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Feature Articles: Space Integrated Computing Network

Vol. 20, No. 12, pp. 28–35, Dec. 2022.

Studies toward Practical Application of HAPS in the Space RAN

Yuki Hokazono, Yoshihisa Kishiyama, and Takahiro Asai


The space radio access network is regarded as a communication infrastructure for the 5th-generation mobile communication system (5G) Evolution and 6G, and extreme coverage extension is being studied for use cases in all locations including air, sea, and space. For early implementation of extreme coverage extension, we are focusing on low-latency communication services using high-altitude platform stations (HAPSs). In this article, we present use cases and technical issues of wireless system technology with HAPSs and propose a three-dimensional-cell control technology for frequency sharing between a HAPS and terrestrial networks.

Keywords: HAPS, Space RAN, extreme coverage extension


1. Introduction

The space radio access network (Space RAN) is regarded as a communication infrastructure for the 5th-generation mobile communication system (5G) Evolution and 6G, and aims to achieve extreme coverage extension*1 to all locations, including the sky, sea, and space, which have not been sufficiently covered by conventional mobile communication networks, using non-terrestrial networks (NTNs)*2 based on geostationary (GEO) satellites, low Earth orbit (LEO) satellites, and high-altitude platform stations (HAPSs)*3 [1].

For early implementation of extreme coverage extension, we are focusing on low-latency communication services using HAPSs [2]. HAPSs make it easy to extend communication-service coverage to a wider area; thus, making it possible to provide highly reliable communication in times of disaster, high-capacity communication for ships and aircraft, and communication services for distant islands and remote areas. Mobile carriers can improve the overall cost-effectiveness and energy efficiency of their mobile networks by combining HAPSs with an increase in the number of their terrestrial base stations to extend their service coverage.

This article describes the efforts toward the practical application of HAPSs in the Space RAN. Specifically, we present use cases and technical issues of wireless system technology with HAPSs and propose a three-dimensional (3D)-cell control technology for frequency sharing between a HAPS and terrestrial networks (TNs).

*1 Extreme coverage extension: To extend the area where base stations can communicate with mobile station terminals to all locations, including the sky, sea, and space, not covered by the current mobile communication system.
*2 NTN: Any network in which the communication area is not limited to the ground but extended to other locations such as the air, sea, and space through the use of non-terrestrial equipment such as satellites and HAPSs.
*3 HAPS: An airborne platform that is designed to operate in the stratosphere on board a vehicle such as a solar-powered aircraft or airship.

2. HAPS use cases and network configuration/control technologies

NTT DOCOMO is researching and developing communication methods and network architectures that can flexibly link 5G networks and other TNs with stratospheric HAPS networks [3]. In addition to providing flexible support for a wide range of future use cases as envisioned in 5G Evolution and 6G, this project is conducting studies aimed at the implementation of communication systems that use HAPSs in terms of development and operation costs.

2.1 HAPS use cases

As shown in Fig. 1, for the 5G Evolution 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*4 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.

Fig. 1. Various use cases expected for HAPS.

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 disaster countermeasures, 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.

2.2 Cooperative network configuration and control technology for HAPSs and TNs

2.2.1 Classification of HAPS-mounted stations

Regarding the network configuration and control technology used when implementing backhauls to 5G base stations via HAPSs, we are focusing on the categorization of HAPS-mounted stations. They can be roughly divided into two types: (1) 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 (2) base stations, which are made by installing 5G network base-station equipment (or at least part of it) in a HAPS [4]. 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 are strictly 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 the advantages of lower development costs and ease of operation, but implementing these functions on the HAPS results in greater resilience to natural disasters. In terms of performance, a HAPS-mounted station should at least implement certain functions, such as 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 HAPS-equipped stations, their development and operation costs, the ability of these HAPS platforms to be shared by backhaul use and direct-to-device communication systems, and their ability to cooperate with GEO/LEO satellites.

