## 2 Evolution of Maritime Communication Networks
### Communication Links and Technologies  
The reference MCN architecture incorporates a rich set of multi-domain communication links, each with specific performance targets, propagation characteristics, and suitable technologies [20; 32]. This subsection elaborates on the functional roles and enabling technologies of each major link category in the space-air-ground-sea-underwater network, as shown in Figure 1. Below, a vision of the different link categories is described, whereas a summary of all communication links per MCN domain is listed in Table 3[32]. 1. **Satellite to Ground Communication:** This link connects satellites to terrestrial stations or control centers, serving as the global backhaul for data traffic, remote control, and coordination. Satellite Base Stations (Sat-BS) use L, Ku, Ka, and Q/V bands, with newer 6G systems exploring optical satellite communication for extremely high-throughput and low-latency links. These links require robust atmospheric compensation and precise beam steering to maintain high performance [33]. 2. **Satellite to UAV/HAP Communication:** Satellites also interface with HAPs and UAVs to extend coverage in areas with no direct ground access. These links are critical for data offloading, relaying, and supporting edge computing in the sky. Communication is typically realized via mmWave backhaul (e.g., Ka/Q/V bands) or laser communication systems, enabling high-rate data transfers and latency-sensitive coordination. 3. **UAV/HAP to Ship/Sea BS Communication:** Airborne platforms such as UAVs and HAPs establish air-to-sea links with moving vessels and floating platforms (e.g., buoys or Sea BS). These links may leverage sub-6 GHz for robust coverage and mmWave or THz for high-throughput, low-latency applications. Reconfigurable Intelligent Surfaces (RIS) mounted on UAVs or buoys can dynamically optimize beam directionality and link quality in challenging maritime conditions. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Layer** & **Entity** & **Role** \\ \hline Space & Satellites & LEO satellites provide global coverage and serve as the backbone for long-range backhaul communication. They offer connectivity to ships, UAVs, and coastal infrastructure, particularly in deep-sea areas with no terrestrial coverage. \\ Air & HAPs, UAVs & HAPs and UAVs operate in the aerial tier to support dynamic relays, coverage extension, and computation offloading. IRS-aided UAVs further enhance signal propagation using reconfigurable surfaces. \\ Sea Surface & Sea BS, Ships, Buoys & This layer includes passenger ships, cargo vessels, small boats, floating buoys, and Sea BS. These nodes engage in short-range S2S, S2 Sh, and ship-to-aerial communication, forming the core of near-surface maritime networking. \\ Underwater & AUVs & AUVs and underwater sensors rely on acoustic or optical links to connect to sea surface nodes. These systems handle environmental monitoring, subsea inspection, and coordinated missions. \\ Ground & Ground Infrastructure & Terrestrial base stations and satellite gateways connected to business/control centers act as the anchor points for centralized control, data aggregation, and global coordination of the MCN. \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Sixth-generation-enabled MCN entities and their roles.