# Device Free Detection in Impulse Radio Ultrawide Bandwidth Systems
## 1 Introduction  
COVID-19 has had a significant impact on our lifestyle [1]. Physical distancing, wearing masks, avoiding crowds, and maintaining better hygiene are becoming the new norms [2, 3]. It has been advised to maintain at least a 1-meter distance between yourself and others [3]. Despite this, there are no concrete studies on the recommended distance between individuals in the indoor environment, where an intelligent guess would be more than 1 meter. However, at this moment, there are no particular devices that can help or provide accurate information of this distance to the user [4]. Furthermore, measurement of this distance becomes problematic, especially for indoor environments [4, 5]. For indoor environments, when trying to measure this distance, two different approaches have been proposed: wearable [6] and device-free [7]. The wearable one is the most widely used approach, and the users are tracked by the sensor placed on the device [8, 9, 10]. The device-free approach has the ability to track people passively without having any contact withthe person, resulting in a device-free communication [11; 12; 13]. Furthermore, device-free communications can provide more flexibility, comfort, and mobility compared to the wearable approach [14; 15]. It is well known that the global positioning system (GPS) is a promising technology to provide outdoor positioning [16]. However, its limited penetration though solid objects makes it unreliable and inaccurate for indoor positioning [17]. To address this, several techniques including Wireless Fidelity (WiFi), Radio Frequency Identification(RFID), Bluetooth Low Energy (BLE) and ZigBee, where the localization is performed based on the received signal strength have been explored [18; 19; 20; 21; 22]. However, these techniques have drawbacks such as low accuracy, low data-rate and/or short range communication [18; 19; 20; 21; 22]. Furthermore, when using narrow-band device-free indoor communication, multipath fading and a cluttered environment make it difficult to design a low-cost and low-complexity receiver [23; 24; 25]. Ultrawide bandwidth (UWB) is a revolutionary low-power communication technology [26; 27]. A UWB system offers many advantages such as high data rate, fading robustness, high precision ranging, obstacle penetration capability, and low-cost transceiver implementation [27; 28; 29; 30; 31]. All these advantages have made UWB a promising technique for device-free communications [24; 25; 26; 32]. Initially, a UWB system was implemented with the help of a slow time hopping (STH) pulse position modulation (PPM) also known as the impulse radio (IR) [33]. However, different modulation schemes such as on-off keying (OOK) [34], pulse amplitude modulation (PAM) [34] and pulse shape modulation (PSM) [35; 36], and different multiple access schemes such as direct-sequence (DS) [37] and hybrid Direct-Sequence and Time-Hopping (DS-TH) [38] have been implemented for UWB systems. In contrast to the received signal strength based positioning, UWB exploits either the time of arrival (ToA) or time difference of arrival (TDoA) for real-time positioning [39; 40]. In [41], authors proposed an indoor positioning system based on UWB and long-range (LoRa) wireless technologies. Simulation results showed that the proposed system can achieve an accuracy of 15 cm. However, the proposed system was not investigated experimentally. The respiratory movement of a person is detected in laboratory conditions by using UWB radar, in [42]. The influence of a user state and environment was carried out in [43]. The performance of a UWB-based system by deploying anchor nodes on the floor was experimentally investigated in [44]. An elderly people tracking system based on the combination of UWB and Bluetooth low energy has been proposed in [45]. Artificial intelligence and machine learning techniques were developed in [39; 40] to improve the indoor localisation to less than 10 cm by classifying them into line of sight (LoS) and non-LoS environments.