Low earth orbit (LEO) satellite constellation-enabled communication networks are expected to be an important part of many Internet of Things (IoT) deployments due to their unique advantage of providing seamless global coverage. In this paper, we investigate the random access problem in massive multiple-input multiple-output-based LEO satellite systems, where the multi-satellite cooperative processing mechanism is considered. Specifically, at edge satellite nodes, we conceive a training sequence padded multi-carrier system to overcome the issue of imperfect synchronization, where the training sequence is utilized to detect the devices' activity and estimate their channels. Considering the inherent sparsity of terrestrial-satellite links and the sporadic traffic feature of IoT terminals, we utilize the orthogonal approximate message passing-multiple measurement vector algorithm to estimate the delay coefficients and user terminal activity. To further utilize the structure of the receive array, a two-dimensional estimation of signal parameters via rotational invariance technique is performed for enhancing channel estimation. Finally, at the central server node, we propose a majority voting scheme to enhance activity detection by aggregating backhaul information from multiple satellites. Moreover, multi-satellite cooperative linear data detection and multi-satellite cooperative Bayesian dequantization data detection are proposed to cope with perfect and quantized backhaul, respectively. Simulation results verify the effectiveness of our proposed schemes in terms of channel estimation, activity detection, and data detection for quasi-synchronous random access in satellite systems.

翻译：低地球轨道卫星星座赋能的通信网络，因其提供无缝全球覆盖的独特优势，有望成为众多物联网部署的重要组成部分。本文研究了基于大规模多输入多输出低轨卫星系统中的随机接入问题，其中考虑了多卫星协作处理机制。具体而言，在边缘卫星节点处，我们设计了一种训练序列填充多载波系统以克服非完美同步问题，其中训练序列用于检测设备活跃度并估计其信道。考虑到地面-卫星链路的固有稀疏性及物联网终端的零星流量特征，我们利用正交近似消息传递-多测量向量算法来估计时延系数和用户终端活跃度。为进一步利用接收阵列结构，采用基于旋转不变技术的信号参数二维估计来增强信道估计。最后，在中央服务器节点处，我们提出了一种多数投票方案，通过聚合来自多颗卫星的回程信息来增强活跃度检测。此外，分别提出了多卫星协作线性数据检测和多卫星协作贝叶斯去量化数据检测，以应对完美回程和量化回程情况。仿真结果验证了所提方案在卫星系统准同步随机接入的信道估计、活跃度检测及数据检测方面的有效性。