Electrification turned over a new leaf in aviation by introducing new types of aerial vehicles along with new means of transportation. Addressing a plethora of use cases, drones are gaining attention in the industry and increasingly appear in the sky. Emerging concepts of flying taxi enable passengers to be transported over several tens of kilometers. Therefore, unmanned traffic management systems are under development to cope with the complexity of future airspace, thereby resulting in unprecedented communication needs. Moreover, the long-term increase in the number of commercial airplanes pushes the limits of voice-oriented communications, and future options such as single-pilot operations demand robust connectivity. In this survey, we provide a comprehensive review and vision for enabling the connectivity applications of aerial vehicles utilizing current and future communication technologies. We begin by categorizing the connectivity use cases per aerial vehicle and analyzing their connectivity requirements. By reviewing more than 500 related studies, we aim for a comprehensive approach to cover wireless communication technologies, and provide an overview of recent findings from the literature toward the possibilities and challenges of employing the wireless communication standards. After analyzing the proposed network architectures, we list the open-source testbed platforms to facilitate future investigations by researchers. This study helped us observe that while numerous works focused on cellular technologies to enable connectivity for aerial platforms, a single wireless technology is not sufficient to meet the stringent connectivity demands of the aerial use cases, especially for the piloting operations. We identified the need of further investigations on multi-technology heterogeneous network architectures to enable robust and real-time connectivity in the sky. Future works should consider suitable technology combinations to develop unified aerial networks that can meet the diverse quality of service demands of the aerial use cases.

A Survey of Wireless Networks for Future Aerial Communications (FACOM)

A review of technological solutions and advances in the framework of a Vertical Heterogeneous Network (VHetNet) integrating satellite, airborne and terrestrial networks is presented. The disruptive features and challenges offered by a fruitful cooperation among these segments within a ubiquitous and seamless wireless connectivity are described. The available technologies and the key research directions for achieving global wireless coverage by considering all these layers are thoroughly discussed. Emphasis is placed on the available antenna systems in satellite, airborne and ground layers by highlighting strengths and weakness and by providing some interesting trends in research. A summary of the most suitable applicative scenarios for future 6G wireless communications are finally illustrated.

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Space-Air-Ground Integrated 6G Wireless Communication Networks: A Review of Antenna Technologies and Application Scenarios

This paper overviews point-to-point (P2P) links for integrated satellite-aerial networks, which are envisioned to be among the key enablers of the sixth-generation (6G) of wireless networks vision. The paper first outlines the unique characteristics of such integrated large-scale complex networks, often denoted by spatial networks, and focuses on two particular space-air infrastructures, namely, satellites networks and high-altitude platforms (HAPs). The paper then classifies the connecting P2P communications links as satellite-to-satellite links at the same layer (SSLL), satellite-to-satellite links at different layers (SSLD), and HAP-to-HAP links (HHL). The paper overviews each layer of such spatial networks separately, and highlights the possible natures of the connecting links (i.e., radio-frequency or free-space optics) with a dedicated overview to the existing link-budget results. The paper, afterwards, presents the prospective merit of realizing such an integrated satellite-HAP network towards providing broadband services in under-served and remote areas. Finally, the paper sheds light on several future research directions in the context of spatial networks, namely large-scale network optimization, intelligent offloading, smart platforms, energy efficiency, multiple access schemes, and distributed spatial networks.

Point-to-Point Communication in Integrated Satellite-Aerial Networks: State-of-the-art and Future Challenges

High-altitude platform station (HAPS) has attracted considerable attention as a new promising platform to provide mobile wireless communication services with ultra-wide coverage and resilience against disaster. Because of its low-delay characteristics, HAPS can provide wireless service directly to smartphones used in terrestrial networks. In a HAPS system, it is required to employ multiple cells to accommodate high communication traffic. Furthermore, the HAPS movement caused by its turning flight or variations in the stratospheric wind is known to result in the displacement of cells projected on the ground. Therefore, the HAPS movement with multicells configuration results in several handovers and disconnections. Some research has shown that this challenge can be addressed by employing antenna beamforming based on a planar or flat-shaped array antenna. Because the target diameters of coverages are only 60 to 80 km, these simple antennas can easily cover the whole coverage area; thus, they can compensate for the displacement easily. However, to achieve ultra-wide coverage over 80-km diameter, drastic change of its antenna design is needed. Therefore, this paper proposes a new antenna structure and its beamforming methods assuming a solar-plane- based HAPS with a 200-km diameter. Computer simulation quantitatively reveals that the proposed antenna can well compensate for the effect of a displaced cell.

A Study on Antenna Beamforming Method Considering Movement of Solar Plane in HAPS System

This paper surveys the literature on point-to-point (P2P) links for integrated satellite-aerial networks, which are envisioned to be among the key enablers of the sixth-generation (6G) of wireless networks vision. The paper first outlines the unique characteristics of such integrated large-scale complex networks, often denoted by spatial networks, and focuses on two particular space-air infrastructures, namely, satellites networks and high-altitude platforms (HAPs). The paper then classifies the connecting P2P communications links as satellite-to-satellite links at the same layer (SSLL), satellite-to-satellite links at different layers (SSLD), and HAP-to-HAP links (HHL). The paper surveys each layer of such spatial networks separately, and highlights the possible natures of the connecting links (i.e., radio-frequency or free-space optics) with a dedicated survey of the existing link-budget results. The paper, afterwards, presents the prospective merit of realizing such an integrated satellite-HAP network towards providing broadband services in under-served and remote areas. Finally, the paper sheds light on several future research directions in the context of spatial networks, namely large-scale network optimization, intelligent offloading, smart platforms, energy efficiency, multiple access schemes, distributed spatial networks, and routing.

