Antenna Tilt Guidelines

Antenna Downtilt Guideline

ANTENNA DOWNTILT GUIDELINE

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TABLE OF CONTENTS 1

INTRODUCTION .............................................................................................. 3

2

ANTENNAS...................................................................................................... 3

2.1 The Antenna diagram ............................................................................................... 3 2.1.1 Gain .................................................................................................................... 3 2.1.2 Horizontal beamwidth .......................................................................................... 3 2.1.3 Vertical beamwidth .............................................................................................. 3 2.1.4 First null beamwidth............................................................................................. 5 2.1.5 Null-fill ................................................................................................................. 5 2.1.6 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....................................... 13 3.1.1 18 dBi Antennas ................................................................................................ 13 3.1.2 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....................................................................................... 22 4.2.1 Areas with large cells......................................................................................... 22 4.2.2 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 more efficiently. High capacity frequency planning techniques are often based on tight frequency reuse. The networks become interference limited, and in order to maximise the capacity, every available technique to minimise interference becomes important. As the capacity demand increases, the network plans also become tighter. A macro-cell site-site distance down to 400 meter or less is not unusual. With shorter site-to-site distances, limiting the interference from each cell becomes more and more important. When the cells are very small, down tilt can be applied without loss of coverage. Compared to no tilt at all, downtilt can even improve coverage in these dense networks. A well-chosen overall tilt-strategy can lower the overall interference in the network. A too aggressive down tilting strategy will however lead to an 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, for example 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 be increased by narrowing the horizontal and/or the vertical beam width. Typical for a three sector site is a 65°horizontal beam width with a maximum 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 border between 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 tilting is 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

-25

-20

-15

-10

dB

5 0 -5 -5 0 -10 -15 -20 -25 -30 -35 -40

5

10

15

20

25 GAIN 15 dBi GAIN 18 dBi

Degrees

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 mean approximately –1.5 dB for the 15.5 dBi antenna, and –8.5 dB for the 18 dBi 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) 0 -10

-5

0

5

10

15

20

-5

dB

-10 GAIN 18 dBi, 5 degrees GAIN 18 dBi, 0 degree tilt GAIN 18 dBi, 8 degree tilt

-15 -20 -25 -30 degrees

Figure 2. Vertical antenna diagram for 18 dBi antenna. 0°, 5°and 8°tilt applied. Note that the antenna diagram is valid only in the forward direction. When mechanical downtilt is applied, the tilt effect in directions other than straight forward is different, which means that the horizontal antenna 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 the main 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 may vary 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, suppress the 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 mobile may experience if it is located at a position where the vertical angle from the basestation antenna corresponds to the first null under the horizon in the antenna diagram. Such antennas do however tend to loose some of its maximum gain. Most of the large antenna manufacturers such as Kathrein, has a the first null specified to be > -25 dB relative the maximum gain. This figure is however somewhat theoretical, since the actual antenna diagram for these low power dips is effected by the antenna mounting. Moreover, the reflections and diffractions in the wave propagation will even out the dip in the antenna diagram, and the receiving mobile will not experience such a dramatic decrease in signal strength.

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2.1.6

Back-lobe The theoretical back-lobes for two typical 65°antennas are shown in the picture below. However, the actual antenna gain for different vertical directions is very difficult to estimate. Things like the mounting masts and the near environment on the roof has a large impact on the radiation in the backward direction. The actual signal strength behind the cell may also be the result of reflections from the energy transmitted in the forward direction. It is therefore difficult to theoretically predict the effect that down tilting has on the signal strength in the backward direction.

Vertical antenna gain for Backlobe 0 160

165

170

dB

155

175 -5180 -10 -15 -20 -25 -30 -35 -40 -45 -50 -55 -60

185

190

195

200

205

GAIN 15 dBi GAIN 18 dBi

degrees

Figure 3. Vertical gain for the back-lobe for two typical 15 dBi and 18 dBi antennas. 2.2

MECHANICAL VERSUS ELECTRICAL TILT Mechanical tilt When using mechanical tilt, the antenna is mounted with adjustable brackets 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 all horizontal directions. Electrical tilt can be combined with additional mechanical tilt.

