WIRELESS COMMUNICATION

1. WIRELESS COMMUNICATION LEC 13-14 bitwali.com

2. Basics: How do Satellites Work • When two Stations on Earth want to communicate through radio broadcast but are too far away to use conventional means. • The two stations can use a satellite as a relay (ریلے کرنا۔ دوسرے اسٹیشن کے پروگرام نشر کرنا) station for their communication • One Earth Station sends a transmission to the satellite. This is called a Uplink. (the link from ground station to satellite) • The satellite Transponder converts the signal and sends it down to the second earth station. This is called a Downlink( the link from satellite to one or more ground stations or receivers) • Some companies sell uplink and downlink services to • television stations, corporations, and to other telecommunication carriers. • A company can specialize in providing uplinks, downlinks, or both. bitwali.com

3. Motivation to use Satellites bitwali.com

4. Communication Satellite A Communication Satellite can be looked upon as (خیال کرنا) a large microwave repeater. It contains several transponders which listens to some portion of spectrum, amplifies the incoming signal and broadcasts it in another frequency to avoid interference with incoming signals. bitwali.com

5. Satellite Uplink and Downlink bitwali.com

6. Satellite Missions Source: Union of Concerned Scientists [www.ucsusa.org] bitwali.com

7. Satellite Signals Used to transmit signals and data over long distances Weather forecasting Television broadcasting Internet communication Global Positioning Systems bitwali.com

8. Satellite Transmission Bands The C band is the most frequently used. The Ka and Ku bands are reserved exclusively for satellite communication but are subject to rain attenuation bitwali.com

9. Communications System Successive satellites have become larger, heavier and more costly but the rate at which the traffic capacity has increased has been much greater with each succeeding generation of satellite. The designer of a satellite communication system is not free to select any frequency and BW. The frequency and BW regulations are administered by regulatory agencies such as FCC in the United States. The bands currently used for majority of services are 4-6 GHz and 11-14 GHz with 20-30 GHz coming into service. bitwali.com

10. Coverageof a Satellite bitwali.com

11. Circular Orbit Centre at earth centre. This is the only orbit that can provide full global coverage by one satellite. In communications where the instantaneous transfer of information is required, full global coverage could be achieved with a series of satellites, separated in time and angle. Elliptically Inclined Orbit An orbit of this type has unique properties that have been successfully used by some communications satellite systems, For thissystem, the elliptical orbit has an angle of inclination of 63° and a 12-hour orbit period. By using three satellites, suitably phased, continuous coverage of the polar region can be provided that would not be covered by other orbits.( mostly 0<i<90) Circular Equatorial orbit (Geostationary) A satellite in a circular orbit at 35,800 km has a period of 24 hours, and consequently appears stationary over a fixed point on the Earth's surface. Polar Orbit A polar orbit is one in which satellite passes above both poles of the body. (i=90) bitwali.com

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13. Source: Union of Concerned Scientists [www.ucsusa.org] Satellite Orbits bitwali.com

14. Why Satellites remain in Orbits bitwali.com

15. Space Segment Satellite Launching Phase Transfer Orbit Phase Deployment Operation Retirement Phase bitwali.com

16. Satellite System Elements bitwali.com

17. Ground Segment Collection of facilities, Users and Applications Earth Station = Satellite Communication Station (Fixed or Mobile) bitwali.com

18. Transponders Communication equipment in satellites composed of one or more antennas which receive and transmit over wide bandwidths at microwave frequencies and a set of receivers and transmitters that amplify and retransmit the incoming signals The receiver transmitter units are Transponders Signals transmitted by an earth station are received at the satellite by either a zone beam or a spot beam antenna. Zone beams can receive from transmitters anywhere within the coverage zone, whereas spot beams have limited coverage bitwali.com

19. Frequency Bands Allocated to the FSS Different Frequency bands are allocated to different services at World Radio-communication Conferences (WRCs). Allocations are set out in Article S5 of the ITU Radio Regulations. It is important to note that (with a few exceptions) bands are generally allocated to more than one radio services. CONSTRAINTS Bands have traditionally been divided into “commercial" and "government/military" bands, although this is not reflected in the Radio Regulations and is becoming less clear-cut as "commercial" operators move to utilize "government" bands. bitwali.com

20. Advantages of Satellite Communication • Can reach over large geographical area • Easy to install new circuits • Circuit costs independent of distance • Broadcast possibilities • Temporary applications (restoration) • Mobile applications • Terrestrial (زمینی ۔ زمین سے متعلق ۔ زمین پر موجود) network "by-passs" • Provision ( مہیا کرنا) of service to remote or underdeveloped areas • User has control over own network bitwali.com

