Beyond 5G White Paper
Supplementary Volume“NTN Technologies”

Version 1.0
March 7, 2024
Beyond 5G Promotion Consortium
White Paper Subcommittee

Preface .............................................................................................................................. 3
I. HAPS Technology: HAPS Flight and Communication Test Results Show Path toUnlock Stratospheric Communications ................................................................... 5
I-1. Introduction .............................................................................................................. 5
I-2. About HAPS.............................................................................................................. 5
I-3. HAPS Aircraft and Communication Equipment Development ............................ 6
I-4. Communications Tests ............................................................................................ 8
I-5. Challenges Towards Realizing HAPS................................................................... 11
I-6. Conclusion .............................................................................................................. 12
II. Extreme Coverage Extension in Beyond 5G and 6G: Cooperative HAPS Architecture
Integrating Terrestrial Networks .......................................................................... 13
II-1. Introduction .......................................................................................................... 13
II-2. HAPS use cases and network configuration/control technologies .................... 13
II-3. Cooperation between multi-layered NTN system and TN ................................ 16
II-4. Conclusion ............................................................................................................. 17
II-5. Acknowledgements ............................................................................................... 17
III. Very Low Earth Orbit Satellite Networks for 6G [1] .............................................. 18
III-1. Introduction ......................................................................................................... 18
III-2. Driving factors and motivations ........................................................................ 18
III-3. Usage scenarios and motivations ....................................................................... 18
III-4. Challenges and Solutions ................................................................................... 20
III-5. Conclusion............................................................................................................ 23
IV. Multi-layer non-terrestrial network and its routing management for Beyond 5G /6G................................................................................................................................ 25
IV-1. Introduction ......................................................................................................... 25
IV-2. Multi-layer satellite network (Space RAN) ....................................................... 26
IV-3. Traffic control for multi-layer satellite network ............................................... 27
IV-4. Conclusion ............................................................................................................ 29
Abbreviation List ........................................................................................................... 31

1

【Revision History】
Ver.
Date
1.0 2024.3.7
Initial version

Contents2

NotePreface
In our published white paper “Beyond 5G White Paper ~Message to the 2030s~”, weare discussing that Japan is at the forefront of developing 6G technology, focusing on the
integration of non-terrestrial network (NTN) to revolutionize connectivity fromterrestrial to aerial and space domains. This endeavor aims to address several key
challenges and unlock new use cases in various sectors such as unmanned systems,
monitoring systems, IoT, backhaul for emergency services, and smartphone integration
for human lives.
A primary challenge is achieving high throughput and capacity in NTN
communications, especially with the adoption of higher frequency bands like millimeterand terahertz wave. These bands are susceptible to weather conditions, requiring Japanto develop robust communication solutions.
Reducing latency is crucial, as Japan's reliance on geostationary satellites introduces
significant delays. The exploration of Low Earth Orbit (LEO) satellites and High
Altitude Platform Station (HAPS) is seen as a solution to minimize these delays for timesensitive applications.
We also recognize the need for massive IoT connectivity in Beyond 5G networks,
necessitating efficient network configurations using a mix of satellite types to manage
the surge in connected devices.
Optimal route connection via multiple terrestrial networks and NTN components withmulti-connectivity technology are also required. Multi-routing with optimal routeselection and simultaneous multi-connections provides better reliability and extension
coverage, especially for higher demand areas.
Edge computing technology is another key area, crucial for handling large data
volumes from IoT devices and ensuring low-latency communication in NTN environment.
To tackle these challenges and to realize new use cases, a lot of research anddevelopment activities in Japan. In this white paper, we show these research anddevelopment activities and its results with a lot of figures as follows.
-“HAPS Technology: HAPS Flight and Communication Test Results Show Path toUnlock Stratospheric Communications” describes a concept of HAPS and resultsof flight and communication tests using a fixed-wing type of HAPS aircraft.

-“Extreme Coverage Extension in Beyond 5G and 6G: Cooperative HAPS
Architecture Integrating Terrestrial Networks” presents HAPS use cases and acooperative architecture that focuses on the link between NTNs and terrestrial
networks (TNs).

3

-“Very Low Earth Orbit Satellite Networks for 6G [1]” proposes the vision for theevolution of Very Low Earth Orbit (VLEO) satellites-based NTN towards 6G anddescribes the technical challenges and potential solutions.

-“Multi-layer non-terrestrial network and its routing management for Beyond 5G/6G” presents the concept of multi-layer non-terrestrial network (multi-layer
NTN), which consists of HAPS, LEO and GEO and shows our routing
management technology for multi-layer NTN.

In conclusion, Japan's engagement in NTN and Beyond 5G technology is not only
addressing current challenges but also paving the way for a transformative leap in global
communication networks, promising to unlock new applications and services.