2.2.2 Examples of network configuration in conjunction with the 5G network

An example of a HAPS base station in a network configuration linked to the 5G network is shown in Fig. 2. The distributed unit (DU) and radio unit (RU) of the 5G base station are mounted on the HAPS in accordance with Open RAN (O-RAN) ALLIANCE specifications [5]. In this configuration, availability is ensured by installing a centralized unit (CU) at a disaster-resistant point on the ground. Information received by the HAPS from the CU in the feeder link is transmitted via 5G radio to a small terrestrial base-station device (relay station) in the service link, enabling the use of portable 5G base stations without having to use a wired backhaul. In this configuration, it is also possible to provide direct communication from the HAPS to 5G terminals without the need for intervening relay stations. As a further extension, site diversity*5 can be implemented using multiple CUs on the ground side to reduce the impact of bad weather and natural disasters, and mobility support*6 can be implemented by switching the communication target to a different HAPS when the terminal moves from one communication area to another.

Fig. 2. Example of cooperative configuration when HAPS is used for backhaul.

Another promising configuration using a relay-type configuration where a 5G radio repeater is installed in a HAPS is shown in Fig. 3. In this configuration, the TN is used from the core network to the fronthaul*7, and the HAPS terrestrial system equipped with the RU function bundles and communicates signals for multiple beams. A broadband frequency, such as the Q-band, is used in the feeder link*8, and the HAPS relay system executes frequency conversion and power control. The HAPS can then establish service-link*9 communications using multiple beams at the same time. As the service link, certain frequency bands below 2.7 GHz already identified for International Mobile Telecommunications (IMT) should be used according to the specifications approved at the World Radiocommunication Conference 2019 (WRC-19) [6] and the agenda item for WRC-23 [7].

Fig. 3. Example of cooperative configuration when HAPS is used for direct access.

In addition to the configurations shown in Figs. 2 and 3, we consider other promising configurations in which a HAPS is used to carry a standalone*10 5G base station. For each configuration, it is necessary to conduct a comprehensive study that takes into account various attributes such as mobility support, site diversity technology, frequency-sharing technology*11, and HAPS installation requirements such as links with GEO/LEO satellites, the equipment weight, and power consumption.

*4 Backhaul: In a mobile communication network, a backhaul is a fixed line that supports high-speed, high-capacity transmission of information between a large number of wireless base stations and the core network.
*5 Site diversity: A technique for improving communication quality by switching between multiple ground stations when radio waves are highly attenuated due to rain or obstacles.
*6 Mobility support: Technology that allows communication to continue when a terminal moves across a communication area by switching it to a different base station before communication is interrupted.
*7 Fronthaul: The line between the baseband processing unit of the base station and the wireless device, such as optical fiber.
*8 Feeder link: A communication path between a satellite or HAPS and a terrestrial base station (gateway) in an NTN communication system.
*9 Service link: A communication path between a satellite or HAPS and a terminal in an NTN communication system.
*10 Standalone: A deployment scenario using only New Radio (NR), in contrast with non-standalone operation which uses Long Term Evolution (LTE)-NR Dual Connectivity to coordinate existing LTE/LTE-Advanced and NR.
*11 Frequency-sharing technology: Technology that makes it possible to share frequencies by suppressing the interference effects that occur when two systems use the same frequency at the same location. In this article, we are mostly concerned with frequency sharing between HAPS systems and terrestrial mobile communication systems.

3. 3D-cell control technology for frequency sharing between a HAPS and TNs

For mobile applications in which general user equipment (UE) communicates directly with HAPS base stations, the use of frequency bands below 2.7 GHz specified for IMT is being discussed as International Telecommunication Union (ITU) agenda item 1.4 at WRC-23 [8]. By sharing the same frequency between a HAPS and a TN, sets of UEs connected to the terrestrial IMT network can directly connect to the HAPS and conserve frequency resources. In this section, we present our proposed 3D-cell control technology that avoids interference between a HAPS and TNs. We also present a HAPS-performance evaluation in the 2-GHz band using the 6G system-level simulator developed as a stepping stone for actualizing HAPS technology.

3.1 Evaluation of interference avoidance in the 2-GHz band using 3D-cell control technology

3.1.1 3D-cell control technology

Our 3D-cell control technology suppresses the intersystem interference between a HAPS and TNs. As shown in Fig. 4, the HAPS suppresses the intersystem interference and achieves load balancing by not directing the beam to the distance threshold X [km] around the terrestrial next-generation NodeB (gNB) (hereafter, X denotes gNB connection threshold).

