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

TL;DR

This paper surveys hybrid satellite-terrestrial maritime communication networks, highlighting key technologies, challenges, and future directions for improving connectivity, coverage, and service reliability in maritime IoT environments.

Contribution

It provides a comprehensive overview of current maritime communication networks and discusses innovative solutions and open issues for future integrated satellite-air-ground systems.

Findings

Satellite networks face high latency and low data rates.

Terrestrial base stations and UAVs can extend coverage.

Future challenges include environment-aware, smart MCNs.

Abstract

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.

Tables

Table 1. TABLE I: Comparison between cellular and maritime communications.

Characteristics	Cellular communications	Maritime communications \bigstrut
Single BS coverage	Small	Wide \bigstrut[t]
	4G: 500–2000 m for a single cell in urban area	Shore-based MCN: 10–100 km for a single BS
	5G: 100–300 m for a single cell in urban area	MCN using ship-borne/UAV-enabled BSs: 1–50 km for a single BS
	(Achieved by building a large number of BSs)	(Due to the limited number of geographically available BSs) \bigstrut[b]
Wireless channel	Lower propagation loss (Due to small cell radius\bigstrut)	Higher propagation loss (Due to long-distance transmission \bigstrut)
	Mainly affected by blocks and scatteres \bigstrut	Mainly affected by sea surface conditions (such as tidal waves), and atmospheric conditions (such as temperature, humidity, and wind speed) \bigstrut
	Mostly multi-path channels (Rician channels in open areas)	Mostly Rician channels (with a direct path, a specularly reflected path, several diffusely reflected paths, and several atmospheric scattering paths) \bigstrut
One-way transmission delay	Low 4G: less than 10 ms 5G: less than 1 ms	High Onshore BSs: Similar to 4G/5G GEO: approx. 270 ms MEO: approx. 130 ms (e.g., for O3b) LEO: less than 40 ms (e.g., 10–30 ms for Globalstar) \bigstrut
Service	Mainly for mobile communications services	Mainly for maritime affairs, fisheries, ports, shipping, and coastal defence E.g., accurate and intelligent navigational communications for all vessels, real-time operational data communications for offshore drilling platforms, high-throughput multimedia downloading services for passenger/crew infotainment, and emergency communications with low-latency and high-reliability for maritime rescue\bigstrut

Table 2. TABLE II: Existing Maritime Communication Networks.

%ͨ — ʾ Ƿ Ҫ Project/System						Sponsor	Frequency	Maximum rate	Coverage	Feature
WISEPORT	Singapore	5.8 GHz	5 Mbps	15 km	WiMAX
TRITON	Singapore	5.8 GHz	6 Mbps	27 km	mesh
Maritime-MANET	Japan	27/40 MHz	1.2 kbps	70 km	Ad Hoc
BLUECOM+	Portugal	500/800 MHz	3 Mbps	100 km	balloons, 2-hop
Digital VHF TMR	Norway	87.5-108/174-240 MHz	21/133 kbps	130 km	broadcasting
Qingdao TD-LTE Trial Network	China Mobile & Ericsson	2.6 GHz	7 Mbps	30 km	LTE
Norwagian Offshore LTE Network	Tampnet & Huawei	1785-1805 MHz	2 Mbps	50 km	LTE
Internet.org	Facebook	laser	unknown	wide	UAVs & laser
Loon	Google	2.4 GHz	10 Mbps	wide	balloons
Inmarsat-4	Inmarsat	L/S band	492 kbps	wide	GEO
Iridium NEXT	Iridium & Motorola	L band	30 Mbps	wide	LEO
Tiantong-1	China	S band	9.6 kbps	wide	GEO

Table 3. TABLE III: Maritime Channel Measurements and Models.

%ͨ — ʾ Ƿ Ҫ Ref.

