While 5G networks are being rolled out, the definition of the potential 5G-Advanced features and the identification of disruptive technologies for 6G systems are being addressed by the scientific and academic communities to tackle the challenges that 2030 communication systems will face, such as terabit-capacity and always-on networks. In this framework, it is globally recognised that Non-Terrestrial Networks (NTN) will play a fundamental role in support to a fully connected world, in which physical, human, and digital domains will converge. In this framework, one of the main challenges that NTN have to address is the provision of the high throughput requested by the new ecosystem. In this paper, we focus on Cell-Free Multiple Input Multiple Output (CF-MIMO) algorithms for NTN. In particular: i) we discuss the architecture design supporting centralised and federated CF-MIMO in NTN, with the latter implementing distributed MIMO algorithms from multiple satellites in the same formation (swarm); ii) propose a novel location-based CF-MIMO algorithm, which does not require Channel State Information (CSI) at the transmitter; and iii) design novel normalisation approaches for federated CF-MIMO in NTN, to cope with the constraints on non-colocated radiating elements. The numerical results substantiate the good performance of the proposed algorithm, also in the presence of non-ideal information.

翻译：尽管5G网络正在部署，科学界和学术界正着手定义潜在5G-Advanced特性并识别6G系统的颠覆性技术，以应对2030年通信系统将面临的挑战，如太比特容量和永远在线网络。在此框架下，全球公认非地面网络（NTN）将在支撑物理、人类和数字领域融合的全连接世界中发挥基础性作用。其中，NTN需解决的主要挑战之一是为新生态系统提供所需的高吞吐量。本文聚焦于NTN的无小区多输入多输出（CF-MIMO）算法，具体包括：i) 讨论支持NTN中集中式和联邦式CF-MIMO的架构设计，后者通过同编队（集群）的多颗卫星实现分布式MIMO算法；ii) 提出一种新颖的基于位置的CF-MIMO算法，该算法无需发射端信道状态信息（CSI）；iii) 设计适用于NTN联邦式CF-MIMO的新型归一化方法，以应对非共址辐射单元的约束。数值结果验证了所提算法在理想信息及非理想信息条件下的良好性能。