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

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

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

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

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
¾ 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

Operating freqeuncies
B.6

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

Terrestrial Microwave
B.8

Terrestrial Microwave
– With no intervening obstacles, the maximum distance between antennas is d = 7 .
14 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
¾ two antennas at a height of 100m may be as far as
82km apart
B.9

Terrestrial Microwave
Applications
– Long-distance telecommunication service
• requires fewer amplifiers or repeaters than coaxial cable
• requires line-of-sight transmission
• Example
¾ telephone system
¾
TV distribution
– Short point-to-point links
• Data link between local area network
• closed-circuit TV
• bypass application
B.10

Terrestrial Microwave
B.11

Terrestrial Microwave
B.12

Terrestrial Microwave
Transmission characteristics
– The higher the frequency used, the higher the potential bandwidth and therefore the higher the potential data rate
Band (GHz)
2
6
11
18
Bandwidth (MHz)
7
30
40
220
Data rate (Mbps)
12
90
90
274
B.13

Terrestrial Microwave
Attenuation
L = 10 log


4
d
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 logarithmically with distance
Why ?
– repeaters or amplifiers may be placed farther apart for microwave systems - 10 to 100 km is typical
B.14

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.15

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).
Why different frequencies are used?
• A orbiting satellite operate on a number of frequency bands, called transponder channels
B.16

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.17

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-topersonal-terminal connectivity.
B.18

Applications
– Television distribution
• Direct broadcast satellite
¾ 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

Applications
– private business networks
• very small aperture terminal (VSAT) systems
¾ subscriber stations equipped with low cost VSAT antennas share a satellite transmission capacity for transmission to a hub station
¾ the hub station can exchange messages with each of the subscribers
B.20

Applications
B.21

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

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

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

Physical description
– omnidirectional
Broadcast Radio
Applications
– AM broadcasting
• operating frequencies
¾
MF (medium frequency): 300 kHz - 3 MHz
¾ HF (high frequency): 3 MHz - 30 MHz
B.25

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

Broadcast Radio groundwave
B.27

Broadcast Radio
Applications
– FM broadcasting
• operating frequencies
¾ VHF (very high frequency): 30 MHz - 300 MHz
– TV broadcasting
• operating frequencies:
¾ VHF
¾ UHF (ultra high frequency): 300 MHz - 3000MHz
B.28

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