A Primer on HIBS -- High Altitude Platform Stations as IMT Base Stations
1 Abstract — Mobile communication via high-altitude platforms operating in the stratosphere is an idea that has been on the table for decades. In the past few years, however, with recent advances in technology and parallel progress in standardization and regulatory bodies like 3GPP and ITU, these ideas have gained considerable momentum. In this article, we present a comprehensive overview of HIBS – High Altitude Platform Stations as IMT Base Stations. We lay out possible use cases and summarize the current status of the development, from a technological point of view as well as from standardization in 3GPP, and regarding spectrum aspects. We then present preliminary system level simulation results to shed light on the performance of HIBS. We conclude with pointing out several directions for future research. I. INTRODUCTION Over the past decades, mobile operators have greatly expanded the coverage of broadband wireless service, with the total number of mobile subscriptions exceeding 8 billion in 2020 [1]. Despite the wide deployment of terrestrial mobile networks, there is still a need for greater broadband connectivity services in remote communities. If non-terrestrial technologies could be deployed at a competitive cost, and if interworking with terrestrial networks could be achieved, they might have the potential of providing connectivity in remote areas, thereby complementing the terrestrial networks. Non-terrestrial networks (NTN) refer to networks utilizing spaceborne or airborne payloads for communication. The recent interest in spaceborne satellite communication has been centered on Low Earth Orbit (LEO) NTN that feature large constellations with thousands of satellites to provide global broadband access [2]. The focus of this article is on airborne NTN utilizing the same frequency bands as ground based International Mobile Telecommunications (IMT) base stations (BS). This concept is known under the designation High Altitude Platform Stations (HAPS) as IMT base stations, or HIBS. By using the same spectrum as already identified for IMT and where deployments already exist today, HIBS can extend the operator’s coverage area and benefit from the already existing device ecosystem. While HAPS and HIBS both refer to High Altitude Platforms, they differ in the type of spectrum that they will be using. Often, the term HAPS is used also for the aircraft carrying the communications payload. Throughout this article, however, we reserve it (and the term HIBS) for the complete communications platform. For the aircraft alone, we use the term “high - altitude platform”. The objective of this article is to https://hapsalliance.org provide a state-of-the-art primer on HIBS, considering the latest developments in this area. HIBS operate in the stratosphere, usually at an altitude of about 20 km. HIBS may be airplanes, airships or balloons, typically unmanned. When compared to a terrestrial network, a HIBS system may provide wider coverage. When compared to a satellite network, a HIBS system may provide lower latency. Thus, in addition to satellite systems, HIBS can play a role for expanding mobile coverage to remote communities. The studies of high-altitude platforms for telecommunications and remote sensing can be traced back to the 1990s [3]. A milestone for HAPS initiatives was the identification of the first frequency bands for HAPS in the Fixed Service in the International Telecommunication Union (ITU) Radio Regulations in 1997, followed by the identification of additional frequency bands for HAPS to provide IMT service (HIBS) in 2000. Despite the early interest, the development of high-altitude platforms for commercial connectivity in the 1990s and 2000s was limited due to immaturity of technical solutions. A resurgence of interest in providing connectivity using high-altitude platforms started around 2014, mainly driven by the Internet companies Google and Facebook that invested in new technology to beam connectivity through the atmosphere to reach remote areas. Admittedly, technology advancements in connectivity, as well as in areas such as solar panel efficiency, power storage, lightweight composite materials, avionics, microelectronics, and antennas have now made HAPS/HIBS systems more viable, leading to the creation of HAPS Alliance . A high-level overview of HAPS communications was provided in [4] back in 2007. A more recent survey on high-altitude platforms was presented in [3], which focused on technologies directly related to airborne platforms but did not address the connectivity aspect. The work [5] presented an investigation into the constellation design methodology of HAPS systems. A method for maximizing sum rate in a HAPS system, based on interference alignment, was proposed in [6]. In parallel with the academic studies about HAPS/HIBS, there have been notable new developments in standardization and on the regulatory front. In particular, the World Radiocommunication Conference 2019 (WRC-19) defined an agenda item for WRC-23 on HAPS as IMT BS, i.e., HIBS, in certain frequency bands already allocated to the Mobile Service as well as identified for IMT [7]. In addition, the 3rd Generation Partnership Project (3GPP) has been working on evolving the fifth generation (5G) radio access technology – known as New Radio (NR) – to support NTN which include HIBS systems. Sebastian Euler, Xingqin Lin, Erika Tejedor, Evanny Obregon Ericsson Emails: {sebastian.euler, xingqin.lin, erika.tejedor, evanny.obregon}@ericsson.com A Primer on HIBS – High Altitude Platform Stations as IMT Base Stations
USE
CASES OF HIBS This section outlines some of the potential use cases for HIBS.
