Inter-frame Ambiguity Analysis and Suppression of LTE Signal for Passive Radar
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摘要: LTE (Long Term Evolution)信号具有大带宽、高覆盖率、强通用性等优点,是一种新型的外辐射源雷达机会照射源。该文从FDD-LTE (Frequency Division Duplexing Long Term Evolution)信号结构入手,探讨了该信号作为第三方机会照射源的模糊函数特性;根据实测FDD-LTE信号,阐述了模糊函数中帧间模糊带抑制的必要性,并对该帧间模糊带的形成机理进行了详细的分析,分析结果表明LTE信号结构中的确定性特征既是引起模糊带的主要因素,同时也是信号相干积累的主要能量来源。对此,该文提出了基于OFDM (Orthogonal Frequency Division Multiplexing)符号子载波系数归一化的帧间模糊带抑制方法,该方法能够在抑制帧间模糊带的同时,又不影响信号相干积累进行目标探测。仿真和实测结果验证了该抑制方法的有效性,为LTE外辐射源雷达目标探测奠定了基础。Abstract: Long Term Evolution (LTE) is a new type of illuminators of opportunity for passive radars, with the advantages of broad bandwidth, high coverage, and strong generality. In this paper, the ambiguity function of Frequency Division Duplexing Long Term Evolution (FDD-LTE) signal is analyzed as an illuminator of opportunity. According to the measured signal, it was found that it is necessary to suppress the inter-frame ambiguity strips in the ambiguity function. Furthermore, themechanism of these inter-frame ambiguity strips was analyzed in detail, which revealed that the LTE signal frame structure is the main factor that causes these inter-frame ambiguity strips and is the major energy source of coherent integration. Thus, a method based on Orthogonal Frequency Division Multiplexing (OFDM) symbol subcarrier coefficient normalization is proposed to suppress these inter-frame ambiguity strips. Simulation and experimental results show that the method can suppress inter-frame ambiguity strips effectively, but does not affect coherent integration, which is the foundation of target detection.
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表 1 FDD-LTE信号模糊函数典型副峰
Table 1. Typical ambiguity peaks in the AF of the FDD-LTE signal
副峰位置 产生原因 帧内模糊 66.67 μs 循环前缀 11.11 μs整数倍 CRS 1 kHz整数倍 控制区域信号 2 kHz整数倍 CRS 帧间模糊 10 ms CRS及同步信号 表 2 下行物理信道及物理信号的调制方式
Table 2. Modulation modes of the downlink physical channels and physical signal
物理信道及物理信号 调制方式 CRS QPSK PCFICH QPSK PHICH BPSK PDCCH QPSK 表 3 FDD-LTE信号仿真参数
Table 3. Parameters used for the simulated FDD-LTE signal
参数 数值 采样率 23.04 MHz 带宽 15 MHz 子载波间隔 15 kHz OFDM符号数据体长度 66.67 μs 循环前缀长度 4.69/5.12 μs 无线帧数 50 信噪比 25 dB PCI 0 控制格式指示信息CFI 1 PHICH组内用户数量 3 表 4 多径和目标仿真参数
Table 4. Parameters used for the simulated the multipath and target
多径杂波 目标 距离元 [0, 1, 2, 5, 9] 30 多普勒频率(Hz) [0, 0, 0, 0, 0] 100 信噪比(dB) [25, 20, 15, 10, 5] –20 -
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