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Simultaneous Utilization of Multiple Radio Access Networks in Ubiquitous 6G Connectivity for Autonomous Ships: Opportunities and Challenges

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The growing significance of ubiquitous 6G connectivity within the maritime sector is a consequence of its evolution into an era characterized by the adoption of autonomous ships. This evolution necessitates the development of adaptable communication capabilities, even in the face of increasing heterogeneity in Radio Access Networks (RANs). This heterogeneity is a consequence of the extended lifespans of maritime communication technologies used by both legacy and emerging ships at sea, in contrast to the generational shift seen in terrestrial communication technologies. This paper undertakes a comprehensive examination and provides an insightful overview of communication technologies within the framework of 3rd Generation Partnership Project (3GPP) standards with the aim of preparing for the forthcoming 6G standardization to enable ubiquitous 6G connectivity in the maritime domain. The primary focus of this paper is the mobile RAN entities (e.g., satellites and uncrewed aerial vehicles (UAVs)) positioned for integration into the 5G and beyond systems. These entities are distinguished by their differences from conventional terrestrial RAN entities, which are typically stationary on land. This integration enables User Equipment (UE) to connect to various RAN entities, including mobile RANs, interconnected with core networks, thereby granting UE secure access to external internets through 5G and beyond systems, enabling them to enjoy a diverse range of application services, even in areas beyond terrestrial coverage, such as at sea. This paper further conducts an in-depth analysis of a transport layer-level solution known as the Access Traffic Steering, Switching, and Splitting (ATSSS) feature enabling a concurrent connection to multiple RANs for data-traffic delivery. Furthermore, this paper explores opportunities and challenges for future research in the realm of forthcoming 6G standardization within 3GPP, especially when combined with the ATSSS features for the success of autonomous ships within the maritime sector. These considerations encompass the concept of autonomous ships as mobile RAN entities, the integration of legacy maritime communications into the 6G framework, and the variability in maritime channel measurements, generally employed as one of the criteria for selecting an appropriate RAN among multiple options, influenced by uncontrollable factors such as climate change.
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Citation: Koo, H.; Ryoo, C.; Kim, W.
Simultaneous Utilization of Multiple
Radio Access Networks in
Ubiquitous 6G Connectivity for
Autonomous Ships: Opportunities
and Challenges. J. Mar. Sci. Eng. 2023,
11, 2106. https://doi.org/10.3390/
jmse11112106
Academic Editors: Zaili Yang, Sean
Loughney, Eduardo Blanco-Davis
and Milad Armin
Received: 4 October 2023
Revised: 31 October 2023
Accepted: 1 November 2023
Published: 3 November 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
Journal of
Marine Science
and Engineering
Review
Simultaneous Utilization of Multiple Radio Access Networks in
Ubiquitous 6G Connectivity for Autonomous Ships:
Opportunities and Challenges
Hyounhee Koo 1,2, Changho Ryoo 1and Wooseong Kim 2,*
1SyncTechno Inc., Seoul 06628, Republic of Korea; henary@gachon.ac.kr (H.K.)
2Department of Computer Engineering, Gachon University, Seongnam-si 13120, Republic of Korea
*Correspondence: wooseong@gachon.ac.kr
Abstract:
The growing significance of ubiquitous 6G connectivity within the maritime sector is a
consequence of its evolution into an era characterized by the adoption of autonomous ships. This
evolution necessitates the development of adaptable communication capabilities, even in the face of
increasing heterogeneity in Radio Access Networks (RANs). This heterogeneity is a consequence of
the extended lifespans of maritime communication technologies used by both legacy and emerging
ships at sea, in contrast to the generational shift seen in terrestrial communication technologies. This
paper undertakes a comprehensive examination and provides an insightful overview of communica-
tion technologies within the framework of 3rd Generation Partnership Project (3GPP) standards with
the aim of preparing for the forthcoming 6G standardization to enable ubiquitous 6G connectivity in
the maritime domain. The primary focus of this paper is the mobile RAN entities (e.g., satellites and
uncrewed aerial vehicles (UAVs)) positioned for integration into the 5G and beyond systems. These
entities are distinguished by their differences from conventional terrestrial RAN entities, which are
typically stationary on land. This integration enables User Equipment (UE) to connect to various
RAN entities, including mobile RANs, interconnected with core networks, thereby granting UE
secure access to external internets through 5G and beyond systems, enabling them to enjoy a diverse
range of application services, even in areas beyond terrestrial coverage, such as at sea. This paper
further conducts an in-depth analysis of a transport layer-level solution known as the Access Traffic
Steering, Switching, and Splitting (ATSSS) feature enabling a concurrent connection to multiple RANs
for data-traffic delivery. Furthermore, this paper explores opportunities and challenges for future
research in the realm of forthcoming 6G standardization within 3GPP, especially when combined
with the ATSSS features for the success of autonomous ships within the maritime sector. These
considerations encompass the concept of autonomous ships as mobile RAN entities, the integration
of legacy maritime communications into the 6G framework, and the variability in maritime channel
measurements, generally employed as one of the criteria for selecting an appropriate RAN among
multiple options, influenced by uncontrollable factors such as climate change.
Keywords:
heterogeneity; concurrent connection; maritime communication; simultaneous utilization;
6G; autonomous ship; mobile RAN
1. Introduction
Traditional analog maritime communications still remain a dominant means of com-
munication at sea, primarily serving voice communication, distress calls, and critical
navigational functions. These systems leverage their extensive communication coverage,
a characteristic derived from their unique frequency properties. However, as maritime
communication requirements evolve, these systems reveal limitations in radio performance.
This evolution necessitates the adoption of high-performance and secure communication
technologies tailored to the specific challenges of maritime environments.
J. Mar. Sci. Eng. 2023,11, 2106. https://doi.org/10.3390/jmse11112106 https://www.mdpi.com/journal/jmse
J. Mar. Sci. Eng. 2023,11, 2106 2 of 24
Satellite communication has emerged as a prevalent method for maritime commu-
nication, particularly for vessels navigating globally. This shift began with the adoption
of the convention on the International Mobile Satellite Organization (formerly known as
Inmarsat) by the International Maritime Organization (IMO) in 1976. This convention was
established to oversee and regulate the provision of specific satellite-based maritime dis-
tress communication services, primarily integrated within the Global Maritime Distress and
Safety System (GMDSS) [1]. Presently, the IMO is in the process of reviewing the GMDSS,
with plans for its modernization expected to take effect in 2024. However, existing maritime
communication systems prove inadequate in meeting the performance requirements neces-
sary to support imminent advancements in the maritime industry. These advancements
encompass the operation of autonomous ships on extended ocean voyages and the devel-
opment of intelligent offshore aquaculture, etc. Despite offering broader communication
coverage than terrestrial cellular communications, current satellite communications, which
rely on proprietary solutions tied to manufacturers, are inadequate in meeting the evolving
demands of the maritime sector due to their lack of global interoperability, unlike 3GPP
standards-based products and solutions with a dominant economy of scale in the market.
In stark contrast, first-generation (1G) analog terrestrial communication has become
obsolete. Second-generation (2G) and third-generation (3G) digital communication tech-
nologies, such as Global System for Mobile Communications (GSM) and Universal Mobile
Telecommunication System (UMTS), have been globally phased out. Fourth-generation (4G)
communication, represented by LTE, has achieved widespread deployment. Furthermore,
fifth-generation (5G) communication is exerting its influence by driving the digitalization
and mobilization of various industries while catalyzing the evolution of the Information
and Communication Technology (ICT) sector [2].
In addition, the 3rd Generation Partnership Project (3GPP) [
3
] has been actively en-
gaged in the systematic development of advanced and promising evolutionary features,
custom-tailored to meet the stringent requirements of industries venturing beyond the
boundaries of 5G [
4
6
]. Notably, a diverse range of industry sectors, including automotive,
factory automation, and public safety, has persistently advocated for enhanced cover-
age. This expanded coverage extends across both network infrastructure and off-network
scenarios, encompassing indoor and outdoor environments.
In response to these persistent demands, 3GPP has embarked on a rigorous standard-
ization process, characterized by an unwavering commitment to enhancing performance
and optimizing user experiences. This comprehensive effort includes the extension of
communication coverage, such as the introduction of satellite components as part of 5G
New Radio (NR), and the integration of various advanced features aimed at improving
reliability and reducing latency. This groundbreaking initiative commenced with the in-
troduction of Release 15, representing a pivotal milestone in 3GPP’s specifications for 5G.
Release 15 established the foundational architecture capable of accommodating the diverse
requirements of multiple industries [7].
Furthermore, the dynamic and evolving requirements originating from these industry
sectors continue to exert a profound influence on the trajectory of ongoing standardization
efforts within 3GPP. This transformative journey began with the introduction of Release
18, marking the inaugural version of 3GPP specifications for 5G-Advanced [
8
]. These
cutting-edge technologies operate seamlessly within a globally harmonized communica-
tion framework, strategically positioned to deliver benefits across multiple industries by
leveraging the economies of scale inherent in 3GPP standards-based products and solutions.
Moreover, these advancements are poised to effectively address evolving needs within the
forthcoming standardization framework, often referred to as 6G. This framework is antici-
pated to closely align with the framework and overall objectives of the future development
of International Mobile Telecommunications (IMT) for 2030 and beyond, encompassing
six distinct usage scenarios, including the realization of ubiquitous connectivity, and four
overarching aspects, notably the imperative of connecting the unconnected [
9
]. These
J. Mar. Sci. Eng. 2023,11, 2106 3 of 24
objectives hold particular significance for the maritime sector, given its distinctive and
evolving communication demands.
It is clear that various industry sectors share common demands in terms of enabling
technologies within the context of 3GPP standardization, and some of these demands may
have relevance within the maritime sector. This relevance is particularly evident when
considering the need to enhance communication coverage at sea, which can be achieved
through the adoption of cutting-edge enabling technologies and solutions within the 3GPP
framework. The inclusion of the maritime domain within the 3GPP standardization frame-
work, starting from Release 16 [
10
12
], has brought a wealth of enabling technologies from
5G and beyond into the scope of maritime transformation. However, it is crucial to acknowl-
edge that these technologies may require further optimization to seamlessly align with the
specific challenges posed by the maritime environment. This recognition emphasizes the
need for additional work within 3GPP to address maritime-specific requirements, which
should be carefully formulated with significant input from key stakeholders representing
the global maritime sector, including maritime safety aspects. These requirements are
poised to assume a pivotal role in the forthcoming 6G standardization, with a core objective
of delivering evolved performance and tailored user experiences customized to meet the
distinctive challenges of maritime environments.
