###### Abstract  
The growing significance of ubiquitous 6G connectivity within the maritime sector is a consequence of its evolution into an era characterized by the adoption of autonomous ships. This evolution necessitates the development of adaptable communication capabilities, even in the face of increasing heterogeneity in Radio Access Networks (RANs). This heterogeneity is a consequence of the extended lifespans of maritime communication technologies used by both legacy and emerging ships at sea, in contrast to the generational shift seen in terrestrial communication technologies. This paper undertakes a comprehensive examination and provides an insightful overview of communication technologies within the framework of 3 rd Generation Partnership Project (3 GPP) standards with the aim of preparing for the forthcoming 6G standardization to enable ubiquitous 6G connectivity in the maritime domain. The primary focus of this paper is the mobile RAN entities (e.g., satellites and uncrewed aerial vehicles (UAVs)) positioned for integration into the 5G and beyond systems. These entities are distinguished by their differences from conventional terrestrial RAN entities, which are typically stationary on land. This integration enables User Equipment (UE) to connect to various RAN entities, including mobile RANs, interconnected with core networks, thereby granting UE secure access to external internets through 5G and beyond systems, enabling them to enjoy a diverse range of application services, even in areas beyond terrestrial coverage, such as at sea. This paper further conducts an in-depth analysis of a transport layer-level solution known as the Access Traffic Steering, Switching, and Splitting (ATSSS) feature enabling a concurrent connection to multiple RANs for data-traffic delivery. Furthermore, this paper explores opportunities and challenges for future research in the realm of forthcoming 6G standardization within 3 GPP, especially when combined with the ATSSS features for the success of autonomous ships within the maritime sector. These considerations encompass the concept of autonomous ships as mobile RAN entities, the integration of legacy maritime communications into the 6G framework, and the variability in maritime channel measurements, generally employed as one of the criteria for selecting an appropriate RAN among multiple options, influenced by uncontrollable factors such as climate change. heterogeneity; concurrent connection; maritime communication; simultaneous utilization; 6G; autonomous ship; mobile RAN +
Footnote †: journal: Marine Science and Engineering  
Satellite communication has emerged as a prevalent method for maritime communication, particularly for vessels navigating globally. This shift began with the adoption of the convention on the International Mobile Satellite Organization (formerly known as Inmarsat) by the International Maritime Organization (IMO) in 1976. This convention was established to oversee and regulate the provision of specific satellite-based maritime distress communication services, primarily integrated within the Global Maritime Distress and Safety System (GMDSS) [1]. Presently, the IMO is in the process of reviewing the GMDSS, with plans for its modernization expected to take effect in 2024. However, existing maritime communication systems prove inadequate in meeting the performance requirements necessary to support imminent advancements in the maritime industry. These advancements encompass the operation of autonomous ships on extended ocean voyages and the development of intelligent offshore aquaculture, etc. Despite offering broader communication coverage than terrestrial cellular communications, current satellite communications, which rely on proprietary solutions tied to manufacturers, are inadequate in meeting the evolving demands of the maritime sector due to their lack of global interoperability, unlike 3 GPP standards-based products and solutions with a dominant economy of scale in the market.