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Hybrid Satellite-Terrestrial Communication Networks for the Maritime Internet of Things: Key Technologies, Opportunities, and Challenges

Te Wei, Wei Feng, Yunfei Chen, Cheng-Xiang Wang, Ning Ge, Jianhua Lu

arXiv:1903.11814v2cs.NI

TL;DR

Maritime IoT needs high-speed, reliable connectivity, but maritime networks face distinctive propagation, coverage, topology, and service constraints. This paper surveys hybrid satellite-terrestrial MCNs and their enabling technologies, then identifies open issues for environment-aware, service-driven satellite-air-ground integration. The review concludes that existing 4G/5G and satellite methods generally require tailoring rather than direct maritime deployment.

  • Problem

    Maritime communication networks face dynamic electromagnetic environments, limited BS sites, and rigorous mission-critical service demands that existing communication theories do not directly accommodate.

  • Method

    The paper surveys maritime communication demand, MCN architectures, challenges, and technologies for transmission efficiency, coverage extension, and maritime-specific services.

  • Results

    The review categorizes enabling technologies and identifies environment-aware, service-driven satellite-air-ground integration using external auxiliary information as a future direction.

  • Takeaways & Limitations

    Conventional 4G/5G and satellite communication methods need tailoring to maritime characteristics rather than direct application.

Abstract

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With the rapid development of marine activities, there has been an increasing number of maritime mobile terminals, as well as a growing demand for high-speed and ultra-reliable maritime communications to keep them connected. Traditionally, the maritime Internet of Things (IoT) is enabled by maritime satellites. However, satellites are seriously restricted by their high latency and relatively low data rate. As an alternative, shore & island-based base stations (BSs) can be built to extend the coverage of terrestrial networks using fourth-generation (4G), fifth-generation (5G), and beyond 5G services. Unmanned aerial vehicles can also be exploited to serve as aerial maritime BSs. Despite of all these approaches, there are still open issues for an efficient maritime communication network (MCN). For example, due to the complicated electromagnetic propagation environment, the limited geometrically available BS sites, and rigorous service demands from mission-critical applications, conventional communication and networking theories and methods should be tailored for maritime scenarios. Towards this end, we provide a survey on the demand for maritime communications, the state-of-the-art MCNs, and key technologies for enhancing transmission efficiency, extending network coverage, and provisioning maritime-specific services. Future challenges in developing an environment-aware, service-driven, and integrated satellite-air-ground MCN to be smart enough to utilize external auxiliary information, e.g., sea state and atmosphere conditions, are also discussed.

I. INTRODUCTION

Maritime IoT requires high-speed, reliable connectivity, but satellite, terrestrial, and emerging maritime networks face distinct coverage, propagation, topology, and service challenges. This survey integrates these issues and reviews technologies and recommendations for hybrid satellite-terrestrial maritime communication networks.

  • Motivation: Maritime activities and growing numbers of vessels, platforms, and buoys create demand for high-speed and ultra-reliable communications supporting navigation, operations, and multimedia services.Applications include navigational information, operational data, and multimedia services for passengers, crew, and fishermen.
  • Existing Networks: Existing maritime connectivity relies mainly on satellites and coastal or island base stations, while conventional shore-based systems cannot provide high-speed data services.Satellite systems offer limited rates, and terrestrial coverage is constrained by the earth’s curvature and maritime environment.
  • Hybrid Architecture: An efficient hybrid satellite-terrestrial MCN is needed to combine satellites’ wide coverage with shore-based systems’ high capacity for sparsely distributed maritime terminals.Ship-to-ship links and UAVs can complement satellite and shore-based backhaul connectivity.
  • Key Challenges: Maritime transmission efficiency is challenged by sea-surface and atmospheric conditions, as well as changing ship-borne antenna height and angle, which make fading channels especially sensitive.These conditions can cause frequent link interruptions and require maritime-specific communication techniques.
  • Key Challenges: Limited BS sites and mobile ship-borne, aerial, and LEO-satellite platforms create highly dynamic, irregular topologies with blind zones and severe co-channel interference.Increasing transmit power to reach remote users can intensify interference for neighboring BSs.
  • Service Provisioning: Maritime applications across affairs, fisheries, ports, shipping, and coastal defence have different service requirements, making reliable service provisioning a major MCN challenge.The paper argues that conventional communication and networking theories must be tailored to maritime scenarios and reviews solutions across link, network, and service levels.
  • Survey Scope: The survey addresses MCN demand, state of the art, challenges, and technologies while connecting topics that earlier surveys generally treated separately.It recommends an environment-aware, service-driven, satellite-air-ground MCN that uses auxiliary information such as sea-state conditions.