21. Disadvantages of Satellite Communication • Large up front capital costs (space segment and launch) • Interference and propagation delay • Congestion of frequencies and orbits bitwali.com

22. When to use Satellites • When the unique features of satellite communications make it attractive • When the costs of communication are lower than terrestrial (زمینی ۔ زمین سے متعلق ۔ زمین پر موجود) routing • When it is the only solution • Examples: • Communications to ships and aircraft (especially safety communications) • TV services - contribution links, direct to cable head, direct to home • Data services - private networks • Overload traffic • Delaying terrestrial investments • 1 for N diversity • Special events bitwali.com

23. When to use Terrestrial(زمینی ۔ زمین سے متعلق ۔ زمین پر موجود) • PSTN - satellite is becoming increasingly uneconomic for most trunk telephony routes but, there are still good reasons to use satellites for telephony such as: very long distance traffic and remote locations. • Land mobile/personal communications - in urban areas of developed countries new terrestrial infrastructure is likely to dominate (e.g. GSM, etc.) • but, satellite can provide fill-in as terrestrial networks are implemented, also provide similar services in rural areas and underdeveloped countries bitwali.com

24. Basics: Factors in satellite communication (cont.) • Other impairments(نقائص) to satellite communication: • The distance between an earth station and a satellite (free space loss). • Satellite Footprint: The satellite transmission’s strength is strongest in the center of the transmission, and decreases farther from the center as free space loss increases. • Atmospheric Attenuation caused by air and water can impair the transmission. It is particularly bad during rain and fog. bitwali.com

25. Atmospheric Losses • Different types of atmospheric losses can disturb radio wave transmission in satellite systems: • Atmospheric absorption • Atmospheric attenuation • Traveling ionospheric disturbances bitwali.com

26. Atmospheric Absorption • Energy absorption by atmospheric gases, which varies with the frequency of the radio waves. bitwali.com

27. Atmospheric Attenuation • Rain is the main cause of atmospheric attenuation (hail, ice and snow have little effect on attenuation because of their low water content). bitwali.com

28. Traveling Ionospheric Disturbances • Traveling ionospheric disturbances are clouds of electrons in the ionosphere that provoke (اکسانا) radio signal fluctuations which can only be determined on a statistical basis. bitwali.com

29. Types of Waves Ionosphere (80 - 720 km) Sky wave Mesosphere (50 - 80 km) Stratosphere (12 - 50 km) Space wave Groundwave Troposphere (0 - 12 km) Transmitter Receiver Earth 29 bitwali.com

30. Radio Frequency Bands 30 bitwali.com

31. Propagation (پھیلانا ٬ افزایش کرنا ٬ مشتہر کرنا,) Mechanisms • Reflection • Propagation wave impinges (اوپر گرنا۔ ٹکرانا۔ لگنا۔ تصادم ہونا) on an object which is large as compared to wavelength - e.g., the surface of the Earth, buildings, walls, etc. • Diffraction • Radio path between transmitter and receiver obstructed (رکاوٹ ڈالنا) by surface with sharp irregular edges • Waves bend around the obstacle, even when LOS (line of sight) does not exist • Scattering • Objects smaller than the wavelength of the propagation wave - e.g. foliage, street signs, lamp posts 31 bitwali.com

32. Radio Propagation Effects Building Direct Signal Reflected Signal hb Diffracted Signal hm d Transmitter Receiver 32 bitwali.com

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34. Propagation Modes • Ground-wave propagation • Sky-wave propagation • Line-of-sight propagation bitwali.com

35. Ground Wave Propagation bitwali.com

36. Ground Wave Propagation • Follows contour of the earth • Can Propagate considerable distances • Frequencies up to 2 MHz • Example • AM radio bitwali.com

37. Sky Wave Propagation bitwali.com

38. Sky Wave Propagation • Signal reflected from ionized layer of atmosphere back down to earth • Signal can travel a number of hops, back and forth between ionosphere and earth’s surface • Reflection effect caused by refraction bitwali.com

39. Line-of-Sight Propagation bitwali.com

40. Line-of-Sight Propagation • Transmitting and receiving antennas must be within line of sight • Satellite communication – signal above 30 MHz not reflected by ionosphere • Ground communication – antennas within effective line of site due to refraction • Refraction – bending of microwaves by the atmosphere • Velocity of electromagnetic wave is a function of the density of the medium • When wave changes medium, speed changes • Wave bends at the boundary between mediums bitwali.com