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Future non-terrestrial networks aim to achieve a throughput of Terabits/s. Therefore, free-space optical (FSO) communications have been adopted as a candidate solution due to their ability to achieve an extremely high data rate. Nonetheless, FSO communications are sensitive to the adverse effects of beam scintillation, beam-wander-induced pointing errors, free-space loss, and weather conditions. Space-air-ground (SAG) FSO transmission and hybrid single-hop (SH) FSO/radio frequency (RF) transmission are promising solutions to improve the performance of FSO links and can be integrated into a satellite communication (Satcom) system. In this work, we carry out a thorough capacity analysis of the resulting integrated SAG-FSO/SH-FSO/RF Satcom system, where Gamma-Gamma and Rician distributions are used to characterize FSO and RF links, respectively. The exact analytical expressions are derived and validated by Monte-Carlo simulations. We also obtain asymptotic expressions for the ergodic capacity in the high signal-to-noise ratio region. The numerical results highlight the significant potential of the integrated SAG-FSO/SH-FSO/RF Satcom system over existing solutions. We also show that the integrated Satcom system with intensity modulation and direct detection can achieve a capacity gain over that with a heterodyne detection technique over all satellite zenith angles.

Future non-terrestrial networks aim to achieve a throughput of Terabits/s. Therefore, free-space optical (FSO) communications have been adopted as a candidate solution due to their ability to achieve an extremely high data rate. Nonetheless, FSO communications are sensitive to the adverse effects of beam scintillation, beam-wander-induced pointing errors, free-space loss, and weather conditions. Space-air-ground (SAG) FSO transmission and hybrid single-hop (SH) FSO/radio frequency (RF) transmission are promising solutions to improve the performance of FSO links and can be integrated into a satellite communication (Satcom) system. In this work, we carry out a thorough capacity analysis of the resulting integrated SAG-FSO/SH-FSO/RF Satcom system, where Gamma-Gamma and Rician distributions are used to characterize FSO and RF links, respectively. The exact analytical expressions are derived and validated by Monte-Carlo simulations. We also obtain asymptotic expressions for the ergodic capacity in the high signal-to-noise ratio region. The numerical results highlight the significant potential of the integrated SAG-FSO/SH-FSO/RF Satcom system over existing solutions. We also show that the integrated Satcom system with intensity modulation and direct detection can achieve a capacity gain over that with a heterodyne detection technique over all satellite zenith angles. [ABSTRACT FROM AUTHOR]

An efficient aeronautical telecommunication network (ATN) is proposed in this article based on adaptive multibeam antenna arrays and high-altitude platforms (HAPs). First, the network structure is demonstrated, and its geometry is analyzed based on a geocentric coordinate system that converts the global positioning system (GPS) and altitude data of aircrafts and HAPs into direction-of-arrival (DOA) information which is required for subsequent beamforming operation. The antenna array is then formed by designing a low-profile dual concentric conical array (DCCA) with uniform elements distribution and is used at the aircraft and HAP to achieve bidirectional beamforming. The antenna array elements are fed by an adaptive-exponent sine profile function to reduce the sidelobe levels while the low-profile dual conical array structure reduces the secondary major lobe and backlobe levels. The proposed beamforming technique is also capable of providing multibeam towards several aircrafts at the same time for effective resource sharing and management. The radiation performance of the array is demonstrated, analyzed, and compared with the concentric circular array where it is found that the secondary major lobe has been reduced by 13 dB with sidelobe level down to −40 dB relative to the mainlobe level. In addition, the bit energy-to-noise power spectral density and probability of bit error of the proposed aeronautical network has been investigated at different operating frequencies including 3.5 GHz and millimeter wave (mm-wave) frequencies of 28 and 39 GHz. The simulation results have shown that a channel capacity of about 3 Gbps can be achieved for BPSK and QPSK signals using bidirectional beamforming for a channel bandwidth of 400 MHz at 28 and 39 GHz mm-wave frequencies while it is about 773 Mbps at 3.5 GHz for 100 MHz bandwidth that can be provided over a line-of-sight distance of 896 km between an aircraft and HAP.

