Microwave Good

8/3/2019 Microwave Good

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Introduction toIntroduction toMicrowave LinkMicrowave Link


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Microwave Link introduction :

In its simple form the Microwave link can be one hopeconsisting of one pair of antenna spaced as little as one ortwo kilometres apart, or can be a backbone, includingmultiple hops , spanning several thousand of kilometres. Asimple hop is typically 30 to 60 km in relatively flat regionsfor frequencies in the 2 to 8 GHz bands.


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In its simple form the Microwave link can be one hopeconsisting of one pair of antenna spaced as little as one ortwo kilometres apart, or can be a backbone, includingmultiple hops , spanning several thousand of kilometres. Asimple hop is typically 30 to 60 km in relatively flat regionsfor frequencies in the 2 to 8 GHz bands.

Line of sight is a term which is only partially correct whendescribing microwaves paths. atmospheric conditions andterrains effects modify the propagation of the microwaves sothat even if the designer can see from point A to point B(true line of sight), it may not be possible to place antennasat those two points and achieve a satisfactorycommunication performance.


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The objective of microwave communication systems is totransmit information from one place to another withoutinterruption and clear reproduction at the receiver.

The voice ,video, data channels are combined by atechnique known as multiplexing to produce a BB signal.This signal is frequency modulated to an IF then upconverted to the RF for transmission through theatmosphere.

The microwave transmission frequencies are within theapproximate range 2 to 50 GHz.



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Microwave Link Notions :

1. Antenna

Since antennas play a central role in microwavecommunications they will be considered first. their areseveral shapes of antenna available for transmittingmicrowaves ex: Flat antenna, flat grid antenna, sect oralantenna ,parabolic dish antenna, parabolic grid dishantenna.


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Microwave Link Antenna Types

Flat Antenna Sectoral Antenna Parabolic Antenna Parabolic gridAntenna


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1. Antenna

Since antennas play a central role in microwavecommunications they will be considered first. their areseveral shapes of antenna available for transmittingmicrowaves ex: Flat antenna, flat grid antenna, sectoralantenna ,parabolic dish antenna, parabolic grid dishantenna.

Telecommunication systems almost always use theparabolic type and sometimes the horn type

These antennas are highly directional, the energy isfocused into a very narrow beam by the transmittingantenna and aimed at the receiving antenna, which

concentrates the received power by a mechanism analogueto the telescope.


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Parabolic antennafront feed diagnostic


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The most important characteristic of antennas is the gain.This is a measure of the antennas ability to transmit thewaves in a specific direction instead of in all directions. Its ameasure of directionality.

Since the antenna dish is not fabricated perfectly parabolicin shape. The waveguide feed and focus causes somereduction of the transmitted or received power since it is apartial blockage to the microwaves. Commercially availableparabolic antennas have efficiencies in the region of 50 to 70percent.

For a conservative efficiency of approximately 50 percentthe gain equation can be written:

dBDfG )4.7(log20 10!

1. Antenna

Where

D: antenna diameter (m)

f = frequency (GHz)


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1. Antenna

Example: in our case the antenna diameter is 648 mm

the frequency range is 5.8 GHz

what will be the gain of the antenna?????

Solution:

G=20log(7.4*0.648*5.8)=28.88 dB


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1. Antenna

The beamdwidth is another important characteristics ofantennas, for parabolic antennas the beamwidth is :

Where:

= beamwidth measured at the half-maximum power

points (3dB down points).= frequency (GHz).

= antenna diameter (m).

Df.3.21!*

*

f

D


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1. Antenna

Example: in our case the antenna diameter is 648 mm

the frequency range is 5.8 GHz.

what will be the 3 dB beamwidth angle of the

antenna ?????

Solution:G= 21.3/(0.648*5.8)=5.667 degree


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1. Antenna

An important characteristic of antenna is the front-to-back ratio.

It is defined as the ratio of maximum gain in the forwarddirection to the maximum gain in the backward direction (back

lobes). this backward radiation has great significance with respectto noise and interference. signals reach the receiver afterreflection from the ground behind the antenna, this introducesunwanted noise. Secondly, at repeaters where the samefrequency is used for incoming and outgoing signals, couplingoccurs between the receiver and transmitter antennas. since

transmitter power are usually at least 60 dB higher than receiverlevels, it is evident that there must be a high degree of isolationbetween antennas in order to avoid severe interference.

An electromagnetic wave does not travel in a straight line


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1. Antenna (polarization)

If a horn feeder is used to illuminate the antenna ,it isoriented in one of tow positions

1. horizontal : the electric field is in the horizontal plane and

the antenna is said to be horizontally polarized .

2. vertical: the electric field is in the vertical plane andthe antenna is said to be vertically polarized .


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2. Fresnel zones

An electromagnetic wave does not travel in a straight line, thewave spreads out as it propagates. also the individual waves thatmakes up a radio signal do not travel at the same phase velocity.

