Satellite Links Research Articles

Performance Analysis of HSTC Network with Non-Static Terrestrial Nodes in a Fading Environment

In this paper, Hybrid satellite-terrestrial systems (HSTC) are preferred in dynamic type environments with high mobility of nodes due to the performance enhancement features in multiple relay-based selective decode-and-forward (DF) approach. Because of the satellite link, aerial satellite to destination and satellite to relay links, are not the same due to time-selective shadowed Rician fading. Time-selective Rician fading depends on parameters like the angle of elevation of the satellite, the terrestrial relay node, and the node destination links, considered to be distinctively time-selective Nakagami faded. Here, per-frame average symbol error rate (SER) and outage probability are derived in a closed-form expression with consideration of M-ary PSK modulated symbols transmission. After evaluation, it was detected that the system performance is significantly degraded due to the time-varying nature of the links (dynamic environment). After various simulations and calculations, it was established that the error rate of the HSTC is significantly low by increasing the elevation angle of the satellite on the relay point. The performance enhancement can be observed by enhancing the satellite angle at the destination node of user equipment.

EyePod-Mini: Constellations, Urban Launches and Buoy Landings

Small and low-cost balloons using EyePod-Mini radios are ideal for urban and long duration launches for improving the learning experience. The EyePod-Mini is an all-in-one data and tracking pod with a mass of 150 g including 2.2 AHr batteries. It is significantly less than the mass of the balloon and fits into the balloon neck (3 cm). It communicates with Globalstar satellites for 24/7 global coverage via the internet. An additional low power 900 MHz module and a 2m APRS ham module are also available for real time data and tracking that also fit in the 3 cm tube. Another EyePod option is its design to float in water as a buoy surface monitor and tracked or as a live ground probe in remote regions for additional geo-learning. Because the probe is small it is ideal for balloon constellations making many measurements over an interesting set of time and space coordinates. Multipoint measurements help visualize the dynamics of the atmosphere to better understand weather fronts, thunderstorms, eclipses, and turbulence to improve models and learning. External instrument Pods can be attached to the EyePod mini as well and the data wirelessly connected to the EyePod links. Use of two small balloons for a launch ensures good linear altitude data collection for ascent and decent (no free fall). Small balloons with small radios are ideal for urban launches near water and forest since the experiments can continue to work as ground probes with the satellite link and solar array charging. It is reasonable to launch the EyePod-Mini without the expense of chase and recovery in difficult terrain. Because the EyePod-Mini is a rod like spear without attachment cords it is much easier to recover with treetop landings. For lake landings data can be acquired on the surface as the payload drifts to the shore for pick-up. Because the Globalstar link works globally, long duration flights are possible with useful data over oceans and remote areas.

A Simulated and Experimental Analysis of Evaporation Duct Effects on Microwave Communications in the Irish Sea

This article presents a simulated and experimental analysis of the effects of the evaporation duct on microwave propagation in the Irish Sea. The evaporation duct is a phenomenon that occurs almost permanently over all the world’s oceans and allows electromagnetic waves to travel beyond the horizon. Weather data logged over several years from four buoys off the east coast of Ireland has been analyzed to find the probability and strength of the evaporation duct. Signal propagation in the evaporation duct has been simulated using the parabolic equation model and compared to results obtained from an experimental setup in the Irish Sea. The best antenna heights and frequencies to maximize signal propagation at this location are also found. Results show that the evaporation duct can be used to provide high bandwidth communications beyond the horizon with an uptime of approximately 40%, and that weather data from buoys can be used to predict the performance of the communications link. While this method of communication is not one that can be relied on all the time, it could be useful to reduce dependency on expensive satellite links or provide nontime critical communications.

Outage Performance of Multi-UAV Relaying-Based Imperfect Hardware Hybrid Satellite-Terrestrial Networks

In this paper, we consider an imperfect hardware hybrid satellite-terrestrial network (HSTN) where the satellite communication with a ground user equipment (UE) is aided by the multiple amplify-and-forward (AF) three-dimensional ($3$D) mobile unmanned aerial vehicle (UAV) relays. Herein, we consider that all transceiver nodes are corrupted by the radio frequency hardware impairments (RFHI). Further, a stochastic mixed mobility (MM) model is employed to characterize the instantaneous location of $3$D mobile UAV relays in a cylindrical cell with UE lying at its center on ground plane. Taking into account the aggregate RFHI model for satellite and UAV relay transceivers and the random $3$D distances-based path loss for UAV relay-UE links, we investigate the outage probability (OP) and corresponding asymptotic outage behaviour of the system under an opportunistic relay selection scheme in a unified form for shadowed-Rician satellite links' channels and Nakagami-\emph{m} as well as Rician terrestrial links' channels. We corroborate theoretical analysis by simulations.