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Electrical Downtilt vs. Mechanical Downtilt The largest advantage of electrical antenna down tilt is that the horizontal beam width is not affected. With mechanical down tilt, the tilting effect is greater in the 0°direction. At for example +/- 60°, the effective tilt angle becomes lower. This effect can be very difficult to predict. 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 the overall interference level in the network becomes higher. Electronical vs mechanical downtilt 0 -10

-5

0

5

10

15

20

-5 -10

dB

6 degrees EDT 18 dBi

-15

6 degrees mechanical -20 -25 -30

degrees

Figure 4. Comparison of vertical antenna gain for mechanical and electrical down tilt. Note that the graph above is only valid in the forward, 0°horizontal directon.

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Mechanical vs Electrical Downtilt Horizontal gain at zero degree vertical angle 20 15 dB Gain

10 MDT

5

No tilt

0 -90

-60

-30

-5 0

30

60

90

EDT

-10 -15 Degrees

Figure 5. Comparison of horizontal antenna gain at 0°vertical angle for mechanical and electrical down tilt. Because no 3D antenna patterns are was available, the Mechanical downtilt antenna diagram is estimated from 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 vertical beam width, around half the beam width of an 18 dBi antenna. These antennas are larger than the standard 18 dBi 1800 MHz antennas. The effect that these antennas have on coverage in urban areas has not been verified, but with such narrow beam width, at least in theory an even larger tilting effect can be achieved.

2.4

THEORETICAL TILT-EFFECTS When selecting the optimum tilt angle, the goal is to have as high signal strength 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 low as possible. The basic theory is that down tilting an antenna increases the signal strength in the area close to the site, whereas the signal strength becomes lower at far distances. The relation between the signal strength and 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 fairly accurately estimated by calculating the vertical angle between the antenna and the mobile at various distances from the site. Example Tilt effect, in terms of antenna gain experienced by a mobile, calculated given 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: α=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 antenna is 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 distances from the site have been calculated for a typical 18 dBi gain antenna. The antenna gain has been added to a simple path propagation model in order to show the signal strength in relation to the distance from the site for different antenna tilt angles. In the calculations, a 50 meter antenna height has been assumed. A different antenna height will change the scale of the X-axis, but the relative gain for the different antenna tilt angles will remain. In order to make the figure easier to read, two different scales have been plotted.

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Theoretical signalstrength, 15 dBi antenna -40 -50 0

500

1000

1500 Max Gain

dBm

-60

0 degrees

-70

8 degrees

-80

14 degrees

-90 -100 Distance from site (meter)

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

Theoretical signalstrength, 15 dBi antenna

-50 0

2000 4000 6000 8000 10000 12000 Max Gain

dBm

-70

0 degrees 8 degrees

-90

14 degrees -110 -130 Distance from site (meter)

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

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Theoretical signalstrength, 18 dBi antenna -40 -50

0

500

1000

1500 Max Gain

dBm

-60

0 degrees

-70

5 degrees

-80

9 degrees

-90 -100 Distance from site (meter)

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

-50 0

2000

4000

6000

8000 10000 12000 Max Gain

dBm

-70

0 degrees 5 degrees

-90

9 degrees -110 -130 Distance from site (meter)

Figure 7. The theoretical signal-strength from a 50 meter high site using a 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 no down tilt, a 18 dBi antenna will have its first null at around 400 meter from the site, and a 15 dBi antenna will have its first null around 200 meter from the site. Down tilting the antenna moves the first null closer to the site. At far distance, the signal strength is lower with down tilt, which means less coverage (if the cell is serving there) or reduced interference (if the cell is not serving). This is the basic theory behind all antenna down tilting.

