Low Earth Orbit Satellite Systems Research Articles

Compensation System Design and Comparison of Very High Doppler Frequency Effect

One of the serious problems in the low earth orbit (LEO) satellite communication system is the very high Doppler frequency effect since the LEO satellite system runs very fast to keep the orbit altitude. Especially, this problem produces the critical performance degradation when the broad bandwidth is necessary for the very high speed satellite transmission of the very high resolution image data. As the data speed is increased, the higher efficient communication technology will be required to transmit the high-capacity of data to the ground station. In the LEO satellite communication systems, very large Doppler influence is generated because of the very fast movement of satellite. This Doppler spectrum shift effect, similar to the carrier frequency offset effect, can cause performance degradation in the satellite communication system. Therefore, it is very important to effectively compensate for this Doppler frequency shift. So, in this paper, we analyze this Doppler effect of LEO satellite receiver and propose an efficient compensation system design against the Doppler frequency. Two compensation methods are proposed: pilot method and phase locked loop (PLL) method. As simulation results show, when the SNR is approximately 5 dB or less, the pilot method provides better performance, compared with the PLL method. When the SNR is approximately 6 dB or more, it can be confirmed that the PLL method outperforms than the pilot method, which is almost same to the theoretical performances. Furthermore, it can be said that the PLL method is very spectrum-efficient than the pilot method since it does not require any pilots. However, it is very difficult and expensive to design the accurate, agile, broadband operating PLL system to compensate the very wide range of Doppler frequency shift.

Beam Coverage Comparison of LEO Satellite Systems Based on User Diversification

To achieve global network coverage and the need for high-speed communication, the idea of providing Internet access from space has made a strong comeback in recent years. The low earth orbit (LEO) communication satellite constellation is once again on the stage of the world with its unique features and new technology. In order to provide faster and more affordable communication resources, low-orbit satellites need be customized to design satellites. The beam coverage design is essential to the user-customized design. This paper combines the user traffic demand model and the low-orbit satellite beam coverage model to analyze the impact of beam coverage characteristics on the performance of low-orbit satellite systems. The user traffic model bases on the user simulative distribution (uniform, normal) and the user geographic distribution (according to the AIS and ADS-B historical data acquired by STU-2B and STU-2C which are the LEO satellites launched in Sep, 2015, Jiuquan, China). The beam coverage model compares the OneWeb system to the SpaceX system. The beam coverage model takes the variability in performance induced by atmospheric conditions for the user links into account. Follow that this paper proposes a system method to simulate the two satellite system which described by the throughput, delay, access probability. Finally, the sensitivity of beam coverage to user diversification is summarized and discussed.

A Novel Dynamic Spectrum-Sharing Method for Integrated Wireless Multimedia Sensors and Cognitive Satellite Networks.

With the growing demand, Wireless Multimedia Sensor Networks (WMSNs) play an increasingly important role, which enhances the capacity of typical Wireless Sensor Networks (WSNs). Additionally, integrating satellite systems into WMSNs brings about the beneficial synergy, especially in rural and sparsely populated areas. However, the available spectrum resource is scarce, which contradicts the high-speed content required for multimedia. Cognitive radio is a promising solution to address the conflict. In this context, we propose a novel spectrum-sharing method for the integrated wireless multimedia sensor and cognitive satellite network based on the dynamic frequency allocation. Specifically, the Low Earth Orbit (LEO) satellite system plays the role of the auxiliary to connect sensor nodes and the remote control host, and it shares the same frequency with the Geostationary Earth Orbit (GEO) system in the downlink. Because the altitudes of GEO and LEO satellites differ greatly, the beam size of GEO is much larger than that of LEO, which provides the opportunity for LEO beam to reuse the frequency that was allocated to the GEO beam. A keep-out region is defined to guarantee the spectral coexistence based on the interference analysis in the worst case. In addition, a dynamic frequency allocation algorithm is presented to deal with the dynamic configuration caused by the satellite motion. Numerical results demonstrate that the dynamic spectrum-sharing method can improve the throughput.