This White Paper was prepared with the generous support of many people who
participated in the White Paper Subcommittee. The cooperation of telecommunicationsindustry players and academia experts, as well as representatives of various industries
other than the communications industry has also been substantial. Thanks to everyone’s
participation and support, this White Paper was able to cover a lot of useful information
for future business creation discussions between the industry, academia, andgovernment, and for investigating solutions to social issues, not only in thetelecommunications industry, but also across all industries. We hope that this White
Paper will help Japan create a better future for society and promote significant global
activities.
Kazunori Sakumoto, Fujitsu

4I. HAPS Technology: HAPS Flight and Communication Test Results Show
Path to Unlock Stratospheric Communications
Yusuke Segawa, SoftBank Corp.
Kyohei Sakamoto, SoftBank Corp.
Shiro Fukumoto, SoftBank Corp.

Abstract— In the Beyond 5G and 6G, "Ubiquitous Connectivity" is being considered
for communication in all locations including air, sea, and space. NTN technologies,
especially High Altitude Platform Station (HAPS), hold great potential in providing
mobile broadband to unserved and underserved areas that have lacked sufficient
coverage from terrestrial mobile networks. This article describes a concept of HAPS andresults of flight and communication tests using a fixed-wing type of HAPS aircraft.
I-1. Introduction
In order to realize HAPS, stable flight of the aircraft mainly in the stratosphere andappropriate communication from the aircraft hovering at a fixed point in thestratosphere to the ground are essential. To prove these points, tests have been
conducted by various companies and organizations.
As an example, this article introduces the aircraft and communication equipment
being developed by SoftBank Corp. (formerly HAPSMobile Inc.), as well as the HAPS
flight and communication tests that SoftBank conducted in the United States from 2019
to 2020.
I-2. About HAPS
HAPS is an acronym for High Altitude Platform Station, which is a system that utilizes
an unmanned vehicle, such as an aircraft, flying in the stratosphere as a communications
base station. In doing so, the system is capable of providing communications services
over an extensive area. The stratosphere is a layer of the atmosphere far above the clouds.
Because of this, it is not affected by rain or snow and air currents have little influence.
These characteristics enable the flight of a stratospheric platform to be more stable ascompared to flight in other layers of the atmosphere. HAPS can provide a wide array ofservices which include connectivity, earth, atmosphere and climate monitoring, disaster
response, mapping and humanitarian missions, search and rescue, infrastructure
inspection, and more. HAPS can either be stationary (or quasistationary), or be mobile
across large regions (e.g. for survey missions). In the field of telecommunications, HAPSis classified as a non-terrestrial network, equivalent to that of geostationary and lowearth-orbit satellites. This new network system is capable of covering a wider area more5efficiently when compared to traditional ground base stations. It is also unaffected bydamage caused by disasters such as earthquakes and tsunamis.

Fig. I-1 Non-Terrestrial Networks
I-3. HAPS Aircraft and Communication Equipment Development
I-3.1. HAPS Aircraft Development
The Sunglider is one of the large fixed-wing HAPS aircraft currently being developed
and put into practical use. Its major feature is that it has no tail section, like Pathfinderand Helios, which minimizes induced resistance at the wingtips, thus enabling efficient
flight. Additionally, meticulous planning ensued in order to reduce the weight of theairframe. As a result, despite its large size, Sunglider is capable of flight over an
extended period of time.
The future use of Sunglider is illustrated below. A ground crew will control it visuallywhen taking off from an airport. It will then gradually climb to the stratosphere whilecircling over the airport. Once it has reached the stratosphere, operation will besupervised remotely via a satellite command and control link. After reaching thedestination, it will remain there circling at a fixed point in the sky. At this point in time,
it will begin providing service as a network for terrestrial communications.
Sunglider is powered by solar energy. During the daytime, it generates electricity
using the solar panels attached on the top of its wing. Using this solar energy, its
propellers turn and communication devices mounted on the airframe are powered. At
the same time, any surplus energy is stored in battery cells mounted in the airframe. As
the stratosphere is located above the clouds, stable power generation is ensured during
the daytime. At night, the propellers are kept turning by consuming the energy stored
during the daytime. This is how it is planned to maintain continuous flight in thestratosphere for approximately six months.

6

Fig. I-2 Sunglider operation
I-3.2. Payload Development
Here “payload” collectively means the equipment mounted on an unmanned aircraft,including communications devices, cameras and measuring instruments. In this section,
it specifically refers to communications equipment used for a cellular phone networkbase station.
Payloads must be developed based on the assumption of being able to function in allenvironments from the ground to the stratosphere, the point from which services aredelivered. Among all of the requirements, resistance to significant changes intemperature and air pressure is paramount. In addition, as the payload is to be mounted
on a lightweight unmanned aircraft powered by solar energy, power consumption andweight need to be minimized. Payloads also need to be resistant to the vibration uniqueto lightweight unmanned aircrafts in order to provide a reliable signal.

