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Điện thoại di động mạng lưới Radio P14

Mobile Satellite Communication∗ In principle communications satellites provide the same connectivity as terrestrial (wireless and wireline) networks. The advantages of satellites, such as fast wide-area coverage, flexible transmission parameters and cost independence due to distance, are compared with the disadvantages, such as restricted channel capacity because of the frequencies available, orbital positions, need for line-of-sight connectivity and high initial investment besides relatively lon

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  1. Mobile Radio Networks: Networking and Protocols. Bernhard H. Walke Copyright©1999 John Wiley & Sons Ltd ISBNs: 0-471-97595-8 (Hardback); 0-470-84193-1 (Electronic) 14 Mobile Satellite Communication∗ 14.1 Fundamentals In principle communications satellites provide the same connectivity as ter- restrial (wireless and wireline) networks. The advantages of satellites, such as fast wide-area coverage, flexible transmission parameters and cost indepen- dence due to distance, are compared with the disadvantages, such as restricted channel capacity because of the frequencies available, orbital positions, need for line-of-sight connectivity and high initial investment besides relatively long signal propagation times. As a result, only certain application areas have been developed for satellites in the past. 14.1.1 Application Areas Satellites are being widely used for distribution functions, e.g., for transmit- ting television and radio programmes as well as for data. Existing communi- cations networks can be totally bridged through the use of satellite systems. Satellite communication, which until recently was almost only exclusively used for navigation and aviation as well as in land vehicles, is a branch that opens up a totally new world of applications. Satellite paging along with GPS (Global Positioning System) and GLONAS have recently been introduced for civil use. The interest in global personal communications is leading to big efforts in the development of new satellite systems that operate at the low orbital heights. Currently mobile satellite systems are being used mainly in areas where no other terrestrial communications systems are available (on the open seas, in the desert, in rural regions, etc.). These systems are also attractive to users who operate internationally and otherwise use different kinds of terres- trial mobile radio systems, requiring them to carry terminals with different standards. A differentiation is made between worldwide, regional and national systems, depending on the coverage area of a satellite system. In terms of institutional and organizational structure, a distinction is made between international, na- tional and private operators of satellite systems. Tables 14.1–14.5 present a compilation of the known parameters of all systems currently in development. ∗ With the collaboration of Branko Bjelajac and Alexander Guntsch
  2. Table 14.1: Narrowband satellite systems concentrating on telephony applications—Part 1 716 System Globalstar ICO IRIDIUM Odyssey Ellipso ECCO (later: Aries) Company Loral, Qual- ICO Global Motorola TRW, Teleglobe Mobile Constellation comm, Alcatel Communications Canada Communications Inc., Telebras Espace Holdings Orbit LEO (circular) MEO (circular) LEO (circular) MEO (circular) MEO LEO (circular) (circ. + ell.) Path altitude [km] 1414 10 355 780 10 354 520–7846 (ell.) 2000 8040 (circ.) No. of sats. + spare 48 + 8 10 + 2 66 + 12 12 + 2 on the 14 + 3 11 + 1 (later sats. Ground add’l 35 + 7) No. of orbits 8 2 11 3 3 (2 ellipt. + 1 1 equat. (later circular) add’l 7) No. of ground 100 12 21 7 ≥ 20 11 (more later) stations 14 Inclination [°] 52 45 86 50 116.5 (ellipt.)/ 0 0 (later 62) (circ.) Min. elevation [°] 10 10 10 20 25–30 5 Cells/sat. 16 163 48 37 61 32 ISL — — 4/sat. — — — Access methods CDMA FDMA/TDMA FDMA/TDMA CDMA CDMA CDMA Duplex method FDD TDD Cluster size 6 180 (global) 6.3 5 (USA) 4 (Europa) Error handling FEC FEC FEC FEC 1 r = 1–2 3 MS: r = 43 r= 1 2 Voice: r = 1 2 1 Grnd.st.: r = 2 Data: r = 14 Mobile Satellite Communication
