Hybrid Self-Organizing Networks: Evolution, Standardization Trends, and a 6G Architecture Vision.

Self-organizing networks (SONs) need to be endowed with self-coordination capabilities to manage the complex relations between their internal components and to avoid their destructive interactions. Existing communication technologies commonly implement responsive self-coordination mechanisms that can be very slow in dynamic situations. The sixth generation (6G) networks, being in their early stages of research and standardization activities, open new opportunities to opt for a design-driven approach when developing self-coordination capabilities. This can be achieved through the use of hybrid SON designs. A hybrid architecture combines the centralized and distributed management and control. In this article, we review the history of SONs, including the inherent self-coordination feature. We then delve into the concept of hybrid SONs (H-SONs), and we summarize the challenges, opportunities, and future trends for H-SON development. We provide a comprehensive collection of standardization activities and recommendations, discussing the key contributions and potential work to continue the evolution and push for a wide adoption of the H-SON paradigm. More importantly, as a key 6G architectural feature we propose that H-SONs should be loosely coupled networks. Loose coupling refers to the weak interaction of different layers and weak interaction between users in the same layer, that is, the various feedback loops must be almost isolated from each other to improve the stability and to avoid chaotic situations. We finally conclude the article with the key hints about the future landscape and the key drivers of 6G H-SONs.