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3

MEASUREMENTS This chapter contains some measurements that where performed in a large Asian City. The topography is very flat, and has no significant impact on the results. The area refereed to as “Urban”is dense, but not extremely dense. The buildings are of various heights, including skyscrapers up to 100 meter or 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 the buildings are not as densely built. Five sites were selected for the measurements. These sites where all 3sector, using 65°horizontal beamwidth antennas with no Electrical downtilt. This means that all tilts were done using mechanical downtilt. For each site, two cells where selected. Two of the sites had 18 dBi antennas, 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 0° tilt, 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 BCCH frequencies configured with “clean”test-frequencies. The signal strength was measured by a TEMS phone, using “Scan-mode”, and logged together with GPS readings. After the measurements, the signal strength was plotted in Map-info, and the result was analysed. In addition to this, the measurements were also post-processed in Mat-lab. The signal strength was filtered out for different directions, and plotted as a function of distance from the site. These plots are presented in this chapter. These kind of measurements are time consuming. In this measurement project, a large number of cells have been prioritised rather than performing a larger number of different tilt-angles for each cell. Due to the fact that the different tilt-angles were not logged simultaneously, the measurements are not exact enough to compare the signal-strength very close to the site. The accuracy of the positioning is limited by the GPS readings. Each drive-test was not performed with exactly the same speed. This does also have an impact on the accuracy of the compared signal strengths for the different tilt-angles. In the measurement graphs, the signal-strength in each point is the middle value of all samples in a 50 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 in graphs, 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. In addition 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)

dB

20 10 0 -10 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 -20 -30 -40 -50 -60 -70 -80 -90 -100 -110

SS_0 SS_9 SS_14 SS_9 - SS_0 SS_14 - SS_0

Disance form site, km

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Cell 1B dBi antenna, Semi-Urban environment, 45 meter antenna height Forward direction (+/- 40 degrees) 20 10 0 -10 0 -20 -30 dB -40 -50 -60 -70 -80 -90 -100 -110

0.5

1

1.5

2

2.5

3

3.5

4

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 Forward direction (+/- 40 degrees) 20 10 0 -10 0 -20 -30 dB -40 -50 -60 -70 -80 -90 -100 -110

0.5

1

1.5

2

2.5

SS_5 SS_9 SS_14 SS_9 - SS_5 SS_14 - SS_5


Cell 2B: 18 dBi antenna, Urban environment, 50 meter antenna height Forward direction (+/- 40 degrees) 20 10

dB

0 -10 0 -20

0.5

1

1.5

2

2.5 SS_0

-30 -40 -50 -60

SS_5 SS_12 SS_5 - SS_0 SS_12 - SS_0

-70 -80 -90 -100 -110


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15 dBi antennas

dB

Cell 3A: 15.5 dBi antenna, Urban environment, 30meter antenna heigth Forward direction (+/- 40 degrees) 20 10 0 -10 -20 0 -30 -40 -50 -60 -70 -80 -90 -100 -110

SS_0 0.5

1

1.5

2

2.5

3

3.5

4

4.5

SS_8 SS_14 SS_8-SS_0 SS_14-SS_0


dB

Cell 3B: 15.5 dBi antenna, Urban environment, 50 meter antenna height Forward direction (+/- 40 degrees) 20 10 0 -10 -20 0 -30 -40 -50 -60 -70 -80 -90 -100 -110

SS_0 0.5

1

1.5

2

2.5

3

3.5

SS_8 SS_14 SS_8-SS_0 SS_14-SS_0


Cell 4A: 15.5 dBi antenna, Urban environment, 30 meter antenna height Forward direction (+/- 40 degrees)

dB

3.1.2

20 10 0 -10 -20 0 -30 -40 -50 -60 -70 -80 -90 -100 -110

SS_0 0.5

1

1.5

2

SS_8 SS_14 SS_8-SS_0 SS_14-SS_0

Distance from site, km

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Cell 4B: 15.5 dBi antenna, Urban environment, 30 meter antenna height Forward direction (+/- 40 degrees) 20 10 0 -10 -20 0 -30 dB -40 -50 -60 -70 -80 -90 -100 -110

SS_1 0.5

1

1.5

2

2.5

SS_8 SS_14 SS_8-SS_1 SS_14-SS_1

Distance form site, km

Cell 5A: 15.5 dBi antenna, Urban environment 50 meter antenna height Forward direction (+/- 40 degrees) 20 10 0 -10 -20 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 -30 dB -40 -50 -60 -70 -80 -90 -100 -110

SS_0 SS_8 SS_14 delta_8 delta_14


Cell 5B: 15.5dBi antenna, Sub-urban environment 50 meter antenna height Forward direction (+/- 40 degrees) 20 10 0 -10 -20 0 -30 dB -40 -50 -60 -70 -80 -90 -100 -110