Fig. I-3 Payload Development7I-4. Communications Tests
I-4.1. Network Structure
The payload transmitted service link (LTE) radio waves earthward to be received bysmartphones. SoftBank secured two frequency bands to be used as the feeder link fortransmitting data from the smartphones to terrestrial Internet lines, enabling the feeder
link to operate with main and sub-channels and thereby having structure redundancy.
Two more frequency bands were allocated exclusively for controlling the payload anddata collection. The frequency bands used in the tests are listed below.
<Frequency bands>
・Service Link : 700MHz (3GPP Band28)
・Feeder Link : 70-80GHz (Main), 5.8GHz (Sub)・Payload control & data collection channel : 902-928MHz, 1200-1700MHz

Fig. I-4 Network Structure
A ground gateway serves as the hub that connects a ground base station and a HAPS.
A shipping container was temporarily transformed into a ground gateway (shown below).
Feeder link antennas were installed on the roof of the container to conduct thecommunications test.

8

Fig. I-5 Terrestrial Gateway
I-4.2. Executing Communications Tests
SoftBank conducted communications speed tests, ping tests, and a video call using the
network structure described above. For the communications speed tests, the presence ofany fading effect was checked by comparing the difference between estimated signal
received and that actually measured. The effectiveness multiple-input multiple-output(MIMO) in the HAPS environment was also verified. For the ping tests, messages weresent from a host computer to another computer. Round-trip time (RTT), which is theamount of time it takes for the host computer to receive a reply, was estimated, as wellas the amount of delay in commercial use.
The testing environment for communications speed and ping tests is illustrated below.
User equipment (UE) was controlled using personal computer connected via USB cable.
Communications speed and pinging were measured using Iperf and a test server on theInternet.

Fig. I-6 Communications Tests
I-4.3. Test Results
I-4.3.1. Communications Speed Results
SoftBank conducted the tests using one UE connected at time, disconnecting the UE
tested before testing the next. The results confirmed successful connectivity to all of theUEs, including the one located indoors. Additionally, fluctuation in signal reception waspredicted, which would result in fading due to the HAPS being in motion. However, an9analysis of the time variation in down-link (DL) reference signal received power (RSRP)
obtained during the tests revealed that there was no fading effect. The analysis ofcommunications speed also indicated that there were many cases in which the Rank
Indicator values sent from UEs maintained a value of 2, which means MIMO is possible.
There had been regarding MIMO capabilities due to the open-air nature of the HAPS
environment; however, the data revealed that MIMO is feasible.
I-4.3.2. Ping Test Results
The following illustration shows from which node to which node verification was
carried out during the ping test. The test was performed from a UE to the test server, as
well as from the gateway to the test server, from the gateway to the Evolved Packet Core
(EPC), and from Sunglider to the EPC. Using the values obtained, the RTTs for the
Feeder Link (FL) and Service Link (SL) could be determined.
The FL RTT value turned out to be very small. Even if the transmission distance isfarther than used in the test at SpA, HAPS systems can achieve RTT values significantly
lower than 1ms. On the other hand, for the SL, partly because of the LTE protocols used,
the scheduling request transmission cycle and grant allocation negatively affected theamount of delay. In order to expand the application of HAPS, it is imperative to lower
the latency. SoftBank is also considering the utilization of 5G networks as a solution to
reduce latency.

Fig. I-7 Ping Test
I-4.3.3. Video Call Test Results
Using a communications payload resistant to the demanding stratospheric conditions
and smartphones equipped with the Zoom video-conferencing application, members ofLoon and the AeroVironment team at Spaceport America successfully completed a video
call with members of SoftBank in Tokyo, Japan.

10Fig. I-8 Voice Call Test Success
I-5. Challenges Towards Realizing HAPS
I-5.1. Flight Challenges and Measures Moving Forward
There are currently no established, internationally recognized, regulatory pathwaysfor HAPS to operate in the stratosphere. In order to achieve the commercial use of HAPS
systems in the near future, it is imperative that a regulatory framework, suitable for
HAPS operations, be developed.
It is also important to analyze and clarify meteorological phenomena in the
troposphere and stratosphere as a means of contributing to the safe flight of HAPS
aircraft. Sharing meteorological information is one of the industry-wide issues for those
involved in HAPS projects, as well as being required for improving the meteorologicalobservation framework.
In order to address these issues, in cooperation with meteorological authorities inrespective countries, a global platform should be established for obtaining meteorological
data in the troposphere and stratosphere.
I-5.2. Communication Challenges and Measures Moving Forward
Since HAPS will provide coverage over extensive areas from the stratosphere,measures will need to be taken for radio-wave interference in the proximity ofneighboring nations’ borders. In order to conduct communications tests and provide
smooth, stable services using HAPS at the global level, rules need to be established sothat negotiations and arrangements with neighboring countries are conducted undercordial circumstances.
The public and private sectors should work together to establish rules that resolve theproblems in each country.

11I-6. Conclusion
This article describes the concept of HAPS and the results of a communications tests
from the stratosphere using a fixed-wing type of HAPS aircraft.
This test demonstrated the concept that HAPS can be used with ordinal user terminals
such as smartphones without modification, and that it can also be used for adequate data
communications such as video calls. Based on these results, further technologicaldevelopment, standardization and regulatory activities are planned for implementing
HAPS technology in the future.

