Satellite Personal Communication Services (S-PCS)

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1 Satellite Personal Communication Services (S-PCS) Thomas Olsson Dept. of Communication Systems Satellite personal communication services (S-PCS) are one of the recent systems available to global mobile user. The benefits with using satellites are greater coverage, especially in remote areas such as the poles or in the middle of the oceans. However, satellite communication is expensive and for personal use still in its infancy. Today there exist a number of operators delivering more or less global S-PCS. 1. Introduction In the last years of the 20 th century the first satellite-based communication systems for personal use were launched. In the beginning of the 21 st century several systems have gone operational, offering more or less global services [2]. Examples on operators that are offering global mobile voice communication include Iridium 1, Globalstar 2,ICO 3 and Ellipso 4 [1]. Other operators provide broadband services, such as Teledesic 5 and SkyBridges 6 [1], which the focus on data communication and backbone networks (i.e. mostly non-mobile users). In addition to these global systems, there are a number of data only operators [1] (OrbComm 7 ) and regional systems around the world [1] (North American M-SAT (or AMSC/TMI), the Australian OPTUS, the Japanese N-Star, Asian ACeS and Arab world Thuraya). In addition to the mentioned operators there exist several other systems, some which are operational, some that are just in the planning stages. Satellites can be an effective complement to terrestrial systems in remote areas, such as the poles and the oceans, terrestrial systems cannot be used and in low traffic areas, such as deserts and deep forests where terrestrial systems are not competitive [4]. A satellite system has the benefit of covering a larger area than the equivalent terrestrial system. Though, it comes at a cost of a more difficult communication environment with longer transmission times and less favourable conditions for the links. This report focuses on satellite-based system for the mobile users, using a hand-held device, wanting global coverage. 2. Evolution of satellite personal communication services 2.1. The first GEOs The first commercial satellite system for mobile communication was the Marisat system, launched in 1976 [1, 2, 6]. This system utilized geostationary orbits, meaning that the satellites always are in the same spot in the sky above earth. The main purpose of this system was to provide ships in the Atlantic and Pacific Ocean with mobile communication. The system was later extended to include satellites in the Indian Ocean, which made the system global and it changed name to Inmarsat [1, 2]

2 The first generation of satellites are basically repeaters [2]. They contain little intelligence and do little processing to the signals. Basically, uplink information is received in one frequency. This information is transformed into another frequency for the downlink. The focus is on OSI layers 1 and 2. The advantages with using geostationary satellites are several [5, 6, 7]. The high orbit of GEO (about km) means that a satellite sees almost a third of the earth (the poles are somewhat discriminated). Hence, it only takes three satellites to cover (almost) the whole earth. Further, the routing can, to a large extent, be static, since the position of the satellites is constant and there never have to be any problems with complex handover situations. However, there are a couple of obstacles as well [5, 6, 7]: As the satellites orbit about km about earth, the link has problems. Either a very high power is needed or large antennas (or using them very intelligently). It takes the signals about 0.25s to travel the distance from earth to the satellite. This makes the round-trip for signals to be at least half a second, depending on how fast the satellite can process and re-send the signal. As one satellite covers such a large area, this results in an inefficient use of frequencies. (This can be improved by using spot beams, but will still never be as efficient as the equivalent LEO system.) 2.2. Turning to LEOs With the demands on the communication systems today, the system designers turned to LEOs (Low Earth Orbit) instead. The orbit for LEOs range from 700km to 1800km. By being closer to earth, the problems with the GEOs are reduced. The propagation delay is reduced, as is the needed signal power. As the orbit is closer to earth it is also easier to launch the satellites. The period for a LEO satellite ranges from 1.5h to 10h. The first kind of LEO satellites were little LEO satellites. These do not provide a full range of service, only data communication and paging possibilities [2]. Typical data rates for these systems are 100 to 600 bps. An example on a little LEO system is OrbComm, see section 4.1. The little LEO systems are very simple, small and easily deployed. They do not have a lot of intelligence and do not demand a very high throughput or quality of service. The second generation of LEO systems were the so called big LEOs [2]. These systems provide both data and voice communication. Iridium (section Error! Reference source not found.) and GlobalStar (section 4.3) are examples of this kind of system. The complexity of these systems varies a lot depending on the design decisions taken by the operators. Iridium employs a quite complex solution with a lot of signal processing and network routing done in the satellites [2]. GlobalStar, on the other hand, has chosen a solution allowing simpler satellites and thereby not as heavy satellites and technology simpler systems [2]. The latest addition to the LEO world is the broadband satellites [2]. These provide broadband communication using low orbiting satellites. Teledesic (section 4.6) is one broadband LEO system. These systems usually target stationary users, rather than mobile ones. The aim is to provide a backbone network in the sky with high data-rates. The targeted customers are international companies and organisations requiring high throughput. As the user is assumed to be stationary, the antennas can be larger and thereby allowing higher data rates and more reliable communication. However, with LEOs a couple of new problems are introduced [1, 2, 4, 6, 7]: As the satellites are closer to earth, they cannot cover as big area. With GEOs, three satellites are enough to attain a global coverage. With LEOs there is a need of satellites depending on the altitude.