Scenario

Frequency (GHz)

Tx-Rx Distance (km)

Tx/Rx Antenna Heights (m)

Channel Statistics

Environmental Factors Considered

Channel Model

[74]

ship to shore

2.1

13-41

10,25,50,100/10

PL

earth curvature

FSPL model

[75]

ship to shore

2.075

45

9.5/11.2

RSL, PDP, SC

not mentioned

ITU-R model

[77]

buoy to ship

5.8

10

1.7/9.8

PL

not mentioned

modified 2-ray model

[78]

ship to ship

35/94

20

5/9.7

PDP, PL, RMS-DS

not mentioned

FSPL model, modified 2-ray model

[79]

shore to ship

2.4

2

3/4.5

RSL, PL

not mentioned

FSPL, ITU-R, and 2-ray models

[80]

ship to shore

5.15

10

3-4/7.6,10,20

PL

evaporation duct

FSPL, 2-ray, and 3-ray models

[86]

ship to shore

2.075

45

9.5/11.2

RSL, PDP, SC

not mentioned

ITU-R model

[87]

air to ground

5.7

10

370,1830/2.1,7.65

PL

evaporation duct

FSPL model, 2-ray model

[88]

ship to shore

2.075

15.5

6.5/23

RSL, PDP, DFO, SRC

sea state

2-ray model

[95]

shore to ship

1.95

16

22/2.5

RSL, PL

not mentioned

FSPL model

[96]

flight to ship

5.7

27.7

1000/5.5

PL

evaporation duct

ducting-induced enhancement model

[97]

island to island

0.248/0.341

33.3,48

18.5/16,14

RSL, PL

sea state

FSPL model, ITU-R model

[98]

buoy to ship

5800

0.2

1.9/3.3

PDP, RMS-DS

not mentioned

not mentioned

[99]

ship to ship

1900

5-30

8/8

PDP, PL

not mentioned

log-distance PL model

Tx: transmitter; Rx: receiver; PL: path loss; RSL: received signal level; PDP: power delay profile; SC: spatial correlation; RMS-DS: root-mean-square delay spread; DFO: Doppler frequency offset; SRC: sea reflection coefficient; FSPL: free-space path loss

Table 4. TABLE IV: Technologies to Enhance Transmission Efficiency for MCNs.

Goal	Scheme	Characteristics of MCNs Used	Contributions      \bigstrut
reducing transmission loss	microwave scattering	evaporation duct over sea	setting up beyond-LOS maritime communications: 78-km [120], 100-km [121][122] \bigstrut
	frequency-time scheduling	two-path maritime channels	exploiting vertically spaced multiple antennas at the Rx. to overcome deep fading [124] \bigstrut
reducing transmission delay	routing	delay-tolerant maritime services	a service-oriented framework and three policy-based routing schemes [125], DTN technologies [126][127] \bigstrut
improving network throughput	traffic scheduling	content-aware maritime applications	data traffic task scheduling [128] \bigstrut
	resource allocation	heterogeneous networking	joint backhaul and access link resource management [129] \bigstrut
improving data delivery ratio	routing	user behaviours (such as shipping lanes)	a model of ship encounter probability [133], an architecture integrating the AIS [134] \bigstrut
improving energy efficiency	traffic scheduling		energy-and-content-aware scheduling algorithms for video uploading based on the deterministic network topology [135] and the ship route traces [136] \bigstrut

Equations

$$L_{2-\mathrm{ray}}=-10{\log }_{10}\left\{{\left(\frac{\lambda }{4\pi d}\right)}^2{\left[2\sin \left(\frac{2\pi H_1{H}_2}{\lambda d}\right)\right]}^2\right\}$$

