# Positioning-Based Uplink Synchronization Method for NB-IoT in LEO Satellite Networks
## 1 Introduction  
As the core direction of the development of next-generation information technologies, the Internet of Everything (IoE) is a key enabler for future digitalization and intelligent transformation. To realize the vision of IoE in future 6G networks, achieving global network coverage is a fundamental prerequisite. However, the construction and maintenance costs of terrestrial base stations in remote areas are prohibitively high. Currently, more than 80% of land areas and over 95% of oceans are not covered by terrestrial networks. As a result, the use of satellites for global coverage of wireless networks has become a research hotspot [1, 2].  
For terminals in these regions that require Positioning, Navigation, and Timing (PNT) services, Global Navigation Satellite System (GNSS) technologies are commonly usedto obtain location and related information [3]. Nevertheless, GNSS suffers from several drawbacks, including high power consumption, frequent signal unavailability, and reliance on external systems. As an alternative to GNSS, PNT services via low earth orbit (LEO) satellites are gaining increasing attention [4; 5]. Compared to medium earth orbit GNSS satellites, LEO satellites are much closer to the earth, allowing for stronger satellite signals that can provide PNT services even under GNSS-denied conditions--such as signal blockage, inability to receive signals from four GNSS satellites simultaneously, or signal interference. Although LEO satellite-based positioning has lower accuracy than GNSS, it is more valuable for IoT terminals with low power consumption requirements and less stringent positioning accuracy needs [6; 7].  
NB-IoT is a mature, widely deployed low-power wide-area (LPWA) technology in terrestrial networks. It is one of the communication technologies designed for the Internet of Things, targeting application scenarios characterized by low data rates, low power consumption, and massive device connectivity. Similar technologies include LoRa. Earlier this year, Iridium Communications (McLean, VA, USA), a well-established satellite operator, officially launched Project Stardust, a direct-to-satellite NB-IoT Non-Terrestrial Network (NTN) initiative based on 3 GPP standards. The project has begun to foster collaboration across the NB-IoT industry ecosystem.  
One of the major challenges in LEO satellite-to-ground networks is uplink synchronization. Terrestrial networks use Timing Advance (TA) mechanisms to achieve uplink synchronization [8]. Traditional Timing Advance strategies require user equipment to first capture downlink synchronization signals and extract synchronization information. However, in satellite networks, the vast distance between satellites and ground terminals exceeds the maximum allowable TA value. Furthermore, the high-speed movement of satellites causes rapid TA value obsolescence, necessitating frequent updates. Traditional methods in which terminals transmit preamble signals to obtain TA values are not feasible in satellite channels [9]. To address this, 3 GPP has proposed that terminals may use GNSS to obtain their positions and then calculate TA values using satellite ephemeris data for synchronization [10; 11]. While this approach is viable for 5G NR terminals, it is unsuitable for NB-IoT devices due to GNSS's high power consumption [12; 13]. Therefore, it is essential to develop a novel uplink synchronization solution tailored to IoT devices with low-power requirements.