planning tips when migrating frequencies

8/3/2019 Planning Tips When Migrating Frequencies

1/23

Planning tips when migrating

frequencies from 10 Ghz, 13 GHz, 22

GHz and 25 GHz

Bill Williamson:

1Written and prepared by Bill [email protected]


2/23

These are the subjects that I have

been asked to address

1. Frequency band Selection

2. Dish size

3. Dish position (height)

4. Polarity

5. TX power

6. Natural clutter

7. Reliability including fade margin.

8. Instability9. Propagation

Other topics which are continual causes of confusionhave also been included.



3/23

1.1 DISTANCE Vs FREQUENCY

One of the first items to consider for any microwave path is theactual distance from antenna to antenna. The further a microwavesignal must travel, the greater the signal loss. This form ofattenuation is termed free space loss (FSPL). Assuming anunobstructed path, only two variables need to be considered in FSL

calculations: The frequency of the microwave signalnumerically higher

frequencies require more power to cover a given distance.

The actual path distancethe greater the distance the greater thesignal loss.

A signal transmitted at a frequency of 6 GHz will have moreavailable power than a signal transmitted at 11 GHz. For example, amicrowave system at 6 GHz can expect to cover about 40 Kmbetween communication points. The same system using a frequencyof 11 GHz will only cover about 16 Km.



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Comparison of frequency Vs Free

Space Path LossFrequency (GHz)

10

10

10

13

13

13

22

22

22

25

25

25

Distance

5

10

20

5

10

20

5

10

20

5

10

20

FSPL

126.38

132.40

138.42

128.66

134.68

140.70

133.23

139.25

145.27

134.34

140.36

146.38

Written and prepared by Bill [email protected]

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5

17

13.5

9.6

7

0

2

4

6

8

10

12

14

16

18

0 5 10 15 20 25 30

Distance

Frequency

Frequency Vs Distance with 64 QAM & 20 Db Fade

Series1


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1.2 FREQUENCY

The sensitivity of digital radio equipment to

frequency-selective fading can be described by

the signature curve of the equipment

Use higher frequency bands for shorter hopsand lower frequency bands for longer hops

Avoid lower frequency bands in urban areas

In areas with heavy precipitation , if possible,use frequency bands below 10 GHz.


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2.0 Antennae in general

Adding a better antenna to a communicationssystem is the single best way to increaseperformance in almost every parameter. Beforeyou increase the power of your transmitter, you

should first make sure your antenna system is thebest it can be.

A FAQ is Can you go through a window? Theanswer is yes but with added losses, a good

starting point is allow 10dB as an initial guess thiscan be greater for metallic or tinted glass



8/23

3.1 Antennae height

Resolve excessive antenna height. Go only as

high as required to give you 0.6 clearance of

the first Fresnel zone above all obstacles.

Use a larger antenna. Larger antennas have

more gain, narrower beam width, and better

side lobe suppression.


8


9/23

3.2 ANTENNAE TOO HIGH

When antennas are placed higher than required,unpredictable system outages can occur. Excessiveheight allows more sources of RF interference fromdistant sites as well as to multipath problems. For

example, a 2.4 GHz system operating over a 48 Kmflat-terrain path requires the antennas to be placed 48 mup a tower just to clear the earths bulge. And, using anantenna with beam width of 3.6 degrees, which, at 48Km away, is 3.05 Km wide. The receiving radios

window for receiving interference is enormous.



10/23

4.1POLARITY

By not having the same polarity on your network'santennas, you can receive a 20 -30 dB loss of signalstrength. This is an enormous loss, but can also be veryuseful. It is worth considering changing antenna

polarization because you can help eliminate certaintypes of radio interference, or allow many antennas inone location. Horizontal antenna polarization atmicrowave frequencies will generally provide lessmultipath and may also provide lower path loss in non

line-of-sight situations. Horizontal polarity: attenuation tends to be a little

higher than vertical because of the shape of fallingraindrops



11/23

4.2 Polarization

Vertical polarization is far less susceptible to

rainfall attenuation (40 to 60%) than are

horizontal polarization frequencies

On water paths at frequencies above 3 GHz,

it is advantageous to choose vertical

polarization


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12/23

5.1 Too much power

Interference can be caused by energy that is at thesame frequency as the signal that you wish toreceive, or can be at a nearby frequency withenough energy to leak into the receiver

Interference can also be caused by energy that is acompletely different frequency from that whichyou wish to receive. The reason for this is thathigh powered transmitters can radiate harmonics

where they are also inadvertently transmittingenergy that is a multiple of the intendedtransmitter frequency



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6.0 CLUTTER

Ground clutter is a term used to describe the amount ofmicrowave energy scattered to the antennae from stationaryobjects on the ground like towers, hills, high tension lines,trees, buildings, etc.

for example, scattering from the sea may be particularlystrong at frequencies where there is some sort of matchbetween the signal wavelength and the wavelength of thewater waves - either the main waves which are immediately

apparent, or the small waves that are superimposed on themain waves. These in turn will depend upon the windstrength and direction and the depth of the water, so thatthere will often be correlations between the cluttercharacteristics and the meteorological conditions


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7.1 SUMMARY OF LINK RELIABILITY

A common misunderstanding is that system availability numbers,such as 99.999%, derived from path analysis software, areachievable as long as the calculated fade margin is met. They do notinclude the effects of reflective fading (multipath) or interference

from other intentional radiators.

