High Altitude Platform Research Articles

A Non-Stationary 3-D Wideband Channel Model for Intelligent Reflecting Surface-Assisted HAP-MIMO Communication Systems

Intelligent reflecting surface (IRS) composed of unit cells with tunable reflection coefficients, amplitude, phase, and frequency has received great interest from wireless communication communities. In this paper, a non-stationary three-dimensional (3-D) wideband IRS-assisted channel model, including spatial and temporal non-stationarity, is proposed for high altitude platform (HAP) multiple-input multiple-output (MIMO) communication systems. The proposed channel model considers the propagation environment around the receiver (Rx) and that around both the IRS and Rx, which are modeled using multiple 3-D cylinders and confocal elliptic-cylinders, respectively. In the proposed model, the optimal reflection phases of the IRS reflection units are proposed, and the number and the size of the IRS reflection units are also considered, which are used to investigate received signal strength, multipath fading effect, and spatial non-stationarity of IRS-aided communication systems. The numerical results show that the IRS reflection units can improve the received signal strength by adjusting the reflection phases and multipath fading effect can be effectively reduced by increasing the number and the size of IRS reflection units. Also, it is found that the spatial correlation significantly decreases with the increase of the number and size of the IRS reflection units, and the proposed IRS-aided channel model presents spatial non-stationarity. These observations can provide guidelines and references for systems design and performance analysis of IRS-aided HAP-MIMO communication systems.

On the Design of Rate Adaptation for Relay-Assisted Satellite Hybrid FSO/RF Systems

This paper addresses the design of hybrid free-space optical/radio frequency (FSO/RF) systems for a high-altitude platform (HAP)-aided relaying satellite communication for mobile networks supported by unmanned aerial vehicle (UAV). While prior work primarily focused on fixed-rate design, which frequently switches between FSO and RF lead reduced the system performance, we propose a rate adaptation design that gradually adjusts the data rate in each link when its channel state fluctuates, under a target bit error rate (BER) constraint. The proposed design’s downlink performance is analyzed, taking into account many challenging issues, including beam spreading loss, cloud attenuation, statistical behaviors of the atmospheric turbulence in the dual-hop channel, and pointing misalignment due to the UAV hovering. Different performance metrics are analytically derived based on channel modelings, including outage probability, average transmission rate, and achievable spectrum efficiency. In addition, the average system BER, which satisfies the design constraint, is also numerically obtained. The results quantitatively confirm the effectiveness of our proposed system under the impact of UAV hovering misalignment and atmospheric-related issues like clouds and turbulence. Finally, Monte-Carlo simulations validate the accuracy of theoretical results.

Research on anti-interference based on particle swarm optimization algorithm in high altitude platform stations

Research on anti-interference based on particle swarm optimization algorithm in high altitude platform stations

Capacity, Coverage and Power Profile Performance Evaluation of a Novel Rural Broadband Services Exploiting TVWS From High Altitude Platform

The coverage and capacity of exploiting TV White Spaces (TVWS) from High Altitude Platforms (HAPs) with multiple antenna payloads using uniform rectangular phased array (URA) antennas are explored in this article. A scenario is suggested in which a HAP and TVWS base station are installed on the HAP within a HAP coverage area with a radius of 100 km and a cell radius of 10.5 km. A scenario is suggested in which TVWS base stations are installed on a HAP providing coverage to an area of 100 km radius. This area is consisted of a number of cells where each of them has a radius of 10.5 km. The formation of HAP cells with various reuse patterns has been established. An analysis of coverage, capacity and power profile function for TVWS link from 25×25 URA HAP arrays is presented. To minimize the sidelobe levels in the antenna array synthesis, window function has been assumed. Power profile within a cell using URA is highlighted. Channel to Interference (CIR) performance contour, coverage probability and channel capacity have been determined. The channel capacity for the theoretical Shannon bound and practical truncated Shannon bound are determined and compared. Results show that a maximum channel capacity of 53 Mbps can be attained from the proposed model.

An Efficient HAPS Cross-Layer Design to Mitigate COVID-19 Consequences

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.

