Time Variant Routing D. King
Internet-Draft Lancaster University
Intended status: Informational K. Shortt
Airbus
Expires: July 18, 2023 January 17, 2023
Time Variant Challenges for Non-Terrestrial Networks
draft-king-tvr-ntn-challanges-00
Abstract
Advanced networks, including the Internet, will utilise an increasing
amount of Non-Terrestrial Network (NTN) infrastructure. NTNs include
Low Earth Orbit (LEO) satellites, High Altitude Long Endurance (HALE)
aviation, and High-Altitude Platform Stations (HAPS). In addition,
NTN infrastructure will facilitate the deployment of advanced 5G use
cases and services.
NTNs infrastructure is typically mobile, with various links and nodes
operating at different altitudes and latencies. Some NTN nodes and
links are temporal and need to be scheduled and established at
specific times based on line-of-sight availability, traffic demand
and power budgets.
This document summarises time variant NTN requirements and
challenges not met by existing routing and traffic engineering
techniques.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . .2
1.1 Terminology . . . . . . . . . . . . . . . . . . . . .5
2. 3GPP NTN Use Cases and Requirements . . . . . . . . . .6
2.1 Architecture . . . . . . . . . . . . . . . . . . . . .7
2.1.1 Physical Layer Control . . . . . . . . . . . . . . .7
2.1.2 Uplinks and Downlinks . . . . . . . . . . . . . . .8
2.1.3 Feeder Links . . . . . . . . . . . . . . . . . . . .8
2.2 Satellite Service Continuity . . . . . . . . . . . . .9
2.3 Satellite-based NG-RAN Architectures . . . . . . . . .9
2.4 NB-IoT and eMTC Support . . . . . . . . . . . . . . .10
3. Routing and Traffic Engineering Challenges for NTNs. . .10
3.1 Link and Routing Resilience for NTNs. . . . . . . . . .12
3.2 Multi-layer Networking in NTNs. . . . . . . . . . . . .13
4. NTN Management and Operation . . . . . . . . . . . . . .13
5. Security Considerations . . . . . . . . . . . . . . . . .14
6. IANA Considerations . . . . . . . . . . . . . . . . . . .14
7. Acknowledgements . . . . . . . . . . . . . . . . . . . .14
8. Contributors . . . . . . . . . . . . . . . . . . . . . .14
9. Informative References . . . . . . . . . . . . . . . . .14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . .15
1. Introduction
Exponential increases in Internet speed have facilitated an entirely
new set of applications and industry verticals underpinned by
evolving fixed network infrastructure. However, the costs of
deploying new fixed fibre networks are a limiting factor.
Therefore, as 5G and Internet infrastructure build-out continues,
we must look up, both figuratively and physically, to our next
networking enabler.
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Non-Terrestrial Network (NTN) infrastructure, including
Geosynchronous Equatorial Orbit, Low Earth Orbit (LEO) satellites
Starlink, Kuiper, and OneWeb), High Altitude Long Endurance (HALE)
aviation, and High-Altitude Platform Stations (HAPS) and Unmanned
Aerial Vehicles (UAV), are providing a significant role in
Internet communications in terms of both access and backhaul
services. These new networks will continue to increase in size and
scale.
The NTN definition has become an umbrella term for a network that
may involve non-terrestrial flying objects. Approximate altitudes
and latencies for NTN nodes, include:
o GEO 36,000 km (600-800 ms)
o MEO 20,000 km (120-300 ms)
o LEO 400 km (30-50 ms)
o HAPS 20km (<3 ms)
o HALE 10 km (<3 ms)
o UAV 1 km (<3 ms)
As defined in 3GPP [TR38.82118], a satellite-based NTN typically
feature the following elements:
o One or several satellite gateways that connect the
Non-Terrestrial Network to a public data network;
o A feeder link or radio link between a satellite-gateway and the
satellite;
o A service link or radio link between the user equipment (UE) and
the satellite.
Satellite nodes may have one of two orbits. Firstly, moving in a
circular orbit around the Earth. Secondly, keeping a notional
station with its position fixed in terms of azimuth to a given
Earth point.
