Satellite Microwave


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Satellite Microwave

  1. 1. Fundamentals of Microwave & Satellite Technologies Presented By Engr. Abdul Khaleque
  2. 2. Historical Perspective <ul><li>Founded during WWII </li></ul><ul><li>Used for long-haul telecommunications </li></ul><ul><li>Displaced by fiber optic networks </li></ul><ul><li>Still viable for right-of-way bypass and geographic obstruction avoidance </li></ul>
  3. 3. Microwave Spectrum <ul><li>Range is approximately 1 GHz to 40 GHz </li></ul><ul><ul><li>Total of all usable frequencies under 1 GHz gives a reference on the capacity of in the microwave range </li></ul></ul>
  4. 4. Microwave Impairments <ul><li>Equipment, antenna, and waveguide failures </li></ul><ul><li>Fading and distortion from multipath reflections </li></ul><ul><li>Absorption from rain, fog, and other atmospheric conditions </li></ul><ul><li>Interference from other frequencies </li></ul>
  5. 5. Microwave Engineering Considerations <ul><li>Free space & atmospheric attenuation </li></ul><ul><li>Reflections </li></ul><ul><li>Diffractions </li></ul><ul><li>Rain attenuation </li></ul>
  6. 6. Microwave Engineering Considerations-cont’d <ul><li>Skin affect </li></ul><ul><li>Line of Sight (LOS) </li></ul><ul><li>Fading </li></ul><ul><li>Range </li></ul><ul><li>Interference </li></ul>
  7. 7. Free Space & Atmospheric Attenuation <ul><li>Free space & atmospheric attenuation is defined by the loss the signal undergoes traveling through the atmosphere. Changes in air density and absorption by atmospheric particles. </li></ul>
  8. 8. Reflections <ul><li>Reflections can occur as the microwave signal traverses a body of water or fog bank; cause multipath conditions </li></ul>
  9. 9. Diffraction <ul><li>Diffraction is the result of variations in the terrain the signal crosses </li></ul>
  10. 10. Rain Attenuation <ul><li>Raindrop absorption or scattering of the microwave signal can cause signal loss in transmissions. </li></ul>
  11. 11. Skin Affect <ul><li>Skin Affect is the concept that high frequency energy travels only on the outside skin of a conductor and does not penetrate into it any great distance. Skin Affect determines the properties of microwave signals. </li></ul>
  12. 12. Line of Sight Fresnel Zone Clearance <ul><li>Fresnel Zone Clearance is the minimum clearance over obstacles that the signal needs to be sent over. Reflection or path bending will occur if the clearance is not sufficient. </li></ul>
  13. 13. LOS & FZC-cont’d Fresnel Zone D 1 D 2 72.2 D 1 X D 2 F x D secret formula
  14. 14. Microwave Fading Normal Signal Reflective Path Caused by multi-path reflections and heavy rains
  15. 15. Range <ul><li>The distance a signal travels and its increase in frequency are inversely proportional </li></ul><ul><li>Repeaters extend range </li></ul><ul><ul><li>Back-to-back antennas </li></ul></ul><ul><ul><li>reflectors </li></ul></ul>
  16. 16. Range-cont’d <ul><li>High frequencies are repeated/received at or below one mile </li></ul><ul><li>Lower frequencies can travel up to 100 miles but 25-30 miles is the typical placement for repeaters </li></ul>
  17. 17. Interference <ul><li>Adjacent Channel Interference </li></ul><ul><ul><li>digital not greatly affected </li></ul></ul><ul><li>Overreach </li></ul><ul><ul><li>caused by signal feeding past a repeater to the receiving antenna at the next station in the route. Eliminated by zigzag path alignment or alternate frequency use between adjacent stations </li></ul></ul>
  18. 18. Components of a Microwave System <ul><li>Digital Modem </li></ul><ul><li>Radio Frequency (RF) Unit </li></ul><ul><li>Antenna </li></ul>
  19. 19. Digital Modem <ul><li>The digital modem modulates the information signal (intermediate frequency or IF). </li></ul>
  20. 20. RF Unit <ul><li>IF is fed to the RF unit which is mounted as close physically to the antenna as possible (direct connect is optimal). </li></ul>
  21. 21. Antenna <ul><li>The antenna is a passive device that radiates the modulated signal. It is fed by direct connect of the RF unit, coaxial cable, or waveguides at higher frequencies. </li></ul>
  22. 22. Waveguides Waveguides are hollow channels of low-loss material used to direct the signal from the RF unit to the antenna.