Analysis of Wet Antenna Losses on 11.843 GHz Slant Path in Nigeria and Comparison with Some Tropical Climates

The amount of water deposits on the receiving dish antenna can cause additional losses and consequently contaminate the actual slant-path attenuation. Only a few research works have been reported in the literature on the measurement of wet antenna attenuation (WAA) and the technique(s) of extracting the losses from the total attenuation in tropical and equatorial climates, characterized with heavy rainfall intensities of convective kind. Therefore, the adverse effects of antenna losses due to rain on a 11. 843 GHz satellite link and the methodology of extracting the losses from the measured rain-induced attenuation have been reported in this article. A vertically polarized parabolic dish with diameter and elevation of 0.3 m and 420, respectively have been used in the study. The measured results at 11.843 GHz were frequency-scaled in order to obtain their equivalents at 18.585 and 20.2 GHz; and then compared with those reported from Malaysia and Canada. The study will provide useful information in the planning and designing of an efficient and reliable Earth-satellite communication link in tropical climates. The experimental results also can further enrich the ITU-R databank.

Performance analysis of decoy state quantum key distribution over underwater turbulence channels

Decoy state quantum key distribution protocols have been studied for atmospheric, fiber, and satellite links; however, those results are not directly applicable to underwater environments with different channel characteristics. In this paper, we investigate the fundamental performance limits of decoy state BB84 protocol over turbulent underwater channels and provide a comprehensive performance characterization. We adopt a near field analysis to determine the average power transfer over a turbulent underwater path and use this to obtain a lower bound on the secret key rate. We quantify the performance of decoy BB84 protocol in different water types assuming various turbulence conditions. We further investigate the effect of system parameters such as transmit aperture size and detector field of view on the performance.

Physical Layer Security in Cybertwin-Enabled Integrated Satellite-Terrestrial Vehicle Networks

In this paper, we investigate the secure vehicle communications in cybertwin-enabled integrated satellite-terrestrial networks, where the digital twins (DTs) in the cybertwin space reflects the physical entities (i.e., satellite, terrestrial base station (BS), and vehicles). Particularly, considering the channel similarity between different satellite links versus the randomness difference in terrestrial links, it is challenging to reach the secure transmission in satellite and terrestrial links independently with limited resources. Considering the information exchange in the cybertwin space can support an information sharing between such physical entities, the secure transmission design by using the heterogeneous satellite-terrestrial resources can be conducted from a global perspective. With the channel feedback information of vehicles gathered at the cybertwin, the co-channel interference caused by the spectrum sharing is leveraged to assist the implementation of secure transmissions in the integrated satellite-terrestrial vehicle network. Specifically, the problems of maximizing the secrecy rate of satellite-to-vehicle link and the terrestrial BS-to-vehicle link are formulated, respectively. To solve such two problems, we propose two corresponding beamforming optimization approaches, where semi-definite relaxation (SDR) and semi-definite programming (SDP) are adopted due to the non-convexity. In addition, the tightness of SDR is proved and the complexity of proposed approaches is also analyzed. Finally, extensive numerical simulations are carried out and results show the effectiveness of our proposed approach.

An Ant Colony Optimization-Based Routing Algorithm for Load Balancing in LEO Satellite Networks

Satellite networks can provide a wider service range and lower delay than traditional terrestrial optical fiber networks. However, due to the bursty characteristic of the Internet traffic and the distributive feature of satellite links, traffic-intensive areas often suffer from link congestion while links in other areas are underutilized, i.e., the traffic imbalance problem in LEO satellite networks. In this paper, an ant colony optimization routing algorithm with window reduction for LEO satellite networks, ACORA-WR, is proposed to achieve load balancing. ACORA-WR limits the movement of the ant colony to a specific range and comprehensively considers the path distance, transmission direction, and link load to find a path with low delay and overhead. Simulation results verify that the proposed ACORA-WR scheme demonstrates high data delivery ratio and network throughput, while ensuring low average delay and network transmission overhead.

