CS601 - Data Communication - Lecture Handout 27

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Propagation of Specific Signals

  • The type of propagation used in radio transmission depends upon the frequency of the signal
  • Each frequency is suited for a specific layer of atmosphere and is most efficiently transmitted and received by technologies adapted to that layer

Very Low Frequency (VLF) (Figure)

  • VLF waves are propagated as surface waves through air
  • Do not suffer much attenuation in TX but are susceptible to high levels of atmospheric noise I.e. electricity and heat
  • Used for Long-range radio navigation and Submarine Communication
Very Low Frequency (VLF) (Figure)

Low Frequency (LF) (Figure)

  • Also propagated as surface waves
  • Used for Long-range radio and for navigational locators
  • Attenuation is greater in the day time when absorption of waves by natural obstacles increases
Low Frequency (LF) (Figure)

Middle Frequency (MF)

  • Propagated in the Troposhere
  • These frequencies are absorbed by Ionosphere
  • The distance they cover is limited by the angle needed to get the signal reflect from the troposhere and not enter ionosphere
  • Absorption increases during the day time
  • Used for AM Radio
Middle Frequency (MF)

High Frequency (HF) (Figure)

  • Use ionospheric propagation
  • These frequencies move into the ionosphere where the density difference reflects them back on earth
  • Used for Citizen’s Band Radio, International Broadcasting, Military Communication, Telephone, Telegraph and Fax
High Frequency (HF) (Figure)

Very High Frequency (VHF) (Figure)

  • Most VHF waves use line-of-sight propagation
  • Used for VHF Television, FM Radio, Aircraft AM Radio
Very High Frequency (VHF) (Figure)

Ultra High Frequency (UHF) (Figure)

  • Always use line-of-sight propagation
  • Used for UHF Television, Mobile Telephone, Cellular Radio, Paging, Microwave Links
  • Note that microwave communication begins at 1GHz in UHF and continues into SHF and EHF band
Ultra High Frequency (UHF) (Figure)

Super High Frequency (SHF) (Figure)

  • SHF waves are TX using mostly line-of-sight and some Space propagation
  • Used for Terrestrial and Satellite Microwave and Radar Communication
Super High Frequency (SHF) (Figure)

Extremely High Frequency (EHF) (Figure)

  • Use space propagation
  • Used for Radar , Satellite and Experimental Communication
Extremely High Frequency (EHF) (Figure)

Terrestrial Microwa

  • Microwaves do not follow the curvature of earth and therefore require line-of-sight TX and RX equipment
  • Distance covered by line-of-sight signal depends to a large extent on the height of the antennas
  • Height allows the signal to travel farther by crossing a lot of obstacles like low hills and buildings
  • Microwave signals propagate in one direction at a time, which means that two frequencies are necessary for 2-way communication such as telephone conversation
  • One frequency is reserved for MICROWAVE communication in one direction and the other for TX in the other direction
  • Each frequency requires its own transmitter & receiver combined in a Transceiver nowadays
Terrestrial Microwave

Repeaters

  • To increase distance for terrestrial microwave, a system of repeaters can be installed with each antenna
  • A signal received by one antenna can be converted back to the transmittable form and relayed to the next antenna
  • The distance required b/w repeaters varies with frequencies of the signal and the environment in which the antennas are found
  • A repeater may broadcast the regenerated signal either at original frequency or a new frequency depending on system
  • Used in Telephone systems worldwide

Antennas

Two types of Antennas are used for Microwave communication:
  • Parabolic Dish
Antennas

Parabolic Dish

  • Based on the geometry of a Parabola
  • Every line parallel to the line of symmetry (line of sight) reflects off the curve at an angle such that they intersect in a common point called FOCUS
  • Parabolic dish works like a funnel catching a wide range of waves and directing them to a common point
  • In his way most of the signal is recovered than would be possible with a single-point receiver

HORN

  • Outgoing transmissions are broadcast through a horn aimed at the dish. The microwaves hit the dish and and are deflected outward in a reversal of the receipt path
  • A horn antenna looks like a gigantic scoop
  • Outward TXs are directed upward a stem and are deflected outward in a series of narrow parallel beams

Satellite Communication

  • Satellite TX is much like line-of-sight transmission in which one of the stations is the satellite orbiting around the earth
  • The principle is similar to the terrestrial microwave with a satellite acting as a Super tall antenna and Repeater
Satellite Communication
  • Although in satellite TX, signals must still travel in straight line, the limitations imposed on distance by curvature of earth are reduced
  • In this way satellites can span Continents and oceans with one bounce off the satellite
  • Satellite can provide TX capability to and from any location on earth no matter how remote
  • This advantage makes high quality communication available to underdeveloped parts of the worked at almost no cost
  • Satellites themselves are very expensive but leasing a freq or time on one can be cheap

Geosynchronous Satellite

  • Line of sight propagation requires the sending and receiving antennas must be locked into each other
  • To ensure continuous communication, satellites must move with the same speed as earth. So that they seem fixes w.r.t earth
  • These satellites are called Geosynchronous Satellites
Geosynchronous Satellite
Figure shows 3 satellites in geosynchronous orbit each 120 degree from one another so that whole earth can be covered

Satellite Frequency Bands

Each satellite sends and receives over two bands
  • Uplink: From the earth to the satellite
  • Downlink: From the satellite to the earth
Satellite Frequency Bands

Cellular Telephony

  • Each service area is divided into small ranges called cells
  • Each cell office is controlled by a switching office called MTSO

Operations of Cellular Telephony

Operations of Cellular Telephony

Transmitting

  • Mobile phone sends the number to the closest cell office
  • Cell office
    • MTSO
    • Telephone office
  • MTSO assigns an unused voice channel

 

Receiving

  • Telephone office sends the signal to MTSO
  • MTSO sends queries to each cell (paging)
  • If mobile phone is found and available, assigns a channel

Handoff

  • MTSO monitors the signal level every few seconds
  • If the strength diminishes, MTSO seeks a new cell and changes the channel carrying the call

Transmission Impairments

Transmission media are not perfect What is sent is not what is received
Transmission Impairments

Attenuation

  • Attenuation means loss of energy
  • Some of electrical energy is converted to heat

Decibel (dB)

Relative strengths of two signals or a signal at two points
Decibel (dB)

Example 7.1

Imagine a signal travels through a transmission medium and its power is reduced to half. This means P2=(1/2)P1. Calculate Attenuation?
Solution:
Example 7.1

Distortion

  • Distortion means that the signal changes its form or shape
  • Distortion occurs in a composite signal

Summary

  • Frequency Ranges
  • Microwave Communication
  • Satellite Communication
  • Cellular Telephony
  • Transmission Impairments

Reading Sections

  • Section 7.2, 7.3 “Data Communications and Networking” 4th Edition by Behrouz A. Forouzan
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