Network coverage expansion : HIBS can cover sparsely populated or hard to reach geographical areas where terrestrial infrastructure is impossible or too costly to build (e.g. mountains, deserts, oceans, etc.). With the expected wide coverage from HIBS solutions, it might be possible to expand the network coverage area in a timely and cost-effective manner in order to narrow the infrastructure gap between urban and remote areas.
Disaster resiliency : Since HIBS provide connectivity from the Stratosphere, they represent a highly resilient network infrastructure against natural disasters such as earthquakes, floods, and bushfires. A HIBS-based network can be used as a back-up network in the situation where terrestrial infrastructure is not operative as a consequence of a natural disaster. This can ensure a high-level of availability for critical operations.
Fostering IoT deployment : The deployment of mobile networks traditionally follows the population density A HIBS-based network could be built to provide coverage for IoT devices and goods, considering their more diverse uses and locations (crop, forest, wildlife, oil & gas, logistics). HIBS-based networks could be a complement to terrestrial networks to expand the IoT deployment.
Drones : As HIBS are operating from the Stratosphere, they can also provide 3D coverage of a large geographical area to facilitate drone operations. Terrestrial networks may not be optimal for serving drones, due to their antennas being down-tilted to optimize coverage on the ground [9]. A HIBS-based network can provide connectivity from the ground up to the sky. III.
CHARACTERISTICS OF HIGH-ALTITUDE
PLATFORMS Overall, high-altitude platforms can be classified into three groups with different characteristics and challenges: airplanes, balloons, and airships. Airplanes need to move through the surrounding mass of air to generate lift and thus need part of their energy to power their engines. Balloons and airships, on the other hand, are lighter than air and thus do not need to spend energy to stay airborne. Airships are equipped with engines and are thus fully steerable, too, which means that they can be flown to their operation areas and positioned as desired. Balloons, on the other hand, move only with the winds, resulting in limited steerability at best. Common to these three classes is that the vehicles are generally unmanned and fully automated, which is a prerequisite to allow economical operation and flight durations of weeks or even months. The operational altitude of about 20 km is chosen mainly because of the favorable atmospheric conditions in this part of the atmosphere. Almost all weather phenomena happen below, and in particular wind speeds are very low and comparable to those at the surface. An additional benefit is that the airspace above approximately 20 km height is not regulated by air traffic control in most countries [3]. A further requirement for long-duration missions and thus another common feature is that the vehicles are solar-powered. While the desired consequence of this is the independence of any carried fuel, it also highlights one of the technological challenges of HIBS: operation at high latitudes during the winter months, with little sunlight, is challenging or – in the polar regions – even impossible. The current generation of HIBS is restricted to a latitude band of approximately 35 degrees north and south of the equator, if year-round operation is desired. Engineering challenges are plenty in high altitude platforms, and each vehicle type has its own. Since the lift force generated by an airplane ’s wing is proportional to the density of the surrounding air, which is reduced to about 5% at 20 km height, these airplanes need to employ extreme lightweight construction methods and are largely made of carbon fiber. At the same time, a large wingspan is necessary to generate the required lift. The results of these engineering constraints are airplanes which have an extremely large wingspan (up to >70 m, more than a Boeing 747), but weigh only a few hundred kg. They are quite fragile and susceptible to turbulence, as was demonstrated by the loss of several airplanes, e.g. the loss of the Helios Prototype in 2003, and more recently those of Google’s Solara 50 in 2015, and
