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					Global Mobile Satellite




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Global Mobile Satellite


                                                  Abstract:
          To make a satellite phone call today from a location that does not offer terrestrial wire line
          or wireless coverage requires the use of a large, costly terminal, and entails very high per
          minute charges. Further, the quality of service is relatively poor because of annoying
          echoes, large transmission delays, over talk associated with satellite communications using
          geostationary satellites.


          There is a trend for mobile satellite system architectures aimed at the deployment of
          multi-satellite constellations in Non-Geostationary Earth Orbits. This allows the user
          terminals to be small size, low cost and having low power demand. In present and next
          generation satellite systems, CDMA has been proposed as the multiple access technique for
          a number of mobile satellite communication systems. To enhance the coverage and quality
          of service, Low Earth Orbiting (LEO) constellations are usually selected. Here, we analyze
          the performance of the downlink of a LEO satellite channel. The provision of such a
          service requires that the user have sufficient link quality for the duration of service. To
          have sufficient link quality, the user must have an adequate power to overcome the path
          loss and other physical impairments to provide acceptable communication and improve the
          performance of the system.

          Thus, in many parts of the world, the demand for communications mobility can be met
          effectively only through global mobile satellite services. Handheld satellite phones are
          therefore forecast as the emerging mobile communications frontier with growth that could
          parallel recent growth in cellular mobile industry. In order to guarantee the service quality
          and reliability for mobile satellite communication systems, we have to take into account
          outages due to obstruction of the line-of-sight path between a satellite and a mobile
          terminal as well as the signal fluctuation caused by interference from multipath radio
          waves. Thus, we need a good characterization for the satellite propagation channel. It is
          commonly accepted that satellite communications systems (in particular, low earth orbit
          LEO systems) are the de facto solution for providing the real personal communications
          services (PCS’s) to the users either stationary or on the move anywhere, anytime and in any
          format (voice, data, and multimedia).

          The satellite segment is a network of GEO or LEO satellites arranged in orbital planes (i.e.
          different parts of the sky) in such a way that they have a communications link with end-
          user equipment, ground gateways and other satellites. The gateway connects the satellites
          to the local telephone network. The gateway also transmits signals to the satellites and
          receives transmission from the satellites. Due to the high mobility of low earth orbit (LEO)
          satellites, there is a significant number of handover attempts in a LEO-based mobile
          satellite communication system, causing a high handover failure rate. This paper proposes
          to extend the period of which a handover request is valid, and thus rendering higher
          probability of successful handover.

          Satellite communication service can be provided by geostationary earth orbit (GEO),
          medium earth orbit (MEO) or low earth orbit (LEO) satellites. Because of its much shorter

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          distance from earth, lower power requirement and thus smaller mobile terminal (MT) size,
          LEO satellite system is a preferable choice. Differences between satellite and terrestrial
          systems exist in spite of common objectives for high quality services and excellent
          spectrum efficiency. Some differences arise because:- user costs are closely related to
          satellite transmit power the satellite propagation channel is highly predictable satellite paths
          introduce significant propagation delays and Doppler shifts frequency co-ordination has to
          be on a global basis frequency re-use options are more limited, hence bandwidth is a tight
          constraint satellite beam shaping and sizing opportunities are limited.

          The most significant attribute of any satellite communication system is the wide area
          coverage that can be provided with very high guarantees of availability and consistency of
          service. Satellite communication systems are designed to provide voice, data, fax, paging,
          video conferencing and internet services to users worldwide. Through satellite based
          systems, users will be able to make a phone call from an African safari or while sailing
          around the world. No matter where users are, they will be able to communicate with
          clients, customers, associates, friends, and family anywhere in the world. In addition,
          satellite communications will allow countries to provide phone services without large
          investments in landline or wireless systems. Satellite communications will be one of the
          fastest growing areas within the communications industry.




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                                              INDEX
          TOPIC                                                         PAGE NO.

          1. INTRODUCTION                                                 1
          2. CLASSIFICATION OF MOBILE SATELLITE COMMUNICATION SYSTEMS     4
                  2.1 GEOSTATIONARY SATELLITE.                            4
                  2.2 QUASI-ZENITH SATELLITE.                             4
                  2.3 NON-GEOSTATIONARY SATELLITE.                        4
                          2.3.1 HIGHLY ELLIPTIC ORBIT (HEO).              6
                          2.3.2 MEDIUM EARTH ORBIT (MEO).                 7
                          2.3.3   LOW EARTH ORBIT (LEO).                  7
          3. SOME LEO SATELLITE SYSTEM                                    8
                  3.1 THE IRIDIUM SYSTEM                                  8
                  3.2 THE GLOBALSTAR SYSTEM                              10
                  3.3 THE TELEDESIC SYSTEM                               12
          4. MEO SATELLITE SYSTEM                                        13
              4.1 THE ICO SYSTEM                                         13
          5. OVREVIEW OF HOW A MOBILE SATELLITE SYSTEM WORKS             15
             5.1 HOW A SATELLITE CALL GETS ROUTED                        15
             5.2 HANDOVER MANAGEMENT IN MOBILE SATELLITE SYSTEMS         17
          6. GENERAL ASPECTS OF MOBILE SATELLITE SYSTEMS                 19
          7. CAPABILITIES OF MOBILE SATELLITE SYSTEMS                    20
          8. LIMITATIONS OF MOBILE SATELLITE SYSTEMS                     21
          9. GROWTH DRIVERS OF MOBILE SATELLITE COMMUNICATION SERVICES 26
          10. WHO WILL USE SATELLITE COMMUNICATIONS SYSTEMS?             27
          11. GLOBAL MOBILE SATELLITE COMMUNICATIONS SERVICES            28
          12. APPLICATION OF MOBILE SATELLITE SYSTEM                     29
          13. CONCLUSION                                                 32
          14. BIBLIOGRAPHY                                               32




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          INTRODUCTION
            To make a satellite phone call today from a location that does not offer terrestrial wireline
            or wireless coverage requires the use of a large, costly terminal, and entails very high per
            minute charges. Further, the quality of service is relatively poor because of annoying
            echoes, large transmission delays, overtalk associated with satellite communications
            using geostationary satellites. The next generation of satellite communication systems
            will use advances in satellite systems, wireless technologies, and miniaturization, to
            provide global mobile satellite services that will make calls between any two locations on
            earth much easier, much more affordable and much more user friendly.
            Even in the year 2000, the terrestrial cellular coverage is available to less than 60% of the
            world’s population and only about 15% of the earth’s total surface. More than 3 billion of
            the world’s population have no phone service. The waiting list of landline telephone
            service has over 50 million names with the average wait greater than 1.5 years. Rural
            areas, regions, that are sparsely populated in developed countries and large parts of the
            developing world are destined to be underserved or to remain out of reach of terrestrial
            mobile services altogether. Thus, in many parts of the world, the demand for
            communications mobility can be met effectively only through global mobile satellite
            services. Handheld satellite phones are therefore forecast as the emerging mobile
            communications frontier with growth that could parallel recent growth in cellular mobile
            industry. Regardless of how you look at the numbers, there is a significant amount of
            people without phone service throughout the world. Mobile Satellite communication
            services will solve the need of worldwide travelers and provide phone services to many
            areas of the world that currently do not have phone service. The emerging next
            generating mobile systems are generally referred as GMPCS, for Global Mobile Personal
            Communication by Satellites.
             Until now Communication Satellites have operated using Geo-Stationery Orbits (GEO),
            lying above 36,000 kilometers above the earth’s surface. From this Orbit the satellite
            appears to be stationery (fixed) above a specific location from earth, thereby ensuring
            continuous, uninterrupted coverage to that location. The primary role of a geostationary
            communications satellite is to act as a wireless repeater station in space that operates in a
            broadcast mode and provides a microwave link between two remote locations on earth.
            The key components of a communication satellite include various transponders,
            transceivers, and antennas that are tuned to the allocated frequency channels. Although
            the Geostationary Satellites have a large footprint, so that the entire surface of the earth
            can be covered by few such satellites, their high altitude leads to very long roundtrip
            signal delays and resultant degradation in service quality.
            There is a trend for mobile satellite system architectures aimed at the deployment
            of multi-satellite constellations in Non-Geostationary Earth Orbits (NGEOs).This allows
            the user terminals to be small size, low cost and having low power demand.To enhance
            the coverage and quality of service, Low Earth Orbiting (LEO) constellations are usually
            selected. To supports a wide range of services and to provide superior service quality
            comparable to that available from terrestrial wireless and wireline networks,
            constellations of satellites operating in Low Earth Orbits (LEO) or Medium Earth Orbits
            (MEO) are considered more suitable.

