{ {F\left[ {{S_i}(t)} \right]{{\rm{e}}^{j2\pi f{\tau _i}}}} \right\}_{f = 2kr/c}} \end{split}$$ (1) 式中:F(·)表示快速傅里叶变换;Si(t)是第i个接收机接收到的去斜信号(i=1,2,···,N);τi为第i个回波与参考信号对应的延时差;k为发射的线性调频信号的啁啾率;c是信号在空气中的传播速度。这里f=2kr/c是一个与距离相关的函数,能保证对所有探测距离均能完成真延时补偿,从而克服宽带波束倾斜问题。完成数字真延时补偿后,依据阵列的导向矢量,即:
$${\bf{\varPhi }}(\theta ) = \left[ {\begin{array}{*{20}{c}} 1&{{{\rm{e}}^{j2\pi fd\sin \theta /c}}}&{\cdots}&{{{\rm{e}}^{j2\pi f(N - 1)d\sin \theta /c}}} \end{array}} \right]$$ (2) 通过改变θ可实现数字波束扫描。需要注意的是,与窄带数字波束扫描中载波频率视为定值的情况不同,公式(2)中的f为需要考虑发射信号的各个频率。
基于以上原理,搭建了1×4的宽带微波光子相控阵雷达,其工作带宽为4 GHz (22~26 GHz),每个接收通道的采样率为500 MSa/s。图5(a)所示为DPMZM四倍频调制后输出的光信号频谱图,可以看出此信号主要包含±2阶调制光边带,光载波的功率抑制比大于20 dB。经过光电转换后得到的线性调频信号频率范围为22~26 GHz,其频谱与时域波形如图5(b)所示。图6(a)为对单个小尺寸金属板进行探测的实验场景图片。在完成真延时数字补偿后进行波束扫描成像,结果如图6(b)所示。需要说明的是,由于使用的天线尺寸较大(间距约为5倍中心波长),导致成像结果中存在栅瓣。对主瓣成像目标进行分析,获得图7所示的距离向与角度向的点扩散函数(Point spread function, PSF),发现距离分辨率与角分辨率分别为3.85 cm和2.68°,与理论值相符。将四个间距较近的小目标依次排开进行多目标探测成像实验,结果如图8所示。可以发现,四个目标可以清晰地分辨出来,进一步证明了微波光子宽带相控阵雷达的高分辨探测能力。
图 5 (a) DPMZM输出的信号光谱图;(b)发射的雷达信号频谱和时域波形
Figure 5. (a) Optical spectrum of the signal after DPMZM; (b) Spectra and waveform of the transmitted radar signal
图 6 (a)相控阵天线与探测目标实物图;(b)数字波束扫描成像结果
Figure 6. (a) Photograph of the phased array antennas and target; (b) Image constructed by digital beamforming
Broadband array radar based on microwave photonic frequency multiplication and de-chirp receiving (Invited)
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摘要:
微波光子雷达利用光子学方法实现雷达信号的产生与处理,具有突出的宽带工作能力,能显著提升雷达距离分辨率。为了提升雷达角度分辨能力并实现灵活波束控制,将微波光子雷达技术与阵列技术相结合是必然的发展趋势。目前研究较多的宽带阵列雷达采用光真延时技术克服宽带波束倾斜问题,通常面临复杂度高、灵活性差、延时精度有限等问题。近年来,基于微波光子倍频与去斜接收的宽带雷达收发架构得到了广泛关注,基于此技术构建的阵列雷达,在实现宽带工作的同时具备实时数字补偿与处理功能,为宽带阵列雷达的发展提供了新的思路。文中针对作者在此方面的最新研究进展进行了综述,在阐明基于微波光子倍频与去斜接收实现宽带雷达收发机理的基础上,介绍了构建宽带相控阵雷达的方法以及实现数字波束扫描与成像的性能。然后,将阵列形式扩展至多输入多输出(MIMO)形式,介绍了基于光波分复用技术实现宽带微波光子MIMO雷达的方法,并分析了微波光子MIMO雷达在目标探测与成像方面的性能。
Abstract:Microwave photonic radar enables the generation and processing of broadband radar signals, which can significantly improve the range resolution of the radar system. To improve the radar angle resolution and realize flexible beam control, combining microwave photonic radar technology with array radar technology is an inevitable development trend. Previously, the optical truth delay technology is intensively investigated to achieve squint-free beam steering in broadband phased array radars, which usually face the problems of high complexity, poor flexibility, and limited delay accuracy. In recent years, the broadband radar architecture based on microwave photonic frequency multiplication and de-chirp receiving has received extensive attention. The array radar constructed based on this technology has wide operation bandwidth while enabling real-time digital compensation and processing functions, which provides a new idea for the development of broadband array radars. In this paper, the research progress of the broadband array radar based on microwave photonic frequency multiplication and de-chirp processing was reviewed. After expounding the transceiver mechanism of microwave photonic broadband radar, the method for constructing broadband phased array radar and the performance of digital beam scanning and imaging were introduced. Then, the radar array was extended to MIMO architecture. The broadband microwave photonic MIMO radar based on optical wavelength division multiplexing technology was introduced and its performance in target detection and imaging was analyzed.
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Key words:
- microwave photonics /
- radar /
- phased array /
- MIMO /
- beamforming
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