  3. Table 14.2: Narrowband satellite systems concentrating on telephony applications - Part 2 14.1 System Globalstar ICO IRIDIUM Odyssey Ellipso ECCO (later: Aries) Modulation QPSK QPSK QPSK BPSK No. of channels/sat. 2700/ 4500/ 4070/2.4 kbit/s 2800 1000 2.4 kbit/s 4.8 kbit/s Fundamentals No. of channels in entire 130 000 45 000 283 000 27 600 11 000 (later system 46 000) Channel bandwidth [kHz] 25.2 Transmission rate [kbit/s] 2.4–9.6 4.8–38.4 2.4 2.4–9.6 0.3-9.6 bis 9.6 Voice transmission rate 2.4/4.8/9.6 4.8 2.4 9.6 [kbit/s] Bit-error ratio voice/data 10−3 /10−6 10−2 /10−3 10−3 /10−5 10−3 /10−6 Frequency UL [MHz] 1610–1621.35 1980–2010 1621.35–1626.5 1610–1621.35 1610–1621.35 1610–1621.35 Frequency DL [MHz] 2483.5–2500 2170–2200 1621.35–1626.5 2483.5–2500 2483.5–2500 2483.5–2500 Bandwidth UL+DL [MHz] 27.85a 70 5.15 27.85a 27.85a 27.85a Frequency GW–Sat [GHz] 5.091–5.25 5.15–5.25 29.1–29.3 29.1–29.4 15.45–15.65 5.05–5.25 Frequency Sat–GW [GHz] 6.875–7.055 6.975–7.075 19.4–19.6 19.3–19.6 6.875–7.075 6.825–7.025 Satellite weight [kg] 450 2300 689 2000 689 & 877 425 Antenna type Planar horn Planar Dual refl. Satellite intelligence Available Available Available No No No Satellite transmission mode Transparent Regenerative Regenerative, Transparent Transparent Transparent switching Transmitter power [W] 1100 5000 1430 4500 2300 815 Power reserve [dB] 3–10 dyn. 16 6 Max. delay [ms] 11.5 8.22 44.3 Eavesdropping security Possible High Possible 717
  4. 718 Table 14.2: Narrowband satellite systems concentrating on telephony applications—Part 2 (continued) System Globalstar ICO IRIDIUM Odyssey Ellipso ECCO (later: Aries) Terminal (mobile) Dual-mode Dual-mode Dual-mode Dual-mode Dual-mode Dual-mode Terminal trans. power [W] 3.8 Services Voice, data, Voice, data, Voice, data, Voice, data, Voice, data, Voice, data, fax, RDSS, fax, RDSS, fax, RDSS, fax, RDSS, fax, RDSS, fax, RDSS, SMS SMS SMS SMS SMS SMS RDSS accuracy [km] 0.3–2 0.5 Coverage area 74° S–74° N Global Global Global 55° S–90° N 23° S–23° N (later global) Availability [%] 90–95 90–95 99.5 (User) 99.9 (Grnd.st.) No. of users [Mill] 2–5 1.4 2.3 1.0 > 1.0 14 System cost [Mill. US$] 2.6 2.6 4.4 3.2 0.9 0.55 (later 1.7) Price of terminal [US$] 750 1000 2000–3000 550 1000 1500 Cost/min [US$] 0.35–0.53 1–2 3 0.65 0.12–0.5 Start comm. operations 1998 2000 1998 1998 2000 2000 Useful life [years] 7.5 12 5–10 15 5 5 German partners DB Aerospace Vebacom Licence 01/95 granted 10/95 01/95 granted 01/95 granted 06/97 granted 06/97 granted by FCC frequency alloc. by FCC by FCC by FCC by FCC by ITU a The UL and DL frequency bands are occupied by CDMA systems simultaneously Mobile Satellite Communication
  5. Table 14.3: Narrowband satellite systems concentrating on message transfer 14.1 System Orbcomm E-Sat Faisat GE Starsys GEMnet LEO One Company Magellan Sys- Echostar Final Analysis, GE Ameri- CTA Orbital dBX Corp. tems, Teleglobe, Communica- Polyglott Enterp., can Comm., Sciences Orbital Science tions VITA CLS North America Fundamentals Orbit LEO LEO LEO LEO LEO LEO Path altitude [km] 775 1260 1067 1000 950 No. of satellites 28 + 8 6 26 + 4 24 38 48 No. of orbits 8 6 4 8 No. of ground stations min. 1 per country USA 3, others in other countries Inclination 45° 50° Intersatellite links — — — — — — Access method CDMA CDMA Transmission rate [kbit/s] 0.3–2.4 UL: 1.2–19.2 2.4–9.6 DL: 1.2–38.4 Frequency uplink [MHz] 148–149.9 Fin. An.: 455–456, 148–149.9 148–150.05 459–460 VITA: 148–149.9 Frequency downlink [MHz] 137–138 400–401 (Fin. An. 137–138 137–138 and VITA) Bandwidth UL+DL [MHz] 2.9 2.9 3.05 Frequency GW–Sat [MHz] 148–149.9 148–150.05 Frequency Sat–GW [MHz] 137–138 400.15–401 Satellite weight [kg] 46 80 125 719
  6. 720 Table 14.3: Narrowband satellite systems concentrating on message transfer (continued) System Orbcomm E-Sat Faisat GE Starsys GEMnet LEO One Terminal (communicator) × × × × × × Services (Monitoring, × × × × × × control, message transfer) Coverage area Global N. America Global Global Global 65° S–65° N 14 System costs [Mill. US$] 0.35 0.05 0.25 0.17 0.16 0.25 Price of terminal [US$] 100–500 Start comm. operations 1998 1998 2002 1999 1999 2000 Useful life 4 5–7 5 Mobile Satellite Communication