SS_0 0.5 1

1.5 2

2.5 3

3.5 4

4.5 5

5.5 6

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 as could be expected from the theoretical calculations.At far distances, the signal strength becomes lower when the antenna is tilted, but not quite as much as can be expected from the theoretical calculations based on the antenna diagram. One reason for this could be that the signal strength that reaches the mobile actually is transmitting more “straight forward”(at a lower vertical angle), above the rooftops, and later diffracted down to the mobile station. For the 18 dBi antennas, down tilting increases the signal strength at around 500 meter and closer to the site. For 15 dBi antennas, down tilting increase the signal strength at closer than around 2 – 300 meter from the site. It should be kept in mind that these sites where mostly around 50 meter high. If the sites are lower, the “break point”where the down 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 the network, can give a positive effect on the signal-to-interference ratio, C/I. This is however only true if the cell size does not exceed the distance where down tilting will reduce the coverage. For typical Urban cellplans, with sites that are 50 meter or lower, this means that the cell ranges should 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 a significant impact on the overall C/I levels at the cell-boarders. This is due to that downtilt will lower the signal strength at the cell-boarders in almost the same extent as it will lower the signal-strength further away from the site where the cell is causing interference. Hence, the conclusion is that a general down tilting strategy, down tilting all cells more 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 of around 500 meter or less. This corresponds to a site/site distance of around 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 in order to represent the signal strength at the cellborder towards the cosited cells. The results were consistent for every site, therefore one graphs, with a typical result, are presented here. The graph for the forward direction is included as a reference.

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dB

Cell 5A: 15.5 dBi antenna, Urban environment 50 meter antenna height Side direction (+/- 50-70 degrees) 20 10 0 -10 -20 0 0. 1 1. 2 2. 3 3. 4 4. 5 5. 6 6. 7 7. 8 8. -30 5 5 5 5 5 5 5 5 5 -40 -50 -60 -70 -80 -90 -100 -110



Cell 5A: 15.5 dBi antenna, Urban environment 50 meter antenna height Forward direction (+/- 40 degrees) 20 10 0 -10 -20 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 -30 dB -40 -50 -60 -70 -80 -90 -100 -110



Evalutaion of Signals strength measurments in side direction All measurements where done with mechanical down tilt. The result shows that the down tilt has a similar effect on the signal strength at an angle corresponding to the cell border to the co-sited cells, but at a lower degree. The signal strength becomes stronger close to the site, and weaker further away, but not as much as in the forward direction. This result is in-line with what can be expected from a theoretical point of view. In practise, this result tells us that if the cell diameter is larger than approximately 500 meter, the horizontal beam width of the cell becomes wider. The cells become “shorter and wider”. This will have a negative impact on frequency planning and the C/I relations between the cells. It may for example make be impossible to plan a 4/12 plan with sufficient C/I levels.

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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 backlobe +/- 40°. Cell 5B: 15.5dBi antenna, Sub-urban environment 50 meter antenna heigth Backward direction (180 +/- 40 degrees)