Facebook’s Aquila in 2016 [10]. The payload capacity is typically in the range of tens of kg. A benefit of the large wing is a lot of area that can be covered with solar panels, and a large HIBS airplane can easily produce electrical power in excess of 10 kW. The largest fraction of this power is needed to power the plane’s engines and to load the batteries for operation during night. Still, typically a few
Figure 1: HIBS architecture
NASA’s Pathfinder in the to Google’s Solara
50 in 2015. In 2018, the Airbus Zephyr was the first high-altitude platform to enter serial production, albeit still in single-digit numbers. An Airbus Zephyr plane also holds the record for the longest flight by any aircraft with almost 26 days in 2018 [11]. The AeroVironment HAWK30 (a descendent of the NASA Pathfinder/Helios program) also completed its first test flight in 2019 [12]. It is of particular interest because it is developed in a joint venture with the Japanese mobile network operator SoftBank and is intended to be used as a HIBS. In contrast to airplanes, high-altitude superpressure balloons, as used by Loon , are a proven technology and have been used for scientific purposes for decades. In contrast to the more common variable-volume balloons (e.g. weather balloons), they consist of stronger and less flexible material. This allows to keep the volume of the balloon approximately constant independent of outside air pressure. As a result, they do not need to vent gas or release ballast to keep a stable altitude, resulting in much longer possible flight durations. In 2019, one of Loon’s balloons set the record with a flight duration of 223 days. At the end of the flight, they are landed and can be recovered. The biggest disadvantage of balloons is certainly their relative inflexibility since they are not steerable and cannot easily be flown to a new region when demand changes. However, Loon have developed methods to keep their balloons within their desired area of operation by utilizing different wind directions at different heights [13]. T he balloons’ altitude is regulated by pumping air in or out, which increases or decreases the weight of the balloon, and thus decreases or increases the floating height. Recently, Loon started to provide commercial 4G LTE service in Kenya, making it the wo rld’s first HIBS operator. Airships, finally, seem at a first glance to combine the strengths of airplanes (their steerability) with those of balloons (their robustness and their ability to stay airborne without consuming electrical power). However, they come with their own share of engineering challenges. To allow for a reasonable payload mass, they need to be of very substantial size (typical lengths are >100 m), and thus require large and expensive ground infrastructure. Thermal modeling of such large airships is rather complicated and not yet fully understood. So far, only very few prototypes have been built, and these have been plagued by technical problems. Both the HiSentinel80 and the HALE-D airships made only a single flight. The only project in active development seems to be the Stratobus by Thales Alenia Space [14]. The main characteristics of the three types of high-altitude platforms are summarized in Table I. Apart from the difficulties related to the construction and operation of the platforms, operating a communications payload in the stratosphere is a challenge in itself. In addition to the strict weight and power limitations, the equipment might need to cope with low temperatures (below -60 °C), low air pressure, and high levels of UV radiation, unless the platform provides a fully climate-controlled payload compartment. This might put requirements on the electronics that are not very different from operating a satellite payload in space. IV.
