Spectral Efficiency and Scalability Analysis for Multi-Level Cooperative Cell-Free Networks

This paper proposes an easy-to-deploy multi-level cooperative architecture to balance the spectral efficiency and scalability of cell-free massive multiple-input multiple-output (mMIMO) systems. In the proposed architecture, spatial expansion units (SEUs) and the central processing unit (CPU) form a multi-level decoding implementation, which avoids the deployment difficulties of existing scalable schemes and achieves a better trade-off between the complexity and the performance of the cooperative transmission. Firstly, we design the access point selection service and pilot allocation joint optimization (ASPAJO) algorithm, scalable partial zero-forcing (SPZF) receiver and ameliorated large-scale fading decoding (A-LSFD) strategy to support the proposed architecture. Then, closed-form expressions of the uplink user achievable rates under multi-level cooperative architecture with maximal ratio combining (MRC) and SPZF receivers are derived. Numerical results validate the theoretical derivation and showcase the superior performance and scalability of the multi-level cooperative architecture.