dB

3.3

20 10 0 -10 -20 0 -30 -40 -50 -60 -70 -80 -90 -100 -110

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

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 antenna down tilt did not have any significant effect on the signal strength behind the cell. Even at cells with large nearby buildings, where reflections are expected, down tilting did not have an effect on the signal strength in the backward direction. The “risk”of being served by the back-lobe is however effected by tilting since the signal strength in the forward direction changes. The risk of being served by a back-lobe is reduced close to the site since the signal strength in the forward direction (from the cell that should be serving) is stronger. Further away from the site, the effect is the opposite since the signal strength in the forward direction becomes weaker when the antenna is down tilted. However, far away from the site, back-lobe problems are not very common. The conclusion is therefore that tilting does 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 all cells in an area. The reason for this is that: 1. It becomes difficult to to fix specific problems, e.g. interference problems or cells that are too large. If a cell that already has a large donwntilt applied, needs to be further down tilted due to for example a local interference problem, this cell would need to have a very large tilt angle. The effects from very large downtilts are difficult to predict, and may lead to a significant loss of coverage in the area. 2. If mechanical downtilt is used, the horizontal beam-width of the antenna becomes wider (see chapter 2.2). This effect is difficult to consider in the frequency planning, and the wider antenna diagram probably creates an overall worse C/I distribution in the network. 3. If the cellplan is not very tight (around 700 meter site-to-site distance or more depending on antenna heights and type), antenna down tilting will reduce the overall coverage in the network. This is of course not good for the quality of the network, especially for indoor areas with weak coverage. • One good strategy is to have a few default tilt values that are implemented on every site. The default value can be different depending on the area, the size of the cell, antenna height and which type of antenna that is used. The general recommendation is however to keep it simple, and not do to many theoretical calculations for every site. It is better to start with a low tilt for every cell (see further the recommendations in chapter 4.2.1, 4.2.2), and study the actual coverage and interference situation. On a case by case bases, apply further down-tilt can be applied (and verified through 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 15 dBi antenna, and around 3.5°tilt for an 18 dBi antenna. A smaller tilt gives 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 corresponds to the 3dB point towards the horizon, and the angle that corresponds to 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 the first null towards the horizon. Further down tilting can be done in extreme cases, but if there is a need for further reduction of interference or cell-size, a reduction of output power, or possibly lowering of the antenna height, should also be considered. Very large down tilts (beyond the first null towards the horizon) should be carefully verified since the effect of such large tilts is difficult to predict. • Define, for every antenna type, four or five tilt-angles, and do only use these tilts. This makes it easier to work in a structured way, and to have better control over all the down tilts in the network. An example of such fixed tilt-values could be:

Theoretical gain at horizon (relative max. Gain)

Default Tilt-angles (exact values depends on the antenna diagram) 15 dBi antenna

18 dBi antenna

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 how much each cell is down tilted, but also WHY the down tilt was performed. If an antenna tilt was performed in order to solve a local interference problem due to for example a bad co-channel, this tilt should possibly be removed when a new frequency plan (without this co-channel) is implemented. Another example is a cell that has been down tilted because of congestion. If the cell is expanded with additional transceivers, it might be possible to reduce the down tilt. • A new site effects the coverage area of all cells that are neighbours to the new site. The downtilt angles in these sites should be revised. Additional downtilt should be considered in neighbouring cells that gets 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 is just as important to check the coverage and quality in the down tilted cell, 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 might have occurred. • It is better to put a lot of effort tilting and verifying the result on a few cells compared of doing a “quick and dirty”job tilting a larger number of cells. • Consider the nearby environment. Use common sense. If there for example is a close by building, which is of almost equal height as the antenna, a down tilt will make the coverage beyond that building to almost disappear. 4.2

RECOMMENDED TILT-VALUES

4.2.1

Areas with large cells (approximately 800 meter site-site distance or more): • Around 3.5°for an 18 dBi antenna, and 7°for 15 dBi antenna could be 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 tilt is however minimal. If the cells in an area currently have no down tilt at all, it might be better to leave them that way and to put the effort and resources that it takes to apply downtilt on cells that are more important. • Cells that are very large and cause congestion can be further down tilted. A cell with a very large number of handovers creates problems with frequency planning, and is a sign that a cell may cause interference 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 distance or less): • With smaller cells, there is a better chance to get an overall improvement of the interference situation in the network by adapting a 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 antennas should definitely be downtilted, or the first null in the antenna diagram might create poor coverage at the cell boarder. This may lead to interference problems in the cell, and the quality will definitely benefit from antenna down tilt.

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• In areas where interference is a large problem, and the cells are very small (often the same area), additional down tilt can be applied. Additional tilt should be decided on a case-by-case basis, and the result should always be verified. • Consider using 6°Electronic Down Tilt (EDT) antennas (18 dBi, 6° EDT is available in Ericsson’s product package for 1800 MHz). The 6°EDT antenna may result in a overall loss of coverage if used in every 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 larger tilt-angle is desired, the EDT antenna can be down tilted. When additional mechanical tilt is applied, this mechanical tilt angle is small, and will not effect the horizontal antenna diagram to a great extent. In the forward direction, for example a 2°downtilt on a 6° EDT antenna will give approximately the same effect as a 6°+ 2 °= 8°mechanical downtilt. An 8°mechanical downtilt does however have a distorted horizontal antenna diagram (see chapter 2.2, Figure 5), while the 2°additional mechanical tilt on the EDT antenna will only have a minor impact on the horizontal antenna diagram.

5

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

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Appendix

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

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Antenna Tilt Guidelines

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