# Error Investigation on Wi-Fi RTT in Commercial Consumer Devices
## 2 Related Works
### Ranging with Wi-Fi RTT in Android  
The protocol of FTM has been implemented in Wi-Fi RTT API and introduced since Android 9 (API level 28). Such an API can estimate the distance from the RTT-enabled smartphones to RTT-enabled Wi-Fi access points [17] and other neighboring RTT-enabled smartphones [21]. As officially claimed by Google, more than 40 flagship models of smartphones from different manufacturers are RTT-enabled, such as Google, Xiaomi, LG, Samsung, and Sharp [17]. Particularly, all the phones from the Google Pixel series (from Google Pixel 1 to Google Pixel 6 Pro) support such functions. Moreover, there are also multiple retail, warehousing, and distribution center devices that support Wi-Fi RTT, such as Zebra PS20 and Skorpio X5 [17]. In contrast, the only three consumer APs that officially claim to support RTT are the APs from the Google Wi-Fi series, including Google Wi-Fi, Google Nest Wi-Fi Router, and Google Nest Wi-Fi Point [17]. Although Google also officially claims that Compulab WILD AP developed by Compulab [22] is RTT-capable, it is not recommended for new projects and is not available as advertised on its website [22]. There are also some other enterprise options from Aruba that are RTT-enabled. Nevertheless, only three models of Google APs have proven standard compatibility officially and are available to consumers at the moment, which narrows the options.  
Besides the hardware, some apps have been introduced by Google and other developers for exploring the Wi-Fi RTT API with Android smartphones. The first app developed by Google, named WiFiRtScan, can discover nearby RTT-enabled APs and estimate the distance using Wi-Fi RTT API [23]. It can provide various information of the discovered AP, such as BSSID, SSID, RSSI, range, mean range, and range standard deviation. It also supports data logging by storing the measurements/estimations in an Excel file. The code of the original (old) version of this open-source project can be found in GitHub [24], Israel and Enrica [25] developed a new app based on WiFiRtScan by adding some new functions, such as conducting sampling campaigns and showing RTT capabilities of all nearby APs. However, one major limitation of these two apps is they cannot handle the simultaneous reading of multiple nearby RTT-capable APs. Compulab released one app named WILD Minimal [26] which can read from multiple APs at the same time but with limited information, including BSSID, range, range standard deviation, and RSSI. In June 2021, Google released a new app named WiFiRtLocator to demonstrate indoor positioning with Wi-Fi RTT [27]. Rather than only providing distance estimations, such an app can even provide location information by calculating from multiple (at least three) Google  
Figure 1: Overview of FTM protocol (one FTM request gives one burst with \(n\) FTMs (\(n\leq 31\)) [20].  
APs. However, the complex configuration of the app should be completed before starting the Wi-Fi scanning. This includes locating the floor plan on the map, obtaining precise ground truth (latitude and longitude) of the APs, and so on. A new Android app named FTMRTT developed by [PERSON] was released [28] in 2022. The location of the APs can be provided in any convenient Cartesian coordinate system (in meters) in the app, which is much easier to configure than WiFiRttLocator. Moreover, this app supports more functions, such as showing the measurement error and working with uncooperative APs in one-sided RTT mode [29].