# Technology Trends for Massive MIMO towards 6G
## 5 Cell-Free Massive MIMO
Then, dividing the access points into network-centric clusters and letting each user device select a preferred subset of those clusters in a user-centric manner was proposed in [32]. In [33], a joint pilot assignment and cooperation cluster formation algorithm was proposed by analyzing the scalability of different signal processing algorithms. For a scalable (in terms of signal processing complexity and fronthaul signaling load) cell-free massive MIMO system, an access point is allowed to serve only a finite amount of user equipment [22; 33]. As illustrated in Figure 3, each user equipment is served by multiple access points with the preferable channel conditions, which are the ones in the colored shaded circular regions. * **Deployment in an end-to-end network architecture:** The physical-layer aspects of cell-free massive MIMO such as receiver combining design, transmit precoding design, and power allocation algorithms in line with a futuristic scalable system design have now been well-established [22]. In addition to the radio site aspects, the centralized computational processing unit and the fronthaul links between it and access points are two major layers in a practical cell-free massive MIMO operation envisioned to be built in 6G communication systems. When edge clouds are placed between the access points and the center cloud, as shown in Figure 3, the midhaul transport and the collaborative processing unit consisting of the edge and center cloud are the additional components in a cell-free network. Hence, the imperfections, limitations, and energy consumption should be analyzed from an end-to-end (from radio edge to the center cloud) perspective. Conducting an end-to-end study of a low-cost and energy-efficient cell-free massive MIMO implementation is critical to accelerating its practical deployment in 6G. The network architecture of a cell-free massive MIMO system with access points connecting to central processing units via fronthaul links is entirely in line with the wave of cloudification in mobile communications networks. Hence, it is expected from the very beginning to envision prospective cell-free networks on top of a cloud radio access network (C-RAN). In [34], the test results of a cloud-based cell-free massive MIMO implementation were reported. General-purpose processors were utilized in the central cloud. Test results demonstrate the capability of cloud-based cell-free massive MIMO to achieve 5G new radio (NR) requirements. In [35], the authors have explored the performance of cell-free massive MIMO on the virtualizedC-RAN network architecture. Virtualized C-RAN enables centralizing the digital units of the access points in an edge or central cloud with virtualization and computing resource-sharing capabilities. Since digital units are located in a shared cabinet with a single cooling system, and they are orchestrated to share different computational tasks, virtualized C-RAN architecture provides several energy-saving opportunities. Going beyond virtualized C-RAN, the implementation options of cell-free massive MIMO have been discussed on top of open radio access networks (O-RAN) aiming for an intelligent, virtualized, and fully interoperable 6G architecture [36]. In [37], the end-to-end power consumption of cell-free massive MIMO in O-RAN architecture is considered together with resource allocation algorithms that jointly optimize the radio, fronthaul, and cloud processing resources in O-RAN architecture. * **Low-cost fronthaul/midhaul transport:** Fronthaul/midhaul transport technology is one of the vital components in the low-cost deployment of cell-free massive MIMO onto the legacy network. In a large-scale cell-free massive MIMO system, deploying a dedicated optical fiber link between each access point and the edge or central cloud would be highly costly and infeasible. The so-called \"radio stripes\"-based fronthaul architecture developed by Ericsson reduces the cabling cost by sequentially integrating the access points into the shared fronthaul lines.