NTN infrastructure provides tremendous potential in benefiting
the augmentation of terrestrial infrastructures in providing
flexible connectivity for a wide variety of use cases, including:
NG-Radio Access Network (NG-RAN), Enhanced Mobile Broadband
(eMBB) NTN, Internet of Things (IoT) NTN, Massive Machine-type
Communications (mMTC) NTN.
NTNs must also cooperate with the current terrestrial network
infrastructure (Integrated Space and Terrestrial Networks - ISTNs)
and exploit existing heterogeneous devices, systems and
networks. Thus, providing much more effective services than
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traditional Earth-based infrastructure, and greater reach and
coverage than proprietary and isolated NTN environments.
An NTN-based system will be compromised of end devices at different
altitude layers, each with a corresponding set of link
characteristics. For example, GEO satellites provide stable and
continuous links to ground devices with a considerable propagation
delay. In contrast, LEO satellites may be characterised by low-delay
interfaces but may suffer service discontinuity depending on the
constellation density. The type of service provided by each layer
will require specific link management and scheduling.
By their nature, GEO satellites differ from LEO satellites in terms
of location, connectivity, redundancy capabilities, antenna designs,
transceivers, operational frequency, and internal resources (e.g.,
hardening, storage, processing, and power availability).
The variance in design and capabilities of Unmanned Aerial Vehicles
(UAVs) is apparent with crewless aerial vehicles (HALE and HAPS), as
they are conceived for different purposes. In addition, they are
designed for varying use cases and environments and terminals whose
antennas range from small and isotropic to active ones capable of
tracking. The above further exacerbates the need for efficient link
and time management to guarantee a near-optimal use of resources
while leveraging overall heterogeneity.
Beyond current power-triggered procedures for link management,
specific NTN and asymmetric approaches will be required, which
must consider the handover direction, e.g., within a vertical
layer (within an LEO constellation or inter-HAPS) or across
technologies (ground-to-air/space or vice versa). Furthermore,
network topologies will be created based on anticipated traffic
patterns. Finally, links will be planned and scheduled based on
node liveliness, line-of-sight availability and link energy costs,
prioritising node energy conservation over link data rates.
In addition, link management policies must trade off reliability,
spectral and energy-efficient operation and load balancing, and
signaling overhead caused by conditional handover preparations,
planned outages, and radio or optical link failures.
In summary, NTN consists of mobile nodes, where the topology is
dynamic as nodes and links are removed and re-established due to
the nature of the devices. In space and aerial networks, without
fixed power sources, such as battery-operated or powered by wave,
wind and solar, node aliveness, and link availability will be
restricted and planned for in advance of traffic being forwarded.
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This document summarises time-variant NTN topology problems; it
outlines the use cases and key requirements, for link management
and topology creation and routing, when network connectivity is
temporal, where nodes and links must be managed to maximise power
efficiency.
1.1 Terminology
ATG: Air to Ground
eNodeB: A 4G base station
e-MTC: enhanced Machine Type Communication
FSO: Free Space Optics
GEO: Geosynchronous orbit with the altitude 35786 km
gNB: A 5G base station
HAPS: High Altitude Platform System
IGP: Interior gateway protocol
ISL: Inter Satellite Link
ISLL: Inter Satellite Laser Link
ISTN: Integrated Space Terrestrial Network
LEO: Low Earth Orbit with the altitude from 180 km to 2000 km.
MEC: Multi Edge Computing
MEO: Medium Earth Orbit
NG-RAN: Next Generation Radio Access Network
NGSO: Non-Geostationary Satellite Orbit
NTN: Non Terrestrial Networks
NTN-Gateway: An earth station for accessing NTN nodes
RSRP: Reference Signal Receive Power
SNO: Satellite Network Operator
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SRI: Satellite Radio Interface
TN: Terrestrial Networks
2. 3GPP NTN Use Cases and Requirements
Discussion on Non-Terrestrial Networks (NTN) started in 3GPP with a
Study Item in Release-15 in 3GPP RAN WG1 in 2018. 3GPP is involved
in investigating the NTN physical layer aspects, protocols, and
architecture, as well as the radio resource management, link
requirements, and frequency bands to be used. Work continued, and
in 2019 in Release-16 [TR 38.821] detailed deployment scenarios
and channel models for NTN. 3GPP Release-17 has also introduced
new network topologies into the specifications for NTN.
