Beam Domain Random Access for NB-IoT Integrated LEO Satellite Communications

Integrating narrowband Internet of things (NB-IoT) into low-Earth orbit (LEO) satellite communications plays a promising role in serving massive devices and extensive coverage. In the receiver, the active user equipments (UEs) detection and timing synchronization in the random access procedure is essential but greatly challenged by the large frequency offset and long delay characterized by the LEO satellite. This article presents the preamble allocation and receiver design of beam domain random access for NB-IoT integrated LEO satellite communications equipped with large-scale array antenna. Leveraging the inherent sparsity of the LEO satellite beam domain channel, we first design a UE grouping-based preamble allocation scheme to alleviate the collision by channel orthogonality for UEs simultaneously accessing. Building on the designed scheme, we propose a joint Doppler-beam domain active UE detection method that resorts to large frequency offset, which significantly improves the detection rate and obtains the coarse estimation of frequency offset. To achieve accurate timing synchronization, we formulate the timing offset estimation problem based on the maximum likelihood criterion. Following this, a modified and refined phase difference timing offset estimation algorithm is proposed, which employs the complete frequency hopping pattern of the preamble. To reduce the computational complexity, we propose an estimation algorithm possessing high performance with the prior information of estimated frequency offset. In comparison with the conventional method, the simulation demonstrates the superior performance of the proposed active UE detection and timing synchronization algorithms.