The predicted amount of time the system will be operating withouterror the usual level is a BER


15/23

7.2 RELIABILITY VARYING RSL

If the RSL varies periodically look at the

surrounding topology. Is there a factory nearby

which might have large impulses?

Is there any building work going on in the LOS?

Perhaps a crane is moving in and out of shot.

Is the shot over water? Perhaps you are the victim

of reflections at high tide. Is the dish loose? Look for RSLs which vary with

high winds.



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7.3 RELIABILITY RAIN

A common question regarding radio wavesused for outdoor communications is in regardsto the effect of rain, snow, fog or any

inclement weather, there is a substantialdifference in the effect of rain on systemsoperating at or below 6 GHz frequencies

compared to 11 GHz or higher frequencies. Ina cloudburst condition the attenuation of thesignal level can be substantial.



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EXAMPLE CALCULATION OF FADE

MARGIN

For a vertically polarized 15 km 23 GHz link nearLondon, calculate the fade experienced for more than0.01 % of the time.

London is in rainfall region E with a rainfall of 22 mm

Hr-1

Dedd= 15/1+15/25.16 = 9.4 Km

At 23 GHz and vertical polarization a = 0.09544 and b =1.055

Lr = a x Rb(0.01%)X Deff

= 22.6 dB. This is the require fade margin


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18/23

8. 1 System Instability.

The three leading causes of system instability,

other than equipment failure, are as follows:

1. Excessive path length

2. Excessive antenna height

3. RF interference



19/23

9.1 PROPAGATION A microwave beam can also be reflected by water or relatively smooth terrain, very

much in the same way a light beam can be reflected from a mirror. Again, since thewavelength of a microwave beam is much longer than that of a visible light beam,the criteria for defining smooth terrain is quite different between the two. While alight beam may not reflect well off of an asphalt road, a dirt field, a billboard, or theside of a building, to a microwave beam these can all be highly reflective surfaces.Even gently rolling country can prove to be a good reflector.

A microwave beam arriving at an antenna could effectively be cancelled by its ownreflection, causing signal loss. Long microwave paths can also be affected byatmospheric refraction, the result of variations in the dielectric constant of theatmosphere.

For relatively short 2.4GHz microwave paths, only reflection points andobstructions are usually of real concern. The effects of atmosphere and earthcurvature will not usually come into play, so the engineering of these paths is quite

straightforward. For long or unusual paths, however, all aspects of path engineeringmust be considered.



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9.2 PROPAGATION TERRAIN

CONSIDERATIONS Mountainous terrain is best

Many multipath reflections will not reach the other end,thus reducing the potential for out-of-phase reflectedsignals that may have degraded the integrity of the direct

signal Flat, smooth terrain is worst Many multipath reflections

may reach the other end, thus increasing the potential forout-of-phase reflected signals.

Remember that Fresnel zones are three-dimensional. When

designing a link that goes down a street between buildings,or between groves of trees, these are also potential sourcesof reflection points and need to be evaluated in the design.



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9.3 PROPAGATION.REFRACTION AND

SCATTERING Youve probably seen the apparently bent straw or spoon in

a glass of water, or reached into water to touch something,and it is not exactly where you think it is. This is becausethe light waves refract through water compared to air.Again, radio waves behave in a similar manner.

Scattering is best visualized by considering looking at alight source when it is foggy versus when it is clear. When itis foggy, you will see less energy overall because some of itis absorbed and/or reflected away from you by the watermolecules and the light that you do see will be distorted in

comparison. Again, radio waves behave in a similar manner.



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A SET OF BASIC RULES1.

Always select the proper frequency band. Long links should use the lower frequency bands(e.g. 13 or 15 GHz), short links should use as high frequencies as possible (23, 38 or 58

GHz). In several countries there are local regulations forcing all the network operators for

efficient band selection.

2.

Avoid High-Low (H/L) conflict on sites to eliminate the near-field interference. Frequency

Division Duplex (FDD) radios have high transmit/low receive and low transmit/highreceive frequency sub-band variants. As a general rule, it is recommended to use always

the same sub-band of the radio links on a given site. In some cases the near-field

interference may be shadowed by obstacles, e.g. concrete walls on building rooftops and

the H/L conflict rule can be disregarded. The rule is not relevant for TDD radios that

transmit and receive in the same frequency.



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3:

Preferably use high gain and high performance antennae High antenna gain can compensate for

low output power. High performance antennae reduce the transmission of power density into

unwanted directions.

4:

Use proper polarization to increase discrimination between neighbouring links.

5:

Balance RSLat nodal points terminating several MW links. Decreasing the difference between RS

of different links reduce the probability of harmful interference.

6:

Calculate interference with IQ Link. Use the correct set of interference matrices from real radio

measurements.

Following these basic rules should keep you out of most difficult scenarios

23Written and prepared by Bill Williamsonbillengineer@btinternet com

planning tips when migrating frequencies

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