Follow-up work in the 3GPP Technical Specification Groups (TSGs)
SA (Systems Aspects) provided use cases for satellite-based NTN
in Release-17. The work identified three main use cases for
satellite-based NTN:
o Service Continuity: Use cases where 5G services cannot be
offered by Terrestrial Networks (TN) alone. A combination
of terrestrial and nonterrestrial networks, such as
commercial or private jet, and maritime platforms, would
be required;
o Service Ubiquity: Use cases address unserved or under-served
geographical areas where terrestrial networks may not be
available. Use cases include industrial agriculture, asset
tracking, emergency networks, and smart home;
o Service Scalability: Use cases that maximise the satellite's
extensive coverage and capability, and use multicasting or
broadcasting techniques to distribute content.
According to the architecture outlined in [TR 38.821], the
satellite payload implements frequency conversion and a radio
frequency amplifier in both uplink and downlink direction.
The 3GPP 5G system is expected to support service continuity
between terrestrial 5G access networks and 5G satellite access
networks owned by the same operator, or owned by two different
operators having an agreement.
Connectivity is implied between TN and NTN nodes, a GEO, MEO, or
LEO satellite will communicate with HAPS or UAV nodes, or
terrestrial Next Generation NodeB (gNB or ng-eNB), or
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satellite-enabled 5G User Terminals (UE).
It is expected that the next generation (NG) based mobility
should work to transition between NTN, TN and B5G NTN. It is
anticipated that NTN can interact with 5G, and 4G terrestrial
networks via legacy inter-RAT (Radio Access Technology)
procedures.
2.1 Architecture
Typically, the NTN architecture comprises one or several
satellite gateways that connect the NTN to a public data network.
In addition, several link elements exist:
o A feeder link or radio link between a satellite gateway
and the satellite or the UAS platform;
o A service link or radio link between the user equipment
(UE) and the satellite or the UAS platform;
A satellite or a UAS platform may implement either
a transparent or a regenerative (with onboard processing)
payload. The satellite or the UAS platform typically generates
several beams over a given service area bounded by its field of
view. The footprints of the beams are typically of an elliptic
shape. The field of view of the satellite or the UAS platform
depends on the onboard antenna diagram and the minimum elevation
angle.
Additionally, Inter-Satellite Links (ISL) exist in a constellation
of satellites; their interfaces have traditionally been RF-based,
but increasingly Free-Space-Optics (FSO) are being deployed.
This will require regenerative payloads on board the satellites.
The ISL may then operate in an RF or optical band.
The logical architecture described in [TS 38.401] may be used as a
baseline for NTN scenarios, which include but are not limited to:
o Transparent Satellite Based NG-RAN;
o Regenerative Satellite Based NG-RAN;
o Regenerative Satellite with gNB on Board;
o Regenerative Satellite with gNB-DU on Board.
2.1.1 Physical Layer Control
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The propagation delays in terrestrial mobile systems are usually
less than 1 ms. In contrast, the propagation delays in NTN are
much longer, ranging from several milliseconds to hundreds of
milliseconds depending on the altitudes of the spaceborne or
airborne platforms and payload type.
2.1.2 Uplinks and Downlinks
Several NTN uplink power control methods have been proposed in
Release-16:
o Beam-specific configuration for power control parameter and
common parameter for all beams;
o A UE prediction of its own transmission power using other
available information such as satellite ephemeris and UE
trajectory;
o Adaptive uplink power control based on adaptive UE
configuration of Layer 3 filter coefficients (i.e., configuring
multiple Layer 3 filter coefficients and letting UE select one
of the Layer 3 filter coefficients based on measured Reference
Signal Receive Power (RSRP);
o A UE can be configured with different uplink power control
parameters such as P0 and alpha parameters for disabled and
enabled HARQ processes;
o The transmission power of different UEs can be adjusted as a
group with a reference UE transmission power.
