Antenna Down Tilting

8/6/2019 Antenna Down Tilting

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Antenna Downtilt Guideline

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ANTENNA DOWNTILT GUIDELINE


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TABLE OF CONTENTS

1 INTRODUCTION .............................................................................................. 3

2 ANTENNAS...................................................................................................... 3

2.1 The Antenna diagram ............................................................................................... 32.1.1 Gain .................................................................................................................... 32.1.2 Horizontal beamwidth.......................................................................................... 32.1.3 Vertical beamwidth.............................................................................................. 32.1.4 First null beamwidth............................................................................................. 52.1.5 Null-fill ................................................................................................................. 52.1.6 Back-lobe ............................................................................................................ 6

2.2 Mechanical versus electrical tilt .............................................................................. 6

2.3 Super high gain antennas........................................................................................ 8

2.4 Theoretical tilt-effects............................................................................................... 8

3 MEASUREMENTS ......................................................................................... 12

3.1 Signal strength MEASUREMENTS in forward direction....................................... 133.1.1 18 dBi Antennas ................................................................................................ 133.1.2 15 dBi antennas ................................................................................................ 15

3.2 Signal strength MEASUREMENTS in side direction............................................. 17

3.3 Signal strength MEASUREMENTS in Backward direction ................................... 19

4 RECOMMENDATIONS .................................................................................. 20

4.1 General recommendations..................................................................................... 20

4.2 Recommended tilt-values....................................................................................... 224.2.1 Areas with large cells......................................................................................... 224.2.2 Areas with small cells ........................................................................................ 22

5 CONCLUSION................................................................................................ 23


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1 INTRODUCTION

With an increasing capacity demand, and a limited frequency spectrum,the operators are forced to utilise the frequency spectrum moreefficiently. High capacity frequency planning techniques are often basedon tight frequency reuse. The networks become interference limited, andin order to maximise the capacity, every available technique to minimiseinterference becomes important.

As the capacity demand increases, the network plans also becometighter. A macro-cell site-site distance down to 400 meter or less is not

unusual. With shorter site-to-site distances, limiting the interference fromeach cell becomes more and more important. When the cells are verysmall, down tilt can be applied without loss of coverage. Compared to notilt at all, downtilt can even improve coverage in these dense networks.

A well-chosen overall tilt-strategy can lower the overall interference inthe network. A too aggressive down tilting strategy will however lead toan overall loss of coverage. In addition to a general down tilt strategy,applied in all cells, down tilt can be used to solve specific problems, forexample local interference problems or cells that are too large.

2 ANTENNAS

2.1 THE ANTENNA DIAGRAM

2.1.1 Gain

The antenna diagrams show the antenna gain, in a given direction,relative an isotropic antenna. The maximum gain for an antenna can beincreased by narrowing the horizontal and/or the vertical beam width.

Typical for a three sector site is a 65 horizontal beam width with amaximum gain of 15 or 18 dBi.

2.1.2 Horizontal beamwidth

The standard antennas for a three-sector site has a horizontal beam

width, also referred to as the half power beam width, of 65. This

means that the gain is 3 dB less at +/- 32.5 (i.e. half power) than the

maximum gain in the 0 direction. At 60 (i.e. the theoretical cell borderbetween the sectors), the gain is suppressed typically 10 dB.

2.1.3 Vertical beamwidth

The most interesting part of the antenna pattern when it comes to tiltingis the vertical antenna-gain pattern in the forward direction. A 15 dBi

antenna usually has a vertical half power beam width of around 15 (i.e.


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+/- 7.5 ). The high gain 18 dBi antennas have a narrower vertical beam

width, typically 6-8 (i.e. +/- 3 - 4).

Below is an example of two typical antennas with 15.5 and 18 dBi gain.

Vertical antenna gain in forward direction

-40

-35

-30-25

-20

-15

-10

-5

0

5

-25 -20 -15 -10 -5 0 5 10 15 20 25

Degrees

dB GAIN 15 dBi

GAIN 18 dBi

Figure 1. Vertical gain for two typical 15 dBi and 18 dBi antennas.

If the antenna tilted for example 5, the gain in the horizontal direction,

relative the maximum gain, equals the gain at 5 in the antenna

diagram. For the antennas above, a 5 downtilt would meanapproximately 1.5 dB for the 15.5 dBi antenna, and 8.5 dB for the 18dBi antenna.