3 The speed of LEOs is significantly higher. The speed increases as the orbit decreases. The period of a LEO ranges from 1.5 to 10h. Hence, the satellite is only in sight for minutes (from about 5 to 30 minutes), forcing many handovers. When the number of satellites increases, the network management becomes more complex, especially if inter-satellite links is used (see below). 3. Issues in SPCS The modern satellite systems being launched for global mobile communication services are generally LEO or MEO. The discussion below is focused on these types of systems, 3.1. Coverage models for LEO systems The area that a satellite can service is referred to as the footprint of the satellite [5, 7]. The footprint of a satellite is similar to the cell in, for example, GSM. The size of the footprint depends on the altitude of the orbit and the characteristics of the antennas. In the case of Iridium, the footprint has a diameter of 4021km. The difference from the GSM case is that the footprint of the satellite is moving. In fact, it takes around 5-15 minutes for a satellite to pass a user on the ground [5]. Since the satellites are moving so fast, movement of the mobile user on the ground can be neglected. Even if the user moves at several hundred kilometres per hour, the speed of the satellite is still much faster. Hence, the opposite situation to the landbased system is derived: The (mobile) user is considered to be stationary and the network serving the user is moving. As with terrestrial-based systems, one way of improving the capacity is to divide the service area into smaller cells. In the satellite case, a footprint is divided into spot beams [5, 6, 7]. Basically, the footprint of a satellite is divided into several sections or cells. By dividing the footprint into several cells, frequency reuse is possible. Further, by making the area smaller from which the antenna on the satellite has to cover, a better signal-to-noise ratio is achieved, increasing the speed, reliability and power consumption [6]. The satellite does not actually have separate antennas for each spot beam. Rather, by using phased or matrix antennas, one single antenna can cater several spot beams, as many as a couple of hundreds [5]. Given the speed of the footprint at a large, a user is covered only for 1-2 minutes by a spot beam Handover issues Since the network is moving, not the user, the handover models for a satellite-based system is very different from the land-based ones [5, 7]. The SPCS also has a more dynamic character since how the routing is done changes over time as the satellites move. The equation becomes even more difficult when inter-satellite links (ISL) are used. There are three kinds of handover situation in a satellite network [5]: Inter-satellite handover The connection is handed over to a different satellite. This typically happens when a satellite moves out of sight for a user on the ground. Link handover If ISL is used, then a link between two satellites might be interrupted, due to the movement of the satellites. This happens, for example, when the paths of the satellites are crossed in an inter-plane link situation. In this situation, a rerouting is necessary of the signal path to maintain the connection. Intra-satellite or spot beam handover In the case when spot beams is used, as the satellite moves, so do the spot beams. As a user is no longer covered by a spot beam, there is a handover to another spot beam with the same satellite. A user is only covered by a spot beam for a few minutes [5]. Hence, this type of handover is very common. The management of spot beams, the routing of connections and the speed of the satellites makes algorithms for optimising the network difficult [5]. Still, since the satellites generally have a limited computing power, the algorithms have to be simple and efficient enough not to take too long to execute.

