antenna+separation+guideline

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1 (15) Prepared (also subject responsible if other) No.

Approved Checked

2001-09-14 A

Antenna Configuration Guidelines

Guidelines for mounting and separation of base station antennas for GSM900, GSM1800 and GSM1900 and UMTS

Contents 1 Introduction 2 2 General 2 3 Isolation 2 3.1 General 2 3.2 Separation Requirements 3 3.2.1 Directional Antennas, Horizontal Separation 3 3.2.2 Directional Antennas, Vertical Separation 5 3.2.3 Directional Antennas, Combined Vertical/Horizontal Separation 6 3.2.4 Omni Antennas, Horizontal Separation 7 3.2.5 Omni Antennas, Vertical Separation 8 3.2.6 Omni Antennas, Combined Vertical/Horizontal Separation 9 4 Diversity 9 4.1 Brief description of diversity 9 4.2 Methods for Diversity 10 4.3 Guidelines for Polarization Diversity 10 4.4 Guidelines for Space Diversity, Omni and Directional Antennas 10 4.4.1 General 10 4.4.2 Diversity Separation and Antenna Height 11 4.4.3 Horizontal Separation for Space Diversity 11 4.4.4 Vertical Separation for Space Diversity 12 5 Nearby obstacles 12 5.1 Definition 12 5.2 Roof Mounting 12 6 Wall mounted antennas 14


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2001-09-14 A

1 Introduction

This document describes requirements on mounting of antennas in order to achieve specified isolation values as well as separation values for Space diversity.

The document is an update of Antenna Configuration Guidelines for GSM systems, LV/R-97:029 Rev B 1997-08-19. The values given for GSM in this document is still valid.

2 General

There is a range of parameters that have to be considered in an antenna installation. This document does not include considerations like, height above ground level, clearance to avoid shadowing effects from surrounding buildings other radio propagation criteria etc.

A key parameter in an antenna installation is the isolation between antennas. By maintaining a certain isolation value, Intermodulation, blocking, etc, can be kept under control. Isolation must be specified within a system as well as between systems. Co-siting with old and new systems must be considered.

Isolation is defined as the attenuation of a signal applied on one antenna connector and measured on any other antenna connector. It can be discussed if the losses in the feeder cables should be included in the measurement. That should be correct in case it is only the RBS Tx power and Rx sensitivity to consider. As many systems use active parts (Tower Mounted Amplifiers) close to antennas, it is safer to exclude the feeder loss and see that as an extra margin. It is also so that the feeder loss varies from site to site and in some cases can be very low.

Space diversity has been replaced by Polarization diversity in many applications. The information on required separation for Space diversity is still in the document. At least it will be needed for sites with omni directional antennas.

3 Isolation

3.1 General

The required isolation value is 30 dB. It is valid for GSM900, GSM1800 and GSM1900 using RBSs within the RBS2000 family and for UMTS using RBS3000. The same value is needed also between systems like: GSM900 and GSM1800 GSM900 and GSM1900 GSM900 and UMTS GSM1800 and UMTS


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A value lower than 30 dB can only be accepted for pure Rx antennas.

The required isolation value must then be converted to a separation distance. The resulted distance depends on frequency, antenna type and type of mounting. In general, higher frequency requires less spacing (wavelength dependence). Omni antennas and directional antennas with a wide horizontal beam width require more spacing compared with narrow beams. Mounting positions is important. In general, vertically separated antennas provide a higher isolation for a comparable horizontal spacing. Slant mounting gives higher isolation than horizontal separation.

It can be noted that the gain of the antennas has only a small impact. That due to the fact that for 30 dB, the antenna separation is small compared to the wavelength and by that the antenna interaction is in the near field.

3.2 Separation Requirements

3.2.1 Directional Antennas, Horizontal Separation

Figure 3.1: Horizontal Separation

Requirements: Tx - Tx and Tx - Rx: 30 dB

Horizontal separation: See Table 3.1 below


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Table 3.1 Horizontal separation (d in Figure 3.1, centre-to-centre)

Horizontal beam-width (deg./-3 dB)

GSM900 GSM1800/GSM1900/UMTS

GSM900 + GSM1800/GSM1900/UMTS

65±10 0.4 m 0.2 m 0.3 m

90±10 1.0 m 0.5 m 0.5 m

105±10 1.5 m 0.7 m 0.7 m

120±10 2.0 m 1.0 m 1.0 m

180±10 5.0 m 2.5 m 2.5 m

The antenna gain has no noticeable influence compared with the radiation in the 90 degrees direction if the antennas are on the same frontline. That means that an antenna with a broader beam requires more distance, due to the fact that the broad-beam antenna has more power radiating in the 90 degrees direction, compared to an antenna with a narrower beam.

