# Seamless 5G Multi-Hop Connectivity Architecture and Trials for Maritime Applications
## 1 Introduction
Since the finalization of Release 17, equipment vendors have been quick to start ambitious development programs for suitable chipsets, components and units with remarkable success [8; 9; 10; 11]. These advances will allow for rapidly bringing full 5G NTN implementations, providing direct access from the user equipment (UE) to satellites to the market with the promise to revolutionize connectivity for mobility applications. Already available today are 5G-compatible satellite backhauling solutions between larger vessels, aircraft or trains with their own 5G micro-cell or non-public network (NPN) and a 5G core. Thus, it is clear that hybrid satellite-terrestrial communication networks will play, as expected, a pivotal role in providing connectivity in maritime environments. Keytechnologies, open issues, opportunities and challenges for these types of networks are described in [12; 13]. Besides the coverage extension thanks to the aerial/space segment, another key technology that is paramount in providing coverage extension is multi-hop. The idea of multi-hop cellular networks gained a lot of attention for 4G networks, also known as IMT-advanced [14] networks. One of the most cited publications [15] dates back to 2004 and describes the related challenges for the technical and commercial success of multi-hop cellular networks using relays: the cost of a relay must be low enough to compensate for the loss of capacity resulting from dedicating part of the radio spectrum to a wireless backhaul link. To the best of our knowledge, no commercial relays are available on the market and logically, therefore, no commercial deployments exist. Instead, wireless backhauling in high-frequency bands [16], also known as millimeter wave (mmWave), has been widely deployed. It combines the advantage of high bandwidths and reduced deployment costs and effort, as no fibers connecting to the base station sites are required while not having to dedicate any spectrum for backhauling, as the cell operates in other, lower-frequency bands. Relay-like solutions regained momentum with 5G new radio (NR), as it introduced regular cell operation in mmWave high bands [17]. The corresponding solution is called integrated access backhaul (IAB), and it is motivated by applying existing mmWave spectrum licenses that are used for wireless backhauling to also be used for the actual cell (Uu-interface) or, alternatively, to also use the mmWave spectrum, which is mainly intended for the Uu-interface, for backhauling. Additionally, besides hybrid satellite-terrestrial communication networks and technologies such as multi-hop, trends such as mobile edge computing/cloud (MEC) and NPNs, also known as private networks, have facilitated the research and commercial deployment of mobile radio networks with different geographical locations of network functions. In the context of MEC, which applies to many use cases [18], different splits are introduced to 4G and 5G networks, as described in [19]. This particularly includes the idea of having the core user plane locally, allowing for connecting MEC servers while keeping the control plane components of the core central. This principle was evolved for the 5G core, assuring a lightweight user plane that only consists of the user plane function (UPF). For example, the 5G cross-border service continuity project 5 GCroCo discussed different split options and their implications in Deliverable D3.3 [20]. The typical motivation behind MEC is to enable deploying edge servers close to already-existing base station sites. NPNs typically aim at deploying networks on premises while keeping costs and complexity reasonable, which requires selecting an appropriate option for the \"split\" between the RAN and the core and between the core user and control planes. This can include the option of reusing the existing base stations of the public network to create public network integrated NPNs (PNI-NPNs), as discussed with an economic focus in [21]. A comprehensive discussion of different core split options is provided in [22].