Consequently, there is substantial value in conducting a comprehensive review of
standardized 5G enabling technologies, with a particular focus on integrating mobile RAN
entities like satellite access components into 5G and beyond systems and supporting the
concurrent utilization of multiple Radio Access Networks (RANs) including those mobile
RAN entities. This effort takes into account the extended operational lifespans of the
maritime communication technologies, which result in a diverse landscape of multiple
access networks within the maritime sector. Effectively, harnessing multiple RANs simulta-
neously and efficiently is crucial for achieving ubiquitous connectivity, which is an essential
foundation for the successful deployment of autonomous ships in the maritime sector. The
key contributions of this paper are summarized as follows:
This paper contributes to the insightful analysis of Access Traffic Steering, Switching,
and Splitting (ATSSS) features, which enable the simultaneous resource utilization of
multiple RANs including mobile RANs within the 3GPP standardization framework.
This transport-layer solution offers substantial advantages to the maritime sector,
preparing for impending shifts, including the adoption of autonomous ships that
require ubiquitous connectivity during their voyages.
In addition, this paper makes a significant contribution by conducting a thorough
examination of standardization trends and technologies within 3GPP, with a specific
focus on the integration of satellites and low-altitude Uncrewed Aerial Vehicles (UAVs)
as mobile RAN entities. These mobile RAN entities, integrated into 6G systems, are
poised to play a crucial role in ensuring uninterrupted connectivity and optimizing
concurrent resource utilization across multiple RANs when combined with ATSSS
features, given the anticipated increase in RAN heterogeneity in the near future as the
maritime sector undergoes digitalization and mobilization, including the introduction
of autonomous ships.
Finally, this paper contributes significantly by exploring opportunities and challenges
for future research within the context of ATSSS features in the maritime communication
scenarios in terms of three aspects encompassing the concept of autonomous ships
as mobile RAN entities, the integration of legacy maritime communications into the
6G framework, and the variability in maritime channel measurements, generally
employed as one of the criteria for selecting an appropriate RAN among multiple
options, influenced by uncontrollable factors such as climate change.
This paper is organized as follows: in Section 2, we present an overview of related
work from academic research and global standardization trends, with a particular focus
on ITU-R [
13
] and 3GPP, for the provision of seamless connectivity over 6G systems for
the evolution of the maritime sector including autonomous ships. Section 3provides
J. Mar. Sci. Eng. 2023,11, 2106 4 of 24
a comprehensive exploration of integrating satellite and low-altitude UAVs as mobile
RAN entities into 5G and beyond. Following this, Section 4explores transport-layer
solutions for efficiently distributing data traffic across two access networks, catering to
various applications within the 3GPP standardization framework. In Section 5, we navigate
the opportunities and challenges that warrant further investigation in the ever-evolving
landscape of providing ubiquitous connectivity optimized for autonomous ships in the
maritime sector. Finally, Section 6offers our conclusions.
2. Related Works
2.1. Academic Research
Since the advent of wireless communication technologies, achieving seamless commu-
nication has been a fundamental objective pursued by nearly all communication technolo-
gies. Conventional wireless communication networks incorporate radio access network
selection algorithms, including handover mechanisms, which predominantly prioritize
signal strength as the key determinant in network switching decisions, with the overarching
objective of maintaining device connectivity throughout transitions [
14
]. In the trajectory
of mobile communication systems progressing beyond 5G, often referred to as 6G, con-
siderable academic research has been conducted to formulate frameworks and protocols
that facilitate seamless provisioning of wireless communication anywhere, anytime [
15
23
].
This pursuit seeks to not only enhance throughput and bolster reliability but also to opti-
mize radio resource utilization across various radio access technologies (RATs) within a
unified communication ecosystem.
Within the realm of maritime communication, pertinent related works elucidate ap-
proaches for incorporating autonomous ships into a cohesive communication framework,
encompassing advancements like 5G and its evolutionary successors (i.e., 6G) with AI-aided
solutions for ubiquitous connectivity in the maritime communication environment [
24
29
].
The investigation into the harmonious coexistence of autonomous and conventional vessels
has also been explored, aiming to facilitate the autonomous ships’ effective conveyance of
their operational status and intentions to traditional ships via diverse maritime communica-
tion methods [30].
2.2. Global Standardization in ITU-R and 3GPP
ITU-R and 3GPP have a cooperative relationship in the development of global mobile
communication standards. ITU-R establishes comprehensive guidelines and recommenda-
tions to steer the evolution of IMT families, and ITU-R Working Party 5D (WP 5D) bears
the responsibility for the comprehensive radio-system aspects of the terrestrial component
within IMT systems, which encompass IMT-2000, IMT-Advanced (referred to as 4G or
LTE), IMT-2020 (commonly known as 5G), and IMT-2030 (indicative of the forthcoming
6G) [
31
]. 3GPP is responsible for crafting standards for radio interfaces of IMT families,
aligning them with requirements stipulated by ITU-R. Figure 1a summarizes the ITU-R
Recommendation and corresponding radio interfaces based on 3GPP standards for IMT
families. Furthermore, 3GPP undertakes the formulation of technical standards for core
networks and terminals, expanding upon ITU-R’s recommendations as shown in Figure 1b.
This collaborative effort ensures the worldwide compatibility and efficiency of communica-
tion systems, optimizing the utilization of allocated radio frequencies, which contribute
to establish the unified communication ecosystems based on 3GPP standards prevalent in
the market.
J. Mar. Sci. Eng. 2023,11, 2106 5 of 24
J. Mar. Sci. Eng. 2023, 11, x FOR PEER REVIEW 5 of 25
Figure 1. Relations between IMT systems and 3GPP systems.
2.2.1. Standardization Trends for IMT-2030 System in ITU-R
In June 2023, ITU-R WP 5D reached a consensus on the draft new recommendation
titled Framework and overall objectives of the future development of IMT for 2030 and
beyondthat addresses the trends, usage scenarios, and capabilities of IMT-2030, serving
as the basis for the forthcoming standardization eorts in the development of the next
generation of IMT standards [9]. Ubiquitous connectivity and connecting the uncon-
nectedhave been incorporated into the IMT-2030 framework. Their inclusion aims to fa-
cilitate the extension of communication coverage beyond terrestrial networks, spanning
into integrated networks that encompass space, air, ground, and sea, transitioning from
the conventional terrestrial-based paradigm. Upon the approval of this draft new recom-
mendation during the fourth quarter of the year 2023 by ITU-R Study Group 5 (SG5), the
subsequent standardization eorts will encompass the specication of precise require-
ments according to the IMT-2030 standardization timeline in ITU-R [32]. This undertaking
involves elaborating on exact technical specications, functionalities, and performance re-
quirements. These particulars will serve as guiding principles for the advancement of
IMT-2030 standards and their practical implementations in 3GPP.
2.2.2. Standardization Trends for 5G and Beyond in 3GPP
For seamless communication and the improvement of communication quality, a
range of features including multi-connectivity solutions have been developed in 3GPP
across successive generational advancements [9,33,34]. Preceding the establishment of 5G
standards, the prevalent approaches to enable multi-connectivity predominantly
Figure 1. Relations between IMT systems and 3GPP systems.
2.2.1. Standardization Trends for IMT-2030 System in ITU-R
In June 2023, ITU-R WP 5D reached a consensus on the draft new recommendation
titled “Framework and overall objectives of the future development of IMT for 2030 and
beyond” that addresses the trends, usage scenarios, and capabilities of IMT-2030, serving as
the basis for the forthcoming standardization efforts in the development of the next genera-
tion of IMT standards [
9
]. ‘Ubiquitous connectivity’ and ‘connecting the unconnected’ have
been incorporated into the IMT-2030 framework. Their inclusion aims to facilitate the ex-
tension of communication coverage beyond terrestrial networks, spanning into integrated
networks that encompass space, air, ground, and sea, transitioning from the conventional
terrestrial-based paradigm. Upon the approval of this draft new recommendation during
the fourth quarter of the year 2023 by ITU-R Study Group 5 (SG5), the subsequent standard-
ization efforts will encompass the specification of precise requirements according to the
IMT-2030 standardization timeline in ITU-R [
32
]. This undertaking involves elaborating
on exact technical specifications, functionalities, and performance requirements. These
particulars will serve as guiding principles for the advancement of IMT-2030 standards and
their practical implementations in 3GPP.
J. Mar. Sci. Eng. 2023,11, 2106 6 of 24
2.2.2. Standardization Trends for 5G and Beyond in 3GPP
For seamless communication and the improvement of communication quality, a range
of features including multi-connectivity solutions have been developed in 3GPP across suc-
cessive generational advancements [
9
,
33
,
34
]. Preceding the establishment of 5G standards,
the prevalent approaches to enable multi-connectivity predominantly stemmed from as-
sessments within RAN domains. These approaches were designed to ascertain the optimal
selection of radio channels, both within a communication system with a single connection
to a certain RAN and in interworking scenarios involving 3GPP systems (e.g., LTE) and
non-3GPP systems (i.e., Wi-Fi) [
35
]. With the integration of satellite access components
as part of the 5G landscape in 2022 [
36
38
], a critical challenge has arisen concerning the
efficient utilization of multiple RANs within a cohesive communication framework. This
challenge aims to harness the potential for augmented throughput, enhanced reliability,
and diminished energy consumption in the context of 5G and subsequent standardization
efforts [
39
,
40
]. The forefront of 5G technology pertaining to the efficient resource utilization
of multiple RANs will undergo continuous advancement in the subsequent generation.
These capabilities are of paramount importance in meeting the demands for ubiquitous
connectivity within the IMT-2030 system. A comprehensive exploration of these features
will be presented in the forthcoming sections.
3. Mobile RAN Entity Integrated into 5G and Beyond
In contrast to its predecessors, 5G has been designed to cater to the diverse require-
ments stemming from various industries, including the automotive, railway, public safety,
broadcasting, and maritime sectors. Furthermore, continuous efforts are devoted to advanc-
ing these technologies, facilitating seamless communication across diverse deployment
scenarios that are indispensable for the mobilization and automation of various industries.
It is crucial to delve into the cutting-edge advancements in 5G technologies related to the
efficient utilization of multiple RANs, which assume a pivotal role in providing ubiquitous
connectivity to autonomous ships within the maritime sector considering the increase in
the heterogeneity in RANs, spanning the 5G and beyond systems.
Traditionally, RAN entities based on 3GPP standards were perceived as stationary
and lacking mobility, while User Equipment (UE) was recognized as a mobile entity,
transitioning between different spatial points. This dichotomy necessitated solutions such
as handovers for UE in connected mode or cell reselection for UE in idle mode to ensure
uninterrupted communication as the UE moves. However, with the advent of the 5G and
beyond era, mobile entities like satellites and UAVs are poised to be integrated into RAN
equipment, despite the myriad challenges arising from their departure from conventional
assumptions concerning the mobility capabilities of both RANs and UE. This section
provides a comprehensive exploration of the integration of satellites and UAVs into the 5G
and beyond system, formulated within the framework of 3GPP standardization.
