# Remote Radio Frequency Sensing Based on 5G New Radio Positioning Reference Signals
## 1 Introduction
In general, the idea of joint application of pilot reference signals in the communication part of ISAC systems for channel estimation and correction, as well as in the radar part for target sensing, is one of the most important research challenges in future multi-antenna and mmWave communication [27; 28; 29]. The main purpose of this paper is to further investigate the feasibility of using PRS--positioning reference signals--for OFDM radar sensing in 5G mobile networks, based on the example of high-speed train (HST) detection by a 5G base station. The PRS is a highly configurable reference signal, more than others. The present paper is more communication-oriented than previous works [20; 21; 22; 23; 24; 25]. Only monostatic radar is considered here: the signal is transmitted, received, and analyzed at the 5G NR base station. In contrast to other works, the HST is assumed to be a moving object--channel modeling in this case is more difficult [30]. Only the results from simulations are presented in this work; however, the simulation parameter values are based on measurements [31; 32; 33]. Our studies show that it is possible to build the OFDM-radar for the HST, which detects both the train range and velocity, using the PRS reference signals. In contrast to our previous works [31; 32; 33], where the 5G NR waveform was used for passive coherent location (PCL) with the help of the CAF-based approach, currently, the OFDM-based active monostatic radar that exploits 5G PRS signals is tested. Before, the receiver used two antennas: a reference one (REF) looking at the 5G NR base station, and a surveillance one (SURV) looking at the region of interest. Instead of the OFDM-based radar methodology, the CAF-based one was applied: the REF signal was delayed, modulated/shifted in frequency, multiplied by the SURV signal, and integrated. In this way, the range-velocity maps (RVMs) were calculated and the vehicles detected. Alternatively, system information was recovered from the REF signal, and based on this, the 5G NR reference signals, such as PSS, SSS, SSB, and CSI-RS (without PRS), were identified and re-synthesized. They were then correlated with the SURV signal using the CAF approach. In this paper, the OFDM-based approach is applied, and PRS pilots are used. The main innovations of this paper are as follows:  
* A greater number of different 5G PRS signal configurations are tested during simulations compared to other papers on this topic;
* The HST scenario/channel is adopted from [30] for the purpose of this study;
* Complete RVMs are calculated in comparison with [21];
* The simulations parameters are set according to the values observed in our previous field experiments [31; 32; 33] and with respect to the 3 GPP standard ;
* An iterative high-resolution spectral analysis method proposed by [PERSON] and [PERSON] [34] is used for a precise estimation of velocity;
* the findings indicate that utilizing PRS pilots in remote sensing is feasible, and the densest configurations do not offer significant enhancements while requiring substantially more resources. The outline of this paper is as follows. In Section 2, a 5G NR grid and waveform structures are presented. Following that, in Section 2, the positioning reference signal, its advantages, and its configuration parameters are described. In Section 3, the simulation scenario and propagation channel model are defined, whereas in Section 4, the OFDM radar that uses the PRS is introduced. Finally, the simulation results are presented in Section 5. This paper ends with conclusions and proposals for future work.