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            A number of various global mobile satellite communications systems have already been
            in development stages. With the first global mobile satellite services initiated in 1998.
            The four such systems that are in advanced stages of planning or early implementations
            are Iridium, Globalstar, ICO and Teledesic.
            The era of satellite-based mobile communications systems started with the first
            MARISAT satellite which was launched into a geostationary orbit over the Pacific Ocean
            in 1976 to provide communications between ships and shore stations. The combination of
            high cost and unacceptably large equipment has kept mobile satellite communications
            (MSC) systems from appealing to the wider market of personal mobile communications.
            However, the progress made over the last ten years in digital voice processing, satellite
            technology, and component miniaturization has resulted in the viability of MSC systems
            in responding to the growing market in personal mobile
            Communications.The system architectures of each system are presented along with a
            description of the satellite and user handset designs, the multi-access techniques
            employed, and an analysis of their respective cost structures.It is concluded that the
            technical feasibility of satellite-based mobile communications systems seems to be
            secure. It will be challenging however, for the vendors to actually develop and deploy
            these systems in a cost effective, timely, and reliable way that meets a continually
            evolving set of requirements driven by user expectations fueled by a rapidly changing
            technology base.
            In order to guarantee the service quality and reliability for mobile satellite
            communication systems, we have to take into account outages due to obstruction of the
            line-of-sight path between a satellite and a mobile terminal as well as the signal
            fluctuation caused by interference from multipath radio waves. Thus, we need a good
            characterization for the satellite propagation channel. It is commonly accepted that
            satellite communications systems (in particular,low earth orbit LEO systems) are the de
            facto solution for providing the real personal communications services (PCS’s) to the
            users either stationary or on the move anywhere, anytime and in any format (voice,
            data,and multimedia).Satellite communication systems have provided international
            telecommunications services since the 1960’s. These systems were augmented in the
            1970’s and 1980’s with regional satellite systems, national systems, and private network-
            based very small aperture terminals (VSAT’s). Throughout this period, systems have
            been based exclusively on satellites in geosynchronous orbit communicating with earth
            stations using high gain fixed antennas. As the systems have evolved, the original 30-m-
            diameter Intelsat earth stations have evolved into 1.2-m -band VSAT’s for business and
            home TV usage, but the basic system architecture explaining a geosynchronous
            spacecraft has not changed during this period. With the launch of the first Iridium
            spacecraft in 1997 and 1998, a significant new architecture has been introduced into the
            field of satellite communications. These systems are based upon the use of LEO and
            medium earth orbiting (MEO) systems. hese LEO and MEO systems have several
            advantages over geosynchronous systems. The most significant advantages are:
          1) The reduction in range provides a large decrease in path loss resulting in much small
          receiving antennas and



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          2) The reduction in range provides a significant reduction in propagation delay making
          voice conversation more pleasing to the user and increasing the throughput of most data
          communication protocols. These systems can and will serve mobile and portable users
          With small near omni antennas.
           However, the use of the small antennas as well as the motion of the transmitter and the
          receiver introduces the possibility of multipath and path blockage into the link budget of
          these satellite systems. Moreover, the propagation channel will be time varying due to
          different shadowing and scattering phenomena, so traditional channel models may
          not work well.This is concerned with the statistical modeling of the propagation
          characteristics of LEO and MEO systems. Since in these systems, satellites and mobile
          users are all allowed to move during communication sessions, the channel characteristics
          will be different from the geostationary systems (GEO’s). Due to the movement of
          receivers or transmitters, the received signals may fluctuate very rapidly from time to time.
          This fluctuation results from the combining effects of random multipath signals and
          obstruction of the line-of-sight path, which induces various fading phenomena. The
          communication quality of service(QoS) parameters such as the word-error rate will be
          affected in great deal in such communication environment. For effective mobile satellite
          communications system design, we must quantitatively know the propagation
          characteristics such as signal fading due to reflection; shadowing from trees,
          buildings,utility poles, and terrain; Doppler effects due to movement of mobile terminals,
          mobile satellites, or the communication effects; and other effects such as the rainfall. Such
          characteristics can be studied by the statistical distribution of the received
          signal envelope or received power in mobile communication systems.




          MOBILE SATELLITE COMMUNICATION SYSTEM CAN
          BE BROADLY CLASSIFIED BY ORBIT PERIOD


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          1. Mobile Satellite Communication System by the Geostationary Satellite

          The geostationary satellite is the artificial satellite which looks stationary from the ground.
          3-4 geostationary satellites can cover almost the entire surface of the earth. Most of the
          artificial satellites actually used for communications or broadcasting are geostationary
          satellites.

                 i. Altitude: about 36,000km
                 ii. Orbit: the circle orbit cycle on the equator is the same as the earth's autorotation
                  time.
                 iii. Number of Satellites: four (service areas are duplicated.)
                 iv. Principle Satellite System: Inmarsat Communication System, N-STAR
                  Communication System, Omunitrucks Communication System

          2. Mobile Satellite Communication System by the Quasi-Zenith Satellite
          The quasi-zenith satellite is an artificial satellite of the satellite system where one satellite
          always stays near the zenith in Japan by positioning at least three satellites synchronously
          on the orbit inclined at 45 degrees from the geostationary orbit. As the ground surface orbit
          draws the shape of number 8, it's also called "Number 8 Orbit Satellite". It can obtain a
          high elevation angle to reduce the influence of buildings and so forth (blocking.)

                 i. Altitude: about 36,000km
                 ii. Orbit: circle orbit crossing with the equator by the angle of 45 degrees
                 iii. 3 as the minimum
                 iv. The research and development of the satellite communication system is in
                  progress

          3. Mobile Satellite Communication System by the Non-Geostationary
          Satellite

          This is roughly divided into three kinds of orbits: highly elliptic orbit, medium earth orbit,
          and low earth orbit. The medium and low earth orbits have lower satellite altitudes to
          shorten the radio transmission delay, enabling more speedy and smooth communication.
          Specifically, the highly elliptic orbit can obtain a higher elevation angle. It is currently
          being researched and developed.




          i. Highly Elliptic Orbit (HEO)

              1. Altitude: about 40,000km
              2. Orbit: about 5-6 hours
              3. Number of Satellites: 2-3 as the minimum

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              4. The system planning is in progress.

          ii. Medium Earth Orbit (MEO)

              1.   Altitude: several thousand - 20,000km (about 10,000km)
              2.   Orbit: about 5-6 hours
              3.   Number of Satellites: 8-10 (for the entire world)
              4.   The system planning is in progress.

          iii. Low Earth Orbit (LEO)

              1.   Altitude: 500km - several thousand km (about 1,000km)
              2.   Orbit: about 5-6 hours
              3.   Number of Satellites: several dozen (for the entire world)
              4.   Principle Satellite System: Globalstar Mobile Satellite Communication System,
                   Orbcomm Mobile Satellite Communication System (IRIDIUM Mobile Satellite
                   System (abolished))

          Satellites in GSO

          GSO satellites orbit the Earth in the equatorial plane with the same angular velocity as the
          Earth at a height of about 36 000 km above the equator. Geostationary satellites therefore
          appear stationary to an earth-bound observer and a single satellite can provide continuous
          service to roughly one third of the Earth's surface (but excluding
          polar regions above ± 75 degrees of latitude). The maximum distance the satellite can "see"
          on the Earth's surface is about 42 000 km and means the propagation delay for a single hop
          via the satellite (once up and down) can be up to 280 ms. Geostationary satellites also move
          about their nominal positions causing a small but noticeable Doppler shift on both the
          feeder and mobile links.For personal and vehicle terminals, handover during a call between
          GSO satellites is unnecessary because the coverage is static and wide. However handover
          might be contemplated for aircraft terminals between different spot beams of the same
          satellite. In the latter case there is practically no difference in path length to consider.
          Within Europe, GSO satellites appear at low elevation angles. For the geographical latitude
          of 50°North (e.g.Luxembourg), the satellites reach approximately 31° elevation as a
          maximum when the satellite is due South: either East or West of this position the elevation
          slowly reduces. Frequent blocking of the line-of-sight signal therefore occurs from trees,
          buildings and hills. GSO satellites can work in such a shadowed environment but the
          satellite Equivalent Isotropic Radiated Power (EIRP) would have to be increased by 15 dB
          to 20 dB or more depending on the coverage required.This could be achieved but has a
          serious impact on the size and cost of the satellite. In addition, assuming that the mobile
          EIRP is limited, the satellite receive sensitivity also has to increase and this can only be
          done with very large spacecraft dish antennas. For this reason, only very low bit rate
          services (i.e. paging, alerting, etc.) might be viable under such circumstances until the user
          moves to a more favourable position to receive a voice call.