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.

This paper proposes a new cross-layer communication system for the provision of Internet services and applications to mitigate the negative impacts of COVID-19, due to which the massive online demands are affecting the current communication systems’ infrastructures and capabilities. The system requirements and model are investigated where it utilizes high-altitude platform (HAP) for fast and efficient connectivity provision to bridge the communication infrastructure gap in the current pandemic. The HAP is linked to the main server or gateway station located on ground and can provide communication narrow beams towards isolated areas which suffer from poor terrestrial radio coverage or lack of communication infrastructure. The vital e-learning applications using Internet services provision from the proposed HAP system are described and modelled including system adaptation parameters such as the application and physical layers to control the data rates of different e-learning applications and the overall cell data rate. On the other hand, the provision of high-speed Internet services from the proposed system is supported by using adaptive antenna arrays onboard HAP which provides high-gain beams to achieve the required high-quality transmission data rates at the student premises and provides the capability of coverage cell area adaptation for load balancing. The concentric circular antenna arrays with tapered feeding are proposed in this adaptive antenna system to control the cell mainlobe gain and reduce the out-of-coverage radiation as well. In addition, the system feasibility has been proved in two coverage scenarios including single-beam and multibeam HAP communications. [ABSTRACT FROM AUTHOR]

Abstract High-altitude platforms (HAPs) are quasi-stationary aerial wireless communications platforms meant to be located in the stratosphere, to provide wireless communications and broadband services. They have the ability to fly on demand to temporarily or permanently serve regions with unavailable infrastructure. In this paper, we consider the development of an efficient method for resource allocation and controlling user admissions to multicast groups in a HAP system. Power, frequency, space and time domains are considered in the problem. The combination of these many aspects of the problem in multicasting over an OFDMA HAP system were not, to the best of our knowledge, addressed before. Due to the strong dependence of the total number of users that could join different multicast groups on the possible ways we may allocate resources to the different multicast groups, it is important to consider a joint user to multicast group assignments and radio resource management across the groups. From the service provider’s point of view, it would be in its best interest to be able to admit as many users as possible, while satisfying their quality of service requirements. The problem turns out to be a mixed integer non-convex non-linear program for which branch and bound solution framework is guaranteed to solve the problem. Branch and bound (BnB) can be also used to obtain sub-optimal solutions with desired quality. Even though branch and bound is guaranteed to find the optimal solution, the computational cost could be extremely high, which is why we considered different types of enhancements to BnB. Mainly, we consider reformulations by linearizing a specific set of quadratic constraints in the derived formulation, as well as the application of different branching techniques to find the one that performs the best. Based on the conducted numerical experiments, it was concluded that linearization, applied for at least 100 presolving rounds, and cloud branching achieve the best performance.

Current ground-based transportation systems are subjected to various challenges, including the high cost of infrastructure development, limited land space, and a growing urban population. Therefore, the automotive and aviation industries are collaborating to develop flying cars, also known as electric, vertical, takeoff, and landing aircrafts (eVTOLs). These eVTOLs will allow for rapid and reliable urban and suburban transportation. Safe operation of eVTOLs will require well-developed wireless communication networks; however, existing communication technologies need enhancement in order to provide services to flying cars. We describe several potential innovations that make communication between eVTOLs and the ground feasible. These innovations include three-dimensional cellular networks on-ground, tethered balloons, high-altitude platforms, and satellites.

In this paper, we propose a novel wireless scheme that integrates satellite, airborne, and terrestrial networks aiming to support ground users. More specifically, we study the enhancement of the achievable users’ throughput assisted with terrestrial base stations, high-altitude platforms (HAPs), and satellite stations. The goal is to optimize the resource allocations and the HAPs’ locations in order to maximize the users’ throughput. In this context, we formulate and solve an optimization problem in two stages: a short-term stage and a long-term stage. In the short-term stage, we start by proposing an approximated solution and a low complexity solution to solve the associations and power allocations. In the approximated solution, we formulate and solve a binary linear optimization problem to find the best associations and then we use the Taylor expansion approximation to optimally determine the power allocations. In the latter solution, we propose a low complexity approach based on a frequency partitioning technique to solve the associations and power allocations. On the other hand, in the long-term stage, we optimize the locations of the HAPs by proposing an efficient algorithm based on a recursive shrink-and-realign process. Finally, selected numerical results underline the advantages provided by our proposed optimization scheme. [ABSTRACT FROM AUTHOR]

