低軌道(low-earth orbit, LEO)衛星覆蓋範圍廣、移動速度快的特點，導致時間延遲不確定性、都卜勒偏移，對隨機接入(random access, RA)前導訊號(preamble)的傳輸造成影響，這將導致時間延遲估計不正確和前導訊號檢測率不佳。為了解決這個問題，如果用戶端在上行傳輸時沒有進行時間和頻率偏移的補償，則需要加強物理隨機接入通道格式和前導訊號。現有的方法是基於更大的子載波間距(subcarrier spacing, SCS)和重複多個扎德奧夫-朱(Zadoff-Chu, ZC) 序列，不然就是使用多個具有不同根的ZC序列。前者對子載波間距要求高，後者需要使用很多對根，根可能會不夠。在本文中，我們採用後一種方法。為了解決根不足的問題，我們提出了一種基於集合系統(quorum)的隨機機入前導訊號設計。通過最少的同步時間，採用基於quorum的空符號插入來生成更多的前導訊號。模擬結果驗證了新的前導訊號設計在低軌道衛星通訊環境中的有效性，並且能在大幅載波頻率偏移下維持前導訊號偵測的效果，以及在多用戶場景下有很好的效能。;The characteristics of broad coverage, fast-moving speed of LEO results in delay uncertainty and Doppler shift to random access (RA) preamble, which will lead to poor time estimation and preamble detection rate. To address this issue, in case pre-compensation of timing and frequency offset is not performed at the user side for UL transmission, enhanced PRACH formats and/or preamble sequences are needed. Existing approaches either a single Zadoff-Chu (ZC) sequence based on larger subcarrier spacing (SCS) and repetition number, or multiple ZC sequences with different roots. The former requires high SCS, while the latter is required to specify new root pairs, and there may not be enough root pairs. In this paper, we adopt the latter approach. To solve insufficient root issue, we proposed a quorum-based RA preamble approach. By the least time to synchronization, quorum-based null symbol insertion is adopted to generate more preambles. The simulation result validates the effectiveness of the new preamble in a LEO satellite communication environment, and reveal that the proposed method can achieve the robustness for CFO and provide outstanding performance improvements especially in multi-user scenarios.
