Chapter 4 of GSM RNP&RNO Application of Antenna Feeder System-20060327-A-1.0

7/28/2019 Chapter 4 of GSM RNP&RNO Application of Antenna Feeder System-20060327-A-1.0

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Table of Contents

Chapter 4 Application of Antenna Feeder System .................................................................... 2

4.1 Overview........................................................................................................................... 2

4.2 Antenna Basics................................................................................................................. 3

4.2.1 Types...................................................................................................................... 3

4.2.2 Working Principles.................................................................................................. 4

4.2.3 Important Technology Characteristics.....................................................................5

4.2.4 Antenna diversity..................................................................................................13

4.2.5 Relationship between antenna lobe width and antenna gain................................17

4.3 Antenna Tilt Planning......................................................................................................184.3.1 Antenna Tilt design............................................................................................... 19

4.3.2 Application............................................................................................................23

4.4 Antenna Selection........................................................................................................... 26

4.4.1 Problems Present in Antenna Selection................................................................26

4.4.2 Principles for Selecting Base Station Antenna in Urban Areas.............................26

4.4.3 Principles for Selecting Base Station Antenna in Suburban Areas........................27

4.4.4 Principles for Selecting Base Station Antenna in Rural Areas..............................28

4.4.5 Principles for Selecting Base Station Antenna along Highroads...........................28

4.4.6 Other Considerations for Antenna Selection.........................................................30

4.4.7 Antenna Selection Reference............................................................................... 30

4.5 Combiner and Divider Unit.............................................................................................. 32

4.5.1 Principles..............................................................................................................32

4.5.2 Configuration of Combiner and Divider Unit......................................................... 35

4.6 Tower Amplifier................................................................................................................ 37

4.7 Feeder............................................................................................................................. 38

4.8 Distributed Antenna System............................................................................................ 39

4.8.1 Composition Principle of a Distributed Antenna System.......................................39

4.8.2 Types of Distributed Antenna Systems.................................................................40

4.8.3 Key Technical Indexes for Antenna Components..................................................434.9 New Antenna Technology—Smart Antenna Overview.....................................................47

4.9.1 Smart antenna...................................................................................................... 47

4.9.2 Smart Antenna Application.................................................................................... 51



GSM Radio Network Planning and OptimizationChapter 4 Application of Antenna Feeder System

For internal use only

Χηαπτερ 4 Application of Antenna Feeder System

4.1 Overview

In a wireless telecommunication system, the antenna provides the interface between

base transceiver station (BTS) and outside propagation mediums. One set of antenna

can both radiate and receive radio waves. When radiating radio waves, it converts

high frequency current into electromagnetic wave; when receiving radio waves, it

converts the electromagnetic wave into high frequency current.

During network planning, the right antenna is selected according to the radio

environment of the BTS. The parameters, such as antenna height, antenna azimuth

angle, tilt angle, are decided based on the selected antenna.

Antenna is directly related to uplink and downlink converges, so are the radio

frequency (RF) components, such as feeder cable, combiner, and duplexer.

Figure 1.1 shows the composition of an antenna feeder system

Figure 1.1 Composition of an antenna feeder system

6/27/2013 All rights reserved Page2 of 54





4.2 Antenna Basics

4.2.1 Types

Generally, antennas for mobile communication are passive. Table 1.1 lists the

antenna types in terms of different division standards.

Table 1.1 Antenna type and division standard

Division standard Type

Radiation direction Omni-directional antenna

Directional antenna

Structural feature Linear antenna

Dish antenna

Cap antenna

Polarization way Vertical polarization antenna (unipolarization antenna)

Cross polarization antenna (dual polarization antenna)

Figure 1.2 shows the antennas commonly used in mobile communication. They are

directional antenna, omni antenna, and indoor cap antenna from left to right.

Figure 1.2 Antennas commonly used in mobile communication






4.2.2 Working Principles

According to Maxwell equation, electromagnetic wave radiation can be generated if

alternate current is present in the conductor. The radiation capability is related to the

length and shape of the conductor.