On the Design of RIS-UAV Relay-Assisted Hybrid FSO/RF Satellite-Aerial-Ground Integrated Network

Satellite-aerial-ground integrated network (SAGIN) has been widely envisioned as a promising network architecture for 6 G. In the SAGINs, high altitude platform (HAP)-aided relaying satellite systems using hybrid free-space optics (FSO)/radio-frequency (RF) communications have recently attracted research efforts worldwide. Nevertheless, the main drawback of hybrid FSO/RF systems is the restricted bandwidth of the RF connection, especially when the FSO one is blocked by cloud coverage. This paper explores a novel solution for the hybrid FSO/RF HAP-based SAGIN under the impact of weather and atmospheric conditions. Specifically, an additional unmanned aerial vehicle (UAV) is deployed to diverse the FSO link from the HAP-to-ground station to avoid cloud blockage while maintaining a high-speed connection of the FSO link. A mirror array constructed by re-configurable intelligent surface (RIS), an emerging technology, is mounted on the UAV to reflect the signals from the HAP. The channel model of RIS-UAV takes into account both atmospheric turbulence and hovering-induced pointing errors. Furthermore, we present a novel link switching design with a multi-rate adaptation scheme for the proposed network under different weather and turbulence conditions. Numerical results quantitatively confirm the effectiveness of our proposal. Additionally, we provide insightful discussions that can be helpful for the practical system design of RIS-UAV-assisted HAP-based SAGIN using hybrid FSO/RF links. Monte Carlo simulations are also performed to validate the accuracy of theoretical derivations.

Topology of a Three-Port Converter for High-Altitude Platforms

Three-port converter (TPC) with non-regulated bus cannot meet the voltage stability demands of high-altitude platforms (HAPs) during daytime operations. In contrast, full-regulated bus TPC is complex in topology and inferior in power density, although it can maintain voltage stability across whole day operations. Therefore, semi-regulated bus TPCs are more suitable for this scenario, which can not only maintain voltage stability during daytime operations, but also have simple topology. However, conventional semi-regulated bus structure converts the power twice, leading to lower efficiency and power density. In order to overcome this problem, the paper proposes a novel semi-regulated bus TPC that can be used on high-altitude platforms. The topology only has one conversion between the ports. It regulates the bus voltage during the day and prevents the loss of diode during the night by directly discharging through the MOSFET and achieving high efficiency. The paper adopts GaN devices to elevate the working frequency to further improve the power density, and conduct a theoretical analysis for all working modes and provide key equations. The closed-loop transfer function of the system is deduced for stability analysis. The results show that the proposed topology has high efficiency and power density.

Service-Aware User Association and Resource Allocation in Integrated Terrestrial and Non-Terrestrial Networks: A Genetic Algorithm Approach

In 6G networks and beyond, multiple radio access networks (RANs), including; the satellite, high altitude platforms, low altitude platforms, and the terrestrial network, will co-exist. These networks are characterized by different capabilities and limitations in meeting the envisioned 6G contrasting user requirements. Therefore, associating users with the appropriate radio access network (RAN) in such an integrated network is rigorous and complex. In this work, the user association and resource allocation problem is formulated as a multi-objective optimization problem (MOOP), aiming to maximize data rate while minimizing mobility-induced handoff in the integrated network. Moreover, the problem is formulated in such a way as to prioritize the service provisioning of mission-critical users. The weighted sum method is adopted to simplify and transform the MOOP into a single-objective optimization problem (SOOP). In order to solve the formulated NP-hard SOOP, a genetic algorithm (GA) whose fitness value is based on the user’s service group is proposed. The performance of the proposed algorithm is evaluated by comparing it to the optimal solution, the greedy signal-to-interference-plus-noise ratio (SINR) based association, and the random user association algorithms. Simulation results show that as the number of access nodes in the network increases, the GA’s spectrum efficiency (SE) remains within 0.4% of the optimal solution. Moreover, the GA outperforms all three schemes in user acceptance ratio (AR) and handoff probability.

Wireless Backhaul in 5G and Beyond: Issues, Challenges and Opportunities

With the introduction of new technologies such as unmanned aerial vehicles (UAVs), high altitude platforms (HAPS), millimeter wave (mmWave) frequencies, massive multiple-input multiple-output (MIMO), and beamforming, wireless backhaul is expected to be an integral part of the 5G networks. While this concept is nothing new, it was shortcoming in terms of performance compared to the fiber backhauling. However, with these new technologies, fiber is no longer the foremost technology for backhauling. With the projected densification of networks, wireless backhaul has become mandatory to use. There are still challenges to be tackled if wireless backhaul is to be used efficiently. Resource allocation, deployment, scheduling, power management and energy efficiency are some of these problems. Wireless backhaul also acts as an enabler for new technologies and improves some of the existing ones significantly. To name a few, rural connectivity, satellite communication, and mobile edge computing are some concepts for which wireless backhauling acts as an enabler. Small cell usage with wireless backhaul presents different security challenges. Governing bodies of cellular networks have standardization efforts going on especially for the integrated access and backhaul (IAB) concept, and this is briefly mentioned. Finally, wireless backhaul is also projected to be an important part of the beyond 5G networks, and newly developed concepts such as cell-free networking, ultra-massive MIMO, and extremely dense network show this trend as well. In this survey, we present the aforementioned issues, challenges, opportunities, and applications of wireless backhaul in 5G, while briefly mentioning concepts related to wireless backhaul beyond 5G alongside with security and standardization issues.