Below is an example of what an 18 dBi vertical antenna diagram will look

like with 0, 5 and 8 down tilt.


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Vertical diagram for different tilts (18 dBi gain antenna)

-30

-25

-20

-15

-10

-5

0-10 -5 0 5 10 15 20

degrees

dB

GAIN 18 dBi, 5 degrees

GAIN 18 dBi, 0 degree tilt

GAIN 18 dBi, 8 degree tilt

Figure 2. Vertical antenna diagram for 18 dBi antenna. 0, 5 and 8 tiltapplied.

Note that the antenna diagram is valid only in the forward direction.When mechanical downtilt is applied, the tilt effect in directions otherthan straight forward is different, which means that the horizontalantenna diagram is changed (see further chapter 2.2).

2.1.4 First null beamwidth

The first null beam width is the angle between the nulls adjacent to themain lobe. In the antenna diagram in Figure 1, it can be seen that the 15

dBi antenna has a First null Beam width of 32 (+/- 16). The 18 dBi

antenna has a First null Beam width of 15 (+/- 7,5). These figures mayvary a little bit for different antennas models, but the figures are roughly

the same for all 65 antennas with 15 or 18 dBi gain.

Tilting half of the First null Beam width will, at least in theory, suppressthe antenna gain towards the horizon with up to 20 dB or more.

2.1.5 Null-fill

Some antennas use null-fill in order to make the first null under the

horizon smaller. This is to limit the loss of signal strength that the mobilemay experience if it is located at a position where the vertical angle fromthe basestation antenna corresponds to the first null under the horizon inthe antenna diagram. Such antennas do however tend to loose some ofits maximum gain. Most of the large antenna manufacturers such asKathrein, has a the first null specified to be > -25 dB relative themaximum gain. This figure is however somewhat theoretical, since theactual antenna diagram for these low power dips is effected by theantenna mounting. Moreover, the reflections and diffractions in the wavepropagation will even out the dip in the antenna diagram, and thereceiving mobile will not experience such a dramatic decrease in signalstrength.


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2.1.6 Back-lobe

The theoretical back-lobes for two typical 65

antennas are shown in thepicture below. However, the actual antenna gain for different verticaldirections is very difficult to estimate. Things like the mounting mastsand the near environment on the roof has a large impact on the radiationin the backward direction. The actual signal strength behind the cell mayalso be the result of reflections from the energy transmitted in theforward direction. It is therefore difficult to theoretically predict the effectthat down tilting has on the signal strength in the backward direction.

Vertical antenna gain for Backlobe

-60

-55

-50

-45

-40

-35

-30

-25

-20

-15

-10

-50155 160 165 170 175 180 185 190 195 200 205

degrees

dB

GAIN 15 dBi

GAIN 18 dBi

Figure 3. Vertical gain for the back-lobe for two typical 15 dBi and 18 dBiantennas.

2.2 MECHANICAL VERSUS ELECTRICAL TILT

Mechanical tilt

When using mechanical tilt, the antenna is mounted with adjustablebrackets in a way that the tilt can be adjusted on site.

Electrical tilt

Electrical tilt means an in-built tilt that lowers the vertical beam in allhorizontal directions. Electrical tilt can be combined with additionalmechanical til t.


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Electrical Downtilt vs. Mechanical Downtilt

The largest advantage of electrical antenna down tilt is that thehorizontal beam width is not affected. With mechanical down tilt, the

tilting effect is greater in the 0 direction. At for example +/- 60, theeffective tilt angle becomes lower. This effect can be very diff icult topredict. With an overall, very high mechanical tilt level in the network, the

cells become shorter and wider, more comparable to maybe 90 antennas. The frequency planning becomes more difficult, and theoverall interference level in the network becomes higher.

Figure 4. Comparison of vertical antenna gain for mechanical andelectrical down tilt. Note that the graph above is only valid in the forward,

0 horizontal directon.

Electronical vs mechanical downtilt

-30

-25

-20

-15

-10

-5

0

-10 -5 0 5 10 15 20

degrees

dB

6 degrees EDT

18 dBi

6 degrees mechanical


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Mechanical vs Electrical Downtilt

Horizontal gain at zero degree vertical angle

-15

-10

-5

0

5

10

15

20

-90 -60 -30 0 30 60 90

Degrees

dBGain MDT

No tilt

EDT

Figure 5. Comparison of horizontal antenna gain at 0 vertical angle formechanical and electrical down tilt. Because no 3D antenna patterns arewas available, the Mechanical downtilt antenna diagram is estimatedfrom the vertical antenna pattern in the forward direction.