II. STATE-OF-THE-ART MARITIME COMMUNICATIONS

Maritime communication networks have evolved from early long-distance radio and narrowband systems toward satellite-based and broadband architectures. Existing MCNs span satellite, shore, island, vessel, and air-based designs, with satellites offering broad coverage but varying data rates and costs.

  • Development History: Maritime communications began with Marconi’s long-distance radio experiments and later provided narrowband telegraph, telephone, and fax services.His experiments included a 6-km open-sea link in 1897 and trans-Atlantic communication exceeding 3000 km in 1901.
  • Development History: Broadband MCN development includes MARCOM, BLUECOM+, and TRITON projects using terrestrial, multi-hop, and mesh networking technologies.These projects targeted broadband communication for remote ocean areas or connected vessels, beacons, and buoys.
  • Network Categories: MCNs are categorized as satellite-based, shore-based, island-based, vessel-based, and air-based networks according to their architectures.The paper discusses these categories in subsequent sections.
  • Satellite-based Networks: Inmarsat progressed from analogue voice, fax, and low-speed data to digital voice, fax, and low- or medium-speed data services across its generations.Inmarsat-1 entered service in 1982, Inmarsat-2 in 1990, and Inmarsat-3 in 1996.
  • Satellite-based Networks: Satellite systems range from narrowband services such as Tiantong-1’s 9.6 kbps peak rate to high-throughput systems such as Shijian-13 and EchoStar-19.Shijian-13 has more than 20 Gbps total capacity, while EchoStar-19 has more than 200 Gbps capacity.
  • Satellite-based Networks: Satellite communications provide wide coverage and bandwidth-dependent data rates but face environmental, reliability, and affordability constraints.Inmarsat Fleet 77 installation costs approximately $28000, while data service costs $2.8 per minute; fewer than 25,000 of nearly 80,000 simultaneously sailing ships may afford high-throughput equipment.

B. Shore-based Maritime Communications

Shore-based maritime networks extend terrestrial broadband services offshore, while island, vessel, and mesh systems expand reach through additional fixed or mobile nodes. Their practical boundaries arise from coverage geometry, environmental instability, infrastructure density, and equipment cost.

  • Narrowband Systems: NAVTEX provides 300 bps direct-printing services within 200 nautical miles offshore but cannot support broadband or user-originated real-time information.It broadcasts navigational messages, meteorological warnings and forecasts, and emergency information.
  • Narrowband Systems: PACTOR-IV supports text-only email over thousands of kilometres at up to 10.5 kbps using 2.4 kHz bandwidth, but large delay prevents real-time communication.The system uses adaptive channel equalization, channel coding, and source compression for severe multipath channels.
  • Shore-based Networks: Offshore LTE trials demonstrated broadband shore-based services, including 1 Mbps uplink and 2 Mbps downlink over 20–50 km offshore.The network served platforms, tankers, and FPSO facilities and supported video surveillance uploading and wireless trunking.
  • Shore-based Networks: A Qingdao TD-LTE trial operated at 2.6 GHz, covered up to 30 km offshore, and reached a 7 Mbps peak rate.It targeted maritime transportation and offshore fisheries applications.
  • Shore-based Networks: Shore-based MCNs provide broadband services but have limited coverage determined largely by geometrically available BS sites and are most suitable for densely clustered maritime applications.Their services include multimedia file downloading and video surveillance data uploading.
  • Island-based Networks: Island-based BSs expand terrestrial coverage and support communications for nearby ships, but their coverage remains limited and vulnerability to typhoons and rainstorms is higher.They can provide voice and video calls between the coast and islands.
  • Vessel-based Networks: Maritime-MANET reaches up to 70 km offshore at 1.2 kbps, while TRITON reaches 27 km with 98.91% coverage and 6 Mbps broadband service but not the high seas.Both approaches use maritime nodes to extend shore or island network coverage.
  • Vessel-based Networks: Vessel-based mesh networks are constrained by sea and weather variability, require dense vessel populations, and impose expensive equipment requirements on participating vessels.The paper identifies more reliable protocols and more cost-effective ship-borne terminals as needed.

