A Weighted Semi-Distributed Routing Algorithm for LEO satellite networks

https://doi.org/10.1016/j.jnca.2015.08.015Get rights and content

Highlights

  • A Weighted Semi-Distributed Routing Algorithm for LEO satellite networks.

  • A new antenna structure to support routing across Counter-Rotating Seam.

  • Proved robustness and effectiveness of WSDRA.

Abstract

Satellites play a major role in the development of a global information infrastructure. Developing a specialized, efficient and robust routing algorithm in Low Earth Orbit (LEO) satellite networks is a challenge in both research and practice. For example, algorithms based on topology can find the shortest path but the complexity of time and space is high; some algorithms based on Datagram Routing Algorithm (DRA) have a high computation overhead; others based on Dynamic Source Routing (DSR) store all route information in datagram, which however negatively impacts routing efficiency and robustness. In this paper, we propose a Weighted Semi-Distributed Routing Algorithm (WSDRA) that discovers a path with minimum propagation delay by using delicate information of two routing hops. This design can reduce computation overhead up to 50% compared to DRA. Low-cost ground networks are also embraced in WSDRA by applying a weighed cost function to describe path cost. In particular, we design a new antenna structure to cross Counter-Rotating Seam in order to shorten a routing path. We evaluate the propagation delay through both simulations and practical tests. The results show that the proposed algorithm is more efficient and robust than DRA and the delay can be reduced about 50% after adopting the new antenna structure.

Introduction

Satellites play a major role in the development of a global information infrastructure. Due to wide coverage and independence on the actual land distance between any pair of communication nodes, the satellites are widely applied to support robust and efficient worldwide networking. Obviously, satellites will become an essential part of the Next Generation Internet (NGI).

Low Earth Orbit (LEO) satellite networks have been paid special attention in both research and commercial business due to its advantages of low round-trip propagation delays, low launch cost, flexibility, and survivability (Maral et al., 1991, Long et al., 2011, Stenger, 1996). An LEO satellite network system is made up of a constellation consisting of a number of satellites in circular orbits at altitudes ranging from 500 km to 1500 km. The constellation can be divided by geometry, as inclined or polar orbits (Gavish, 1997). The area and duration of coverage highly depend on the number of satellites in the constellation, their altitude and orbit inclination.

For commercializing a satellite networking system, a number of technical obstacles should be overcome. One of existing challenges is developing a specialized, efficient and robust routing algorithm for Low Earth Orbit (LEO) satellite networks. Although LEO satellites have lower propagation delay and launch costs than that of Geostationary Earth Orbit (GEO) satellites, and they use Ka frequency band, which would assemble smaller antennas vs. C and Ku frequency band of GEO, more LEOs are required to constitute networks for a global coverage. In an LEO satellite network, the satellites connect to each other via Inter-Satellite Links (ISLs). The ISL must be maintained due to satellite movement and cyclical network topology changes. In addition, transmission technique affects the design of a routing algorithm in an LEO satellite network. The above specific characteristics of LEO satellite networks make existing routing algorithms infeasible to be applied due to impropriety, heavy complexity, or inefficiency (Kershenbaum, 1993, Ekici et al., 2001, Johnson, 2003).

On the other hand, Counter-Rotating Seam (CRS) of ISLs should be seriously considered in the routing of LEO satellite networks. A seam is a border of two special longitudinal regions where the directions of revolution of neighbor satellite planes are opposite. However, establishing ISLs across CRS in a LEO satellite system is rarely discussed in the literature (Werner, 1997, Hu et al., 2002, Gavish and Kalvenes, 1997). Prior research showed that it is difficult to maintain the communication links between satellites with a high relative movement speed in opposite directions (Yeo and Turner, 2004). Almost all researches on LEO satellite routing algorithms are based on this assumption. The separation of source–destination pair by the seam makes the existing algorithms very complicated (Ekici et al., 2001, Chang et al., 1998, Werner, 1997, Chen et al., 2002).