High-altitude platforms (HAPs) have been proposed previously for transmitting radio frequency signals from ground stations over large distances of several hundred kilometers. These HAP architectures suffer from the limitations of data rate. The availability of the system has also not been evaluated for these architectures. In this paper, we propose a high-capacity and highly reliable optical wireless multi-hop HAP network configuration with free-space optical links from the ground stations and between the serial HAPs in the stratosphere. In the proposed scheme, communication services for long distance requiring very high bandwidth are transmitted optically from the ground station to the HAP station in the stratosphere, where it can cover distances of several hundreds of kilometers on the free-space optical links with serial multiple hops and be transmitted back optically to the distant ground receiver. In order to increase reliability and availability, particularly with reference to fading of vertical links due to atmospheric conditions, alternate slant links are added as redundant paths. The closed-form expressions for the average bit error rate and average channel capacity are derived assuming the composite channel model, which includes atmospheric attenuation, turbulence, and pointing errors. The gamma-gamma distribution with beam wander effect is used to model the atmospheric turbulence. Beam optimization is also done using the Nelder-Mead simplex algorithm. The effects of beam wandering and random pointing error are mitigated by using multi-hop HAP architecture. Link availability and system downtime of the complete ground-to-ground FSO network and reconfigured architecture has been calculated for component failure and atmospheric turbulence, moderate fog, rain, and snow. The proposed reconfigured architecture has high availability, meeting telecom requirements, and high mean time between failures and lower system downtime. [ABSTRACT FROM PUBLISHER]

High-altitude platform (HAP) is promising for providing efficient wireless communications services. One of the critical issues in HAP communications is the difficulty to localize users through two-dimensional direction-of-arrival (2D-DOA) estimation due to the high computational complexity and the large covered area. Therefore, in this paper, an efficient technique is proposed to determine user location through 2D-DOA with a reduced processing time. The proposed technique estimates the 2D-DOA in two successive stages; the first will utilize a low-resolution 2D-DOA estimation technique such as Bartlett algorithm performed on a low-resolution distance, then applying a suitable threshold on the normalized Bartlett 2D-DOA spectrum to define ground windows over which the next high-resolution 2D-DOA stage will be performed such as MUSIC algorithm. Two scenarios are examined for the proposed technique to investigate the reduction in calculations time compared to the conventional 2D-DOA MUSIC algorithm without windowing. Simulation results show that at a resolution of 40 m, the required processing time is reduced to only 20 % of the conventional MUSIC algorithm and can be further reduced to 4 % at resolution of 100 meters at the same array size. In addition, the proposed technique is flexible and can be applied for any other efficient low-complexity 2D-DOA algorithms. [ABSTRACT FROM AUTHOR]

High-altitude platforms are one of the most promising alternative infrastructures for realizing next generation high data rate wireless networks. This paper presents a three-dimensional (3-D) scattering model for land mobile stratospheric multipath-fading channel with its complex faded envelope. From the scattering model and the complex envelope second-order statistics are derived for a 3-D non-isotropic scattering environment. When we discuss on the second-order statistics we refer to the level crossing rate and the average fade duration, whichare two main parameters in describing the fading severity over time and are very important in assess system characteristics such as hand off, velocities of the transmitter and receiver and fading rate. Numerical calculations have been carried out to demonstrate theoretical derivations and the utility of the proposed model. [ABSTRACT FROM AUTHOR]

