Dedicated AP-Based Interference Cancellation in Cell-Free Massive MIMO Systems: Architecture and Algorithm

Since a cell-free massive multiple-input multiple-output (mMIMO) system employs a large number of access points (APs), which can collaboratively serve user equipments (UEs), its energy and spectral efficiency can be much higher than that of a conventional cellular system. Typically, to improve the performance of the cell-free mMIMO system, the inter-user interference (IUI) should be suppressed appropriately, which usually requires high computational complexity. To resolve this issue, we propose a new cell-free mMIMO architecture, called a dedicated AP-based interference cancellation (DAP-IC) architecture which consists of two types of APs: DS-APs that transmit the data signal to UEs and IUI-APs that transmit the signal to eliminate the IUI. This new architecture can help make low-complexity algorithms for cell-free mMIMO systems while maintaining good performance. In our proposed architecture, we formulate a joint optimization problem to determine the AP roles and precoding vectors for maximizing spectral efficiency while ensuring user fairness. To solve this problem, we develop a precoding and dedicated AP selection (PADAS) algorithm, which jointly optimizes the AP roles and precoding vectors, and evaluate its performance through simulations. To demonstrate the effectiveness of the proposed DAP-IC architecture, we compare our proposed algorithm with the weighted minimum mean square error (WMMSE) algorithm, which is widely used in conventional mMIMO systems since it can provide high performance. Simulation results demonstrate that our PADAS algorithm tailored to the DAP-IC architecture requires significantly lower computational complexity than the WMMSE algorithm for the conventional cell-free mMIMO architecture, while providing comparable performance. These results confirm that the proposed DAP-IC architecture provides an effective balance between performance and complexity, making it a practical solution for large-scale cell-free mMIMO systems.