OPTIMIZATION TECHNIQUES

RADIO NETWORK OPTIMIZATION

“To Tilt or Not To Tilt”?

ANTENNA TILTING• Antenna downtilting is one of the main optimization and performance improvement techniques in tuning tools RF propagation in GSM networks.

• For several reasons, antenna downtilting must be considered separately from the other parameter adjustments made during the interactive optimization process.

•First, antenna downtilting is dependent upon the antenna type and cell layouts specific to each cluster.

•Second, downtilting involves physical changes to the antenna as performed by a tower crew; the additional costs and delays associated with downtiltingwarrant special attention.

•Third, downtilting involves complex tradeoffs to balance interference and coverage; for downtilting to be effective, it must be applied appropriately.

• Sites that need more than 4 degrees down tilt should have electrical down tilt antennas installed. Excessive mechanical tilting has no effect on cell boundary coverage, and is only of limited use in controlling interference.

• Although it is not practical to change out all existing antennas in the network, it is recommended that all future sites be designed with appropriate default electrical down tilt antennas (either 2 or 4 degrees default).

ANTENNA TILTING CONSIDERATIONS

• It is also recommended that the antennas at all KV identified high sites be changed to a minimum 2 to 4 degree electrical down tilt to have a Rcell ~ 1000m in deep urban/urban and some suburban areas .

• The final required tilt depends on the antenna model and the Gain vs. tilt table as required by the cell shaping guidelines and by the Optimization teams.

ADDITIONAL CONSIDERATIONS WHEN TILTING:• Cell antenna height and terrain height.

• If the cell antenna is high and can cause overshoot into tier two neighbor cells, the sector is identified as a possible candidate for antenna downtilt.

• Look at the terrain data. If a sector is covering a low elevation region near the cell, downtilt its antenna may both eliminate the interference in areas far from the cell and increase the coverage in areas near the cell.

• Make sure there are enough neighbor cells around the proposed site that will take possible signal level coverage.

ANTENNA TILTING CONSIDERATIONS, cont.

• Check for Neighbor cells TCH CONG% to make sure that when traffic shifts from the proposed tilted cell to these neighbor cells, it does not add additional TCH CONGESTION to the cells becoming the best ranked cell for during handover process.

• Check for distance separation between cells. If a sector is close to the next cell, downtilting its antenna may both eliminate the interference in areas far from the cell and increase the coverage in areas near the cell.

ANTENNA TILTING CONSIDERATIONS, cont.

There are two theories when evaluating tilt.

1) The first one reduces the interference at the base of the neighbor cell (r = 2R) by 3 dB , or the HPBW (Half Power Beamwidth).

The antenna downtilt is:

(FIRST FORMULA)

where δ is the downtilt angle in degree, R is the radius of the cell, h is the antenna height, and VBW is the vertical beam width of the antenna.

2) To preserve the coverage in the fringe of the cell (r = R), the second downtilt angle formula is:

(SECOND FORMULA)

ANTENNA DOWNTILT GUIDELINES

δ = − ⎛⎝⎜

⎞⎠⎟+90 2 2arctan / ,R

hVBW

δ = − × ⎛⎝⎜

⎞⎠⎟

180 2 arctan .Rh

Antenna Downtilt Angle (°)

0

1

2

3

4

5

6

20 40 60 80 100 120

R/h (Cell radius / Antenna Height)

Dow

ntilt

Ang

le (°

) First Formula

Second

Formula

ANTENNA DOWNTILT GUIDELINES

In the plot, the solid line is the first downtilt formula and the dashed line is the second formula. The triangles represent the example downtilts for the example above

R (Cell Radius) (mile) 1.5h (Antenna+Terrain Height) (ft) 160VBM (vertical beam width)(deg) 5.5mtk (mile to kilometer) 1609ftm (foot to meter) 0.3048Downtilt Using First Formula Downtilt Using Second Formula Average

3.3 2.3 2.8

As the plots show, for small R/h (small cell radius and/or high antenna), the second formula predicts larger downtilt angle; while for large R/h (large cell radius and/or low antenna), the second formula predicts smaller downtiltangle. The first formula for downtilt angle prediction shows less dependence on the R/h ratio. The two curves intersect at approximately R/h = 30.

ANTENNA DOWNTILT GUIDELINES

PARAPET ANTENNA CLEARANCE REQUIREMENTSAntennas should be installed in such a way that the main axis (maximum Gain at the horizon) does not hit the roof and cause reflections that can attenuate the signal and create additional multipath and interference.Some also required that when tilted, not only the main beam (maximum gain) but also the – 3dB points do not hit the edge of the antenna.Below is a diagram indicating the distance requirements:D (distance from the edge of the building)Main Gain of antenna does not hit the roof edge for a given THETA tilt,Or THETA tilt + 3dB point in the Vertical BW plane does not hit the edge of the buildingTHETAtilt

PARAPET ANTENNA CLEARING

Main Gain of antenna does not hit the roof edge for a given THETA tilt,Or THETA tilt + 3dB point in the Vertical BW plane does not hit the edge of the building

THETAtilt

HcenterAntenna

MAX ALLOWABLE TILT1 = {ATAN (( H center of antenna)/D)} – default ET.