E. Air-based Maritime Communications

Air-based maritime networks use UAVs, balloons, and tethered platforms to extend communications over remote ocean areas. They can cover wider areas than vessel-based networks, but severe weather and maritime propagation conditions remain important constraints.

  • Aerial Platforms: Internet.org used UAVs at altitudes of 55–82 km as aerial BSs connected through laser communications for remote-area Internet access.The project targeted impoverished areas in Latin America, Asia, and Africa.
  • Aerial Platforms: Loon used super-pressure balloons at 20 km altitude and 2.4/5.8 GHz to provide communication services at 10 Mbps, including emergency communications after natural disasters.The project was not commercial at the time described.
  • Aerial Platforms: BLUECOM+ used tethered balloons, TV white spaces, and multi-hop relaying to cover ocean areas up to 150 km from shore at 3 Mbps.The approach extended land-based communications toward remote ocean areas.
  • Air-based Network Characteristics: Air-based MCNs can cover wider areas than vessel-based MCNs and provide high-rate but low-reliability communication services.Their aerial BSs are easily damaged by severe weather.
  • Cross-cutting Challenges: Existing MCNs combine satellite, shore and island BS, and aerial approaches, but coverage depends on available BS sites and transmission efficiency is affected by maritime weather.The paper identifies transmission enhancement, coverage extension, and service-oriented techniques as necessary directions.
  • Cross-cutting Challenges: Sea conditions, atmospheric conditions, and rapidly changing ship-borne antenna height and angle make maritime electromagnetic propagation distinct from terrestrial propagation.These factors motivate maritime channel measurement and modelling for MCN design.

A. Characteristics and Models of Maritime Channels

Maritime channels exhibit environment-sensitive large- and small-scale fading shaped by sea conditions, atmospheric effects, antenna geometry, and propagation paths. Existing models include terrestrial adaptations, two-ray formulations, evaporation-duct effects, and statistical fading descriptions, but their combined environmental impact remains insufficiently known.

  • Maritime fading is classified into large-scale fading, which changes slowly with user location, and small-scale fading, driven by rapid signal amplitude, phase, and multipath-delay fluctuations.
  • Two-ray maritime models represent propagation mainly through direct and sea-reflected paths, incorporating carrier wavelength, transmission distance, and antenna heights.
  • Evaporation ducts can provide additional scattering gain and influence path loss when transmitter–receiver distance exceeds a threshold related to antenna heights.
  • Maritime propagation is vulnerable to sea-surface fluctuations, weather, atmospheric refraction, antenna-height variation, and reflection effects.
  • Existing terrestrial models may not suit environment-sensitive maritime channels, while the combined effects of frequency, distance, antenna geometry, weather, and sea fluctuations remain unknown.

B. Reducing Transmission Loss: Exploiting Evaporation Duct for Remote Transmissions

Atmospheric ducts can reduce maritime propagation loss by trapping electromagnetic waves in refractive layers, with evaporation ducts offering a mechanism for beyond-line-of-sight links. Their practical use remains constrained by weather-dependent duct-height prediction and limited development.