Figure 1.1 shows the principles of antenna radiation

Figure 1.1 Principles of antenna radiation

As shown in Figure 1.1 (a), when the distance of the two conductors is short, the

induced electromotive force generated on the ideal conductors will offset the effect of

each other, so only a small amount of energy is radiated beyond the two conductors.

As shown in Figure 1.1 (b), there is a flare angle between the two conductors.

Because the current is generated in the same direction, the induced electromotive






force is generated in the same direction. In this case, a larger amount of energy is

radiated beyond the two conductors.

As shown in Figure 1.1 (c), when the flare angle is wide enough to match wavelength,

the amount of current flowing through the conductors will be greatly increased. Thus

intensive radiation is generated.

Generally, the direct conductor that can generate intensive radiation is called dipole. If

the length of the two arms of a dipole is 1/4 wavelength, the dipole is called

symmetrical half-wave dipole.

The symmetrical half-wave dipole is a basic element of a mobile telecommunication

antenna. As shown in Figure 1.2, an actual antenna consists of multiple dipoles.

Figure 1.2 Composition of an actual antenna

4.2.3 Important Technology Characteristics

I. Antenna gain

The antenna is passive equipment, so the concept of antenna gain is different from


Unit dipole

Feeding networkFeedingnetwork

Antenna connector Antenna connector

Unit dipole

Feeding network

Directional antenna Omni-directional antenna





that of the power amplifier gain. The power amplifier can amplify power, but the

antenna does not increase the energy for radiated signals. It concentrates the energy

into a certain direction by changing the feeding mode of antenna dipoles throughassembling the antenna dipoles.

Antenna gain is an important antenna index, indicating the antenna capability (the

directional antenna) of concentrating energy into a certain direction.

The dBi and dBd are two units of antenna gain, and the relationship between the two

is as follows:

15.2+= dBd dBi

Where,

The dBi indicates the energy concentration capability of the antennas withdirections (including omni antennas) as compared with that of the isotropic

antennas. “i” stands for “isotropic”.

The dBd indicates that the energy concentration capability of the antennas with

directions (including omni antennas) as compared with that of the symmetrical

dipole antennas. “d” stands for “dipole”.

Figure 1.1 shows the relationship between dBi and dBd.

Isotropic antenna

Symmetrical dipole

antenna Actual antenna

dBd

dBi

2.15d

B

Figure 1.1 Relationship between dBi and dBd

The dBi indicates the gain of actual antennas as compared with that of isotropic

antennas; the dBd indicates the gain of actual antennas as compared with that of

half-wave dipole antennas.

II. Directional diagram

The radiation intensity is related to radiation direction. If the relationship between the

two is drawn according to relative scale, it is an antenna directional diagram, or






radiation diagram.

Three relative scales are available for drawing a directional diagram. They are:

Linearity (power directional diagram)

Square root (field strength diagram)

Decibel

The decibel scale is more often used among the three, because it expresses the side

lobe level in a simpler way.

The antenna directional diagram is space solid figure, but the one in common use is a

directional diagram within two principle planes perpendicular to each other, known as

plane directional diagram. For the linear antenna, since the ground effect is great, it

adopts the vertical plane and horizontal plane as its principle plane. For the planeantenna, it adopts two principles planes, namely, E plane and H plane.

Essentially, the dipole arrangement and the change of the feeding phase of each

dipole work together to determine antenna direction, and the principle of which is

similar to that of the light interference effect. Therefore, the energy in some directions

is amplified, but the energy in other directions is weakened. In this case, lobes (or

beams) and zero points are formed. The lobe with the fullest energy is the major lobe.

The lobe with the second fullest energy lobe is the first side lobe, and so on. For the

directional antenna, it has a back lobe.

Figure 1.1 shows a horizontal plane and a vertical plane of a directional antenna.