3.1. Integration of Satellite Components into the 5G and Beyond Landscape
The incorporation of satellite components into the 5G system began with its inclu-
sion in the specifications of 3GPP Release 17 in 2022. As outlined in Figure 2, after this
pivotal integration, a series of investigations and initiatives followed, aiming at further
enhancement. The initial design objective primarily aimed at establishing the connectivity
between traditional mobile phones and a satellite component intricately woven into the
architecture of the 5G framework depicted in Figure 3, functioning within the sub-6GHz
frequency band. This integration sought to extend 5G’s reach to mobile users situated in
geographically challenging areas, such as maritime expanses. The satellite component,
illustrated in Figure 3a as a transparent payload, was designed to serve as a repeater, limited
to radio frequency (RF) processing tasks, including frequency conversion, amplification,
and beam management [3638].
J. Mar. Sci. Eng. 2023,11, 2106 7 of 24
J. Mar. Sci. Eng. 2023, 11, x FOR PEER REVIEW 7 of 25
Figure 2. Studies and works for satellite access as a 5G New Radio and a backhaul entity in 3GPP.
Figure 3. Comparison between 5G system architectures with transparent satellite access and with
terrestrial NR radio access and typical satellite network architecture.
However, since the inception of 5Gs design in 3GPP Release 15 specications, oper-
ating under the presumption of a terrestrial-based radio interface environment [7,33], nu-
merous challenges emerged due to the unique characteristics associated with satellites.
Within the framework of conventional 3GPP standards-based communication systems,
these challenges encompass various critical dimensions [41,42]. They include signicant
considerations like the substantial round-trip delays inherent to especially Geostationary
Earth Orbit (GEO) satellites. Additionally, complexities have arisen from rapid Doppler
Shift and intricate mobility paerns not typically encountered in terrestrial-based RAN
equipment, particularly evident within the realm of Low Earth Orbit (LEO) satellites.
Moreover, a formidable obstacle has emerged in the form of establishing communication
coverage across geographical regions that extend beyond the boundaries of a single
Figure 2. Studies and works for satellite access as a 5G New Radio and a backhaul entity in 3GPP.
J. Mar. Sci. Eng. 2023, 11, x FOR PEER REVIEW 7 of 25
Figure 2. Studies and works for satellite access as a 5G New Radio and a backhaul entity in 3GPP.
Figure 3. Comparison between 5G system architectures with transparent satellite access and with
terrestrial NR radio access and typical satellite network architecture.
However, since the inception of 5Gs design in 3GPP Release 15 specications, oper-
ating under the presumption of a terrestrial-based radio interface environment [7,33], nu-
merous challenges emerged due to the unique characteristics associated with satellites.
Within the framework of conventional 3GPP standards-based communication systems,
these challenges encompass various critical dimensions [41,42]. They include signicant
considerations like the substantial round-trip delays inherent to especially Geostationary
Earth Orbit (GEO) satellites. Additionally, complexities have arisen from rapid Doppler
Shift and intricate mobility paerns not typically encountered in terrestrial-based RAN
equipment, particularly evident within the realm of Low Earth Orbit (LEO) satellites.
Moreover, a formidable obstacle has emerged in the form of establishing communication
coverage across geographical regions that extend beyond the boundaries of a single
Figure 3.
Comparison between 5G system architectures with transparent satellite access and with
terrestrial NR radio access and typical satellite network architecture.
However, since the inception of 5G’s design in 3GPP Release 15 specifications, op-
erating under the presumption of a terrestrial-based radio interface environment [
7
,
33
],
numerous challenges emerged due to the unique characteristics associated with satellites.
Within the framework of conventional 3GPP standards-based communication systems,
these challenges encompass various critical dimensions [
41
,
42
]. They include significant
considerations like the substantial round-trip delays inherent to especially Geostationary
Earth Orbit (GEO) satellites. Additionally, complexities have arisen from rapid Doppler
Shift and intricate mobility patterns not typically encountered in terrestrial-based RAN
equipment, particularly evident within the realm of Low Earth Orbit (LEO) satellites.
Moreover, a formidable obstacle has emerged in the form of establishing communication
coverage across geographical regions that extend beyond the boundaries of a single coun-
try. As delineated in Release 17 [
34
], the solution adopted has illuminated suboptimal
J. Mar. Sci. Eng. 2023,11, 2106 8 of 24
design principles from the perspective of satellite components. These principles have been
implemented through the adaptation of key procedures, notably encompassing aspects
such as timing advance, hybrid automatic repeat request (HARQ) operations, and tracking
area management. Importantly, this adaptation has been executed while concurrently
preserving established procedural norms that have conventionally underpinned radio
interfaces within the framework of 3GPP standardization.
Furthermore, ongoing advancements are currently under way within 5G-Advanced
standardization [
43
45
]. Aligned with the goals of the IMT-2030 framework including
‘ubiquitous connectivity’ and ‘connecting unconnected’, it is anticipated that satellite com-
ponents will be seamlessly and comprehensively integrated into the foundational design of
the 6G communication system within the framework of 3GPP standardization [36,46,47].
3.2. Integration of Low-Altitude UAVs into 5G and Beyond
The integration of low-altitude UAVs and Uncrewed Aerial System (UAS) traffic
management into 5G systems has become a prominent research focus in both academic
and global standardization communities [
48
51
]. As illustrated in Figure 4, UAS Phase 1
defines the architecture and protocols for the UAS application layer, encompassing UAS
application enablers within the 3GPP standardization framework [5256]. UAS Phase 2 is
currently under standardization, with a specific focus on meeting regulatory requirements
for broadcast remote ID and detect and avoid (DAA) solutions [
57
,
58
]. Concurrently, UAS
Phase 3 is in the process of standardization to develop additional service requirements.
J. Mar. Sci. Eng. 2023, 11, x FOR PEER REVIEW 8 of 25
country. As delineated in Release 17 [34], the solution adopted has illuminated suboptimal
design principles from the perspective of satellite components. These principles have been
implemented through the adaptation of key procedures, notably encompassing aspects
such as timing advance, hybrid automatic repeat request (HARQ) operations, and track-
ing area management. Importantly, this adaptation has been executed while concurrently
preserving established procedural norms that have conventionally underpinned radio in-
terfaces within the framework of 3GPP standardization.
Furthermore, ongoing advancements are currently under way within 5G-Advanced
standardization [4345]. Aligned with the goals of the IMT-2030 framework including
ubiquitous connectivity and connecting unconnected’, it is anticipated that satellite
components will be seamlessly and comprehensively integrated into the foundational de-
sign of the 6G communication system within the framework of 3GPP standardization
[36,46,47].
3.2. Integration of Low-Altitude UAVs into 5G and Beyond
The integration of low-altitude UAVs and Uncrewed Aerial System (UAS) trac
management into 5G systems has become a prominent research focus in both academic
and global standardization communities [4851]. As illustrated in Figure 4, UAS Phase 1
denes the architecture and protocols for the UAS application layer, encompassing UAS
application enablers within the 3GPP standardization framework [5256]. UAS Phase 2 is
currently under standardization, with a specic focus on meeting regulatory requirements
for broadcast remote ID and detect and avoid (DAA) solutions [57,58]. Concurrently, UAS
Phase 3 is in the process of standardization to develop additional service requirements.
Figure 4. Studies and works for Uncrewed Aerial Systems (UAS) in 3GPP.
As depicted in Figure 5, within the context of 3GPP standardization, UAS Phase 1
comprehensively outlined the functional architecture, procedures, and information ows
that govern the integration of the UAS Application Enabler (UAE) layer with 5G/LTE sys-
tems. This integration eectively harnessed the capabilities oered by the Service Enabler
Architecture Layer (SEAL).
Figure 4. Studies and works for Uncrewed Aerial Systems (UAS) in 3GPP.
As depicted in Figure 5, within the context of 3GPP standardization, UAS Phase 1
comprehensively outlined the functional architecture, procedures, and information flows
that govern the integration of the UAS Application Enabler (UAE) layer with 5G/LTE
systems. This integration effectively harnessed the capabilities offered by the Service
Enabler Architecture Layer (SEAL).
J. Mar. Sci. Eng. 2023,11, 2106 9 of 24
J. Mar. Sci. Eng. 2023, 11, x FOR PEER REVIEW 9 of 25
Figure 5. System architecture and UAS application enabler layer for UAS over 5G/LTE system in
UAS Phase 1.
The following functionalities were specied within the UAS Phase 1 framework, con-
tributing to the enhancement of precision and versatility in UAV monitoring within the
network:
UAV remote identicationIncorporated into 5G/LTE systems, the civil aviation ad-
ministration (CAA)-Level UAV ID serves as a globally unique and easily readable
identier for UAVs. This identication plays a crucial role in ensuring the accurate
routing of requests to the designated UAV service supplier (USS) for the retrieval of
UAV-related information. During the initial registration of a UAV by its owner with
the USS, the CAA-Level UAV ID is assigned to the UAV. It encompasses essential
aviation-level data, including the UAVs serial number, pilot information, and UAS
operator details. Subsequently, this information is transmied to the USS for metic-
ulous record keeping and management;
UAV USS authentication and authorization (UUAA)After successful 3GPP authen-
tication of the UE using the credentials of a mobile network operator (MNO), a ded-
icated UUAA procedure is established to verify the UAVs registration with the USS.
This procedure relies on the CAA-Level UAV ID for USS authentication. In 5G sys-
tems, it can occur during 3GPP registration or packet data unit (PDU) session setup
Figure 5.
System architecture and UAS application enabler layer for UAS over 5G/LTE system in
UAS Phase 1.