  • Atmospheric ducts form when refractive-index changes meet certain conditions, trapping signals where propagation loss is much smaller than in free space.
  • Surface, elevated, and evaporation ducts occur over the sea; evaporation ducts form approximately 0–40 m above sea level and occur only in oceanic atmospheres.
  • Evaporation ducts have supported beyond-line-of-sight maritime links spanning 78 km and 100 km.
  • Evaporation-duct height depends on temperature difference, humidity, air pressure, wind speed, and wave height, limiting prediction accuracy under weather-sensitive formulations.

C. Improving Resource Utilization: Resource Management and Allocation Schemes

Maritime resource management must account for rapidly changing channels, service diversity, vessel movement, and sparse propagation paths. Surveyed approaches use scheduling, routing, ship-behaviour information, and large-scale channel state information, with environmental awareness identified as a future direction.

  • Traditional resource allocation based on service statistics can be inefficient for random, rapidly changing maritime channels.
  • Prior work addresses maritime management through synchronization and scheduling, service-oriented routing, ship-encounter models, AIS-assisted networking, and energy- or content-aware video scheduling.
  • Sparse maritime propagation paths make large-scale fading dominant, motivating power allocation and user scheduling using location-obtainable large-scale CSI.
  • Future MCNs should sense sea level, temperature, humidity, and wind speed to predict CSI more accurately and exploit dynamic channel changes.

IV. INCREASING BROADBAND COVERAGE

Broadband maritime coverage is limited by sparse and mobile BS locations, irregular topology, blind zones, and interference. Coordinating shore, shipborne, aerial, and satellite platforms through integrated multi-hop networks is therefore central to extending coverage.

  • Link-level improvements alone cannot ensure seamless wide-area coverage because maritime networks have limited BS sites.
  • Shore and island BSs, ship-borne BSs, UAVs, offshore lighthouses, and satellites provide complementary maritime coverage resources.
  • Limited fixed sites and mobility create highly irregular topology, blind zones, and strong co-channel interference when BS transmission power increases.
  • Multi-hop wireless networking and satellite-terrestrial integration can coordinate available BSs, while dynamic beamforming and microwave scattering can extend individual-BS coverage.
  • Maritime ad hoc and mesh networks use directional antennas, virtual MIMO, relaying, handover, adaptive time slots, and cognition-enhanced MAC protocols.
  • Terrestrial delay-tolerant routing schemes often perform poorly in maritime communications, motivating maritime-specific routing based on lane intersections and environmental sensitivity.
  • Traffic-aware BS switching can deactivate low-loaded BSs to save energy and reduce interference while supporting current users.
  • UAVs can act as aerial BSs or relays, with coordinated flight trajectories, routing, and transmission forming multi-hop maritime networks.

B. Utilizing High-throughput Satellites: Multi-spot Beams and Satellite-terrestrial Cooperation

Satellite-terrestrial cooperation combines satellites’ broad coverage with terrestrial networks’ higher capacity, while multi-spot beams improve maritime broadband efficiency. Additional approaches include caching, beamforming, microwave scattering, and large LEO constellations, each with deployment trade-offs.

  • Multi-spot beams: Multi-spot beams increase spectral efficiency through narrower beam widths and enable smaller maritime satellite terminals.Multi-spot beams can also reuse frequencies across separated beams, although side lobes may create inter-beam interference.
  • Satellite-terrestrial cooperation: Integrated satellite-terrestrial networks combine terrestrial high capacity with satellite wide coverage to support seamless broadband maritime coverage.Proposed mechanisms include intelligent middleware, link-specific protocols, multi-hop routing, and SNR-based backhaul selection.
  • Delay reduction: Backhaul activation and caching strategies target satellite networks’ long round-trip times and reduce traffic delivery or access delays.The cited work includes traffic-delivery minimization, back-tracing partition-based caching, QoE-driven placement, and secure in-network caching.
  • LEO constellations: Large LEO constellations are presented as a route toward broader maritime broadband coverage, with Starlink plans targeting 25 ms–35 ms delays.The passage describes approximately 12,000 planned Starlink satellites across two constellations.
  • Beamforming: Phased-array beamforming can exploit clustered maritime users by steering directional beams according to geographical locations.AIS can provide user locations, while dynamic beam scheduling supports higher throughput or wider coverage.
  • Microwave scattering: Microwave scattering supports long-distance over-the-horizon maritime links because oceanic tropospheres contain more scatterers than terrestrial ones.Experimental links have operated in the 5.8 GHz and 2.2 GHz bands, but high-power antennas or large arrays can reduce cost-effectiveness.

