## 1 Introduction
### Indoor Localization Systems  
Conventional radio indoor localization systems can be divided into two groups based on the type of used signals: narrowband systems and ultra-wideband (UWB) systems. Narrowband systems, which usually use widely spread standards (e.g., WiFi [5] and Bluetooth [6]) typically allow to localize the user with accuracy from one to several meters. UWB-based solutions [7] are much more accurate. Their localization accuracy can be as high as a dozen centimeters, but since the employed technology is not as known as than the aforementioned, they are less popular. In the UWB positioning systems [7], the users are usually localized based on the time of arrival measurements of signals transmitted from the localized tags to the system infrastructure. Due to the short duration of the UWB signals (less than 1 ns), the time measurements can be performed with a resolution as high as 15.65 ps [8], which under the right conditions, can allow for precise and accurate localization. The two typically used localization methods in the UWB based systems are Time of Arrival (ToA) and Time Difference of Arrival (TDoA). In Time of Arrival-based UWB systems [9] users location is derived based on the signal propagation times between the localized tag and the anchors comprising the system infrastructure. Range to the anchor is usually measured using the Two-Way Ranging procedure (TWR) which consists of exchanging packets between the tag and the anchor, noting their transmission and reception times [9]. The main advantage of this method is that, since the distance between the tag and each of the anchors is measured independently, anchor synchronization is not needed. On the other hand, ranging with each of the anchors on its own takes time and in case of extensive packet exchange, the infrastructure's radio interface might become cluttered, which would limit the number of localized tags. The Time Difference of Arrival method consists of localizing the user based on the difference of times, at which the anchors receive signals transmitted from the tag [10]. Typically, the only transmitting devices in this method are the tags, which helps to organize the transmission and allows to achieve higher system capacities when compared to ToA. The main problem with implementing TDoA is the requirement to achieve accurate and reliable anchor synchronization. In commercial systems, it is usually done by transmitting synchronization signals to the anchors with cables [11] but can also be achieved wirelessly by adding another reference anchor to the system infrastructure [10]. Both ToA and TDoA allow for precise localization in Line-of-Sight propagation conditions. The typically obtained accuracies are at the level of a dozen centimeters [12]. When the propagation occurs in Non-Line-of-Sight Conditions (NLOS) the accuracy is usually lower. Localization in narrowband systems is typically based on measurements of Received Signal Strength (RSS). In tracking and monitoring systems, where the transmitting tags are being localized, they are usually performed by the anchors. Those systems usually employ well known and widely spread standards such as WiFi or Bluetooth (more recently Bluetooth Low Energy BLE [6; 13]). Most of the systems localize the users with one of two methods: fingerprinting or RSS ranging. Fingerprinting is a method which consists of comparing the signal levels received by the anchors with a previously prepared radio map [14] containing information on signal levels received from particular places. Besides its easy implementation, the main advantage of the method is that it does not require any information on a system infrastructure placement or location. Unfortunately, achieving high accuracy localization demands preparing accurate high-density radio maps, which is usually done manually by placing the tag in points distributed in the area and storing received signal level values.