# Positioning-Based Uplink Synchronization Method for NB-IoT in LEO Satellite Networks
## 2 System Model  
### Problem Scenario and Technical Challenges  
Figure 1 illustrates the system architecture of an LEO and GNSS satellite-based IoT network built upon the NB-IoT framework. The NB-IoT system adopts an Orthogonal Frequency Division Multiplexing (OFDM) transmission scheme. During uplink synchronization, the base station (BS) requires that the uplink signals from all user equipment (UE)  
arrive nearly simultaneously, ideally within one CP duration, to ensure correct demodulation. To achieve this, NB-IoT employs a TA mechanism: terminals located farther from the BS are instructed to transmit earlier, so that their signals arrive at the BS in alignment with others. In terrestrial NB-IoT systems, the terminal first performs time and frequency synchronization by detecting the downlink synchronization signal broadcast by the BS. It cross-correlates the received signal with a local reference to identify the peak position, enabling synchronization. This is followed by a random access procedure, where the terminal sends a preamble sequence to the BS. Upon receiving the Message 1 (MES1), the BS detects the preamble, calculates the required TA value, and sends it back to the UE via Message 2 (MES2).  
However, this conventional TA-based synchronization strategy is not directly applicable in satellite NB-IoT systems. First, the large Doppler frequency shift in satellite channels causes traditional correlation-based synchronization algorithms to fail. Second, the high mobility of satellites results in a very short validity period for the TA value, necessitating frequent updates and thus consuming excessive signaling resources.  
To address this, 3 GPP proposes that most NTN terminals be equipped with GNSS receivers for uplink synchronization. With knowledge of its own position and satellite ephemeris data broadcast by the satellite, a terminal can compute the required TA value and the pre-compensated Doppler shift. However, in practical NB-IoT LEO satellite communication scenarios, GNSS signal reception may be blocked due to environmental obstructions, or the terminal may be unable to simultaneously receive signals from four GNSS satellites. Additionally, GNSS signals may suffer from interference. For NB-IoT devices that are  
Figure 1: Illustration of LEO and GNSS satellite-based IoT network.  
power-constrained, relying on a GNSS that is external to the system results in substantial additional power consumption. This has motivated research into leveraging the communication signals themselves for PNT services. In this approach, the communication system provides terminal positioning capabilities without GNSS assistance. However, realizing positioning using communication signals in LEO satellite systems faces significant technical challenges, including large Doppler shifts and time-varying numbers of visible satellites.  
To this end, this paper explores the use of the NB-IoT downlink broadcast signal, namely the Narrowband Primary Synchronization Signal, to enable terminal positioning. By combining this with satellite ephemeris data, a pre-compensation-based uplink synchronization scheme is proposed, specifically tailored to LEO satellite scenarios.