Seamless 5G Multi-Hop Connectivity Architecture and Trials for Maritime Applications

Abstract

This paper provides a study of the different alternatives being considered in the 5G-ROUTES project to establish seamless 5G connectivity in a maritime environment, both from an architectural point of view and from the definition of field trials to evaluate the performance and dependability of the proposed solution. As expected, the main challenge in providing 5G connectivity at sea is to provide coverage over large areas of open water. Thus, as a starting point, this paper presents a measurement campaign conducted to assess the current coverage in the Baltic Sea, which concluded that the current terrestrial networks cannot guarantee sufficient coverage.

Next, the paper describes the solution architecture and trials proposed by 5G-ROUTES, based on integrating satellite backhaul with leading-edge multi-hop connectivity in 5G networks. Satellite backhaul can potentially overcome the connectivity challenge from the terrestrial to the maritime domain, while multi-hop connectivity extends coverage among the different ships navigating the sea. The paper further describes how the project will evaluate, in field trials tailored to this maritime environment, common connectivity key performance indicators (KPIs) such as latency, throughput, availability and reliability [36, 37].

It concludes with a vision for applying the results and insights to maritime transportation and other remote areas where deploying suitable 5G infrastructure may be challenging or costly. The findings will guide the design of future 5G networks for marine applications and identify the most effective methods for secure, dependable maritime communication.

1. Introduction

Exploiting the oceans can enable huge economic growth, job creation and innovation. Ref. [1] showed that by 2030 the marine economy will contribute around USD 3 trillion in global value added, creating about 40 million jobs. The maritime sector frontruns digitalization and onboard ICT usage, with major class-renewal inspections (typically every five years) used for substantial ICT modernization. Yet radio communication — besides analogue voice, satellite and WiFi — has received only limited attention. Fifth (5G) and sixth generation (6G) mobile communications promise seamless connectivity at sea, directly impacting the quality of life and service for shipping and logistics workforces.

A major legislative push is forcing the maritime cluster into a new digitalization era: EU initiatives such as the European maritime single window, applying from 15 August 2025 [2], and the EU single-window environment for customs [3], in EU law since December 2022, will increase the need for data and data exchange. 5G and 6G fit naturally as enabling technologies through their wide fit-for-purpose radio spectrum and customer-oriented features, but dependable high-speed marine data rates remain challenging.

The air/space segment in mobile networks has recently gained attention as one solution, mostly studied theoretically so far. In [4], unmanned aerial vehicles (UAVs) enhanced the coverage of a hybrid satellite–terrestrial maritime network through a difficult optimization problem; in [5], a space–air–sea non-terrestrial network (NTN) was designed for energy-efficient resource allocation. From a practical perspective, new satellite production, propulsion and launching technologies have drastically cut costs, and the key technologies, opportunities and challenges of such hybrid satellite–terrestrial networks are described in [12, 13].

Besides coverage extension via the aerial/space segment, multi-hop is paramount: the multi-hop cellular concept gained attention for 4G (IMT-advanced [14]) networks, with one of the most cited publications [15] dating to 2004 on relay-based deployment challenges. Relay-like solutions regained momentum with 5G new radio (NR) in mmWave high bands [17] as integrated access backhaul (IAB). Trends such as mobile edge computing/cloud (MEC) and non-public networks (NPNs, private networks) further facilitated research and commercialization [18–22]. This paper's main contributions are: a 5G coverage measurement campaign at 3.5 GHz and 700 MHz, architectural solutions for seamless connectivity, and field-trial definitions in the Baltic corridor under the 5G-ROUTES project [24].

2. Maritime Scenario and Problem Statement

The coverage measurement campaign ran on the Helsinki–Tallinn ferry line using a 5G base station at 3.5 GHz in Helsinki harbor and a 4G station at 700 MHz in Vuosaari harbor, with Rohde & Schwarz FR4 Freerider 4 backpack equipment (Figure 1). The 5G signal reached 8.11 km from shore at −105 to −110 dBm then deteriorated rapidly — better than expected but insufficient for shore-to-shore coverage. The 700 MHz signal reached 45.85 km at the same strength, but the technical limit of 700 MHz 5G new radio (NR) is currently 35 km, so shore-to-shore coverage is impossible even with coastal stations at Muuga (Estonia) and Vuosaari (Finland). Numerical tables appear in Appendix A.