SPECTRUM
ASPECTS High-altitude platforms may provide user traffic in specific bands according to the ITU Radio Regulations (RR) [15]. Some of these bands are allocated to the Fixed Service (referred to as HAPS in the RR), while others are allocated to the Mobile Service (i.e. HIBS). In particular, the bands 1885-1980 MHz, 2010-2025 MHz and 2110-2170 MHz in Region 1 (Europe, the former Soviet Union, Africa, and the Middle East,) and Region 3 (most of Asia and Oceania) and the bands 1885-1980 MHz and 2110-2160 MHz in Region 2 (North and South
Table I: High-altitude platform types
Airplane Balloon Airship Heavier or lighter than air
Heavier than air Lighter than air Lighter than air
Steerability
Fully steerable No or limited steerability Fully steerable
Power source
Solar powered
Operation altitude
About 20 km
Technical challenges
Large wingspan needed, fragile construction Limited steerability, cannot easily be flown to their area of operation Large ground infrastructure, thermal management
Table II: HIBS spectrum overview
ITU Region 1 ITU Region 2 ITU Region 3 HIBS frequency bands as per RR (Res.221)
Frequency bands for consideration under AI 1.4 at WRC-23 (Res.247)
HIBS
INITIATIVE IN Figure 2: Coupling loss (left) and Geometry SIR (right) distribution over the simulation area
SYSTEM
LEVEL
PERFORMANCE OF HIBS In this section, we present initial system simulation results to shed light on the performance of HIBS. Our simulations are based on the assumptions described in 3GPP TR 38.811, including the overall geometry of the setup, the antenna modeling, and the modeling of the radio propagation through the atmosphere. Scintillation losses occurring in the ionosphere above a HIBS are not considered. We simulate 19 cells, served by a single HIBS at an altitude of 20 km and an elevation angle of 90° as seen from the central cell. The beam footprint diameter of the innermost cell is approximately 10 km, resulting in a total service area of roughly 4000 km . The left panel of Figure 2 shows the coupling loss distribution over the simulation area, including the 16.5 dBi antenna gain of the HIBS antenna. It can be observed that the coupling loss values in outer cells are much larger than those in central cells. This is a purely geometric effect and a result of the comparatively low altitude of a HIBS (vs. a satellite), which leads to a large variation in the HIBS-to-ground distance between the center and the outer regions of the coverage area. In the example simulated here, the distance to the central cell is 20 km (the flight altitude of the HIBS), while the distance to the outer edge of the simulated area is larger than 40 km. As a result, the outermost layer of cells shows a coupling loss that is at least 4 dB larger than that of the central cell. To counteract this effect, applying a different antenna beam pattern with a higher gain or a higher output power for the outer cells may be considered. Nonetheless, the corresponding geometry signal-to-interference-ratio (SIR), shown in the right panel of Figure 2, is quite high throughout the simulation area. The median SIR is about 5.7 dB, with the 10 th and 90 th percentiles at -0.35 dB and 10.6 dB, respectively. Figure 3 shows the signal-to-interference-and-noise (SINR) distribution for different user densities. We simulate full-buffer data traffic, i.e. all users are receiving (DL) or transmitting (UL) all the time. In the DL, it can be observed that the SINR degrades with increasing user density. When the user density is below 1, there is on average less than one user per cell. In this regime, as the user density increases, inter-beam interference increases rapidly, leading to rapid degradation of the SINR. When the user density increases further, the system on average approaches a fully loaded state and enters an interference-limited scenario, making SINR insensitive to the user density. The UL SINR does not depend on user density. Compared with the DL SINR, it is always low, even with very low user densities. The reason is that we assume a regular handheld and thus power-limited terminal. As a result, the UL SINR is noise-limited and not much affected by the increased interference resulted from increased user density. To reach better SINR, a terminal with higher transmit power or equipped with a high gain antenna (e.g. a VSAT (very-small-aperture terminal) as defined in 3GPP TR 38.811) could be considered. Finally, we also study throughput performance for HIBS networks as a function of user density, shown in Figure 4. A similar pattern can be seen for DL and UL. The average user throughput (throughput measured at the individual users) decreases monotonically with increasing user density, since the available system capacity has to be shared by ever more users Figure 3: Downlink and uplink SINR
Figure 4: Downlink and uplink throughput
CONCLUSIONS
AND
RESEARCH
DIRECTIONS Connectivity everywhere and at any time is critical and mobile networks are key to achieve this goal. 5G is not just an evolution of mobile networks but a revolutionary technology that will further advance society. As part of 5G, NR includes non-terrestrial connectivity. In particular, HIBS can expand the current mobile network operators ’ coverage area to remote areas where terrestrial deployments are challenging or impossible, while reusing mobile spectrum assets and taking advantage of the available ecosystem. In this article, we have presented a state-of-the-art primer on HIBS. The overview provided herein covers use cases, key characteristics of HIBS systems, spectrum aspects, 3GPP standardization, and system level performance evaluations. Our results illustrate how the coupling loss between a HIBS and a terminal on the ground varies across the beam footprint. The results also show that, in the DL, the SINR decreases with increasing user density. In the UL, in contrast, the SINR is always low because of the power limited terminals. This is also reflected in the throughput results, where the UL user throughput at low user densities does not reach as high as in the DL. Making HIBS systems become a commercially available option to serve the underserved faces many challenges. We conclude by pointing out some fruitful avenues for future research. Power consumption optimization : Aerial platforms in HIBS systems usually rely on solar power systems to supply the necessary power for operations including telecommunications transceivers and antennas. As a result, the power availability might vary across daytime and nighttime and over months/seasons. Adapting and optimizing the communications design to the power constraint of HIBS systems is a largely under-explored research area.