The following functionalities were specified within the UAS Phase 1 framework,
contributing to the enhancement of precision and versatility in UAV monitoring within the
network:
UAV remote identification—Incorporated into 5G/LTE systems, the civil aviation
administration (CAA)-Level UAV ID serves as a globally unique and easily readable
identifier for UAVs. This identification plays a crucial role in ensuring the accurate
routing of requests to the designated UAV service supplier (USS) for the retrieval of
UAV-related information. During the initial registration of a UAV by its owner with the
USS, the CAA-Level UAV ID is assigned to the UAV. It encompasses essential aviation-
level data, including the UAV’s serial number, pilot information, and UAS operator
details. Subsequently, this information is transmitted to the USS for meticulous record
keeping and management;
UAV USS authentication and authorization (UUAA)—After successful 3GPP authenti-
cation of the UE using the credentials of a mobile network operator (MNO), a dedicated
UUAA procedure is established to verify the UAV’s registration with the USS. This
procedure relies on the CAA-Level UAV ID for USS authentication. In 5G systems,
it can occur during 3GPP registration or packet data unit (PDU) session setup for
J. Mar. Sci. Eng. 2023,11, 2106 10 of 24
UAS services and in LTE systems, it is during packet data network (PDN) connec-
tion establishment. The UUAA procedure comprises a service-level-AA container,
a Service-level-AA procedure for authentication/authorization, and an API-based
method. Security material specifics are considered outside the scope of 3GPP as they
are application layer concerns;
Command and Control (C2) communication over 5G/LTE—To facilitate C2 commu-
nication over 5G/LTE systems, specifically involving a UAV user plane connection
with the UAV controller, it is imperative to obtain authorization from the USS. This
authorization encompasses several key aspects, including permission for the UAV and
UAV controller pairing and, optionally, flight authorization for the UAV, a process
overseen by the USS. To support these vital functions, the existing non-access stratum
PDU session establishment and modification procedures have been expanded. These
enhancements enable the inclusion of critical elements such as the CAA-level UAV ID
and other application-layer authorization information within the service-level-AA con-
tainer, ensuring a robust framework for secure and controlled cellular communication
in UAV operations;
UAV location reporting and tracking—The specification of UAV location reporting
and tracking extensively relies on the reuse of existing location procedures [
59
]. In
this framework, the UAS network function (UAS NF/NEF) engages with network
functions such as the gateway mobile location center and the mobility management
functions (e.g., access and mobility management function (AMF) in the case of 5G
systems, mobility management entity (MME) in the case of LTE systems). Various
UAV tracking modes have been established to cater for diverse operational scenarios:
(1)
UAV location reporting—The USS subscribes to the UAS NF, specifying prefer-
ences for accuracy and timing, to receive continuous UAV location updates;
(2)
UAV presence monitoring—The USS subscribes to event reports triggered
when a UAV enters or exits a defined geographical area;
(3)
List of aerial UEs—The USS can request the UAS NF to provide a compre-
hensive list of UAVs within a geographical area served by the PLMN. This
multifaceted tracking system enhances the precision and versatility of UAV
monitoring within the network.
These efforts are aimed at strengthening network support for UAVs, encompassing
aspects such as predictive monitoring, flight-path planning, reliable communication, and
enhancing UAV operational control and safety. This includes the ability to detect connected
UAVs, manage flight paths, and identify non-3GPP flying objects. Notably, the incorpo-
ration of UAVs into the 5G and beyond systems to extend coverage is currently outside
the scope of consideration within 3GPP. However, ongoing research endeavors are actively
exploring the integration of UAVs as a mobile RAN entity into the forthcoming 6G system,
with the goal of extending communication coverage beyond regional boundaries [
48
,
60
62
].
4. Simultaneous Utilization of Multiple RANs within a 3GPP Framework
The effective utilization of radio resources has become a significant focal point in
academic research and global standardization efforts, increasingly incorporating machine
learning algorithms [
63
65
]. With the emergence of 5G and beyond systems, a multitude of
RANs have become accessible, expanding their coverage even to non-terrestrial regions,
thanks to the addition of satellite components such as mobile RANs integrated into 5G
and beyond. Furthermore, UAVs are poised to play a pivotal role as a mobile RAN, facili-
tating expanded coverage in the forthcoming 6G standardization. Traditionally, cellular
networks like LTE connect to just one access network at a time [
14
] or utilize unlicensed
spectrum bands as part of 3GPP-based radio access resources for offloading wireless data
traffic [
66
,
67
], alongside a range of radio access interface-level solutions [
68
]. However, the
simultaneous connection and utilization of multiple access networks now empowers the
distribution of data traffic [
69
,
70
]. This section explores transport layer-level solutions for
J. Mar. Sci. Eng. 2023,11, 2106 11 of 24
efficiently distributing data traffic simultaneously across two RANs for several applications
in the context of 3GPP standardization, as outlined in Table 1.
Table 1. Summary of ATSSS functions in the context of 3GPP standardization.
Function ATSSS Phase 1
(Release 16)
ATSSS Phase 2
(Release 17)
ATSSS Phase 3
(Release 18)
Steering
functionality
MPTCP functionality for
TCP traffic
ATSSS-LL functionality for all
types of traffic
Same as ATSSS Phase 1
Same as ATSSS Phase 1
MPQUIC steering
functionality over HTTP
Steering
Mode (SM)
Active-standby SM
Smallest delay SM
Load-balancing SM with
pre-defined or fixed split
percentages
Priority-based SM
Same as ATSSS Phase 1 Same as ATSSS Phase 1
Redundant SM
N/A
Steering Mode Indicator
indicating either autonomous
load-balance indicator or
UE-assistance indicator for
load-balancing SM
Threshold Values indicator in
case of load-balancing SM
with fixed split percentage or
priority-based SM
Same as ATSSS Phase 2
Performance measurement
function (PMF) to assist
access selection
Round Trip Time (RTT)
measurement in case of smallest
delay mode in use
UE’s reporting on access
availability/unavailability
to UPF
RTT measurement same as
ATSSS Phase 1
Packet Loss Rate (PLR)
measurement for a service
data flow (SDF) over
both accesses
Same as ATSSS Phase 2
N/A
Access performance
measurement over a certain
QoS flow
Same as ATSSS Phase 2
Combination of two access
networks and core networks
a 3GPP access path over 5GC/a
non-3GPP access path over 5GC
Same as ATSSS Phase 1
a 3GPP access path (e.g.,
E-UTRAN) over EPC/a
non-3GPP access path
over 5GC
Same as ATSSS Phase 2
a 3GPP access path over
5GC/a non-3GPP access
path over ePDG/EPC
4.1. Simultaneous Utilization of 3GPP-Based and Wi-Fi Access Networks over 5GC or EPC
Commencing with Release 16 and onwards, a transport layer-level solution referred
to as ATSSS was initially introduced to integrate non-3GPP access networks (e.g., Wireless
Local Area Network (WLAN)) into the 3GPP ecosystem. This innovation facilitates the
distribution of data traffic related to multiple applications through various pathways,
encompassing both a 3GPP access network (e.g., NG-RAN) and a non-3GPP access network
(e.g., WLAN) within the framework of the 5G system. In the subsequent sections, we delve
into the distinct functions of ATSSS in each phase, enabling the simultaneous distribution
of data traffic across both 3GPP and non-3GPP access networks, integrated into either 5G
Core (5GC) or Evolved Packet Core (EPC) network.
4.1.1. ATSSS Phase 1
Incorporating the ATSSS Phase 1 feature into the 5G system involves the introduction
of a Multi-Access PDU (MA PDU) session into the existing 5G network functions. This
integration is designed to ensure alignment with the foundational structure of the 5G system
architecture, as depicted in Figure 6. The MA PDU session enables the seamless exchange
of PDUs between the UE (application client) and the internet (remote application server
host) for a wide range of applications. This exchange is made possible by simultaneously
J. Mar. Sci. Eng. 2023,11, 2106 12 of 24
utilizing both a 3GPP access network and a non-3GPP access network, each operating
within two independent N3/N9 interfaces. Regarding the MA PDU session type, ATSSS
Phase 1 exclusively accommodates structured types, including IPv4, IPv6, IPv4v6, and
Ethernet, while it does not provide the support for unstructured types of a MA PDU session.
J. Mar. Sci. Eng. 2023, 11, x FOR PEER REVIEW 12 of 25
4.1.1. ATSSS Phase 1
Incorporating the ATSSS Phase 1 feature into the 5G system involves the introduction
of a Multi-Access PDU (MA PDU) session into the existing 5G network functions. This
integration is designed to ensure alignment with the foundational structure of the 5G sys-
tem architecture, as depicted in Figure 6. The MA PDU session enables the seamless ex-
change of PDUs between the UE (application client) and the internet (remote application
server host) for a wide range of applications. This exchange is made possible by simulta-
neously utilizing both a 3GPP access network and a non-3GPP access network, each op-
erating within two independent N3/N9 interfaces. Regarding the MA PDU session type,
ATSSS Phase 1 exclusively accommodates structured types, including IPv4, IPv6, IPv4v6,
and Ethernet, while it does not provide the support for unstructured types of a MA PDU
session.
Figure 6. Multi-Access PDU (MA PDU) session with user-plane resources across both 3GPP access
and non-3GPP access networks (The gure does not show entities related to the control plane in a
5G system).
The three following procedures are dened in the context of an ATSSS Phase 1 fea-
ture, which is applicable between one 3GPP access and one non-3GPP access.
ATSSS trac steeringThis procedure is employed to select an access network for a
new data ow to transfer the data trac of this data ow over the selected access
network;
ATSSS trac switchingThis procedure is employed to seamlessly transition all
data trac of an ongoing data ow from one access network to another access net-
work;
ATSSS trac spliingThis procedure is employed to split the data trac of a data
ow across two access networks. This spliing allows some data trac of the data
ow to be routed through one access network, while simultaneously directing other
data trac of the same data ow through another access network.
The network-provided policy, which includes ATSSS rules for UE and N4 rules for
UPF, is derived by the UEs serving Session Management Function (SMF) within the 5GC.
Concurrently, the UE reports the availability of the access network. In light of the network-
provided policy and the access network availability reported from the UE, the UPF deter-
mines the appropriate procedure for distributing downlink data trac across the two ac-
cess networks. Simultaneously, the UE governs uplink data-trac distribution, selecting
the appropriate procedure based on the network-provided policy, access network availa-
bility, and local factors, including signal strength, user preferences, and signal loss.
The ATSSS Phase 1 feature encompasses two steering functionalities: Multi-Path
Transmission Control Protocol (MPTCP) functionality for TCP trac, facilitated by an
MPTCP proxy in the UPF, and ATSSS Low Layer (ATSSS-LL) functionality, which extends
Figure 6.
Multi-Access PDU (MA PDU) session with user-plane resources across both 3GPP access
and non-3GPP access networks (The figure does not show entities related to the control plane in a
5G system).
The three following procedures are defined in the context of an ATSSS Phase 1 feature,
which is applicable between one 3GPP access and one non-3GPP access.
ATSSS traffic steering—This procedure is employed to select an access network
for a new data flow to transfer the data traffic of this data flow over the selected
access network;
ATSSS traffic switching—This procedure is employed to seamlessly transition all data
traffic of an ongoing data flow from one access network to another access network;
ATSSS traffic splitting—This procedure is employed to split the data traffic of a data
flow across two access networks. This splitting allows some data traffic of the data
flow to be routed through one access network, while simultaneously directing other
data traffic of the same data flow through another access network.
The network-provided policy, which includes ATSSS rules for UE and N4 rules for
UPF, is derived by the UE’s serving Session Management Function (SMF) within the
5GC. Concurrently, the UE reports the availability of the access network. In light of the
network-provided policy and the access network availability reported from the UE, the
UPF determines the appropriate procedure for distributing downlink data traffic across
the two access networks. Simultaneously, the UE governs uplink data-traffic distribution,
selecting the appropriate procedure based on the network-provided policy, access network
availability, and local factors, including signal strength, user preferences, and signal loss.
