隨著近期低軌道衛星之進展，衛星通訊已被納入第五代通訊中之非地面網路，並寄望於補足地面基地台無法完整布建的缺口，實現無縫覆蓋之願景。3GPP 也著手於基於 New Radio (NR) 技術之非地面網路之研究與排定其於NR Rel-17 中之工作項目。然而，由於衛星通訊的固有特性，部分 NR 功能需要重新評估設計。其中，影響隨機接入程序最多即是低軌道衛星通訊系統之長傳輸延遲。因應長傳輸延遲，隨機接入程序中之部分參數與資源安排都應重新審視。在 NR Rel-17 中，假設非地面網路之用戶均配備GNSS功能，意即用戶知曉自己的位置訊息。基於上述前提，本論文為基於 NR 之低軌道衛星系統提出了新的隨機接入時槽 ( RO ) 設計。
在設計過程中，考量到場景中可能具有或不具有 GNSS 能力的用戶（G-UE 或 nG-UE），分別排程它們的隨機接入時槽，G-UE 使用G-RO；nG-UE 使用nG-RO。基於此獨立排程設計，本論文提出了三種可以由網路配置的 RO 模式：ROpair, Overlapped RO 與 Non-Overlapped RO。ROpair即是以相同的週期排程 G-RO 和 nG-RO；Overlapped RO 和Non-Overlapped RO則是使用不同的週期排程 G-RO 和 nG-RO，差別於允許G-RO 和 nG-RO 重疊與否。
為了驗證提出之 RO 模式的有效性，本論文透過模擬評估存取所需之延遲。模擬結果顯示分別調度 G-RO 和 nG-RO 的優點有兩個。其一，所有用戶的平均接入延遲明顯降低。再者，nG-UE之平均接入延遲較不會受未來G-UE數量增長影響。此外，本論文亦分析了在不同前導碼分配比情況下，Overlapped RO 之性能有何差異。

With the recent advances in low earth orbit (LEO) constellation, satellite communication (SATCOM) has been included in Non-Terrestrial Network (NTN) and recognized as a promising solution to extend services to unserved area in 5G networks. 3GPP has also considered to develop NR-based NTN, however, part of NR functions should be re-worked duo to SATCOM inherent characteristics. In NR-based NTN, user equipments (UEs) are anticipated to be GNSS-enabled in NTN scenarios in NR Rel-17. Based on the aforementioned features, we propose three novel RACH Occasions (RO) patterns for LEO-NTN.
Though user terminals are assumed to have location information, we design RO patterns with backward compatibility, namely, users who is not aware of its own location are also considered. For UEs with or without GNSS capability (G-UE or nG-UE), we propose to schedule their RO separately, G-RO for G-UE, nG-RO for nG-UE. In this work, we propose three RO patterns that could be flexibly configured by the network. Specifically, scheduling G-RO and nG-RO with same periodicity, we propose RO pair, contrarily with different periodicity, we propose overlapped RO and non-overlapped RO.
To validate the effectiveness of proposed RO patterns, we review the preamble transmission delays through computer simulation. Advantages of scheduling G-RO and nG-RO separately are two-fold. Firstly, simulation results show that average delay of successful preamble transmission for all UEs is reduced. Secondly, nG-UEs are relatively more insusceptible to expected increment of G-UE number in the future. Moreover, we investigate the performance of overlapped RO with different preamble division ratio.

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