The Efficiency of a Cellular Network Depends of Its Correct Configuration and Adjustment of Radiant Systems

8/10/2019 The Efficiency of a Cellular Network Depends of Its Correct Configuration and Adjustment of Radiant Systems

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The efficiency of a cellular network depends of its correct configuration andadjustment of radiant systems: their transmit and receive antennas.

And one of the more important system optimizations task is based on correct

adjusting tilts, or the inclination of the antenna in relation to an axis. With the tilt,

we direct irradiation further down (or higher), concentrating the energy in the new

desired direction.

When the antenna is tilted down, we call it 'downtilt', which is the most common use.If the inclination is up (very rare and extreme cases), we call 'uptilt'.

Note: for this reason, when we refer to tilt in this tutorial this means we're talking

about 'downtilt'. When we need to talk about 'uptilt' we'll use this nomenclature,explicitly.

The tilt is used when we want to reduce interference and/or coverage in somespecific areas, having each cell to meet only its designed area.

Although this is a complex issue, let's try to understand in a simple way how all ofthis works?

Note: All telecomHall articles are originally written in Portuguese. Following wetranslate to English and Spanish. As our time is short, maybe you find some typos(sometimes we just use the automatic translator, with only a final and 'quick'

review). We apologize and we have an understanding of our effort. If you want tocontribute translating / correcting of these languages, or even creating andpublishing your tutorials, please contact us:contact.

But Before: Antenna Radiation Diagram
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Before we talk about tilt, it is necessary to talk about another very importantconcept: the antennas radiation diagram.

The antenna irradiation diagram is a graphical representation of how the signal isspread through that antenna, in all directions.

It is easier to understand by seeing an example of a 3D diagram of an antenna (inthis case, a directional antenna with horizontal beamwidth of 65 degrees).

The representation shows, in a simplified form, the gain of the signal on each of

these directions. From the center point of the X, Y and Z axis, we have the gain in alldirections.

If you look at the diagram of antenna 'from above', and also 'aside', we would seesomething like the one shown below.


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These are the Horizontal (viewed from above) and Vertical (viewed from the side)diagrams of the antenna.

But while this visualization is good to understand the subject, in practice do not workwith the 3D diagrams, but with the 2D representation.

So, the same antenna we have above may be represented as follows.

Usually the diagrams have rows and numbers to help us verify the exact 'behavior' ineach of the directions.

The 'straight lines' tells us the direction (azimuth) as the numbers 0, 90, 180 and 270 inthe figures above.

And the 'curves' or 'circles' tells us the gain in that direction (for example, the larger circletells you where the antenna achieves a gain of 15 db).

According to the applied tilt, we'll have a different modified diagram, i.e. we affect

the coverage area. For example, if we apply an electrical tilt of 10 degrees toantenna shown above, its diagrams are as shown below.


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As we have seen, when we apply a tilt, we change the antenna radiation diagram.

For a standard antenna, without Tilt, the diagram is formed as we see in thefollowing figure.

There are two possible types of Tilt (which can be applied together): the electrical

Tilt and Mechanical Tilt.

The mechanical tilt is very easy to be understood: tilting the antenna, throughspecific accessories on its bracket, without changing the phase of the input signal,the diagram (and consequently the signal propagation directions) is modified.


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And for the electrical tilt, the modification of the diagram is obtained by changing thecharacteristics of signal phase of each element of the antenna, as seen below.

Note: the electrical tilt can have a fixed value, or can be variable, usually adjustedthrough an accessory such as a rod or bolt with markings. This adjustment can be

either manual or remote, in the latter case being known as 'RET' (Remote ElectricalTilt) usually a small engine connected to the screw stem/regulator that does thejob of adjusting the tilt.

With no doubt the best option is to use antennas with variable electrical tilt ANDremote adjustment possibility, because it gives much more flexibility and ease to theoptimizer.

However these solutions are usually more expensive, and therefore the antennaswith manual variable electrical tilt option are more common.

So, if you don't have the budget for antennas with RET, choose at least antennas

with manual but 'variable' electrical tilt only when you have no choice/options,choose antennas with fixed electrical tilt.

Changes in Radiation diagrams: depends on the Tilt Type

We have already seen that when we apply a tilt (electrical or mechanical) to an

antenna, we have change of signal propagation, because we change the 3D diagramas discussed earlier.

But this variation is also different depending on the type of electrical or mechanical

tilt. Therefore, it is very important to understand how the irradiated signal is affectedin each case.

To explain these effects through calculations and definitions of db, null and gains onthe diagram is possible. But the following figures shows it in a a much moresimplified way, as horizontal beamwidth behaves when we apply electrical andmechanical tilt to an antenna.

See how is the Horizontal Irradiation Diagram for an antenna with horizontalbeamwidth of 90 degrees.


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Of course, depending on the horizontal beamwidth, we'll have other figures. But the

idea, or the 'behavior' is the same. Below, we have the same result for an antennawith horizontal beamwidth of 65 degrees.


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Our goal it that with the pictures above you can understand how each type of tiltaffects the end result in coverage one of the most important goals of this tutorial.

But the best way to verify this concept in practice is by checking the final coveragethat each one produces.