2.3 SUPER HIGH GAIN ANTENNAS

For 1800 MHz, there are extremely high gain (around 21 dBi) 65

antennas available. These antennas have an even narrower verticalbeam width, around half the beam width of an 18 dBi antenna. Theseantennas are larger than the standard 18 dBi 1800 MHz antennas. Theeffect that these antennas have on coverage in urban areas has notbeen verified, but with such narrow beam width, at least in theory aneven larger tilting effect can be achieved.

2.4 THEORETICAL TILT-EFFECTS

When selecting the optimum tilt angle, the goal is to have as high signalstrength as possible in the area where the cell should be serving traffic.Beyond the serving area of the cell, the signal strength should be as lowas possible.

The basic theory is that down tilting an antenna increases the signalstrength in the area close to the site, whereas the signal strengthbecomes lower at far distances. The relation between the signal strengthand distance from the site depends on:

Down tilt angle

Antenna type


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Antenna height

Near environment (topography and obstacles)

In an open environment, the effects of antenna down tilting can be fairlyaccurately estimated by calculating the vertical angle between theantenna and the mobile at various distances from the site.

Example

Tilt effect, in terms of antenna gain experienced by a mobile, calculatedgiven the following circumstances:

Effective antenna height: (antenna height mobile height): 50 meter

Distance, site mobile: 500 meter

Antenna down tilt: 8

The vertical angle to the mobile is:

=arctan(50/500) = 5.7

It the antenna was not down tilted, the antenna gain for the mobile would

correspond to 5.7 in the antenna diagram. However, since the antennais down tilted, the corresponding angle in the vertical antenna diagram

is:

5.7 - 8 = -2.3

In the figure below, the theoretical antenna gain for different distancesfrom the site have been calculated for a typical 18 dBi gain antenna. Theantenna gain has been added to a simple path propagation model inorder to show the signal strength in relation to the distance from the sitefor different antenna tilt angles.

In the calculations, a 50 meter antenna height has been assumed. Adifferent antenna height will change the scale of the X-axis, but the

relative gain for the different antenna tilt angles will remain. In order tomake the figure easier to read, two different scales have been plotted.


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Theoretical signalstrength, 15 dBi antenna

-100

-90

-80

-70

-60

-50

-40

0 500 1000 1500

Distance from site (meter)

dBm

Max Gain

0 degrees

8 degrees

14 degrees

Figure 6. The theoretical signal-strength from a 50 meter high site, usinga 15 dBi Gain antenna. The Max Gain is the theoretical signal-strength ifa dipole antenna, with 18 dBi Gain was used. The Max Gain is includedin the graph in order to be able to see the effect that the vertical antennadiagram has on the signal-strength for the different tilt-angles.


-130

-110

-90

-70

-50 0 2000 4000 6000 8000 10000 12000


dBm

Max Gain

0 degrees

8 degrees

14 degrees

Figure 7. The theoretical signal-strength from a 50 meter high site usinga 15 dBi antenna. Compared to Figure 6, the scale is different in order tosee the effect at far distance.


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-100

-90

-80

-70

-60

-50

-40

0 500 1000 1500


dBm

Max Gain

0 degrees

5 degrees

9 degrees

Figure 8. The theoretical signal-strength from a 50 meter high site, usinga 18 dBi Gain antenna.


-130

-110

-90

-70

-50 0 2000 4000 6000 8000 10000 12000


dBm

Max Gain

0 degrees

5 degrees

9 degrees

Figure 7. The theoretical signal-strength from a 50 meter high site usinga 18 dBi antenna. Compared to Figure 6, the scale is different in order to

see the effect at far distance.

As can be seen from the figures, with a 50 meter antenna height and nodown tilt, a 18 dBi antenna will have its first null at around 400 meterfrom the site, and a 15 dBi antenna will have its first null around 200meter from the site. Down tilting the antenna moves the first null closerto the site. At far distance, the signal strength is lower with down tilt,which means less coverage (if the cell is serving there) or reducedinterference (if the cell is not serving). This is the basic theory behind allantenna down tilting.


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3 MEASUREMENTS

This chapter contains some measurements that where performed in alarge Asian City. The topography is very flat, and has no significantimpact on the results.