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Point-to-Point Communication in Integrated Satellite-Aerial 6G Networks: State-of-the-Art and Future Challenges

High-altitude platform stations (HAPSs) are expected to provide ultrawide-coverage areas and disaster-resilient networks from the stratosphere at around 20 km by installing wireless equipment on HAPS. Because their altitude is much lower than that of communications satellites, HAPSs can provide mobile communications services directly to smartphones, which are commonly used in terrestrial networks, such as fourth generation Long Term Evolution. Considering the widespread nature of mobile broadband communications and the importance as a backup line in case of disaster, HAPSs are expected to provide a large capacity in the future. A cellular system with single-cell frequency reuse using multiple cells similar to terrestrial mobile communications should be introduced to achieve such a capacity. The number of cells that a HAPS can accommodate ranges from 1 to more than 100, depending on unmanned aerial vehicle (UAV) ability. By contrast, the optimal cell configuration, which depends on the number of available cells, has not been clarified in previous research. In this paper, we propose an optimization method for the cell configuration for HAPS mobile communications using a genetic algorithm, which can be generally applied regardless of the number of cells and can clarify the optimal cell configuration. Although many cells are required to achieve gigabit-class HAPS mobile communications, the heightened power consumption due to the large number of cells is a critical problem for UAVs. Thus, we also investigate the reduction of the total transmission power and demonstrate the feasibility of energy-efficient gigabit HAPS mobile communications with wide coverage.

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System Design of Gigabit HAPS Mobile Communications

A High-Altitude Platform Station (HAPS), which provides communications services from an altitude of 20 km by using an Unmanned Aerial Vehicle (UAV) such as a balloon, airship, or other aircraft, is attracting much attention as a new mobile communications platform for an ultra-wide coverage area and disaster-resilient networks. A HAPS can provide mobile communications services directly to smartphones, which are commonly used in terrestrial mobile communications networks with Fourth Generation Long Term Evolution (4G LTE), and in the near future, Fifth Generation New Radio (5G NR). To achieve wide coverage and large capacity, it is desirable to introduce a cellular system that uses one- cell frequency reuse much like terrestrial mobile communications. However, a HAPS can provide various numbers of cells ranging from one to more than 100 depending on HAPS and UAV type, which differs from terrestrial base stations that have three to six sectors at most. Additionally, conventional studies on HAPS have paid little attention to cell configuration optimization. In this paper, we propose cell configuration optimization method for a HAPS cellular system with LTE, that can be generally applied regardless of the number of cells. We also clarify the cell configuration with antenna parameters that are suitable for different numbers of cells a HAPS can provide.

A Study on Cell Configuration for HAPS Mobile Communications

In mobile communications networks, if we can estimate the location of each user equipment (UE) with high accuracy, efficient cell planning and network optimization become possible. However, it is difficult for operators to estimate the location of most commercial UEs because they cannot feedback their location information directly to their serving base stations with the exception of UEs with Minimization of Drive Test (MDT) functions specified in LTE Release 10. Radio Frequency (RF) fingerprint method is known as an effective localization method, with which we can estimate the location of UEs with only RF signature and location information beforehand. In LTE, RF fingerprint database can be collected by conducting drive tests or using measurement data from UEs compatible with MDT. Although the estimation accuracy can be better with more RF signatures, the increase in the amount of RF signatures causes the increase in the feedback, which consumes the uplink capacity and UE battery. Furthermore, keeping all RF signatures in database is inefficient because some RF signatures do not have effect on improving estimation accuracy. Hence, it is important to clarify the effect of each RF signature and use only effective ones in the localization. In this paper, we conducted a field experiment to create an RF fingerprint database in dense urban area, and evaluated the effectiveness of each RF signature by making several localization models based on machine learning. Eventually, we clarified minimum RF signatures required to operate RF fingerprint localization in LTE network.

Field Experiment of Localization Based on Machine Learning in LTE Network

A System Design of Gigabit HAPS Mobile Communications with Energy Efficiency

In the above article [1] , there was an incorrect statement on antenna pattern related to (1) in Section III-B. The sentence just after (1) should be read as “where $\Psi_{b}$ is the half-power beamwidth divided by 2, $\Psi_{1}=\Psi_{b} \sqrt{-L_{N} / 3}, \Psi_{2}=3.745 \Psi_{b}, X=L_{N}+A \log _{10}\left(\Psi_{2}\right)$ , and $\Psi_{3}=10^{\left(X-L_{F}\right) / A}$ .” The definitions of $\Psi_{b}$ and $\Psi_{3}$ are modified.

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Noboru Kanazawa

Papers

System Design of Gigabit HAPS Mobile Communications

A Study on Cell Configuration for HAPS Mobile Communications

Field Experiment of Localization Based on Machine Learning in LTE Network

A System Design of Gigabit HAPS Mobile Communications with Energy Efficiency

Correction to “System Design of Gigabit HAPS Mobile Communications”