           Satellites in HEO
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          Satellites in HEO constellations orbit the Earth in planes that are inclined nominally 63,4°
          against the equatorial plane.This is necessary in order to keep the apogees in the most
          northern (southern) positions within their elliptical orbits.Typically HEO orbital periods are
          between 8 and 24 hours. HEO satellites are normally active only about their apogees where
          they appear nearly stationary to an earth observer for about eight hours, and then have to
          hand over to a following satellite.The satellites belonging to one particular system appear
          in time shift in the same celestial region. In the HEO track is sketched in profile showing at
          every point the true distance to the Earth's surface. In this specially depicted case, the
          orbital period is 12 hours and the satellites appear
          alternatively at the opposite sides of the rotating globe. Therefore the illustrated HEO track
          reaches a maximum height at both ends above the geographical latitude of 63,4° North. At
          both upper ends (solid line), the satellite payloads are active. The dotted line constitutes the
          part where the satellite payloads are (typically) switched off. For comparison, see figure 2,
          where two HEO loops are indicated corresponding
          to the two ends in profile in figure 1.Under the above conditions, the HEO apogee
          (maximum height above the Earth's surface) can be up to 42 000 km.However the
          maximum range to the Earth's surface is in the order of 47 000 km resulting in a maximum
          propagation delay of the order of 310 ms. HEO satellites reach high relative speeds during
          their active phase (order of magnitude:2 km/s), so that the Doppler shift (1.3 x 10-5 of
          radio frequency and bit rate) cannot be neglected: the radio frequency
          shift is mainly due to the microwave feeder link and is of the order of 50 kHz for C-band
          feeder links. The satellite motion is mainly radial relative to the user community, so that
          common compensation of the Doppler main component is feasible.Irrespective of any user
          roaming, HEO systems require handover from the descending to the ascending satellite
          typically every eight hours. Depending on the specific system design, the distance to the
          two satellites at handover could be significant and a jump in path length cannot be
          excluded. However, a large Doppler jump will always happen.Within Europe HEO
          satellites can appear near the zenith. Therefore the user can work under vertical line-of-
          sight condition for most of the time, with blockage only being experienced in tunnels or
          under bridges, trees, etc. However vertical propagation is not very good within multi-storey
          buildings and hence paging, alerting, etc. may not be satisfactory.
          Because vertical propagation can be in principle multipath-free, high data rate services are
          possible for outdoor operation.A number of HEO orbits have been studied extensively and
          given names such as "Molnya", "Tundra", and "Loopus".




          Satellites in MEO
          MEO satellites are in principle the same as LEO satellites. The differences are that MEO
          systems cause more propagation delay (80 ms to 120 ms), their Doppler shift is smaller,
          and handover happens less frequently and is less problematic. MEOs also need to work in a



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          multipath environment as the number of satellites is usually smaller than for LEO but the
          average margins can be lower since many calls will be at a continuously high
          elevation angle.The typical MEO altitude is between 10 000 km and 20 000 km, just
          outside the Van Allen belts with an orbital period of around 6 to 12 hours. A complete
          MEO constellation would probably require between 10 to 15 satellites. MEO satellites are
          used to provide current global radio navigation services and are optimum for such services.

          Satellites in LEO
          LEOs are typically circular orbits where satellites fly low above the Earth's atmosphere
          typically 700 to 1 500 km, bounded by outer atmospheric drag and the Van Allen radiation
          belts with an orbit time of about 90 minutes. For orbits near 1 500 km, inclinations near 50
          degrees reduce the risk of debris collisions. Whereas polar orbits provide a whole Earth
          coverage including the poles themselves, inclined orbits can provide improved coverage
          over the populated areas located between latitudes -75 to +75 degrees. One proposed
          system is known to stay 700 km above the surface (see figure 1; LEO) where the coverage
          area at any point in time may measure up to 3 000 km in radius for about 10 degree
          elevation. This implies a maximum propagation delay of 20 ms and while higher altitude
          LEO systems would have higher propagation delays, they will never approach the values
          associated with GSO or HEO systems for a single satellite hop. However, on-board
          processing and Inter-Satellite Links (ISL) can increase delays considerably.LEO satellites
          move at very high speeds relative to the Earth's surface (7 km/s) and produce large Doppler
          frequency shifts (4,7x10-5 of radio frequency and bit rate). As the velocity is tangential to
          the Earth, Doppler compensation may need to be applied individually for each user.LEO
          systems, in common with HEO systems, also require to handover between adjacent
          satellites, but at a much more frequent rate of about ten minutes. Although the two LEO
          satellites are widely spaced, the individual path lengths can be similar and it is possible to
          minimise any path length jump. However, the Doppler shift jump will still always happen.
          As LEO satellites orbit very close to Earth, they can be considered as moving base stations.
          For the user the satellites appear most of the time below 30 degree elevation. Therefore
          LEO satellites work much of the time in a multipath environment. The additional satellite
          EIRP and receive sensitivity to compensate for multipath losses are achieved witha much
          smaller antenna on a LEO spacecraft (compared to GSO) because of the much shorter
          range (roughly 1/12th).
          Diversity techniques may offset some of these multipath effects.The total number of
          satellites required to give total global coverage depends on many factors including quality
          of service and system capacity but the total could be as high as 70. Lower numbers are
          possible using special orbits or by using a mixture of LEO and GSO (for example). The
          cost for large numbers of LEO satellites is offset to some extent by their lower complexity
          and easier launch requirements. However their orbital life tends to be half that of typical
          GSO satellites (10 - 13 years). Another factor in LEO design is the required battery
          capacity and solar panel size to allow operation for nearly 50% of time in eclipse.

          SOME LEO SATELLITE SYSTEM


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          1. THE IRIDIUM SYSTEM

                  The Iridium System is not proposed to be a replacement for existing terrestrial
          cellular systems, but rather as an extension of existing wireless systems to provide mobile
          services to remote and sparsely populated areas that are not covered by terrestrial cellular
          services. It provides more capacity (large no of channels) and better quality of service
          (shorter transmission delays) to areas that currently receive mobile services from
          geostationary satellites. It can also provide emergency service in the event that terrestrial
          cellular services are disabled in disaster situations(earthquakes,fires,floods,etc.).
                  The concept of using a constellation of low earth orbit satellites to provide global
          telecommunication services to mobile users. Because the initial proposal called for 77
          satellites in the constellation, the system was called IRIDIUM after the element, which has
          77 electrons in its orbit.later studies indicated that only 66 satellite would be adequate to
          provide the targeted services and performance. The 66 satellites are are grouped in six
          orbital planes; there are 11 active satellites in each plane with uniform nominal spacing of
          32.7”. the satellites have circular orbits at an altitude of 783 km, and for each plane an in-
          orbit satellite is provided.
          Satellites in one plane are placed to travel out of phase with those in the adjacent planes.
          Except for the first and last planes, which are counterrotating where they are adjacent, all
          remaining planes are corotating, The distance between corotating planes is 31.6, and the
          distance between the counterrotating planes is 22. The reduced seperation between
          counterrotating planes is needed to compensate for the reduced coverage provided by
          satellites on counterrotating planes. In the Iridium system, each satellite is equipped with
          four two-way communication links(intersatellite links, or ISLs), one each with its
          neighbors in the same plane and with those in the adjacent planes.
          Each Iridium satellite uses a 48-beam antenna pattern, and each beam, which has a
          minimum diameter of 600 km, can be individually switched. For example, only about two-
          thirds of the beams will be active at any given time because some the beams will be
          switched off when the satellites are in the vicinity of the poles, where beam patterns tend to
          overlap, or when the satellites are over countries or regions in which, Iridium does not have
          regulatory arrangements to operate. The switched of beams is referred a cell management.
          In a LEO-based system like Iridium, the beams are equivalent to cells associated with
          terrestrial mobile systems. However, in case of the Iridium system, it is the beams that
          move rapidly relative to the subscriber, who is considered to be stationary with respect to
          the satellite. Thus, switching of beams or cell management to provide continuity of an
          existing call is equivalent to handoff in terrestrial cellular mobile systems. This requirement
          for cell management is, of course, and additional complexity associated with LEO- based
          systems compared with MEO or GEO systems.
          The Iridium system supports links of three types:up- and downlinks from the space vehicle
          (SV) to the gateway (GW) [or to the telemetry, tracking, and control (TT & C) center],
          using the ka band; up- and downlinks between the SV and the Iridium subscriber unit
          (ISU), using the L band; and two-way inter-satellite links between the SVs using the Ka
          band.