On the performance of UNB for machine‐to‐machine low earth orbit (LEO) satellite communications

SummaryUltra narrow band (UNB) is a widely used technology for machine‐to‐machine and low‐power wide‐area communications. Its properties, long range with small RF power, make it naturally attractive for satellite communications but also draw new challenges compared with terrestrial systems where this technology is already deployed. Indeed, the main advantage of UNB signals, their small bandwidth, makes them more sensitive to frequency drifts that are particularly present in the case of LEO satellite systems. It also implies the use of a random access method in which the carrier frequency is a parameter unknown by the receiver. In this paper, we propose a general semianalytical model to evaluate the performance of a terrestrial or satellite system using UNB technology, taking into account the multiuser interference and the frequency drift. This model is then used to assess the performance (packet loss ratio and throughput) on the return link medium access control (MAC) of a representative LEO satellite system. With our model, we analyze the effect of frequency drift on the system performance. This paper also proposes to investigate more deeply the multiuser interference modeling in order to estimate accurately the performances of a UNB system in terms of bit error rate (BER). We propose a semianalytical approach to study the interference in presence of arbitrary power imbalance that includes the effect of frequency offset and frequency drift and applicable for any linear modulation and any pulse‐shaping filter. The expression of the multiuser interference is established in the general case. We then propose a methodology to compare this exact model to the Gaussian interference approximation (mainly used through the central limit theorem) in order to assess its accuracy.

Dynamic channel reservation scheme based on priorities in LEO satellite systems

According to low earth orbit (LEO) satellite systems with users of different levels, a dynamic channel reservation scheme based on priorities is proposed. Dynamic calculation of the thresholds for reserved channels is the key of this strategy. In order to obtain the optimal thresholds, the traffic is predicted based on the high-speed deterministic movement property of LEO satellites firstly. Then, a channel allocation model based on Markov is established. Finally, the solution of the model is obtained based on the genetic algorithm. Without user location, this strategy effectively reduces handover failures and improves channel utilization by adjusting dynamically the thresholds according to traffic conditions. The simulation results show that the system's overall quality of service can be improved by this strategy.

A Rain-Attenuation Stochastic Dynamic Model for LEO Satellite Systems Above 10 GHz

In this paper, a new rain-attenuation stochastic dynamic channel model for low Earth orbit (LEO) satellite links operating above 10 GHz is proposed, taking into account the time dependence of elevation angle and its impact on rain-attenuation dynamics. The new synthesizer is based on the first-order stochastic differential equations (SDEs) and extends the well-known Maseng–Bakken (M–B) model for fixed satellite communication links. Moreover, an analytical closed-form expression of the rain-attenuation dynamic parameter is derived and used in the proposed synthesizer. The new channel model is validated in terms of the LEO-slant-path rain-attenuation exceedance probability. Finally, a new analytical model for the calculation of fade-slope conditional exceedance probability is presented, which is then validated with accurate simulations. The new models are directly applicable for the evaluation and design of fade mitigation techniques (FMTs) in LEO satellite systems operating at frequencies above 10 GHz.

Channel Model and Throughput Analysis for LEO OFDM Satellite Communication System

Low earth orbit (LEO) satellite has the advantages of global coverage, low propagation delay and low path losses, etc. Combined with orthogonal frequency division multiplexing (OFDM) transmission, LEO satellite system can support very high capacity suitable for wideband multimedia traffic. This paper analyzes the channel model and throughput for LEO OFDM satellite communication system. First, several channel models for LEO satellite system are expounded with simulations because channel model is the basis of throughput analysis. Second, the beam model and the signaling model for LEO satellite are introduced. Considering high speed movement of LEO satellite, channel fading and the channel quality information (CQI), link throughput is simulated. Finally, compared with MF-TDMA system, simulation results demonstrate that OFDM system outperforms the former because its feasibility and superiority in throughput.

