# Renyi Entropy-Based Adaptive Integration Method for 5G-Based Passive Radar Drone Detection
## 3 5G NR Signal Characteristics  
5G-based passive radar relies on the usage of signals from 5G network traffic between gNodeB (gNB--5G NR base station) and user equipment (UE). The rules of this traffic are defined by the 5G NR standard, developed by the 3 rd Generation Partnership Project (3 GPP), which is described in Technical Specification, series 38 [32]. Insight into the telecommunications standard from the radar perspective is given in [9]. 5G transmission is based on orthogonal frequency division multiplexing (OFDM). This means that the signal is divided into multiple data streams and transmitted simultaneously on orthogonal subcarrier frequencies. Specification [33] defines possible intervals between subcarriers, called subcarrier spacing (SCS): 15 kHz, 30 kHz, 60 kHz, 120 kHz, 240 kHz, 480 kHz and 960 kHz. In the time domain, the signal is divided into frames of 10 ms duration. Each frame consists of 10 subframes, 1 ms long. The number of slots in a subframe varies from 1 to 64 according to Table 1. In each slot, there are always 14 symbols. An example of the frame structure is depicted in Figure 3.  
In 5G, the smallest time interval is one symbol, and the smallest frequency quantum is a subcarrier. The resource element is the smallest time-frequency resource made up from 1 symbol and 1 subcarrier. The resource block (RB) is defined only in the frequency domain, and it consists of 12 subcarriers. All available resource blocks comprise the so-called resource grid (see Figure 4). Resources are allocated for several signaling channels and signals, e.g., the synchronization signal block (SSB) and channel status information--reference signal (CSI-RS), but mainly for user data transmission, which is contained in the physical downlink shared channel (PDSCH) or physical uplink shared channel (PUSCH). Two directions of the traffic can be distinguished: uplink (UL)--from UE to gNB, and downlink (DL)--in the opposite direction. In most cases, the time-division duplex (TDD) is used. The time-division pattern for UL/DL transmission can be configured with up-to-symbol granularity, and the periodicity of that pattern can be defined. The DL-to-UL ratio can be any, but in practical cases most resources are allocated for downlink traffic. The work of the network scheduler is to optimally allocate the resource blocks for the signaling and data transmissions for every user in both the UL and DL directions. \begin{table}
\begin{tabular}{c c} \hline \hline
**Subcarrier Spacing** & **Number of Slots** \\ \hline
15 kHz & 1 \\ \hline
30 kHz & 2 \\ \hline
60 kHz & 4 \\ \hline
120 kHz & 8 \\ \hline
240 kHz & 16 \\ \hline
480 kHz & 32 \\ \hline
960 kHz & 64 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Number of slots per subframe. Figure 3: Frame structure for SCS 30 kHz. Theoretical cases of the allocation of the resource grid are depicted in Figures 5-7. The schemes are obtained using the 5G Waveform Generator app from Matlab. In Figure 5, the orange field is PDSCH, which is the channel responsible for the user data downlink transmission. The whole resource grid is covered with orange, which means that all possible Resource Blocks are allocated for data traffic. Purple rectangles are resources responsible for synchronization--the SSBs grouped into synchronization signal bursts (SS Bursts). For the sake of clarity, the authors did not sketch other signaling channels present in the 5G NR signal. Figure 4: Resource grid illustration. In Figure 6, there are some resources not allocated at all (white background). This is the case when there is less demand for data traffic. The last case (Figure 7) is when there is no data payload. The only allocated RBs are for SSB, which are \"always-on\" the signalling block responsible for synchronization with the base station. The rest of the time-frequency resources are not in use. Figure 5: Resource grid fully allocated for data payload. Figure 6: Resource grid partially allocated for data payload. The above examples (Figures 5-7) show the nature of content-dependent transmission in 5G NR.