## 2 Overview of Positioning Principles Using UWB and Wi-Fi RTT Radio Signals
### Comparison of UWB and Wi-Fi RTT Positioning Technologies  
UWB and Wi-Fi RTT represent two prospective radio positioning technologies for indoor environments. Although both technologies are based on the same geometric positioning principles, they have their respective advantages due to their own signal characteristics and application ecosystem. UWB indoor positioning has the following advantages:  
High positioning accuracy: UWB signals have a time resolution of the picosecond level, enabling centimeter-level positioning accuracy.  
Low positioning latency: UWB signals have a low latency of millisecond or even microsecond level.  
High robustness: UWB signals demonstrate high resilience to multipath effects, obstacles, and interference, offering more stable positioning performance compared to other wireless technologies. However, it is important to note that when significant obstacles, reflections, or strong multipath effects are present in the environment, UWB signals may still experience distortion or delays, particularly in dense urban or indoor environments with complex infrastructure.  
On the other hand, Wi-Fi RTT positioning, based on the IEEE 802.11 mc protocol, has several advantages:  
High compatibility: Wi-Fi RTT is integrated with Wi-Fi network devices, including smartphones, which are used as mobile carrier; hence, Wi-Fi RTT positioning has high accessibility and applicability.  
High security: Wi-Fi RTT protocol leverages Wi-Fi network encryption and authentication mechanisms to guarantee positioning security, effectively preventing from spoofing or replay attacks.  
Low positioning cost: Wi-Fi RTT leverages existing Wi-Fi network infrastructure, such as routers and mobile phones; thus, it requires no additional purchases or dedicated deployment of hardware. However, it is worth noting that the number of devices that support the IEEE 802.11 mc Wi-Fi protocol is limited, but it is gradually increasing. A few manufacturers provide Access Points (APs), phones and other devices that optimize support for Wi-Fi RTT, but it may take several years to achieve full popularization. Table 2 lists the existing devices supporting Wi-Fi RTT [38, 39].  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  
In addition to hardware support, researchers have developed several applications to explore Wi-Fi RTT capabilities using Android smartphones. Google's initial Wi-Fi RTT-based application, WiFiRttScan, enables the discovery of nearby RTT-enabled APs and estimates distances through the Wi-Fi RTT Application Programming Interface (API) [40]. This application can display and log a variety of details about the discovered APs. Based on WiFiRttScan, the authors of [28] developed a new application with additional features, such as sampling and displaying the RTT of nearby APs. However, a major limitation of both applications is their inability to simultaneously handle readings from multiple nearby RTT-enabled APs. Compulab addressed this with its WILD Minimal application [41], which can read information from multiple APs at once. Then, Google released WiFiRttLocator in 2021, a new application to demonstrate indoor positioning based on Wi-Fi RTT [42]. This application not only provides distance estimates but also calculates location information using at least three APs. However, it requires a more complex setup before starting a Wi-Fi scan, including mapping the floor plan and obtaining precise global coordinates for each AP. In 2022, Horn introduced a new Android application called FTMRTT [43]. In FTM RTT, the location of APs can be set using any convenient Cartesian coordinate system, making configuration easier than in WiFiRttLocator. Additionally, the application supports more advanced features, such as displaying measurement errors and working with non-cooperative APs in unilateral RTT mode [44]. Recently, WifiRttScanX was developed in 2023 based on the WiFiRttScan and FTMRTT, which can estimate the unknown offset in FTM RTT results [45].