To do this, then let's take as a reference a simple 'coverage prediction' of a samplecell. (These results could also be obtained from detailed Drive Test measurements in

the cell region).


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Then we will generate 2 more predictions: the first with electrical tilt = 8 degrees(and no mechanical tilt). And the second with only mechanical of 8 degrees.

Analyzing the diagrams for both types of tilt, as well as the results of the predictions(these results also can also be proven by drive test measures) we find that:

With the mechanical tilt, the coverage area is reduced in central direction,but the coverage area in side directions are increased.

With the electrical tilt, the coverage area suffers a uniform reduction in thedirection of the antenna azimuth, that is, the gain is reduced uniformly.

Conclusion: the advantages of one tilt type to another tilt type are very based on itsapplication when one of the above two result is desired/required.

But in General, the basic concept of tilt is that when we apply the tilt to an antenna,

we improve the signal in areas close to the site, and reduced the coverage in moreremote locations. In other words, when we're adjusting the tilt we seek a signal as


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strong as possible in areas of interest (where the traffic must be), and similarly, asignal the weakest as possible beyond the borders of the cell.

Of course everything depends on the 'variables' involved as tilt angle, height andtype of antenna and also of topography and existing obstacles.

Roughly, but that can be used in practice, the tilt angles can be estimated throughsimple calculation of the vertical angle between the antenna and the area of interest.

In other words, we chose a tilt angle in such a way that the desired coverage areasare in the direction of vertical diagram.

It is important to compare:

the antenna angle toward the area of interest;

the antenna vertical diagram.

We must also take into account the antenna nulls. These null points in antenna

diagrams should not be targeted to important areas.

As basic formula, we have:

Angle = ArcTAN (Height / Distance)

Note: the height and distance must be in the same measurement units.

Recommendations

The main recommendation to be followed when applying tilts, is to use it with

caution. Although the tilt can reduce interference, it can also reduce coverage,especially in indoor locations.


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So, calculations (and measurements) must be made to predict (and check) theresults, and if that means coverage loss, we should re-evaluate the tilt.

It is a good practice to define some 'same' typical values (default) of tilt to be

applied on the network cells, varying only based on region, cell size, and antennasheights and types.

It is recommended not to use too aggressive values: it is better to start with a smalltilt in all cells, and then go making any adjustments as needed to improvecoverage/interference.

When using mechanical tilt, remember that the horizontal beamwidth is wider to the

antenna sides, which can represent a problem in C/I ratio in the coverage ofneighboring cells.

Always make a local verification, after changing any tilt, by less than it has been.This means assessing the coverage and quality in the area of the changed cell, andalso in the affected region. Always remember that a problem may have been solved

... but another may have arisen!

Documentation

The documentation is a very important task in all activities of the

telecommunications area. But this importance is even greater when we talk aboutRadiant System documentation (including tilts).

It is very important to know exactly 'what' we have currently configured at each

network cell. And equally important, to know 'why' that given value has changed, oroptimized.

Professionals who do not follow this rule often must perform rework for severalreasons simply because the changes were not properly documented.

For example, if a particular tilt was applied to remove the interfering signal at a VIP

customer, the same should go back to the original value when the frequency plan isfixed.

Other case for example is if the tilt was applied due to problems of congestion. Afterthe sector expansion (TRX, Carriers, etc), the tilt must return to the previous value,

reaching a greater coverage area, and consequently, generating overall greater

revenue.

Another case still is when we have the activation of a new site: all neighboring sitesshould be reevaluated both tilts and azimutes.

Of course that each case should be evaluated according to its characteristics andonly then deciding to aplly final tilt values. For example, if there is a large building infront of an antenna, increasing the tilt could end up completely eliminating the

signal.


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In all cases, common sense should prevail, evaluating the result through all the

possible tools and calculations (as Predictions), data collection (as Drive Test) andKPI's.

Practical Values

As we can see, there's not a 'rule', or default value for all the tilts of a network.

But considering the most values found in field, reasonable values are:

15 dBi gain: default tilt between 7 and 8 degrees (being 8 degrees to smaller cells).

18 dBi gain: default tilt between 3.5 and 4 degrees (again, being 4 degrees to smallercells).

These values have tipically 3 to 5 dB of loss on the horizon.

Note: the default tilt is slightly larger in smaller cells because these are cells are indense areas, and a slightly smaller coverage loss won't have as much effect as in

larger cells. And in cases of very small cells, the tilt is practically mandatory otherwise we run the risk of creating very poor coverage areas on its edges due toantenna nulls.

It is easier to control a network when all cells have approximately the same value onalmost all antennas: with a small value or even without tilt applied to all cells, wehave an almost negligible coverage loss, and a good C/I level.

Thus, we can worry about - and focus - only on the more problematic cells.

When you apply tilts in antennas, make in a structured manner, for example withsteps of 2 or 3 degrees document it and also let your team know this steps.

As already mentioned, the mechanical tilt is often changed through the adjust of

mechanical devices (1) and (2) that fixes the antennas to brackets.


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And the electrical tilt can be modified for example through rods or screws, usually

located at the bottom of the antenna, which when moved, applies somecorresponding tilt to the antenna.


