## 3 Signal Model
### NR Signal Components  
NR specifies different physical channels to utilize radio resources fully [31]. As shown in Figure 1, an NR frame lasts 10 ms and consists of 10 subframes, each lasting 1 ms. Furthermore, each subframe is divided into \(2^{\mu}\) slots and numerology \(\mu\in\{0,\ldots,4\}\) is an NR configuration parameter. Every slot contains 14 (normal CP) or 12 (extended CP) symbols and the subcarrier spacing follows \(\Delta f=2^{\mu}\cdot 15\) kHz. The resource grid further illustrates the allocation of NR physical channels. Each resource element (RE) in the resource grid carries a modulation symbol \(c_{n,k}\), where \(n\) and \(k\) denote the symbol and subcarrier indices, respectively. These modulation symbols \(c_{n,k}\), obtained from higher-layer signaling, can be categorized into two types: (1) unknown communication data and (2) predefined reference signals or predictable broadcast signals. The latter category can serve as navigation signals. NR downlink signals conclude multiple physical channels: a synchronization signal and physical broadcast channel (SSB), physical downlink shared channel (PDSCH), physical downlink control channel (PDCCH), and channel state information reference signal (CSI-RS). The SSB facilitates synchronization of user equipment (UE) and provides system message broadcasting. It comprises three signal components: primary synchronization  
Figure 1: New radio (NR) physical channel structure and resource grid. signal (PSS), secondary synchronization signal (SSS), and physical broadcast channel (PBCH). In the time domain, the SSB occupies four OFDM symbols with specific allocation: one for PSS, one for SSS, and two for PBCH. Its transmission periodicity is configurable with options \(\{5,10,20,40,80,160\}\) ms. Both PSS and SSS are m-sequences and their combination uniquely determines the physical layer cell identity (PCI):  
\[PCI=N_{\text{ID}}^{\text{SSS}}\times 3+N_{\text{ID}}^{\text{PSS}} \tag{1}\]  
where \(N_{\text{ID}}^{\text{PSS}}\in\{0,1,2\}\) denotes the PSS sequence index, and \(N_{\text{ID}}^{\text{SSS}}\in\{0,1,\ldots,335\}\) represents the SSS sequence index. The PSS and SSS are fixed for transmitters and can be used for navigation service. The PBCH comprises a master information block (MIB) and demodulation reference signals (DMRS). UE uses the DMRS, with predefined positions and content, for channel estimation. The MIB carries system information about the base station: system frame number (SFN), subcarrier spacing configurations, subcarrier offset parameters, and PDCCH configurations. The SFN increases over time, and other MIB parameters typically remain static. To enhance interference resistance, the MIB undergoes polar coding followed by scrambling. As a consequence, the coded MIB signal is time-varying. Nevertheless, cooperative receivers can effectively estimate these dynamic MIB components through predefined protocol specifications. Due to the predictable property, the entire SSB signal can serve as a local replica signal for navigation receivers. PDCCH dynamically schedules uplink and downlink radio transmission resources. The downlink control information (DCI) within PDCCH randomly varies according to service requirements. Apart from DCI, PDCCH also contains DMRS, which is used for channel estimation. PDCCH's DMRS are predetermined; thus, it can be used for navigation. PDSCH carries downlink communication data. Its time-frequency resources are flexibly allocated according to service demands. To optimize spectral efficiency, NR allocates most resource grid elements to PDSCH. Beyond random communication data, PDSCH also incorporates fixed DMRS for channel estimation. Once PDSCH is configured, the DMRS is deterministic, allowing it for navigation service. CSI-RS in NR system is dynamically configured by base station according to communication scenarios. UE measures its power to evaluate the wireless channel quality. This protocol-defined determinism signal can be used for navigation.