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Mobile Wireless Systems. Mobile Wireless Comms Lecture 4. Based on: William Stallings, Wireless Communications & Networks, Chapter 2. Satellite Technology. Satellite-Related Terms. Earth Stations – antenna systems on or near earth Uplink – transmission from an earth station to a satellite

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Mobile Wireless Systems


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    1. Mobile Wireless Systems Mobile Wireless Comms Lecture 4 Based on: William Stallings, Wireless Communications & Networks, Chapter 2

    2. Satellite Technology Leeds Metropolitan University Faculty of Art, Environment and Technology

    3. Satellite-Related Terms • Earth Stations – antenna systems on or near earth • Uplink – transmission from an earth station to a satellite • Downlink – transmission from a satellite to an earth station • Transponder – electronics in the satellite that convert uplink signals to downlink signals Leeds Metropolitan University Innovation North – Faculty Of Information And Technology

    4. Ways to CategorizeCommunications Satellites • Coverage area • Global, regional, national • Service type • Fixed service satellite (FSS) • Broadcast service satellite (BSS) • Mobile service satellite (MSS) • General usage • Commercial, military, amateur, experimental Leeds Metropolitan University Innovation North – Faculty Of Information And Technology

    5. Classification of Satellite Orbits • Circular or elliptical orbit • Circular with center at earth’s center • Elliptical with one foci at earth’s center • Orbit around earth in different planes • Equatorial orbit above earth’s equator • Polar orbit passes over both poles • Other orbits referred to as inclined orbits • Altitude of satellites • Geostationary orbit (GEO) • Medium earth orbit (MEO) • Low earth orbit (LEO) Leeds Metropolitan University Innovation North – Faculty Of Information And Technology

    6. Geometry Terms • Elevation angle - the angle from the horizontal to the point on the center of the main beam of the antenna when the antenna is pointed directly at the satellite • Minimum elevation angle • Coverage angle - the measure of the portion of the earth's surface visible to the satellite Leeds Metropolitan University Innovation North – Faculty Of Information And Technology

    7. Minimum Elevation Angle • Reasons affecting minimum elevation angle of earth station’s antenna (>0o) • Buildings, trees, and other terrestrial objects block the line of sight • Atmospheric attenuation is greater at low elevation angles • Electrical noise generated by the earth's heat near its surface adversely affects reception Leeds Metropolitan University Innovation North – Faculty Of Information And Technology

    8. GEO Orbit • Advantages of the GEO orbit • No problem with frequency changes • Tracking of the satellite is simplified • High coverage area • Disadvantages of the GEO orbit • Weak signal after traveling over 35,000 km • Polar regions are poorly served • Signal sending delay is substantial Leeds Metropolitan University Innovation North – Faculty Of Information And Technology

    9. Leeds Metropolitan University Faculty of Art, Environment and Technology

    10. LEO Satellite Characteristics • Circular/slightly elliptical orbit under 2000 km • Orbit period ranges from 1.5 to 2 hours • Diameter of coverage is about 8000 km • Round-trip signal propagation delay less than 20 ms • Maximum satellite visible time up to 20 min • System must cope with large Doppler shifts • Atmospheric drag results in orbital deterioration Leeds Metropolitan University Innovation North – Faculty Of Information And Technology

    11. LEO Categories • Little LEOs • Frequencies below 1 GHz • 5MHz of bandwidth • Data rates up to 10 kbps • Aimed at paging, tracking, and low-rate messaging • Big LEOs • Frequencies above 1 GHz • Support data rates up to a few megabits per sec • Offer same services as little LEOs in addition to voice and positioning services Leeds Metropolitan University Innovation North – Faculty Of Information And Technology

    12. Low Earth Orbit Satellites Leeds Metropolitan University Faculty of Art, Environment and Technology

    13. MEO Satellite Characteristics • Circular orbit at an altitude in the range of 5000 to 12,000 km • Orbit period of 6 hours • Diameter of coverage is 10,000 to 15,000 km • Round trip signal propagation delay less than 50 ms • Maximum satellite visible time is a few hours Leeds Metropolitan University Innovation North – Faculty Of Information And Technology

    14. Medium Earth Orbit Satellites Leeds Metropolitan University Faculty of Art, Environment and Technology

    15. Frequency Bands Available for Satellite Communications Leeds Metropolitan University Innovation North – Faculty Of Information And Technology

    16. Satellite Link Performance Factors • Distance between earth station antenna and satellite antenna • For downlink, terrestrial distance between earth station antenna and “aim point” of satellite • Displayed as a satellite footprint (Figure 9.6) • Atmospheric attenuation • Affected by oxygen, water, angle of elevation, and higher frequencies Leeds Metropolitan University Innovation North – Faculty Of Information And Technology

    17. Satellite Footprint

    18. Satellite Network Configurations

    19. Capacity Allocation Strategies • Frequency division multiple access (FDMA) • Time division multiple access (TDMA) • Code division multiple access (CDMA) Leeds Metropolitan University Innovation North – Faculty Of Information And Technology

