Precise localization is one key element of the Internet of Things (IoT). Especially concepts for position estimation when Global Navigation Satellite Systems (GNSS) are unavailable have moved into the focus. One crucial component for localization systems in general and precise runtime-based positioning, in particular, is the necessity of ultra-precise clock synchronization between the receiving base stations. Our work presents a novel approach for the wireless synchronization of spatially separated base stations. The proposed system estimates the time synchronization, sampling clock offset, and carrier frequency offset using broadcast signals as Signals of Opportunity. In this paper, we derive the theoretical lower bound for the estimation variance according to the Modified Cramer-Rao Bound. We show that a theoretical time synchronization accuracy in the range of ps and a frequency synchronization precision in the range of milli-Hertz is achievable. An algorithm is presented that estimates the desired parameter based on evaluating the Cross-Correlation Function between base stations. Initial measurements are conducted in a real-world environment. It is shown that the presented estimator nearly reaches the theoretical bound within a time and frequency synchronization accuracy of down to 200 ps and 6 mHz, respectively.

翻译：精确的定位是物联网（IoT）的关键要素之一。当全球导航卫星系统（GNSS）不可用时，位置估计的概念尤其成为研究焦点。定位系统（尤其是基于精确时间测量的定位）的一个核心组成部分是需要实现接收基站之间的超精密时钟同步。本文提出了一种新颖的无线同步方法，用于空间分离的基站。所提出的系统利用广播信号作为机会信号，估计时间同步、采样时钟偏移和载波频率偏移。本文根据修正的克拉美-罗下界推导了估计方差的理论下限。我们证明，理论上可达到皮秒级的时间同步精度和毫赫兹级的频率同步精度。提出了一种基于基站间互相关函数评估来估计所需参数的算法。在真实环境中进行了初步测量，结果表明，所提出的估计器几乎达到了理论界限，时间同步精度低至200 ps，频率同步精度低至6 mHz。