Global Formulation of the Synthetic Storm Technique Oriented to Satellite Link–Budget Design

We have updated the global Synthetic Storm Technique (referred to as the global SST) by reformulating it according to a larger database of rain rate time series collected in several sites in different climatic regions. For each site, the average annual probability distribution of rain attenuation obtained with the global SST, PSST,gloA, in a slant path, was compared with that given by the full SST, PSSTA, which we have considered as experimental data. The test was performed for frequency ranging from 10 to 100 GHz, for elevation angle θ ranging from 20° to 60° and for annual probabilities 10% to 0.01%. The global SST tends to underestimate the attenuation by approximately 10% for elevation angle θ≤30° and about 20% for 30°<θ<60° in the probability range 10% to 0.1%, and approximately 15% in the probability range 0.1% to 0.01%. For any probability, the error is zero for θ=90° because at the zenith, the global SST coincides with the full SST.

Uplink Outage Performance of NOMA-Based Hybrid Satellite-Terrestrial Relay Networks Over Generalized Inhomogeneous Fading Channels

In this paper, we investigate the outage performance of an uplink (UL) non-orthogonal multiple access (NOMA)-based hybrid satellite-terrestrial relay network (HSTRN), in which two users communicate with the satellite through a decode-and-forward (DF) relay due to the lack of direct link. To provide a comprehensive yet hitherto unexplored outage analysis framework, we consider a more generalized channel model, i.e., the terrestrial and satellite links, respectively, undergo <inline-formula> <tex-math notation="LaTeX">$\alpha -\mu $ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$\kappa -\mu $ </tex-math></inline-formula> shadowed fadings. Under fixed power allocation (FPA) for both multiple access phase and relaying phase, we firstly study three successive interference cancellation (SIC) decoding schemes, of which the first two are refined from existing schemes, while the third one, named as <i>extended SIC (ESIC)</i>, is proposed in this paper to satisfy the quality of service (QoS) decoding criterion, which is shown to offer better performances for both users as compared to the former two SIC schemes. We also propose a novel dynamic power allocation (DPA) scheme for the multiple access phase, termed as <i>enhanced DPA (EDPA)</i>, to overcome both users&#x2019; error floor (EF) issue yet provide better user fairness than the conventional DPA in the literature. We then analyze the exact and asymptotic outage performance for three SIC and the <i>EDPA</i> schemes under the generalized channel setting. It is shown that both the proposed <i>ESIC</i> and <i>EDPA</i> can circumvent the EF issue and moreover, each has its own advantage in terms of the diversity order (DO). Our results also reveal that there exists a trade-off between <i>ESIC</i> and <i>EDPA</i>, since the former requires a premise on users&#x2019; targets rates to overcome the EF and once this premise is satisfied, no DO degradation will occur, while the latter does not entail such a premise but may face potential DO degradation. Finally, we present numerical results to verify the theoretical analysis, manifest the impacts of key parameters on the system performance, and demonstrate the advantages of our proposed network over other benchmarks.

Improving IoT-Over-Satellite Connectivity Using Frame Repetition Technique

Through Non-Terrestrial Networks (NTN), a global coverage connecting areas with minimal or no terrestrial services is envisaged using satellite and air-borne platforms. However, one of the challenges to delivering NTN communications is the increased levels of path-loss, relative to typical terrestrial scenarios, due to the vast communication distance towards satellite platforms. Typically, cellular IoT technologies adopt frame repetitions where many redundant data transmissions are included to enhance its success rate. This can be also extended to satellite links to counter the effect of long distance communications on the link budget. In this paper, we put forth an analytic framework that captures the repetition behavior based on the probability of line-of-sight (LoS), as it heavily influences the propagation conditions. We analyze the coverage performance in terms of the frame success rate, which allows us to tune the repetitions behavior to suit a given satellite admittance region. Furthermore, we obtain an optimal global average success rate by considering the availability of satellites. The presented framework can facilitate coverage enhancements for IoT-over-Satellite networks.

