Section 1

CDMA RF Design

Section 4Antenna SpecificationsSection 4

Antenna Specifications4-1

Objectives4-3

Antenna Specifications4-4

Tackling Multipath fading4-6

Space Diversity Systems4-8

Antenna Spacing 4-10

Space Diversity4-12

Polarization Diversity Antenna Systems4-14

What are dual polarized antennas4-16

Configurations4-18

Air Combining Systems4-20

General Antenna Specifications4-22

Relationship of antenna height to the number of cells4-34

Voltage Standing Wave Ratio4-36

Objectives

Vertical Separation

Composite Separation

Objectives

At the end of this section the trainee should be able to

Explain antenna placement consideration

Explain antenna diversity and polarization

Explain how to select the antenna

Antenna Specifications

Horizontal Separation

Antenna Specifications

The primary objective for a proper antenna location and choice of an appropriate diversity scheme is to provide a uniform coverage within the cell area and minimum interference to and from other BTS antennae.

Choice of antenna location (cell site) is based on proper containment of coverage and alignment of the sites in to a specific hexagonal pattern. The choice may be limited due to availability of space, links to BSC etc.

Large coverage obtained by keeping an antenna at a height may not satisfy in-building coverage requirements.

In fact, one can rely on the buildings to serve as radio path shields, limiting the coverage area. Also the reflections from the buildings provide coverage to areas, which would not have been possible in the normal LOS mode. These additional paths consequently increase in-building penetration also.

Antenna Considerations:

Uniform Coverage in the cell

Alignment with hexagonal pattern

Space availability

Connectivity to BSC/MSC

Urban areas may have the following conditions:

Several Sites may be needed

Frequency re use is unavoidable

In-building penetration is a must.

Buildings act as RF shield and contain coverage.

Buildings reflect signals and provide coverage to areas where LOS would have failed.

Such additional paths improve in-building penetration.

Antenna at a very high point may not meet In-building coverage requirements.

It may be noted that the highest point in the area may do more harm than good. This is because it may cause interference to other sites and also in-building coverage may be limited because of this.

This is explained in the diagram shown below.

Location of antennae at high points needs careful examination of site coverage, type of area etc.

I

Tackling Multipath fading

(a) View from

boresight

-(broadside view)

separation

(b) View from 45

0

off

boresight

.

Reduced

Separation

Tackling Multipath Fading

In general we have the following methods to combat Multi path fading:

In the Time Domain: Interleaving and Coding

In the Frequency Domain: Frequency Hopping

In the Spatial Domain: Space Diversity

In the Polarization Domain: Polarization Diversity.

The last 2 are related to Antenna Systems.

Diversity Antenna Systems

In general, a diversity antenna system provides a number of receive paths (normally 2). The diverse output from each path is combined by the receiver to give a signal of sufficient S/N.

Thus a Diversity antenna System essentially has:

Two or More antennae

A combiner circuitry.

Another major requirement of Diversity antenna systems is that the signals arriving at the different receive paths/ports should have VERY LOW CORRELATION. This is because if a signal is fading at one port, the chances of it happening in the other port should be LOW.

After all, that is the purpose of diversity!

Diversity Antenna Systems

A Diversity antenna System essentially has:

Two or More antennae

A combiner circuitry.

Space Diversity Systems

View from 90

0

off

boresight

.

Space Diversity systems

There are 3 ways in which Space Diversity could be realized:

Horizontal Separation

Vertical Separation

Composite Separation.

The separation between antennas is a function of the correlation coefficient. To achieve a desired correlation coefficient, say 10

l

Horizontal Separation

Rx 1

Rx 2

Tx

> 10

l

Composite Arrangement

Rx 2

T

R

Rx 1

> 10

l

Two Antennae Diversity System

Antenna Spacing

H/V type

45

0

Slant type

Dual

Polarisation

Antenna Types

Antenna Spacing

Separation

d/(

900 Mhz

1800 MHz

Horizontal

10

3.3 m

1.7 m

Vertical

17

5.7 m

2.8 m

Figures given in the table are for minimum required separation.

If space is not a constraint, larger separation is always recommended.

Horizontal separation is preferred because it provides low correlation values.

However, horizontal separation suffers from angular dependence (demonstrated in the diagram, next page).

Vertical separation does not suffer much from angular dependence.

It also requires minimum supporting fixtures and does not occupy a lot of space.

But, as the distance increases, the correlation between the RF signals at the antenna points increases rapidly, thereby negating the very advantage of space diversity.

Scatter

Scatter

T

R

Filter

Dual

Pol

. Antenna

Required only if

the isolation between

the two ports of the

antenna is < 30 dB.

Single Antenna System.

Space Diversity

Dual

Pol

. Antenna

Single

Polarised

Antenna

Rx 1

Rx 2

Space Diversity

Space Diversity can be achieved using:

3 antennae systems

2 antennae systems

The 3 antennae system provides very good spatial separation between the two receive antennae and avoids the use of Duplexers. This reduces the risk of generating inter modulation products.