REFERENCE[1] HAPS Alliance, “Bridging the Digital Divide with Aviation in the Stratosphere,” Dec
2021.
https://hapsalliance.org/wpcontent/uploads/formidable/12/SpA_Flight_Communication_Tests_White_Paper_En
glish_Final_2021.pdf

12

II. Extreme Coverage Extension in Beyond 5G and 6G: Cooperative HAPS
Architecture Integrating Terrestrial Networks
Yuki Hokazono, NTT DOCOMO, INC.
Kenji Fukasawa, NTT DOCOMO, INC.
Satoshi Nagata, NTT DOCOMO, INC.
Yoshifumi Morihiro, NTT DOCOMO, INC.

Abstract—The space radio access network is regarded as a communication
infrastructure for the Beyond 5G and 6G, and extreme coverage extension is being
studied for use cases in all locations including air, sea, and space. To achieve thisextreme coverage extension, non-terrestrial network (NTN) technologies that use
geostationary (GEO) satellites, Low Earth Orbit (LEO) satellites, and a High Altitude
Platform Station (HAPS) are promising tools for providing high-quality communications
services to areas that cannot be covered by conventional mobile communications
networks. In this article, we present HAPS use cases and a cooperative architecture that
focuses on the link between NTNs and terrestrial networks (TNs).
II-1. Introduction
A key issue in the Beyond 5G and 6G eras is expected to be ubiquitously expanding
the communications area where its benefits can be enjoyed. We are conducting researchand development aimed at actualizing extreme coverage extension to all locations,
including the sky, sea, and space, which have not been sufficiently covered byconventional mobile communication networks, using non-terrestrial networks (NTNs)
based on geostationary (GEO) satellites, Low Earth Orbit (LEO) satellites, and High
Altitude Platform Stations (HAPSs). For early implementation of extreme coverage
extension, we are focusing on direct-to device communication services using HAPSs. In
this article, we present HAPS use cases and a cooperative architecture that focuses on
the link between NTNs and terrestrial networks (TNs).
II-2. HAPS use cases and network configuration/control technologies
We are researching and developing communication methods and networkarchitectures that can flexibly link 5G networks and other TNs with stratospheric HAPS
networks [1]. In addition to providing flexible support for a wide range of future use cases
as envisioned in Beyond 5G and 6G, this project is conducting studies aimed at theimplementation of communication systems that use HAPSs in terms of development andoperation costs.
HAPS use cases

13

As shown in Fig. II-1, for the Beyond 5G and 6G era, it is expected that various use
cases will involve using HAPSs to relay radio waves or emit radio waves as a base station.
These use cases include fixed systems that provide services for backhaul applications
and mobile systems that provide services to terminals either directly or via repeaters
and relays. There is a need for flexible communication methods and systems that can
support all use cases of fixed and mobile systems.
It is also necessary to flexibly control lines so that they can be adapted from normal
business applications to public safety applications in the event of a disaster. Current
disaster countermeasures are geared toward providing basic communication services
such as voice calls and short message services, but it may also be necessary to consider
use cases that require faster communication speeds, such as remote control of equipment
at disaster sites, video transmission, and communication via drones. For disastercountermeasures, it will also be necessary to study network configurations and control
technologies that assume the ability of a system to operate even if certain devices become
unavailable.
Cooperative network configuration and control technology for HAPSs and TNs
Regarding the network configuration and control technology used when implementing
backhauls to 5G base stations via HAPSs, we are focusing on the categorization of HAPSmounted stations. They can be roughly divided into two types: (i) relay stations, which
receive signals from ground stations and relay them back to other ground stations after
executing necessary processes such as frequency conversion, and (ii) base stations, which
are made by installing 5G network base-station equipment (or at least part of it) in a
HAPS [2]. The relay type is effective when the number of onboard devices is relatively
small and the size, weight, and power consumption of the HAPS-mounted station arestrictly limited. The base-station type is formed by equipping a HAPS with an antenna
device, together with many base-station functions. The more of these functions it
includes, the greater the amount of control that can be executed within the HAPS,
making it possible to reduce the amount of feeder-link information. However, installing
more functions results in a station that is larger, heavier, and consumes more power.
Implementing more base-station functions on the ground-network side has theadvantages of lower development costs and ease of operation, but implementing thesefunctions on the HAPS results in greater resilience to natural disasters. In terms of
performance, a HAPS-mounted station should at least implement certain functions, suchas beam control, when using millimeter waves. It is also necessary to comprehensively
study a wide range of requirements to be considered when incorporating HAPS systems
into a 5G network. These include the size, weight, and power consumption of HAPSequipped stations, their development and operation costs, the ability of these HAPS

14

platforms to be shared by backhaul use and direct-to-device communication systems, andtheir ability to cooperate with GEO/LEO satellites.
An example of promising configuration using a relay-type configuration where a 5G
radio repeater is installed in a HAPS is shown in Fig. II-2. In this configuration, the TN
is used from the core network to the fronthaul, and the HAPS terrestrial systemequipped with the radio unit (RU) function bundles and communicates signals for
multiple beams. A broadband frequency, such as the Q-band, is used in the feeder link,
and the HAPS relay system executes frequency conversion and power control. The HAPS
can then establish service-link communications using multiple beams at the same time.
As the service link, certain frequency bands below 2.7 GHz already identified forInternational Mobile Telecommunications (IMT) should be used according to thespecifications approved at the World Radiocommunication Conference 2023 (WRC-23)[3].