Using a 3.5 GHz carrier brings several advantages: suitability for phased-array antenna radios given the wavelength characteristics [25], enabling beamforming for optimal coverage and interference rejection along ferry lines; more bandwidth for capacity-demanding use cases such as dense harbors; high propagation loss allowing better control of designated coverage; and flexible TDD configuration of uplink/downlink resources. Drawbacks include poor penetration of obstacles such as buildings and ships, high propagation loss over required distances, and difficult interference management in non-synchronized deployments.

The 700 MHz band offers low-loss propagation covering long distances with less transmit power [26], mainly uses FDD without synchronization challenges supporting ultra-low-latency use cases, and reduces user-equipment power consumption for battery-powered IoT devices. Its disadvantages are limited available bandwidth, immutable UL/DL bandwidths without dynamic-management flexibility, hardware design challenges making beamforming inefficient, and coverage-control difficulties from strong diffraction and reflection causing interference.

Further aspects complicate offshore radio planning: weather conditions — fog, waves, ice — strongly affect radio waves, making coverage inconsistent [27], with greater impact at greater distances. The future vision (Figure 4) is 5G as the primary seamless-connectivity technology for maritime: coastal coverage, ship-to-ship multi-hop, and satellite-based non-terrestrial connectivity in offshore waters.

3. Maritime Network Architecture and Solutions for Seamless Connectivity

Vessels must always be (best) connected, with communication needs rising in crowded or challenging waters. Since no single solution fits all requirements, this paper studies complementary connectivity scenarios: terrestrial coverage in ports and near shore, ship-to-ship 5G multi-hop, and 5G non-terrestrial offshore connectivity. Coverage is only one aspect — the key is assuring the QoS each use case needs via KPIs such as latency, throughput and availability. Three link types (A, B, C) are defined among the elements in Figures 4–5 according to the geographical nature of the communication path.

Section 3.1 presents network-split alternatives for coast-to-vessel connectivity (Link A). The first option (Figure 7), wireless access backhaul, places the baseband unit (BBU) onboard with N2/N3 interfaces to the onshore 5G core realized over ship–shore IP connectivity, and data-network (DN) access via the onshore N6 interface. Losing IP connectivity causes an almost instant total outage through control-plane timeouts; this split costs least (BBU plus antennas only). The second split (Figure 8) keeps only core control-plane functions onshore and moves the data plane onboard, enabling access to an onboard DN and MEC servers so application services survive an outage by a few seconds — at medium cost (BBU, antennas, UPF and MEC servers).

The third option (Figure 9) hosts the full 5G network (RAN plus core) onboard, so losing external connectivity only breaks Internet services while MEC-hosted services keep running — at the highest cost. Table 1 summarizes: wireless backhaul (onboard BBU, onshore core, no MEC, immediate outage, lowest cost); control-plane-onshore (onboard data plane, MEC yes, delayed outage, medium cost); core-aboard (all onboard, local services survive, highest cost).

5G multi-hop offers an attractive coverage-extension option: using real-time vessel-movement information and historical data, a multi-hop network can be planned over key maritime corridors using operative vessels as moving base stations or multi-hop nodes, saving infrastructure investment and adding flexibility, including emergency and rescue support through shared connectivity capacity. A "dynamic" 5G mesh in intensive-traffic corridors can backhaul ships far from shore, building on multi-hop link-quality analysis [28] and latency studies in multi-hop satellite networks [29].

Seamless link switching is handled by an always-best-connected (ABC) router selecting among satcom or 5G options with legacy fallback, optionally aggregating links (Figure 10). For satellite with 5G, the suitable ETSI [31] options are Scenario A4 (indirect access via NTN transport network) and Scenario A5 (indirect untrusted access via N3IWF), with A4 implemented as the baseline for easy 5G-core integration. Cross-border/MNO service continuity (Section 3.3), studied in 5GCroCo, 5G-MOBIX and 5GCARMEN [32, 33], offers three solutions: inter-PLMN handover, release with redirect (RwR) with or without S10/N14 interface, and modem-based solutions with one or two modems.

4. Trials in the Baltic Sea Corridor (Tallinn–Helsinki)

To validate the architectural solutions, trials are planned in the Muuga/Tallinn–Vuosaari/Helsinki corridor under 5G-ROUTES using Eckerö Line's Finbo vessel as a radio-equipped test platform, supplemented by a second, more freely operating boat since Finbo follows a tight daily schedule. The maritime use cases will use 700 MHz, 3.5 GHz and satellite communication as backbone and ferry coverage, extended with multi-hop links, all tested separately and jointly in a real maritime operating environment. The cases cover most user groups: passengers, freight companies and ferry operators/staff.