  7. Table 14.4: Broadband satellite systems concentrating on data transfer 14.1 System Teledesic Celestri LEOa M-Star SkyBridge Company Teledesic Motorola Motorola Alcatel Espace, Loral Space & Comm. Orbit LEO LEO LEO LEO Path altitude [km] 1375 1400 1350 1457 Fundamentals No. of satellites 288 63 72 64 No. of orbits 12 7 12 16 No. of ground stations 2 control + 6 antenna 6 centres, many GWs Inclination 40° 48° 47° No. of cells per sat. 576 260 Intersatellite links 8 per sat. (visual) 6 per sat. (visual) 4 per sat. No Access method DL: Asynch. TDMA DL: FDM/TDM UL: MF-TDMA UL: FDM/TDMA No. of channels per satellite 125 000 at 16 kbit/s No. of channels in total Simult. 36 Mill. Simult. 395 000 at 64 kbit/s, system in total 1.8 Mill. users at 64 kbit/s Transmission rate [Mbit/s] UL: up to 2, DL: up to Up to 155.52 2.048–51.84 0.016–60 64 Frequency UL [GHz] 28.6–29.1 and 28.6–29.1 & 29.5–30 47.2–50.2 14–14.5 27.6–28.4a Frequency DL [GHz] 18.8–19.8 and 18.8–19.8 & 19.7–20.2 37.5–40.5 11.7–12.7 17.8–18.6a a Motorola in 1998 has joined the Teledesic consortium 721
  8. 722 Table 14.4: Broadband satellite systems concentrating on data transfer (continued) System Teledesic Celestri LEO M-Star SkyBridge Bandwidth UL+DL [MHz] 2600 2000 6000 1500 Frequency GW–Sat [GHz] In above band In above band In user-sat. band 12.75–13.25 and 13.75–14 and 17.3–17.8 Frequency Sat–GW [GHz] In above band In above band In user-sat. band 10.7–11.7 Terminal Fixed terminal (Ant.-Ø Fixed terminal Terminal Fixed terminal 0.16–1.8 m) (Ant.-Ø 0.66–1.5 m) Trans. power terminal [W] 0.01–4.7 Services Multimedia, video, data Multimedia, video, data Data, high-rate network Multimedia, video, data connection 14 Coverage area 95 % of area and 100 % 70° S–70° N (with Global Global of population Celestrib global) System costs [Mill. US$] 9 12.9 6.1 3.5 Start comm. operations 2002 2002 2002 2002 Useful life [years] 10 8 Licence from FCC Granted March 1997 Application accepted Application accepted Application accepted a Gigalinks b The Celestri system consists of the Celestri-LEO, M-Star and Millenium (GEO systems) Mobile Satellite Communication
  9. Table 14.5: Broadband GEO satellite systems concentrating on data transfer 14.1 System Spaceway Express- Millenium Astrolink GE Star PAC 1–8 Inmarsat-3 way (Celestri- and Galaxy GEO) Sat. Company Hughes Hughes Motorola, Lockheed GE PanAmSat Inmarsat Vebacom Martin Comm. Americom Contractors Orbit GEO GEO GEO GEO GEO GEO GEO Fundamentals Path altitude [km] 35 786 35 786 35 786 35 786 35 786 35 786 35 786 No. of satellites 9 (Type 14 4 9 9 16 5 HS702) No. of different satellite 5 5 positions No. of ground stations 7 No. of cells per sat. 48 44 Intersatellite links Yes Yes No Access method UL: TDMA/ UL: TDMA/ FDMA FDMA DL: TDM DL: TDM No. of channels per sat. 276 480 at 64 at 16 kbit/s 125 MHz, 4 at 250 MHz No. of channels in total 248 832 588 000 576 at system 125 MHz Transmission rate [kbit/s] 16–6000 16–9600 384–40 000 Frequency uplink [GHz] 28.35–28.6 V- and 29.5–30 28.35–28.6 and 28.35–28.6 5.925–6.425 and Ku-band 29.25–30 and 29.25–30 29.25–30 Frequency downlink [GHz] 19.7–20.2 V- and 18.55–18.8 19.7–20.2 19.7–20.2 3.7–4.2 + 5 GHz Ku-band and + 5 GHz + 5 GHz in-band 19.7–20.2 in-band in-band 17.7–18.8 17.7–18.8 17.7–18.8 723
  10. Table 14.5: Broadband GEO satellite systems concentrating on data transfer (continued) 724 System Spaceway Express- Millenium Astrolink GE Star PAC 1-8 u. Inmarsat-3 way (Celestri- Galaxy Sat. GEO) Bandwidth UL+DL [GHz] 2a 1.25 2a 2a 1 Frequency GW–Sat [GHz] In-band In-band In-band 14.0–14.5 user-sat. user-Sat. user-sat. Frequency Sat–GW [GHz] In-band In-band In-band 11.7–12.2 user-sat. user-sat. user-sat. Terminal Terminal Terminal Terminal Terminal Terminal Terminal Terminal (Ant.-Ø 0.7 m) Services Multimedia, Video, data Data, video, Data, video, Voice, data, Voice, data, video, data, broadcast Internet audio fax, multi- fax, Pos. det. voice media, RDSS 14 Coverage area Continents Global America Global America, Global 70°§–70° N ex. parts of Europe, Russia Asia, West Pacific, Caribbean System costs [Mill. US$] 3.2 3.9 2.3 9 4 0.69 Price of terminal [US$] 1000 Several 100 Start comm. operations 2000 2001 2000 In operation since 1997 Useful life [years] 15 11 13 Licence from FCC 05/97 05/97 05/97 a Several systems share frequency band Mobile Satellite Communication