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          2. THE GLOBALSTAR SYSTEM
          Globalstar is a global mobile satellite system based on a constellation of 48 LEO satellites.
          Unlike the Iridium system, Globalstar system does not use intersatellite links but rather
          depends on a large number of interconnected earth stations or gateways for efficient call

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          routing and delivery over the terrestrial network. It is designed to complement the
          terrestrial cellular mobile networks to provide telephony and messaging services to
          subscribers in locations that are not covered, or inadequately covered, by conventional
          wireline or wireless networks.
          Globalstar’s constellation of 48 LEO satellites is designed t orbit at an altitude of 1414 km
          above earth’s surface in eight orbital plans inclined at 52. With each plane to be occupied
          by six satellited with a provision for one in-orbit spare satellite in each plane. The nominal
          weight of each satellite is 450 kg, with a deployed span of 7 meters and working life of 7.5
          years. Since Globalstar satellites do not employ intersatellite communication, they
          essentially provide only transponder functions, making their design and operation less
          complex and perhaps more reliable. Each satellite supports a 16-beam antenna pattern with
          an average beam diameter of 2250 km. To mitigate blocking and shadowing, Globalstar
          will deploy path diversity, whereby multiple satellites may be used to complete a call.
          In the absence of intersatellite links, the Globalstar system makes maximum use of the
          international terrestrial networks (wireline and wireless). Calls from a subscriber are routed
          via a satellite to the nearest earth station/gateway, and from there they will be routed over
          the existing terrestrial network. To provide the interface between the ground segment
          (terrestrial networks) and space segment (Globalstar satellites) Globalstar design deploys
          100 or more gateway stations distributed around the world with each station equipped with
          three or five antennas that can track the trajectories of the satellites. A Globalstar gateway
          is designed to serve an area 3000 km in diameter and will be designed to take into account
          the technical and administrative requirements of the coverage area. These requirements
          may include such factors as coverage, quality of service, and satellite visibility, as well as
          regulatory and contractual factors associated with national boundaries.
          Globalstar uses two types of communication links: service links in the L/S band for
          communication between the terminals and the space vehicle, and the gateway links in the C
          band for communication between the earth stations and the space vehicle.




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          ]3. THE TELEDESIC SYSTEM
          Currently the high bandwidth, high quality fiber connectivity needed to support Internet
          access, computer internetworking, video conferencing, and so on is restricted to major
          commercial and population centers. Outside these application areas, such facilities are
          either too expensive or simply not available. The aim of the Teledesic network is to extend
          the existing terrestrial, fiber-based infrastructure to provide advanced information and
          communication services anywhere on earth. Whereas the target application for Iridium,
          Globalstar, and ICO is voice, with support of low bit rate data for facsimile and messaging
          for mobile subscribers, the primary target application for the Teledesic system is the
          provide worldwide, seamless, fiber like connectivity to support multimedia, video, and high
          bit rate data services. In a strict sense, Teledesic does not fall in the category of global
          mobile satellite systems or GMPCS because its focus is not on worldwide terminal
          mobility, but rather on providing the so-called Internet in the sky function. The planned


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          target for Teledesic service availability is end of year 2002. Rather than individual end
          users, primary customers for the Teledesic system will be service providers in countries
          around the world wishing to extend their network capabilities in terms of geographic scope
          and the range of services, and also multinational corporations needing to extend the
          capabilities of their enterprise networks.
          The design of the Teledesic system has not been finalized. According to the original plans,
          the Teledesic satellite segment was to use 840 LEO satellites in 21 planes at altitudes of
          700 km. The Teledesic system now intends to deploy only 288 active LEO satellites placed
          in 12 planes (24 satellites per plane) at altitudes around 1350 km. Each satellite in the
          Teledesic constellation will have connections to eight of its neighboring satellites through
          intersatellite links operating in the connectionless packet mode, with each satellite in this
          interconnected mesh network providing necessary switching functions. The Teledesic
          network is designed for dual-satellite visibility with at lest one insight satellite at a
          minimum elevation of 40. This high elevation angle ensures an unobstructed and
          omnidirectional view of the sky from most building tops where Teledesic terminals may be
          located. Besides eliminating shadowing effects from neighboring buildings and terrain, the
          high elevation angel greatly reduces the fading effects of rain at high frequencies.




          MEO SATELLITE SYSTEM

          1. THE ICO SYSTEM
          The ICO is a medium earth orbit (MEO) mobile satellite system, which is designed
          primarily to provide services to handheld phones. ICO will use TDMA as the radio
          transmission technology. The system is designed to offer digital voice, data, facsimile, and
          short-targeted messaging services to its subscribers. ICO’s primary target customers are
          users from the existing terrestrial cellular systems who expect to travel to locations in
          which coverage is unavailable or inadequate. Other customer groups potentially served by
          ICO include road transport, maritime, and aeronautical users, as well as users of semifixed
          terminals in rural areas and develo0ping countries, where conventions terrestrial wireline or



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          wireless mobile satellite communications capability with the public land mobile networks
          like GSM, D-AMPS, and PDC and their PCS variants.
          ICO system is designed to use a constellation of 10 MEO satellites in intermediate circular
          orbit (ICO), at an altitude of 10,355 km above the earth’s surface. The nominal weight of
          these satellites at launch is less than 2000 kg. The satellites, with an expected life of 12
          years, are arranged in two planes with five satellites (and one spare) in each plane: orbital
          planes inclined at 45 relative to the equator. Each satellite has antennas to provide 163
          transmit and receive service link beams. The orbital configuration provides coverage of
          earth’s entire surface at all times and ensures significant overlap so that two or more
          satellites are visible to the user and the satellite access node (SAN) at any time. Further, at
          least one of the satellites appears at the high elevation angle, thereby minimizing the
          probability of blocking due to shadowing effects.
          The ground segment in the ICO system, which will link the ICO satellites to the terrestrial
          networks, will consist of the 12 interconnected SANs located in various parts of the world.
          Each SAN consists of earth stations with multiple antennas for communication with the
          satellites, switching equipment, and databases to facilitate interconnection with public
          telephone, data, and mobile networks. The interconnection to the public networks is
          through appropriate gateways. Whereas each SAN supports VLR functions, the HLR
          function can reside in one (or more) of the SANs. A SAN tracks the satellites within its
          sight and will direct communication traffic to the satellite, which can provide reliable,
          uninterrupted link for a given call, in terms of angle of elevation and duration of satellite
          visibility. SANs also have the capability to execute handoffs from an area covered by one
          satellite to another satellite’s coverage. Such handoffs are expected to be very infrequent in
          ICO’s MEO-based system. Besides the SANs, the ICO system deploys TT & C stations
          connected to a satellite control center (SCC) for monitoring and controlling the satellites, as
          well as one or more network control centers (NCC) for overall management and control of
          the ICO system. The TT & C functions are associated with 6 of the 12 interconnected
          SANs.