The 2 antennae system is preferred where the space for the antenna structure is limited or where the operators want to use less number of antennae

Dual

Pol

. Antenna

T

T

R

R

Polarization Diversity Antenna System

Operating Principles

A plane polarized wave has two components namely Vertical and Horizontal components.

The two fields exhibit a good degree of de correlation. This means that dual polarization can be used as a diversity system.

A Dual- Polarization antenna consists of 2 sets of radiating elements, which radiate, or in reciprocal, receive, 2 orthogonal fields. The antenna has 2 input connectors, which separately connect to each set of the elements.

The antenna has therefore the capability to transmit and receive two orthogonally polarized fields simultaneously.

q

3 dB point

3 dB point

What are dual polarized antennas

What Are Dual Polarized Antennas?

A dual polarized antenna is actually a two antennas encased within one housing.

Polarization is defined as the direction of the E vector (electric field vector) with respect to a given plane (earth in our case).

Dual polarized antennas are used in CDMA systems as methods of receive diversity on the reverse link.

The two antennas within a dual polarized antenna housing are orthogonal to each other and designed with the E vectors placed both horizontal and vertical to the earth or at ( 45 (slant 45) to the earth plane.

Advantages of Dual Polarisation:

Reduced support structure for the antenna

Reduced weight

Slim Towers and hence quicker construction and low cost.

Cost of One dual polarised antenna is generally lower than the cost of two space diversity antennae.

Choice of Dual Polarisation type:

H/V type:

As most mobiles are held at an angle of 450, H/V is more likely to cause balanced signals at the two branches.

The diversity performance is less dependent on the mobiles location.

Slant type:

Correlation between the two elements is angular dependent.

Unbalanced signals at the two arms of the receive antenna, since one of the signals could be at the same angle as the mobile.

What are dual polarized antennas

A dual polarized antenna is actually a two antennas encased within one housing.

Polarization is defined as the direction of the E vector (electric field vector) with respect to a given plane (earth in our case).

Dual polarized antennas are used in CDMA systems as methods of receive diversity on the reverse link.

The two antennas within a dual polarized antenna housing are orthogonal to each other and designed with the E vectors placed both horizontal and vertical to the earth or at ( 45 (slant 45) to the earth plane.

Advantages of Dual Polarisation:

Reduced support structure for the antenna

Reduced weight

Slim Towers and hence quicker construction and low cost.

Cost of One dual polarised antenna is generally lower than the cost of two space diversity antennae.

Choice of Dual Polarization type:

H/V type

Slant type

Configurations

One Antenna System:

Needs a Duplexer for the port transmitting and Receiving.

Needs a Duplexer or an External Filter for the, receive only arm.

The filter can be avoided if the isolation between the two ports Is better than 30 dB.

Two Antenna System:

Receiving antenna is the Dual Polarized antenna.

Transmitting antenna is the conventional vertical polarized antenna.

Both transmit and receive antennae should have identical characteristics such as beam width, gain etc.

Transmit antenna is usually mounted below the receive antenna. As most of the systems have down tilts, keeping the transmit antenna below the receive antenna minimizes shadowing of the receive antenna by the transmitter.

Air Combining Systems

Air Combining Systems

This enables both the ports of the antenna to simultaneously transmit and receive.

The air combining system requires a duplexer. The system has the following advantages:

It reduces combining losses.

Ideal for SFH as it minimizes the losses caused by hybrid combiners.

Enhances isolation, because of the Duplexer.

The use of the duplexer increases the risk of high inter-modulation products. It is essential ot choose low inter-mod product duplexers for this configuration.

General Antenna Specifications

Typical parameters of importance:

Polarization:

Linear Polarization: E-Vector contained in one plane.

Horizontal Polarization: H Vector parallel to the horizontal plane.

Vertical Polarization: E-vector parallel to the vertical plane.

Circular/Elliptical Polarization: The extremity of the E or H field describes a circle or an ellipse in the direction of propagation.

Radiation Pattern:

This is a plot of electric field intensity as a function of direction from the antenna, measured at a fixed distance.

Antenna Beam width:

This is measured in degrees between the half power points (3 dB ) points of the major lobe of the antenna.

The beam width can be expressed in terms of Azimuth (horizontal or H-Plane) or the elevation (Vertical or the E-Plane).

General Antenna Specifications

Typical parameters of importance (contd..):

Front to Back Ratio:

Is the ratio of power radiated in the Forward direction to that radiated in the backward direction through the back lobe of the antenna.

The antenna pattern uses the logarithmic scale. To show the front/back ratio, look only at the 0 and 180 degree line. Each ring represents 5 dB. If you count 7 rings on the 0/180 0 line, from the center to the front, and 2 rings from the center to the back, subtract the 2 from the 7 and get 5. That is a 25 dB front/back ratio. (5dB * 5 rings = 25 dB).