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For example in the above figure, we have a dual antenna (two frequency bands), and

of course, 2 rods (1) and (2) that are moved around, and have a small display (3)indicating the corresponding electrical tilt one for each band.

And what are the applications?

In the definitions so far, we've already seen that the tilts applications are several, asto minimize neighboring cells unwanted overlap, e.g. improving the conditions for

the handover. Also we can apply tilt to remove local interference and increase the

traffic capacity, and also cases where we simply want to change the size of certaincells, for example when we insert a new cell.

In A Nutshell: the most important thing is to understand the concept, or effect ofeach type of tilt, so that you can apply it as best as possible in each situation.

Final Tips

The tilt subject is far more comprehensive that we (tried to) demonstrate heretoday, but we believe it is enough for you to understand the basic concepts.

A final tip is when applying tilts in antennas with more than one band.

This is because in different frequency bands, we have different propagation losses.For this reason, antennae that allow more than one band has different propagationdiagrams, and above all, different gains and electrical tilt range.

And what's the problem?


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Well, suppose as an example an antenna that has the band X, the lower, and a bandY, highest.

Analyzing the characteristics of this specific antenna, you'll see that the ranges ofelectrical tilt are different for each band.

For example, for this same dual antenna we can have:

X band: electrical tilt range from 0 to 10 degrees.

Y band: electrical tilt range from 0 to 6 degrees.

The gain of the lower band is always smaller, like to 'adjust' the smaller loss that this

band has in relation to each other. In this way, we can achieve a coverage arearoughly equal on both bands of course if we use 'equivalent' tilts.

Okay, but in the example above, the maximum is 10 and 6. What would beequivalent tilt?

So the tip is this: always pay attention to the correlation of tilts between antennaswith more than one band being transmitted!

The suggestion is to maintain an auxiliary table, with the correlation of these pre-defined values.

Thus, for the electrical tilt of a given cell:

X Band ET = 0 (no tilt), then Y Band ET = 0 (no tilt). Ok.

X Band ET = 10 (maximum possible tilt), then Y Band ET = 6 (maximum possible tilt). Ok.

X Band ET = 5. And there? By correlation, Y Band ET = 3!

Obviously, this relationship is not always a 'rule', because it depends on each band

specific diagrams and how each one will reach the areas of interest.

But worth pay attention to not to end up applying the maximum tilt in a band (Y ET

= 6), and the 'same' (X ET = 6) in another band because even though they havethe same 'value', actually they're not 'equivalent'.

After you set this correlation table for your antennas, distribute it to your team so,when in the field, when they have to change a tilt of a band they will automatically

know the approximate tilt that should be adjusted in the other(s).

And how to verify changes?

We have also said previously that the verifications, or the effects of tilt adjustments

can be checked in various ways, such as through drive test, coverage predictions,

on-site/interest areas measurements, or also through counters or Key PerformanceIndicators-KPI.

Specifically about the verifications through Performance counters, in addition to KPIdirectly affected, an interesting and efficient form of verification is through Distancecounters.


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On GSM for example, we have TA counters (number of MR per TA, number of RadioLink Failure by TA).

Note: we talked about TA here at telecomHall, and if you have more interest in thesubject,click here to read the tutorial.

This type of check is very simple to be done, and the results can be clearlyevaluated.

For example, we can check the effect of a tilt applied to a particular cell throughcounters in a simple Excel worksheet.

Through the information of TA for each cell, we know how far the coverage of eachone is reached. So, after we change a particular tilt, simply export the new KPI data(TA), and compare the new coverage area (and also the newdistributions/concentrations of traffic).

Another way, perhaps even more interesting, is plotting this data in a GIS program,

for example in Google Earth. From the data counters table, and an auxiliary tablewith the physical information of cells (cellname, coordinates, azimuth) can have aresult far more detailed, allowing precise result checking as well.
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Several other interesting information can be obtained from the report (map) above.

When you click some point, we have its traffic information. The color legend alsoassists in this task. For example, in regions around the red dots, we have a traffic

between 40 and 45 Erlangs. In the same logic, light yellow points between 10 and 15resulting Erlangs according to legend see what happens when we click at thatparticular location: we have 12.5 Erlangs.


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Another piece of information that adds value to the analysis, also obtained by

clicking any point, is the percentage of traffic at that specific location. For example,

in the yellow dot we have clicked, or 12.5 Erlangs = 14% out of a total of 88.99Erlangs that cell has (the sum of all points).

Also as interesting information, we have the checking of coverage to far from thesite, where we still have some traffic. In the analysis, the designer must take into

account if the coverage is rural or not. If a rural coverage, it may be maintained

(depends on company strategy). Such cases in sites located on cities, are most likelysignal 'spurious' and probably should be removed for example with the use of tilt!

The creation and manipulation of tables and maps processed above are subject of

our next tutorial 'Hunter GE TA', but they aren't complicated be manually obtained

mainly the data in Excel, which already allow you to extract enough information andhelp.

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The Efficiency of a Cellular Network Depends of Its Correct Configuration and Adjustment of Radiant Systems