The ATSSS Phase 1 feature encompasses two steering functionalities: Multi-Path
Transmission Control Protocol (MPTCP) functionality for TCP traffic, facilitated by an
MPTCP proxy in the UPF, and ATSSS Low Layer (ATSSS-LL) functionality, which extends
to all types of data traffic, including TCP, User Datagram Protocol (UDP), Ethernet, and
more. Furthermore, four distinct steering modes, included in ATSSS rules, have been
defined to guide UE in routing data traffic for MA PDU sessions, and three steering modes,
except for active standby, are applicable only to non-Guaranteed Bit Rate (non-GBR) [
71
,
72
]
data flow:
Active-standby steering mode: When an access network, indicated as ‘active’, is
available, all data traffic for the MA PDU session is routed through this active access
network. If the active access network becomes unavailable while another access
network, indicated as ‘standby’, remains accessible, the data traffic for the MA PDU
J. Mar. Sci. Eng. 2023,11, 2106 13 of 24
session is then directed through the standby access network. The determination of the
active or standby status of the 3GPP access network and non-3GPP access network is
conveyed through the ATSSS/N4 rules;
Smallest delay steering mode: The data traffic for the MA PDU session is routed
through the access network with the smallest RTT if two access networks are available;
Load-balancing steering mode: Data traffic within the MA PDU session is intelli-
gently distributed across both 3GPP and non-3GPP access networks, with a specified
percentage allocation when both access networks are concurrently accessible. This
distribution of data-traffic percentages between 3GPP and non-3GPP access networks
is communicated via the ATSSS/N4 rules;
Priority-based steering mode: Data traffic within the MA PDU session is primarily
directed over a high-priority access network, unless the high-priority access network
experiences congestion or becomes unavailable. In instances of congestion on the
high-priority access network, new data traffic is redirected to the low-priority access
network. In the event of the high-priority access network being unavailable, all data
traffic for the MA PDU session is seamlessly rerouted to an alternative low-priority
access network if available. The designation of which access network holds the high-
priority status is specified within the ATSSS/N4 rules.
In addition, for the assistance of access network selection, the Performance Measure-
ment Function (PMF) is introduced to support two types of measurements, i.e., Round
Trip Time (RTT) measurement per access network from the UE and UPF in the case of the
smallest delay steering mode, and access network availability measurement from the UE
to report it to the UPF. Moreover, the 5G Quality of Service (QoS) model designed for a
single-access PDU session is extended to the MA PDU session. This ensures that the same
QoS Flow Identifier (QFI) is employed by the SMF for both 3GPP and non-3GPP access
networks, thereby facilitating consistent QoS support across both access networks.
4.1.2. ATSSS Phase 2
In ATSSS Phase 2, two significant advancements have been achieved: the refinement
of steering modes and the integration of MA PDU session support with a 3GPP access
network connected to the EPC, which serves as the LTE system’s core network.
In the realm of steering-mode enhancement, a new indicator, referred to as the Steering
Mode Indicator, has been incorporated into the ATSSS/N4 rules [
73
], specifically designed
for the load-balancing steering mode. This indicator signifies that the UE is empowered to
modify the default steering parameters and autonomously adjust traffic steering based on
its own decision making. In situations where ATSSS/N4 rules do not provide predefined
split ratios for the load-balancing steering mode, this innovation allows both the UE and
the UPF to independently and autonomously determine the data-traffic split percentage
between two access networks. To indicate the absence of predefined or fixed percentages,
the parameter called the autonomous load-balance indicator [
74
] is included within the
Steering Mode Indicator. The primary goal of this enhancement is to optimize data flow
throughput in both the uplink and downlink directions. It operates under the assumption
that the UE and UPF typically select percentages that maximize aggregate throughput,
considering RTT measurements that can vary depending on the radio environment in
which the UE is situated. In ATSSS Phase 1, data-traffic distribution policies across two
access networks were primarily controlled by the network infrastructure, irrespective of
the specific circumstances encountered by the UE, regardless of their criticality. ATSSS
Phase 2 has effectively addressed this limitation by including the parameter called the
UE-assistance indicator [
74
] within the Steering Mode Indicator. This indication empowers
the UE to make informed decisions regarding data-traffic distribution based on its internal
status, such as battery consumption. Furthermore, the UE can request the UPF to consider
its preferences when determining data-traffic distribution in the downlink direction, thereby
providing increased control and flexibility to the UE in optimizing its network experience.
J. Mar. Sci. Eng. 2023,11, 2106 14 of 24
It is important to note that the Steering Mode Indicator from ATSSS/N4 rules includes only
one parameter, either the autonomous load-balance indicator or the UE-assistance indicator.
Furthermore, ATSSS Phase 2 introduces a new element called Threshold Values within
the ATSSS/N4 rules. This information establishes specific conditions for adjusting the
data-traffic distribution for the load-balancing steering mode with fixed split percentages
and the priority-based steering mode. A threshold value can represent either an RTT
value or a Packet Loss Rate (PLR), and it applies to both access networks. Both the UE
and the UPF utilize these threshold values. For instance, in the load-balancing steering
mode with fixed split percentages, if at least one measured parameter (either RTT or PLR)
on one access network surpasses the designated threshold value, the UE and UPF may
cease sending data traffic on that access network or reduce the data traffic on that access
network in a manner specific to the implementation. Consequently, the reduced data-traffic
amount is redistributed to the other access network. Importantly, it should be noted that
the Steering Mode Indicator and the Threshold Values are not concurrently included within
the ATSSS/N4 rules.
To enhance the precision of QoS flow control, ATSSS Phase 2 introduces the capability
to conduct performance measurements, such as RTT and PLR measurements, over the same
QoS flow used to carry the data flow. This improvement addresses the challenges associated
with less-optimized control that stem from relying on rough estimates of RTT/PLR, which
are obtained through measurements over the default QoS flow in ATSSS Phase 1. The
network determines whether measurements are conducted ‘over the default QoS flow’ or
‘over a certain QoS flow’ during MA PDU session establishment.
Regarding the configuration of the connectivity between access networks and core
networks, ATSSS Phase 1 establishes connections between both access networks and the
5GC, which serves as the core network for the 5G system. ATSSS Phase 2 extends support
for a different connectivity scenario, where a 3GPP access network is linked to the EPC, the
core network of the LTE system, while a non-3GPP access network is connected to the 5GC.
4.1.3. ATSSS Phase 3
ATSSS Phase 3 feature is currently undergoing standardization within the 5G-Advanced
framework in 3GPP, with a target completion date set for March 2024. The architectural
aspects have already been fully specified in Stage 2 specifications, which were finalized
in June 2023. Building upon the achievements in architectural specification, this section
provides an in-depth exploration of the four noteworthy enhancements introduced in
ATSSS Phase 3 [75,76].
First, ATSSS Phase 3 introduces a new steering mode known as the redundant steering
mode integrated into both the UE and the UPF. This steering mode enables the distribution
of duplicated data traffic between both a 3GPP access network and a non-3GPP access
network. It offers the flexibility to seamlessly suspend or resume this distribution through
a message transmitted via the user plane of any available access network within the MA
PDU session. Additionally, this steering mode operates efficiently, whether configured
with or without Threshold Values for RTT and PLR. It is specifically designed for non-GBR
service data flow and is not configured with ATSSS-LL steering functionality.
The roaming scenario involving network entities (i.e., Non-3GPP InterWorking Func-
tion (N3IWF) or Trusted Non-3GPP Gateway Function (TNGF)) among non-3GPP access
networks is designed to facilitate the transition of data-traffic distribution within the MA
PDU session. This transition shifts the data flow from the user-plane resource of an existing
non-3GPP access network to that of a new non-3GPP access network, while keeping the
data-traffic distribution over the 3GPP access network connected for the MA PDU session.
When both the UE and the 5G network components (i.e., AMF, SMF) have the capability
to switch the non-3GPP access path, such path redirection becomes achievable through
the procedure of the mobility registration update initiated by the UE over the user-plane
resource of the new non-3GPP access network. Furthermore, this capability for non-3GPP
J. Mar. Sci. Eng. 2023,11, 2106 15 of 24
access-path switching extends to the user-plane resources of non-3GPP access networks,
even in the context of a single-access PDU session.
In pursuit of enhancing the configuration of connectivity to the core network, ATSSS
Phase 3 introduced additional improvements aimed at facilitating the MA PDU session
over a 3GPP access network linked to the 5GC and non-3GPP access network connected to
the evolved Packet Data Gateway (ePDG) [77,78] via the EPC.
In addition to two steering functionalities introduced in ATSSS Phase 1, namely
MPTCP functionality and ATSSS-LL functionality, a new steering functionality referred
to as Multi-Path Quick UDP Internet Connection (MP-QUIC) has been defined to steer,
switch and split UDP traffic flows according to ATSSS/N4 rules based on policies decided
by the Policy Control Function (PCF). The MP-QUIC steering functionality consists of three
components as follows:
QoS flow and steering mode selection: Following the establishment of an MA PDU
session, this component within the UE initiates the setup of one or more multipath
QUIC connections. The UE and UPF components then determine the QoS flow, steering
mode, and transport mode based on ATSSS/N4 rules, respectively;
HTTP/3 layer: It supports the HyperText Transfer Protocol version 3 (HTTP/3)
HTTP/3 protocol [
79
], along with extensions for UDP proxying over HTTP [
80
],
HTTP datagrams [
81
], and extended CONNECT [
82
]. This layer selects a multipath
QUIC connection to be used for each UDP flow and allocates a new QUIC stream on
this connection linked with UDP flow;
QUIC layer: It supports the QUIC protocol [
83
85
] and the extensions [
86
,
87
] defined
by the Internet Engineering Task Force (IETF) [88].
As depicted in Figure 7, the MP-QUIC steering functionality within the UE com-
municates with a corresponding MP-QUIC proxy steering functionality in the UPF. This
communication employs the QUIC protocol and its multipath extensions over the HTTP/3.
Furthermore, a new indicator, called Transport Mode, is introduced exclusively for MP-
QUIC steering functionality. ATSSS/N4 rules specify one of three transport modes to
determine how a UDP flow is transmitted between the UE and UPF according to the
selected transport mode:
Datagram mode 1 (An extension of the mode defined in [
80
]): This mode encapsulates
UDP packets within QUIC Datagram frames, providing unreliable transport with
sequence numbering and packet reordering/duplication;
Datagram mode 2 [
80
]: This mode encapsulates UDP packets within QUIC Data-
gram frames, offering unreliable transport without sequence numbering or packet
reordering/duplication;
Stream mode: This mode encapsulates UDP packets within QUIC Stream frames, deliv-
ering reliable transport with sequence numbering and packet reordering/duplication.
