## 3 Overview of 6G Technologies
### General Multi-Level 6G Architecture  
6G networks are envisioned as multi-level hierarchical architectures comprising different layers of computation, communication, control, and intelligence [8; 55]. Figure 2 illustrates a generic hierarchical 6G architecture, which can be described as follows:  
1. **Physical Infrastructure Layer:** At the bottom of the architecture lies the physical infrastructure, consisting of diverse communication and sensing elements such as terrestrial BSs, UAVs, HAPs, satellites, underwater nodes, vehicles, and RISs. This layer is responsible for signal generation and propagation, physical layer access, and 3D coverage deployment across ground, air, sea, and underwater domains. Key enabling technologies at this layer include THz communication links for ultra-high-speed transmission, massive MIMO for spatial multiplexing and beamforming, RIS for energy-efficient signal redirection, UAVs/HAPs/satellites for non-terrestrial and 3D global coverage [47].  
2. **Network Control and Orchestration Layer:** The next layer hosts the control and management functions needed to operate and coordinate the underlying physical resources. It introduces intent-based networking, where high-level objectives are translated into optimized network configurations, potentially using AI-driven orchestration [55]. This layer performs resource allocation and slicing, mobility management and handover control, topology reconfiguration and adaptive routing, and energy-aware and reliability-sensitive decisions. Key technologies that are supported in this layer include AI-based orchestration frameworks, federated learning for distributed control, autonomous network management agents and policy engines.
3. **Edge-Cloud and Distributed Intelligence Layer:** A key innovation of 6G lies in the distribution of intelligence and computation across multiple layers of the network--from cloud to edge to far edge [39]. This allows network nodes such as ships, UAVs, or buoys to process data locally, reducing latency and improving resilience in disconnected or delay-tolerant scenarios. This layer supports real-time AI model inference and training at the edge, offloading decisions between ship-side, UAV-based, or cloud-based nodes, and local predictive analytics (e.g., for route optimization system failures). Enabling technologies include MEC, digital Twins for ships and platforms, FL for privacy-preserving AI, Swarm intelligence and Deep Reinforcement Learning (DRL) for adaptive control.
4. **Applications and Services Layer:** At the top of the stack, this layer interfaces directly with vertical sectors that will benefit from 6G's enhanced capabilities. These services are not only bandwidth-intensive but also latency-critical and reliability-sensitive, requirements which are central to 6G's design. Typical applications include smart shipping and autonomous navigation, healthcare (e.g., remote diagnostics, tele-surgery), Extended Reality (XR) and immersive entertainment onboard, remote control of autonomous maritime assets, predictive maintenance of ships and offshore platforms [15]. As such, this layer emphasizes cross-domain integration, enabling AI models and services to span edge devices, the cloud, and application endpoints seamlessly.