  11. 14.1 Fundamentals 725 14.1.2 Satellite Organizations Until now, the commercial operations of international satellite systems have almost exclusively been carried out by state-run operating companies such as Intelsat, Eutelsat and Inmarsat (see Appendix B). The primary responsibility of Intelsat is to provide regular radio services worldwide, namely telephony and data links as well as the transmission and distribution of TV and radio programmes. Satellites for regular radio services specifically for Europe are operated by Eutelsat (European Telecommunications Satellite Organization). The organization Inmarsat (International Maritime Satellite Organization) was set up to provide a worldwide maritime satellite radio service. These operating organizations are all structured in a similar way. They are based on international agreements that were ratified as national law by the participating countries. These member countries establish the objectives of the organization and entrust national telecommunications authorities or operating companies with creating satellite earth stations. The satellite or- ganizations of the countries have no customer contact, but instead provide satellite capacity and, among other things, specify the frequency ranges avail- able for the operation of satellite systems. The operating companies design the satellite services for the customers. State-run operators of satellite systems are competing against private op- erators, international companies and consortia, which mainly concentrate on implementing new concepts such as worldwide satellite-based mobile commu- nications systems. These organizations focus on providing almost complete wide-area coverage of the earth’s surface using Low Earth Orbit (LEO) and Medium Earth Orbit (MEO) satellite systems. Some of these satellite systems are introduced in the following sections, accompanied by advantages and dis- advantages of geostationary and non-geostationary satellite systems. 14.1.3 Satellite Orbits Path height is possibly the most important aspect of describing satellite sys- tems. As shown in Figure 14.1, the categorization breaks down into LEO, MEO, Highly Elliptical Orbit (HEO) and Geostationary Orbit (GEO) systems. LEO system satellites are located between a height of 200 km and the inner Van Allen belt (named after James Alfred van Allen, b. 1914, an American physicist. The Van Allen belts are two radiation belts of the earth (zones with high-intensity ionizing radiation)) at a height of 1500 km. The satellites of the MEO systems are located between the two Van Allen belts, between 5000 and 13 000 km. LEO and MEO satellites are in circular orbits. In addition to the circular systems, there are also HEO systems that are able to achieve a better coverage of densely populated areas using elliptical orbits without having to forfeit the advantages of low orbits. During the 1960s, developers of the first systems started discussing the advantages and disadvantages of LEO, MEO and GEO systems. The technical