The area refereed to as Urban is dense, but not extremely dense. Thebuildings are of various heights, including skyscrapers up to 100 meteror more. A photo from a typical Urban site is shown in Appendix.

Most of these measurements are from non line-of-site. Areas referred to

as Suburban do not have that many high-rise buildings, and thebuildings are not as densely built.

Five sites were selected for the measurements. These sites where all 3-

sector, using 65 horizontal beamwidth antennas with no Electricaldowntilt. This means that all tilts were done using mechanical downtilt.For each site, two cells where selected. Two of the sites had 18 dBiantennas, the other three sites where equipped with 15.5 dBi antennas.For each cell, the signal strength was measured for three different tilt

angels. If possible, the first measurement was always performed with 0tilt, as a reference. For some cells, the antenna mounting was however

such that 0 tilt could not be applied.

Measurement procedures

Prior to the measurements, the cells to be measured got the BCCHfrequencies configured with clean test-frequencies. The signal strengthwas measured by a TEMS phone, using Scan-mode, and loggedtogether with GPS readings. After the measurements, the signal strengthwas plotted in Map-info, and the result was analysed. In addition to this,the measurements were also post-processed in Mat-lab. The signalstrength was filtered out for different directions, and plotted as a functionof distance from the site. These plots are presented in this chapter.

These kind of measurements are time consuming. In this measurementproject, a large number of cells have been prioritised rather thanperforming a larger number of different tilt-angles for each cell. Due tothe fact that the different tilt-angles were not logged simultaneously, themeasurements are not exact enough to compare the signal-strengthvery close to the site. The accuracy of the positioning is limited by theGPS readings. Each drive-test was not performed with exactly the samespeed. This does also have an impact on the accuracy of the comparedsignal strengths for the different tilt-angles. In the measurement graphs,the signal-strength in each point is the middle value of all samples in a50 x 50 meter square.


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3.1 SIGNAL STRENGTH MEASUREMENTS IN FORWARD DIRECTION

The signal strength from the three different tilt angles are plotted ingraphs, as a function of the distance from the site. Only the

measurement samples with a horizontal angle of +/- 40 has been used.

The tilt angles used for each cell can be seen in the graph labels. Inaddition to the signal strength plots, the relative difference, compared to

0 tilt (or the lowest measured tilt where not applicable) is also plotted.

3.1.1 18 dBi Antennas

Cell 1A: 18 dBi antenna, Semi Urban environment,

45 meter antenna height

0, 9 and 14 degree tilt, Forward direction (+/- 40 degrees)

-110-100-90-80-70-60-50-40-30-20-10

01020

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5

Disance form site, km

dB

SS_0

SS_9

SS_14

SS_9 - SS_0

SS_14 - SS_0


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Cell 1B dBi antenna, Semi-Urban environment,


Forward direction (+/- 40 degrees)

-110-100-90-80-70-60-50-40-30-20-10

01020

0 0.5 1 1.5 2 2.5 3 3.5 4


dB

SS_0

SS_4

SS_12

SS_4 - SS_0

SS_12 - SS_0

Cell 2A: 18 dBi antenna, Urban environment, 50 meter antenna

height


-110-100-90-80-70-60

-50-40-30-20-10

01020

0 0.5 1 1.5 2 2.5


dB

SS_5

SS_9

SS_14

SS_9 - SS_5SS_14 - SS_5

Cell 2B: 18 dBi antenna, Urban environment, 50 meter antenna height


-110

-100

-90

-80

-70

-60

-50

-40

-30

-20

-100

10

20

0 0.5 1 1.5 2 2.5


dB

SS_0

SS_5

SS_12

SS_5 - SS_0

SS_12 - SS_0


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3.1.2 15 dBi antennas

Cell 3A: 15.5 dBi antenna, Urban environment,30meter antenna heigth


-110

-100-90-80-70-60-50-40-30-20-10

01020

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5


dB

SS_0

SS_8

SS_14

SS_8-SS_0

SS_14-SS_0

Cell 3B: 15.5 dBi antenna, Urban environment,



-110-100

-90-80-70-60-50-40-30-20-10

01020

0 0.5 1 1.5 2 2.5 3 3.5


dB

SS_0

SS_8

SS_14

SS_8-SS_0

SS_14-SS_0

Cell 4A: 15.5 dBi antenna, Urban environment,



-110-100

-90-80-70-60-50-40-30-20-10

01020

0 0.5 1 1.5 2

Distance from site, km

dB

SS_0

SS_8

SS_14

SS_8-SS_0

SS_14-SS_0


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Cell 4B: 15.5 dBi antenna, Urban



