# Technology Trends for Massive MIMO towards 6G
## 5 Cell-Free Massive MIMO  
After the seminal paper [20] was published, massive MIMO took place in the latest mobile 5G communications in less than ten years. Such an immediate success of a theoretical concept resulted from the foreseen performance improvement and the extreme effort put in by researchers in the last decade. Although traditional cellular massive MIMO with co-located antennas increases the data rate significantly, the user devices at the cell edges (places much far away from the serving base station) may suffer from low rates. Currently, when data communications are more dominant than voice communications in mobile systems, large data rate variations across the coverage area of the base stations are not desired. To solve the cell-edge user issues, massive MIMO scholars have developed the idea of \"cell-free massive MIMO\" [21], where there are no cell boundaries from the user perspective and many low-cost access points at the radio edge serve each user device [22]. Cell-free massive MIMO is envisioned as a promising technology for beyond 5G systems due to the highly improved spectral and energy efficiency it would provide. In fact, it complements the ultra-massive MIMO systems operating at THz frequencies by providing solutions to some of the aforementioned challenges. As a natural consequence, not only academia but also the industry has a great interest in cell-free massive MIMO, which is also named \"distributed MIMO\" or \"distributed massive MIMO\" by industrial researchers [23]. In a cell-free massive MIMO system, a large number of distributed low-cost access points serve multiple users' equipment coherently using the same time-frequency resources in line with the massive MIMO operation. While densely deployed access points provide macro-diversity that increases coverage, the joint transmission/ reception processing enables mitigation of the adverse effects of cell-edge interference. Indeed, cell-free massive MIMO has been developed as a clean-slate look at the coordinated multi-point processing(CoMP) in 4G. Its focus is on user-centric operation rather than network-centric to provide uniformly good service throughout the coverage. In [21], it has been shown that cell-free massive MIMO can improve the 95%-likely per-user spectral efficiency by an order of magnitude higher than that of the traditional cellular small-cell system. In one of the following works in [24], it has been concluded that cell-free massive MIMO can also improve energy efficiency by an order of magnitude compared to cellular co-located massive MIMO. After these pioneering works were published, cell-free massive MIMO gained great importance in wireless communications. Currently, cell-free massive MIMO is treated as one of the key technologies for the next-generation mobile communication systems by the leading researchers in the area [25; 26; 27; 28]. In [29], all the operation modes of cell-free massive MIMO have been outlined. In addition to the distributed mode that was considered in the early works, centralized cell-free massive MIMO and signal processing schemes for that are elaborated on. In the sequel, we elaborate on different aspects and the corresponding challenges for the implementation of cell-free massive MIMO in next-generation networks. * **Scalability:** In the original form of cell-free massive MIMO, all the access points in the network area serve all the user devices. However, such an operation of cell-free massive MIMO is not scalable in terms of signal processing complexity and fronthaul signaling load. Several network-centric and user-centric clustering methods have been developed to address the scalability issue. Although network-centric clustering can be applied in a simpler manner, user-centric clustering avoids the problem of low data rates at the cluster edges. From the CoMP literature, user-centric clustering was first considered in the cell-free context in [30; 31].