# Analysis of the Scalability of UWB Indoor Localization Solutions for High User Densities
## 2 Related Work  
### Scientific Literature  
UWB is a very well known technology in the scientific community. Originally designed for military communication in the late 1960s [5], it rapidly became popular for other application domains. Its peculiar characteristics allow for precise timing resolution, which is crucial to accurately estimate the Time of Flight (ToF) of the radio signal. Consequently, many researchers started to investigate this promising technology and its relevance in indoor positioning system. Immediately, important studies were published, showing that UWB was capable of localizing a node with a typical error of around 30 cm. Centimeter level accuracy is a significant performance improvement compared to other technologies such as WiFi and Bluetooth low energy (BLE), which can typically achieve localization accuracy in the order of a few meters [6]. The list of studies aiming to improve UWB accuracy is existing and they focus on small-scale setups with a limited number of mobile nodes in a controlled environment such as office or residential rooms. In contrast, very little research has been published that focuses on the scalability of UWB systems. An analysis of the performance of two standard MAC protocol is presented in [7]. It is shown how the overall performance is affected by frame sizes, packet arrival rates and acquisition time. MAC protocol such as the carrier-sense multiple access with collision avoidance (CSMA/CA) protocol in a UWB have weaker performance than in narrowband system due to longer channel acquisition time. To support large numbers of simultaneously transmitting devices, several UWB TDMA solutions have recently been proposed. An overview of these techniques is presented in [8]. In addition, the combination of TDMA with other approaches such as Code Division Multiple Access (CDMA) is investigated in [9], wherein, despite the MAC analysis in terms of latency, it is not shown whether this approach could further enhance the scalability to support higher user densities. A distributed localization scheme is proposed in [10]. The maximum number of users involved in the simulation is 200. The study focuses on collaborative localization based on power measurements (RSSI). However, the impact of their design choices on the overall scalability is not discussed. In [11], an UWB positioning system that supports up to 30 UWB tags in a TDMA access scheme has been developed, but their system is not designed for allowing larger user densities and they only investigate the use of TDoA approaches. A study of the benefits of a hybrid TDoA-TWR approach is illustrated in [12]. In particular, a hybrid algorithm was simulated and afterwards the results were compared with a standard TDoA approach. The experimental results show that in terms of localization precision, the TDoA-TWR approach performs better than the standard TDoA one, but this analysis does not involve any MAC feature nor the study of the system scalability. Thus, it is not possible to evaluate the impact of this solution on the number of supported users. Also in most practical evaluations of commercial systems, the user density is not considered at all. For example, in [13], the authors compare two existing solutions in terms of accuracy, position update rates and end-to-end delays only.  
In [14], a system architecture that scales towards more users and has larger coverage areas was proposed. In this architecture, the managing and control part of the network (including allocating UWB resources such as time slots and device roaming) are separated from the actual positioning system. UWB is only used for ranging and position estimation, whereas the other functions are managed through a second non-interfering WiFi network. Nonetheless, coupling communication and localization requirements using UWB only, is also feasible. A possible approach is explained in [15] wherein a contention-based MAC protocol is adapted to gather position information. However, again the scalability of the system in terms of user density is not evaluated.