$$\displaystyle\!\!\!\!{L_{{\rm{3-ray}}}}\!\!=\!\!\left\{\!\!\!\!\begin{array}[]{l}-10{\log_{10}}\!\left\{{{{\left({\frac{\lambda}{{4\pi d}}}\right)}^{2}}{{\left[{2\sin\left({\frac{{2\pi{H_{1}}{H_{2}}}}{{\lambda d}}}\right)}\right]}^{2}}}\right\},d\!<\!{d_{break}}\\ -10{\log_{10}}\!\left\{{{{\left({\frac{\lambda}{{4\pi d}}}\right)}^{2}}{{\left[{2\left({1+\Delta}\right)}\right]}^{2}}}\right\},d\!>\!{d_{break}}\end{array}\right.\!\!\!\!\!\!\!\!\!\!$$

$$p\left(a\right)={\int }_0^{\infty }\frac{a}{{\sigma }^2}\exp \left(-\frac{a^2}{2{\sigma }^2}\right)\frac{1}{\sqrt{2\pi }\sigma {\sigma }_l}\exp \left(-\frac{{\left(\ln \sigma -\mu \right)}^2}{2{\sigma }_l^2}\right)d\sigma$$

Full text

I Introduction

Maritime activities, such as marine tourism, offshore aquaculture, and oceanic mineral exploration, have seen rapid development in recent years. With the increasing number of vessels, offshore platforms, buoys, etc., there has been a growing demand for high-speed and ultra-reliable maritime communications to connect them [1]–[3]. For example, navigational information and operational data are required for the safe navigation of all vessels, and multi-media communication services are needed for passengers, crew, and fishermen onboard. Similarly, offshore drilling platforms require real-time operational data communications, and buoys also have a large amount of meteorological and hydrological data to upload [4]–[6]. For maritime rescue, in addition to information exchange using texts and voices, real-time videos are often required for better ship-to-ship and ship-to-shore coordination [7]. Therefore, building a broadband maritime communication network (MCN) for the maritime Internet of Things (IoT) is of great significance for marine transportation [8][9], production safety [10] and emergency rescue [11].

Currently, mobile terminals on the ocean mainly rely on maritime satellites or base stations (BSs) on the coast/island to acquire services. Narrow-band satellites, represented by International Maritime Satellites (Inmarsat), mainly provide services such as telephone, telegraph, and fax, at a low communication rate. For example, the annual throughput of Inmarsat was only 66 Gbps in 2016, while the number of ships has exceeded 2 million. Thus, the average communication rate per ship is less than 33 kbps [12]. To meet the demand for broadband satellite communication services, several companies have launched high-throughput satellites, such as EchoStar-19 (also known as Jupiter-2) by EchoStar and the Starlink project by SpaceX. In addition to maritime satellites, shore & island-based BSs are also used to extend the coverage of terrestrial fourth-generation (4G)/fifth-generation (5G) networks for maritime activities [13]. The existing shore-based communication systems, such as the Navigation Telex (NAVTEX) system and the Automatic Identification System (AIS), mainly provide services for information broadcasting, voice, and ship identification, but they cannot provide high-speed data services [14]. To improve the communication rate, several companies, such as Huawei and Ericsson, have carried out long-distance shore-to-ship transmission tests based on Worldwide Interoperability for Microwave Access (WiMAX) or Long Term Evolution (LTE) networks [15][16]. However, the coverage of these systems is limited by the earth curvature and maritime environment.

To provide a practically affordable solution for broadband maritime communications, an efficient hybrid satellite-terrestrial MCN is urgently required to combine the advantage of satellites’ wide coverage with shore-based systems’ high capacity. It is believed that an arm-hand-like network architecture is beneficial to cover the widely but sparsely distributed maritime mobile terminals. In this framework, satellites and shore-based systems provide backhauls for dynamic global coverage (like the arms), while ship-to-ship interconnections and unmanned aerial vehicles (UAVs) can be exploited for enhancing local coverage (like the hands) [17]. However, different from terrestrial cellular networks, the MCN still faces many challenges due to the complicated electromagnetic propagation environment, network topology patterns, and service demands from mission-critical applications [18]–[25]:

$\cdot$ Transmission efficiency: Compared with the terrestrial environment, the atmosphere over the sea surface is unevenly distributed due to the large amount of seawater evaporation. Shore-to-ship and ship-to-ship communications are very vulnerable to both sea surface conditions, such as tidal waves, and atmospheric conditions, such as temperature, humidity, and wind speed. In addition, the height and the angle of ship-borne antennas vary greatly with the ocean waves. Thus, the fading channel is particularly sensitive to parameters, such as antenna height and angle, which may cause frequent link interruption. Therefore, the transmission efficiency in these applications is often low, due to these complicated time-varying factors.

$\cdot$ Coverage performance: In a terrestrial network, it is possible to increase the broadband coverage by installing more BSs. However, in an MCN, the available BS sites are very limited. Due to the limited onshore BS sites and the strong mobility of the ship-borne BSs, aerial BSs, and low-earth orbit (LEO) satellites, the topology of the hybrid satellite-terrestrial MCN is highly dynamic and irregular. Blind zones always exist in the planned coverage area. Additionally, if the BS covers remote users using high power, it will generate strong co-channel interference to the users served by neighbouring BSs. The coverage performance of MCNs is thus restricted by blind zones and areas with severe interference.

$\cdot$ Service provisioning: Marine information network contains several industries, such as maritime affairs, fisheries, ports, shipping, and coastal defence. Their maritime application scenarios are also quite different with unique service requirements. Providing reliable services for all of these maritime-specific applications is a major challenge for the MCN.

In Table I, we illustrate the difference between traditional cellular communications and relevant maritime communications. To address the unique challenges in maritime communications, conventional communications and networking theories and methods need to be tailored for maritime scenarios, leading to an emerging area of communications. To date, a number of studies have been conducted on MCNs. To enhance the maritime transmission efficiency, various channel measurement and modelling projects have been performed to analyse the impact of important system parameters (frequency, antenna height, etc.) and maritime environments (sea state, weather conditions, etc.) on the maritime channel. Moreover, advanced resource allocation schemes, such as dynamical beamforming and user scheduling techniques, have been studied to adapt to the dynamic changes in maritime channels. In addition, several studies have exploited the evaporation duct effect to improve the transmission efficiency, especially for remote ship-to-ship/shore transmissions. To expand the network coverage, various BSs have been utilized, including onshore BSs, ship-borne BSs, aerial BSs, and satellites. For these BSs, advanced beamforming and microwave scattering techniques have been studied to reduce the signal attenuation and extend the coverage. In addition, interference mitigation and satellite-terrestrial coordination schemes have been studied to overcome the interference due to the irregular network topology. To satisfy the unique maritime service requirements, different systems and their transmission and resource allocation techniques have been studied for different service requirements, such as bandwidth, latency, and criticality.

Although there have been a large number of works on the above topics, there are very few survey papers on MCNs. Additionally, most of them are focused on a specific issue, such as channel models [18]–[20], network management [21], or existing systems [22]–[25]. These issues are closely related to the characteristics of MCNs, but they were addressed separately without considering their interplays. For example, the surveys on maritime channel models have pointed out the challenges faced by environment-sensitive maritime channels but have not discussed any technologies to enhance transmission efficiency in such maritime scenarios. Moreover, many other important issues for MCNs, such as resource allocation, service provisioning and network integration, are not completely discussed by any of these surveys. Although the survey papers on some relevant topics, such as 5G channel measurements and models [26], space-air-ground integrated networks [27], and cognitive-radio-based IoT [28], could shed light on the development of an efficient hybrid satellite-terrestrial MCN, in general they have not specialized in maritime scenarios. To the best of the authors’ knowledge, a survey paper dedicated to hybrid satellite-terrestrial MCNs with a complete picture of maritime communications is not available in the literature but is crucial to pave the way for the understanding of the unique features of MCNs. To fill in the gap, this paper provides a survey on maritime communications, which not only includes the topics that have not been previously covered, such as maritime service provisioning, but also extends the existing surveys by addressing the unique features of MCNs and the inner connections among the topics.