Connectivity for high-latitude regions : Aerial platforms in HIBS systems relying on solar power may face challenges when flying at high-latitude regions during the winter months when daylight hours are few. In order to provide global coverage with HIBS systems, how to provide connectivity to high-latitude regions is an important challenge to overcome.
Coordination and coexistence between HIBS systems and terrestrial mobile networks : As HIBS systems aim to use the same frequency bands as ground-based IMT BS, how to coordinate HIBS systems with terrestrial networks is an important and rich area to look into. Example issues include interference coordination, seamless mobility, and load balancing between HIBS systems and terrestrial networks. Adjacent channel coexistence with neighboring mobile operators is also a key area for further investigation.
Trials and test deployments : Due to the unique characteristics and challenges of deploying HIBS in the stratosphere, it is imperative to conduct extensive early trials and test deployments to collect feedback. Such trials and test deployments will help identify potential enhancement areas. The knowledge obtained can provide guidance to the evolution of 5G NR technology for better supporting HIBS systems. REFERENCES [1]
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BIOGRAPHIES
Sebastian Euler is a Senior Researcher at Ericsson. He joined Ericsson in 2016 and has since focused on the standardization of Non-Terrestrial Networks in 3GPP, extending the LTE and 5G New Radio standards with support for satellite networks and aerial vehicles. He has contributed to academic articles and to the book “5G and Beyond: Fundamentals and Standards” . He has a background in particle physics, and received his Ph.D. from RWTH Aachen University, Germany, in 2014. Before joining Ericsson, he held a postdoctoral position at Uppsala University, Sweden, during which time he worked with neutrino experiments in Antarctica.
Xingqin Lin (M’13 - SM’20) received the Ph.D. in electrical and computer engineering from The University of Texas at Austin, USA. He is currently a Master Researcher and Standardization Delegate at Ericsson. He is a member of Ericsson NextGen Advisory Board. He has contributed to 5G NR, NB-IoT, and LTE standards. He is co-author of the book “Wireless Communications and Networking for Unma nned
Aerial Vehicles” and lead editor of the book “5G and Beyond: Fundamentals and Standards.” He served as an editor of the
IEEE COMMUNICATIONS LETTERS from 2015-2018 and is serving as the Industry Liaison Officer for the IEEE ComSoc Emerging Technology Initiative on Aerial Communications. He received the award of 2020 IEEE ComSoc Best Young Professional in industry.
Erika Tejedor is currently the Director Government and Industry Relations at Ericsson. She has been leading the technical work toward spectrum regulations. She was involved in the ITU-R preparations for WRC-19 as well as in CEPT and FCC regulatory work. She has also taken an active role in the 3GPP RAN4 standardization process for 3G and 4G. She joined Ericsson in 2008 after receiving her Master's degree in wireless communications from the University of Zaragoza, Spain.