On the Performance and Optimization of HAPS Assisted Dual-Hop Hybrid RF/FSO System

In this paper, we derive accurate outage probability and bit error rate expressions for a high altitude platform station (HAPS) assisted terrestrial communication system. In particular, HAPS is deployed as a relay node to assist two ground stations for data transmission. Each ground station-to-HAPS communication link works on a hybrid radio frequency (RF) and free space optics (FSO) mode. Selection combining is performed at the HAPS and the destination ground station to select either RF or FSO link based on the instantaneous channel signal-to-noise ratio (SNR). The accuracy of the derived outage probability and bit error rate expressions are validated with extensive computer simulations. We further develop an optimal power allocation scheme that optimizes the transmit power of source-ground-station and HAPS while satisfying practical energy consumption constraints. The derived expressions provide more insights on system design and assist analyzing HAPS-terrestrial integrated network supported by hybrid RF/FSO system.

HAPS-SMBS 3D installation for 5G FANET performance optimisation using a DNN, WCOP and NetSim-based scheme

High altitude platform stations (HAPSs) can be installed as flying super macro base stations (SMBSs) to serve the existing terrestrial infrastructure as a flying ad hoc network (FANET) keeping enhanced and continuous wireless network connections in normal and disaster scenarios. This dynamic process becomes inefficient if the mobility and locations of the user equipment (UE) are unknown and uncertain. Although increasing the number of HAPSs provides more user connectivity, it is too expensive and impractical. Hence, installations of the available HAPSs in the optimum positions can solve the problem in cost effective manner. This paper proposes a novel combinational scheme consisting of the two following stages, i.e., the deep learning (DL)-based forecasting of the ground user (GU) locations and the weighted combinatorial optimisation problem (WCOP)-based HAPS-SMBS positioning to optimise the 5G FANET performances in both normal and disaster scenarios.

Optimal airfoil design and wing analysis for solar-powered high altitude platform station

The ability of flying continuously over prolonged periods of time has become target of numerous research studies performed in recent years in both the fields of civil aviation and unmanned drones. High altitude platform stations are aircrafts that can operate for an extended period of time at altitudes 17 km above sea level and higher. The aim of this paper is to design and optimize a wing for such platforms and computationally investigate its aerodynamic performance. For that purpose, two-objective genetic algorithm, class shape transformation and panel method were combined and used to define different airfoils with the highest lift-to-drag ratio and maximal lift coefficient. Once the most suitable airfoil was chosen, polyhedral half-wing was modeled and its aerodynamic performances were estimated using the CFD approach. Flow simulations of transitional flow at various angles-of-attack were realized in ANSYS FLUENT and various quantitative and qualitative results are presented, such as aerodynamic coefficient curves and flow visualizations. In the end, daily mission of the aircraft is simulated and its energy requirement is estimated. In order to be able to cruise above Serbia in July, an aircraft weighing 150 kg must accumulate 17 kWh of solar energy per day.

Wireless Technologies to Connect Drones Under Beyond-Line-of-Sight Conditions: From Multi-Hop to High Altitude Platform

Wireless Technologies to Connect Drones Under Beyond-Line-of-Sight Conditions: From Multi-Hop to High Altitude Platform

Two-Step Dynamic Cell Optimization Algorithm for HAPS Mobile Communications

High altitude platform stations are attracting much attention as novel mobile communication platforms for ultra-wide coverage areas and disaster-resilient networks. Multi-cell configurations can increase communication capacity in a wide area. Conventional work optimizes the cell configuration for a multi-cell configuration based on a genetic algorithm (GA). This method identifies an optimal cell configuration in terms of spectral efficiency depending on the number of cells under a uniform user distribution. However, user distributions differ depending on location; thus, cell configuration optimization is also required for non-uniform user distributions. The problem is an increased number of parameters because each cell requires different antenna parameters for non-user distributions compared to a uniform user distribution. This increases the difficulty of optimization, even when using a GA in some cases. Therefore, We propose a GA-based two-step cell optimization algorithm that comprises both search space reduction and antenna parameter optimization steps to address this problem. The proposed method employs the concept of co-evolution, i.e., a divide-and-conquer method. The proposed method divides multiple cells into several groups to reduce the number of optimized parameters and optimizes each group in order. In addition, the search space is reduced based on a marginal histogram obtained via optimization with a large step size. Simulation results demonstrate that this technique can reduce the number of combinations compared to the case without sub-area division. In addition, there are cases where the search space reduction further reduces the number of combinations without degrading the sum of the square root of throughput.