  12. 726 14 Mobile Satellite Communication MEO Systems ICO, Odyssey (10360 km) Inner Van Allen HEO System Belt Earth LEO Systems IRIDIUM, Teledesic LEO System (780, 700 km) Globalstar (1400 km) Outer Van Allen Belt GEO Systems, e.g., INMARSAT (35 786 km) Figure 14.1: Orbits of satellite systems advantages of LEO systems, such as lower signal propagation times and path loss, were weighed up against the practicability of geostationary systems. The first test satellites such as TELSTAR operated in the lower orbits. Before the mobile applications of satellites were considered as a possibility, GEO systems were viewed as being superior. The satellite launches, particularly during the 1960s, were still unreliable and global coverage was less important than it is today, so the GEO systems, which can manage with only one satellite, were favoured. With interest in mobile communications over satellites increasing, there was a renewed effort to speed up the development of systems with low earth orbits. 14.1.4 Elevation Angles and Coverage Zones With LEO satellite systems the radio coverage zone on the ground is divided, following the usual cellular principles, into individual cells by a satellite to allow the reuse of frequency bands within the entire coverage zone. The size of the illuminated zone is established by the minimum elevation angle min , which can be determined from the maximum possible distance between a mobile terminal and the satellite (see Figure 14.2). The elevation is re + h = arccos sin φ (14.1) d
  13. 14.1 Fundamentals 727 §¨¥ © ¨§¦¤¡ © ¥£ h d re ε re φ ¡ ¢  ¥$#¦ §¥§  %  £" !   Figure 14.2: Elevation angle in LEO systems ° Theoretically, elevation angles of 0 are possible. However, there is a tendency to maintain the minimum elevation angle min , which for practical reasons ° is typically at 10 to avoid larger areas without radio coverage because of shadowing [12]. The radius of a coverage zone depends on the earth’s radius re and the path height h of the satellite above the earth’s surface: re rcov = re arccos cos min − min (14.2) re + h With a given maximum distance dmax and a minimum elevation min , the orbit height works out as h= d2 + 2dre sin max min 2 + re − re (14.3) The area covered by the satellite is then 2 Acov = 2πre (1 − cos φ) (14.4) These equations can be used to determine the radius of the coverage zone, the orbit speed of the satellite and the travel time around the earth. If the number of cells per coverage zone is available then the size of the cells can be established and an initial prediction made about the amount of time a user spends in a cell. The above values for some orbit heights are given in Table 14.6. For example, an IRIDIUM satellite (at a 780 km height) travels at approximately 28 500 km/h over a stationary earth point, and is only visible from there for about 10–12 min (see Figure 14.39). 14.1.5 Frequency Regulation for Mobile Satellites Frequency allocation is an important aspect in the planning of satellite sys- tems. Since it is the international authorities who allocate the frequencies,
  14. 728 14 Mobile Satellite Communication Table 14.6: Circular satellite orbits Orbit height Travel time Orbit speed Radius of coverage [km] [km/s] zone [km] 400 1h 32 min 7681 1333 600 1h 36 min 7569 1737 800 1h 40 min 7463 2065 1000 1h 45 min 7361 2349 1200 1h 49 min 7263 2592 1400 1h 54 min 7168 2805 2000 2h 07 min 6906 3320 35786 23 h 56 min 3075 6027 operators of global satellite systems must negotiate licensing agreements with all the countries in which they want to offer their services. Member states of the International Telecommunications Union (ITU) are committed to the resolutions of the World Radio Conference (WRC). Commercial satellite radio is currently using frequencies mainly in the C- and Ku-bands. In the C-band these are 5.925–6.425 GHz range for the uplink and 3.7–4.2 GHz for the downlink; and in the Ku-band they are 11.7–12.2 GHz for the downlink and 14–14.5 GHz for the uplink. Because of the growing interest in satellite communications, additional frequencies in the S- and the K/Ka-bands were reserved for satellite radio at WRC 1992. The frequency bands 1980–2010 for the uplink and 2170–2200 MHz for the downlink were made available in the S-band for the satellite segment of UMTS. With the availability of 3.5 GHz of bandwidth each (27.5–31 GHz and 17.7–21.2 GHz), this means that considerably more bandwidth is now available in the Ka-band than was provided in the previous frequency bands. Since the initial networks are mostly being built by American companies, applications for licences are first made to the American regulatory body, the Federal Communications Commission (FCC). These licences then establish the pattern for regulation in other countries. Accordingly, the first frequencies allocated by the FCC for mobile satellite systems were in the 1610–1626.5 MHz band. As is shown in Figure 14.3, 5.15 MHz was allocated to the IRIDIUM- TDMA system and 11.35 MHz to the CDMA systems to be shared by Odyssey and Globalstar. In case only one of the CDMA systems becomes operational, the frequency band 1616.25–1621.35 MHz will be renegotiated. 