Figure 1.1 Directional diagram of the directional antenna (horizontal plane and

vertical plane)






Table 1.1 list the parameters related to the antenna directional diagram.

Table 1.1 Parameters related to the antenna directional diagram

Parameter Description

Zero

power

point lobe

width

It refers to the included angle

between the zero radiation

directions on both sides of the

maximum major lobe.

Half

power

point lobe

width

It refers to the included angle after

the maximum electrical field falls by

0.707 points (if power falls by half,

the gain falls by 3dB).

It is divided into two types:

horizontal half power point lobe

width and vertical half power lobe

width.

Side lobe

suppressi

on ratio

It refers to the ratio of the maximum

major lobe to the maximum side

lobe.

Front-to-

back ratio

—

Electric tilt

angle

—

III. Polarization

Polarization is a radiation feature describing the space direction for the field strength

vector of electromagnetic wave. Generally, the space direction of the field strength

vector works as the polarization direction of the electromagnetic wave.

The electromagnetic wave with the space direction of the electric field vector

unchanged at any time is called straight line polarized wave. If the land is taken as a

reference, the direction of the electric field vector parallel to the land is called

horizontal polarized wave; the direction of the electric field vector perpendicular to the

land is called vertical polarized wave. The space direction of the electric field vector is

changeable. If the trace of the electric field vector end is a circle, the electromagnetic

wave is called circular polarized wave; if the trace is an ellipse, the electromagnetic

wave is called ellipse polarized wave. Both the circular polarized wave and ellipse

have a feature, which is rotating phase.

The electromagnetic waves of different bands are transmitted by different polarization






modes. Generally, the vertical polarization is used in mobile communication; the

horizontal polarization is used in broadcasting systems; and the ellipse polarization is

used in satellite communication.

The GSM antenna can be divided into two types, namely, single polarization antenna

and dual polarization antenna. With the help of polarization diversity technology, a

dual polarization antenna can promise BTS to receive good signals through reducing

the multi-path effect in mobile communication. Two specifications, 0°/90° and ±45°

are available to the dual polarization antenna. Because GSM bands are more

favorable to the horizontal polarized wave than to the vertical polarized wave, the

0°/90° cross polarization antenna is seldom used at present.

IV. Antenna tilt

Antenna tilt is commonly used to enhance the signal level for the serving cell and

reduces the signal interference on other cells. Table 1.1 lists the antenna tilt type and

related descriptions.

Table 1.1

Antenna type Description

Mechanical tilt It is set by lowering the antenna to a required position

through adjusting antenna mount.

Electrical tilt It is controlled by changing the phase of antenna dipole.

Note:

In actual project implementation, electrical tilt and mechanical tilt can be used

together to control the antenna tilt angle.

The tilt angel of an electrical tilt antenna is fixed, known as preset tilt. The latest

technology enables an electrical tilt antenna to adjust its tilt angles, and this kind of

electrical tilt antenna is called electrical adjustment antenna.

V. Voltage standing wave ratio (VSWR)

For VSWR of the base station antenna in a mobile communication cellular system, its

maximum value must be equal to or less than 1.5:1. If Z A stands for antenna input

impedance, and Z 0 stands for antenna standard characteristic impedance, the

reflection coefficient is:






｜Г ｜ =

｜ Z A− Z 0｜

｜ Z A+ Z 0｜,VSWR =

1+｜Г ｜

1−｜Г ｜, where

Z 0is 50 ohm. The return loss can also

indicate the match characteristic of the port, that is, R. L.(dB

) =20 lglg｜

Г

｜ if VSWR =1.5:1 and R.L. = -13.98dB.

When antenna input impedance is inconsistent with its characteristic impedance, the

reflection wave and incident wave will overlap on feeder cable to form standing wave.

The ratio of the maximum to minimum value of neighbor is the VSWR.

If the ratio is too large, the radiation power will be reduced because part of the power

transmitted into the antenna is reflected back to the power amplifier. Furthermore, the

cable loss is measured when VSWR=1 (it means full match), so the reflection power

increases the cable loss. In addition, the transmitter output power cannot reach the

designed rated value.