  23. 23. Modulation Methods <ul><li>Primarily modulated today with digital FM or AM signals </li></ul><ul><ul><li>Digital signal remains quiet until failure threshold bit error rate renders it unusable </li></ul></ul>
  24. 24. Bit Error Rate (BER) <ul><li>The BER is a performance measure of microwave signaling throughput </li></ul><ul><ul><li>10 or one error per million transmitted bits of information </li></ul></ul><ul><ul><li>Data fail over is at 10 ; voice traffic can withstand this error rate </li></ul></ul>-6 -3
  25. 25. Diversity <ul><li>Space Diversity </li></ul><ul><li>Frequency Diversity </li></ul><ul><li>Hot Standby </li></ul><ul><li>PRI </li></ul>
  26. 26. Space Diversity Normal Signal Faded Signal Transmitter Receiver
  27. 27. Space Diversity-cont’d <ul><li>Space Diversity protects against multi-path fading by automatic switch over to another antenna place below the primary antenna. This is done at the BER failure point or signal strength attenuation point to the secondary antenna that is receiving the transmitted signal at a stronger power rating. </li></ul>
  28. 28. Frequency Diversity Receiver Active XTMR Frequency #1 Protect XTMR Frequency #2 RCVR Frequency #1 RCVR Frequency #2 Transmitter
  29. 29. Frequency Diversity-cont’d <ul><li>Frequency Diversity uses separate frequencies (dual transmit and receive systems); it monitors primary for fail over and switches to standby. Interference usually affects only one range of frequencies. Not allowed in non-carrier applications because of spectrum scarcity. </li></ul>
  30. 30. Hot Standby* Receiver System XTMR Primary #1 System XTMR Standby #2 failure switch Active RCVR #1 Standby RCVR #2 Transmitter *Hot standby is designed for equipment failure only
  31. 31. PRI Receiver Transmitter Connect to PRI interface & PSTN Connect to PRI interface & PSTN To PSTN To PSTN System Transmission Facilities System Receiver Facilities
  32. 32. Availability Formula Percent Availability equals: 1 – (outage hours / 8760 hours per year) Private microwaves have 99.99% availability
  33. 33. Microwave Path Analysis <ul><li>Transmitter output power </li></ul><ul><li>Antenna gain </li></ul><ul><ul><li>proportional to the physical characteristics of the antenna (diameter) </li></ul></ul><ul><li>Free space gain </li></ul><ul><li>Antenna alignment factor </li></ul><ul><li>Unfaded received signal level </li></ul>
  34. 34. Microwave Radio Applications
  35. 39. Satellite Communications
  36. 40. Satellite-Related Terms <ul><li>Earth Stations – antenna systems on or near earth </li></ul><ul><li>Uplink – transmission from an earth station to a satellite </li></ul><ul><li>Downlink – transmission from a satellite to an earth station </li></ul><ul><li>Transponder – electronics in the satellite that convert uplink signals to downlink signals </li></ul>
  37. 41. Ways to Categorize Communications Satellites <ul><li>Coverage area </li></ul><ul><ul><li>Global, regional, national </li></ul></ul><ul><li>Service type </li></ul><ul><ul><li>Fixed service satellite (FSS) </li></ul></ul><ul><ul><li>Broadcast service satellite (BSS) </li></ul></ul><ul><ul><li>Mobile service satellite (MSS) </li></ul></ul><ul><li>General usage </li></ul><ul><ul><li>Commercial, military, amateur, experimental </li></ul></ul>