A French physicist, Augustin Fresnel ,defined the propagation ofa radio wave as three dimensional elliptical path between the

transmitter and receiver.

Fresnel divided the path into several zones based on the phaseand speed of the propagating waves.

The size of each fresnel zone varies based on the frequency ofthe radio signal and the length of the path. As frequency

decreases, the size of the fresnel zone increases.As the length of the path increases, the size of the fresnel zonealso increases. the fresnel zone radius is greatest at the midpointof the path.

Therefore, the midpoint requires the most clearance of any point

in the path.


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2. Fresnel zones


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2. Fresnel zones

The distance (in meters) from the line-of-sight path to theboundary of the nth Fresnel zone is approximated by theequation :

Where d1=distance from one end of the path to the reflectionpoint (Km).

d2=distance from the other end of the path to the

reflection point (Km).D= d1+d2 .

f=frequency (GHz)

n=number of Fresnel Zone(1st,2nd ,3rd ).

nF

fD

dndFn

213.17!


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2. Fresnel zones


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2. Fresnel zones

Exercise : A microwave link installed between two sites apart of35 kilometre, the operational frequency is 5.8 GHz. what is theradius of the first fresnel zone at the distance of 10 km awayfrom the antenna tower????

mFn 19,19358.5

251013.17 !

v

vv!

Solution : d1=10 km , d2=25 km, D=35 km, f=5.8GHz

using the above formula the radius of the first fresnelzone at 10 km away from the tower antenna weobtain :


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3. Propagation free space path loss

Since microwave energy is electromagnetic , it will pass throughspace in a manner similar to that of light. The atmosphere andterrain have modifying effects on the loss of microwave energy,but first only the loss as a result of free space will be considered.

Free Space Loss is the expected attenuation of a signal as ittravels away from the transmitting device. When a signal radiatesfrom the antenna, it spreads out over an increasingly largerdistance. As the area covered increases, the power density (orthe amount of power per unit area) decreases. This effectivelyweakens the radio signal.


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Figure 1




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the following formulas calculate free space in decibels (dB):

or

),log(20),log(204.32 MHzFreqkilometerDistLPL

!

),log(20),log(204.92 GHzFreqkilometerDistLPL !



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exercise:

What will be the path loss of a 5.8 GHz microwave link traversing35 km ??

solution:

What will be the path loss of a 5.8 GHz microwave link traversing35 km ??

D=35 km , F=5.8 Ghz

Using the formula

=92.4 +20log(35)+20log(5.8)

=92.4 +30.88+15.268

=138.5493 dB

PLL



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4. Cable/connectors loss

a directional antenna is connected to the Outdoor Unit with acoaxial cable.

The cable Loss is very small of the range of 3 to 15 dB perhundred feet so it will not contribute significantly to the link loss.

the ANDREW cables that we are using must not introduce a lossgrater of 5 dB per 30 meter (the length of the cable that we areusing does not exceed 2 meter )

The connector loss : the experiments of the communicationbranch have demonstrated that the best connectors that we are

using in fact (Andrew) introduce at least 0.5dB pairs.


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5. System Gain

System gain of the radio system, without any consideration ofthe antennas or cables. is simply the arithmetic differencebetween the transmitters output power and the receiverssensibility threshold .system gain ( ) is measured in dB. Tocalculate the system gain, subtract the receiver sensitivity from

the transmitter power .

dB= (Transmitter Power) dBm (Receiver sensibility) dBm

Example : in our case the transmitter power is 24 dBm

the receiver sensitivity is -88 dBm for BER=

= 24 (-88)

= 112 dB

csG

6

10

csG

csG


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6. Fading

there are two main categories of fading :

1-Flat fading (frequency independent)

2-Frequency selective Fading

Neither type of fading can be predicted accurately because eachis caused by the variations in atmospheric conditions. experiencehas shown that some climates and terrain surfaces are morelikely to cause fading than others , but in all circumstances fadingcan only be defined statistically.


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Two forms of flat fading were indicated earlier: ducting and rainattenuation fading. the microwave beam can be influenced by achange of the refractive index (dielectric constant) of the air;k=4/3 is considered to be the standard atmospheric condition in

which the microwave has one fourth of the true earth curvature(solution space diversity).


6.1 Flat Fading


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Atmospheric multipath fading: When the atmosphericconditions are such that layers of different densities exist, asindicated earlier, ducting can occur, the microwave energy canreach the receiving antenna by paths that are different from the

direct path. this multipath reception produces fading because thetwo waves are rarely received in phase. if they arrive in completeantiphase signal cancellation can occur at the receiver.