SUPPORTING DISTRESS SIGNALS OVER LOW EARTH ORBIT MOBILE SATELLITE SYSTEMS FOR EMERGENCY INFORMATION ACQUISITION

Low earth orbit (LEO) satellite systems allow a broad range of services to be provided using small, lightweight, cellular-like portable telephones. Exploiting LEO satellites to support distress signals for aircrafts, ships and international travelers is explored in the current paper. A multi-service priority-oriented algorithm is proposed for handling voice, data and emergency signals over LEO satellites. The emergency signal is privileged with service priority so that rescue operation can be carried out as soon as possible. The priority mechanism includes channel reservation as well as joining a queue if no free channel is available as long as the request is roaming in the handover area. In addition, a simplified but efficient approach is suggested for locating the object of an imminent danger situation. As LEO satellites are non-geostationary, the visible period of each spot-beam is small. Consequently, a teletraffic model, that accommodates the mobility of spot-beams as well as the resulting handover rate, is developed in order to gauge the performance of the proposed algorithm. Numerical results for access denying and service-dropping rates are presented for nominal system parameters.

On the exploration of time-varying networks

We study the computability and complexity of the exploration problem in a class of highly dynamic networks: carrier graphs, where the edges between sites exist only at some (unknown) times defined by the periodic movements of mobile carriers among the sites. These graphs naturally model highly dynamic infrastructure-less networks such as public transports with fixed timetables, low earth orbiting (LEO) satellite systems, security guards’ tours, etc. We focus on the opportunistic exploration of these graphs, that is by an agent that exploits the movements of the carriers to move in the network.We establish necessary conditions for the problem to be solved. We also derive lower bounds on the amount of time required in general, as well as for the carrier graphs defined by restricted classes of carrier movements.We then prove that the limitations on computability and complexity we have established are indeed tight. In fact we prove that all necessary conditions are also sufficient and all lower bounds on costs are tight. We do so constructively by presenting two optimal solution algorithms, one for anonymous systems, and one for those with distinct node IDs.

New quantum key distribution study for world covers security communications

The use of LEO satellite systems has become widespread throughout the world, not only from the services standpoint but in large scale communication networks as well, as they make it possible to communicate from any point of the globe. Lately, the communications in the satellite field have been markedly developed thanks to the Low Earth Orbit (LEO) satellites and GEO satellite orbit. This paper deals with the main survey of these networks by a demonstration of satellite constellations constitutions and the different interfering factors, namely the number of satellites in each orbit, modelling the HANDCHECK problems or Handover satellite probably blocking. In order to preserve the information, to make sure that a seamless communication between two satellite zones is ensured the Hand check should continuously be present in satellite networks. Although the achievement of such schemes is routine work in the laboratory, nontrivial problems emerge in long-distance applications. At present, the only suitable system for long-distance quantum communication is photons. The essential work carried out in our research laboratory concerns an approach in software development of the implementation of Quantum Key Distribution (QKD) Networks with LEO orbit satellites and a reduction of the telecommunication interruption risks, which indeed will provide a better communication quality.

Simple surrogate models to determine total accessibility duration of low Earth orbit sunsynchronous satellites

