RECENT TRENDS IN IP/NGEO SATELLITE COMMUNICATION SYSTEMS: TRANSPORT, ROUTING, AND MOBILITY MANAGEMENT CONCERNS
ABSTRACT
Non-geostationary (NGEO) satellite communication systems are seen as an attractive solution to realize the vision of anywhere, anytime pervasive access to the Internet. Their design and development have thus gained tremendous interest in the last few years. Commencing with a brief overview of general NGEO satellite configurations, this article next addresses the key technical difficulties in the development of NGEO IP-based satellite communications systems. The article discusses routing concerns, mobility management, and transport protocols with an emphasis on TCP performance in NGEO satellite networks. Some key innovations are presented. The Recursive, Explicit, and Fair Window Adjustment (REFWA) scheme is presented as a solution to improve the efficiency and fairness of TCP in NGEO systems. An improvement to the REFWA scheme, REFWA Plus, is also described to combat page link errors in satellite environments. INTRODUCTION
New multimedia services require more costeffective, high-quality, and high-speed telecommunication technologies. Large-scale deployment of these wideband applications in metropolitan areas with a potentially large number of users is a challenging task for terrestrial technologies. New Internet infrastructure and technologies are required to accommodate these multimedia services with diverse quality of service (QoS) requirements. Furthermore, appropriate mobility support is also needed to provide ubiquitous Internet access. Because of their extensive geographic reach, flexible and rapid deployment features, and inherent multicast capabilities, non-geostationary (NGEO) satellite network systems are seen as an attractive solution to realize the vision of a global broadband multimedia infrastructure. Given the recent advances and ongoing improvements in satellite technologies, broadband satellite- based multimedia services are likely to open a promising and strong market for service providers and operators in the near future. This article surveys the ongoing research efforts tailored to NGEO satellite communications systems. The remainder of this article is structured as follows. We give a brief description of the general configurations of NGEO satellite constellations. We highlight the key technical issues in the development of NGEO broadband satellite communication systems. We also present some recently proposed solutions to these issues. The article concludes with a summary recapping the most important research issues elaborated on in this article.
GENERAL NGEO CONFIGURATIONS
Internetworking with satellites began with the use of individual satellites in geostationary orbits (GEO). For more than three decades, GEO satellite systems have been successful in providing commercial services, such as television distribution and long-distance voice telephony. Despite this wide commercial usage of GEO systems, requirements for lower propagation delays, in conjunction with coverage of high-latitude regions, have turned the light on new NGEO satellite communication systems called low Earth orbit (LEO) or medium Earth orbit (MEO) satellite systems. NGEO systems promise to offer services with much lower latency and terminal power requirements than those offered by GEO systems. Table 1 lists some proposed worldwide NGEO constellations and the distinguishing features of their orbits. Without loss of generality, satellite systems can be classified into two kinds: broadcasting (noninteractive one-way) and interactive bidirectional satellite systems. In a broadcasting satellite system, each terminal has a receive-only satellite dish to collect data delivered from a server in the high-speed satellite channel. Due to the lack of an economical satellite return channel, the reverse path to the server can be provided by a terrestrial link.

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