## 1 Introduction  
The rise of smart city-based applications has brought about a multitude of opportunities for urban environments to become more efficient, sustainable, and people-centric. However, these applications can only be fully realized with the support of precise indoor localization-based services (iLBS). The value of iLBS is becoming increasingly clear in several fields such as intelligent transportation, human activity analysis, COVID-19 contact tracking, and precision marketing. In the context of intelligent transportation, iLBS can facilitate optimized traffic flow within parking garages, airports, train stations, and other venues [1]. By using indoor localization technologies, it is possible to provide guidance to drivers, find available parking spots, minimize congestion, and reduce emissions caused by unnecessary driving. Moreover, iLBS can enhance the user experience when traveling inside buildings, such as guiding passengers to the right gate or providing real-time information on delays. Human activity analysis is another field in which iLBS can bring significant benefits. With its ability to track and monitor individuals' movements within buildings, iLBS can be used to improve workplace safety, optimize retail layouts, and enhance security measures [2]. Inmanufacturing and logistics settings, iLBS can help to ensure that workers are following safe operating procedures, potentially preventing accidents and injuries. In addition, in the retail industry, iLBS can analyze shopping behaviors to optimize store layout, product placement, and ultimately increase sales. The COVID-19 pandemic highlighted the critical need for iLBS for contact tracking purposes [3]. With the virus being highly contagious, fast and accurate tracing of the contacts of infected individuals is vital to contain outbreaks. Indoor localization technologies have been utilized to track the movements and contacts of people within buildings accurately. In the event of an outbreak, iLBS can provide valuable information to health authorities to notify people who may have been exposed to the virus to take necessary precautions. iLBS has tremendous potential in precision marketing. By using indoor localization technologies, retailers can deliver targeted marketing messages to customers based on their location and preferences [4]. For example, a retailer could send a message to a customer's smartphone offering a discount on a product they are currently standing near in the store. This personalized approach has been shown to increase conversion rates and drive sales. At this stage, various indoor positioning systems (IPSs) have been developed to provide indoor location information for users with different precision levels, including Wireless Fidelity (Wi-Fi) [5], Bluetooth Low Energy (BLE) [6], Ultra-wideband (UWB) [7], 5 th Generation Mobile Communication Technology (5G) [8], acoustic source [9], and micro-electro-mechanical systems (MEMS) sensors [10]. However, due to the hardware and signal limitations and differences between consumer-level terminals, the single location source cannot meet the positioning requirements of universality, low-cost, and wide-coverage in large-scale and complex urban indoor environments. Thus, a multi-source fusion structure becomes the main solution for realizing an accurate indoor localization system [11; 12], which also can be applied in other purposes, for instance, occupants' movement tracking [13] and environmental monitoring [14]. The Wi-Fi positioning system (WPS) has become a popular choice for indoor localization owing to its wide coverage and low cost. WPS is supported by existing mobile devices and local wireless facilities, making it a universal location source for indoor positioning. Currently, there are two main methods of implementing WPS: distance measurement and fingerprinting. Distance measurement can be achieved using two primary approaches: received signal strength indicator (RSSI) [15] and round-trip-time (RTT) [16].