# Remote Radio Frequency Sensing Based on 5G New Radio Positioning Reference Signals
## 1 Introduction
Additionally, each sent block of signal samples is preceded by a cyclic prefix, i.e., by the copy of the last samples. This operation guarantees that: (1) the channel impulse response does not cause interference between successive OFDM symbols, (2) linear channel convolution is visible by our communication system as a circular one, and, as a result, (3) channel equalization is straightforward: transmitted carrier states are recovered via the division of received carrier states by the DFT coefficient of the channel frequency response (CFR). The CFR is estimated using the reference carrier states of the so-called pilots, which are known to the receiver. In summary, OFDM-based digital communication systems transmit signals in blocks and have to frequently estimate the CFR using signal fragments in order to correct the received data in highly mobile, time-varying channels. Channel estimates in telecommunication receivers are calculated continuously, so extracting detailed information about reflections of transmitted signals from moving vehicles--such as their delays (vehicle distances) and Doppler shifts (vehicle velocities)--incurs little additional cost (e.g., in ISAC systems dedicated to unmanned aerial vehicles (UAVs) [14]). This is the main working principle of the so-called OFDM-based radar. Its idea is very similar to the radar CAF-by-batches methodology--see the comparison in [12]. Therefore, it is not surprising that about 15 years ago, despite the progress in radar research, an idea emergedin the communication industry to also use the communication 4G/LTE OFDM-based signals for sensing purposes, primarily with the vehicular industry in mind [15; 16]. The idea and technique of OFDM-based ISAC systems are at all times of interest in 5G New Radio (NR) and in the development of behind 5G standards: 5G-A and 6G [5; 6]. Ref. [17] specifies use cases for positioning services in 5G, such as accurate positioning to support augmented reality, emergency services, or UAV missions and operations, that can be simultaneously used with radar sensing or provided by it. The authors in [18] provided a complex and detailed overview of radio sensing using 5G signals, first presenting basic theories and concepts, continuing with state-of-the-art and exemplary applications, and ending with an analysis of open challenges. They also characterized the 5G OFDM-based radar. A comprehensive vision for integrated sensing and communication networks for 6G and beyond is provided in [19], reviewing signal processing, optimization, and machine learning techniques that enable ISAC networks and discussing potential applications of this technology. When 4G or 5G OFDM waveforms are used for radar purposes, the reference pilots, exploited in them, are utilized for the CFR/CIR estimation and detection of moving objects. Currently, different 5G pilots have been investigated. In [20], the usage of 5G NR reference signals was presented for CFR/CIR-based sensing, but with reference to the example of Channel State Information Reference Signals (CSI-RS). Positioning Reference Signals (PRSs) were analyzed in [21; 22]. In this context, however, in [21], RVMs were not calculated; only the velocities of the objects were estimated, whereas in [22], both range and velocity were estimated, but for just one PRS configuration. The Demodulation Reference Signal (DMRS) was also taken into account in [22; 23] regarding radar detection. All three reference signals, along with the Phase Tracking Reference Signal (PT-RS) and the Tracking Reference Signal (TRS), were analyzed in [24] in the context of bistatic vehicular radar, but unfortunately, the simulation results were only provided for the CSI-RS and DMRS signals. Ref. [25] examined the feasibility of using 5G new radio reference signals for target localization in bistatic and multistatic radar configurations using time difference of arrival (TDOA) and angle of arrival (AOA) measurements of 5G NR waveforms for positioning estimation. The idea of using 5G NR reference signals for navigation was discussed in [26].