3GPP Release-17 work develops on earlier studies performed in
Release-16, where NTN channel models and necessary adaptations of
the NR technology to support NTN were identified. The main
challenges identified are related to the mobility and orbital
height of the satellite. The height causes a high path loss and
a large RTT. The mobility of an LEO satellite introduces a very
high Doppler offset on the radio link, and it also inevitably
requires all devices to change their serving nodes frequently.
Furthermore, Release-17 establishes basic mechanisms to manage
these challenges and provides a first set of specifications to
support NTNs based on NR, NB-IoT and LTE-M.
2.1.3 Feeder Links
During the satellite movement in the NTN, the switch-over of the
feeder link between the different NTN gateways will be needed,
especially for non-GEO satellites. The switch-over may happen when
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the satellite moves out of the vision of the current NTN gateway.
A feeder link switchover will occur when the existing feeder link
is changed from a source NTN Gateway, to a target NTN Gateway for a
specific NTN payload. The feeder link switch-over happens at the
transport network layer.
In a soft-feeder link switch-over, an NTN payload can connect to
more than one NTN gateway during a given period, i.e., a temporary
overlap can be ensured during the transition between the feeder
links. A hard-feeder link switch-over, is when an NTN payload only
connects to one NTN gateway at any given time, i.e., a radio link
interruption may occur during the transition between the feeder
links.
2.2 Satellite Service Continuity
Satellites in Earth orbit move at relatively high speed to a fixed
position on Earth. The satellite beam towards the Earth determines
the area coverage that the satellite provides to the user.
There are typically two modes of satellite beam operation:
o Moving-beam: This is the case of a satellite with fixed beams,
which yields a moving footprint on the Earths ground. In this
case, the beam is moving relative to a fixed position on Earth;
o Fixed-beam: This is the case of a satellite with steerable beams.
As the satellites orbit the Earth, the satellite beams are
adjusted so that it can continue to cover the same geographical
area. As long as the satellite is above the horizon relative to
the given geographical area, the beams can be adjusted to cover
that area.
The second scenario, fixed-beam, yields the maximum time a user may
remain under the coverage area of the same satellite. This time is
the time the satellite remains above the horizon relative to the
user's location, which is approximately seven to ten minutes.
2.3 Satellite-based NG-RAN Architectures
The NG-RAN logical architecture is described in [TS 38.401] and is
used as a baseline for NTN scenarios. The satellite payload
implements the regeneration of the signals received from Earth.
o The radio interface (NR-Uu) on the service link between the UE
and the satellite;
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o Satellite Radio Interface (SRI) on the feeder link between the
NTN gateway and the satellite.
Architecture aspects for using satellite access in 5G [TR 23.737]
Specified enhancements for RF and physical layer, protocols, radio
resource management, and frequency bands. Identified a suitable
architecture, addressed TN-NTN roaming and timing-related issues,
enhanced conditional handover, and location-based triggering
2.4 NB-IoT and eMTC Support
A topic discussed in 3GPP Release-17, [TR 36.763] focused on IoT
applications by highlighting issues related to Long Term Evolution
(LTE) timing relationships, uplink synchronization, and HARQ
(Hybrid automatic repeat request).
These use cases may impact requirements for time-variant networking
and will require further study.
3. Routing and Traffic Engineering Challenges for NTNs
Traffic Engineering (TE) has been well investigated for more than
two decades in the context of the traditional terrestrial Internet.
However, TE has not been systematically understood in the NTN and
integrated space and terrestrial network environment, especially
given the district characteristics of the two types of networks
and the mega-constellation behaviors of LEO satellites. It is
generally understood that the inter-satellite link capacity is not
compared to the optical fiber links in the terrestrial Internet.
As such, the traffic injected into the space network has to be
selective [1].
Energy efficiency policies may need to be enforced based on the
node or link type, traffic type and their QoS requirements or
other contexts such as the distance NTN nodes and power
transmission cost. For instance, it may be argued that routing
through a chain of LEO satellites using a currently available
topology is sub-optimal. Instead, a new topology should be
created for the users' end-to-end delay or to meet application
or service bandwidth expectations.
