## 1 Introduction  
With the widespread deployment of fifth-generation (5G) networks, terrestrial mobile communications have made unprecedented progress. However, the maritime domain presents unique challenges, including infrastructure deficiencies and complex electromagnetic propagation characteristics, making the direct transplantation of land-based communication solutions infeasible [1]. Non-terrestrial networks (NTNs) form an essential component of 5G-Advanced and sixth-generation (6G) systems, leveraging large-scale constellations to establish three-dimensional connectivity across the space, aerial, and maritime domains [2]. This multi-layered architecture specifically bridges oceanic coverage gaps while fulfilling 3 GPP's vision for ubiquitous service continuity [3]. Recently, as the research and development (R&D) of Space-Ground Integrated Networks (SGINs) is prioritized in many countries, maritime communication, as a critical application scenario, has attracted much attention in academia and industry [4]. A robust maritime communication network (MCN) is vital for diverse maritime operations, such as environmental monitoring, shipping logistics, and offshore activities within exclusive economic zones (EEZs) [5]. For example, accurate navigation data are a prerequisite for safe voyages, while real-time voice/video communication improves coordination during rescue missions. Additionally, infotainment services (e.g., web browsing and UHD video) are increasingly demanded by crew members and passengers. Therefore, establishing a reliable MCN is imperative to fully unlock the ocean's potential for sustainable human development. Currently, maritime communications mainly rely on coast/island-based cellular networks, maritime satellites, and legacy radio systems [6]. For cellular networks, the base stations (BSs) deployed along the coast can provide long-term evolution (LTE) or 5G services for maritime mobile terminals (MMTs), and their performance requirements can be reliably met. However, the transmission distance of the network is restricted by Earth's curvature and the sea surface condition. At present, the maximum offshore coverage reported in the literature is limited to 100 km from the BS [7]. Maritime satellites are the main means of long-distance ship-to-ship/shore communication. Nevertheless, their transmission rates are low and delays are significant due to limited onboard payload capacity and long round-trip distances. Moreover, the costs of satellite launches, onboard equipment, and communication services are prohibitively high [8]. Maritime radio systems represented by automatic identification systems (AISs) operate in very high-frequency (VHF) bands. Although they support long-range communication, they cannot meet the required services for emerging maritime applications due to bandwidth limitations. Consequently, the existing maritime communication systems fail to satisfy the growing demands of offshore users. To build a high-speed, cost-effective, and ultra-reliable MCN, numerous studies have been conducted. In [9; 10; 11; 12], several maritime wireless channel measurement campaigns were reported, laying the foundation for robust MCN construction. By establishing a wireless mesh network among ships, buoys, and onshore BSs, the terrestrial network coverage can be extended to the sea [13]. In [14], tethered balloons acting as relay nodes (RNs) were deployed over the sea. The experiment results showed that the project could offer broadband services of 3 Mbps for users 150 km from the coast [15]. Similarly, leveraging their high altitude and agile maneuverability, unmanned aerial vehicles (UAVs) can deliver flexible communication services for offshore users in maritime environments [16; 17]. The authors in [18] proposed a network based on distributed antennas (DAs) to meet the service requirements of MMTs and investigated the antenna selection scheme. Unlike the unpredictable movement of users on land, maritime users' positions can be predicted more accurately as most ships follow established shipping lanes.