Microwave Link

Installation & Commissioning

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Introduction •Microwaves are electromagnetic radiations in the frequency range 2GHz to 36 GHz (generally for Telecom).

•Microwaves are widely used for point-to- point communications

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MICROWAVE APPLICATIONS FOR TELECOM INDUSTRY

1.  BTS connectivity E 1 Eg 1 2G , 3G , wimax

2.STM 1 (63 E1) ring closure

3.Ethernet For Etisalat 3G Bts

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Frequency used in MW LinksMicrowave links of short distances are generally allocated with higher frequencies, because high frequency means high losses in air and thus it is good to have short distances in these cases.  While for distances like 20-35 Kms or so we use lower frequencies. 

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Microwave Links can be of two types1.  SDH2.  PDHFrequency allocated to MW link

does not depend on the type of MW link. 

SDH link can carry optical signalsPDH link can carry electrical signals

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POLARIZATION

Type of Polarization1.  Linear - can be sub-divided into Vertical and Horizontal

2.  Circular (We generally do not use this in MW links.)

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VERTICAL POLARIZATION

Vertical Polarization if its electrical component is perpendicular to the horizon of earth

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HORIZONTAL POLARIZATION

Horizontal Polarization if its electrical component is parallel to the horizon of earth

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FACTORS AFFECTING MW LINKFollowing major phenomenon affect

MW Link1.    REFLECTION පරාවර්තනය2.    REFRACTION වර්තනය3.    DIFFRACTION විවර්තනය4.    SCATTERING ප්‍රකිරණය5.    ABSORPTION අවශ�ෝෂණය

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DIVERSITY IN MW LINKSDiversity in MW Links is a sort of redundancy

in network.  They also help overcome various factors which affect MW links.

Two types of Diversity in MW links1.  Frequency Diversity

2.  Space Diversity

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requency Diversity

Frequency Diversity calls for use of two different frequencies for same MW link.

This can overcome frequency interferences and various other factors.

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Space Diversity

Space Diversity uses two MW antennas at each side and is best suited to overcome Reflection of MW waves.  Signal is received by both antennas called Main Antenna and Diversity Antenna and it is IDU to decide which signal to receive.  Generally IDU receives best possible signal.  This diversity also helps a lot in areas of high wind because if one antenna gets misaligned network can function without fail from another. 

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FRESNEL ZONE

From the figure above we can see that apart from direct line of sight (LOS) we need to leave some space above and below it to allow deviation of MW wave from its original path.  This deviation, as already studied, is due to refraction.  Fresnel zone is nothing but distance below and above a point which should be clear for LOS communication.

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where,

Fn = The nth Fresnel Zone radius in metresd1 = The distance of P from one end in metresd2 = The distance of P from the other end in metresλ = The wavelength of the transmitted signal in metres

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examples of how the Fresnel zone can be disrupted.

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FREE SPACE LOSSFree Space Loss is defined as minimum loss an

electromagnetic wave experiences if it travels in atmosphere.Lfs  =  92.45 + 20 log (dist * freq)wheredist = MW hop length in Kms.freq = Frequency of MW link in GHz.

EXAMPLEFor MW link of 15 GHz and hop length 10 Kms free space loss can roughly be calculated as=  92.45 + 20 log ( 10 * 15)=  135.97 dB

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ANTENNA GAINAntenna Gain is the gain antenna provides to the signal

before transmitting it into air.  For parabolic antennas used for MW link, this gain is roughly

Antenna Gain = 17.8 + 20 log (f * dia)wheref     =  Frequency in GHzdia =  Diameter of MW antenna.EXAMPLEFor 18 GHz MW link and 0.3 m size MW antenna,

Antenna Gain will be approx= 17.8 + 20 log (18*0.3)= 32.44 dBi

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LINK BUDGETwe will analyze gains and losses and

calculate received power at other end.

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RxA = TxA + GA - Lfs - Arain + GB

whereTxA = Transmit PowerGA = Gain of Antenna ALfs = Free Space LossArain = Attenuation due to rainGB = Gain of Antenna B

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EXAMPLESuppose we have 6.2 GHz MW link.  Diameter of antenna at both sides is 1.8 m.  Distance is 20 Kms.  Calculate approx received power at point B, if transmitted power at point A is 25 dBm.

First we will calculate Gain of two antennas.  Since diameter is same, both antennas will roughly have gain of

= 17.8 + 20 log (freq * dia)= 17.8 + 20 log (6.2 * 1.8)= 38.753 dBi

Then, we will calculate rough free space loss as= 98.45 + 20 log (dist * freq)= 98.45 + 20 log (20 * 6.2)= 140.318 dBm

Finally we will calculate received power at Point B from above given formula.  We are assuming rain attenuation as zero.

RxB = 25 + 38.753 - 140.318 - 0 + 38.753=    - 37.812 dBm  Answer