NR-NTN Initial Access: Ephemeris-based Approach

This article investigates the initial access (IA) process for the nonterrestrial network (NTN). We highlight the issues that arise when applying the IA process used in the new radio terrestrial network (TN) to NTN. First, satellites need to manage a large number of codebook beams, and the time required for the beam searching process is long. Second, due to the long propagation distance of the satellite, the size of the beam reaching the Earth's surface is very wide. As a result, a massive number of user equipments (UEs) perform random access (RA) using the same resources, leading to a high probability of RA preamble collisions. To address these issues, we propose an IA process utilizing ephemeris information. In addition, we redefined the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">IA-delay</i> to evaluate how quickly UEs can access the satellite. In particular, we considered the propagation delay for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">IA-delay</i> since it has a dominant impact on the IA process in NTNs, unlike in TNs. Simulation results show that the proposed IA process has short <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">IA-delay</i> and reduces IA failures.