From Cells to Freedom: 6G's Evolutionary Shift with Cell-Free Massive MIMO

Cell-free massive MIMO (CF-mMIMO) is emerging as a technological pillar for future sixth generation (6G) mobile networks, promising consistently high spectral and energy efficiencies across the coverage area. Despite the reported advantages of CF-mMIMO over traditional cellular-based massive MIMO (mMIMO), the extensive deployment of access points (APs) and the associated fronthaul links present significant economic and logistical challenges. This paper proposes a transitional framework to facilitate the gradual integration of CF-mMIMO into existing cellular infrastructures, allowing mobile network operators to progressively realize the benefits of a distributed network topology. A comprehensive analysis of the spectral, energy, and computational efficiencies in heterogeneous network scenarios, incorporating both macrocellular and cell-free components, is presented. Our contributions include a unified assessment framework encompassing spectral, energy and computational aspects, a novel channel virtualization mechanism for effective downlink precoding, and a realistic industry-backed power consumption model for joint network operation. The potential performance gains are demonstrated and guidelines for the incremental deployment of CF-mMIMO are provided through detailed numerical results, ensuring a balanced trade-off between integration costs and operational benefits. This approach aims to leverage the capabilities of emerging network architectures to achieve a seamless evolution towards fully distributed 6G networks.