  38. 42. Classification of Satellite Orbits <ul><li>Circular or elliptical orbit </li></ul><ul><ul><li>Circular with center at earth’s center </li></ul></ul><ul><ul><li>Elliptical with one foci at earth’s center </li></ul></ul><ul><li>Orbit around earth in different planes </li></ul><ul><ul><li>Equatorial orbit above earth’s equator </li></ul></ul><ul><ul><li>Polar orbit passes over both poles </li></ul></ul><ul><ul><li>Other orbits referred to as inclined orbits </li></ul></ul><ul><li>Altitude of satellites </li></ul><ul><ul><li>Geostationary orbit (GEO) </li></ul></ul><ul><ul><li>Medium earth orbit (MEO) </li></ul></ul><ul><ul><li>Low earth orbit (LEO) </li></ul></ul>
  39. 43. Geometry Terms <ul><li>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 </li></ul><ul><li>Minimum elevation angle </li></ul><ul><li>Coverage angle - the measure of the portion of the earth's surface visible to the satellite </li></ul>
  40. 44. Minimum Elevation Angle <ul><li>Reasons affecting minimum elevation angle of earth station’s antenna (>0 o ) </li></ul><ul><ul><li>Buildings, trees, and other terrestrial objects block the line of sight </li></ul></ul><ul><ul><li>Atmospheric attenuation is greater at low elevation angles </li></ul></ul><ul><ul><li>Electrical noise generated by the earth's heat near its surface adversely affects reception </li></ul></ul>
  41. 45. GEO Orbit <ul><li>Advantages of the the GEO orbit </li></ul><ul><ul><li>No problem with frequency changes </li></ul></ul><ul><ul><li>Tracking of the satellite is simplified </li></ul></ul><ul><ul><li>High coverage area </li></ul></ul><ul><li>Disadvantages of the GEO orbit </li></ul><ul><ul><li>Weak signal after traveling over 35,000 km </li></ul></ul><ul><ul><li>Polar regions are poorly served </li></ul></ul><ul><ul><li>Signal sending delay is substantial </li></ul></ul>
  42. 46. LEO Satellite Characteristics <ul><li>Circular/slightly elliptical orbit under 2000 km </li></ul><ul><li>Orbit period ranges from 1.5 to 2 hours </li></ul><ul><li>Diameter of coverage is about 8000 km </li></ul><ul><li>Round-trip signal propagation delay less than 20 ms </li></ul><ul><li>Maximum satellite visible time up to 20 min </li></ul><ul><li>System must cope with large Doppler shifts </li></ul><ul><li>Atmospheric drag results in orbital deterioration </li></ul>
  43. 47. LEO Categories <ul><li>Little LEOs </li></ul><ul><ul><li>Frequencies below 1 GHz </li></ul></ul><ul><ul><li>5MHz of bandwidth </li></ul></ul><ul><ul><li>Data rates up to 10 kbps </li></ul></ul><ul><ul><li>Aimed at paging, tracking, and low-rate messaging </li></ul></ul><ul><li>Big LEOs </li></ul><ul><ul><li>Frequencies above 1 GHz </li></ul></ul><ul><ul><li>Support data rates up to a few megabits per sec </li></ul></ul><ul><ul><li>Offer same services as little LEOs in addition to voice and positioning services </li></ul></ul>
  44. 48. MEO Satellite Characteristics <ul><li>Circular orbit at an altitude in the range of 5000 to 12,000 km </li></ul><ul><li>Orbit period of 6 hours </li></ul><ul><li>Diameter of coverage is 10,000 to 15,000 km </li></ul><ul><li>Round trip signal propagation delay less than 50 ms </li></ul><ul><li>Maximum satellite visible time is a few hours </li></ul>
  45. 49. Frequency Bands Available for Satellite Communications
  46. 50. Satellite Link Performance Factors <ul><li>Distance between earth station antenna and satellite antenna </li></ul><ul><li>For downlink, terrestrial distance between earth station antenna and “aim point” of satellite </li></ul><ul><ul><li>Displayed as a satellite footprint (Figure 9.6) </li></ul></ul><ul><li>Atmospheric attenuation </li></ul><ul><ul><li>Affected by oxygen, water, angle of elevation, and higher frequencies </li></ul></ul>
