Horn Berthold K P
Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
Sensors (Basel). 2022 Apr 18;22(8):3091. doi: 10.3390/s22083091.
Indoor localization using fine time measurement (FTM) round-trip time (RTT) with respect to cooperating Wi-Fi access points (APs) has been shown to work well and provide 1-2 m accuracy in both 2D and 3D applications. This approach depends on APs implementing the IEEE 802.11-2016 (also known as IEEE 802.11mc) Wi-Fi standard ("two-sided" RTT). Unfortunately, the penetration of this Wi-Fi protocol has been slower than anticipated, perhaps because APs tend not to be upgraded as often as other kinds of electronics, in particular in large institutions-where they would be most useful. Recently, Google released Android 12, which also supports an alternative "one-sided" RTT method that will work with legacy APs as well. This method cannot subtract out the "turn-around" time of the signal, and so, produces distance estimates that have much larger offsets than those seen with two-sided RTT-and the results are somewhat less accurate. At the same time, this method makes possible distance measurements for many APs that previously could not be used. This increased accessibility can compensate for the decreased accuracy of individual measurements. We demonstrate here indoor localization using RTT with respect to legacy APs that do not support IEEE 802.11-2016. The accuracy achieved is 3-4 m in cluttered environments with few line-of-sight readings (and using only 20 MHz bandwidths). This is not as good as for RTT, where 1-2 m accuracy has been achieved (using 80 MHz bandwidths), but adequate for many applications A wider Wi-Fi channel bandwidth would increase the accuracy further. As before, Bayesian grid update is the preferred method for determining position and positional accuracy, but the observation model now is different from that for two-sided RTT. As with two-sided RTT, the probability of an RTT measurement below the true distance is very low, but, in the other direction, the range of measurements for a given distance can be much wider (up to well over twice the actual distance). We describe methods for formulating useful observation models. As with two-sided RTT, the offset or bias in distance measurements has to be subtracted from the reported measurements. One difference is that here, the offsets are large (typically in the 2400-2700 m range) because of the "turn-around time" of roughly 16 μs (i.e., about two orders of magnitude larger than the time of flight one is attempting to measure). We describe methods for estimating these offsets and for minimizing the effort required to do so when setting up an installation with many APs.
利用精细时间测量（FTM）往返时间（RTT）对协作式Wi-Fi接入点（AP）进行室内定位，已被证明在二维和三维应用中都能很好地工作，并能提供1至2米的精度。这种方法依赖于AP实施IEEE 802.11-2016（也称为IEEE 802.11mc）Wi-Fi标准（“双边”RTT）。不幸的是，这种Wi-Fi协议的普及速度比预期的要慢，可能是因为AP不像其他类型的电子设备那样经常升级，尤其是在大型机构中——而在这些机构中它们会最有用。最近，谷歌发布了安卓12，它也支持一种替代的“单边”RTT方法，该方法也适用于传统AP。这种方法无法减去信号的“往返”时间，因此，产生的距离估计值的偏移量比双边RTT的偏移量要大得多——结果也不太准确。与此同时，这种方法使得对许多以前无法使用的AP进行距离测量成为可能。这种增加的可及性可以弥补单个测量精度的下降。我们在此展示了针对不支持IEEE 802.11-2016的传统AP使用RTT进行室内定位。在杂乱环境中，视线读数较少（且仅使用20MHz带宽）时，实现的精度为3至4米。这不如使用RTT时达到的1至2米精度（使用80MHz带宽），但对许多应用来说是足够的。更宽的Wi-Fi信道带宽会进一步提高精度。和以前一样，贝叶斯网格更新是确定位置和位置精度的首选方法，但现在的观测模型与双边RTT的不同。与双边RTT一样，低于真实距离的RTT测量概率非常低，但在另一个方向上，给定距离的测量范围可能会宽得多（高达实际距离的两倍以上）。我们描述了制定有用观测模型的方法。与双边RTT一样，必须从报告的测量值中减去距离测量中的偏移量或偏差。一个不同之处在于，这里的偏移量很大（通常在2400 - 2700米范围内），因为“往返时间”约为16微秒（即，比试图测量的飞行时间大约大两个数量级）。我们描述了估计这些偏移量的方法，以及在设置有许多AP的装置时尽量减少所需工作量的方法。