In this paper, we propose a Weighted Semi-Distributed Routing Algorithm (WSDRA). It divides satellites in a routing path into two types: Route-Satellite (RS) and Messenger-Satellite (MS). RS is responsible for routing path selection. It generates two routing hops, i.e., it determines the next hop and the hop after the next for the packet forwarding. MS only transmits the packet according to the two-hop routing information carried by the packet, thus it does not need to calculate the route. Obviously, this algorithm can reduce computation overhead up to 50% compared to Datagram Routing Algorithm (DRA) that decides the next hop at each hop. It only uses several bits (3 bits in our algorithm) to indicate the information of the hop after the next one, which is much less than Dynamic Source Routing (DSR) that is a pure source routing algorithm. We evaluate the propagation delay of the proposed algorithm through both simulations and practical tests. The results show that our algorithm is more efficient and robust than DRA. We also consider the network access from space to ground and propose a weighed cost function to describe path cost. In order to solve the issue related to CRS in LEO satellite networks, we propose a new antenna structure to shorten the path by crossing CRS. Specially, the contributions of this paper can be summarized as below:

  • i.

    We propose a novel routing algorithm WSDRA specialized for LEO satellite networks.

  • ii.

    We prove that the robustness and effectiveness of WSDRA are at least similar to or more advanced than DRA by using both simulations and practical tests, while the computation overhead of WSDRA can be reduced up to 50%.

  • iii.

    We investigate the possibility of maintaining links across CRS and propose a practical approach to cross it in LEO satellite network routing.

The rest of the paper is organized as follows. Section 2 gives a brief review of related work. We introduce the LEO satellite networks in Section 3. In Section 4, we describe the detailed design of SDRA and WSDRA. In Section 5, we propose the new antenna structure for crossing CRS. The performance evaluation is given in Section 6. Finally, conclusions and future work are presented in the last section.

Section snippets

Related work

First, we briefly review related work about ground network routing and channel assignment because many techniques used in the satellite networks are based on those for ground ones. Nowadays, the IP protocol that has been maturely adopted in ground networks is also used in the new generation of broadband satellite networks (“IP over CCSDS Space Links,”, 2012). Delay Tolerant Network (DTN) (Vasilakos et al., 2011) has similar characters with satellite networks, where network nodes apply a

LEO satellite network model

Herein, we adopt a constellation that is composed of N polar orbits (planes), each having M satellites with low distance to the Earth (see Fig. 1). We assume that, in this constellation, the inclination angle is 90°. The planes, with an intersection angle 360°/(2×N), cross each other only over the North and South poles. A number of M satellites in a plane are separated from each other with an angular distance of 360°/M. For easy of reference, we introduce the basic concepts used in the rest of

Weighted semi-distributed routing algorithm

In this section, we first introduce a Semi-Distributed Routing Algorithm (SDRA) based on DRA, then we discuss how to support routing from satellites to ground networks by applying Weighted SDRA (WSDRA). Before introducing SDRA, we give a brief description on DRA.

Approach to cross counter-rotating seam

In this section, we will analyze the mistake in Yeo and Turner (2004). Taking the example in Yeo and Turner (2004) about the LEO satellite system, we examine the use of polar orbit constellation. This system contains 6 orbits with 12 satellites per orbit. The satellite is assumed to be placed at an altitude of 1500 km from the Earth surface. Then we can get T=6930 s=115.5 min, the satellite velocity v=7.122 km/s=25,641 km/h and the Lv=4427 km.

The length Lh of interplane ISLs is variable and is

Performance evaluation

Our performance evaluation includes two parts. In the first part, we conducted five experiments to evaluate the performance of SDRA and WSDRA. The first four experiments were conducted to evaluate the performance of SDRA. The purpose is to show SDRA can still keep the overall performance of DRA and at the same time it can reduce computation overhead to at most 50% of DRA. We also show that SDRA can be easily applied in IP protocol through an implementation. In the fifth experiment, we give a

Conclusions and future work

In this paper, we proposed WSDRA based on SDRA for LEO satellite networks. SDRA finds a minimum propagation delay path for each packet at Route-Satellite (RS) and then stores the hop after the next in the packet header. The packet is forwarded to Messenger-Satellite (MS) and RS alternately to the destination. Only at RS the routing path is calculated based on the states of local links. SDRA can find an alternative path for the packet when congestion or failure happens. When a satellite on the

Acknowledgments

This work is sponsored by NSFC (Grant no. 61472308), 111 Project (Grant no. B08038), the Ph.D. grant of Chinese Educational Ministry (Grant no. JY0300130104), the initial grant of Chinese Educational Ministry for researchers from abroad (Grant no. JY0600132901), and the Grant of Shaanxi Province for excellent researchers from abroad (Grant no. 680F1303), as well as Aalto University.

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