-110-100

-90-80-70-60-50-40-30-20-10

01020

0 0.5 1 1.5 2 2.5

Distance form site, km

dB

SS_1

SS_8

SS_14

SS_8-SS_1

SS_14-SS_1

Cell 5A: 15.5 dBi antenna, Urban environment



-110-100-90-80-70

-60-50-40-30-20-10

01020

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7


dB

SS_0

SS_8

SS_14

delta_8

delta_14

Cell 5B: 15.5dBi antenna, Sub-urban environment



-110-100

-90-80-70-60-50-40-30-20-10

010

20

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6


dB

SS_0

SS_8

SS_14

SS_8-SS_0

SS_14-SS_0


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Evaluation of Signals strength measurements in forward direction

The measurements show that the effect of tilting was pretty much ascould be expected from the theoretical calculations.At far distances, thesignal strength becomes lower when the antenna is tilted, but not quiteas much as can be expected from the theoretical calculations based onthe antenna diagram. One reason for this could be that the signalstrength that reaches the mobile actually is transmitting more straightforward (at a lower vertical angle), above the rooftops, and laterdiffracted down to the mobile station.

For the 18 dBi antennas, down tilt ing increases the signal strength ataround 500 meter and closer to the site. For 15 dBi antennas, down

tilting increase the signal strength at closer than around 2 300 meterfrom the site. It should be kept in mind that these sites where mostlyaround 50 meter high. If the sites are lower, the break point where thedown tilted antenna is stronger, is closer to the site. At very high sites,this breakpoint is further from the site.

The measurements show that an overall down tilt, of all cells in thenetwork, cangive a positive effect on the signal-to-interference ratio, C/I.This is however only true if the cell size does not exceed the distancewhere down tilting will reduce the coverage. For typical Urban cellplans,with sites that are 50 meter or lower, this means that the cell rangesshould not exceed around 500 meter. If the cells are larger, an overall

down tilt, for every cell, will reduce the overall coverage, but not have asignificant impact on the overall C/I levels at the cell-boarders. This isdue to that downtilt will lower the signal strength at the cell-boarders inalmost the same extent as it will lower the signal-strength further awayfrom the site where the cell is causing interference. Hence, theconclusion is that a general down tilting strategy, down tilting all cellsmore than the angle that corresponds to a 3dB loss at the horizon,should only be applied in areas where the cells are small, with a range ofaround 500 meter or less. This corresponds to a site/site distance ofaround 700-800 meter.

3.2 SIGNAL STRENGTH MEASUREMENTS IN SIDE DIRECTION

For these graphs, only the measurement samples with a horizontal

angle of +/- 50 70 has been used. This angle has been chosen inorder to represent the signal strength at the cellborder towards the co-sited cells.

The results were consistent for every site, therefore one graphs, with atypical result, are presented here. The graph for the forward direction isincluded as a reference.


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Side direction (+/- 50-70 degrees)

-110-100-90-80-70-60-50-40-30-20-10

01020

0 0.

5

1 1.

5

2 2.

5

3 3.

5

4 4.

5

5 5.

5

6 6.

5

7 7.

5

8 8.

5


dB

SS_0

SS_8

SS_14

delta_8

delta_14




-110-100-90

-80-70-60-50-40-30-20-10

01020

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7


dB

SS_0

SS_8

SS_14

delta_8

delta_14

Evalutaion of Signals strength measurments in side direction

All measurements where done with mechanical down tilt. The resultshows that the down tilt has a similar effect on the signal strength at anangle corresponding to the cell border to the co-sited cells, but at a lowerdegree. The signal strength becomes stronger close to the site, and

weaker further away, but not as much as in the forward direction. Thisresult is in-line with what can be expected from a theoretical point ofview.

In practise, this result tells us that if the cell diameter is larger thanapproximately 500 meter, the horizontal beam width of the cell becomeswider. The cells become shorter and wider. This will have a negativeimpact on frequency planning and the C/I relations between the cells. Itmay for example make be impossible to plan a 4/12 plan with sufficientC/I levels.