E. Interference Alleviation for Irregular Network Topology

Maritime networks face strongly coupled inter- and intra-system interference, irregular topology, earth-curvature effects, and difficult antenna and pilot design. Coverage and resource coordination therefore require maritime-specific interference management and heterogeneous-network integration.

  • Interference coupling: Irregular maritime topology strongly couples inter-system and intra-system interference, making independent interference treatment inadequate.Potential responses include satellite-terrestrial beam scheduling, optimal beam design, and inter-beam interference suppression.
  • Coverage geometry: Antenna height must balance coverage distance against pilot contamination between neighbouring cells.Low antennas reduce base-station coverage, whereas high antennas can increase pilot contamination.
  • Pilot design: Strong near-far interference complicates channel estimation because nearby-user pilots interfere with pilots from remote users.The passage identifies careful pilot design as necessary when high-power service to remote users affects neighbouring cells.
  • Heterogeneous integration: Maritime coverage technologies include shipborne and UAV multi-hop networking, satellite-terrestrial cooperation, dynamic beam scheduling, microwave scattering, and interference management.The paper proposes coordinating satellites, shore and island base stations, shipborne base stations, and UAVs into a heterogeneous network.
  • Service-aware coordination: Heterogeneous maritime networks must allocate spectrum and power according to service requirements such as bandwidth, latency, and reliability.Navigational geographic-information services and passenger video downloading illustrate differing resource demands.

B. Service Provisioning for Typical Maritime Applications

Maritime applications span clustered, delay-sensitive, and delay-tolerant services with different bandwidth, latency, and reliability requirements. The survey links these demands to satellite, shore-based, AIS, radar, and other service-specific communication technologies.

  • Service requirements: Maritime services differ in bandwidth, latency, and reliability, requiring technology choices matched to their operational demands.The paper distinguishes navigational, infotainment, monitoring, rescue, and industrial communication needs.
  • Service categories: The paper organizes maritime-specific provisioning around intelligent navigation, clustered distribution, delay-reliability-sensitive services, and delay-tolerant services.This classification reflects the differing requirements of users and applications at sea.
  • Navigational services: Navigational services require real-time, accurate traffic information to support safe navigation and improve shipping efficiency.Satellite AIS, LRIT, terrestrial AIS, and coastal radar are identified as supporting technologies.
  • Navigational services: Satellite-based AIS architectures address message collisions in dense maritime zones while reducing downlink power, bandwidth, and latency.The cited architecture uses parallel signal processing for satellite-based AIS.

2) Passenger/Crew Infotainment–Clustered Distribution Services:

Maritime service provisioning must accommodate clustered users, emergency reliability demands, and delay-tolerant broadband transfers. Cross-layer coordination is proposed to flexibly match limited network resources to these heterogeneous requirements.

