Analysis of basic system parameters of communication networks based on low earth orbit satellites

Currently many efforts are undertaken to develop and install communication networks based on low earth orbit (LEO) and intermediate circular orbit (ICO) satellites. However, many problems are to be solved until the final operation of such networks. This paper deals with basic design problems of LEO/ICO-based networks. In the first part, estimates for the necessary number of satellites, orbits and number of communication channels per satellite are derived. Since the latter is a crucial quantity, the number of communication links and channels per link are derived with a more elaborate model in the second part of the paper. This includes the radio links from the satellites to mobile users and to gateways, as well as intersatellite links and terrestrial lines. We introduce a formal model for LEO/ICO-based networks and propose a method for the evaluation of link capacities, given the network topology and the traffic requirements. As an example, two constellations are investigated in detail. One of these constellations is the IRIDIUM system proposed by Motorola.     

Performance evaluation of adaptive routing algorithms in packet‐switched intersatellite link networks

This paper addresses the performance evaluation of adaptive routing algorithms in non‐geostationary packet‐switched satellite communication systems. The dynamic topology of satellite networks and variable traffic load in satellite coverage areas, due to the motion of satellites in their orbit planes, pose stringent requirements to routing algorithms. We have limited the scope of our interest to routing in the intersatellite link (ISL) segment. In order to analyse the applicability of different routing algorithms used in terrestrial networks, and to evaluate the performance of new algorithms designed for satellite networks, we have built a simulation model of a satellite communication system with intersatellite links. In the paper, we present simulation results considering a network‐uniform source/destination distribution model and a uniform source–destination traffic flow, thus showing the inherent routing characteristics of a selected Celestri‐like LEO satellite constellation. The updates of the routing tables are centrally calculated according to the Dijkstra shortest path algorithm. Copyright © 2002 John Wiley & Sons, Ltd.     

Next‐generation global satellite system with mega‐constellations

Mega satellite constellations in low earth orbit (LEO) will provide complete global coverage; rapidly enhance overall capacity, even for unserved areas; and improve the quality of service (QoS) possible with lower signal propagation delays. Complemented by medium earth orbit (MEO) and geostationary earth orbit (GEO) satellites and terrestrial network components under a hybrid communications architecture, these constellations will enable universal 5G service across the world while supporting diverse 5G use cases. With an unobstructed line‐of‐sight visibility of approximately 3 min, a typical LEO satellite requires efficient user terminal (UT), satellite, gateway, and intersatellite link handovers. A comprehensive mobility design for mega‐constellations involves cost‐effective space and ground phased‐array antennas for responsive and seamless tracking. An end‐to‐end multilayer protocol architecture spanning space and terrestrial technologies can be used to analyze and ensure QoS and mobility. A scalable routing and traffic engineering design based on software‐defined networking adequately handles continuous variability in network topology, differentiated user demands, and traffic transport in both temporal and spatial dimensions. The space‐based networks involving mega‐constellations will be better integrated with their terrestrial counterparts by fully leveraging the multilayer 5G framework, which is the foundational feature of our hybrid architecture.     

Optical satellite networks

Several nongeosynchronous satellite constellation networks providing broad-band access to end-users are currently under development. The use of multigigabit laser intersatellite links (ISLs) is the enabling factor for routing traffic through the spare segment and creating a global space-based optical backbone network. Optical networking techniques based on wavelength division multiplexing (WDM) ISLs and wavelength routing can allow by-pass of the transit traffic significantly simplifying routing decisions and minimizing processing delays. The paper examines the characteristics of these networks and investigates the applicability of various optical networking schemes based on single hop and multihop approaches. Single hop can be adopted in medium earth orbit (MEO) systems consisting of 10 to 15 satellites whereas double-hop schemes based on the matrix lightpath allocation approach are suited for constellations up to 100 satellites, covering the requirements of most of the proposed low earth orbit (LEO) systems. Multihop will be required for some of the very large in number of satellites constellations. Statistical multiplexing of the transported traffic over the ISLs appears to be a necessary condition to achieve an efficient utilization of the satellite resources. Traffic routing has to take into account the impact of the varying range of the interorbit ISLs on the propagation delays. Although maximum leverage of the technologies developed for fiber optic WDM networks should be made, the technologies to be employed on board the satellites have to be space-qualified that may limit the applicability of some otherwise high-efficiency components     

A model for the handover traffic in low earth-orbiting (LEO) satellite networks for personal communications

The spectacular growth of cellular telephone networks has demonstrated the demand for personal communications. Communication systems based on low earth orbit (LEO) constellations of satellites seem to be an adequate approach to achieve a world-wide network. When defining the capacity in terms of satellite circuits, the network designer has to take into account the handover traffic. Unfortunately, in a LEO communication network where handover is most often due to the network nodes motion, handover traffic models for terrestrial cellular networks cannot be used. Hence specific models must be developed. This paper proposes an analytical model for the handover in LEO satellite networks. This model is applied to different network configurations and compared to discrete-time simulations. Simulation results agree with those obtained from the analytical model.     