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          A broad overview how a mobile satellite system works

          A satellite system consists of a satellite segment, ground segment, and end-user segment.

          Satellite Segment
          The satellite segment is a network of GEO or LEO satellites arranged in orbital planes (i.e.
          different parts of the sky) in such a way that they have a communications link with end-
          user equipment, ground gateways and other satellites. The satellites transmit a continuous
          signal to earth which enables the satellites, end-user equipment and
          gateways to be linked together. The links allow end-users to be transferred between
          satellites as the satellites move overhead (LEO systems). On the ground, there is a ground
          control facility (or facilities) which manage the performance of the satellites and the
          transfer of information from the satellites to the gateways.

          Ground segment - gateways
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          The gateway connects the satellites to the local telephone network. The gateway also
          transmits signals to the satellites and receives transmission from the satellites. The gateway
          tends to have switching capabilities along with software that allows the system provider to
          keep track of billing information and route calls.



          End-user
          The end user terminals, pagers and phones communicate with the gateway equipment,
          satellites, satellite and cellular phones along with the cellular base station equipment. For
          the Iridium and Globalstar systems, the endusers will use a phone slightly larger than the
          average cellular phone. Both Iridium and Globalstar plan to offer dual mode handsets
          which will allow users to connect to the existing cellular systems or their own satellite
          system.Other systems from such companies as American Mobile Satellite and Intelsat, use
          phones which are the size of a briefcase and must be unpacked before use. The paging
          equipment from Iridium, the only satellite company who currently has a paging system in
          place, is your normal run of the mill pager. A few satellite systems (Globalstar included)
          plan to offer fixed satellite terminals which are a telephone booth in rural areas. The phone
          booth will include one or more phones and will not look much different than a phone booth
          you may find on the streets of New York City.



          How a satellite call gets routed
          There are two types of satellite systems under development and each have a different
          approach to routing phone calls. The proposed satellite systems will use either a bent pipe
          or an intersatellite linked system.




          Bent pipe
          In a bent pipe system (Globalstar) a call is placed by a satellite user which is then beamed
          up to the nearest orbiting satellite. The satellite reflects the call to the nearest ground
          gateway. Once at the gateway, the call is routed through the public telephone network to
          the intended receiver of the call. The gateway must be in the line of sight of the satellite, so
          the system operator must have a significant number of ground gateways to provide direct
          satellite links. For the most part, a bent pipe system is less complex than an intersatellite
          linked system because the brains of the system (switching) are on the ground and the
          satellites are just reflectors in the sky. Bent pipe systems are easier to operate because most
          of the call is transferred over the public telephone network, this also reduces the cost of the
          system. Many of the technical features will be located at the gateway which will allows
          most technical problems to be fixed on the ground.




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          Intersatellite links
          In an intersatellite linked system (Iridium) a user’s call is beamed up to the nearest orbiting
          satellite. When it reaches the satellite, the call enters the onboard satellite switching system
          and is routed between satellites up in space. The call is then downlinked to another satellite
          user or the closest local gateway to the end user. Upon
          entering the gateway the call is directed to the intended receiver through the public
          telephone network. The major benefit of the intersatellite linked system is that it minimizes
          the cost of the ground segment (i.e. - the call is switched in the air therefore you do not
          need a ground gateway in the line of sight of each satellite) and it also
          minimizes the long distance and interconnect fees (much of which is bypassed in the air).
          The intersatellite linked system operator is able to keep a larger dollar amount of each call
          as compared to the bent pipe system operator.However, there is added risk and higher costs
          because each satellite must have on-board switching capabilities.On-board switching also
          adds to the complexity of the system because repairs must be made to satellites.




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          Handover Management in Mobile Satellite Systems
          Due to the high mobility of low earth orbit (LEO) satellites, there is a significant number of
          handover attempts in a LEO-based mobile satellite communication system, causing a
          high handover failure rate. This paper proposes to extend the period of which a handover
          request is valid, and thus rendering higher probability of successful handover. Satellite
          communication service can be provided by geostationary earth orbit (GEO), medium earth
          orbit (MEO) or low earth orbit (LEO) satellites. Because of its much shorter distance from
          earth, lower power requirement and thus smaller mobile terminal (MT) size, LEO satellite
          system is a preferable choice. In this paper, only the LEO satellite system is considered.
          The satellite coverage area, or its footprint, is divided into a number of areas, each of which
          spotted by one of the satellite's multiple spotbeams, forming a cell. Since a LEO satellite is
          not located at a geosynchronous orbit, it is mobile with respect to a fixed point on earth.
          Hence an active MT may move from one cell to another and handover occurs. The ground
          velocity of the MT is ignored compared to the
          much higher satellite velocity. Suppose the length of a cell is 400 km and the satellite
          moves at a velocity of 6.6 km/sec, the time taken for a MT to cross a cell, Tcell, is about 60
          seconds. Thus handover is extremely frequent in this system. And it is probable that a call
          is dropped due to unsuccessful handover. Handover is prone to failure when the subsequent
          cell has no unused channel to offer. Drop call is a phenomenon where an ongoing call has
          to be discontinued, which the users find hard to tolerate with, making it a major technical
          issue.There have been some methods proposed to minimise handover failure. It is widely
          accepted that handover requests are to be prioritised over new call requests, either by
          allocating guard channels to the handover requests [1], or by queuing up the handover
          requests when all the channels in one cell are occupied [1] [2]. This is because dropping an
          ongoing call is less desirable than blocking a new call attempt. There are also proposals of
          making the handover request earlier, so that the request has longer time to wait for a free
          channel, thus reducing the handover failure rate [3] [4].
          Gerard Maral et al. have proposed that a handover request is to be made to a cell as early as
          the MT enters the cell located right before the target cell [3]. In [4], the time of sending out
          a handover request during handover process was made available regardless of the location
          of MS in a cell. In all of these cases, a call somehow has to be terminated when the
          originating MT has crossed into the target cell and yet handover is not granted by the target
          cell. The termination is done since no service is provided by either the original cell or the
          target cell. In this paper it is proposed that in a similar situation, the call be only
          temporarily discontinued for a specific amount of time, before it is permanently terminated
          if there is still no available channel. Although no service is provided by both cells to the
          MT, its handover request which has been queued is ‘kept in view’ by the target cell. During
          this idle period, there is a chance that an originally occupied channel in the target cell is
          released. If this is the case, this channel is allocated to the suspended call and handover is
          completed. As a result, the handover failure rate is reduced.
          Mathematical analysis has been carried out to verify the idea and the results are
          encouraging. For a user, he/she only experiences a short period of call discontinuity and is
          notified about the temporary discontinuity through a special tone. In terms of quality of
          service (QoS), this is more tolerable to the users compared to a drop call.

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                 A handover management strategy is proposed to efficiently manage the channel
          resource of a cell in a multibeam mobile satellite system (MSS) and improves its service
          quality by reducing the interbeam handover failure rate (Phf) caused by limited number of
          communication channels. The Extended Queueing of Handover (EQH) technique extends
          the channel reservation time of a handover request into the adjacent cell that the user
          terminal (UT) is subsequently entering (destination cell). Both mathematical analysis and
          simulation show that EQH reduces Phf significantly, without compromising the new call
          admission rate and efficiency of channel utilisation.
                 The footprint of a multibeam satellite is divided into cells where each cell is
          illuminated by a spotbeam. Interbeam handover is frequent in mobile satellite system
          (MSS) due to the high velocity of the satellite (about 7 km/h for a low earth orbit satellite).
          When a user terminal (UT) leaves from one cell and enters the adjacent one, a handover
          process must be completed for the sake of call continuity, where a
          communication channel must be allocated to the UT by the adjacent cell (destination cell).
          In a channel resource limited system, handover is subject to failure when the destination
          cell has no idle channel to offer. In this case the call is dropped and this is intolerable to
          users. In order to provide higher handover quality, system operator has to allocate a larger
          portion of the channel resource to the ongoing call as compared to the new call. A method
          in use is by applying the blocked-calls-queued policy to the calls
          requesting for handover (handover calls) [1]; and on the other hand sacrificing some new
          calls through the blocked-call-cleared policy. The longer a handover call stays in a queue,
          the higher chance of it being handed over successfully. Other methods that prioritise
          handover call over new call are: guard channelallocation [2], and channel reservation in
          advance [3] [4]. The compromise of new call causes inefficiency in channel utilisation
          because channels that are allocated to handover call cannot be taken up by new calls even
          though they are idle.


