This paper investigates the massive access for a satellite-aerial-terrestrial network (SATN), where a high-altitude platform (HAP) is deployed as a relay to assist the uplink transmission from terrestrial user equipment (UE) to satellite. Unlike previous works, we adopt radio frequency (RF) and free space optical for the aerial-terrestrial and satellite-aerial links, respectively. Specifically, by assuming that imperfect angular information (IAI) of each UE is known at the HAP, we develop a space division multiple access (SDMA) scheme to maximize the ergodic sum rate (ESR). To this end, we first exploit the IAI to calculate the analytical expression of channel correlation matrix. Then, by considering the limitation of array freedom, we propose a subspace-based UE grouping and scheduling scheme to cluster all UEs into groups. Next, we present a computationally effective beamforming (BF) scheme for each UE at HAP to efficiently implement SDMA in the RF link. Furthermore, a closed-form expression for the ESR of the SATN is derived to validate the proposed BF and SDMA schemes. Finally, simulation results corroborate the derived theoretical formulas and reveal the impacts of array size, angular estimation error, the number of UEs and scheduling threshold on the system performance.

中文翻译：

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混合RF / FSO卫星-地面-地面网络中的上行链路大规模访问

本文研究了卫星-地面-地面网络（SATN）的大规模访问，其中部署了一个高空平台（HAP）作为中继，以协助从地面用户设备（UE）到卫星的上行链路传输。与以前的工作不同，我们分别对空中地面和卫星空中链路采用射频（RF）和自由空间光学。具体来说，假设在HAP已知每个UE的不完美角度信息（IAI），我们开发了一种空分多址（SDMA）方案以最大化遍历总和率（ESR）。为此，我们首先利用IAI来计算信道相关矩阵的解析表达式。然后，考虑到阵列自由度的限制，提出了一种基于子空间的UE分组和调度方案，将所有UE聚类为组。下一个，我们为HAP上的每个UE提出了一种计算有效的波束成形（BF）方案，以有效地在RF链路中实现SDMA。此外，推导了SATN的ESR的闭式表达式，以验证所提出的BF和SDMA方案。最后，仿真结果证实了推导的理论公式，并揭示了阵列大小，角度估计误差，UE数量和调度阈值对系统性能的影响。