It is also worth noting, the capability of TE in the space network
also largely depends on the specific routing mechanisms that are
deployed, which has been the case in terrestrial network
environments, e.g., IP/MPLS/SDN. As mentioned above, the capability
of TE in integrated space and terrestrial network infrastructures
will also depend on the routing mechanisms deployed in the two
network environments, either with separate protocols
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(the case today) or with a unified protocol suite.
Routing and signaling across emerging NTN infrastructure is far
from static [2]; satellite-to-satellite connectivity
changes frequently, space-based ISL link latencies will vary,
and links from space-to-ground will change regularly. The satellite
that is overhead a particularly ground station changes frequently,
the RF or laser links between space-based satellites change often,
and link latencies for satellite-to-ground links will vary based on
atmospheric conditions [3].
Satellites will also have to contend with predictive routing
capabilities, as links will only be established when optical
alignment is possible or powered and in service. Given that meshes
of 100s and 1000s of satellites are also expected, techniques that
use per-hop Dijkstra calculation will be extremely inefficient [3].
Several link management and control plane challenges have been
identified for NTN infrastructure, these include:
o New link acquisition, predicted link availability, and link
metric dynamicity: as the acquisition and tracking of satellites
and links change, there is a need to adjust basic link and TE
metrics (delay, jitter, bandwidth) and update the existing
routing traffic engineering database;
o Space-based path computation: selection of the best path across
ISLs and direct uplinks and downlinks, consideration of cloud
cover, air turbulence and external object occlusion;
o Temporal routing: consideration of the time-varying topology of
the space network may necessitate frequent routing updates,
unless an SDN-based centralised controller is used;
o Predictive routing: time-scheduled routing paths based on
expected satellite orbits and air-interface alignment;
o Rerouting of paths: which may be required in the event of
projected space-based debris orbits that prevent line-of-sight
between adjacent nodes, interface and node failures, and adverse
weather which may affect space-to-ground communication points;
o Unmanned aerial aircraft link and time availability;
o Resilience: overall, the network must be resilient to failures,
and capable of routing within bandwidth and latency thresholds,
even when traffic levels are significant enough to oversubscribe
the preferred paths.
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Integrating the space-based infrastructure with an existing network
might be achieved using traditional Internet routing techniques
and identifying the extra-terrestrial portion of the network as
a specific domain (such as an IGP area or an AS) [3]. The space-
domain might run a traditional routing control plane, likely
logically within an Earth-based representation which programs the
path via an SDN-programming technique [3]. However, this approach
would not be capable of computing paths based on the unique space
connectivity dynamics. Furthermore, if the space-domain was
connected to traditional Earth-based Internet domains (including
ASes via BGP), it might create unwanted route flapping, causing
routing instability.
Due to the unique characteristics of the space-based nodes (which
may have multiple interfaces and lines of sight to next-hop
satellite nodes or ground stations, may fluctuate), other network
control methods may be needed, especially when power consideration,
and expected link loss, or link activation is planned.
3.1 Link and Routing Resilience for NTNs
Legacy satellites might typically operate independently from their
orbiting counterparts. However, next generation space-based
infrastructure will be utilizing multiple links between
satellite nodes and ground-stations, which leaves potential
network paths susceptible to the consequences of node and link
failures or anomalies. Loss of node payload, communication link,
or other sub-system components might render the entire NTN
nodes inoperable, and planned connectivity is lost.
In a satellite network, there several types of failures a routing
system might be concerned with; these include:
o Failures of components in the forwarding plane, e.g., uplink
and ISL communication failure;
o Control plane malfunction, if the central controller is
destroyed or disconnected, or the distributed control plane
suffers a catastrophic failure or attack;
o Misconfiguration of an NTN node, such as a satellite or ISL
forwarding, or degradation of satellite orbit, and loss of
communication sight to neighbouring node.
In general, node failures or components of the forwarding
plane are problematic but as the latest generation of NTN
infrastructure is highly meshed, routing around node failures
is feasible. Once a failure occurs, the centralized controller,
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or distributed control plane, would have to respond and update
the forwarding state in devices to route traffic around the
failed nodes or links. As failure may be seen as an extreme
case of an unexpected change in traffic level, a traffic
reoptimization mechanism would likely be required.