Short and infrequent access events are inherent characteristics of low Earth orbit sunsynchronous satellites. Furthermore, for such satellites, distribution pattern of access events varies significantly in time. Thus, determination of the metric of total accessibility duration in a given time interval has been a challenge in the field of low Earth orbit satellite systems engineering. In this article, for zero and conventional non-zero minimum elevation angles, surrogate models were developed to determine total accessibility duration of low Earth orbit satellites, based on orbital altitude of the satellite and latitude of the ground segment. For this purpose, concept of repeatability cycle was employed to achieve total accessibility duration in a time-independent manner. Then, a perturbative propagation model was presented to determine pattern of accessibility events and quantify total accessibility duration metric. In order to account for non-zero minimum elevation angles, two distinct approaches were adopted. In the first approach, four modification factors were introduced to modify the surrogate model for zero elevation angle to account for conventional non-zero minimum elevation angles. In the second approach, a dedicated surrogate model was developed to directly determine total accessibility duration for conventional non-zero minimum elevation angles. Numerous examples are examined to verify fidelity of our two proposed approaches. Due to their simplicity, the surrogate models given in this article eliminate the need for professional staff to determine metric of total accessibility duration. The advantage is that considerable saving in required initial staff cost and schedule can be realized, especially in early mission design phases.

A resource allocation scheme of low-earth orbit satellite system based on the traffic modelling

A resource allocation scheme of low-earth orbit satellite system based on the traffic modelling

A Resource Allocation Scheme of Low-Earth Orbit Satellite System Based on the Traffic Modelling

Traffic modelling is the foundation of satellite resource management. Considering the uneven traffic distribution in China, a traffic model of low-earth orbit satellite is established in this study according to the per-capita GDP and the density of population. Base on this model, a new dynamic channel allocation and reservation (DCAR) technique is proposed. The performance of the call blocking probability, the handoff blocking probability and the channel utilisation is evaluated by simulation. Different from the fixed channel allocation, DCAR implements channel allocation by estimating the traffic conditions of all spot beams. The simulation results show that DCAR achieves a significant improvement on call blocking probability and handover blocking probability, particularly under heavy traffic load.

Load Balancing and QoS Provisioning Based on Congestion Prediction for GEO/LEO Hybrid Satellite Networks

While GEostationary Orbit (GEO) satellite systems provide us with a wide coverage area, their long delay serves as a significant constraint for real-time applications. On the other hand, Low Earth Orbit (LEO) satellite systems are best suited to delay sensitive applications. However, the coverage and mobility issues of LEO satellites lead to relatively high management costs. In this paper, we devise a new load balancing and quality of service (QoS) provisioning scheme to accommodate both real-time and non-real-time traffic based on a new congestion-prediction scheme. The effect of this new scheme is expected to improve the efficiency of the GEO/LEO hybrid satellite networks and the QoS satisfaction of end users.

A Trace-Time Framework for Prediction of Elevation Angle Over Land Mobile LEO Satellites Networks

Elevation angle is one of the most significant parameters of land mobile satellite channels, subject to rapid variations in the case of Low Earth Orbit (LEO) satellite systems. In this paper a novel trace-based framework is proposed and analyzed which is capable of predicting elevation angle as a function of time during satellite visibility window. Trace-time based modeling makes the framework practical for real-time evaluation of elevation angle and its alteration incurred by communication links in LEO satellite systems. The proposed method is particularly suitable for development of communication channel models and services in mobile LEO satellite networks where path variability is of great importance.

저궤도 위성망과 FS 시스템의 주파수 공유 방안 연구

본 논문에서는 저궤도 위성망(LEO satellite network)과 FS 시스템(Fixed Service system) 동일한 하향 링크 주파수를 사용하는 통신 환경에서 두 시스템이 간섭 영향 없이 운용될 수 있는 방안을 연구하였다. 저궤도 위성망에 의한 FS 시스템으로의 간섭 시나리오의 경우 동시에 운용되는 저궤도 위성 빔의 수에 따라 FS 시스템의 영역을 정의하여 각 영역에서의 간섭 영향을 분석하였다. 또한 FS 시스템에 의한 지구국으로의 간섭 시나리오에서는 지구국의 보호 영역을 정의하여 보호 영역의 반경에 따른 간섭 영향을 분석하였다. 분석한 결과는 두 스템이 간섭 영향 없이 운용될 수 있는 공유 조건으로 FS 시스템의 설계 단계에 활용될 수 있다. This paper addresses the analysis of the interference produced between the LEO(Low Earth Orbit) satellite constellation and FS(Fixed Service) system operating in the same frequency and area. At first, we calculates the interference of FS system from the LEO satellite constellation depending on the number of LEO satellite antenna beams. Simulation results show that the amount of interference that was calculated from each region. This result can be used to define the carrier level for protecting FS system from total interference by LEO satellite constellation. In the second scenario, we calculates the interference of LEO satellite system earth station by the FS link depending on radius of protection area. The presented results can be used to design FS systems minimizing interference to earth station.

