Evolution of RAN Architectures Towards 6G: Motivation, Development, and Enabling Technologies

In this survey paper, we first provide insights on the evolution of radio access networks (RANs) through investigating the existing paradigms and future trends towards 6G. We then present the fully-decoupled RAN (FD-RAN), which aligns with the trends by integrating existing paradigms and introducing new features such as physical decoupling of uplink and downlink base stations. We also discuss the key technologies enabled by different architectures for further performance improvement and some open issues. We hope that this survey can stimulate more in-depth research on transforming 6G RAN so as to unleash the power of state-of-the-art technologies and meet higher performance requirements in the future.