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3.3 SIGNAL STRENGTH MEASUREMENTS IN BACKWARD DIRECTION

For the graph below, only the measurement samples that had a

horizontal angle of 140 - 220 was used. This corresponds to the back-

lobe +/- 40.

Cell 5B: 15.5dBi antenna, Sub-urban environment

50 meter antenna heigth

Backward direction (180 +/- 40 degrees)

-110-100-90-80-70-60-50

-40-30-20-10

01020

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5


dB

SS_0

SS_8

SS_14

SS_8-SS_0

SS_14-SS_0

Evalutaion of Signals strength measurments in backward direction

The measurements show that in the backward direction, the antennadown tilt did not have any significant effect on the signal strength behindthe cell. Even at cells with large nearby buildings, where reflections are

expected, down tilting did not have an effect on the signal strength in thebackward direction.

The risk of being served by the back-lobe is however effected by tiltingsince the signal strength in the forward direction changes. The risk ofbeing served by a back-lobe is reduced close to the site since the signalstrength in the forward direction (from the cell that should be serving) isstronger. Further away from the site, the effect is the opposite since thesignal strength in the forward direction becomes weaker when theantenna is down tilted. However, far away from the site, back-lobeproblems are not very common. The conclusion is therefore that tiltingdoes generally not make back-lobe problems more critical.


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4 RECOMMENDATIONS

4.1 GENERAL RECOMMENDATIONS

One general recommendation is not to apply a large down tilt for allcells in an area. The reason for this is that:

1. It becomes diff icult to to fix specific problems, e.g. interferenceproblems or cells that are too large. If a cell that already has a largedonwntilt applied, needs to be further down tilted due to for examplea local interference problem, this cell would need to have a verylarge tilt angle. The effects from very large downtilts are difficult topredict, and may lead to a significant loss of coverage in the area.

2. If mechanical downtilt is used, the horizontal beam-width of theantenna becomes wider (see chapter 2.2). This effect is difficult toconsider in the frequency planning, and the wider antenna diagramprobably creates an overall worse C/I distribution in the network.

3. If the cellplan is not very tight (around 700 meter site-to-site distanceor more depending on antenna heights and type), antenna downtilting will reduce the overall coverage in the network. This is ofcourse not good for the quality of the network, especially for indoorareas with weak coverage.

One good strategy is to have a few default tilt values that areimplemented on every site. The default value can be differentdepending on the area, the size of the cell, antenna height and whichtype of antenna that is used. The general recommendation ishowever to keep it simple, and not do to many theoreticalcalculations for every site. It is better to start with a low tilt for everycell (see further the recommendations in chapter 4.2.1, 4.2.2), andstudy the actual coverage and interference situation. On a case bycase bases, apply further down-tilt can be applied (and verifiedthrough drive-tests and analysing statistics!).

There is no point in tilting an antenna less than the angle which gives

a 3 dB loss at the horizon. This corresponds to around 7 tilt for a 15dBi antenna, and around 3.5 tilt for an 18 dBi antenna. A smaller tiltgives a limited impact and is hardly worth the effort.

Study the antenna diagram carefully before selecting the tilt-angle.Most of the tilting effect happens between the angle that correspondsto the 3dB point towards the horizon, and the angle that correspondsto tilting the first null towards the horizon. It is sort of like the ketchup

effect. For example 8 tilt gives far more than twice the effect

compared to 4.


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Avoid down tilting more than the angle that correspond to having thefirst null towards the horizon. Further down tilting can be done in

extreme cases, but if there is a need for further reduction ofinterference or cell-size, a reduction of output power, or possiblylowering of the antenna height, should also be considered. Verylarge down tilts (beyond the first null towards the horizon) should becarefully verified since the effect of such large tilts is difficult topredict.

Define, for every antenna type, four or five tilt-angles, and do onlyuse these tilts. This makes it easier to work in a structured way, andto have better control over all the down tilts in the network. Anexample of such fixed tilt-values could be:

Default Tilt-angles(exact values depends on

the antenna diagram)

Theoretical gainat horizon

(relative max.Gain)

15 dBiantenna

18 dBiantenna

0 dB 0 0

3 dB 7 3.5

6 dB 9.5 5

10 dB 11.5 6

> 15 dB 14 7

Document all antenna down tilts! It is important not only to know howmuch each cell is down tilted, but also WHY the down tilt wasperformed. If an antenna tilt was performed in order to solve a localinterference problem due to for example a bad co-channel, this tiltshould possibly be removed when a new frequency plan (without thisco-channel) is implemented. Another example is a cell that has been

down tilted because of congestion. If the cell is expanded withadditional transceivers, it might be possible to reduce the down tilt.