It ensures strict reliability and in-order delivery with retransmission, which is particu-
larly suitable for applications lacking their own reliability mechanism.
4.2. Simultaneous Utilization of Two 3GPP Access Networks and Future Enhancements
The ATSSS feature was initially designed as a transport-layer solution with a specific
focus on concurrently utilizing user-plane resources across both a 3GPP access network
(such as NG-RAN or E-UTRAN) and a non-3GPP access network (like WLAN), all without
causing any disruptions to the underlying radio access networks. This concept of optimiz-
ing user-plane resources across multiple RANs with different RATs can be extended to
encompass access networks specified exclusively within the framework of 3GPP standard-
ization. The demand for this expansion has become more pressing due to the limitations
of non-3GPP access networks such as WLAN in enhancing communication coverage with
secure reliability and Quality of Experience (QoE) in certain use cases (e.g., maritime usage).
Moreover, as the market evolves, there is an expectation of the coexistence of multiple
3GPP access networks, including NG-RAN, E-UTRAN, and 5G LEO/GEO satellite access
networks, within the framework of 5G-Advanced and future systems.
J. Mar. Sci. Eng. 2023,11, 2106 16 of 24
From the perspective of the maritime sector and considering the diverse wireless com-
munication environments that ships encounter during global navigation, the imperative of
simultaneously utilizing radio resources of multiple RANs, including LEO/GEO satellites,
E-UTRAN, and NG-RAN, becomes even more pronounced, as illustrated in Figure 8. This
is a fundamental requirement for the digitalization of the maritime sector, including au-
tonomous ships, to ensure the secure provision of seamless ubiquitous connectivity within
a single subscription provided by a mobile operator with roaming services, facilitated
through mutual agreements with other mobile or satellite operators.
J. Mar. Sci. Eng. 2023, 11, x FOR PEER REVIEW 16 of 25
Figure 7. Protocol stack of the user plane with MP-QUIC steering functionality within the Release
18 framework of 3GPP standardization (Adapted from Figure 5.32.6.2.2-1 of 3GPP TS 23.501 V18.1.0
[89]).
4.2. Simultaneous Utilization of Two 3GPP Access Networks and Future Enhancements
The ATSSS feature was initially designed as a transport-layer solution with a specic
focus on concurrently utilizing user-plane resources across both a 3GPP access network
(such as NG-RAN or E-UTRAN) and a non-3GPP access network (like WLAN), all with-
out causing any disruptions to the underlying radio access networks. This concept of op-
timizing user-plane resources across multiple RANs with dierent RATs can be extended
to encompass access networks specied exclusively within the framework of 3GPP stand-
ardization. The demand for this expansion has become more pressing due to the limita-
tions of non-3GPP access networks such as WLAN in enhancing communication coverage
with secure reliability and Quality of Experience (QoE) in certain use cases (e.g., maritime
usage). Moreover, as the market evolves, there is an expectation of the coexistence of mul-
tiple 3GPP access networks, including NG-RAN, E-UTRAN, and 5G LEO/GEO satellite
access networks, within the framework of 5G-Advanced and future systems.
From the perspective of the maritime sector and considering the diverse wireless
communication environments that ships encounter during global navigation, the impera-
tive of simultaneously utilizing radio resources of multiple RANs, including LEO/GEO
satellites, E-UTRAN, and NG-RAN, becomes even more pronounced, as illustrated in Fig-
ure 8. This is a fundamental requirement for the digitalization of the maritime sector, in-
cluding autonomous ships, to ensure the secure provision of seamless ubiquitous connec-
tivity within a single subscription provided by a mobile operator with roaming services,
facilitated through mutual agreements with other mobile or satellite operators.
Figure 7.
Protocol stack of the user plane with MP-QUIC steering functionality within the Release 18
framework of 3GPP standardization (Adapted from Figure 5.32.6.2.2-1 of 3GPP TS 23.501 V18.1.0 [
89
]).
Figure 8.
Example of available multiple 3GPP access networks including 5G satellite access ((Adapted
from Figure 5.10-1 of 3GPP TR 22.841 V2.0.0 [90]).
J. Mar. Sci. Eng. 2023,11, 2106 17 of 24
Lately, 3GPP has been investigating pertinent use cases that bring about new service
requirements, requiring corresponding transport-layer solutions to support the utilization
of user-plane resources across two 3GPP access networks [
91
]. With a single subscription,
it is considered that two 3GPP access networks are connected to the same Public Land
Mobile Network (PLMN) [
92
94
], two different PLMNs, or a PLMN and a Non-Public
Network (NPN) [
95
] called a private network on the market. In addition, two 3GPP
access networks may involve combinations such as NG-RAN and 5G LEO satellite access
networks, E-UTRAN and 5G LEO satellite access networks, or 5G LEO and GEO satellite
access networks.
In the traditional approach, data-traffic distribution within the 3GPP standardization
framework relies on the utilization of user-plane resources from a single-access network.
This implies that existing procedures may need to be reconsidered to enable the simultane-
ous utilization of user-plane resources from two separate 3GPP access networks without
disrupting RAN entities. Looking ahead to the progress of 3GPP standardization in Release
19, there is an expectation of further enhancements. These enhancements are expected to
continue into Release 20, which is considered the inaugural version of the 6G communica-
tion system within the 3GPP framework.
5. Opportunities and Challenges
The simultaneous use of multiple RANs within a unified communication system
offers distinct advantages for autonomous ships navigating in the maritime environment
with heterogeneous multiple RANs. This becomes especially pronounced during global
navigation, where the secure connectivity to on-land entities assumes a pivotal role in
monitoring vessel status, tracking location or enabling remote control. The significance
of this connectivity is further underscored by the variations in network infrastructure
capabilities across different countries. The 6G communication systems are poised to
provide a decisive advantage by offering both terrestrial and non-terrestrial networks,
serving as indispensable components for ensuring ubiquitous connectivity in the maritime
communication landscape.
However, it is important to note that legacy maritime communications, such as VHF-
based communication, have endured well beyond legacy cellular communications. This
longevity can be attributed to their extended communication coverage capability stemming
from their frequency characteristics, despite their relatively low performance. This extended
lifespan presents a unique set of challenges that demand meticulous consideration during
the transition from traditional maritime communication methods to the cutting-edge realm
of 6G communications. The latter enables continuous advancements in the concurrent
utilization of multiple 3GPP access networks over 5G and beyond systems, optimizing the
distribution of data traffic for autonomous ships during their navigation in the ocean, in
port areas, or on inland rivers.
As the maritime sector embarks on this transformative journey, thoughtful planning
and collaboration among relevant stakeholders, technology providers, and regulators will
be essential to explore these opportunities and address challenges. In this section, we
provide insightful exploration about opportunities and challenges for further research in
this continually evolving landscape.
5.1. Integration of Autonomous Ships as Mobile RAN Entities into the 6G Framework
The incorporation of autonomous ships into mobile RAN entities leveraged by ATSSS
features presents a substantial opportunity to enhance maritime communication environ-
ments, particularly in cases where deploying essential network infrastructures is challeng-
ing compared to terrestrial communication environments. This integration contributes to
extending network coverage and capabilities, fostering a more adaptable and intercon-
nected communication ecosystem in the maritime environment in addition to the inclusion
of UAVs [
48
,
49
] and more enhanced satellite components as mobile RAN entities within
the framework of forthcoming 6G standardization. Moreover, autonomous ships can be
J. Mar. Sci. Eng. 2023,11, 2106 18 of 24
integrated as network nodes serving as a relay node, like vehicles acting as mobile RAN
entities which are exampled by the mobile Integrated Access and Backhaul (IAB) nodes
mounted on vehicles, as outlined within the 3GPP standardization framework [96,97].
Mobile objects like UAVs and vehicles initially served as UE and later evolved into
mobile RAN entities to enhance coverage and connectivity in the 3GPP framework. Simi-
larly, autonomous ships are poised to revolutionize the concept of RAN entities in maritime
communication environments, albeit with distinctive challenges compared to UAVs and
vehicles, primarily stemming from the various sizes of ships, their specific movement
patterns, and the intricacies of their navigation route, including the number of people
on board.
The conventional cellular network model, known for its characteristic communication
coverage areas resembling hexagons or circles, with signal strength gradually diminishing
from the center outward, has long served as the foundation for maximizing network capac-
ity. When integrating autonomous ships into the realm of mobile RAN entities, applying
this traditional approach becomes intricate. Within the maritime communication environ-
ment, especially with ships of substantial length (e.g., several hundred meters), relying on
the traditional hexagonal or circular shape-based approach may prove suboptimal. Further-
more, the bulk of data traffic may primarily take place within the confines of a ship, with
minimal data traffic extending beyond the ship’s boundaries especially for ships navigating
in oceans such as cruise ships. This suggests a potential need for high capacity of the
network infrastructure in specific areas of the ship, and temporarily along the navigation
route, like a stadium where tens of thousands of people watch concerts together for a few
hours, using their mobile phones for various data exchanges with friends in the stadium
or with family members at home. This can give rise to challenges such as sudden surges
in data traffic along the ship’s navigation routes. Consequently, the adoption of advanced
technologies, such as ATSSS features enabling the simultaneous and efficient utilization of
multiple RANs, including mobile RAN entities such as satellites, UAVs and autonomous
ships, becomes critically necessary to efficiently manage the temporary data-traffic surges
that occur along the ship’s route, within the framework of 3GPP standardization.
5.2. Integration of Legacy Maritime Communications into the 6G Framework
Given the enduring presence of legacy maritime communications, it is evident that
multiple RANs are poised to coexist within the maritime sector, leading to an escalation in
RAN heterogeneity. Thus, it becomes imperative to harness these legacy maritime commu-
nications as non-3GPP access networks, enabling the concurrent use of them together with
3GPP RANs available within the context of the 6G framework. This linkage can empower
autonomous ships to establish connections with external networks, such as the internet,
thereby facilitating access to a diverse range of services during their navigation. The inte-
gration of legacy maritime communications into the context of the 6G framework presents
a landscape brimming with both promising opportunities and formidable challenges.
One of the most compelling prospects lies in the potential to significantly enhance
connectivity at sea. Traditional maritime communication systems have often grappled with
limitations in bandwidth and coverage. However, integrating them into the 6G ecosystem,
alongside mobile RANs with the capability of ATSSS features, enables ships to harness
ultra-high-speed data transmission and reliable communication, even in the open ocean.
