Unguided Media After this lecture, you will be able to describe the physical and transmission characteristics of various unguided media B.

Transcription

1 Unguided Media After this lecture, you will be able to describe the physical and transmission characteristics of various unguided media B.1

2 Unguided media Guided to unguided Transmission the signal is guided to an antenna via a guided medium antenna radiates electromagnetic energy into the medium Reception antenna picks up electromagnetic waves from the surrounding medium. Example a voice signal from a telephone network is guided via a twisted pair to a base station of mobile telephone network the antennas of the base station radiates electromagnetic energy into the air the antenna of the mobile phone handset picks up electromagnetic waves B.2

3 Directional and Omnidirectional Directional the transmitting antenna puts out a focused electromagnetic beam the transmitting and receiving antennas must be aligned Example Satellite communication systems For a satellite located at 35784km above the ground, a 1 beam covers 1962km 2 B.3

4 Directional and Omnidirectional Omnidirectional the transmitted signal spreads out in all directions and can be received by many antennas. In general, the higher the frequency of a signal, the more it is possible to focus it into a directional beam Example mobile communication systems radio broadcasting B.4

5 Operating freqeuncies Microwave Frequencies in the range of about 30 MHz to 40 GHz are referred to as microwave frequencies 2 GHz to 40 GHz wavelength in air is 0.75cm to 15cm wavelength = velocity / frequency highly directional beams are possible suitable for point-to-point transmission 30 MHz to 1 GHz suitable for omnidirectional applications B.5

6 Operating freqeuncies B.6

7 Terrestrial Microwave Physical description limited to line-of-sight transmission. This means that microwaves must be transmitted in a straight line and that no obstructions can exists, such as buildings or mountains, between microwave stations. To avoid possible obstructions, microwave antennas often are positioned on the tops of buildings, towers, or mountains. B.7

8 Terrestrial Microwave B.8

9 Terrestrial Microwave With no intervening obstacles, the maximum distance between antennas is d = kh d is the distance between antennas in kilometers, h is the antenna height in meters k is an adjustment factor to account for the fact that microwaves are bent or refracted with the curvature of the earth k ~ 4/3 Example two antennas at a height of 100m may be as far as 82km apart B.9

10 Terrestrial Microwave Applications Long-distance telecommunication service requires fewer amplifiers or repeaters than coaxial cable requires line-of-sight transmission Example telephone system TV distribution Short point-to-point links Data link between local area network closed-circuit TV bypass application B.10

11 Terrestrial Microwave B.11

12 Terrestrial Microwave B.12

13 Terrestrial Microwave Transmission characteristics The higher the frequency used, the higher the potential bandwidth and therefore the higher the potential data rate Band (GHz) Bandwidth (MHz) Data rate (Mbps) B.13

14 Attenuation Terrestrial Microwave L = 10log 4πd λ 2 db d is the distance λ is the wavelength attenuation varies as the square of the distance for twisted pair and coaxial cable, loss varies Why? logarithmically with distance repeaters or amplifiers may be placed farther apart for microwave systems - 10 to 100 km is typical B.14

15 Terrestrial Microwave Attenuation increases with rainfall, especially above 10 GHz Interference the assignment of frequency bands is strictly regulated OFTA (Office of telecommunications authority) B.15

16 Satellite Microwave Physical description a satellite is a microwave relay station link two or more ground-based microwave transmitter/receivers (known as earth stations or ground stations) The satellite receives transmissions Why on different one frequency band (uplink), amplifies or repeats the frequencies signal, and transmits are it on another frequency (downlink). used? A orbiting satellite operate on a number of frequency bands, called transponder channels B.16

17 Geostaionary Satellites It is launched into an orbit above the equator at km. This orbit distance means that the satellite is orbiting the earth as fast as the earth is rotating. It appears to earth stations that the satellite is stationary, thus making communications more reliable and predictable. Earth stations is less expensive because they can use fixed antennas. B.17

18 Low earth orbit (LEO) and medium earth orbit (MEO) satellites For small mobile personal communications terminals, a network with significantly reduced transmission and processing delay is required. Such a service could be provided by low earth orbit (LEO) and medium earth orbit (MEO) satellite systems. Delay is ms for geostationary satellites These systems can provide direct personal-terminal-topersonal-terminal connectivity. B.18

19 Applications Television distribution Direct broadcast satellite video signals are transmitted directly to the home user long-distance telephone transmission point-to-point trunks between telephone exchange offices in public telephone networks suffers from transmission delay B.19

20 Applications private business networks very small aperture terminal (VSAT) systems subscriber stations equipped with low cost VSAT antennas share a satellite transmission capacity for transmission to a hub station the hub station can exchange messages with each of the subscribers B.20

21 Applications B.21

22 Satellite microwave Frequency allocation Optimum frequency range for satellite transmission is 1-10GHz Below 1 GHz, there is significant noise from nature sources About 10 GHz, the signal is severely attenuated by atmosphere B.22

23 Satellite microwave Fixed satellite service Typical frequency bands for uplink/downlink usual terminology 6/4 GHz C band 8/7 GHz X band 14/12 GHz Ku band 30/20 GHz Ka band B.23

24 Satellite microwave Mobile satellite service Typical frequency bands for uplink/downlink usual terminology 1.6/1.5 GHz L band 30/20 GHz Ka band Broadcasting satellite service Typical frequency bands for uplink/downlink usual terminology 12 GHz Ku band B.24

25 Broadcast Radio Physical description omnidirectional Applications AM broadcasting operating frequencies MF (medium frequency): 300 khz - 3 MHz HF (high frequency): 3 MHz - 30 MHz B.25

26 Broadcast Radio HF is the most economic means of low information rate transmission over long distances (e.g. > 300km) A HF wave emitted from an antenna is characterized by a groundwave and a skywave components. The groundwave follows the surface of the earth and can provide useful communication over salt water up to 1000km and over land for some 40km to 160km. The skywave transmission depends on ionospheric refraction. Transmitted radio waves hitting the ionosphere are bent or refracted. When they are bent sufficiently, the waves are returned to earth at a distant location. Skywave links can be from 160km to 12800km. B.26

27 Broadcast Radio groundwave B.27

28 Broadcast Radio Applications FM broadcasting operating frequencies VHF (very high frequency): 30 MHz MHz TV broadcasting operating frequencies: VHF UHF (ultra high frequency): 300 MHz MHz B.28

29 Infrared Does not penetrate walls no security or interference problems no frequency allocation issue no licensing is required B.29