Fig. 4. Handover and 3D-cell control technology.

3.1.2 Simulation conditions

We assumed a scenario in which the HAPS service link and terrestrial gNB link share the 2-GHz band, as shown in Fig. 5. The HAPS simulator was used to evaluate the interference avoidance of the 3D-cell control technology. Table 1 lists the system-related parameters and Table 2 lists the simulation parameters for each device. The layout of each device is shown in Fig. 5. As an initial evaluation, two Earth stations, two HAPSs, 500 UEs, and two terrestrial gNBs were placed in an area of approximately 60 × 114 km that includes Tokyo, Japan. The two terrestrial gNBs were placed near Tokyo, and UEs were randomly placed at a ratio based on population distribution.

Fig. 5. HAPS simulator.

Table 1. System-related parameters.

Table 2. Parameters for each device.

3.1.3 Simulation results

The values of the cumulative distribution function (CDF) at 5, 50, and 95% for the throughput for all UEs when X was changed are listed in Table 3. We confirmed that load balancing worked as X increased and the average throughput for all UEs improved in the downlink. If X increases to 9 km, the number of UEs with low received power of the desired wave increases, and the throughput deteriorates because even UEs at a point far from the terrestrial gNB are connected to it. When comparing the maximum number of HAPS beams, the throughput for all UEs was higher when the number of beams was 500 than when it was 50. This is because each HAPS-connected UE can select the optimal HAPS beam that yields the maximum gain.

Table 3. Downlink throughput of HAPS-connected UEs.

The values of CDF at 5, 50, and 95% in the interference-to-noise ratio (I/N) of gNB-connected UEs when X was changed are listed in Table 4. The I/N decreased as X increased, so the interference avoidance effects could be confirmed. When comparing the maximum number of HAPS beams, the I/N in the downlink was higher when the number of beams was 50 than when it was 500. This is because the smaller the number of beams, the larger the transmission power per beam of a HAPS.

Table 4. Downlink I/N of gNB-connected UEs.

The same evaluation was conducted on the uplink, and load balancing and interference avoidance effects were confirmed as on the downlink. It is necessary to select an appropriate X considering that the coverage area of a HAPS decreases with the increase in X.

4. Conclusion

As part of our efforts to make HAPSs practical in the Space RAN, we presented use cases and technical issues with wireless system technology with HAPSs and proposed a 3D-cell control technology for frequency sharing between a HAPS and TNs.

NTT DOCOMO will continue developing NTN technology aimed at achieving extreme coverage extension and technology for HAPS networks and promoting demonstration experiments and standardization activities.

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).


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Yuki Hokazono
Research Engineer, 6G-IOWN Promotion Department, NTT DOCOMO, INC.
He received a B.E. and M.E. from Graduate School of Engineering, the University of Tokyo, in 2017 and 2019. Since joining NTT DOCOMO in 2019, he has been involved in research and development activities related to radio-frequency wireless technologies for 5G Evolution and 6G, especially non-terrestrial networks. He also contributed to the 3rd Generation Partnership Project (3GPP) TSG-RAN WG1 as a delegate from NTT DOCOMO.
Yoshihisa Kishiyama
Manager, 6G-IOWN Promotion Department, NTT DOCOMO, INC.
He received a B.E., M.E., and Ph.D. from Hokkaido University in 1998, 2000, and 2010. Since joining NTT DOCOMO in 2000, he has been involved in the research and development of 4G and 5G radio access technologies, including concept development, standardization, and trials. He is currently investigating 5G Evolution and 6G and is the main author of “DOCOMO 6G White Paper.” He has more than 400 granted patents in the area of mobile communications. In 2012, he received the ITU Association of Japan Award for his global contributions to LTE.
Takahiro Asai
General Manager, 6G-IOWN Promotion Department, NTT DOCOMO, INC.
He received a B.E. and M.E. in electrical and electronics engineering and Ph.D. in communications and computer engineering from Kyoto University in 1995, 1997, and 2008. In 1997, he joined NTT Mobile Communications Network, Inc. (currently, NTT DOCOMO, INC.). Since then, he has been engaged in the research of signal processing for mobile radio communication, and is currently investigating 5G Evolution and 6G.