Or even more restrictively find the degree @ which the gain is – 3dB in the Vertical Beamwidthplane:

MAX ALLOWABLE TILT2= {ATAN (( H center of antenna)/D) – default ET} – 3dB VBW.

In the example below for the RFS antenna DPS60-13-XX of Length = 1.2 m and default 2 degrees ET, the gain is 3 dB less at 10 degrees in the VBW plane.

So the maximum allowable tilt for a given distance “D” away from the edge is given by:MAX TILT (main beam formula) = (180/3.14) * ATAN ( H/D) –2.

This antenna pattern gives a gain reduction of 3 dB @ an VBW angle of 10 degreesMAX TILT (3 dB point) = (180/3.14) * ATAN ( H/D) –2 - 10

Notice that the maximum tilt can be large for “D” up to 6 to 7 meters away from the edge. Once the 3 dB gain is allowed in the formula large tilts are allowed only at 4 meters away from the roof edge. The larger the distance from the edge, the less allowable tilt is possible.

PARAPET ANTENNA CLEARING

MODEL 1:

DISTANCE AWAY FROM EDGE (m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35H

BOOMS3 meter 2.35 65 48 36 28 23 19 17 14 13 11 10 9 8 8 7 6 6 5 5 5 4 4 4 4 3 3 3 3 3 2 2 2 2 2 26 meter 5.35 77 68 59 51 45 40 35 32 29 26 24 22 20 19 18 16 15 15 14 13 12 12 11 11 10 10 9 9 8 8 8 7 7 7 78 meter 7.35 80 73 66 59 54 49 44 41 37 34 32 29 27 26 24 23 21 20 19 18 17 16 16 15 14 14 13 13 12 12 11 11 11 10 10

MINI STRUCTURE6 meter 5.35 77 68 59 51 45 40 35 32 29 26 24 22 20 19 18 16 15 15 14 13 12 12 11 11 10 10 9 9 8 8 8 7 7 7 78 meter 7.35 80 73 66 59 54 49 44 41 37 34 32 29 27 26 24 23 21 20 19 18 17 16 16 15 14 14 13 13 12 12 11 11 11 10 1010 meter 9.35 82 76 70 65 60 55 51 47 44 41 38 36 34 32 30 28 27 25 24 23 22 21 20 19 19 18 17 16 16 15 15 14 14 13 13

MODEL 2: Main axes and 10 degree 3db point of RFS TX antenna does not hit the edge at "D"meters away from the edge.

DISTANCE AWAY FROM EDGE (m)

H 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

BOOMS3 meter 2.35 55 38 26 18 13 9 7 4 3 1 0 -1 -2 -2 -3 -4 -4 -5 -5 -5 -6 -6 -6 -6 -7 -7 -7 -7 -7 -8 -8 -8 -8 -8 -86 meter 5.35 67 58 49 41 35 30 25 22 19 16 14 12 10 9 8 6 5 5 4 3 2 2 1 1 0 0 -1 -1 -2 -2 -2 -3 -3 -3 -38 meter 7.35 70 63 56 49 44 39 34 31 27 24 22 19 17 16 14 13 11 10 9 8 7 6 6 5 4 4 3 3 2 2 1 1 1 0 0

MINI STRUCTURE6 meter 5.35 67 58 49 41 35 30 25 22 19 16 14 12 10 9 8 6 5 5 4 3 2 2 1 1 0 0 -1 -1 -2 -2 -2 -3 -3 -3 -38 meter 7.35 70 63 56 49 44 39 34 31 27 24 22 19 17 16 14 13 11 10 9 8 7 6 6 5 4 4 3 3 2 2 1 1 1 0 010 meter 9.35 72 66 60 55 50 45 41 37 34 31 28 26 24 22 20 18 17 15 14 13 12 11 10 9 9 8 7 6 6 5 5 4 4 3 3

D

Main axes of RFS TX antenna does not hit the edge at "D"meters away from the edge.

Maximum Tilt at a distance away from the edge D given the Boom height H

MAXIMUM TILT = THETA - 2 . ( ALSO KNOWN AS KNOB SETTING ON VARIABLE ELECTRICAL TILT).

BOOM=3m, 6m, 8m (Not Typical).

RFS DPS60-13-XX length=1.3m.

GIVEN H=ANTENNA STRUCTURE HEIGHT -1/2 (RFS TX ANTENNA LENGTH)

MAXIMUM ALLOWABLE TILT AT A DISTANCE "D" AWAY FROM BUILDING EDGE

• Note that the higher the Boom height “H” is the LESS restriction on Max Tilt to clear the parapet of the rooftop.

• The larger D from the dege of the rooftop is the MORE restriction on Max Tilt to clear the parapet of the rooftop.

Final Notes on Tilting

Final considerations when implementing MT or ET:

• Audit current antenna model installed , location, azimuth , current tilt default or imeplemented, terrain elevation, clutter type.

• Monitor where you can afford to tilt, where traffic may shift to which NCELL?. Coverage holes may appear and “some” Indoor coverage may be lost for those levels above the antenna installed, but coverage will get stronger for those buildings below the antenna height.

• Perform drive test before and after and compare , notice AVG RXLEV distribution, handover borders shifts to NCELLS, RXQUAL SQI, MOS on cluster.

• Monitor Traffic/DCR%/HOSR% statistics

• Monitor Customer complaints.