  Leveraging non-terrestrial platforms in 6G networks holds immense
significance as it opens up opportunities to expand network coverage, enhance
connectivity, and support a wide range of innovative applications, including
global-scale Internet of Things and ultra-high-definition content delivery. To
accomplish the seamless integration between terrestrial and non-terrestrial
networks, substantial changes in radio access network (RAN) architecture are
required. These changes involve the development of new RAN solutions that can
efficiently manage the diverse characteristics of both terrestrial and
non-terrestrial components, ensuring smooth handovers, resource allocation, and
quality of service across the integrated network ecosystem. Additionally, the
establishment of robust interconnection and communication protocols between
terrestrial and non-terrestrial elements will be pivotal to utilize the full
potential of 6G technology. Additionally, innovative approaches have been
introduced to split the functionalities within the RAN into centralized and
distributed domains. These novel paradigms are designed to enhance RAN's
flexibility while simultaneously lowering the costs associated with
infrastructure deployment, all while ensuring that the quality of service for
end-users remains unaffected. In this work, we provide an extensive examination
of various Non-Terrestrial Networks (NTN) architectures and the necessary
adaptations required on the existing 5G RAN architecture to align with the
distinct attributes of NTN. Of particular significance, we emphasize the
crucial RAN functional split choices essential for the seamless integration of
terrestrial and non-terrestrial components within advanced 6G networks.