14.2 Geostationary Satellite Systems The geostationary orbit is at an altitude of 35 786 km above the earth’s surface. Satellites in this circular orbit have an angular velocity that is the same as the rotational speed of the earth. Therefore to an observer on the ground the satellite is at in a fixed position. Because a geostationary satellite can
  15. easily be bridged. • Coverage of large areas with only one satellite. Large distances can • Simple configuration. geostationary systems include: The advantages of geostationary satellite systems compared with non- can follow these positional changes better than mobile stations. bodies, positional corrections are essential for the satellite. Ground stations drift up to 60 km from its position owing to the influence of other celestrial Figure 14.3: Frequencies for satellite radio © ¨ § ¥£ § ¤¤¢¦¤¢  r ¥ ©¤¨¤§¢¦£¢  s r i€ s tx ir t¤¤¢¦¤¢  s © ix ¨ § ¥ £ ¡ u x f Y„ g e`„ ¤g f d ¤g h `g g`g e ¤g f `c d `c E%U5 & # T    ¤ ¨ ¤  ¨ ¤¤   ¨   ¤¤¢¦¤¢  © ¨ § ¥£ ¡ ¤¤¢¦¢” © ¨ § ¥£ © ¨ § ¥ ¤¢“£ F s yi yi s rir s ii s stx stp tp s u s ur s u s yr s € u x qx vr yr vr uq qiq x u u u u f Yc ƒ Yc h „ `c g Yc c `c e `c – f • d ƒ h &EUT5 % #    ¤ ¨ ¤   ¨    ¨ ¤¤           ¨ ¤¨¤ ¤ ¨ ¤ ¤ ¨    ¨    ¨ ¤¤  WQ 1 R5E'P ( C! ‰7 F ! DE$! 59`7 £ 59 a 6 1CG F % # B A 7 B b '$! b59 ¦£ B`Y¤¦6 D % # B a 79 B  ¡  ’ §   RW Q ! 3 3 4 ¤C E¤C‘¤"E'P ( "5"3 ˆ† … d d '‡Uƒ ‚`¤gY`c ƒ ‚c g Y„ `¤Yd`c ƒ fc `g ‚```d`c ecd ```c EC! `7Y¤B“6 D% # B 9 D% E#C! ¤5@¤86 B A9 7 7 ! 3 3 3 2 1 !  5"45¢$0) X W  § V  T SP RQQP © 2 ! ¤§5'Q ¤¤ U¤E¤'EEIH¢1CG F r © ¨ § ¥ ¤¤¢¦£ ( © ¨ § ¥£ ¤¤¢¦¤ w ee `¤h`g ee g `¤`g `e`¤g e e `e``c e f ipr eed ```c r r ip s €y q ivw s tp iq f u qix iq q u &'#$" % !   ¨  ¤ ¨ ¤¤   ¨   ¤ ¨ ¤¤  ¨ ¤        ¨ ¤ 729 Geostationary Satellite Systems 14.2
  16. 730 14 Mobile Satellite Communication • Minimal routing problems in the coverage areas because of coverage zones of the satellites. • The low relative movement of the earth only causes insignificant Doppler shifts of the signals. The disadvantages of using a GEO system are as follows: • A need for high transmitter power and large receiving antennas (because of the distances attenuation is at approximately 200 dB). • Regions at a very high geographical latitude cannot be serviced. The elevation angle, which decreases the higher the latitude, falls below an elevation of 10◦ at a degree of latitude ϕ > 72◦ , so that perfect connec- tion can no longer be guaranteed. For example, in Germany (47 –55 ° ° ° ° North) elevation angles between 20 and 30 are achieved. • Coverage to urban areas is a problem because of shadowing. Reception in these degrees of latitude is only possible through the use of directional antennas. • Signal propagation times are large because of the large distances. One way propagation delay is approximately 125 ms. Therefore ARQ (au- tomatic repeat request) data link control protocols with error handling designed for wired networks are not suitable. • It is expensive to launch satellites into a geostationary orbit compared with lower orbits. These disadvantages are dominant as far as utility of GEOs are concerned for mobile communication. Such orbits are primarily suitable for stationary services. European telecommunications network operators use the Inmarsat satellite system operated by Inmarsat, an organization established in 1979 with its headquarters in London and supported by 63 member countries. The mobile radio services offered by Inmarsat were initially restricted to maritime appli- cations. Inmarsat is still the only supplier today that offers global mobile satellite communications on the sea, on