  47. 51. Satellite Footprint
  48. 52. Satellite Network Configurations
  49. 53. Capacity Allocation Strategies <ul><li>Frequency division multiple access (FDMA) </li></ul><ul><li>Time division multiple access (TDMA) </li></ul><ul><li>Code division multiple access (CDMA) </li></ul>
  50. 54. Frequency-Division Multiplexing <ul><li>Alternative uses of channels in point-to-point configuration </li></ul><ul><ul><li>1200 voice-frequency (VF) voice channels </li></ul></ul><ul><ul><li>One 50-Mbps data stream </li></ul></ul><ul><ul><li>16 channels of 1.544 Mbps each </li></ul></ul><ul><ul><li>400 channels of 64 kbps each </li></ul></ul><ul><ul><li>600 channels of 40 kbps each </li></ul></ul><ul><ul><li>One analog video signal </li></ul></ul><ul><ul><li>Six to nine digital video signals </li></ul></ul>
  51. 55. Frequency-Division Multiple Access <ul><li>Factors which limit the number of subchannels provided within a satellite channel via FDMA </li></ul><ul><ul><li>Thermal noise </li></ul></ul><ul><ul><li>Intermodulation noise </li></ul></ul><ul><ul><li>Crosstalk </li></ul></ul>
  52. 56. Forms of FDMA <ul><li>Fixed-assignment multiple access (FAMA) </li></ul><ul><ul><li>The assignment of capacity is distributed in a fixed manner among multiple stations </li></ul></ul><ul><ul><li>Demand may fluctuate </li></ul></ul><ul><ul><li>Results in the significant underuse of capacity </li></ul></ul><ul><li>Demand-assignment multiple access (DAMA) </li></ul><ul><ul><li>Capacity assignment is changed as needed to respond optimally to demand changes among the multiple stations </li></ul></ul>
  53. 57. FAMA-FDMA <ul><li>FAMA – logical links between stations are preassigned </li></ul><ul><li>FAMA – multiple stations access the satellite by using different frequency bands </li></ul><ul><li>Uses considerable bandwidth </li></ul>
  54. 58. DAMA-FDMA <ul><li>Single channel per carrier (SCPC) – bandwidth divided into individual VF channels </li></ul><ul><ul><li>Attractive for remote areas with few user stations near each site </li></ul></ul><ul><ul><li>Suffers from inefficiency of fixed assignment </li></ul></ul><ul><li>DAMA – set of subchannels in a channel is treated as a pool of available links </li></ul><ul><ul><li>For full-duplex between two earth stations, a pair of subchannels is dynamically assigned on demand </li></ul></ul><ul><ul><li>Demand assignment performed in a distributed fashion by earth station using CSC </li></ul></ul>
  55. 59. Reasons for Increasing Use of TDM Techniques <ul><li>Cost of digital components continues to drop </li></ul><ul><li>Advantages of digital components </li></ul><ul><ul><li>Use of error correction </li></ul></ul><ul><li>Increased efficiency of TDM </li></ul><ul><ul><li>Lack of intermodulation noise </li></ul></ul>
  56. 60. FAMA-TDMA Operation <ul><li>Transmission in the form of repetitive sequence of frames </li></ul><ul><ul><li>Each frame is divided into a number of time slots </li></ul></ul><ul><ul><li>Each slot is dedicated to a particular transmitter </li></ul></ul><ul><li>Earth stations take turns using uplink channel </li></ul><ul><ul><li>Sends data in assigned time slot </li></ul></ul><ul><li>Satellite repeats incoming transmissions </li></ul><ul><ul><li>Broadcast to all stations </li></ul></ul><ul><li>Stations must know which slot to use for transmission and which to use for reception </li></ul>
  57. 61. FAMA-TDMA Uplink
  58. 62. FAMA-TDMA Downlink