The factors in the previous paragraph will decrease coverage area. Moreover, the

reflection power will return to the power amplifier of the transmitter. If the power is too

high, it will damage the power tube. In this case, the communication system cannot

work normally.

At present, however, the transmitter output power can reach the rated power under

certain mismatch conditions (for example, when VSWR < 1.7 or 2.0). Related

calculation shows that compared with the power loss when VSWR = 1.3, the power

loss is decreased by only 0.23dB when VSWR=1.5, which can be neglectedaccording to mobile communication fading. If the VSWR is too low, however, it will

increase antenna manufacturing cost. Therefore, the balance between the cost and

VSWR must be emphasized.

VI. Front-to-back ratio (F/B)

The difference between the level of the side lobes within back 180°±30° and the

maximum beam is indicated by positive value. Generally, the antenna front-to-back

ratio ranges from 18 dB to 45dB. For densely populated areas, to reduce the

interference generated by back lobes, the antenna with greater front-to-back ratioshould be used.

VII. Port isolation

For the antenna with multiple ports, such as dual polarization antenna and dual-band

dual polarization antenna, the isolation between the ports for both transmission and

reception must be greater than 30dB.






VIII. Power capacity

Power capacity refers to the average power capacity. The antenna contains coupling

devices, such as match, balance, and phase shift, so the power it can bear is limited.

Suppose the power of a single carrier is 20W, if one antenna port can input up a

maximum of six carriers, the total input power of the antenna is 120W. Therefore, the

power capacity of the antenna single port must be greater than 200W when

environmental temperature is 65 degree Celsius.

IX. Zero point filling

To make the radiation level within service areas more even, the first zero point of the

lower side lobe needs to be filled by using the shaped-beam design. Generally, when

the zero depth is -20dB greater than the main beam, it means that the zero point

filling is present in antenna.

It is recommended that the zero point filling technology should be applied to high gain

antennas with great height (for example, the antenna height is 100 meters) to improve

nearby coverage and avoid the unequal coverage caused by signal fluctuation.

X. Upper side lobe suppression

For a cellular system, to reduce the interference between neighbor cells, the base

station antenna should reduce the side lobes aiming at the interference cells. In this

case, the upper side lobe suppression ratio can be enhanced and the ratio of garbagesignals to useful signal (D/U) of the coverage area is improved. The level of the first

upper lobe must be smaller than -18dB relative to the maximum gain of the major

lobe. There is no such requirement for the antenna of macro cell base station.

XI. Antenna inout interface

To improve the reliability of passive intermodulation and RF connection, the input

interface of base station antenna adopts 7/16DIN-Female. Before the antenna is

available, a cover must be installed at the antenna port to prevent oxide and

contamination.

XII. Passive intermodulation (PIM)

It is the inermodulation effect caused by the non-linearity present in the passive

components, such as connector, antenna, feeder, and filter, working under high power

signals of multiple carriers.

Generally, it is granted that passive components are of linearity. Under high power

condition, nonlinearity is present in passive components to some extent mainly due to

the following causes:






Metals of different materials are contacted together.

The contact surface of the same materials is rough.

The components are not tightly connected. Magnetic substances are present.

Intermodulation products will disturb the communication system; especially the

intermodulation products falling within the receiving band have a remarkable negative

effect on the system receptivity. Therefore, the requirements on the passive

components, such as the connector, feeder, antenna, are strict in a GSM system.

Generally, the value of the antenna passive intermodulation index must reach

-150dBc.

XIII. Antenna size and weight

If all electric indicators are met, the antenna should be as small as possible in size

and as light as possible in weight for storage, transport, installation, and security

purposes.

Now carriers have higher requirements on antenna size, weight, and shape.

Therefore, both technical indicators and the previous non-technical factors must be

emphasized in antenna selection. Generally, the antenna installed in urban area

should be small, light, and eye catching.