For the beam width, follow the pattern back 3 dB, starting at the 0 line. Take the point where the pattern hits 3dB (for example, 40 degrees). Multiply that by 2 to get the beam width (for example, 40 degrees*2=80 degrees beam width). Determine the beam width the same way, whether you are looking at a horizontal or a vertical pattern.

General Antenna Specifications

Typical parameters of importance (contd..):

Down tilt:

When the main radiation lobe of the antenna is intentionally adjusted above or below its plane of propagation, the result is known as a beam tilt. When tiled downward, we get the Down Tilt.

Down tilt can be done in 2 ways:

Electrical Down tilt

Mechanical Down tilt.

Electrical down tilt is achieved by changing the phase value of the RF signal at the inputs of a phased array. It is normally factory set and can be field-adjusted by changing externally the phasing cables supplied by the manufacturer.

Mechanical down tilt is done by physically changing the antenna position. Antenna beam tilting helps in confining the signals within a specified area and it also reduces interference. When an antenna is tilted down wards, its radiation pattern also gets tilted, resulting in a drastic reduction of signal strength at far off points.

Down Tilt:

Electrical Down Tilt

Mechanical Down Tilt.

Confines signals within specified area

Minimizes interference.

Down Tilt

Down Tilt

The down tilt required at a given site depends on the coverage planned.

With reference to the diagram opposite, it can be seen that the coverage diminishes rapidly outside the main lobe of the transmitted signal. Keeping the interference objective in mind, it is preferable to limit the outer edge of the main lobe to the cell radius.

In general down tilt angles greater than 5-6 degrees are not recommended. Also not more than 2 degrees difference in down tilt angles between any two adjacent sectors in a given site is desirable.

The down tilt angle for a cell may be calculated from geometry. It is given by the equation:

Down Tilt ( = arctan (h/Dmax) + (vertical beam width / 2 )

Typical vertical beam width of an antenna is 10-12 degrees.

Example

For a BTS height of 30 metres and a cell radius of 3 Kms, this works out to be:

( = arctan (.01) + (8/2), assuming 8 degrees vertical beam width.

= 4.57 degrees.

Down tilt

Outer edge of main lobe limited to cell radius.

Not more than 5-6 degrees.

Not more than 2 degrees difference between adjacent sectors of a site.

Down tilt angle given by:

Down Tilt ( = arctan( h/Dmax) + (vertical beamwidth / 2)

Down Tilt

Introduction to Mechanical Down tilt

When the antenna is physically down tilted, a notch at the centre of the horizontal beam pattern is produced. This notch becomes larger as the down tilt angle increases.

This notch can be effectively used to control interference as shown in the diagram opposite.

Mechanical Down Tilt

Mechanical down tilt of the antenna causes a notch at the centre of the horizontal beam pattern.

This can be used to minimize interference.

Electrical Downtilt

Electrical down tilt is achieved by changing the phase value of the RF signal at the inputs of a phased array. It is normally factory set and can the manufacturer supply fieldadjusted by changing externally the phasing cables. Figure-1 below shows an example of the effect of using different length phasing cables. Figure-2 shows the resultant coverage changes for differing values of electrical down tilt.

Figure 1

Figure 2

Relationship of Antenna Height to the Number of Cell Sites

Relationship of Antenna Height to the Number of Cell Sites

Voltage Standing Wave Ratio

Voltage Standing Wave Ratio

Voltage Standing Wave Ratio (VSWR) is another parameter used to describe an antenna performance. It deals with the impedance match of the antenna feed point to the feed or transmission line. The antenna input impedance establishes a load on the transmission line as well as on the radio link transmitter and receiver. To have RF energy produced by the transmitter radiated with minimum loss or the energy picked up by the antenna passed to the receiver with minimum loss, the input or base impedance of the antenna must be matched to the characteristics of the transmission line. The VSWR of a PCS antenna should be less than 1.5:1.

Return Loss

Return loss is the decibel difference between the power incident upon a mismatched continuity and the power reflected from that discontinuity. Return loss can be related to the reflection coefficient (VSWR) as follows;

RL dB = 20 log (1/p) Where p = VSWR-1/VSWR+1

VSWR = Vmax/Vmin

In other words, the return loss of an antenna can be considered as the difference in power in the forward and reverse directions due to impedance mismatches in the antenna design. All other things being equal, the higher the antenna return loss, the better the antenna. The system engineer should choose an antenna with a return loss of 14 dB or better. Note that 14 dB corresponds to a VSWR of 1.5:1 as per the following example;

VSWR = 1.5/1 = 1.5 p = 1.5 VSWR-1/VSWR+1 = 0.5/2.5 = 0.2

RL dB = 20log (1/0.2)

RL dB = 13.979 dB

VSWR

VSWR = 1.5/1 = 1.5 p = 1.5 VSWR-1/VSWR+1 = 0.5/2.5 = 0.2

RL dB = 20log (1/0.2)

RL dB = 13.979 dB

MOTOROLA I (P) LTD.FOR TRAINING PURPOSES ONLYPage 4 36/37

Section 4: Antenna Specifications