Directional antennas on different cells need less separation distance. 20 mm is enough if the antenna direction deviate 90° or more and the beam width is 90° or less. See Figure 3.2

Figure 3.2: Antennas on different cells

Table 3.1 above is also valid if the antennas are installed as shown in figure 3.3 below.


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Figure 3.3: Mounting axis not perpendicular to the main beam direction

3.2.2 Directional Antennas, Vertical Separation

Figure 3.4: Vertical separation

The following is valid for GSM900, GSM1800/GSM1900/UMTS MHz and co-located GSM900 + GSM1800/GSM1900/UMTS.


Pre-condition: No influence from tower structure in the forward direction of the antennas.

Vertical separation: Tx - Tx and Tx - Rx: Minimum 0.2 m (k in figure 3.4)


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Additional information. The 30 dB isolation will be achieved for a lower value than 0.2 m, but for practical reasons the 0.2 m is used. This value is valid for all types of directional antennas.

3.2.3 Directional Antennas, Combined Vertical/Horizontal Separation

The following is valid for GSM900, GSM1800/GSM1900/UMTS and co-located GSM900+GSM1800/GSM1900/UMTS.


Combined separation: Vertical direction: minimum 0.5 m, Horizontal direction: no limitation.

Additional information: The combined vertical/horizontal separation is common in systems with two receiver antennas for diversity, if the Rx antennas are horizontally separated. The required 30 dB isolation will be achieved if 0.5 m vertical separation, k, is maintained, irrespective of horizontal separation, d, and irrespective of horizontal and vertical beam-width. See Figure 3.5.

Figure 3.5: Combined vertical/horizontal separation


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3.2.4 Omni Antennas, Horizontal Separation

Figure 3.6: Horizontal separation


Horizontal separation: See Table 3.2 below

Table 3.2 Horizontal separation (d in figure)

Omni antenna Gain dBi

GSM900 GSM1800/ GSM1900/UMTS

GSM900 + GSM1800/ GSM1900/UMTS

<10 3.0 m 1.5 m 1.0 m

>10 5.0 m 2.5 m 1.0 m

The horizontal distance between a Tx antenna and an Rx antenna or between two Tx antennas is dependent on the gain of the antennas. The horizontal separation distance is considerably higher compared to vertical separation, as the antennas are in the main radiation fields.


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3.2.5 Omni Antennas, Vertical Separation

Figure 3.7: Vertical separation

The following is valid for GSM900, GSM1800/GSM1900/UMTS and co-located GSM900 + GSM1800/GSM1900/UMTS.


Pre-condition: No influence from tower structure in the forward direction of the antennas.

Vertical separation: Tx - Tx and Tx - Rx: Minimum 0.2 m (k in figure 3.7)

Additional information. Antennas on the same axis with a separation of 0.2 m will actually have more than 40 dB isolation but 0.2 m is a practical separation to use. This value is valid for all types of omni directional antennas, as the near-field radiation in the vertical direction is not dependent on the gain of the antenna.


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3.2.6 Omni Antennas, Combined Vertical/Horizontal Separation

Figure 3.8: Combined Vertical/Horizontal separation

The following is valid for GSM900, GSM1800/GSM1900/UMTS and co-located GSM900 + GSM1800 or GSM1900 or UMTS.


Combined separation: Vertical direction: minimum 0.5 m, Horizontal direction: no limitation.

Additional information: The combined vertical/horizontal separation is common in systems with two receiver antennas for diversity, if the Rx antennas are horizontally separated. The required 30 dB isolation will be achieved if 0.5 m vertical separation, k, is maintained, irrespective of horizontal separation, d, and irrespective of horizontal and vertical beam-width. See Figure 3.8 above.

4 Diversity

4.1 Brief description of diversity

When diversity for reception is used, the radio signal is received by two separate antenna arrays within a dual polarized antenna or by two separate antennas as with space diversity. The output signal from the antennas normally varies strongly due to fading, but since they have different polarization or they are separated in space, the variations are not fully correlated.


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When space diversity is used the antennas must be positioned at a certain minimum distance from each other to give the expected diversity gain. The diversity gain increases with distance, but the extra gain by exceeding the recommended values is marginal and for practical reasons the distance is usually limited. Receiving antennas are usually mounted at the same height above the ground (horizontal separation). Vertical separation can however be used, but normally requires a larger separation for the same diversity effect. Sometimes horizontal and vertical separation is combined.

The height of the antennas above the ground also has an influence on the signal correlation. The higher above ground the antennas are located, the longer apart they must be positioned to obtain required space diversity gain. Polarization diversity already offers a low correlation and the influence of the antenna height is small.

4.2 Methods for Diversity

The two methods for up-link diversity are: 1. Polarization diversity 2. Space diversity

Polarization diversity is achieved by using two Rx antennas with orthogonal polarization that gives the wanted un-correlation of the Rx signals. There are two main antenna designs for dual polarization, slant +-45 degree polarization and Horizontal/Vertical polarization. The first type, +-45, is today the dominating type.