This paper provides a survey on the demand, state of the art, major challenges and key technical approaches in maritime communications. In particular, it focuses on the unique characteristics of maritime communications that are not seen in terrestrial or satellite communications. It discusses the major challenges of MCNs due to unique meteorological conditions and geographical environments, as well as heterogeneous service requirements. Consequently, it illustrates the corresponding solutions from link-level, network-level, and service-level perspectives. Finally, it makes recommendations on developing an environment-aware, service-driven and satellite-air-ground integrated MCN, which is smart enough to utilize the external auxiliary information, e.g., the sea state conditions. The relevant open issues are also pointed out.

The remainder of this paper is organized as follows. Section II briefly reviews the state-of-the-art MCNs, including satellite-based, shore-based, island-based, vessel-based, air-based, and underwater MCNs. In Section III, we introduce the key technologies to enhance maritime transmission efficiency. Section IV introduces the key technologies to extend the coverage of MCNs. The demand for maritime communications, and key technologies for providing maritime-specific services such as low-power communications and cross-layer design, are discussed in Section V. In Section VI, we suggest the architecture and features of future smart MCNs, as well as suggesting future research topics. Section VII concludes this paper. Figure 1 shows the outline of the paper.

II State-of-the-Art Maritime Communications

Maritime communications began at the turn of the 20th century, pioneered by Marconi’s work on long-distance radio transmissions. In 1897, Marconi established a 6-km communication link across the Bristol Channel, which is the first wireless communication over open sea. In 1899, he initiated the transmission across the English channel, from Wimereux, France to Dover, England, approximately 50 km away. In the same year, Marconi and his assistants installed wireless equipment on the Saint Paul, a trans-Atlantic passenger liner, and successfully received telegrams from the coast station 122 km away. In 1901, Marconi achieved trans-Atlantic communications with a transmission distance of over 3000 km, using a 20 kW high-power transmitter and a receiving antenna with a height of 150 metres [29]–[31]. Marconi’s experiments aroused great interest from the shipping industry in Europe and North America. From then on, many countries began to install coast stations and ship-borne radio stations. Narrowband communication services such as telegraph, telephone and fax were provided using data transmissions via intermediate frequency (MF, 0.3–3 MHz), high frequency (HF, 3–30 MHz), very high frequency (VHF, 30–300 MHz), and ultra high frequency (UHF, 0.3–3 GHz) bands. Among them, VHF is mostly used for radio and television broadcast. It is also a licensed band for aviation and navigation, which is important for the safe navigation of ships within 25 nautical miles along the coast. VHF terminals have been widely used on merchant ships, fishing boats, official ships, yachts, and lifeboats. It is the most popular communication equipment for marine vessels [32]–[34].

At present, several works have been conducted on broadband MCNs. Norway and Portugal launched the MARCOM project and the BLUECOM+ project, respectively, to provide broadband communications for remote areas by using Wi-Fi, General Packet Radio Service (GPRS), Universal Mobile Telecommunications System (UMTS), LTE technologies or their combination [35][36]. Singapore launched the TRITON project, where a wireless multi-hop network is formed between adjacent vessels, maritime beacons, and buoys, to ensure wide-area coverage [37]. In addition, the authors in [38]–[40] discussed methods to achieve maritime communications through collaborative heterogeneous wireless networks using terrestrial networks, satellite networks, and other types of wireless networks.

The history of the development of MCNs is depicted in Figure 2. Based on the network architecture, MCNs can be categorized into satellite-based, shore-based, island-based, vessel-based, and air-based networks. They will be discussed in the following sections.