Analysis of handover characteristics in shadowed LEO satellite communication networks

In the near future, low earth orbit (LEO) satellite communication networks will partially substitute the fixed terrestrial multimedia networks especially in sparsely populated areas. Unlike fixed terrestrial networks, ongoing calls may be dropped if satellite channels are shadowed. Therefore, in most LEO satellite communication networks more than one satellite needs to be simultaneously visible in order to hand over a call to an unshadowed satellite when the communicating satellite is shadowed. In this paper, handover characteristics for fixed terminals (FTs) in LEO satellite communication networks are analysed. The probability distribution of multiple satellite visibility is analytically obtained and the shadowing process of satelites for FTs are modelled. Using the proposed analysis model, shadowing effects on the traffic performance are evaluated in terms of the number of intersatellite and interbeam handovers during a call. Copyright © 2001 John Wiley & Sons, Ltd.     

Implementation of a geographical routing scheme for low Earth orbiting satellite constellations using intersatellite links

A major problem for low Earth orbit (LEO) constellations with intersatellite links is the efficient routing of the data packets through such a highly dynamic network. In order to achieve a worldwide coverage even in remote areas and Internet access with a limited amount of gateway stations, intersatellite links are a promising approach. Since LEO constellations represent a distinct, highly dynamic routing environment, specific strategies are needed. To this end, a suitable geographical routing scheme is proposed and investigated in two Walker Star constellations. The proposed scheme targets reliable transmissions with low latency and high data rates. The approach is based on a geographical address identifier in Layer 2 of the communication stack. The globe is thus divided into geographical areas that determine this identifier in the MAC address of the terminals. As mobile terminals are considered, the MAC addressing scheme is flexible, whereas the IP addresses of the terminals remain static. This decoupling allows for flexibility in the choice of the address resolution scheme. Moreover, the geographical identifier in the MAC address enables fast routing table lookups and switching. The proposed routing scheme also takes possible overloads of the satellites due to traffic into account and applies a rerouting procedure. When a packet arrives in the geographical area of the destination terminal, a local rerouting scheme is applied if needed. The proposed approaches take handover events that possibly occur during a transmission into account. Furthermore, the scan angles of the satellites have been adapted to the constellations to provide full coverage and high elevation angles. So a robust and adaptable routing scheme is provided for a dynamic environment where satellites and terminals are constantly moving. The proposed definitions and procedures have been implemented in a system level simulator, which allows for comparisons with adjustable parameters in various scenarios. In this work, an Iridium‐like constellation and a megaconstellation are investigated and compared regarding the address resolution procedures, the average end‐to‐end transmission delay, and the dropping and rerouting rates. Additionally, the signaling overhead is compared with other approaches. The simulator and results of the simulations provide grounds for further research w.r.t. the routing in satellite constellations using intersatellite links.     

A dynamic routing concept for ATM-based satellite personalcommunication networks

Satellite systems are going to build a part of the future personal communications infrastructure. The first-generation candidates for satellite personal communication networks (S-PCN) will rely on low Earth orbit (LEO) and medium Earth orbit (MEO) constellations. A noticeable trend in this field is toward broadband services and the use of ATM. For LEO satellite systems employing intersatellite links (ISLs), this paper proposes an overall networking concept that introduces the strengths of ATM to their operation. The core of the paper is the design of a new routing scheme for the periodically time-variant ISL subnetwork, discrete-time dynamic virtual topology routing (DT-DVTR), and its ATM implementation. DT-DVTR works completely off line, i.e., prior to the operational phase of the system. In a first step, a virtual topology is set up for all successive time intervals of the system period, providing instantaneous sets of alternative paths between all source-destination node pairs. In the second step, path sequences over a series of time interval are chosen from that according to certain optimization procedures. An ATM-based implementation of DT-DVTR in LEO satellite ISL networks is presented with some emphasis on the optimization alternatives, and the performance in terms of delay jitter is evaluated for an example ISL topology     