3.2 Multi-layer Networking in NTNs
Low-altitude UAVs, HALE and HAPS nodes, and LEO satellite provide
latency benefits, but will typically have more dynamic connectivity
and oscillating link characteristics, and therefore more planned or
expected link outages and re-activations. They may also connect to
higher-altitude nodes, such as the Medium Earth Orbit (MEO) and
Geostationary Orbit (GEO) satellites, which also provide more
physical stability, and reduced dynamicity of the links
as the satellites remain static.
The current GEO satellite system mostly provides relay function;
however, in the next generation, satellite systems could interact
providing multi-layer routing and forwarding functions between
satellite layers, akin to multi-layer networking in
terrestrial networks.
4. NTN Management and Operation
In 2019, 3GPP SA5 started a study on management and orchestration
aspects with integrated satellite components in a 5G network. The
main objective is to study business roles and service,
network management, and orchestration of a 5G network with
integrated NTN components. The scope includes both NTN RAN
based satellite access, and non-3GPP defined satellite access, as
well as HALE and HAPS aspects.
An entity, distributed or centralised, will be required that
dynamically manages planned resources at NTN nodes according to
their availability, power consumption and recharge rate, mobility
patterns, architecture hierarchy, incoming and expected traffic,
ensuring seamless service continuity to the end-user despite of
intermittent link availability, topology changes and possible
link disruptions. Ultimately, link utilisation, across TN and NTN
nodes will need to be optimally allocated and leveraged, based on
the time-variant requirements outlined in this document.
Further discussion of management and operation will be included
in future versions of this document.
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5. Security Considerations
Several existing 5G security requirements and procedures will need
to be considered, including the impact on confidentiality and
integrity protection of user traffic and control plane traffic.
The control plane traffic between UE and the radio access network
(i.e. the gNB) may be protected in two ways:
o Integrity protected, i.e., it cannot be tampered with;
o Confidentiality protected, i.e., it cannot be eavesdropped.
The user plane (i.e. data) traffic between UE and the radio access
network (i.e. the gNB) will also need to be protected. Additionally,
there may also be encryption requirements for NTN interfaces
between NTN nodes, such as a satellite node and the gNB, to
prevent man-in-the-middle attacks.
Further discussion of security will be included in future versions
of this document.
6. IANA Considerations
This document makes no requests for IANA action.
7. Acknowledgements
To be added.
8. Contributors
To be added.
9. Informative References
[TR 38.821] Solutions for NR to Support Non-Terrestrial Networks
(NTN), document TR 38.821, Release 16, 3GPP, Jan.
2020. [Online]. Available: https://www.3gpp.org/
[TS 38.401] 5G NG-RAN Architecture Description, document TR 38.401,
Release 16, 3GPP, Nov. 2020. [Online]. Available:
https://www.3gpp.org/
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[TR 23.737] Study on Architecture aspects for using Satellite
Access in 5G, document TR 23.737, Release 17,
3GPP, Mar. 2021. [Online]. Available:
https://www.3gpp.org/
[TR 36.763] Study on Narrow-Band Internet of Things (NB-IoT) /
enhanced Machine Type Communication (eMTC) support
for Non-Terrestrial Networks (NTN)
[1] Curzi, Giacomo & Modenini, Dario & Tortora, Paolo. (2020).
Large Constellations of Small Satellites: A Survey of Near
Future Challenges and Missions. Aerospace. 2020.
[2] M. Handley, "Delay is not an option: Low latency routing
in space," in Proceedings of the 17th ACM Workshop on Hot
Topics in Networks, 2018, pp. 85-91.
[3] King, D. and Wang, N. "Integrated Space-Terrestrial
Networking and Management", Future Networks,
Services and Management: Underlay and Overlay, Edge,
Applications, Slicing, Cloud, Space, AI/ML, and Quantum
Computing, Springer International Publishing, 2021.
Authors' Addresses
Daniel King
Lancaster University
UK
Email: d.king@lancaster.ac.uk
Kevin Shortt
Airbus
Germany
Email: kevin.shortt@airbus.com
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