II-A Satellite-based Maritime Communications

Inmarsat is an international geostationary Earth orbit (GEO) satellite communications system. It aims to provide worldwide voice and data services for various applications, such as ocean transport, air traffic control, and emergency rescue [41]. The first generation of Inmarsat systems (Inmarsat-1) was put into use in 1982. The system is composed of several satellites and transponders rented from other companies and organizations, mainly providing analogue voice, fax, and low-speed data services [42]. The second-generation system (Inmarsat-2) was put into use in 1990. It has a total of four satellites, each of which is equipped with a single global beam, providing digital voice, fax, and low/medium-speed data services [43][44]. The third-generation system (Inmarsat-3) was put into use in 1996. There are 5 satellites, and each satellite has 4–6 regional spot beams in addition to the global beam. Inmarsat-3 can support mobile packet data service (MPDS), with a capacity 8 times that of Inmarsat-2 [45]–[47]. The state-of-the-art Inmarsat-4 system consists of 3 satellites. Each satellite has a global beam, 19 regional beams, and approximately 200 narrow spot beams. Inmarsat-4 can provide Internet services with a peak rate of 492 kbps [48][49]. The future Inmarsat-5 system, also known as Global Xpress, aims to provide worldwide customers with downlink services at 50 Mbps and uplink services at 5 Mbps [50].

O3b is the first medium Earth orbit (MEO) satellite communications system that has been commercially used. It consists of 16 active satellites, providing standard and limited services for areas within latitudes of 45 degrees and 62 degrees, respectively. At present, the O3b company is actively promoting maritime satellite communication services and has installed O3b satellite communication terminals on several Royal Caribbean cruise ships. The maximum data rate of a single ship is 700 Mbps, while the delay is approximately 140 ms [51].

Iridium is an LEO satellite communications system providing voice and low-speed data services for users with satellite phones and pagers. The second generation of Iridium satellite constellations, Iridium Next, started in 2017. It consists of 66 active satellites, 9 in-orbit spare satellites, and 6 on-ground spare satellites. At present, Iridium Next provides data services of up to 128 kbps to mobile terminals and up to 1.5 Mbps to Iridium Pilot marine terminals. In the future, it will support more bandwidth and higher rate, reaching a transmission rate of 1.4 Mbps for mobile terminals and up to 30 Mbps for high-speed data services when large user terminals are available [52].

Tiantong-1 is China’s first mobile satellite communications system, which is also known as the Chinese “Inmarsat”. The system was launched into orbit in 2016 and put into commercial use in 2018. It mainly covers the Asia-Pacific region, including most of the Pacific Ocean and the Indian Ocean. It provides voice, short message, and low-speed data services, with a peak rate of 9.6 kbps [53].

The Shijian-13 communications satellite is China’s first high-throughput communication satellite. It is a multi-beam broadband communication system using the Ka-band, and its total communication capacity is more than 20 Gbps, approximately 10 times higher than before. The satellite is designed with 26 user spot beams, covering nearly 200 km of China’s offshore areas [54].

Another high-throughput satellite EchoStar-19 has a capacity of more than 200 Gbps and is equipped with 138 customer communications beams and 22 gateway beams. The satellite provides users in North America with high-speed Internet services and emergency response. In addition, Ka-band-based airborne broadband services will be available on the EchoStar-19 satellite [55].

The satellite-based communication systems have wide coverage and can provide low-speed or high-speed data services depending the bandwidth. However, satellite-based communications are easily affected by climate and the marine environment, resulting in low reliability [56]–[59]. In addition, the cost of ship-borne equipment and the communication charges are also very high. For example, the cost of installing ship-borne equipment for Inmarsat (Fleet 77) is approximately $28000, including the antenna, terminal, handset, manuals, SIM card and power supply, and the data service costs$2.8 per minute [60]. Data from the AIS show that there are nearly 80,000 ships sailing simultaneously around the world, less than 25,000 of which are high-end ships (with a load of more than 10,000 tons) that may afford the ship-borne equipment for high-throughput satellite communications.