land and in the air. The Inmarsat satellite system consists of 11 satellites: 4 operating satel- lites and 7 reserve satellites. Reserve satellites can be leased from different organizations. All Inmarsat satellites are located in a geostationary orbit. The four oper- ating satellites are distributed in the equatorial plane so that they can cover the entire globe, except for the polar regions. The reserve satellites are located in positions in the immediate vicinity of the operating satellites so that they can be replaced if necessary. The four coverage zones of the operating satellites are the West Atlantic, the East Atlantic, the Indian Ocean and the Pacific. As can be seen in Figure 14.4,
  17. 14.2 Geostationary Satellite Systems 731 Figure 14.4: Configuration of Inmarsat operating satellites there is a difference in how much the coverage zones overlap each other. The largest overlap is between the West Atlantic and the East Atlantic. The reason for this is to provide better servicing of the large traffic between America and Europe. 14.2.1 Inmarsat-A The Inmarsat-A service caters for the following applications: • Self-dialled telephone connections from/to user terminals. • Self-dialled (half-duplex) telephone connections from/to user terminals. • Calls to terminals and retrieval of data stored in terminals. The infor- mation is provided only if the caller provides unique identification. • Use of telephone connections to transmit fax messages and data (up to 9600 bit/s with a modem). • Data transfer from terminals at a data rate of 64 kbit/s. • Special applications for broadcast transmission, video freezing frames and commentator transmission routes. Inmarsat-A is the first of six systems being operated by Inmarsat. It went into operation in 1979, and transmits analogue modulated signals. Portable termi- nals require a parabolic antenna of approximately 1 m diameter and weighing 20–60 kg. With stationary terminals the antennas are adjusted by hand in the direction of the desired satellite.
  18. 732 14 Mobile Satellite Communication Table 14.7: Technical data on Inmarsat systems Inmarsat A B M C Aero Channel access Aloha Aloha S-Aloha Aloha Aloha Modulation BPSK QPSK BPSK BPSK BPSK Coding BCH 1/2 FEC 1/2 FEC 1/2 FEC 1/2 FEC Transmis. rate [kbit/s] 4.8 24 3 0.6 0.6 14.2.2 Inmarsat-B The Inmarsat-B system has been in existence since 1993 as a further develop- ment of Inmarsat-A, the major improvement being the change over to digital transmission. The traffic channels transmit digitally using forward error cor- rection. It has greatly improved the quality of voice and data transmission. The frequency band and the capacity of the satellites are being used more economically. Improvement in satellite technology have produced a reduction in the transmission costs for a call. The communications services offered by Inmarsat-B are comparable to those of Inmarsat-A (see Table 14.7). 14.2.3 Inmarsat-C Inmarsat-C was developed for worldwide telex and data transmission (X.25) from small mobile terminals. The following applications are some of those available: • Data transmission in X.25 mode from/to mobile terminals (Mobile Earth Station, MES) at data rates of up to 600 bit/s. • Text transmission from the MES in X.25 mode and output of messages on fax equipment. • Data transmission in X.400 mode to mailboxes. • Position location of MES by a central switching centre. • Formation of closed user groups through the allocation of appropriate telephone numbers. The net bit rate transmitted in both directions is 600 bit/s. Dynamic inter- mediate storage (store-and-forward transmission) located in the earth station guarantees reliable data transfer. An earth station automatically makes an- other request for erroneous or missing data blocks. The data blocks are not forwarded to the receiver until the message has arrived error-free at the des- tination. Signals are scrambled and spread by the sender to reduce tramsmission errors that occur owing to fast fading. The length of messages is limited to 32 kbytes, primarily because of the intermediate storage.
