Coupled Access-Fronthaul Feasibility Analysis for Wireless-Fronthauled Cell-Free Massive MIMO Under an Option-8 Functional Split

Abstract

In this paper, we consider a wireless-fronthauled cell-free massive MIMO network and analyze the coupling between the access and fronthaul links under the Option-8 functional split. The two links are coupled through the design parameters, namely the access bandwidth $B_{\text{ac}}$ and the number of access point (AP) antennas $N$ in the access link. The per-AP Option-8 fronthaul rate demand defines the feasibility criterion: an AP is supported only when its achievable fronthaul rate meets this demand. The access link power allocation is always performed by solving a sum spectral efficiency (SE) maximization problem with a weighted minimum mean-squared error blockcoordinate descent (WMMSE-BCD) algorithm. For the fronthaul link, we study two operating models. In the first, the same sum-SE WMMSE-BCD algorithm is augmented with a mechanism that drops the APs whose fronthaul links cannot satisfy the required Option-8 demand. In the second, we replace the sum-SE objective on the fronthaul link with a max-min formulation that strictly satisfies the fronthaul demand at all APs without dropping any AP. We characterize the realized access rate and the feasibility region of both models on the $(N, B_{\text{ac}})$ plane.