XIV. Wind loading

The base station antenna must be installed on the top of high buildings and towers. In

coastal areas, where the wind is strong all the year around, so it is required that the

antenna can work normally against the wind at the speed of 36m/s and are not

damaged when the wind speed reaches 55m/s.

The antenna itself can stand strong wind. In areas where the wind is strong, the

antenna is damaged mainly because the tower or the supporting bar is damaged.

XV. Work temperatire and humidity

The base station antenna works normally when environment temperature ranges

-40°C to +65°C and environmental relative humidity ranges from 0 to 100%.

XVI. Lightning protection

All RF input ports of the base station antenna are required to be directly grounded

through direct current.






XVII. “Three proof” capability

The base station antenna must have “three proof” capability, that is, moisture proof,

salt atmosphere proof, and mildew proof. For the omni antenna, it can be installed in

reverse direction according to installation instructions and the “three proof”

requirements.

4.2.4 Antenna diversity

I. Diversity features

Signal fading in mobile radio environment will give rise to serious problems. With the

movement of the mobile station, the Raleigh fading varies rapidly with signal

instantaneous value, while the logarithm normal fading varies with signal average

value (median value). The two values will deteriorate receiving signals greatly, so they

are the main factors that attributed to unstable receiving signal in mobile

communication.

Though signal stability can be improved through increasing transmitting power,

antenna size and height, such methods cost much in mobile communication, and they

are sometimes far beyond reality.

This problem can be solved, however, with the help of diversity technology. The

diversity technology enables antenna to receive the signals with little dependencies

but carrying the same information on several tributaries. After that, the antenna

combines the signals from each tributary and output the combined signal. In this

case, the deep fading probability can be greatly reduced at the receiving end.

Generally, diversity technology is applied at base station side.

The diversity can be divided into two types, namely, explicit diversity and implicit

diversity. The implicit diversity implies the diversity function in the signals to be

transmitted through signal processing technologies, such as RAKE receiving

technology, channel interleaving technology, anti-fading error correction technology,

and so on, but only the explicit diversity technology is introduced hereunder. The

explicit diversity can also be divided into two types, namely, base station explicit

diversity and common explicit diversity.

According to base station explicit diversity, several base stations distributed in space

fully or partially cover the same area. Because multiple signals are available, the

effect of signal fading can be greatly reduced. Because electric waves are transmitted

on different paths and the shadow effect of landforms and ground objects varies, the

multiple slow fading signals transmitted via independent fading paths are irrelevant to

each other.






Because each signal is not likely to undergo deep fading simultaneously, if the

diversity combination is used to select the tributary with the best signal-to-noise ratio

from all tributaries, namely, the base station and mobile station with the best signalare selected to establish the communication, the shadow effect and other geographic

effect can be eased. Therefore, base station explicit diversity is also called multiple

base station diversity.

Generally, the explicit diversity is used for suppressing Raleigh fading. The

suppression ways include:

Traditional space diversity

Frequency diversity

Polarization diversity

Direction diversity Time diversity

Field component diversity

If the space diversity, polarization diversity, and direction diversity are used, at least

two set of receiving antennas are needed at the diversity receiving side; if the

frequency diversity, field component diversity, and time diversity are used, only one

set of receiving antenna is enough.

The explicit diversity, however, improves the uplink signal quality only. For the mobile

station is restricted in terms of size, price, and battery capacity, and so on, the space

diversity of multiple antenna cannot be realized.

To improve the transmitting quality of downlink signals, you can employ the reciprocity

principle for linear system to equally shift the diversity technology of the mobile station

receiving end to base station transmitting end. And this technology is called transmit

diversity technology.

For the transmit diversity technology, one problem is present. That is, the reciprocity

principle is applicable only when a mobile channel is simplified to an approximately

linear time-variant system. Moreover, when the reciprocity principle for reception and

transmission of the linear system is applicable, the signals must be transmitted andreceived on the same band so that the fading features of the signals are the same.

