6. long distance links

7/30/2019 6. Long Distance Links

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Long-Distance Links

Telecommunication Engineering

www.ee.ui.ac.id/wasp


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Propagation of Wave

The propagation of electromagnetic waves is dependenton the frequency

The propagation characteristics are the result of changesin the radio-wave velocity as a function ofaltitude andboundary conditions

The wave velocity is dependent on air temperature, airdensity, and levels of air ionization

Ionization (free electrons) of the rarified air at highaltitudes has a dominant effect on wave propagation in

the MF and HF bands The ionization is caused by ultraviolet radiation from the

sun, as well as cosmic rays


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Propagation of Wave


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Propagation of Wave


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Propagation of Wave


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Propagation of Wave

The dominant ionized regions are D, E, F1, and F2 The D layer is located closest to the Earths surface at an

altitude of about 45 or 55 miles

For f > 300 kHz, it acts as a RF sponge to absorb(attenuate) the waves

The attenuation is inversely proportional to frequencyand becomes small for frequencies above 4 MHz

For f < 300 kHz, it provides refraction (bending) of RFwaves

It is most pronounced during the daylight hours, withmaximum ionization when the sun is overhead, andalmost dissapears at night


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Propagation of Wave

The E layer has a height of 65 to 75 miles, has maximumionization around noon and disappears after sunset

The F layer has a height of 90 to 250 miles

It ionizes rapidly at sunrise, with its peak ionization in

early afternoon and decay slowly after sunset The F region splits into two: F1 and F2 during the day and

combines into one layer at night

The F region is the most predominant medium in

providing reflection of HF waves


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Propagation of Wave

Three dominant propagation characteristics: Ground wave

Sky wave

LOS

Ground wave propagation is the dominant mode ofpropagation for frequencies < 2 MHz

Here, the wave tends to follow the contour of the Earth

because the diffraction of wave causes it to propagate

along the surface of the Earth What is the lowest radio frequency that can be used?


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Propagation of Wave

For efficient radiation, the antenna needs to be longer

than 1/10 of a wavelength For example, for signaling with a carrier frequency of 10

kHz, the antenna length is minimum 3000 m


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Propagation of Wave

Sky wave propagation is the dominant mode ofpropagation in the 2 to 30 MHz frequency range

Here, long-distance coverage is obtained by reflecting the

wave at the ionosphere and at the Earths boundaries

In the ionosphere the waves are refracted gradually in aninverted U shape because the index of refraction varies

with altitude as the ionization density changes

The refraction index of the ionosphere is given by

2

811

Nn

f refractive indexn

3free electron density (electrons/m )N frequency (Hz)f


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Propagation of Wave

Typical N values range between 10^10 and 10^12

depending on the time of day, season, sunspots In an ionized region because and outside the

ionized region because

1n 0N

1n 0N


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Propagation of Wave

In the ionized region the waves will be bent according toSnells law

The layer D is present during the day and absorbsfrequencies below 4 MHz

Thats why in AM broadcast, the distant stations cannotbe heard during the day, but at night the layer disappearsand distant AM stations can be heard via sky wavepropagation

Sky wave propagation is caused by reflection from the Flayer

sin r in

angle of incidencei angle of refractionr


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Propagation of Wave

LOS propagation is the dominant mode for frequenciesabove 30 MHz

Here the electromagnetic wave propagates in a straight

line


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LOS Systems

The LOS mode has the disadvantage that forcommunication between two terrestrial stations, the

signal path has to be above the horizon

Otherwise, the Earth will blockthe LOS path

The antennas need to be placed on tall towers so thatthe receiver antenna can see the transmitting antenna


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LOS Systems

To the optical horizon, k (bending characteristic) = 1

To the radio horizon, k =4/3

The design of an LOS microwave linkinvolves 5 basic steps: Setting performance requirements

Site selection and preparation of a path profile to determine antennatower heights

Carrying out a path analysis (link budget) Physically running a path/site survey

Installation of equipment and test of the system prior to cutting itover to carry traffic

3 [ ][ ] 2.9

2o

h md km

[ ] 2.9 2 [ ]rd km h m


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Site Selection

Select operational site where we will install and operate radioequipment

Path profile of each link to determine the heights of radiotower to achieve LOS

Steps to obtain path profile:

Obtain good toplogical maps of the region Draw a straight line with a long straight edge connecting the two

sites identified

Follow along down the line identifying obstacles and their heights

Calculate earth curvature (EC)

Calculate the Fresnel zone clearance for each obstacle

Add a value of additional height for vegetation and a growth factor

Draw a straight line from left to right connecting the two highestobstacle locations on the profile


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Site Selection

To calculate the EC we must account for the radio raypath bending by using K-factor

When K-factor is greater than 1, the ray beam bendstowards the earth

Whne K-factor is less than 1, the ray beam bends awayfrom the earth

The following formula applies

where is the distance from the transmit site to theobstacle in question and is the distance from thatobstacle to the receive site

1 20.078d d

h mK

1d

2d


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Site Selection

How to find the K-factor?

Fresnel zone clearance

1 2

[ ] 17.3d km d km

R mF GHz D km

1 2D d d


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Link Budget

A path analysis is carried out to dimension the link Establishing operating parameters such as transmitter

power output, parabolic antenna aperture (diameter),

receiver noise figure


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Link Budget


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Link Budget


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Link Budget

For operating frequencies up to about 10 GHz, path lossis synonymous with free-space path loss

Free-space path loss is given by

EIRP is calculated by adding decibel units: transmittedpower (in dBm or dBW), the transmission line losses in

dB, and antenna gain in dBi

Example: If a microwave transmitter has 1 W of power

output, the waveguide loss is 3 dB and the antenna gain is

34 dBi, the EIRP is

[ ] 92.4 20log [ ] 20log [ ]PL dB F GHz D km

.output trans lineEIRP dBW P dBW Loss dB G dBi

0 3 34 31EIRP dBW dB dBi dBW


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Link Budget

Isotropic Receive Level (IRL) is the RF power levelimpinging on the receive antenna

Receive signal level (RSL) is the power level at the input

port if the first active stage in the receiver

IRL dBW EIRP dBW PL dB

. .rec ant trans lineRSL dBW IRL dBW G dB Loss dB


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Link Budget

Example: Suppose the IRL was -121 dBW, the receiveantenna gain was 31 dB, and the line losses were 5.6 dB.

The RSL would be

The thermal noise level of a receiver is a function of thereceiver noise figure and its bandwidth

The thermal noise power level in a 1-Hz bandwidth of a

perfect receiver operating at absolute zero is

The thermal noise level of a perfect receiver operating at

room temperature is

121 31 5.6 95.6RSL dBW dB dB dBW

228.6nP dBW Hz

228.6 10log 290 204nP dBW Hz K dBW Hz


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Link Budget

We can convert noise figure to noise temperature inkelvins with the following formula

The thermal noise power level of a device operating atroom temperature is

Example: A microwave receiver has a noise figure of 8 dB

and its bandwidth is 10 MHz. The thermal noise level is

10log 1 290eNF dB T

the effective noise temperature of a deviceeT

204 10lognP dBW Hz NF dB BW Hz

6204 8 10log 10 10 126nP dBW Hz dB dBW


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Link Budget

S/N is widely used in analog transmission systems as onemeasure of signal quality

In digital systems the basic measure of transmission

quality is BER

In digital radio links, the ratio Eb/N0 is used as themeasure of signal quality

Eb/N0 means energy per bit per noise spectral density

ratio

N0 is simply the thermal noise in 1 Hz of bandwidth or

0 204N dBW Hz NF dB


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Link Budget

Example: Suppose a receiver has a noise figure of 2.1 dB,what is the thermal noise level in 1 Hz of bandwidth. The

N0 is

Eb is the signal energy per bit and defined as

Example: The RSL into a certain receiver was -89 dBW

and bit rate was 2.048 Mbps. The Eb value is

Then, the formula for Eb/N0 is

0 204 2.1 201.9N dBW Hz dB dBW Hz

or 10logbE RSL dBm dBW bit rate

689 10log 2.048 10 152.11bE dBW dBW

0 10log 204bE N RSL dBW bit rate dBW NF dB

6. long distance links

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