Multiservice on-demand routing in LEO satellite networks

In this paper, a distributed on-demand routing protocol for Low Earth Orbit (LEO) satellite systems, named multiservice on-demand routing (MOR), is proposed and evaluated. The proposed protocol adjusts the routing procedure to the QoS requirements of different traffic classes. The performance of the MOR protocol is compared to the unique proposal for traffic class dependent routing in the literature and the good characteristics of the proposed scheme are corroborated by ample simulation experiments, where significant gains in performance are witnessed.

Iteration-based identification of faulty links in LEO/MEO satellite communication networks

Satellite systems are going to be an important part of the future personal communication infrastructure. The first-generation candidates for satellite personal communication networks (S-PCN) will rely on low earth orbiting (LEO) and medium earth orbiting (MEO) constellations. For LEO satellite systems employing intersatellite links (ISLs), we present a iteration-based algorithm to identify faulty links, which applies to a satellite network assuming a connection-oriented network structure, e.g., ATM or ATM-type switches on-board satellites. In the algorithm a management satellite can autonomously and real-time identify a ranked list of the most probable failed network links through building discrete-time dynamic virtual topology graph (DT-DVTG), then testing can be used to quickly pinpoint the actual faulty links. The performance of the algorithm is evaluated through simulations.

Distributed on-demand routing for LEO satellite systems

Notwithstanding the limited commercial success of the first narrowband low earth orbit (LEO) satellite systems, the interest of the scientific community in this type of systems has been revived on the basis of the current trend toward the migration to all IP-based services. LEO systems can play a pivotal role in providing services to areas where there is no substantial terrestrial infrastructure. Above all, LEO satellite systems can be used as backbone networks to interconnect autonomous systems worldwide. Such an approach provides flexibility in managing the resulting integrated network infrastructure and supporting innovative applications. In this context, routing data from the source all the way to the destination constitutes a daunting challenge. In this paper, a location-assisted on-demand routing (LAOR) protocol is proposed and evaluated. The proposed protocol introduces for the first time in satellite systems the concept of on-demand routing. However, its implementation is tailored to the requirements imposed by the characteristics of the topology of LEO satellite systems. The performance of the LAOR protocol is assessed for different link-cost metrics and compared to the one of centralized routing protocols proposed in the literature so far. Simulation studies further document and confirm the positive characteristics of the proposed protocol.

Reinforcement Learning for Resource Allocation in LEO Satellite Networks

In this paper, we develop and assess online decision-making algorithms for call admission and routing for low Earth orbit (LEO) satellite networks. It has been shown in a recent paper that, in a LEO satellite system, a semi-Markov decision process formulation of the call admission and routing problem can achieve better performance in terms of an average revenue function than existing routing methods. However, the conventional dynamic programming (DP) numerical solution becomes prohibited as the problem size increases. In this paper, two solution methods based on reinforcement learning (RL) are proposed in order to circumvent the computational burden of DP. The first method is based on an actor-critic method with temporal-difference (TD) learning. The second method is based on a critic-only method, called optimistic TD learning. The algorithms enhance performance in terms of requirements in storage, computational complexity and computational time, and in terms of an overall long-term average revenue function that penalizes blocked calls. Numerical studies are carried out, and the results obtained show that the RL framework can achieve up to 56% higher average revenue over existing routing methods used in LEO satellite networks with reasonable storage and computational requirements.