Polarization diversity has more or less been overtaking Space diversity today.

4.3 Guidelines for Polarization Diversity

No guidelines are required for mounting of antennas based on Polarization diversity as long as recommended antenna types are used. The isolation requirement must of course be fulfilled.

4.4 Guidelines for Space Diversity, Omni and Directional Antennas

4.4.1 General

Guidelines for Space diversity are provided in case of Polarization diversity is not acceptable. Space diversity is also used on sites with omni directional antennas as no (real) omnis exists with two polarisations. So-called quasi-omni with two polarisations is a construction based on four directional antennas within a common radome. It is not recommended due to its size and weight.

Situations may occur where the operator prefers space diversity on sector sites instead of polarization diversity or for sites with Omni directional antennas. To obtain the required space diversity gain the receiving antennas in a diversity pair must be positioned at a sufficiently large distance from each other. In determining this distance the antenna height above the ground must be considered.


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4.4.2 Diversity Separation and Antenna Height

The relationship between the desirable diversity separation and the antenna height is shown in the figure below:

Figure 4.1: Diversity separation and antenna height

The calculated distance shall be used together with the conditions in the next paragraph. (If Space diversity is used for UMTS, use the 1800/1900 MHz value.)

4.4.3 Horizontal Separation for Space Diversity

The horizontal separation between the antennas shall be as stated in Table 4.1 below:

Table 4.1 Horizontal separation for space diversity

Frequency band Minimum separation

Recommended separation

GSM900 4 m 6 m

GSM1800/ GSM1900/UMTS

2 m 3 m

To avoid practical difficulties and excessive costs, the diversity separation may be limited, but a greater distance gives a slightly higher diversity gain and may be used where practical.


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4.4.4 Vertical Separation for Space Diversity

Diversity improvement is also achieved by vertical separation of the antennas. For the same improvement as for the horizontal separation, the vertical separation should be approximately 5 times the horizontal value. This is valid for medium and large cells. For small cells in built up areas, a lower value can be acceptable, in extreme cases down to the same value as for horizontal separation. In other words, 4-6 m (GSM900) or 2-3 m (GSM1800/GSM1900/UMTS) vertical separation can be sufficient for very small cells.

5 Nearby obstacles

5.1 Definition

“Nearby obstacles” are defined in this document as reflecting or shadowing materials that can obstruct the beam in a negative way. Only obstacles within 30 m distance are considered, in reality the building if the antennas are to be installed on the roof of a building. Obstacles further away, e.g. surrounding buildings close to the base station, can also act as reflecting or shadowing obstacles but are not considered in this document.

5.2 Roof Mounting

The dominating obstacle is the roof itself. It is possible that the antenna beam will be distorted, if the antenna is too close to the roof. In other words, the antenna must be installed at a minimum height above the roof or other obstacles, see Figure 5.1 below.

Figure 5.1: Height of antenna above roof

A practical planning rule is to keep the first Fresnel zone free. For GSM900 the minimum recommended height above the roof is given by Table 5.1. For GSM1800/GSM1900/UMTS the minimum height is given by Table 5.1. These values are valid irrespective of antenna tilt.


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Table 5.1: Height of antenna above roof

Frequency Distance (d) to obstacle edge

Height (h) above roof (obstacle)

0-1 m 0.5 m (Note)

1-10 m 2 m

10-30 m 3 m GSM900

>30m 3.5 m

0-2 m 0.5 m (Note)

2-10 m 1 m (Note) GSM1800/ GSM1900/UMTS

>10 m 2 m

Note: If possible, use 2 m as the minimum height if there is a risk that people can walk close to the antenna.


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2001-09-14 A

6 Wall mounted antennas

Directional antennas can be installed on a wall, if considerations are taken with respect to the direction of the antennas. The ideal forward direction is when the antenna is perpendicular to the wall. See Figure 6.1. If the front-to-back ratio of the antenna is good, say better than 20 dB for the ”45° backward direction, there is a low risk for distortion of the beam due to reflections.

Figure 6.1: Ideal direction (perpendicular to the wall)

It is also important that the cell sector is free. A safety margin of 15° should also be added on both sides of the sector borderlines. If this sector, ” 75° for a standard 120° cell, is free and if the horizontal beam width is 105° or less, relative to the -3 dB-points, there is no risk for shadowing effects and beam distortion due to reflections.

Figure 6.2: Accepted deviation


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For most of the wall-mounted applications, the forward direction deviates from the perpendicular direction. The maximum acceptable deviation is 15° for a standard 120° cell. See Figure 6.2. If the antenna is close to a corner of the wall and if the antenna is mounted at a certain distance from the wall, then more than 15° can be acceptable as long as the cell sector including the safety margin is free from the wall. See Figure 6.3.

Figure 6.3: Antenna close to corner

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