B.1

Unguided Media

After this lecture, you will be able to – describe the physical and transmission characteristics of

various unguided media

B.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.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 1962km2

B.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.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.6

Operating freqeuncies

B.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.8

Terrestrial Microwave

B.9

Terrestrial Microwave

– With no intervening obstacles, the maximum distance between antennas is

• 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

• Exampletwo antennas at a height of 100m may be as far as 82km apart

khd 14.7=

B.10

Terrestrial Microwave

Applications– Long-distance telecommunication service

• requires fewer amplifiers or repeaters than coaxial cable• requires line-of-sight transmission• Example

telephone systemTV distribution

– Short point-to-point links• Data link between local area network• closed-circuit TV• bypass application

B.11

Terrestrial Microwave

B.12

Terrestrial Microwave

B.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)2 7 126 30 9011 40 9018 220 274

B.14

Terrestrial Microwave

Attenuation

• d is the distance• λ is the wavelength

– attenuation varies as the square of the distance• for twisted pair and coaxial cable, loss varies

logarithmically with distance– repeaters or amplifiers may be placed farther apart for

microwave systems - 10 to 100 km is typical

dB4log102

=λπdL

Why ?

B.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)

• www.ofta.gov.hk

B.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 on one frequency band (uplink), amplifies or repeats the signal, and transmits it on another frequency (downlink).• A orbiting satellite operate on a number of frequency

bands, called transponder channels

Why different frequencies are used?

B.17

Geostaionary Satellites

– It is launched into an orbit above the equator at 35786 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.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 250 -500ms for geostationary satellites

– These systems can provide direct personal-terminal-to-personal-terminal connectivity.

B.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.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 stationthe hub station can exchange messages with each of the subscribers

B.21

Applications

B.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.23

Satellite microwave

Fixed satellite serviceTypical frequency bands for uplink/downlink usual terminology6/4 GHz C band8/7 GHz X band14/12 GHz Ku band30/20 GHz Ka band

B.24

Satellite microwave

Mobile satellite serviceTypical frequency bands for uplink/downlink usual terminology1.6/1.5 GHz L band30/20 GHz Ka band

Broadcasting satellite serviceTypical frequency bands for uplink/downlink usual terminology12 GHz Ku band

B.25

Broadcast Radio

Physical description– omnidirectional

Applications– AM broadcasting

• operating frequenciesMF (medium frequency): 300 kHz - 3 MHzHF (high frequency): 3 MHz - 30 MHz

B.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 ionosphericrefraction. 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. Skywavelinks can be from 160km to 12800km.

B.27

Broadcast Radio

groundwave

B.28

Broadcast Radio

Applications– FM broadcasting

• operating frequenciesVHF (very high frequency): 30 MHz - 300 MHz

– TV broadcasting• operating frequencies:

VHFUHF (ultra high frequency): 300 MHz - 3000MHz

B.29

Infrared

– Does not penetrate walls• no security or interference problems

– no frequency allocation issue• no licensing is required