激光星间链路发展综述：现状、趋势、展望

由于激光通信在空间传输中波长短且方向性强,已成为下一代卫星通信与导航的重要手段。激光星间链路的高速率、高带宽、高安全性等特点,可以提供高质量卫星空间通信,同时其还可以提高星间测距的精度,因此,构建激光星间链路成为下一代卫星网络的研究重点之一。文中首先从技术层面介绍激光星间链路的基本组成,主要介绍了卫星激光建链模式、卫星激光信号调制模式及卫星激光载波波长三个重要技术点。从技术到现象,根据不同轨道高度和不同的任务需求,按照发射时间顺序综合调研并总结了近年来国内外典型中高轨和低轨卫星激光通信成果的发展现状与未来计划。通过调研,进一步从宏观角度分析出卫星激光通信发展标准化、兼容化、网络化和商业化四个趋势,并从微观角度总结了卫星激光终端弹性化和模块化的发展方向。最后,除了作为通讯手段,展望了星间激光链路用于卫星激光测距的良好前景。通过对激光星间链路的现状、趋势和展望的综合分析,旨在为未来激光星间链路的设计与优化提供一定的借鉴和参考,并为我国未来星间激光通信和测距技术的发展及研究提供方向参考。     

中低轨卫星跨层激光链路二次同步切换方法

中低轨卫星之间跨层激光链路的无缝切换直接决定了双层卫星光网络的稳定性.异步切换方法会导致网络拓扑频繁重构,而集中同步切换将造成两层间连接中断,网络运行状态失控.为此,本文提出了中低轨卫星星座激光链路的二次同步切换方法,在保证中低轨道卫星连通的基础上,可降低网络拓扑重构频率.研究了整数周期比的中轨道和低轨道卫星空间位置特性,建立了中低轨卫星星座构形二阶非球摄动模型,确定了中低轨道之间轨道周期比为3的双层卫星星座构形.按连接和切换顺序将该星座构形中跨层激光链路分为两组,以相对周期的1/4为基准,每次令其中一组同步切换,通过交替完成切换.研究结果表明,二次同步切换方法使得网络拓扑重构频率降低到链路切换频率的1/7,比集中切换方法在网络平均时延方面降低了30ms.     

An enhanced MAC protocol based on the IEEE 802.11 for data relay satellite systems

Data relay satellite (DRS) systems play an important role in space information networks. Characterized by highly dynamic topology and discontinuous communication links, it is suggested that the IEEE 802.11 protocol employed in such a network could be more flexible. However, such a terrestrial network protocol could not be applied to DRS systems directly, nor supports a fast response due to the long propagation delay and severe packet collision. To address this challenge, we proposed an enhanced media access control (MAC) protocol based on the IEEE 802.11 protocol providing multiaccess for low earth orbit (LEO) distributed constellations. In this paper, we investigated the access delay performance of the proposed protocol in our model. Then, we derived a contention window adaption by using an iteration algorithm that can dynamically adjust the values of the contention window depending on the number of user satellites in the communication coverage. Simulation results show that the average access delay does not exceed 20 seconds, which is significantly lower than the standard protocol. Moreover, the traffic threshold is increased to 0.6, and the maximum throughput has doubled compared with the standard protocol. It is proved that the enhanced MAC protocol shows a better performance in DRS systems.     

一种基于链路稳定性模型的LEO/MEO双层卫星网星间链路设计方法

卫星网动态拓扑、长变时延等特性及其所处的复杂空间环境带来的通信干扰是影响网络通信稳定的重要因素。综合考虑这些影响因素,提出了一种衡量星间链路稳定性的模型,并基于此模型给出了LEO/MEO双层卫星网的星间链路设计方案。结合具体的星座实例对该方案进行了仿真分析,结果表明通过适当的参数调整该模型可以适应不同的网络性能要求。     

MLSR: a novel routing algorithm for multilayered satellite IPnetworks

Several IP-based routing algorithms have been developed for low-Earth orbit (LEO) satellite networks in recent years. The performance of the satellite IP networks can be improved drastically if multiple satellite constellations are used in the architecture. A multilayered satellite IP network is introduced that consists of LEO, medium-Earth orbit (MEO) and geostationary Earth orbit (GEO) satellites. A new multilayered satellite routing (MLSR) algorithm is developed that calculates routing tables efficiently using the collected delay measurements. The performance of the multilayered satellite network and MLSR is evaluated through simulations and analysis     

An Optimum Traffic Loading to Intersatellite Links

An algorithm has been developed to evaluate the benefits of intersatellite links (ISL's) quantitatively in satellite communications networks. The developed algorithm derives the minimum satellite capacity, under the condition that the same type of satellites are located onto the geostationary orbit, by assigning traffic among earth stations to the satellites effectively. Some example calculations are presented, based on a current INTELSAT database. The calculated results indicate that the benefits of ISL depend strongly on the distribution pattern of traffic in the network. They demonstrate, furthermore, that ISL's over long distances have significant potential advantages in the Atlantic and Pacific Ocean regions for accommodating a large volume of traffic, and enabling the effective utilization of satellite resources at high elevation angles.     