  • Passenger/Crew Infotainment–Clustered Distribution Services: Passenger and crew infotainment users are sparse across the sea but densely clustered within each vessel or platform.This distribution motivates phased-array antennas and beamforming for targeted service delivery.
  • Delay-reliability-sensitive services: UAVs and low-orbit satellites can provide real-time emergency voice, image, and video transmissions for remote maritime rescue.Underwater emergency communications additionally support underwater rescue, wreck positioning, and search operations.
  • Delay-reliability-sensitive services: Rapidly changing shipborne antenna height and angle can interrupt rescue links, motivating antenna switching to improve link stability and packet delivery ratio.The cited challenge arises from sea-surface fluctuation during ship-to-ship rescue communications.
  • Multimedia Downloading and Data Gathering–Delay-tolerant Services: Multimedia downloading and hydro-meteorological data uploading require broadband but tolerate high latency, enabling delay-tolerant resource scheduling.Proposed methods include opportunistic routing and offline scheduling algorithms.
  • Service integration: Cross-layer joint optimization is required to coordinate physical, MAC, and network layers across multiple rates, delays, reliability levels, channel states, and service requirements.The paper frames this coordination as necessary because maritime base-station sites are limited.
  • Low-power communications: LPWAN supports low-bandwidth, low-power, long-range, massively connected maritime IoT sensing, but its maritime coverage expansion remains unresolved.The passage reports approximately 10 km coverage for NB-IoT and higher power consumption than two other listed technologies.

D. Cross-Layer Design for QoS-guaranteed Maritime Communications

Future maritime networks should jointly account for channel conditions, user services, and heterogeneous infrastructure to improve transmission, coverage, and QoS. Knowledge libraries and learning-based optimization can support environment-aware transmission, dynamic topology management, and personalized service scheduling.

  • Cross-layer QoS design: Cross-layer optimization should jointly consider channel state information and maritime service requirements to adapt communications to differing QoS demands.This approach is motivated by the diversity of maritime application scenarios and service requirements.
  • Service-driven optimization: Service-driven schemes can adapt coverage and resource allocation to changing maritime user locations and demands under constrained resources.They also support joint optimization of service scheduling and base-station transmission.
  • Future MCN capabilities: Future MCNs must coordinate terrestrial, satellite, and ship-borne base stations while developing service-specific techniques for maritime requirements.The paper frames this coordination around improving efficiency, extending coverage, and supporting domain-specific services.
  • Integrated network design: Environmental, positional, and service information can be collected and processed to design integrated satellite-air-ground systems with flexible coverage and service patterns.The framework assigns processing roles to a central processor and base stations acting as edge processors.
  • Knowledge-driven networking: A maritime knowledge library can represent signal propagation, network topology, and service characteristics for optimizing transmission efficiency and coverage.Its inputs include internal communication information and external auxiliary information.
  • Knowledge-driven networking: Machine learning and optimization can use meteorological, hydrological, mobility, and service information for transmission optimization, network management, and service scheduling.Examples include predicting maritime channels, configuring resources around evaporation ducts, modeling topology evolution, and forecasting personalized services.

C. Open Problems

Open problems concern building maritime-specific channel models, coordinating heterogeneous coverage technologies, and supporting diverse services through a shared network. The survey concludes that existing networks and theories cannot simply be integrated without tailoring them to maritime propagation, topology, site, and service constraints.

  • Channel modeling: Environment-sensitive maritime channel models must incorporate sea-surface and atmospheric conditions alongside mathematical statistical analysis.The proposed measurement process synchronizes channel measurements with meteorological observations and supports structural modeling.
  • Heterogeneous network coordination: Coordinating satellites, UAVs, and terrestrial base stations is difficult because their mobility, transmission performance, delay, and rate differ substantially.These differences produce stochastic maritime network topologies and complicate network control.
  • Multi-service support: Limited maritime base-station sites motivate a single flexible network capable of supporting multiple marine services with differing requirements.This differs from terrestrial settings where separate networks commonly serve different services.
  • Survey conclusion: The survey organizes maritime-enabling technologies around transmission efficiency, network coverage, and maritime-specific services, while identifying substantial performance loss from directly applying existing 4G/5G and satellite technologies.The stated constraints include dynamic electromagnetic propagation, limited BS sites, and mission-critical service demands.
  • Future direction: Future MCNs should use external auxiliary information to build environment-aware, service-driven, integrated satellite-air-ground networks rather than simply combining existing networks.The paper presents this direction together with associated open research issues.
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