Reconfigurable Intelligent Surface-Assisted HAPS Relaying Communication Networks for Multiusers Under AF Protocol: A Performance Analysis

This research paper studies the performance analysis of a reconfigurable intelligent surface (RIS)-assisted high altitude platform station (HAPS) relaying communication network for multiple ground users. Owing to transmission blockage between the source and the destination, the source transmits its information to the HAPS via the RIS where the HAPS acts as an amplify-and-forward (AF) relay node for sending the source information to ground users. To characterize the transmission links, the channels between the source-RIS-HAPS are assumed to follow approximate generalized-K fading distribution while the links between the HAPS and multiple ground users experience Shadowing-Rician fading distribution. Since HAPS employs AF relaying scheme, the closed-form expressions of the cumulative distribution function (CDF) and probability density function (PDF) of the system end-to-end signal-to-noise ratio (SNR) are derived by exploiting the Meijer’s G and Fox’s H functions. Based on these expressions, the analytical expressions of the system outage probability, average bit error rate (BER) and average capacity are obtained to examine the system performance. Moreover, the asymptotic expressions are obtained to provide more physical insight about the system. In addition, the results unveil the significant effect of the system and channel parameters on proposed system. The results show that the system with RIS offers better performance compared the system without RIS. The analytical results are verified by Monte-Carlo simulation to justify the accuracy of the derived expressions.

An Accurate Model for Computation Offloading in 6G Networks and a HAPS-Based Case Study

The undeniable potential of computation offloading has been attracting attention from researchers for more than a decade. With advances in multi-access edge computing (MEC), computation offloading has become a more critical issue because of the heterogeneity in the computational power of edge devices and the elevated importance of extending their lifespan. Due to the apparent advantages, the use of MEC in 6G networks, where a vertical heterogeneous network composed of space, air, and ground networks is only natural. The non-terrestrial networking elements constitute effective computational resources. However, recent research investigating the potential of computational offloading in 6G networks has involved models that do not adequately reflect the complexity of the underlying processes. In this study, we propose a realistic computation model for 6G networks that considers crucial properties of the offloaded job, including the inter-dependency of the job tasks and the decomposability of the job. Our model is based on the mature application domain of MEC, where proven solutions are already studied. We also investigate the potential of a high altitude platform station (HAPS)-aided MEC platform using this model. The proposed model allows us to design offloading strategies to enable adaptive computational offloading. Through numerical analyses, we show that the proposed model provides sufficient insight to reduce the total processing time significantly.

Performance Optimization of High-Altitude Platform Wireless Communication Network Exploiting TVWS Spectrums Based on Modified PSO

The Uniform hexagonal array (UHA) sidelobe level (SLL) reduction is a crucial consideration for avoiding co-channel interference and improving Carrier-to-interference Ratio (CIR) performance in HAP wireless networks using the TVWS spectrum. The SLL reduction in UHA has been researched by applying evolutionary algorithms such as genetic algorithm (GA) and Particle Swarm Optimization (PSO). However, such algorithms have the drawbacks such as early convergence and the inability to reach globally. The beam pointing approaches for UHA antennas using Modified PSO (MPSO), for reducing the SLL on finding the optimal current weights is presented in this article. The UHA antenna array with 19 cells fitted with 169 elements in the HAPs cellular structure is considered to simulate the proposed approach. Then, the MPSO approach is employed and compared with the standard PSO algorithm and Uniform Weighting (UW) scheme. As per the simulation findings, the proposed approach drastically decreased sidelobes, with a reduction of up to -6.84dB and -13.3dB when compared to PSO and UW respectively. In other words, the proposed approach outperforms the UW and the PSO algorithms. Moreover, the CIR performance of the proposed approach has been demonstrated in terms of average outage probability and the proposed approach provides a better result.

