This paper proposes a multi-level cooperative architecture to balance the spectral efficiency and scalability of cell-free massive multiple-input multiple-output (MIMO) systems. In the proposed architecture, spatial expansion units (SEUs) are introduced to avoid a large amount of computation at the access points (APs) and increase the degree of cooperation among APs. We first derive the closed-form expressions of the uplink user achievable rates under multi-level cooperative architecture with maximal ratio combination (MRC) and zero-forcing (ZF) receivers. The accuracy of the closed-form expressions is verified. Moreover, numerical results have demonstrated that the proposed multi-level cooperative architecture achieves a better trade-off between spectral efficiency and scalability than other forms of cell-free massive MIMO architectures.

翻译：本文提出了一种多级协作架构，旨在平衡无小区大规模多输入多输出（MIMO）系统的频谱效率与可扩展性。在所提出的架构中，引入了空间扩展单元（SEU），以避免接入点（AP）端产生大量计算负担，并提升AP间的协作程度。我们首先推导了采用最大比合并（MRC）与迫零（ZF）接收机时，多级协作架构下上行链路用户可达速率的闭式表达式，并验证了该闭式表达式的准确性。此外，数值结果表明，与其他形式的无小区大规模MIMO架构相比，所提出的多级协作架构在频谱效率与可扩展性之间实现了更优的权衡。