A new site effects the coverage area of all cells that are neighboursto the new site. The downtilt angles in these sites should be revised.Additional downtilt should be considered in neighbouring cells thatgets a reduced coverage area.

Verify all the effects after having performed a down tilt of more than

4 (18 dBi antennas) or 8 (15 dBi antennas). Remember that it isjust as important to check the coverage and quality in the down tiltedcell, as the area where the down tilt is expected to reduce


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interference. Even if one problem is solved, a new problem mighthave occurred.

It is better to put a lot of effort tilting and verifying the result on a fewcells compared of doing a quick and dirty job tilting a larger numberof cells.

Consider the nearby environment. Use common sense. If there forexample is a close by building, which is of almost equal height as theantenna, a down tilt will make the coverage beyond that building toalmost disappear.

4.2 RECOMMENDED TILT-VALUES

4.2.1 Areas with large cells (approximately 800 meter site-site distanceor more):

Around 3.5 for an 18 dBi antenna, and 7 for 15 dBi antenna couldbe used as default tilting values. Compared to having no tilt at all,this may give a possible minor positive impact on the C/I levels,without any significant loss of coverage. The effect of such small tiltis however minimal. If the cells in an area currently have no down tiltat all, it might be better to leave them that way and to put the effortand resources that it takes to apply downtilt on cells that are moreimportant.

Cells that are very large and cause congestion can be further downtilted. A cell with a very large number of handovers creates problemswith frequency planning, and is a sign that a cell may causeinterference problems. Down tilt the cells in pre-defined steps, e.g. in

steps of 2 or 3 depending on antenna type.

4.2.2 Areas with small cells (approximately 700 meter site-site distanceor less):

With smaller cells, there is a better chance to get an overallimprovement of the interference situation in the network by adaptinga tilt strategy with a general tilt on all cells. A slightly worse coverage

in certain areas is also not as critical with a dense cellplan.Recommended default-values is a tilt that corresponds to around 5

dB loss at the horizon. This means around 4 for an 18 dBi antenna,

and 8 for a 15 dBi antenna.

With very small cells, with a range of 300 meter or less, the antennasshould definitely be downtilted, or the first null in the antennadiagram might create poor coverage at the cell boarder. This maylead to interference problems in the cell, and the quality will definitelybenefit from antenna down tilt.


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In areas where interference is a large problem, and the cells are verysmall (often the same area), additional down tilt can be applied.

Additional ti lt should be decided on a case-by-case basis, and theresult should always be verified.

Consider using 6 Electronic Down Tilt (EDT) antennas (18 dBi, 6EDT is available in Ericssons product package for 1800 MHz). The

6 EDT antenna may result in a overall loss of coverage if used inevery cell in an area. As a default setting, the EDT antennas can

therefore be mechanically up-tilted maybe 2. This corresponds to

around 4 down tilt from a conventional non-EDT antenna. If a largertilt-angle is desired, the EDT antenna can be down tilted. Whenadditional mechanical tilt is applied, this mechanical tilt angle issmall, and will not effect the horizontal antenna diagram to a great

extent. In the forward direction, for example a 2 downtilt on a 6EDT antenna will give approximately the same effect as a 6 + 2 =

8 mechanical downtilt. An 8 mechanical downtilt does howeverhave a distorted horizontal antenna diagram (see chapter 2.2, Figure

5), while the 2 additional mechanical tilt on the EDT antenna willonly have a minor impact on the horizontal antenna diagram.

5 CONCLUSION

Antenna down tilt can be a good tool in order to keep interference levelsunder control in a network. Antenna down tilt does have most effect withhigh gain, narrow vertical beam-width antennas. Best result is achievedin areas with small cells, and/or high antenna positions. With large cells,antenna down tilt can still be useful in order to solve local interferenceproblems, or to reduce the cell-size. This is however at the cost ofreduced coverage. The result of an antenna down tilt, if not very minor,should always be verified. It is especially important to verify the effectthat the down tilt has on the coverage and quality in the area close to thedown tilted cell itself.


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Appendix

Picture from a typical Urban site. Some of the other Urban cells hadmore nearby high-rise buildings, in some cases partly blocking the cell.

Antenna Down Tilting

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