The use of WiFi signals for sensing purposes has attracted a lot of interest from both the radar and communications communities and several techniques have been explored. In the attempt of meeting the requirements for small sensor size, compactness, and easy deployment, the authors consider reference-free approaches, namely approaches that do not require a good copy of the transmitted waveform to be available at the radar receiver. To this end, the authors first resort to a passive radar-based processing scheme that only exploits the invariant a priori known initial portion of the physical layer protocol data unit, that is, the PHY Preamble, and its limitations in practical applications is investigated. Specifically, the authors show that, with this approach, an accurate time, phase, and frequency synchronization is essential and a possible strategy is investigated. As an alternative solution the authors consider a forward scatter radar-based approach where only the amplitude modulation of the received signal is exploited to detect the presence of a moving target thus avoiding the need to know the transmitted signal. For the first time, the authors comparatively investigate advantages and drawbacks of the two reference-free approaches and present practical strategies to handle the limitations observed. The results are reported for experimental tests with people and drones using WiFi transmissions in the 2.4 and 5 GHz band.

中文翻译：

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比较用于监控人员和无人机的无参考 WiFi 雷达传感方法

使用 WiFi 信号进行传感已经引起了雷达和通信界的极大兴趣，并且已经探索了多种技术。为了满足小传感器尺寸、紧凑性和易于部署的要求，作者考虑了无参考方法，即不需要在雷达接收器处提供传输波形的良好副本的方法。为此，作者首先采用基于无源雷达的处理方案，该方案仅利用物理层协议数据单元的不变的先验已知初始部分，即PHY前导码，并研究了其在实际应用中的局限性。具体而言，作者表明，采用这种方法，准确的时间、相位和频率同步至关重要，并研究了一种可能的策略。作为替代解决方案，作者考虑了一种基于前向散射雷达的方法，其中仅利用接收信号的幅度调制来检测移动目标的存在，从而避免了了解发射信号的需要。作者首次比较研究了两种无参考方法的优点和缺点，并提出了处理观察到的局限性的实用策略。报告的结果是使用 2.4 和 5 GHz 频段的 WiFi 传输对人和无人机进行的实验测试。