Ground reflection multipath fading: ground reflection can causea multipath reception which will be observed as fading if thewaves are received in antiphase. multipath fading is frequency

selective because, for antiphase cancellation , the different wavesmust reach the receive after travelling distances that differ byone half-wavelength. because the size of one half-wavelengthvaries significantly from 1 to 12 GHz, fading conditions that existat one frequency may not exist at another (frequency diversity)


6.2 frequency selective fading


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Multipath Fading


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Experience has shown that all paths longer than 40 km can besubject to multipath fading for frequencies of operation above890 MHz. Atmospheric multipath fading is most pronouncedduring the summer months or , more specifically when the

weather is hot , humid, and wind-free.As the length of a microwave path is increased, there is a rapidincrease in the number of possible indirect paths by which thesignal may be received.




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To achieve reliable communication, the radio link must have anaverage received signal level high enough to protect the linkagainst fluctuations in the signal power due to multipath fadingand other anomalous propagation conditions. this safety factor isreferred to as the fade margin.

The fade margin is a measure of how much additional signalattenuation the system can endure without dropping below therequired BER level. A fade margin of 15 dB or more is sufficient inmost situations .use the following formula to calculate the fademargin:

Fade margin =

Where is the total system gain measured in dB).

is the total antenna gain of both antennas (measured in dBi).

is the total connector/cable loss of all cables (measured in dBi).

is the pass loss (measured in dB).


7. Determining the Fade Margin

PLCLANTSG LLGG SGG

CLL

PLL

ANTG


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Exercise:

Taking our case where the transmitter power is 24 dBm and thereceiver sensibility is -88 dBm . The antenna gain is 28 dBi andneglecting the cable loss but not the connector loss approximatedto 0.5 dB per connector .what will be the fade margin of amicrowave link over 35 km operating on 5.8 GHz.



SGG

CLL

PLL

ANTG

Solution:

=24-(-88)=112 dB

=2*28=56 dBi

=4*0.5=2 dB

=32.4+20log(35)+20log(5800)= 138.5493 dB

Fade Margin = 112+56-2-138.54= 27.46 dB


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Link implementation can be divided into 3 phases:

1. Site Survey.

2. Wireless Design and Engineering.

3. Link installation.


8. Link Implementation


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Line of sight . Distance calculations. GPS coordinates and Topographical studies. Spectrum analyzer readings of ISM and licensed frequencies .

Antenna placement (location and angles) .


8.1 Site Survey


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Site location-Tower-Tall building or other elevated location-Line of Sight (LOS) to potential customers

Obstructions between base station and customers-Location of equipment-Mounting surfaces Antennas and Radios-Cableways

Power availability-Yes or No-Condition--reliability

Physical security at location-How is access attained-Who controls access


8.1 Site Survey


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Line of Sight-VLOS Visual line of sight-RLOS Radio line of sight-Fresnel Zone

Terrain evaluation-What does the area look like (take pictures)-Hills, trees, buildings-Heights (elevations) of obstructions-Lakes, ponds, rivers


8.1 Site Survey


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Spectrum analyzer readings-When and where do I take them-What information will they provide-How experienced must I be to use one

-How do I get one (borrow, rent, buy)

Balloons, flags, lights-Ways to determine LOS without climbing-How high should that antenna be

Physical area description and photos-Take pictures ofEVERYTHING-Document the roads, terrain, rooftop/tower, power


8.1 Site Survey


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Wireless Design and planningWireless Design and planning

Network design documentationNetwork design documentation

Hardware specifications andHardware specifications andrecommendationsrecommendations

Topography graphs and antennaTopography graphs and antenna

plotting informationplotting information Path calculations and linkPath calculations and link

probability statisticsprobability statistics


8.2 Wireless Design and EngineeringWireless Design and Engineering


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GPS coordinatesGPS coordinates

How can they be determinedHow can they be determined

How can they be usefulHow can they be useful

Topographical maps or softwareTopographical maps or software

How easy are they to useHow easy are they to use

How are they usefulHow are they useful

Distance calculationsDistance calculations

How accurate do they need toHow accurate do they need tobebe

Will my link budget sufferWill my link budget suffer




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Link profilesLink profiles

Printouts from topo softwarePrintouts from topo software(Hertz Mapper)(Hertz Mapper)

Hand drawn from mapsHand drawn from maps

Fresnel zone calculationsFresnel zone calculations Different for frequency ofDifferent for frequency of

equipmentequipment

Combined with the profiles ifCombined with the profiles ifpossiblepossible




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Select equipment and start theSelect equipment and start the

ball rolling.ball rolling.

Make final tower/rooftopMake final tower/rooftop

negotiations.negotiations.

Order equipment.Order equipment.

Make arrangements for installers.Make arrangements for installers.

Make arrangements for site(s)Make arrangements for site(s)

access for installation and testing.access for installation and testing.

Prepare network infrastructure forPrepare network infrastructure for

implementation.implementation.


8.3 Installation


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Tower climbing .Tower climbing .

Antenna installation .Antenna installation .

Cable and grounding installation .Cable and grounding installation .

Equipment configuration .Equipment configuration .

Power system analysis .Power system analysis .

Performance testing, tuning .Performance testing, tuning .

Acceptance testing andAcceptance testing andcommissioning .commissioning .


8.3 installation.installation.

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