Fig. II-1 Various use cases expected for HAPS

Fig. II-2 Example of cooperative configuration when HAPS is used for direct access15

II-3. Cooperation between multi-layered NTN system and TN
As shown in Fig. II-3, the multi-layered NTN system, in which satellites and HAPS
are connected to the terrestrial 5G (or future 6G) core network, is a larger scale threedimensional heterogeneity network than previously achieved. It is expected that TNs,
satellites, and HAPS will cooperate and provide seamless communications according to
the location (including air, sea, and space) to offer service and the required
communications speed and latency.
Two systems are being considered for access to mobile terminals in NTNs: (i) a relay
system that accesses the mobile terminal from a satellite and HAPS through the relay
station, and (ii) a direct access system that directly accesses the mobile terminal from asatellite and HAPS. As a derivative of system (i), a cellular backhaul (CBH) system inwhich satellites and HAPS support a backhaul between the core network and the BS as
an independent tunnel line is conceivable. In the Beyond 5G and 6G eras, mobile devices
can be accessed everywhere in various ways depending on use cases and optimization of
the entire network.
The following are problems facing NTNs: expansion of the radio interface that issuitable for long-distance communications, an efficient frequency effective utilization
method for the ground network, and a network design to actualize high-efficiency
cooperation with the ground network. In addition, further investigation into wirelesstechnologies such as handover, carrier aggregation (CA), and dual connectivity (DC)between the NTNs and TNs would be beneficial. On the other hand, since each NTN
platform has different features such as capacity and propagation delay, it is necessary
to examine routing and network construction considering the features of each platform.
NTNs are also promising as a means to advance cost effectively future expansion of 5G
network coverage already introduced, and they yield the possibility to optimize network
development from the beginning in the 6G era.

16Fig. II-3 Multilayer network system using NTN platforms and cooperation withterrestrial 5G network
II-4. Conclusion
As part of our efforts towards implementing extreme coverage extension, we presented
an architecture that integrates NTNs with TNs. Specifically, we described a cooperativearchitecture that focuses on the link between NTNs and TNs, and a cooperative
configuration with a particular focus on HAPSs. We will continue developing NTN
technology aimed at achieving extreme coverage extension and technology for
actualizing HAPS networks.
II-5. Acknowledgements
Part of this research and development was carried out by the Ministry of Internal
Affairs and Communications (Research and Development for Expansion of Radio
Resources; JPJ000254).
REFERENCE[1] NTT DOCOMO, INC., “White paper: 5G Evolution and 6G (version 5.0),” Jan. 2023.
[2] Y. Xing, F. Hsieh, A. Ghosh, and T. S. Rappaport, “High altitude platform stations(HAPS): Architecture and system performance,” Proc. of the 2021 IEEE 93rd
Vehicular Technology Conference, pp. 1–6, April 2021.
[3] ITU-R, “World Radiocommunication Conference 2023 (WRC-23) Provisional FinalActs,” ITU Publications, 2023.

17

III. Very Low Earth Orbit Satellite Networks for 6G [1]
Shi Xueliang, Huawei Technologies
Koshimizu Takashi, Huawei Technologies Japan

Abstract—With the breakthrough of advanced satellite launching and manufacturing
technologies in recent years, both the academia and industrial communities are making
considerably efforts to study mega-constellations for Very Low Earth Orbit (VLEO)
satellites. The non-terrestrial network (NTN) is widely believed to be a part of the 6G
network. In this paper, the vision for the evolution of VLEO satellites-based NTN
towards 6G is proposed, as well as the technical challenges and potential solutions.
III-1. Introduction
The idea of Very Low Earth Orbit (VLEO) has the potential to change the paradigm
for the Internet because it is much lower than the traditional Low Earth Orbit (LEO) or
Geostationary Earth Orbit (GEO). Accordingly, communications based on mega VLEO
constellations are envisioned owing to attractive features such as low transmission delay,
smaller propagation loss, high area capacity, and lower manufacturing and launching
cost when compared with traditional LEO or GEO satellites.
III-2. Driving factors and motivations
Integrating non-terrestrial and terrestrial communications systems will achieve 3D
coverage of the Earth. They will not only provide communications with broadband andwide-range IoT services around the world, but also provide new functions such as
precision-enhanced positioning and navigation and real-time earth observation.
A fundamental integration of TN and NTN will change the status quo and significantly
improve user experience. By achieving unified design of TN and NTN, the barrier among
different satellite systems will also be eliminated, allowing users to freely roam among
terrestrial networks and non-terrestrial networks of different operators.
III-3. Usage scenarios and motivations
The VLEO-based NTN is expected to provide couples of usage scenarios andapplications：III-3.1. Extreme coverage
By using non-terrestrial network nodes, such as satellites, unmanned aerial vehicles,
and high-altitude platforms, non-terrestrial networks can be flexibly deployed,connecting people through various devices such as smartphones, laptops, fixed-line
phones, and televisions.

