PART I: DIGITAL COMMUNICATION SYSTEMS Chapter 3: Transmission Media

Contents  Twisted Pair  Coaxial Cable  Optical Fiber  Terrestrial Radio  Satellite Radio  Radio Spectrum Management

Objectives  To study the various transmission media used for electrical communication.  To study the characteristics and the advantages of each for practical communication systems.  To study the issue of radio spectrum management.

Twisted Pair

 Twisted pair gets its name because a pair of copper wires is twisted to form the transmission medium.  Least expensive transmission medium and hence the most widely used.  Used extensively in the local underground telephone network, in Private Branch Exchanges (PBX’s), and also in local area network.

 Repeater – amplifies the signal to the required level and retransmits on the medium.  Data rate supported by the twisted pair depends on the distance to be covered and the quality of the copper.  Category 5 twisted pair supports data rates in the range of 10Mbps to 100Mbps up to a distance of 100 meters.

Coaxial Cable

 Used extensively for cable TV distribution, long distance telephone trunks and LANs.  Supports a maximum data rate of 500Mbps for a distance of about 500 meters  Repeaters are required every 1 to 10 kilometeres.

The speed of transmission in copper cable is 2.3 x 10 8 meters/second. Propagation Delay Delay = distance/speed Delay = 10,000/(2.3 x 10 8 ) seconds = microseconds

Optical Fiber

 Deployed extensively and is most preferred medium for all types of networks because of the high data rates that can be supported.  Light in a glass medium can carry more information over large distances, as compared to electrical signals in a copper cable or coaxial cable.

 The speed of transmission is 2 x 10 8 meters/second  The two types of optical fiber are single mode and multimode fiber. Single-mode fiber allows only one ray (or mode) of light to propagate at a time whereas multimode fiber allows multiple rays (or modes).

Terrestrial Radio

 Free space as the medium  Free space is called an unguided medium because the electromagnetic waves can travel freely in all directions.  A radio channel consists of a pair of frequencies – one frequency is used for uplink and one frequency is used for downlink.  In some radio systems, a single frequency is used in both directions.

Special Problems Causes a heavy attenuation of the radio signal. The radio receiver should be capable of receiving very weak signals. The receiver should have high sensitivity. Path Loss Multipath fading is predominant in mobile communication systems. The mobile phone receives the signals that traverse different path. Fading Rain affects radio frequency signals Rain Attenuation

Radio Spectrum Electromagnetic Spectrum Radio waves300GHz and lower (frequency) Sub-millimeter waves100 micrometers to 1 millimeter (wavelenght) Infrared780 nanometeres to 100 micrometers Visible light380 nanometers to 780 nanometers Ultraviolet10 nanometers to 380 nanometers X-ray120eV to 120keV Gamma rays120keV and up

Radio Frequency Frequency BandFrequency RangeApplication Areas Very Low Frequency (VLF)3kHz to 30kHzRadio navigation, maritime mobile Low Frequency30kHz to 300kHzRadio navigation, maritime mobile Medium Frequency300kHz to 3MHzAM radio broadcast, aeronautical mobile High Frequency3MHz to 30MHzMaritime mobile and aeronautical mobile Very High Frequency30MHz to 300MHzLand mobile, FM broadcast, TV broadcast, aeronautical mobile, radio paging, trunked radio

Frequency Band Frequency Range Application Areas Ultra-High Frequency 300MHz to GHz TV broadcast, mobile satellite, land mobile, radio astronomy L Band1GHz to 2GHz Aeronautical radio navigation, radio astronomy, earth exploration satellite S Band2GHz to 4GHz Space research, fixed satellite communication C Band4GHz to 8GHz Fixed satellite communication, meteorological satellite communication X Band8GHz to 12GHz Fixed satellite broadcast, space research Ku Band12GHz to 18GHz Mobile and fixed satellite communication, satellite broadcast

Frequency BandFrequency RangeApplication Areas K Band18GHz to 27GHzMobile and fixed satellite communication Ka Band27GHz to 40GHzInter-satellite communication, mobile satellite communication Millimeter40GHz to 300GHzSpace research, Inter- satellite communications

International Tellecommunications Union (ITU) Frequency Bands AM Radio535 to 1605 MHz Citizen band radio27MHz Cordless telephone devices43.69 to 50 MHz VHF TV54 to 72 MHz, 76 to 88 MHz, 174 to 216 MHz Aviation118 to 137 MHz Ham radio144 to 148 MHz, 420 to 450MHz UHF TV470 to 608 MHz, 614 to 806 MHz Cellular Phones824 to 849 MHz, 869 to 894 MHz Personal Communication services MHz, 930 – 931 MHz, 940 – 941 MHz Search for extra-terrestrial intelligence1420 to 1660 MHz Inmarsat satellite phones1525 to 1559 MHz, to

 Some frequency bands as ham radio and the industrial, scientific and medical band are free bands – no prior government approvals are required to operate radio systems in those bands.

Satellite Radio

 Arthur C. Clarke proposed the concept of communication satellites.

Star and Mesh

Radio Spectrum Management

Allotted Spectrum  Radio spectrum management ensures that the allotted spectrum is being used efficiently, to ensure that there is no interference between different radio systems and to allocate new frequency bands for new services.

Several Factors in RSM  There are some frequency bands for the exclusive use of govermental agencies and some for nogovermental agencies.  When frequency band is allocated for a particular application.  Higher user demands.  New application areas emerge.  New technologies.  Agencies will be allocated fixed frequencies for use.

Spectrum Management Activities  Spectrum assignment and selection involves recommending a specific frequency band operation for use in a given location.  Spectrum engineering and analysis involves computations for installations of radio equipment at specific locations and for predicting the system performance in the radio environment.  Spectrum planning involved long-term/ emergency planning, keeping in view, among other things, the demands for new services and technological changes.

Cost of Spectrum  Operators that obtain specific frequency bands for radio services need to pay for the cost of the spectrum.

End of Chapter 3