Abstract: This paper proposes a novel design technique for irregular shaped cells in the high altitude platforms (HAPs) cellular system. This technique is based on dividing the coverage area into a grid of small pixel spots and the desired cell is obtained by grouping some pixel spots to give the desired cell shape. These spots are formed with narrow and low sidelobe level beams using concentric ring array (CRA) to reduce the cochannel interference as well as improving the resolution of the design. The desired cell pattern is converted into a spatial mask that selects the desired pixel spots with a proper pre-weight smoothing function. The proposed cell design technique will optimize the cell shape according to the user distribution and behavior in the covered area and consequently is expected to reduce the frequent handoff and signaling traffic of location updating from moving users. Simulation results show that a cell with any irregular shape can be formed with as low as 40dB sidelobe level using Gaussian CRA. [Copyright &y& Elsevier]

An adaptive power allocation method for signals transmitted from high-altitude platforms is proposed, and justification of its usefulness is provided. The possible structural scheme for the system is given and its performance is investigated. The adaptation algorithm is based on an optimization procedure, which enables the allocation of the available power to a maximal number of users. The proposed algorithm is tested on simulations. It is shown that for Nakagami fading in downlink channels, the adaptive power allocation yields an energetic gain which can attain up to 21 dB. [ABSTRACT FROM AUTHOR]

How space technologies and new integrated telecommunication networks can mitigate the impact of natural and man-made disasters. The objective is to design new, potentially attractive telecommunication architectures to better manage a disaster scenario. The strengths and the weaknesses of the proposed space-based telecommunication architectures for the emergency and recovery will be outlined also, to individuate the needs for an optimal provision of information and accessibility of space-related services in case of disaster. [ABSTRACT FROM PUBLISHER]

A method for predicting movements in cellular coverage caused by lateral drift of a high-altitude platform (a quasi-stationary platform in the stratosphere) is developed. Cells are produced by spot beams generated by horn-type antennas on the platform. It is shown how the carrier-to-interference ratio (CIR) across these cells varies when the antenna payload is steered to accommodate the lateral movement of the platform. The geometry of the antenna beam footprint on the ground is first developed and then applied to a system of many cochannel beams. Pointing strategies are examined, where the pointing angle is calculated to keep, for example, a center cell or an edge cell in the same nominal position before and after the platform drift, and the CIR distribution is calculated. It is shown that the optimum pointing angle depends on the desired level of CIR across the service area, typically lying between 3±0.75° for a platform drift of 2 km and corresponding to a cell in the middle ring. It is shown that it is necessary for a significant proportion of users to perform a handover to maintain a given CIR after platform drift. The analysis reveals that there is an optimum pointing angle that minimizes the probability of handover for a particular value of drift and CIR. [ABSTRACT FROM PUBLISHER]

Terahertz communications offers a massive potential for the prospective beyond 5G wireless systems, as the band offers huge bandwidth and data rates as compared to the existing sub 6 GHz bands, which are almost saturated. In this paper, we investigate the feasibility of wireless communications over the Terahertz-band (0.75–10 THz) at various atmospheric altitudes, considering different transmission distances and directions by realistically calculating the absorption loss, which is the major limiting factor affecting the propagation of THz waves through the earth's atmosphere. Four practical altitudes are considered, corresponding to Drone-to-Drone (D2D), Jet plane-to-Jet plane (J2J), Unmanned Aerial Vehicle (UAV)-to-UAV, and near-space Satellite-to-Satellite (S2S) communications. Following comparison and validation with two real-world experimental results from the literature measured at the sea-level, Line by Line Radiative Transfer Models (LBLRTM) is used to obtain realistic THz-band transmittance values against each altitude case and setting. Subsequently, absorption loss and total path loss values are computed and mean total path loss sensitivity is further observed against a range of transmission directions via zenith angle variations from vertically-up to vertically-down communication. Numerical results show that as the altitude increases, the concentration of the water vapor molecules decreases, enabling the communication over the THz-band (0.75–10 THz) to be more feasible as compared to the sea-level communication. Moreover, the total usable bandwidth results over the THz-band (0.75–10 THz) exhibit that the upper bounds of 8.218 THz, 9.142 THz and 9.25 THz are usable up to the transmission distance of 2 km against the total antenna gains of 80 dBi for J2J, U2U and S2S communication cases, respectively. [ABSTRACT FROM AUTHOR]