The four use cases span two themes: network availability/coverage and network quality (latencies, throughput, download speeds). Use case #3 targets IoT and freight: IoT sensors on goods and vehicles connect to 5G via OBUs, with data flows analyzed while the ferry moves using shore-provided real-time kinematic (RTK) location and temperature data, plus massive machine-type communication (mMTC) pilots via an Android application opening thousands of TCP streams to prove simultaneous mass-device connectivity in ports. Use case #4 targets the smart ferry itself, equipping the vessel with sensors continuously reporting situational awareness — temperature, air quality, humidity — for operator safety and security monitoring.

Figure 12 overviews the trial architecture: the ABC router will be a 5G router also connecting a VSAT modem over Ethernet, with dual-SIM-dual-active evaluation; satellite backhauling uses option A4, sufficient without deploying an N3IWF. This setup evaluates all single-hop (Links A and C) and multi-hop (Link B) options, with seamless switching by the ABC router while multi-link aggregation stays out of scope. Since onshore cross-border continuity options remain undecided, initial trials use modem-based solutions needing no network-side support.

Satellite trials (Section 4.3) consist of an integrated satellite antenna/modem assembly powered by the ferry and Ethernet-connected to the ferry 5G cell's backhaul router on the upper deck, with a Ku-band antenna toward zenith — one candidate terminal being the Kymeta Hawk u8 [35] — over the OneWeb constellation (ferry-to-satellite access at 10.7–12.7 GHz Ku-band). Coastal 5G trials (Section 4.4) proceed stepwise: private-core and base-station installation and integration on Finbo, controlled-environment verification, per-frequency functional tests, joint multi-solution balancing tests, and a final comprehensive phase seeking the best overall solution — including multi-hop simulation with vessels as moving base stations, resilience testing with lost shore links (rescue, disaster-relief, conflict scenarios), and KPI measurements per [36, 37].

The vision (Section 4.6, Figure 13) validates QoS-based connection selection through two-phase demonstrations: single-route vessel connectivity, then redundant routes from several 5G-equipped vessels. Since 5G IAB functionality [38, 39] will be unavailable during the project lifetime, NPN networks on two ships were chosen instead, with Ericsson's compact 5G private network (two DELL VEP4600 controllers, 6651 baseband, AIR 6449 3.5 GHz outdoor antenna, GNSS receiver, cloud management) deployed in a Latvian test lab (Figure 14). Latency, throughput and signal-to-noise measurements will validate QoS factors, using sub-6 GHz spectrum rather than mmWave given its expected cross-border seamless-connectivity disadvantages.

5. Discussion, Conclusions and Future Vision

This paper described auspicious concepts — cellular backhauling via coastal stations, satellite connectivity and multi-hopping — for continuous 5G maritime connectivity in general and the Helsinki–Tallinn ferry route in particular, with analyses and field-trial results expected toward the end of the project in the 2024 timeframe. The logical steps toward prototypical and operational implementation combine diverse elements: base and relay stations on islands, ships, oil platforms and smaller vessels, airborne platforms and satellites connecting to a 5G core.

The future 5G maritime-connectivity ecosystem comprises three main scenarios: short range from land stations, multi-hop as a mid-range solution for maritime corridors and internal waters, and 5G NTN covering international waters [30]. These will enable enhanced situational awareness, improved goods tracking, better ship-to-ship and port-to-ship connectivity, and autonomous/automated vessels — making ocean traffic, travel and transport safer and more efficient. Road and rail transport will likewise benefit from multi-hop and satellite connectivity to close rural and remote coverage gaps.

The 5G-ROUTES concepts and field-trial results will contribute to the vision of continuous 5G connectivity — not just along Baltic ferry lines but ultimately everywhere and all the time — though technological maturity and cost-efficiency need further improvement. The findings will guide future 5G marine-network design and identify the most effective methods for secure, dependable maritime communication.

Appendix A: Measurement Tables

Appendix A lists the numerical values behind the coverage plots of Figures 2 and 3: Table A1 gives received signal strength versus distance at 3.5 GHz (from −105.2 dBm at shore to repeated outages beyond 8.1 km; outages counted below −110 dBm), and Table A2 the values at 700 MHz (extended coverage to 45.85 km with intermittent outage intervals such as 24.4–29.3 km and 66.0–73.3 km). These quantitative data confirm the headline finding that current terrestrial networks cannot guarantee sufficient shore-to-shore coverage.