Preamble Block Design and Collaborative Detection Strategy for LEO Satellite Internet of Things with Grant-Free Random Access

Ensuring massive access for a large number of devices is essential to realize the full potential of satellite Internet of Things (IoT), promising to bridge connectivity gaps and provide services in previously unreachable regions. However, traditional terrestrial access methods have been found inadequate for satellite-terrestrial communications, primarily due to their inability to account for the unique characteristics of space-ground links, such as signal propagation delays and the dynamic nature of satellite orbits. To tackle this issue, we propose an innovative access scheme based on grant-free random access. In order to meet the demand for simultaneous access of multiple terminals, joint detection and parameter estimation are carried out by collaborative satellites, through the linear minimum mean square error (LMMSE) algorithm and Newton's algorithm. Then, this scheme empowers users to select several short sequences to form their own preamble signal. The new scheme can meet the access requirements of a large number of users while reducing computational complexity and the impact of carrier frequency offset (CFO). Additionally, choosing multiple short sequences can effectively lower the probability of preamble conflict. Simulation results demonstrate that the collision probability of the proposed strategy can be reduced by 70 percent and the detection probability can be improved by 38 percent with an equal number of users and preambles.