Research on Spectrum Needs Prediction Method for HAPS as IMT Base Station

The High-Altitude Platform Station (HAPS) is an integral component in Non-Terrestrial Network (NTN) which is potential 6G technology. The HAPS as IMT Base Stations (HIBS) in the mobile service is an emerging and promising approach to achieve vertical heterogeneous network. Therefore, some efforts have been made in order to harmonize the worldwide usage of HIBS, including investigations of the spectrum needs, usage, and technical and operational characteristics of HIBS, among which the spectrum needs has drawn particular attention because it should be determined in advance prior to the commercial deployment of HIBS. However, there are very few investigations devoted to the prediction of the spectrum needs for future HIBS deployments currently due to some tough difficulties. Firstly, there has no actual deployment precedent of HIBS so that the future capacity demands are not clear. Secondly, both the networking and its performance of HIBS deployment are questions to be answered. Therefore, taking into account the study under WRC-23 Agenda Item 1.4 HIBS-CHARACTERISTICS, this paper proposes an evaluation method of the spectrum needs prediction for HIBS deployed as a supplementary for grounded base station, and presents the results of spectrum needs for HIBS in certain frequency bands identified for IMT. The capacity demand of HIBS in 2025 is predicted and the spectral efficiency of HIBS system is obtained by simulation, and the spectrum needs is defined as <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Capacity Demand/Spectral Efficiency</i> . The main contributions of this paper are as follows: Firstly, to obtain the spectral efficiency of HIBS systems, we set up a simulation model with 7-cell networking in accordance with configuration parameters of WRC-23 AI 1.4, which equipped with beamforming antenna to improve spectral efficiency. And calculating the CDF curve of spectral efficiency by using Monte Carlo method allows the simulation result to more closely match the actual deployment. Secondly, in order to solve the problem of no actual HIBS traffic data, we propose a method based on the methodology provided in Recommendation ITU-R M.1768-1 to simulate the capacity demand, and then to predict the capacity demand of HIBS in 2025 by polynomial fitting. Finally, we have predicted the spectrum needs by calculating <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Capacity Demand/Spectral Efficiency</i> . The spectrum demand range of HIBS for China rural area in 2025 predicted by the proposed method is from 80.30 MHz to 122.97 MHz. This method can effectively predict the spectrum needs of HIBS for various deployment scenarios, which plays an important role in driving the commercial deployment of HIBS.

Nullforming-Based Precoder for Spectrum Sharing Between HAPS and Terrestrial Mobile Networks

Ultra-wide coverage is one of the important concepts of the sixth generation (6G) mobile communications. High-altitude platform stations (HAPSs) have attracted considerable attention as a leading candidate for coverage extension of cellular networks. HAPSs should share a frequency band with terrestrial cellular networks and provide users with mobile communication services using the common frequency band to improve spectral efficiency in service links. However, the signals from HAPS strongly interfere with terrestrial cells in the service area of HAPS, which leads to a considerable deterioration in the system capacity of terrestrial cells. This study proposes a novel two-stage precoding scheme that realizes spectrum sharing between HAPS and terrestrial networks. The proposed scheme mitigates HAPS interference with terrestrial cells using beamforming techniques in phased array antenna systems. In the first stage, nulls are created in directions of terrestrial cells. In the second stage, user-specific beams are formed in directions of users connected to HAPS. Results demonstrate that the proposed scheme improves the signal-to-interference-plus-noise ratio of users in terrestrial cells by up to 10 dB compared with a conventional minimum mean square error (MMSE) precoder without nullforming when the HAPS uses a cylindrical array antenna with 196 elements. Furthermore, for ten terrestrial cells, the proposed scheme achieves 128% of total capacity of the conventional MMSE precoder. Total capacity is defined as the sum capacity of HAPS and all terrestrial cells.

Multi-HAPS Network Implementation within 3GPP’s NTN framework for 5G and beyond

High Altitude Platform Station (HAPS) is part of the 3GPP defined non-terrestrial network (NTN) infrastructure for 5G networks. Various technical studies by 3GPP have addressed NTN-based implementations and have significantly studied satellite-based scenarios. However, the study does not sufficiently address HAPS or multi-HAPS based scenarios specifically. Though HAPS, is captured under Unmanned Aerial Systems (UAS), it has unique operational realities that set it apart from other NTN platforms. For instance, HAPS come in different variants of fixed-wing, balloons and airships. This paper highlights the need for expanded studies specifically aimed at HAPS for more seamless integration. The work also analyses the Doppler effect associated with fixed-wing HAPS systems to further demonstrate how operational scenarios may differ for these platforms and the need for targeted studies. HAPS is expected to contribute significantly to the NTN-based implementations and may require more specialised considerations within the 3GPP NTN technical specification process, especially for 5G and beyond 5G (B5G) networks.