Software-Defined Maritime Fog Computing: Architecture, Advantages, and Feasibility

Advances in sensing and communication technologies have enabled emerging applications in ocean research. The stringent resource limitations, however, hinder local task processing on maritime devices. Cloud-based task offloading is also not feasible due to the unaffordable costs induced by satellite links. In this work, we propose a visionary architecture on software-defined maritime fog computing (SD-MFC), which adopts fog computing to provide localized services and uses software-defined networking technology to provide the abstraction of underlying resources. In SD-MFC, coastal infrastructures and large-size vessels are turned into maritime fog nodes to process the tasks generated by surrounding maritime devices. By exploration of a real-world vessel trajectory dataset, the feasibility and effectiveness of the proposed SD-MFC are verified.

Optimal Throughput for Multicast over MANET-Satellite Networks

The convergence of MANET and satellite networks has been considered as one of the potential solutions to provide emergency communication and disaster relief services. However, unreliable MANET and satellite links may influence seriously to data transmission. In this paper, we study the application of packet-level coding for enhancing network throughput for multicast services over integrated MANET-satellite networks. First, we characterize the performance of multicast networks in terms of the probability of packet delivery under different packet-level codes. We then formulate and solve the coding optimization problem for typical values of computational resources at the network nodes given some target probability of successful delivery to the receivers. Finally, several simulation results show significant gains in the average achievable rate at the receivers with respect to routing for representative scenarios with different types of network devices given a strictly-high probability of successful delivery of the multicast networks. Furthermore, the devices with higher computational resources can obtain significantly better throughput gain.

Satellite to Ground Station, Attenuation Prediction for 2.4–72 GHz Using LTSM, an Artificial Recurrent Neural Network Technology

Satellite communication links suffer from arbitrary weather phenomena such as clouds, rain, snow, fog, and dust. Furthermore, when signals approach the ground station, they have to overcome buildings blocking the direct access to the ground station. This work proposes a model to predict the remaining signal strength for the next timeframe after deducting the attenuation and disruption impact caused during its propagation from the satellite to the ground station. The proposed model can be adjusted to comply with any geographic region and a broad spectrum of frequencies. We employ LTSM, an artificial recurrent neural network technology, providing a time-dependent prediction. We can instantly calibrate the satellite outgoing signal strength to overcome the predicted attenuation, resulting in satellite energy saving using this prediction.

Secrecy outage analysis for UAV assisted satellite-terrestrial SWIPT systems with NOMA

This paper studies the physical-layer security for a non-orthogonal multiple access (NOMA) based unmanned aerial vehicle (UAV) assisted hybrid satellite-terrestrial simultaneous wireless information and power transfer system. Specifically, a satellite sends confidential information on a NOMA basis to two terrestrial users through a UAV relay under the wiretapping of an eavesdropper. A power splitting protocol is deployed to coordinate information decoding and energy harvesting. Assuming the satellite link experiences Shadowed-Rician fading and the terrestrial links are subject to Nakagami-m fading, the analytical expressions for the secrecy outage probability and the probability of strictly positive secrecy capacity for two users are derived and the accuracy of the results is verified by Monte-Carlo simulations.

Performance evaluation of metric measures for converting 30-min GPM rain data to 1-min for microwave applications in Tropical region of Nigeria: A multivariate approach

The quest for a one-minute rain rate is high because it is an important input for generating high accuracy of rain-induced attenuation and will in turn earn good use by radio engineers in designing good and high-quality terrestrial and satellite links. This study utilizes eleven (11) years (2008–2018) rain rate data obtained from a satellite-based program, the Global Precipitation Measurement (GPM) over five different locations in the South West, Nigeria, and 2 years (2017 and 2018) gauge measured rain rate data in one of the locations Akure, Nigeria for validation. The GPM platform was an improved satellite-based program over the Tropical Rainfall Measuring Mission (TRMM) for measuring tropical rainfall with spatial and temporal resolution. Hence, a good application of measured GPM rain rate data can be a good tool for estimating rain-induced attenuation if an adequate conversion is made for a one-minute integration regime as recommended by the \\International Telecommunication Union (ITU). The 30 min GPM rain rate data was converted to one-minute integration time based on a multivariate approach. The coefficient of determination of GPM rain rate data closely matches with the gauge and was found to be 0.9194. Analyses were also performed on seasonal bases for rain rate and rain-induced attenuation ranging from frequencies 10 GHz, to 30 GHz, to assess the seasonal trend between the two sources of data. The seasonal analyses were scaled as pre-monsoon, intensive, cessation, and dry months. A seasonal comparison shows that the GPM data presented the highest rain rate and rain attenuation during the intense rainy regime. Attenuations are considerably high at 0.01, 0.005, and 0.001 percentages of time. Overall, a deduction was made that the GPM rain rate measurements offered reliability in the estimation of rain-induced attenuation analyses in the tropical region of Nigeria due to the spatial–temporal coverage when compared with the rain gauge coverage which is sparse and costly to implement. This research has presented a reliable 30 min to one-minute integration time conversion model suitable for tropical locations, as well as information on seasonal rain attenuation statistics which would be utilized for effective satellite communication systems by the radio engineers.