  19. 14.3 Non-Geostationary Satellite Systems 733 The terminals used in the Inmarsat-C service are relatively small and weigh approximately 5 kg. They consist of a transmit/receive antenna unit for all directions, a transmitter/receiver, an operating unit and a printer (if required). The power consumption in receiving mode is 25 W and in sending mode 100 W pulsed. 14.2.4 Inmarsat-Aero The Inmarsat-Aero service, which was introduced in 1992, enables communi- cation from and to airplanes (see also the TFTS service in Section 4.1). Two types of antenna are available: • Low-gain antennas (0 dBi) for data transmission, mainly for internal operating purposes for airplanes. • High-gain antennas (12 dBi), which allow higher data rates, thereby enabling voice links to the ground. 14.2.5 Inmarsat-M The Inmarsat-M service, a digital expansion of the Inmarsat-A service, has been available since 1993. Inmarsat-M terminals are small and considerably lighter in weight than Inmarsat-A terminals. Mobile radio users can make use of the following communications services: • Telephone • Data transfer (up to 4800 bit/s) • Fax • Variations of the Inmarsat-C service Because of the low transmission rate of 3 kbit/s, the voice quality with Inmarsat-M is poorer than in connections with Inmarsat-A and -B. Because of the terminal’s smaller antenna (30–50 cm) and lower transmitter power, the gain in the Inmarsat-M system is 8 dB less than with Inmarsat-A. It was necessary to reduce the data rate to 3 kbit/s to achieve good transmission quality despite the poor power budget. 14.3 Non-Geostationary Satellite Systems The advantages of LEO and MEO/ICO systems compared with GEO systems are: • Lower transmitter power required because of low orbit heights. • Higher average elevation angle. Because of the large number of satellites, the satellite with the shortest distance to the mobile radio user can always be selected.
  20. 734 14 Mobile Satellite Communication • High operating reliability through increased redundancy. • Good coverage of regions with highest geographical latitude (e.g., polar regions). • Small signal propagation time. However, there are also disadvantages due to the lower orbits: • Short link duration to satellites due to changing elevation angles. • Smaller coverage area per satellite. • Complex system control. At their nearest point to earth (perigee), HEO satellites approach up to a couple of hundred kilometres from the earth’s surface in order to be able to reach the furthest distance (apogee) in the area of the geostationary orbit. They are used there for communication purposes because that is where their orbit speeds are the lowest and a satellite therefore remains visible for a long time from a certain point. Satellites in these orbits are always passing through the Van Allen belt and are consequently subject to increased radiation. 14.3.1 ICO In September 1994 Inmarsat founded a company called ICO (Intermediate Circular Orbit) to introduce and operate the Inmarsat-P21 project (see Ta- ble 14.1). ICO’s biggest investors are INMARSAT (150 million US$) and Hughes Space and Communications (94 million US$). The ICO system is the only mobile satellite system that has been initiated by an operator of an existing satellite system. ° ICO will be using 10 satellites in two orbits with 45 inclination to the equatorial plane (see Figure 14.22; the coverage zones of the ICO system are shown in Figure 14.5). Maximum use will be made of path diversity. It is very probable that two satellites will be visible at the lower elevation, and the satellite with the best channel quality will be selected. It is anticipated that the greatest proportion of users of the ICO system will be accessing through GSM dual-mode terminals, and will use the satellite system only if the GSM network cannot guarantee an adequate radio connection. ICO will be dispersing 12 earth stations (Service Area Nodes, SAN) across the globe (see Figure 14.6), and these will be interconnected over a broadband network (P-network). Because there are no plans for links between satellites, the P-network will forward wide-area connections to the nearest network gate- way, which will create the link to other networks such as PSTN, PLMN and PSDN. The terrestrial terminal and control (TT&C) stations are responsible for operation and maintenance of the satellites, as well as for control of the config- uration through position tracking. Data (such as battery state and position)
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