We consider a constellation of low-earth-orbit (LEO) satellites connected to a handheld device on the ground. Due to the very large orbital speed, an effective handover strategy becomes of paramount importance. In particular, we study the benefits of soft handover in the uplink from the physical-layer point of view. We give a realistic model for both the ground-to-satellite and the inter-satellite links, following the 3GPP channel model for the former. We suppose that, during handover from a serving satellite to a target satellite, one of the two satellites forwards the received signal from the ground user to the other, thus acting as a relay. We quantify through simulations the loss of hard handover, compared to soft handover. For the latter, we test both amplify-and-forward (AF) and decode-and-forward (DF) relaying techniques and verify that, at least in the simulated conditions, DF does not repay, in terms of block error rate (BLER), the increase of complexity with respect to AF. Also, we study the effect of the LEO constellation size on the network BLER. Finally, we show that, with soft handover, the impact of misalignment on the inter-satellite link is severe, especially at optical frequencies.

翻译：我们考虑由低地球轨道（LEO）卫星组成的星座与地面手持设备的连接问题。由于轨道速度极大，有效的切换策略至关重要。本文从物理层角度重点研究上行链路中软切换的优势。我们为地面-卫星链路和星间链路建立了符合实际的模型，其中前者遵循3GPP信道模型。假设在从服务卫星到目标卫星的切换过程中，其中一颗卫星将接收到的地面用户信号转发给另一颗卫星，从而充当转发节点。通过仿真量化了硬切换相对于软切换的性能损失。针对软切换，我们测试了放大转发（AF）与译码转发（DF）两种中继技术，并验证：至少在仿真条件下，DF在误块率（BLER）方面相对于AF并未体现出复杂性增加带来的回报。此外，我们研究了LEO星座规模对网络BLER的影响。最后证明，在软切换条件下，星间链路的对准偏差影响显著，尤其在光学频段更为严重。