II-B Shore-based Maritime Communications

The NAVTEX system is a narrow-band system with data rates of 300 bps, providing direct-printing services for ships within 200 nautical miles offshore. It operates at the MF band, using the 518 kHz band to broadcast international information and the 490 kHz band for local messages [61]. The NAVTEX system delivers navigational messages, meteorological warnings and forecasts and emergency information to enhance marine safety, but it cannot provide broadband communication services or obtain real-time information from users [23].

The PACTOR system is also a narrow-band system, which operates at the HF band, using frequencies between 1 MHz and 30 MHz [62]. The first generation of PACTOR (PACTOR-I) was built to provide a combination of direct-printing and packet radio services. Adaptive modulation methods and orthogonal frequency division multiplexing (OFDM) technologies were applied to PACTOR-II and PACTOR-III, respectively, in order to improve the spectral efficiency [63]. The state-of-the-art PACTOR-IV system uses adaptive channel equalization, channel coding, and source compression techniques, and has proven to be suitable for channels with severe multi-path. PACTOR-IV can provide text-only e-mail services for ships thousands of kilometres away from the land with a data rate of up to 10.5 kbps, using a bandwidth of 2.4 kHz [64]. Similar to NAVTEX, the PACTOR system cannot provide real-time communication services due to a large transmission delay.

As wireless communications technologies advance, several broadband MCNs have been constructed. The world’s first offshore LTE network was jointly developed by Tampnet in Norway and Huawei in China. It covers the platforms, tankers, and floating production storage and offloading (FPSO) facilities from 20 km to 50 km offshore on the North Sea, providing voice and data services of 1 Mbps uplink and 2 Mbps downlink. It also supports video surveillance data uploading and wireless trunking services [65].

Ericsson has also been working on connecting vessels at sea with shore-based BSs. It aims to enable maritime services that facilitate crew infotainment, cargo monitoring, and shipping route optimization. Ericsson and China Mobile have constructed a TD-LTE trial network for maritime coverage in Qingdao, China. The network operates at the 2.6 GHz band, covering areas up to 30 km offshore with a peak rate of 7 Mbps. It can provide broadband services for offshore applications, such as maritime transportation and offshore fisheries [66].

The shore-based MCNs, as extensions of terrestrial networks, can provide broadband communications services for offshore applications, such as multimedia file downloading and video surveillance data uploading. However, the shore-based MCNs have limited coverage compared with satellite networks, and the coverage performance depends largely on the geometrically available BS sites. Shore-based communications are suitable for maritime applications that are densely clustered in a small area.

II-C Island-based Maritime Communications

For the remote islands on the sea, high-quality communications can not only provide service for the islanders but also provide strong support for the timely communications and interconnection of border information. In 2015, the U.S. wireless provider Verizon Communications enhanced 4G LTE network coverage on Rhode Island. It can provide the islanders and nearby vessels with web browsing and file downloading services [67]. In 2016, China Mobile set up a 4G BS on the Yongshu Reef, which is more than 1,400 km from mainland China. By building satellite ground stations on the island, the signals from the island can be transmitted to the satellites, then to the backbone gateway on the mainland. The transmission rate often reaches 10 Mbps on the island and 15 Mbps using nearby ship-borne communication equipment. In 2017, China Telecom set up four 4G BSs on the Nansha Islands, which were connected to the mainland using underwater cables. The BSs provide coverage for the islands and reefs such as Yongshu Reef, Qibi Reef, Meiji Reef and nearby sea areas, enhancing broadband communication services [68].

The construction of island-based BSs further expands the coverage of terrestrial mobile signals. Island-based MCNs can support clear voice and video calls from the coast to the island and provide high-quality communication services for the surrounding ships and fishermen. On the other hand, island-based BSs are more vulnerable to extreme climate events, such as typhoons and rainstorms. Their coverage is also limited.