          Extended Queueing of Handover (EQH) Scheme
                    In a handover process, a UT with unfulfilled handover request will have its call
          terminated once it leaves the present cell i.e. when the signal strength of the present beam
          drops below an acceptable level. Under the proposed Extended Queueing of Handover
          (EQH) scheme [5], the policies of queueing and early channel reservation also apply to
          handover calls. In addition to them, the queueing process of an initially unfulfilled
          handover is allowed to be continued in the destination cell and thus lasts longer, promising
          a higher chance of obtaining a free communication channel. In this case, since the UT has
          left the present cell and has not reserved a channel from the destination cell,
          its call has to be discontinued until either a free channel is available on which the call can
          be resumed on,or until the tolerable suspension period is over which the call has to be
          permanently terminated,whichever comes first. Although this suspended call is prioritised
          over new call in getting a channel, it does not significantly affect the blocking rate of new
          call because the probability that a call get suspended is very small. From the viewpoints of
          the two involved communicating parties in an initially unsuccessful handover call,
          the discontinuity can be notified through a special tone / message. In terms of quality of
          service (QoS), a suspended call that eventually gets terminated is better than a disruptive

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          and uninformed drop call. On the other hand, if the call is able to be resumed upon the
          availability of an idle channel, the short term discontinuity makes it worth than having the
          call terminated and followed by setting up a new call again, which is harder because new
          call is less privileged. Hence regardless of the outcome EQH scheme promises a higher
          QoS.


          General aspects of mobile satellite systems
          Differences between satellite and terrestrial systems exist in spite of common objectives for
          high quality services and excellent spectrum efficiency. Some differences arise because:-
          user costs are closely related to satellite transmit power the satellite propagation channel is
          highly predictable satellite paths introduce significant propagation delays and Doppler
          shifts frequency co-ordination has to be on a global basis frequency re-use options are more
          limited, hence bandwidth is a tight constraint satellite beam shaping and sizing
          opportunities are limited.The first two points lead naturally to the emphasis placed on the
          line-of-sight satellite link budget when establishing the system design. The base link budget
          is derived from theoretical path losses to which link margins are added to
          compensate for inevitable impairments in equipment and propagation characteristics. All
          impairments, even if not directly calculable in terms of signal loss (e.g. group delay and
          rate of change of Doppler shift), are converted accurately to dB so that the compensating
          increase in transmit power can be established. The total margin over the theoretical ideal
          path is only a few dB and precision in calculating the contributory impairments is essential.
          The resulting link budget then allows the availability and quality of service to be estimated
          over the coverage area.Large link margins have a major impact on system build cost and
          operating tariffs simply because of the impact of additional power requirements on
          spacecraft size — a 3dB excess margin would almost double user charges. For this reason,
          mobile satellite communication systems have lead the way in very power-efficient
          modulation formats and low bit rate voice codecs (2,4 kbit/s and 4,8 kbit/s) as well as
          adaptive power control. The drive for efficient use of satellite power is noticeably reflected
          in terminal equipment design with:

          - very low loss antennas coupled with very low loss receive filters;
          - very tight transmit/receive filter specifications;
          - very low noise amplifiers;
          - excellent carrier/signal acquisition in presence of Doppler, noise and interference;
          - power-saving and spectrum-efficient forward error correction;
          - multi-path discrimination techniques might facilitate low signal-to-noise demodulator
          operation
          The satellite-mobile uplink and downlink are inevitably more fragile than the
          corresponding feeder links (land earth station-satellite). However the feeder link itself
          needs a very substantial link margin in order that the aggregate up/down performance may
          be largely determined by the mobile link. These feeder links operate in higher frequency
          bands where Doppler and atmospheric/meteorological disturbances can become even more
          significant. The following clauses of this TR focus on particular characteristics, capabilities
          and limitations of mobile satellite systems together with typical values for key parameters

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          where possible. However it must be recognised that most parameters are inter-dependent
          and will also vary with architecture of the ground infrastructure, the satellite orbital
          arrangement, and the user terminal configuration.



          Capabilities of Mobile Satellite Systems (MSS)
          The most significant attribute of any satellite communication system is the wide area
          coverage that can be provided with very high guarantees of availability and consistency of
          service. The satellite component of UMTS can potentially provide the terrestrial service
          user with a global service without regard to incompatible terrestrial standards used
          elsewhere. Existing satellite mobile services have proved very attractive to the maritime
          and aeronautical sectors and they have also been of great benefit to emergency services,
          relief agencies, journalists, and expeditions over recent years.
          Services are now extending to the land mobile market where hand-portable voice terminals
          are now technically feasible. The next subclauses address the key attributes of wide area
          coverage and types of services appropriate for satellite UMTS.

          6.1 Large area coverage
          A single satellite can see very large areas of the Earth: a single LEO can illuminate an area
          of 6 000 km diameter and a GSO can illuminate about 1/3rd of the globe. Within these
          areas, the spacecraft antenna can be designed to maintain a near-constant power flux
          density on the Earth's surface irrespective of range. However for the GSO and HEO (and
          possibly the LEO or MEO), the spacecraft antenna may need to be arranged as a cluster of
          spot beams (1 000 to 2 000 km diameter) in order to make hand-held terminals feasible and
          to achieve spectrum efficiency. Such spot beams require large spacecraft antennas for
          either GSO or HEO systems. The advantages of HEO and GSO are that it is possible to
          deploy a satellite system to fulfil a regional requirement rather than a global one, and
          frequency planning and co-ordination may be relatively straightforward. Furthermore, the
          ground infrastructure to support the satellites could follow traditional Land Earth Station
          (LES) approaches.The only satellite system that cannot provide polar coverage is GSO.
          With this restriction, any satellite constellation can provide assured line-of-sight global
          coverage unaffected by weather. Operation to shadowed or in-building terminals would
          require an additional link margin in the order of 20 dB or more, depending on the coverage
          required. Note that in cities, the terrestrial UMTS service is likely to be available and
          therefore in-building and city coverage may not be essential.
          The line-of-sight case requires polarisation matching between the satellite and the mobile
          terminal. To avoid the need for polarisation tracking, mobile communications have
          traditionally used circular polarisation.

          6.2 Flexible networks and services
               A feature of most present day satellites is the use of "transparent transponders".
          Compared to conventional cellular base stations, the satellite transponder is little more than

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          a frequency shifting amplifier. This does have drawbacks with regard to some aspects of
          system design but it also means that any one satellite is reasonably independent of
          modulation system or access method, or of service data rate or networking. This has led to
          satellites being used for a variety of applications, each with different terrestrial
          architectures. Provided the basic satellite parameters are satisfactory, these services can be
          introduced long after launch. Future satellites may not be quite so flexible as some studies
          propose to use on-board processing to improve capacity, spectrum efficiency and satellite
          payload performance. The transparency concept has however proved extremely
          costeffective and any on-board processing function is likely to be at least re-configurable
          and re-programmable. Another feature that might be introduced for MEO or LEO is the
          inter-satellite link to simplify terrestrial networking between satellites during handover.
          The transparency concept has enabled mobile satellite systems to efficiently support a
          range of services beyond that of voice telephony:
          - high data rate services (up to 64 kbit/s) to larger antenna (0,15 m ~1,0 m, 8 dBi ~20
          dBi) mobile or fixed
          terminals;
          - group call and broadcasting;
          - low data rate paging, alerting and two-way messaging;
          - terminal location finding.
          Some current satellite systems are designed so that extra services can be provided at very
          little additional cost. This is particularly effective when services are offered as a package to
          perhaps offset the requirement for line-of-sight paths for low-cost voice telephony.