基于分布式星群的双层星座设计

当前,陆地通信系统已无法满足日益复杂的信息需求,利用空间信息网络实现全球范围内的无缝覆盖和高效容量传输成为研究热点。现有卫星通信系统以单层星座为主,缺少高低轨卫星之间的协同。提出了一种基于分布式星群的双层星座设计,以基于分布式星群的低轨卫星作为网络架构的基础,采用星间链路实现低轨卫星之间的通信,通过高轨卫星实现中低纬度地区覆盖性能加强。仿真结果表明,所提方法在仅依靠在国内部署卫星地面站的前提下可实现全球多重覆盖。     

LEO/MEO卫星通信系统发展展望

本简述了LEO/MEO卫星通信系统的主要特点，并介绍了几个典型的LEO/MEO卫星通信系统，如Iridium、Globalstar、ICO、Teledesic、Skybridge系统。还从星座、频率、星上处理及星际链路、无线技术、网络技术等方面讨论了LEO/MEO卫星通信系统的发展趋势和研究热点。     

Network pruning for extending satellite service life in LEO satellite constellations

We address the problem of network pruning for extending the service life of satellites in LEO constellations. Satellites in LEO constellations can spend over 30 % of their time under the earth’s umbra, time during which they are powered by batteries. While the batteries are recharged by solar energy, the depth of discharge they reach during eclipse significantly affects their lifetime—and by extension, the service life of the satellites themselves. For batteries of the type that power Iridium satellites, a 15 % increase to the depth of discharge can practically cut their service lives in half. In this paper, we present the design and evaluation of two forms of network pruning schemes that reduce the energy consumption of LEO satellite network. First, we propose a new lightweight traffic-agnostic metric for quantifiying the quality of a frugal topology, the Adequacy Index (ADI). After showing that the problem of minimizing the power consumption of a LEO network subject to a given ADI threshold is NP-hard, we propose heuristcs to solve it. Second, we propose traffic-aware metric for quantifiying the quality of a frugal topology, the maximum link utilization (MLU). Also, with the problem being NP-hard subject to a given MLU threshold, we propose heuristics to solve it. We evaluate both forms using realistic LEO topologies and traffic matrices. Results show that traffic-agnostic pruning and traffic-aware pruning can increase the satellite service life by as much as 85 and 80 %, respectively. This is accomplished by trading off very little in terms of average path length and congestion.     

Satellite selection scheme for reducing handover attempts in LEO satellite communication systems

Low earth orbit (LEO) satellite communication systems perform frequent intersatellite handovers for both fixed and mobile users. This paper proposes a satellite selection scheme for new/handover call requests when two or more satellites can be seen simultaneously. Each satellite in this scheme has a non-uniform transmitter antenna gain according to its relative position inside the coverage area. The antenna gain is proportional to the residual distance in the satellite's direction of movement and it compensates for the difference in path losses between satellite links. The residual distance distribution of the selected satellite and the mean number of intersatellite handovers during a call connection are calculated and compared with the results based on conventional methods. The proposed scheme can reduce the intersatellite handover call attempt rate without increasing system load and terminal complexity. Furthermore, this scheme can be extended to reduce both intersatellite and interbeam handover call attempt rates in a multiple spot beam environment. Especially, the average number of intersatellite and interbeam handovers during a call can be significantly reduced by using a hybrid algorithm with the proposed non-uniform power transmission scheme. © 1998 John Wiley & Sons, Ltd.     

基于STK的LEO卫星光网络设计及性能分析

文中设计了适合星间激光链路的LEO层卫星网络,并对同轨道内和轨道间链路进行了分析,建立了每个轨道内一颗主卫星,轨道间通过主卫星相连的网络。最后,对链路的稳定性以及覆盖性能进行了研究。     

多层卫星通信网络自适应路由策略

单层卫星网络由于轨道高度和覆盖能力的不同,以至构成通信的单层系统往往不能满足不同业务服务质量的需求。分析了Walkerdelta型星座构建多层卫星通信网络的拓扑结构和ISL性能,提出了在统计分布模型下的多层卫星自适应路由策略,综合考虑了路径时延和ISL链路负载。仿真结果表明了多层网络自适应路由策略能够更加有效地分配网络通信量,网络具有较小的丢包率、网络平均归一化链路负载和特定路径综合路径权重,有利于降低网络平均阻塞概率和特定路径阻塞概率,获得更高的可靠性,较传统的单层非自适应路由更加有效、可靠。