This enhancement serves as a cornerstone for modern maritime operations, spanning ship-
ping, fishing, research endeavors, and emergency response initiatives. Another noteworthy
opportunity arises from the convergence of the Internet of Things (IoT) and sensor technolo-
gies within the maritime sector. This synergy, facilitated by the seamless amalgamation of
legacy systems with 6G capabilities, paves the way for real-time data transmission and mon-
itoring. Ships can seamlessly gather and analyze multifaceted data, encompassing aspects
such as weather conditions, equipment status, and cargo management. This data-centric
approach holds the potential to significantly enhance safety, fortify security measures, and
streamline operational efficiency. Furthermore, the integration process can be orchestrated
J. Mar. Sci. Eng. 2023,11, 2106 19 of 24
with minimal disruption to the existing infrastructure. This approach ensures a seamless
transition, allowing maritime organizations to harness their previous investments while
reaping the benefits of 6G connectivity. Moreover, the potential of 6G to deliver ‘ubiquitous
connectivity’ and ‘connecting unconnected’ carries profound implications for the maritime
sector. Ships can maintain uninterrupted communication, irrespective of their location,
thereby promoting safer navigation and more streamlined logistics.
However, in tandem with these opportunities, a series of notable challenges emerges.
Foremost among these is the critical issue of interoperability. The integration of legacy
maritime communications into the 6G framework presents a formidable task in terms of
achieving seamless communication between different generations of technology. Older
equipment may necessitate adjustments or upgrades to ensure compatibility with the 6G
infrastructure. Navigating the complex regulatory landscape stands as another substantial
challenge. The maritime sector operates within a multifaceted regulatory framework, and
adapting to the intricacies of 6G may demand alterations in spectrum allocation, licensing
protocols, and compliance with international maritime regulations. Effectively addressing
these regulatory challenges is pivotal to a successful integration. Additionally, as connec-
tivity intensifies, an augmented need for cybersecurity becomes apparent. Safeguarding
maritime communication networks against cyber threats takes on paramount significance
to uphold the safety and integrity of sea operations. Legacy maritime communications
may lack the robust security features intrinsic to 6G, necessitating a meticulous approach
to planning and executing cybersecurity upgrades.
5.3. Variability in Maritime Channel Measurements Affected by Climate Change
The channel measurements such as RTT or PLR are used to select RANs among
multiple RANs simultaneously utilized in the context of ATSSS features despite having no
impact on RAN aspects. In the realm of communication systems, channel measurement
metrics such as RTT serve as standard criteria for assessing the suitability of one method
over another. Moreover, statistical channel models often stem from these fundamental
channel measurements. In the context of 6G communication within the maritime sector,
several studies explored the development of 6G channel models, considering the distinctive
characteristics that arise from the maritime communication environment, which differs
significantly from terrestrial communication environments [
3
,
98
100
]. When examining the
data in [
101
] (Figures 16 and 18), it becomes apparent that wireless channel conditions in
the maritime communication environment tend to stay relatively stable on sunny and calm
days. In contrast, they show significant variability on windy days, primarily attributed to
the presence of high waves, even in coastal regions. This variability implies that channel
models may not sustain consistent effectiveness, even within the same maritime region
geographically. The outcome of channel measurements is intrinsically dynamic, influenced
by specific conditions like weather or the increasingly unpredictable temperature patterns
resulting from the significant effects of climate change currently experienced by the global
community. This highlights a disconnect between the current communication environment
and what lies ahead in the maritime sector. As a result, channel models developed based on
channel measurement during the research phase may not be optimally suited for real-world
deployment scenarios of the maritime communication environment, which usually come
to be implemented in at least five years to a decade after the research phase.
In recent times, there has been a remarkable surge in interest surrounding the utiliza-
tion of Artificial Intelligence (AI) technologies for channel modeling within the academic
research community [
102
104
]. Additionally, the 3GPP standards body has undertaken
the exploration of AI and Machine Learning (ML) technologies to enhance the overall
functionalities of 5G and beyond systems. This exploration encompasses AI/ML-based
network energy conservation, data-traffic load balancing, and mobility optimization within
3GPP systems, while considering the dynamic nature of air interfaces [105,106].
Emphasizing the critical nature of acquiring a substantial volume of actual channel-
measurement data for AI/ML-based approaches is imperative. In the maritime sector,
J. Mar. Sci. Eng. 2023,11, 2106 20 of 24
autonomous ships can play a pivotal role in collecting this data, spanning diverse weather
conditions, including severe weather situations when conventional navigation is typically
constrained. Furthermore, it is worth noting that the maritime communication environment
varies significantly across different regions, such as polar sea regions, earthquake-prone
areas, open-ocean expanses, and coastal regions. This variability underscores the challenges
of creating a single universal channel model of a 6G communication system capable of
providing ubiquitous connectivity in all locations, either terrestrial or non-terrestrial areas.
The availability of a comprehensive global dataset of real channel measurements collected
in the maritime communication environment becomes increasingly crucial. It will enable
the development of optimized channel models integrated with AI/ML technologies for the
maritime sector within the context of the 6G framework. These models can be considered
when global standards bodies like 3GPP work towards establishing comprehensive 6G
channel model frameworks, catering to scenarios encompassing space, air, and sea as well
as terrestrial environments.
6. Conclusions
As we embark on the journey towards the next generation of mobile communication
systems, notably 6G, it has become imperative to seamlessly integrate the demands of
digitalization and mobilization within the maritime sector, particularly for autonomous
ships, within the framework of 6G standardization. This integration holds paramount
significance due to the myriad of advantages that the 6G framework offers, centered around
the establishment of unified communication ecosystems, built upon the widely embraced
3GPP standards that have come to dominate in market for the digital revolution of various
industries since the advent of 5G. Notably, the realization of ‘ubiquitous connectivity’ in 6G
is poised to play a pivotal role in expanding communication coverage with secure reliability
and optimal QoE in the maritime communication environment, a critical requirement for
autonomous ships.
This article contributes significantly by providing a comprehensive overview of the
integration of satellites and UAVs as mobile RANs into 5G and beyond systems, which are
envisioned as integral components of the 3GPP access networks supporting the simultane-
ous utilization of multiple RANs in the forthcoming 6G era, within the framework of 3GPP
standardization. Additionally, we have explored the ATSSS features within 3GPP as a trans-
port layer-level solution enabling the seamless steering, switching, and splitting of data
traffic across multiple access networks over the core networks of 5G and beyond systems.
Given the unique characteristics arising from the maritime communication environ-
ment and the ships themselves, we have highlighted the opportunities and challenges for
further exploration and research as we look ahead to the forthcoming 6G era. To succeed
in the 6G-based maritime evolution, it is imperative to foster close collaboration among
maritime stakeholders including authorities responsible for vessel traffic management and
control, and maritime safety. Additionally, the collaboration between maritime stakehold-
ers and 3GPP experts is vital in developing the relevant requirements that align with the
maritime sector’s needs. These requirements should be meticulously formulated with
substantial input from key stakeholders capable of representing the global maritime sector,
including authorities in charge of vessel traffic management and control, and maritime
safety, etc., considering both maritime safety and commercial usage perspectives. This col-
laborative effort serves as a pivotal milestone towards establishing more efficient, adaptable,
and reliable communication within the 6G ecosystem for the maritime sector.
Author Contributions:
Conceptualization, H.K., C.R.; formal analysis, H.K.; investigation, H.K.;
writing—original draft preparation, H.K.; writing—review and editing, H.K., C.R., W.K.; visualization,
H.K.; supervision, W.K.; project administration, H.K. All authors have read and agreed to the
published version of the manuscript.
Funding:
This research was supported by the ‘Development of Autonomous Ship Technology
(20200615)’ funded by the Ministry of Oceans and Fisheries (MOF, Republic of Korea).
J. Mar. Sci. Eng. 2023,11, 2106 21 of 24
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement:
No new data were created or analyzed in this study. Data sharing is
not applicable to this article.
Conflicts of Interest:
The funders had no role in the design of the study; in the collection, analyses,
or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
References
1.
Convention on the International Maritime Satellite Organization (IMSO). Available online: https://www.imo.org/en/About/
Conventions/Pages/Convention-on-the-International- Maritime-Satellite-Organization.aspx (accessed on 17 September 2023).
2.
Ericsson Mobility Report, June 2023. Available online: https://www.ericsson.com/49dd9d/assets/local/reports-papers/
mobility-report/documents/2023/ericsson-mobility-report-june-2023.pdf (accessed on 17 September 2023).
3. 3GPP. Available online: www.3gpp.org (accessed on 14 August 2023).
4.
Changing 3GPP to Help the Verticals. February 2020. Available online: https://vimeo.com/393459786 (accessed on
17 September 2023).
5.
5G Vertical Use Cases, 5G Americas. October 2021. Available online: https://www.5gamericas.org/wp-content/uploads/2021/1
0/5G-Vertical-Use-Cases-WP-InDesign.pdf (accessed on 17 September 2023).
6.
Enterprise Evolution with 5G Adoption, 5G America. June 2023. Available online: https://www.5gamericas.org/wp-content/
uploads/2023/01/Enterprise-Evolution-with-5G- Adoption-Id.pdf (accessed on 17 September 2023).
7.
3GPP TR 21.915 for Release 15. Available online: https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.
aspx?specificationId=3389 (accessed on 15 August 2023).
8.
3GPP Release 18 Overview: A World of 5G-Advanced. 2 February 2023. Available online: https://www.atis.org/?smd_process_
download=1&download_id=1732913 (accessed on 17 September 2023).
9.
IMT Towards 2030 and Beyond. Available online: https://www.itu.int/en/ITU-R/study-groups/rsg5/rwp5d/imt-2030/Pages/
default.aspx (accessed on 14 August 2023).
10.
Maritime Communication Services over 5G Systems. 19 December 2018. Available online: https://www.3gpp.org/news-events/
3gpp-news/maritime2 (accessed on 17 September 2023).
11.
3GPP TR 22.819, Feasibility Study on Maritime Communication Services over 3GPP System. Available online: https://www.
3gpp.org/ftp//Specs/archive/22_series/22.819/22819-g20.zip (accessed on 17 September 2023).
12.
3GPP TS 22.119, Maritime Communication Services over 3GPP System (Release 16). Available online: https://www.3gpp.org/
ftp//Specs/archive/22_series/22.119/22119-g20.zip (accessed on 17 September 2023).
13. ITU-R. Available online: https://www.itu.int/en/ITU-R/information/Pages/default.aspx (accessed on 14 August 2023).
14. Pollini, G.P. Trends in handover design. IEEE Commun. Mag. 1996,34, 82–90. [CrossRef]
15.
Latva-aho, M.; Leppänen, K. Key Drivers and Research Challenges for 6G Ubiquitous Wireless Intelligence; University of Oulu: Oulu,
Finland, 2019.
16.
Ahmad, I.; Shahabuddin, S.; Kumar, T.; Okwuibe, J.; Gurtov, A.; Ylianttila, M. Security for 5G and Beyond. IEEE Commun. Surv.