          7 Limitations of Mobile Satellite Systems

          7.1 Delay and Doppler
          The delay and Doppler effects associated with satellite links are due entirely to the
          mechanical laws governing the satellite orbit. Any system design must take full account of
          these effects. For example, simple delay has an impact on speech quality that will require
          echo cancellers to be used at interfaces with the analogue network. Delay also requires
          allowances to be made in signalling protocols and power level control.
          Changes in delay are the result of integrated Doppler shifts on the bit data rate and are
          significant for all orbits except GSO during a call and particularly during satellite
          handover. Such changes are likely to require a data buffer to maintain the delay at a
          constant maximum value. The data buffer can reside in either the LES or the mobile
          terminal between the two echo control devices and is required for both receive and
          transmit. Doppler shift itself complicates signal acquisition and spectrum management. The
          Doppler shift will not be identical for the in-bound and out-bound links due to the different
          feeder and mobile link microwave frequencies. Furthermore, the shift is in different
          directions if corrected at the mobile terminal. For LEO and MEO orbits, the shift may need
          to be individually corrected for each mobile; for HEO, common Doppler compensation can
          be incorporated in the LES or onboard the satellite.



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          7.2 Low link margins
          Emphasis has already been made on the importance of keeping impairment margins low.
          An illustration can be based on the calculation of Carrier-to-Noise (C/N) ratios for uplink,
          downlink, and the total link. Assuming the downlink has C/Nd = 10 dB and is near the
          performance threshold, the feeder will need a 13 dB margin (C/Nu = 23 dB) to maintain
          the degradation to less than 0,2 dB (i.e. C/Nt = 9,8 dB). Operation at levels just above
          threshold are only feasible for satellite because of the stable propagation path and because
          most impairments (including the large noise contribution) can be considered to be random.
          These low margins, compared to the terrestrial environment, result in longer signal
          acquisition times.
          All impairments must be carefully analysed and include: imperfect in-band filtering, group
          delays, out-of-channel emissions (which demand very tight power amplifier linearity
          requirements, carrier to interference ratios, etc.) Multi-path also requires especial attention.
          In the terrestrial environment, multipath propagation normally results in inter-symbol
          interference that can be compensated with equalisers. The effects of multipath fading itself
          are often negligible within the main service areas because the detected signal level is
          sufficiently above threshold. In the satellite path, multipath delays are often short enough to
          be ignored (except for aircraft and ships) due to the comparatively high elevation angle of
          the radio path. However multi-path fading, in which a multipath signal partially cancels the
          main signal, can reduce the final signal below the modem operating threshold. Hand-offs
          between successive LEO, MEO, or HEO satellites will be more complex because of the
          small operating margins which makes it difficult to promptly detect signal disappearance.
          Satellite signal qualities often cannot be assessed from signal level (which is swamped by
          thermal noise) but are often estimated from the activity within the forward error correction
          algorithm. This requires time averaging and cannot be an instantaneous measurement.
          Satellite diversity reception
          might alleviate some of these issues. Mobile terminals with high gain (directive) antennas
          have further problems with signal acquisition as the antenna may need to be mechanically
          or electronically steered towards the satellite before the signal rises above the detection
          threshold.

          7.3 Spectrum and orbit matters
          Limited spectrum availability will constrain the potential capacity of the satellite
          component and hence will orientate personal satellite services towards low bit rate voice
          and data. Spectrum issues are very complex but can be broadly classified into three areas:
          - feeder link planning;
          - mobile frequency co-ordination;
          - mobile frequency re-use and spectrum efficiency.


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          Global agreements exist for planning GSO systems via the ITU RS (formerly IFRB) for
          designated frequency bands. Feeder links are normally in one of the established Fixed
          Satellite Service (FSS) bands and are straightforward except for the large bandwidths
          required to support peak traffic on each satellite. Mobile frequency co-ordination is not
          simple however, particularly as their antenna patterns are near omni-directional and any
          mobile system is likely to require exclusive access to a frequency band. The next problem,
          that of re-using the frequencies as frequently as possible, is very similar in concept to
          terrestrial cellular planing except that isolation is provided by satellite antenna beam
          shaping rather instead of geographical spacing. Feeder links for non-GSO satellites are
          more complex, particularly because of the lack of established procedures for the
          many possible orbits. Furthermore, there is no orbital registration akin to that in the GSO
          orbit where orbital positions are assigned to particular operators and countries. LEO and
          MEO may require several widely spaced feeder LESs per satellite sector or inter-satellite
          links to prevent the feeder link interfering with the geostationary orbit. In either case,
          there will be additional delay and Doppler jumps. For HEO orbits where the satellites
          appear to operate at the same part of the celestial sphere, feeder link planning may not be
          difficult as GSO-type procedures could be applied. The magnitude of the orbit and
          spectrum planning problems is partly illustrated by figure 2 which shows an azimuth -
          elevation diagram for a fixed land earth station site at a latitude of approximately 50° North
          (It is not computed from simulated systems but shows only the principle. Therefore slight
          differences to simulated orbit constellations may exist)


          The dotted line, extending from East to West in the shape of an arc, represents the
          geostationary orbit with two fixed GSO satellites designated 1 and 2. The three LEO tracks
          belong to one system of approximately polar orbits. The LEO satellites designated 1 and 2
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          travel North-South. LEO satellite 1 is about to hand over to LEO satellite 2. The LEO
          satellite 3 travels South-North, but this satellite No. 3 appears here at this track, due to the
          Earth's revolution, only at a time shift of half a day with respect to the satellites travelling
          North-South. The slight drift of the three LEO satellites towards West is caused by the
          Earth's rotation, and hence the rotation of the earth station site, towards East.
          At the north-north-western horizon and in the East of the zenith there are two loop-shaped
          tracks of the HEO satellites designated 1, 2 and 3. The dotted lines extending from the loop
          near the zenith show the branches of the track where the communication payloads are
          inactive, as is here the case for the HEO satellite 2. From the diagram one can conclude that
          a fixed earth station (according to CCIR Recommendation 465 [1]) at this site can
          communicate with GSO satellite 1 and HEO satellite 1, even when the LEO system is in
          operation. On the contrary, the links with GSO satellite 2 and HEO satellite 3 could not co-
          exist with LEO satellite 3, since it passes both the other satellite positions.
          Assuming, the LEO satellites' orbit period were not adjusted with the Earth's rotation, then
          the LEO satellite tracks would scan across the sky like the lines on a television screen, and
          co-existence with neither GSO nor HEO satellites on the same frequencies would be
          possible.




          7.4 Scope for technical developments

          7.4.1 Signal to Noise (S/N) levels
          Satellite systems operate very close to theoretical signal to noise demodulation thresholds.
          There is virtually no scope for reduction in receive thermal noise levels at the satellite or at
          the mobile terminal as noise levels are dominated by the Earth's background thermal noise
          (290 K). The noise performance of modern amplifiers is almost
          insignificant against this background. The only scope to improve signal to noise margins
          (for example to provide shadow or in-building operation) is to improve satellite antenna
          gain.

          7.4.2 Hand-held terminal antennas
          Present operational mobile satellite systems provide voice services with medium gain
          steered antennas in the gain range 8 dBi to 15 dBi. Low data rate services can use unsteered
          lower gain antennas with gains between 0 dBi and 4 dBi.The challenge for UMTS is to
          provide voice telephony to hand-held terminals using unsteered low gain antennas. The
          hand-held target imposes practical limitations on the form of antenna and it is unlikely that
          antennas will have usable gains greater than 0 dBi. However this does not prohibit the use
          of higher gain antennas for particular applications or circumstances.

          7.4.3 Satellites
          Satellite technology and commercial launcher capabilities have matured over the past ten
          years allowing systems planners to design complex systems with confidence. However,

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          reliability is paramount for commercial satellite services and therefore only well
          established technology, proven in space, is normally considered for major projects.
          The satellite antenna is a critical system element. In order to allow operation with low-
          performance hand-held PES's, the satellite antenna must provide a high gain. This can only
          be achieved by using advanced array-type antenna technology, including electronic beam
          forming and beam steering. The resulting spot (cell) diameters on the Earth's surface are
          typically in the range 1 000 km to 3 000 km.

          7.4.4 Digital modulation techniques
          The potential capacity of any satellite system is limited essentially by the availability of
          frequency spectrum and onboard satellite DC power. Hence, for most cost effective
          operation, it is of paramount importance that power and spectrally efficient transmission
          schemes are employed. Current research is continuing to make worthwhile progress in
          this area.