Dynamic optimization of intersatellite link assignment based on reinforcement learning

Intersatellite links can reduce the dependence of satellite communication systems on ground networks, reduce the number of ground gateways, and reduce the complexity and investment of ground networks, which are important future trends in satellite development. Intersatellite links are dynamic over time, and different intersatellite topologies have a great impact on satellite network performance. To improve the overall performance of satellite networks, a satellite link assignment optimization algorithm based on reinforcement learning is proposed in this article. Different from the swarm intelligence method in principle, this algorithm models the combinatorial optimization problem of links as the optimal sequence decision problem of a series of link selection actions. Realistic constraints such as intersatellite visibility, network connectivity, and number of antenna beams are regarded as fully observable environmental factors. The agent selects the link according to the decision, and the selection action utility affects the next selection decision. After a finite number of iterations, the optimal link assignment scheme with minimum link delay is achieved. The simulation results show that in 8 or 12 satellite network systems, compared with the original topology, the topology calculated by this method has better network delay and smaller delay variance.

A Single-Layer 10–30 GHz Reflectarray Antenna for the Internet of Vehicles

A novel ultra-wideband (UWB) reflectarray antenna for the Internet of Vehicles (IoV) is introduced in this paper. By simply connecting the neighboring reflectarray elements, the proposed reflectarray antenna achieves a remarkable radiation pattern bandwidth of 20 GHz, ranging from 10 GHz to 30 GHz. To explain the operating principles of the proposed reflectarray antenna, the equivalent circuit (EQC) model of the unit cell is built, which also provides an efficient and rapid way to analyze the performance of the proposed reflectarray element. It is found from the EQC analysis that the connected elements can achieve better reflection phase responses than conventional separated elements, thereby improving the array bandwidth. As a proof of concept, a 503-element reflectarray antenna simultaneously covering the vehicle-to-satellite bands (12.25–12.75 GHz/14.0–14.5 GHz/19.6–21.2 GHz/29.4–31.0 GHz), the 24-GHz short-range vehicle radar band (24.25–26.65 GHz) and the 5G millimeter-wave band (27.5–28.35 GHz) is designed, fabricated, and characterized. The experimental results demonstrate that the presented reflectarray antenna can maintain undistorted beams, high antenna gain, low cross-pol level, and moderate aperture efficiency over a bandwidth of 100%, i.e., from 10 to 30 GHz. With its simple and planar aperture as well as excellent performance, the proposed reflectarray antenna can be a promising candidate for vehicles that require reliable high data-rate satellite links and 5G millimeter-wave connections simultaneously.

A compact, transportable optical clock with 1×10−17 uncertainty and its absolute frequency measurement

We report a robust, compact, and transportable optical clock (TOC-729-2) based on a trapped single 40Ca+ ion with a systematic uncertainty of 1.1×10−17, which is limited by the black-body radiation shift uncertainty at room temperature. By comparing it with the previous transportable optical clock (TOC-729-1) similar but completely independent, the instability was measured to be better than 1.2×10−14/τ. Benefiting from the modular and integrated design, this TOC was constructed in a volume of ∼0.33 m3 excluding the controlling electronics in 19-in. racks. After being moved ∼1200 km away by express delivery, the single-ion signal was restored within 24 h. With the TOC uptime of 92% in 35-day period, the absolute frequency of the 729 nm transition of 40Ca+ was measured using a satellite link to International Atomic Time (TAI) to provide traceability to the SI second, and the result is 411 042 129 776 400.15(22) Hz, corresponding to a relative uncertainty of 5.3×10−16.

Studies of scintillations and TEC variations with GPS satellite links together with soil radon anomalies preceding Nepal earthquakes of April–May 2015

Studies of scintillations and TEC variations with GPS satellite links together with soil radon anomalies preceding Nepal earthquakes of April–May 2015