18Fig. III-1 Usage Scenarios and Applications
III-3.2. Mobile broadband for the unconnected
Satellites can directly connect to mobile phones, providing broadband connectivity,with data rates similar to those of cellular networks in remote areas.
III-3.3. Broadband connection on the move
People should be able to access the Internet anytime, anywhere, no matter what kindof transportation they take. Future communications systems should provide MBB
experience connections for all aircraft passengers.
III-3.4. Wide-range IoT services extended to unconnected locations
IoT devices should be able to connect and report information anytime, anywhere, suchas the status of Antarctic penguins, the living conditions of polar bears, and animal andcrop monitoring, from remote and uninhabited areas.
III-3.5. High-precision positioning and navigation
The integrated network can implement high-precision positioning and navigation andimprove the positioning accuracy from meters to centimeters.

19

III-3.6. Real-time earth observation and protection
Earth observation can be introduced to more scenarios, such as real-time traffic
dispatch, real-time remote sensing maps, and quick response to disasters.
III-4. Challenges and Solutions
III-4.1. Integrated network architectures
The following potential solutions are needed to provide unified services involving both
TN and NTN with a single device, 1) 3D UE-centric cell-free communication where thecell boundaries can be eliminated efficiently; 2) Network slicing enables multiple logicalnetworks to run as independent tasks on a common shared physical infrastructure; 3) A
hierarchical control framework with very few ground stations and GEO satellites is used
to achieve global network control, while MEO satellites and LEO/VLEO satellites with
ISL capabilities are used for regional and local control.
III-4.2. Air interface technologies
Two potential solutions are provided to address this fundamental challenge and fully
unleash the service capabilities.
III-4.2.1. On-demand coverage for imbalanced requirements
The beam-hopping concept is introduced to adapt the imbalanced requirements overthe satellite coverage area [2]. Beam-hopping technology can use all the available
satellite resources to provide services to specific locations or users. By adjusting thebeams' illumination duration and period, different offered capacity values can beachieved, i.e., the imbalanced requirements in different beams can be satisfied, as shownin Fig. III-2.

Fig. III-2 Beam-Hopping Scheduling scenario (left) and performance (right)

20

Multi-satellite cooperative transmission is another enabler to achieve on-demand
coverage, as shown in Fig. III-3. Such a scheme makes sense considering the fact that
only a very small percentage of the covered area is in service and multiple satellites areusually visible with a mega constellation. This technology enables one user to receive
multi-satellite signals simultaneously. Accordingly, the user transmission rate or the
peak capacity density can be increased when a user receives signals from multiple
satellites at the same time, or when multiple satellites receive signals from the user.

Fig. III-3 Multi-satellite Cooperative Transmission
III-4.2.2. Multi-beam precoding for high spectral efficiency
Multi-color frequency reuse is usually adopted to mitigate the co-channel interferencein satellite communications, which leads to very low system spectrum efficiency. Multibeam precoding can provide full-frequency reuse and improve the spectrum efficiency in
VLEO/LEO satellite communications scenarios [3]. As the main characteristic of thesatellite channel is Line of Sight, the multi-beam precoding matrix can be calculated
based on the large-scale channel which is approximately decided by the relative location
between the UE and the satellite. Multi-beam precoding can result in a huge gain interms of total throughput during the time the satellite provides services.
III-4.3. Dynamic topology and routing algorithm.
Routing table sizes can grow dramatically as the satellite network grows in size, andthe unpredicted link failure will lead the affected nodes with outdated topology for a long
period of time. Orthodromic Routing (OR) is a promising solution to address the above
problems by trading some packet losses against massive scalability [4], as shown in Fig.

21

III-4. OR consists of an addressing and forwarding plane, a path computation algorithm,
and a limited flooding algorithm. The addressing plane of OR embeds the <X,Y,Z>
coordinates of a point on the unit sphere for both the source and destination into the IP
header thus obviating the need for constant translation of identification and location.
The data plane then forwards packets to the closest satellite within a relatively small
flooding vicinity along the shortest path to that satellite. All satellites also havecoordinate-based addresses which are a strict function of time. Therefore, all satellites
can calculate their own addresses and the addresses of the satellites in their flooding
region as a function of time.

Fig. III-4 Orthodrome Relative to the Great Circle
III-4.4. Powerful on-board capabilities
The on-board capabilities call for thorough enhancements to accommodate
communication requirements of NTN for 6G, mainly in on-board processors, radio
frequency subsystem, antennas, and data transmission algorithms. Massive-beamsatellites with on-board data processing capabilities and advanced algorithms will playa key role in future low-orbit satellite communications, providing more linking
capabilities for users over the coverage area through frequency and beam traffic
reconfiguration. In future NTN, massive-beam high-gain phased array antennas will beequipped to prevent the extremely high path loss from space to ground.
III-4.5. Low-cost manufacturing & service
A full integration of satellite communications into the cellular system is expected to be
the most effective way to reduce the cost of communications components in groundsegment devices like UEs, gateways, as well as the on-board processing system.

