# Error Investigation on Wi-Fi RTT in Commercial Consumer Devices
## 1 Introduction  
The Global Positioning System (GPS) has served for years for providing reliable Location-Based-Services (LBS) in outdoor scenarios. However, the performance of GPS signal degrades in indoor scenarios caused by blockers, such as walls and roofs. In recent years, Wi-Fi has been widely explored to provide LBS in complex indoor environments due to easy implementation and no requirements for extra infrastructure. Researchers have explored multiple Wi-Fi characteristics to estimate user locations, including Received Signal Strength Indication (RSSI) [1], Channel State Information (CSI) [2], Time of Arrival (TOA) [3], and Angle of Arrive (AoA) [4]. In recent years, a new protocol of Fine Time Measurement (FTM) has been standardized by IEEE 802.11-2016 to provide meter-level positioning accuracy [5] using the new technology of Round-Trip Time (RTT). Such a protocol is able to calculate the distance between the RTT-enabled smartphone and the Access Point (AP) by measuring the round-trip time of flight of the signal. Compared to other techniques (such as TOF), Wi-Fi RTT does not require clock synchronization between APs and smartphones. Wi-Fi RTT shows great potential to provide indoor positioning service for mobile users. Some researchers have shown efforts to calibrate the signal or develop reliable positioning algorithms in recent years [6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16]. In 2018, Google introduced the Wi-Fi API to the Android system for smartphones running Android 9 (Pie) or later [17]. Promoted by Google, new smartphones from many manufacturers with up-to-date Android systems support Wi-Fi RTT. Such smartphonescan send Wi-Fi ranging requests to nearby APs to measure their distance according to the Wi-Fi signal traveling time in a very short latency. In addition, Google has also introduced some apps for developers and researchers to test and explore the RTT signal for distance estimation and positioning use. Wi-Fi RTT has great potential to become a prevailing positioning method in the near future. Although Google claims that Wi-Fi RTT provides highly accurate distance estimations (meter-level accuracy), the error in Wi-Fi RTT distance measurements varies according to different environments and settings. The study from [PERSON] [18] shows that the Wi-Fi RTT distance can be affected by hardware and position. It also suggests that position-dependent error caused by user motion usually leads to a significant error in the ranging results. Moreover, the non-line-of-sight/line-of-sight (NLOS/LOS) condition is also the main factor that leads to inaccurate Wi-Fi RTT distance measurements. [PERSON] et al. [19] conducted some experiments to block the RTT signal using different materials of blockers to evaluate the Wi-Fi RTT distance measurements in NLOS conditions. However, the experiments and analysis are not enough to cover the possible situations in complex indoor environments, particularly when considering the complicated settings of different user devices, motion status, and signal pass losses. Therefore, this study explores the errors of Wi-Fi RTT distance measurements for mobile localization with extensive experiments. The contributions are as follows:  
* This paper fully investigates, analyzes, and summarises four categories of errors in Wi-Fi RTT distance measurements, including hardware-dependent bias, blocker-dependent bias, fluctuations, and outliers. * Two cases of keeping the smartphone static or varying its position during continuous measurement of the Wi-Fi RTT distance are stipulated to analyze the errors against user movements. * Different materials are used to block the signal path to evaluate the errors against complex NLOS conditions. The rest of the paper is organized as follows. In Section 2, we introduce the background knowledge of using RTT signals to measure the distance between RTT-enabled smartphones and APs. Capable hardware and software proposed in recent years for such research topics are summarised.