Research on 5G mobile wireless technologies has been very active in both academia and industry in the past few years. While there has been certain consensus on the overall requirements of 5G wireless systems (e.g., in data rate, network capacity, delay), various enabling wireless technologies have been considered and studied to achieve these performance targets. It has been quite clear, however, that there would be no single enabling technology that can achieve all diverse and even conflicting 5G requirements. In general, many fundamental changes and innovations to re-engineer the overall network architecture and algorithms in different layers and to exploit new system degrees of freedom would be needed for the future 5G wireless system. In particular, we may need to consider other potential waveform candidates that can overcome limitations of the orthogonal frequency multiple access (OFDM) waveform employed in the current 4G system, develop disruptive technologies to fulfill 5G rate and capacity requirements including network densification, employment of large-scale (massive) multiple input multiple output (MIMO), and exploitation of the millimeter wave (mmWave) spectrum to attain Gigabit communications. In addition, design tools from the computer networking domain including software defined networking, virtualization, and cloud computing are expected to play important roles in defining the more flexible, intelligent, and efficient 5G network architecture. This paper aims at describing key 5G enabling wireless mobile technologies and discussing their potentials and open research challenges. We also present how papers published in our special issue contribute to the developments of these disruptive 5G technologies.

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Enabling 5G mobile wireless technologies

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

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

High-Altitude Platform Station (HAPS) is a new mobile communication platform, which can provide mobile communication services such as Long Term Evolution (LTE) or 5th Generation New Radio (5G NR) from stratosphere to terrestrial User Equipments (UEs) directly by utilizing aerial vehicles such as balloon, airplane or airship with radio equipment at the altitude of 20km. It has been attracting much attention for its ultra-wide coverage as wide as 50km to 100km in radius and disaster-resilient networks. The service area of HAPS overlaps that of existing terrestrial mobile network due to its ultra-wide coverage and the terrestrial network can be affected by the interference from a HAPS. Although one of the simplest ways to avoid the interference is to separate spectrum for HAPS and terrestrial network exclusively, it is not desirable from the viewpoint of spectral efficiency. Another approach is co-channel spectrum sharing, which enables higher spectrum reuse. However, conventional co-channel spectrum_sharing techniques introduced in LTE or LTE-Advanced require muting data transmission partially in order to mitigate the interference. If the conventional techniques are applied to the HAPS, it leads to a decrease in available radio resource of the HAPS. This paper proposes to apply a co-channel downlink spectrum sharing between HAPS and terrestrial networks without muting HAPS’ transmission. We also make an outdoor propagation measurement to clarify the received signal power from both HAPS and the terrestrial networks in real outdoor fields and show the feasibility of the proposed co-channel spectrum sharing.

A Study of Co-Channel Spectrum-Sharing System between HAPS and Terrestrial Mobile Communication Networks

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

With the increase in the population of elderly, a technique for monitoring them is receiving more and more attention all over the world. In particular, it is highly demanded to develop the system that detects a fainting elderly in a toilet, since such an accident is likely to get worse due to the late detection. In this paper, we propose an anomaly detection method in toilet using an FMCW(Frequency Modulated Continuous Wave) radar, which can measure the distance between an object and an FMCW radar. As anomalies, we determine as the situations where a subject faints while stooping on the toilet seat, leaning on the backrest, leaning on the side wall, and lying on the floor after the fall. With an FMCW radar, a fainting subject could be detected based on whether the estimated distance between a subject and an FMCW radar is almost constant. However, a subject might be sitting still, and also the distance to not a subject but a floor could be estimated depending on a subject's posture. To address this problem, in the proposed method, the time when the estimated distance is almost constant is detected, and then the detected time is classified into the time due to a faint or a non-faint by a supervised machine learning classifier. The used features are extracted based on the data, e.g., the estimated distance, just before the detected time. To evaluate the anomaly detection accuracy of the proposed method, we conducted the experiments to observe not only the aforementioned abnormal behavior but also the normal behavior, e.g., the button operation and sitting down. As a result, in terms of the classification of abnormal and normal behavior, our proposed method achieved the F- measure of 0.91, where F-measure is the overall score of recall and precision rates.

FMCW Radar-Based Anomaly Detection in Toilet by Supervised Machine Learning Classifier

Superposed multicarrier transmission scheme is known to improve frequency utilization efficiency when several wireless systems share the same spectrum. To suppress the effect of interference, forward error correction (FEC) metric masking is proposed. In this technique, the log-likelihood ratio (LLR) that corresponds to the superposed band is set to zero, because received bits that correspond to the superposed band are unreliable. However, to apply FEC metric masking, the information about superposed band must be known at the receiver beforehand. Furthermore, the received bits contain channel estimation errors, which are the cause of performance degradation. In this paper, we propose an iterative estimation technique for undesired signal power (noise, interference, and channel estimation error) for superposed multicarrier transmission. We use the estimated power of the undesired signal to calculate the LLR that takes the channel estimation error into account, since including this extra information about the channel improves the bit error rate (BER). The proposed scheme estimates the power of undesired signal on each subcarrier, and thus, the information about superposed band is not required. Simulation results show that the accuracy of estimating undesired signal becomes more reliable as the number of estimations increases, so that BER becomes better as a result of iterative estimation.

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Yohei Shibata

Papers

System Design of Gigabit HAPS Mobile Communications

A Study of Co-Channel Spectrum-Sharing System between HAPS and Terrestrial Mobile Communication Networks

A Study on Cell Configuration for HAPS Mobile Communications

FMCW Radar-Based Anomaly Detection in Toilet by Supervised Machine Learning Classifier

Iterative estimation of undesired signal power for superposed multicarrier transmission with channel estimation error