In fact, frequency division duplex (FDD) are more often used in a mobile

communication system, where the interval between reception and transmission is far

greater than coherence bandwidth. To prevent FDD from deteriorating the transmit

diversity, you can realize the transmit diversity through controlling the closed loop.

In 3G, the transmit technology is widely used.






II. Diversity and synthesis

The relevant coefficients between the quantity of the diversity tributaries and the

receiving diversity determine diversity features. If the relevant coefficients of each

tributary are identical, various diversity solutions can realize the same relevant

performances. In addition, we must consider how to synthesize the multiple signals

received through the diversity technology because proper synthetic technology can

bring forth desirable performance.

For example, if the Q multiple diversity is adopted, the Q signals before synthesis are

S 1(t ),S 2(t ), ...S q(t ). Because the synthesis can be performed at the baseband output

end between each diversity antenna and receiver and behind the intermediate

frequency output end of the receiver and the detection, the S i(t ) here can be

understood as a general form of the high frequency signal, intermediate frequency

signal and baseband signal. The synthesis concerns how to combine and sum up the

S i(t ). The synthesized signal is expressed as follows:

S (t ) = k 1S 1(t ) + k 2S 2(t ) + ... + k qS q(t )

Where k 1, k 2, ..., k q indicates weighting coefficient. If different weighting coefficients are

selected, different synthesis methods are produced.

The four synthetic techniques commonly used are as follows:

Maximum ratio compound (MRC)

Equal gain compound (EGC)

Selective compound (SEC)

Switch compound (SWC)

Where the MRC is defined as follows:

After weighting the voltage amplitude of the useful signals, perform relevant synthesis

for the signals and non-relevant synthesis for noise power. When the weighting

coefficient is equal to the signal-to-noise ratio of each signal, the maximum

synthesized signal-to-noise ratio of the synthesized signals is equal to the sum of the

signal-to-noise ratio of each tributary signal. In this case, the MRC is present.

For the details of the previous synthesis technologies, they are not introduced in thistextbook. In mobile communication, the space diversity and polarization diversity are

commonly used. The diversity gain designed in a project in about 3.5dB. The

following three sections introduce the space diversity and polarization diversity in

detail.

III. Space diversity

It is performed using the random change of the field intensity in space. In mobile

communication, any space change will result in great change of the field intensity. The






larger the space distance, the less relevance of the signals transmitted on multi-paths

is and the less likely for the deep fading to occur simultaneously, so the space

technology can reduce the effect of the deep fading to the minimum. For this purpose,the antenna space distance must be determined. Generally, the space distance

between two receiving antennas ranges from 12λ to 18λ.

In actual project implementation, the horizontal distance between diversity antennas

should be 0.11 times higher than the valid height of the antenna. If is used to

indicate this parameter, the relationship between and the actual antenna height (h)

is=h D

. For antennas placed horizontally, when ≤ 10, two signal can be irrelevant

to each other. For example, if the antenna is 30 meters in height, a good space

diversity gain can be obtained when the antenna distance is about 3 meters.

IV. Polarization diversity

At present, dual polarization antenna is widely used in actual projects. Theoretically,

because the medium does not give rise to coupling effect, no mutual interference is

present when a frequency carries the signal with two polarization modes. In actual

mobile communication, however, the coupling effect is always present. This means

that after the signals are transmitted via mobile radio mediums, the energy of thevertical polarized wave will leak into the horizontal wave, and vice versa. Compared

with the amount of the main energy, however, the amount of the leaked energy is

little. Therefore, good diversity gain can still be obtained with the help of polarization

diversity.

Because the effect of the ±45° polarization antenna is better than that of the 0°/90°

polarization antenna, the communication networks now mainly use the ±45°

polarization antenna. The greatest advantage of the polarization antenna is that one

set of polarization antenna can meet the requirements, thus reducing the installation

cost.