22

The service cost will also benefit from the full integration since it is also related to theeconomies of scale and the network capacity of a satellite system can be much better
utilized to reduce the overall service cost.
There have been some explorations in recent years on using commercial-class devices
i.e. the Commercial-Off-The-Shelf (COTS) parts in spacecrafts. Optimized processes,
such as a better balance of the cost and reliability in the screening, new shield designs,
and a fault detection and recovery mechanism, are needed to ensure the stability andcommercial efficiency of spacecrafts.
III-4.6. Interference reduction and co-existence
Considering the fact that very limited frequency resources are available, it is more
important than ever to design a frequency sharing mechanism from a technical neutral
perspective. There are several hierarchical frequency sharing technologies
Space isolation: the same frequency resource can be allocated to both cellular andsatellite networks that are geographically far away from each other.
Angle isolation: For scenarios targeting mmWave bands where only UEs withdirectional antenna are deployed, angle isolation can be considered to prevent theinterference caused from different systems.
Scheduling-based interference coordination: with close interaction among neighboringbase stations, joint decisions can be made among those stations to prevent interference.
III-5. Conclusion
The successful realization of LEO/VLEO-based NTN communications calls for joint
efforts from the academia and industrial communities. The ongoing development of newtechnologies and the growing interest and investments in space applications is extending
the boundaries of potential LEO/VLEO-based communications to new heights. In
addition to the technical aspects of satellite communications itself, a fundamental
integration of cellular- and satellite-based communications at the physical layer fromday- one is also the key to the commercial success of LEO/VLEO-based satellite
communications in 6G. The NR-based NTN discussion in 3GPP provides an excellent
platform that traditional cellular and satellite communities can use to work together tobuild a fully integrated network.
REFERENCE[1] Very-Low-Earth-Orbit Satellite Networks for 6G,
https://www.huawei.com/en/huaweitech/future-technologies/very-low-earth-orbitsatellite-networks-6g

23

[2] J. Tang, D. Bian, G. Li, J. Hu and J. Cheng, "Resource allocation for LEO beambopping satellites in a spectrum sharing scenario," IEEE Access, vol. 9, pp. 5646856478, 2021.
[3] G. Zheng, S. Chatzinotas and B. Ottersten, "Generic optimization of linear precoding
in multibeam satellite systems," IEEE Transactions on Wireless Communications,vol. 11, no. 6, pp. 2308-2320, June 2012.
[4] Huawei, “Integrated Non-terrestrial and Terrestrial Networks - Routing and Delay
Analysis in LEO Constellations”.

24IV. Multi-layer non-terrestrial network and its routing management for
Beyond 5G /6G
Munehiro Matsui, NTT corporation.
Hisayoshi Kano, NTT corporation.
Junichi Abe, NTT corporation.
Fumihiro Yamashita, NTT corporation.

Abstract—This paper presents the concept of multi-layer non-terrestrial network(multi-layer NTN), which consists of geostationary orbit satellite, low earth orbit
satellite and High Altitude Platform Station (HAPS). We also show our routing
management technology for multi-layer NTN.
IV-1. Introduction
Next-generation mobile communication systems such as Beyond 5G and 6G have been
attracting the attention of mobile carriers and research institutes around the world [1–4]. One of the requirements for 6G is extreme coverage extension that spans places themobile ground networks cannot reach, such as the sea, remote areas, and the sky [4].
The key technology to achieve this extreme coverage extension is the non-terrestrial
network (NTN). In the NTN, satellites or High Altitude Platform Station (HAPS)
networks are used to form service areas on the ground from space [4–6].
NTT is also studying to achieve the space integrated computing network shown in Fig.
IV-1, which is an infrastructure that integrates multiple orbits from the ground to HAPS,
Low Earth Orbit (LEO) and geostationary orbit (GEO) satellites [7]. They are connectedto the ground by an optical wireless communication network to form a constellation, anddistributed computing enhances various data processing. The space integrated
computing network also provides access to terrestrial mobile devices for ultra-wideservice coverage using space radio access network (Space RAN). We introduce the multilayer non-terrestrial network (multi-layer NTN) as Space RAN in chapter 2. In chapter
3, we also show our traffic route control technology, which is one of our research results
for multi-layer NTN.

