Low Earth orbit (LEO) satellite systems enable close to global coverage and are therefore expected to become important pillars of future communication standards. However, a particular challenge faced by LEO satellites is the high orbital velocities due to which a precise channel estimation is difficult. We model this influence as an erroneous angle of departure (AoD), which corresponds to imperfect channel state information (CSI) at the transmitter (CSIT). Poor CSIT and non-orthogonal user channels degrade the performance of space-division multiple access (SDMA) precoding by increasing inter-user interference (IUI). In contrast to SDMA, there is no IUI in orthogonal multiple access (OMA), but it requires orthogonal time or frequency resources for each user. Rate-splitting multiple access (RSMA), unifying SDMA, OMA, and non-orthogonal multiple access (NOMA), has recently been proven to be a flexible approach for robust interference management considering imperfect CSIT. In this paper, we investigate RSMA as a promising strategy to manage IUI in LEO satellite downlink systems caused by non-orthogonal user channels as well as imperfect CSIT. We evaluate the optimal configuration of RSMA depending on the geometrical constellation between the satellite and users.

翻译：低地球轨道卫星系统能够实现近乎全球覆盖，因此有望成为未来通信标准的重要支柱。然而，低轨卫星面临的一个特殊挑战是高轨道速度，这导致精确信道估计变得困难。我们将这种影响建模为错误的离开角，对应发射端非理想的信道状态信息。较差的CSIT和非正交用户信道会因增加用户间干扰而降低空分多址预编码的性能。与SDMA相比，正交多址接入不存在IUI，但需要为每个用户分配正交的时间或频率资源。速率分割多址接入统一了SDMA、OMA和非正交多址接入，已被证明是在非理想CSIT条件下实现鲁棒干扰管理的灵活方法。本文研究了RSMA作为管理低轨卫星下行链路系统中由非正交用户信道及非理想CSIT引起的IUI的一种有前景策略。我们根据卫星与用户之间的几何星座配置评估了RSMA的最优配置。