fundamental microwave links

8/3/2019 Fundamental Microwave Links

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MICROWAVE LINK -FUNDAMENTALS

DIVERSITY IN MW LINKS

Diversity 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 links

1. Frequency Diversity

2. Space Diversity

Fig MW.10.1 and MW.10.2 shows these diversities respectively.



Fig MW. 10.1 Frequency Diversity Fig MW.10.2 Space Diversity

Frequency Diversity calls for use of two different frequencies for same MWlink. This is normally avoided because two frequency allocation meansdouble the annual fee payable for frequency. Frequency diversity is generally

meant to overcome frequency interferences and various other factors.

Space Diversity uses two MW antennas at each side and is best suited toovercome Reflection of MW waves. Signal is received by both antennas called

Main Antenna and Diversity Antenna and it is IDU to decide which signal toreceive. Generally IDU receives best possible signal. This diversity also helpsa lot in areas of high wind because if one antenna gets misaligned network

can function without fail from another. Thus this provides a sort ofredundancy to our network.



FREE SPACE LOSS

Free Space Loss is defined as minimum loss an electromagnetic wave

experiences if it travels in atmosphere. It depends from place to place. Itsvalue for Kerela and Rajasthan will be different due to various factors one ofwhich can be humidity. However, we may roughly define free space loss forMW link as

Lfs = 92.45 + 20 log (dist * freq)

where

dist = MW hop length in Kms.

freq = Frequency of MW link in GHz.

EXAMPLE

For MW link of 15 GHz and hop length 10 Kms free space loss can roughly becalculated as

= 92.45 + 20 log ( 10 * 15)

= 135.97 dB

Antenna Gain is the gain antenna provides to the signal before transmitting itinto air. For parabolic antennas used for MW link, this gain is roughly

Antenna Gain = 17.8 + 20 log (f * dia)

where

f = Frequency in GHz

dia = Diameter of MW antenna.

EXAMPLE

For 18 GHz MW link and 0.3 m size MW antenna, Antenna Gain will beapprox

= 17.8 + 20 log (18*0.3)



= 32.44 dBi

(Don't worry about unit dBi, refer tutorial "Introduction to dB" elsewhere on

this website. To learn more about antennas refer tutorial on it.)

FRESNEL ZONE

To understand Fresnel zone we need to first refer Fig MW.12.1

Fig MW.12.1 MW Communication

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 wavefrom its original path. This deviation, as already studied, is due torefraction. Fresnel zone is nothing but distance below and above a pointwhich should be clear for LOS communication.

where

rn = radius of fresnel zone. Generally we consider n=1 i.e. first fresnel zone

clearance.

d1 = distance of point from Point A

d2 = distance of point from Point B

Lambda = Wavelength



LINK BUDGET

Now we will see link budget of MW link i.e. we will analyze gains and losses

and calculate received power at other end.

Refer Fig MW.13.1 before moving further.

Fig MW.13.1 Link Budget for MW Link

From Fig MW.13.1 it can be seen clearly that received power at Point B can becalculated as

RxA = TxA + GA - Lfs - Arain + GB

where

TxA = Transmit Power

GA = Gain of Antenna A

Lfs = Free Space Loss

Arain = Attenuation due to rain

GB = Gain of Antenna B

EXAMPLE

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

SOLUTION



First we will calculate Gain of two antennas. Since diameter is same, bothantennas 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 givenformula. We are assuming rain attenuation as zero.

RxB = 25 + 38.753 - 140.318 - 0 + 38.753

= - 37.812 dBm Answer

NOTE

Receiver sensitivity is gener ally around -65 dBm and hence the receive power we are getting is good and also take care of r ain attenuation margin

during r ainy season. It is good pr actice to leave around 30 dB as r ainmargin.

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