25

Fig. IV-1 Space integrated computing network.
IV-2. Multi-layer satellite network (Space RAN)
Fig. IV-2 shows an overview of the multi-layer NTN. It consists of GEO satellite, LEO
satellite, and HAPS network. The GEO and LEO satellites in the GEO/LEO network andthe aircrafts in the HAPS network are connected by a wireless link such as radio or freespace optics (FSO). These satellites and aircrafts form a service area that covers thesurface of the earth and the sky. User equipments (UEs) on the ground connect to thesatellite/HAPS networks, and the traffic generated by UEs are transferred to 5G core
network (5GC) via ground base station.
The network controller controls the link connection between the satellites and aircrafts
to build the optimum network topology on the basis of the position of thesatellites/aircrafts, the state of the wireless link between satellites/aircrafts, and theground base station. This improves the availability of the NTN. For example, a highfrequency such as the Ka band and Q band is required to widen the bandwidth of a feeder
link. However, the feeder link may be broken due to heavy rain because the attenuation
loss of radio wave increases as the frequency increases. Therefore, the network controller
controls the link connection on the basis of the link connection status and the rain
conditions so that the network does not disconnect between UE and 5GC.
The network controller also controls the traffic routing. When feeder link is broken due
to heavy rain, traffic might be concentrated on a specific link. When the traffic flows
exceed the network capacity of a link, packet loss or transfer delay occurs due to traffic
congestion. To tackle the problem, we proposed a traffic-control scheme. In the next
chapter, we present our traffic control technology in detail.

26

Fig. IV-2 Multi-layer NTN.
IV-3. Traffic control for multi-layer satellite network
In the proposed scheme, traffic route between UE and mobile network is derived at
network controller. The network controller selects the optimal route from among thecandidate routes by calculating the route cost, taking into account traffic congestion anddelay time at inter satellite links (ISLs).
Route cost C is calculated using (1).
𝑛

𝐶 = ∑ 𝐶𝑖 𝐶𝑖 =𝑖=1𝐵𝑟
𝑅（1 − 𝛼）

+

𝐷
,
𝐵𝑑

… (1)

where  is defined as (2)
α=

𝑟
,
𝑅

… (2)

Other variables are shown in Table IV-1.
Route costs are calculated for all candidate routes between UE and mobile network, andthe route with the minimum route cost is derived as the optimal route. Finally, thenetwork controller distributes routing tables to satellites and HAPS aircrafts.
We compared the performance with conventional OSPF (Open shortest path first)
routing scheme and proposed scheme by computer simulation. Simulation model andparameters are shown in Fig. IV-3 and Table IV-2. Time-varying rainfall areas varied
the communication availability of the feeder links, and then the network controller
calculates the optimal route and update the route. Fig. IV-4 shows the throughput anddelay time performance of conventional and proposed routing scheme. The results showthat the performance of the proposed scheme is superior to that of conventional scheme
in throughput and delay time.

27

Table IV-1 Variables in (1) and (2)Fig. IV-3 Simulation model
Table IV-2 Simulation Parameters28

Fig. IV-4 Simulation results
IV-4. Conclusion
In this paper, we presented our concept of a multi-layer satellite network for NTNs
(Multi-layer NTN). In addition, we introduced our routing management technology for
multi-layer NTN.
REFERENCE[1] M. Giordani, M. Polese, M. Mezzavilla, S. Rangan, and M. Zorzi, “Toward 6G
Networks: Use cases and Technologies,” IEEE communication Magazine, PP. 55-61,
vol. 33, March 2020.
[2] https://www.ericsson.com/en/6g, Ericsson.

29

[3]

https://www.qualcomm.com/content/dam/qcomm-martech/dmassets/documents/web-why-and-what-you-need-to-know-about-6G-in-2022.pdf,
Qualcomm[4]

https://www.docomo.ne.jp/english/corporate/technology/whitepaper_6g/,

NTT

Docomo.
[5] https://beyond5g.nict.go.jp/en/download/index.html, NICT.
[6] X. Lin, S. Rommer, S. Euler, E. A. Yavuz, and R. S. Karlsson, “5G From Space: An
overview of 3GPP Non-terrestrial Networks,” IEEE communications Standards
Magazine, pp. 147-153, December 2021.
[7] https://group.ntt/en/newsrelease/2021/05/20/210520a.html, NTT.

30

Abbreviation List
Abbreviation

Explanation

5GC

5G Core network

3GPP

Third Generation Partnership ProjectCA

Carrier Aggregation

CBH

Cellular Backhaul

COTS

Commercial-Off-The-Shelf

DC

Dual Connectivity

DL

Down Link

EPC

Evolved Packet CoreFL

Feeder Link

FSO

free-space optics

GEO

Geostationary Earth Orbit

HAPSHigh Altitude Platform Station

IMTInternational Mobile Telecommunications

IoT

Internet of Things

IP

Internet Protocol

IPv6

Internet Protocol version 6

ISLInter Satellite Link

LEO

Low Earth Orbit

LTELong Tern Evolution

MaaS

Mobility as a service

MBB

Mobile Broadband

MEO

Medium Earth Orbit

MIMO

Multiple-Input and Multiple-Output

NR

New Radio

NTN

Non-Terrestrial Network

OR

Orthodromic Routing

OSPF

Open Shortest Path FirstRAN

Radio Access Network

RSRP

Reference Signal Received Power

RTT

Round Trip Time31

Abbreviation

Explanation

RU

Radio Unit

SL

Service Link

TN

Terrestrial Network

UE

User Equipment

VLEO

Very Low Earth Orbit

WRC

World Radiocommunication Conference32