    20. Frequency-Division Multiplexing • Alternative uses of channels in point-to-point configuration • 1200 voice-frequency (VF) voice channels • One 50-Mbps data stream • 16 channels of 1.544 Mbps each • 400 channels of 64 kbps each • 600 channels of 40 kbps each • One analog video signal • Six to nine digital video signals • FDMA = subdivision of channels into further FM channels Leeds Metropolitan University Innovation North – Faculty Of Information And Technology

    21. Frequency-Division Multiple Access • Factors which limit the number of subchannels provided within a satellite channel via FDMA • Thermal noise • Intermodulation noise • Crosstalk Leeds Metropolitan University Innovation North – Faculty Of Information And Technology

    22. Forms of FDMA • Fixed-assignment multiple access (FAMA) • The assignment of capacity is distributed in a fixed manner among multiple stations • Demand may fluctuate • Results in the significant underuse of capacity • Demand-assignment multiple access (DAMA) • Capacity assignment is changed as needed to respond optimally to demand changes among the multiple stations Leeds Metropolitan University Innovation North – Faculty Of Information And Technology

    23. FAMA-FDMA • Logical links between stations are pre-assigned • Multiple stations access the satellite by using different frequency bands • Uses considerable bandwidth Leeds Metropolitan University Innovation North – Faculty Of Information And Technology

    24. Example: FAMA FDMA Leeds Metropolitan University Faculty of Art, Environment and Technology

    25. DAMA-FDMA • Single channel per carrier (SCPC) – bandwidth divided into individual VF channels • Attractive for remote areas with few user stations near each site • Suffers from inefficiency of fixed assignment • DAMA – set of subchannels in a channel is treated as a pool of available links • For full-duplex between two earth stations, a pair of subchannels is dynamically assigned on demand • Demand assignment performed in a distributed fashion by earth station using CSC Leeds Metropolitan University Innovation North – Faculty Of Information And Technology

    26. FAMA-TDMA Operation • Transmission in the form of repetitive sequence of frames • Each frame is divided into a number of time slots • Each slot is dedicated to a particular transmitter • Earth stations take turns using uplink channel • Sends data in assigned time slot • Satellite repeats incoming transmissions • Broadcast to all stations • Stations must know which slot to use for transmission and which to use for reception Leeds Metropolitan University Innovation North – Faculty Of Information And Technology

    27. Reasons for Increasing Use of TDM Techniques • Cost of digital components continues to drop • Advantages of digital components • Use of error correction • Increased efficiency of TDM • Lack of intermodulation noise Leeds Metropolitan University Innovation North – Faculty Of Information And Technology

    28. Example: TDMA Frame Format Leeds Metropolitan University Faculty of Art, Environment and Technology

    29. FAMA-TDMA Uplink

    30. FAMA-TDMA Downlink

    31. Satellite-Switched TDMA Operation Leeds Metropolitan University Faculty of Art, Environment and Technology

    32. Global Positioning System (GPS) • Satellite system for providing location and time. • Development started in 1973. • Functional since 1994. • Is maintained by the US government. • Other GPS systems: • Russian GLONASS • European Galileo (planned) Leeds Metropolitan University Faculty of Art, Environment and Technology

    33. Technical Background • Originally 24 satellites simultaneously flying around the earth. Now there are 32. • At least four satellites need to be in sight of the user receiver. System is designed so that most times there are 6 satellites. • The satellites orbit in 20,200 km (12,600 mi). One orbit takes ½ day. • Receivers: can receive typically 20 channels (20 satellites). Leeds Metropolitan University Faculty of Art, Environment and Technology

    34. Deeper Technical Background • Two carriers L1 at 1575.42 Mhz and L2 at 1227.6 Mhz. • Each satellite transmits its code. • Data rate: 50 Hz. • User data rate: 1 Hz. • Precision improvements: • Differential GPS (using the difference to a known reference station to compensate fluctuations). • WAAS augmentation. Leeds Metropolitan University Faculty of Art, Environment and Technology

    35. NMEA 0183 • NMEA = National Marine Electronics Association. • NMEA 0183 = protocol for position data (from receiver to PC). • Standard: 4800 bit / sec. Newer versions also allow higher data rate. Leeds Metropolitan University Faculty of Art, Environment and Technology

    36. TMC – Traffic Message Channel • Is a feature of GPS Navigation Systems. • Uses FM-RDS (Radio Data System) technology. • 2048 event phrases. • Each message: 37 data bits, 1-3 times per second. • 16 bits for location (no latitude/longitude, but indices to table) • 11 bits for event code • 5 bits for extend • 5 bits for duration and system management Leeds Metropolitan University Faculty of Art, Environment and Technology

    37. Literature about GPS • DKD (1990). A GPS Data Receiver. ARRL/CRRL publication "9th Computer Networking Conference 1990“. http://www.dkdinst.com/articles/Gps_art.html [last accessed: 29.11.2011] • Dale DePriest (2000). NMEA data. http://www.gpsinformation.org/dale/nmea.htm [last accessed: 29.11.2011] Leeds Metropolitan University Faculty of Art, Environment and Technology