Federated Cell-Free MIMO in Nonterrestrial Networks: Architectures and Performance

While 5G networks are being rolled out, the definition of 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 sys- tems will face, such as terabit capacity and always-ON networks. In this framework, it is globally recognized that nonterrestrial networks (NTN) will play a fundamental role in support to a fully connected world, in which physical, human, and digital domains will converge. Notably, one of the main challenges that NTN have to address is the provision of the high throughput requested by the new ecosystem. In this article, we focus on cell-free massive multiple input multiple output (CF-MIMO) algorithms for NTN. In particular: we discuss the architecture design supporting centralized and federated CF-MIMO in NTN, with the latter implementing distributed MIMO algorithms from multiple satellites in the same formation (swarm), we design a location-based CF-MIMO algorithm, which does not require channel state information at the transmitter, and we design normalization approaches for federated CF-MIMO in NTN, to cope with the con- straints on noncolocated radiating elements. The numerical results substantiate the good performance of the proposed algorithm, also in the presence of nonideal information.