# Peer-to-Peer Ultra-Wideband Localization for Hands-Free Control of a Human-Guided Smart Stroller
## 1 Introduction
### Human Tracking Methods  
Recognizing and localizing the target person is critical in a human-following robot system. Optical sensors are widely used to track a moving target [4; 5]; however, their performance is affected by occlusion and light variation [6]. Laser range finders (LRFs) provide higher accuracy in terms of distance measurement, but they also suffer from occlusion. Optical sensors and LRFs are used in conjunction with algorithms for target recognition and tracking, which is not only computationally expensive but also can fail to track the target person or even reassign the target person under some circumstances [7]. Such risks make the optical sensors and LRFs unfavorable for the smart stroller application. On the other hand, peer-to-peer localization systems using radio frequency benefit from identifying the target person carrying a tag and even under no line-of-sight (NLOS) conditions. [PERSON] et al. built a human tracking system by fusing vision data and RFID data to enable the robot to follow a human in the crowd [8]. [PERSON] et al. [9] proposed a human tracking method using passive ultra-high-frequency (UHF) RFID technology. RFID tags are cheap and small but their signal transmission can be easily hindered by physical obstructions and interference from other radio frequencies. [PERSON] et al. [10] proposed a human-follow robot based on the WiFi RSSI-trilateration method by tracking the desired user with the phone. Conveniently, the user does not need to wear an extra sensor instead of carrying their phone in the human following robot situation. Other works [11; 12] attempted to improve the localization accuracy of WiFi-based indoor positioning systems, and the proposed WiFi-based localization systems in the study [13] demonstrated an accuracy characterized by a one-meter spatial resolution in the indoor environment with obstacles. However, the signal strength of WiFi is affected by the obstacle, and localization performance is relatively inaccurate with meter-level accuracy. The Bluetooth low energy (BLE)-based localization system is a popular solution in indoor positioning and object tracking [14; 15]. It boasts advantages of being low cost and low power and is available in all recent smartphones. Reference [16] proposed a BLE-based human-following robot. However, like other RF technology, BLE localization can be affected by physical barriers and signal interference. UWB sensors excel in localization precision, resistance to multipath interference [17], and minimize interference with other radio-frequency devices compared to other radio-frequency sensors above. Several works explored the possibility of UWB technology in the human-following robot field in recent years. [PERSON] et al. proposed a person-tracking system only based on a UWB sensor for mobile robot navigation [18]. [PERSON] et al. mounted UWB radio transceivers on flying robots to build a target human-tracking environment [19],and [PERSON] et al. proposed a robust human-tracking method by sensor fusion of UWB and LRFs [7]. The application scenario of a human-guided stroller extends to both indoor and outdoor environments. UWB sensors perform well in a line-of-sight (LOS) outdoor environment [20] and are minimally impacted by light exposure. Additionally, the UWB signal consists of various frequency components, which enhances its ability to penetrate obstacles due to its wide bandwidth [21], enabling it to perform well in the indoor environment with obstacles. It offers centimeter-level accuracy, is energy-efficient [17; 22; 23], and robustly enables tracking of the person even under NLOS conditions. In the smart stroller context, where it's crucial to maintain proximity to the parent, a high degree of localization accuracy and reliable tracking is necessary. Additionally, major smartphone manufacturers now include UWB technology in their devices [24] to achieve precise localization and enhance device functionality, offering a good opportunity to leverage this technology for innovative applications, such as smart strollers.