Tutor. 2019,21, 3682–3722. [CrossRef]
17.
Zhang, Z.; Xiao, Y.; Ma, Z.; Xiao, M.; Ding, Z.; Lei, X.; Karagiannidis, G.K.; Fan, P. 6G Wireless Networks: Vision, Requirements,
Architecture, and Key Technologies. IEEE Veh. Technol. Mag. 2019,14, 28–41. [CrossRef]
18.
Polese, M.; Jornet, J.M.; Melodia, T.; Zorzi, M. Toward End-to-End, Full-Stack 6G Terahertz Networks. IEEE Commun. Mag.
2020,58, 48–54. [CrossRef]
19.
Vanelli-Coralli, A.; Guidotti, A.; Foggi, T.; Colavolpe, G.; Montorsi, G. 5G and Beyond 5G Non-Terrestrial Networks: Trends and
research challenges. In Proceedings of the 2020 IEEE 3rd 5G World Forum (5GWF), Bangalore, India, 10–12 September 2020.
20.
Leyva-Mayorga, I.; Soret, B.; Röper, M.; Wübben, D.; Matthiesen, B.; Dekorsy, A.; Popovski, P. LEO Small-Satellite Constellations
for 5G and Beyond-5G Communications. IEEE Access 2020,8, 184955–184964. [CrossRef]
21.
Lu, Y.; Zheng, X. 6G: A survey on technologies, scenarios, challenges, and the related issues. J. Ind. Inf. Integr.
2020
,19, 100158.
[CrossRef]
22.
Chowdhury, M.Z.; Shahjalal, M.; Ahmed, S.; Jang, Y.M. 6G Wireless Communication Systems: Applications, Requirements,
Technologies, Challenges, and Research Directions. IEEE Open J. Commun. Soc. 2020,1, 957–975. [CrossRef]
23.
Imoize, A.L.; Adedeji, O.; Tandiya, N.; Shetty, S. 6G Enabled Smart Infrastructure for Sustainable Society: Opportunities,
Challenges, and Research Roadmap. Sensors 2021,21, 1709. [CrossRef]
24.
Alqurashi, F.S.; Trichili, A.; Saeed, N.; Ooi, B.S.; Alouini, M.-S. Maritime Communications: A Survey on Enabling Technologies,
Opportunities, and Challenges. IEEE Internet Things J. 2022,10, 3525–3547. [CrossRef]
25.
Höyhtyä, M.; Martio, J. Integrated Satellite–Terrestrial Connectivity for Autonomous Ships: Survey and Future Research
Directions. Remote Sens. 2020,12, 2507. [CrossRef]
26.
Wang, J.-B.; Zeng, C.; Ding, C.; Zhang, H.; Lin, M.; Wang, J. Unmanned Surface Vessel Assisted Maritime Wireless Communication
Toward 6G: Opportunities and Challenges. IEEE Wirel. Commun. 2022,29, 72–79. [CrossRef]
J. Mar. Sci. Eng. 2023,11, 2106 22 of 24
27.
Nomikos, N.; Gkonis, P.K.; Bithas, P.S.; Trakadas, P. A Survey on UAV-Aided Maritime Communications: Deployment Considera-
tions, Applications, and Future Challenges. IEEE Open J. Commun. Soc. 2022,4, 56–78. [CrossRef]
28.
Saafi, S.; Vikhrova, O.; Fodor, G.; Hosek, J.; Andreev, S. AI-Aided Integrated Terrestrial and Non-Terrestrial 6G Solutions for
Sustainable Maritime Networking. IEEE Netw. 2022,36, 183–190. [CrossRef]
29.
Fang, X.; Feng, W.; Wang, Y.; Chen, Y.; Ge, N.; Ding, Z.; Zhu, H. NOMA-Based Hybrid Satellite-UAV-Terrestrial Networks for 6G
Maritime Coverage. IEEE Trans. Wirel. Commun. 2023,22, 138–152. [CrossRef]
30.
Alsos, O.A.; Hodne, P.; Skåden, O.K.; Porathe, T. Maritime Autonomous Surface Ships: Automation Transparency for Nearby
Vessels. In Journal of Physics: Conference Series; IOP Publishing: Bristol, UK, 2022; Volume 2311.
31.
ITU-R Working Party 5D (WP 5D)—IMT Systems. Available online: https://www.itu.int/en/ITU-R/study-groups/rsg5/rwp5
d/Pages/default.aspx# (accessed on 14 August 2023).
32.
Relationship and Timelines. Available online: https://www.itu.int/en/ITU-R/study-groups/rsg5/rwp5d/imt-2030
/Documents/PDF_Relationship%20and%20Timelines.pdf (accessed on 14 August 2023).
33.
3GPP TR 21.916 for Release 16. Available online: https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.
aspx?specificationId=3493 (accessed on 15 August 2023).
34.
3GPP TR 21.917 for Release 17. Available online: https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.
aspx?specificationId=3937 (accessed on 15 August 2023).
35.
Suer, M.T.; Thein, C.; Tchouankem, H.; Wolf, L. Multi-Connectivity as an Enabler for Reliable Low Latency Communications—An
Overview. IEEE Commun. Surv. Tutor. 2019,22, 156–169. [CrossRef]
36.
Lin, X.; Cioni, S.; Charbit, G.; Chuberre, N.; Hellsten, S.; Boutillon, J.F. On the Path to 6G: Embracing the Next Wave of Low Earth
Orbit Satellite Access. IEEE Commun. Mag. 2021,59, 36–42. [CrossRef]
37.
Rinaldi, F.; Maattanen, H.L.; Torsner, J.; Pizzi, S.; Andreev, S.; Iera, A.; Koucheryavy, Y.; Araniti, G. Non-Terrestrial Networks in
5G & Beyond: A Survey. IEEE Access 2020,8, 165178–165200.
38.
Using 3GPP Technology for Satellite Communication; Ericsson Technology Review. Available online: https://www.ericsson.com/
49bb0c/assets/local/reports-papers/ericsson-technology-review/docs/2023/3gpp-satellite-communication.pdf (accessed on
15 August 2023).
39. Lin, X. An Overview of 5G Advanced Evolution in 3GPP Release 18. IEEE Commun. Stand. Mag. 2022,6, 77–83. [CrossRef]
40.
Dogra, A.; Jha, R.K.; Jain, S. A Survey on Beyond 5G Network With the Advent of 6G: Architecture and Emerging Technologies.
IEEE Access 2020,9, 67512–67547. [CrossRef]
41.
3GPP TR 23.737, Study on Architecture Aspects for Using Satellite Access in 5G (Release 16). Available online: https://portal.
3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=3485 (accessed on 16 August 2023).
42.
3GPP TR 38.821, Solutions for NR to Support Non-Terrestrial Networks (NTN) (Release 16). Available online: https://portal.
3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=3525 (accessed on 16 August 2023).
43.
SWS-230081, Satellite Access—SA Rel-19 Topics and Priorities. Available online: https://www.3gpp.org/ftp/tsg_sa/TSG_SA/
Workshops/2023-06-13_Rel-19_WorkShop/Docs/SWS-230081.zip (accessed on 16 August 2023).
44.
RWS-230048, Consideration on RAN1/2/3 led NTN Topics for Release 19. Available online: https://www.3gpp.org/ftp/TSG_
RAN/TSG_RAN/TSGR_AHs/2023_06_RAN_Rel19_WS/Docs/RWS-230048.zip (accessed on 16 August 2023).
45.
RWS-230049, Consideration on RAN4 led NTN Topics for Release 19. Available online: https://www.3gpp.org/ftp/TSG_RAN/
TSG_RAN/TSGR_AHs/2023_06_RAN_Rel19_WS/Docs/RWS-230049.zip (accessed on 16 August 2023).
46. Araniti, G.; Iera, A.; Pizzi, S.; Rinaldi, F. Toward 6G Non-Terrestrial Networks. IEEE Netw. 2021,36, 113–120. [CrossRef]
47.
Chen, W.; Lin, X.; Lee, J.; Toskala, A.; Sun, S.; Chiasserini, C.F.; Liu, L. 5G-Advanced toward 6G: Past, Present, and Future. IEEE J.
Sel. Areas Commun. 2023,41, 1592–1619. [CrossRef]
48.
Geraci, G.; Garcia-Rodriguez, A.; Azari, M.M.; Lozano, A.; Mezzavilla, M.; Chatzinotas, S.; Chen, Y.; Rangan, S.; Di Renzo, M. What
Will the Future of UAV Cellular Communications Be? A Flight From 5G to 6G. IEEE Commun. Surv. Tutor.
2022
,24, 1304–1335.
[CrossRef]
49.
Abdalla, A.S.; Marojevic, V. Communications Standards for Unmanned Aircraft Systems: The 3GPP Perspective and Research
Drivers. IEEE Commun. Stand. Mag. 2021,5, 70–77. [CrossRef]
50.
Ullah, Z.; Al-Turjman, F.; Mostarda, L. Cognition in UAV-Aided 5G and Beyond Communications: A Survey. IEEE Trans. Cogn.
Commun. Netw. 2020,6, 872–891. [CrossRef]
51.
Mishra, D.; Natalizio, E. A survey on cellular-connected UAVs: Design challenges, enabling 5G/B5G innovations, and experimen-
tal advancements. Comput. Netw. 2020,182, 107451. [CrossRef]
52.
3GPP TS 22.125, Unmanned Aerial System (UAS) Support in 3GPP; Stage 1 (Release 17). Available online: https://www.3gpp.
org/ftp/Specs/archive/22_series/22.125/22125-h60.zip (accessed on 4 September 2023).
53.
3GPP TS 23.256, Support of Uncrewed Aerial Systems (UAS) Connectivity, Identification, and Tracking; Stage 2 (Release 17).
Available online: https://www.3gpp.org/ftp/Specs/archive/23_series/23.256/23256-h60.zip (accessed on 4 September 2023).
54.
3GPP TS 23.255, Application Layer Support for Uncrewed Aerial System (UAS); Functional Architecture and Information
Flows (Release 17). Available online: https://www.3gpp.org/ftp/Specs/archive/23_series/23.255/23255-h50.zip (accessed on
4 September 2023).
55.
3GPP TS 24.257, Uncrewed Aerial System (UAS); Application Enabler (UAE) layer; Protocol Aspects; Stage 3 (Release 17).
Available online: https://www.3gpp.org/ftp/Specs/archive/24_series/24.257/24257-h30.zip (accessed on 4 September 2023).
J. Mar. Sci. Eng. 2023,11, 2106 23 of 24
56.
3GPP TS 29.257, Application Layer Support for Uncrewed Aerial System (UAS); UAS Application Enabler (UAE) Server
Services (Release 17). Available online: https://www.3gpp.org/ftp/Specs/archive/29_series/29.257/29257-h10.zip (accessed