          7.4.5 Voice coding
          Lower bit rate voice codecs have been widely used in mobile satellite systems compared to
          terrestrial systems to reduce power and spectrum requirements. Continuing codec
          development, coupled with advances in semiconductor integration, is likely to yield
          improved speech quality and some reduction in overall power/spectrum demands. Target
          performances for UMTS speech codecs have been set for both terrestrial and satellite
          components, taking into account the progress that is expected to be made by the time
          UMTS is introduced.

          Growth drivers of mobile satellite communication services

          Deregulation
          Governments throughout the world are opening up their telecommunications systems
          whether it be through spectrum allocations, privatization, competition or access.
          Governments have realized that there is a strong correlation between telecommunications
          services and economic growth. Therefore they have started to knock down the walls that
          existed within their telecommunications markets and are encouraging investment in the
          latest technologies so that their countries do not fall behind in communications.

          Technology
          Technological developments have improved the power and versatility of satellites, today
          they have greater capacity and lower costs. For instance, the smaller size of many of
          today’s satellites lowers the cost of launching satellites. At the same time recent digital
          technologies (TDMA - Time Division Multiple Access, CDMA - Code Division
          Multiple Access) are being applied to satellite systems which a increases capacity and
          lowers the cost of launching a system.

          Globalization


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          People no longer are isolated from the world. People are affected by trade like never
          before; Nike and Gillette are no longer just U.S. companies. Because people are traveling
          halfway around the world on a moment’s notice, there is a demand for communications
          services that allow people to stay in touch no matter where they are. People want
          to be able to make a phone call and receive one -- they want one telephone number that can
          be used anytime, anywhere in the world. Thus, we feel the development of the global
          economy is a key driver of the mobile communications business.

          Economic growth
          Economic growth throughout the world has increased living standards which also drives
          demand for communications services. As individuals increase their economic stature, one
          of the first things they desire is a phone. This is a positive for satellite service providers. As
          developing economies continue to grow and enter the global economy, the demand for
          satellite services will increase because people will be able to afford it, and the need for
          mobile services will increase.


          Demand for phone service
          More than 3 billion of the world’s people do not have phone service. The waiting list for
          landline telephone service has over 50 million names with the average wait greater than 1.5
          years. On average, there are slightly fewer than 12 phones lines per every 100 people in the
          world. This is far lower than what exists in developed countries such as Sweden (68 lines)
          and the U.S. (60 lines). We believe that because the wait is so long, many do
          not even attempt to get service -- this could understate the actual number of people
          waiting for phone services. Atthe same time, Iridium (through research by Booz Allen and
          Gallup) has determined that the demand from worldwide travelers for mobile satellite
          services will be 42 million people by the year 2002. Regardless of how youlook at the
          numbers, there is a significant amount of people without phone services throughout the
          world. Also, phone services are not developed in many countries, so travelers are unable to
          access a reliable phone. Satellite communications services will solve the needs of
          worldwide travelers and provide phone services to many areas of the world that currently
          do not have phone service.

          Mobile communications trends
          Cellular demand continues to explode throughout the world with some estimates of 500
          million subscribers by the year 2002. Cellular phone bills in Third World countries are
          higher than the average bill in the U.S. This suggests that demand for mobile
          communications services continues to grow at a very fast pace and that developing
          countries are willing to pay for phone services. An ubiquitous phone service offered by
          satellite companies will benefit from these trends in cellular communications.

          Who will use satellite communications systems?

          Global roamer


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          The first type of satellite user will be the global roamer. The global roamer consists mainly
          of business travelers who want to have the ability to make and receive calls anywhere in
          the world. Iridium has conducted extensive analysis of this market and concluded that this
          market will consist of 42 million people by the year 2002.

          Cellular extension
          The second type of user will be individuals who wish to extend their cellular coverage to
          areas where no service currently exists. Both Globalstar and Iridium plan to offer dual
          mode phones which will work with GSM/TDMA/CDMA cellular systems and satellite
          communications systems. An example of a dual mode user would be would be an
          individual who lives in Chicago and travels to upstate Montana for a hunting trip. The
          person would normally have cellular service from Ameritech but that coverage does not
          include upstate Montana where no cellular coverage exists. To be able to receive service on
          their Ameritech system (same phone number) in Montana, the individual would sign up
          with Iridium for dual mode service. Signing up would mean that once the
          individual got out of the range of their Ameritech systems, they could hit a switch on their
          Iridium phone and make or receive calls outside of their Ameritech coverage zone routed
          through the Iridium satellite system. This would allow for ubiquitous service for cellular
          users even when they are out of range of their current cellular system.




          Landline extension
          The third type of satellite user will an individual who wants landline extension. In this
          instance a satellite company would install a fixed telephone booth in a rural area (e.g. in the
          outskirts of India). This would enable a rural town, which currently has no means of voice
          communications, to communicate with an urban area where medical, police or other
          services exist. The rural town could also use the phone to call suppliers of staple products.
          Fixed satellite service would mostly be used when a landline system is uneconomical or
          technologically incapableor serving a particular location. Vodaphone has been using fixed
          wireless phone booths in South Africa and has averaged 800 minutes of use per booth.

          Global Mobile Satellite communications services
          Satellite communication systems are designed to provide voice, data, fax, paging, video
          conferencing and internet services to users worldwide. Through satellite based systems,
          users will be able to make a phone call from an African safari or while sailing around the
          world. No matter where users are, they will be able to communicate with clients,
          customers, associates, friends, and family anywhere in the world. In addition, satellite
          communications will allow countries to provide phone services without large investments
          in landline or wireless systems. Satellite communications will be one of the fastest growing
          areas within the communications industry.



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          GEO satellite networks offer great potential for multimedia applications with their ability
          to broadcast and multicast large amounts of data over a very large area thus achieving
          global connectivity. Internet via satellites, in particular, GEO satellites, have the following
          merits:


          High bandwidth.
                 A Ka-band (20-30Ghz) satellite can deliver throughput of gigabits per second rates.
          Inexpensive.
                 A satellite communications system is relatively inexpensive because there are no
                 cable-laying costs, and one satellite covers a very large area.
          Untethered communication.
                 Users can enjoy untethered mobile communication anywhere within the footprints
                 of the satellite.
          Simple network topology.
                 Compared with the mesh interconnection model of the terrestrial Internet, GEO
                 satellite networks have much simpler delivery paths. The simpler topology often
                 results in more manageable network performance.
          Broadcast/multicast.
                 Satellite networks are naturally attractive for broadcast/multicast applications (such
                 as MBONE). In contrast, multicast in a mesh interconnection network requires
                 complicated multicast routing. Performance can vary for each multicast group
                 member and is dependent on the route from the source.
          Video Conferencing:
                 GEO satellites can provide better quality in video conferencing due to the
          available bandwidth and simpler network topology.



          APPLICATION OF MOBILE SATELLITE SYSTEM




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          Conclusion
          Satellite networks promise a new era of global connectivity, where geographical isolation
          will no more be the barrier among continent. In this work, we have shown that indeed
          many popular Internet applications perform to user expectation over satellite networks,
          such as video teleconferencing, bulk data transfer, background electronic mail, and non-
          real time information dissemination. Some other applications, especially highly interactive
          applications such as web browsing.

          Even though the systems will eventually be built, conflicts over billing and control still
          remain. Operators of phone services in Europe don't like the idea of systems that would
          bypass their equipment and, probably, taxes and fees. Some very complex negotiating will
          be needed to calm objections by such bodies, and no doubt some interesting compromises
          will be made. The iridium project's future is still uncertain, but looks rather promising as
          far as space ventures go. The use of such a system in helping promote the thought of the
          world as a "global village" deserves thought and, I think, respect. The global
          communications industry has grown like no one ever thought it would, and the future for
          iridium or an iridium-like system is, in the long run, assured

          Thus the field mobile satellite communication have better prospectus in the future. As
          conditions on mars are seems to be favourable for human life.



          BIBLIOGRAPHY

          Mobile and personal communication systems and services. By RAJ PANDYA

          www.google.co.in

          www.howstuffworks.com

          www.isl.com

          www.whatis.com

          www.ee.surrey.ac.uk

          www.tt.dk

           www.gd-decisionsystems.com/satelliteservices

          www.etsi.fr

          www.futron.com


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