## 2 Technologies for Indoor Localization  
From a technology point of view, localization solutions exploit different approaches such as Wi-Fi, Ultra-WideBand (UWB), ZigBee, Bluetooth, Radio-frequency Identification (RFID), Global System for Mobile communication (GSM), Infrared (IR) and visible light Frequency Modulation (FM), ultrasounds and audible sounds, magnetic field, camera and sensors such as inertial sensors available in wearable devices, smartphones or dedicated electronics [1; 11]. In some cases, hybrid systems based on the combination of different techniques, such as multimodal fingerprinting, triangulation-based fusion, and pedestrian dead-reckoning-based fusion, have been presented as solutions to cope with limitations in accuracy, coverage, and complexity while maintaining a low cost [7]. The main drawbacksof several navigation systems in the literature are related to discontinuous assistance for the user, the high cost, and sometimes the arbitrary form of information provided [12]. A comparison of the different location solutions and a review of the approaches used to evaluate their performances are discussed in [13]. IR technology is largely adopted in many communication systems. Anyway, since it relies on the Line-of-Sight (LoS) between transmitters and receivers, this approach can be conveniently adopted in the case of open environments. As an example, in [14], a low-cost localization method based on tracking mobile IR transmitters and an AoA estimation algorithm is presented. The solution was investigated in the context of the supermarket cart navigation, showing accuracies ranging from centimeters in the case of static positions up to 1 m mean localization error obtained for a cart moving at 1.4 m/s. The main problem affecting IR technology and, in general, light-based solutions is the multipath propagation due to light reflections and refractions in the indoor environment. Both the line-of-sight component and the multipath component are received by the receiver, and because signal components can vary in power strength and phase, the localization accuracy can be significantly affected. Strategies for the compensation of the multipath effect have been proposed in literature. The authors in [15] proposed a model of IR signal reflections on any kind of surface material that can be applied for the multipath behavior of optical signals. The same authors, in [16], proposed a model to determine the multipath effect in indoor environments when the shape and characteristics of the environment (e.g., reflection features of the materials) are known a priori. Recently, the effects on the accuracy of IR-based indoor positioning caused by the multipath have been analyzed using Angle of Arrival (AoA) and Phase-of-Arrival (PoA) techniques [17]. In the context of visible light communication systems, fluorescent lamps and Light-Emitting Diodes (LEDs) are used to transmit data as light beams and light pulses. The basic principle is that each light source serves as a beacon whose modulated radiation can be captured by a light sensor and uniquely identified [18]. A survey of LED-based visual light communication systems is reported in [19]. The main drawbacks reported in the literature are the need for significative computational resources for real-time processing and high energy demands. Sound-based indoor tracking systems adopt microphones, speakers, and sound (audible) and ultrasonic sensors to generate and detect signals. Such systems can be used to detect the intensity of sounds or as distance sensors by estimating the travel time of the sound signal from the source to the sensor. To this aim different distance measurement techniques have been proposed, such as the Time of Arrival (ToA) and the Time Difference of Arrival TDoA. Ultrasonic sensors are used as a distance sensor in the location detection systems discussed in [20; 21]. The authors in [21] proposed a high-accuracy ultrasonic indoor positioning system that exploits several wireless ultrasonic beacons in the indoor environment.