4 4. Global SPCS providers and their systems This section presents a more detailed look at 7 SPCS system providers offering (more or less) global coverage. OrbComm is presented as a representative for the little LEO systems. Two providers of full-service, big LEO, systems are described: Iridium and GlobalStar. ICO is then presented as an example of a MEO system. Ellipso is a LEO system utilizing elliptic orbits, which is described in some details. Finally, one system offering broadband services is elaborated on, Teledesic (even though it is not primarily for mobile users) OrbComm OrbComm provides global data services only using LEOs (little LEO system) [2]. The OrbComm constellation consists of 36 satellites weighting 85 pounds. Services provided are messaging, emergency alerts, position determination and remote data collection. The effective throughput is around 300bps Iridium Iridium is one of the most complex and advanced systems currently in operation [2]. Iridium provides full mobile services globally, with a total number of 66 satellites. The constellation consists of 6 equally distanced circular planes at 785km above the surface. One of the main features with Iridium is the onboard processing capability. The satellites have a lot of computing power for signal processing and network routing. This increases the weight and the technical difficulties with developing the system. However, the benefits are better utilization of the bandwidth and more effective network routing. Iridium also features intersatellite links (ISL) [1]. This means that data can be routed along a path using the satellites. One major benefit of this is that the number of ground stations is significantly reduced. If a satellite does not have a ground station in sight, then the data is routed through other satellites to one which has a station in sight. Further, this allows for routing of signals without using a terrestrial network, most likely on hired lines. Another feature with the Iridium system is that both connection-oriented and connection-less links can be routed. Hence, introducing data services, including TCP/IP is easy GlobalStar The GlobalStar system provides nearly global services using 48 satellites at an altitude of 1401km [1]. The system is simpler in the design than Iridium. For example, no ISL is employed and the satellites are traditional bent-pipe repeaters. This means that in the GlobalStar system the number of ground stations is significantly higher than with the Iridium system. To achieve global coverage, about 200 stations are required. GlobalStar is not likely to construct all these. Hence the coverage will not be global. However, GlobalStar is using a network of franchising roamers. The local roamers carry some of the investment cost in arranging with licensing, ground stations, etc, making GlobalStar seem as financially more sound system [1]. Another interesting aspect with GlobalStar is that a user on the ground is covered by more than one satellite at a time a significant amount of the time. This will improve the reliability of the links and make the system less sensitive to shadowing effects [1] ICO ICO is sprung out of the Inmarsat system [1]. ICO stand for Intermediate Circular Orbit and is a MEO system. ICO uses ten satellites to achieve global coverage, possible since the orbit is much higher than of Iridium or GlobalStar. To achieve good links, ICO satellites uses 163 spot beams Ellipso Ellipso differs from most other systems by using elliptic orbit [1]. The reason for using elliptic orbits is to focus where the satellites will provide most coverage [5]. In the case of

5 Ellipso, the satellites spend more time in the northern hemisphere, as most of the population is found in the northern haft of the earth. Ellipso uses 16 satellites in MEO orbits Teledesic Teledesic is an example of broadband system. Teledesic is quite an ambition and technology complex system [2, 7]. The focus is on non-mobile broadband backbone type of communication links. Teledesic is using some 300 satellites at 700m orbits. Like Iridium (section 4.2), Teledesic extensively uses onboard processing to allow for a better utilization of the network, even supporting ATM and ISL [2]. The data-rates range from 16kbps to 2048kbps (sometimes even higher). 5. S-PCS and PLMN The public land-based mobile networks (PLMN) have during the last 10 years seen a tremendous development. Most countries in the world have some kind of PLMN, with varying coverage. The problem with PLMN is that for remote areas or scarcely populated it is not really financially viable to have coverage. Since the antennas for PLMN are, as the name suggests, on the ground just adding more power to the signals don t solve the problem. The variations in the terrain (mountains, forests and even building) and the fact that the earth is round will block the signals. However, in areas with a lot of traffic and where it is possible to have antennas on the ground, land-based systems will be cheaper, more flexible and easier to maintain. S-PCS and PLMN are, to a large, trying to fulfil the same need. That it, provide fast and reliable communication for mobile users. The question is then if they can be integrated, and if they can which is the best solution. UMTSandPCS[4,3] 6. An un-educated guess for the future [4, 3] The derivation of a standard - IntegrationwithPLMN - Development of satellites - The true global system References 1. J.V. Evans, Satellite Systems for Personal Communication. Proceedings of the IEEE, vol. 86, no. 7, July Gary Comparatto, Rafols Ramirez, Trends in Mobile Satellite Technology, IEEE Computer, February Y.F. Hu, R.E. Sheriff, Evaluation of the European market for satellite-umts terminals, International journal of satellite communications, no. 17, pp , Fulvio Ananasso, Francesco Delli Priscoli, Satellite systems for personal communication networks, Wireless Networks, no. 4, Ian Akylidiz, Huseyin Uzunalioglu, Michael Bender, Handover management in Low Earth Orbit (LEO) satellite networks, Mobile Networks and Applications, no. 4, Leonard Golding, Satellite communications systems move into the twenty-first century, Wireless Networks, no. 4, 1998.

6 7. Erich Lutz, Issues in satellite personal communication systems, Wireless Networks, no. 4, Tamer ElBatt, Anthony Ephredimes, Design aspects of satellite-cellular hybrid wireless systems, International journal of satellite communications, no. 20, 2002.

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