V. Comparison between space diversity and polarization diversity

The greatest advantage of the polarization diversity is to save installation space. For

the space diversity, it is realized by two set of receiving antennas with a certain

distance. For the polarization diversity, however, it can be realized by only one set of

antenna, which contains two groups of dipoles.

When the transmitting antenna of the mobile station declines, the polarization

diversity obtains better relevant statistics than the space diversity. Apart from some






special performances of space diversity antenna the polarization diversity antenna

has, the polarization diversity antenna is affected by the local radio transmitting or

scattering.

The environment measurement performed in urban area shows that the diversity gain

of a ±45° dual polarization antenna equals the space diversity gain, but the diversity

gain of a 0°/90° dual polarization is 1dB lower than the space diversity gain. In the

open areas, such as suburban areas, the diversity gain needs to be further measured.

In addition, any group of dipoles of the ±45° dual polarization diversity antennas can

transmit signals, but only the vertical polarized dipoles of the 0°/90° dual polarized

antenna can transmit signals.

4.2.5 Relationship between antenna lobe width and antenna gain

One function of the antenna is concentrating energy, so if the radiation intensity in

some direction is strong; the radiation intensity in other direction is weak. Generally,

the radiation intensity in a direction can be enhanced through reducing the width of

the lobes on the horizontal plane. When the antenna gain is certain, the antenna

horizontal half power angle is reversely proportional to the vertical half power angle,

and the relationship between the two is as follows:

β θ ⋅≈

32400log10

aG

Where,

Ga is antenna gain, in the unit of dBi.

is vertical half power angle, in the unit of degree.

is horizontal half power angle, in the unit of the degree.

According to the formula, when the gain and horizontal half power angle is known, the

vertical half power angle can be obtained.

For example, there is an omni antenna. If the antenna gain is 11dBi, and the

horizontal half power angle is 360°, the vertical half power angle is

°=⋅

≈ 15.710360

3240010/11

β .

Due to the difference of antenna design and manufacturing, slight difference is

present for the vertical half power angle of the actual omni antenna. And such

difference is determined by the focus and implementation ways of the electrical

design.






Figure 1.1 shows the relationship of the antenna gain, vertical half power angle and

horizontal half power angle.

Figure 1.1 Relationship of the antenna gain, vertical half power angle and horizontal

half power angle.

As shown in Figure 1.1, when the antenna gain is small, the vertical half power angle

and horizontal half power angle are large; when the antenna gain is large, the vertical

half power angle and horizontal half power angle are small.

In addition, the antenna gain depends on dipole quantity. The larger the dipole

quantity, the larger the antenna gain is, and the larger the antenna aperture (effectiveantenna receiving area) is. For example, for a 900MHz omni antenna, if the antenna

gain increases by 3dB, the antenna length doubles. Generally, therefore, the gain of

the omni antenna does not exceed 11dBi, and the antenna length now is about 3

meters.

4.3 Antenna Tilt Planning

In cellular communication, coverage theory, frequency multiplexing theory and BSS

functional algorithm are all based on regular cellular layout. The design of project






parameters is the main factor that affects the cellular layout in radio network planning.

In a wireless network system, the macro-BTS layout and the actual location of each

base station, antenna height, lobe width, direction, tilt angle, and EIRP together forma specific cellular network.

Generally, the performance indexes of the antenna itself are selected according to the

radio networking characteristics, such as the base station density and macro

coverage goal. Once the location of a base station is determined, it seldom changes.

For the antenna height, direction and tilt angle, however, they are finally determined

according to the parameters specified previously and the actual coverage goal of a

cell.

Hereunder is the analysis of the relationship among antenna height, direction, tilt

angle, and coverage goal (suppose that the cell radius is R), and the antenna tilt

angel is finally recommended according to this analysis. The propagation of radio

signals is closely related to the environment. For example, dense buildings and the

reflection caused by mountains, water surface, or huge glass walls will affect radio

propagation. Therefore, it is not necessarily that all the environments are favorable to

radio propagation. However, the regularity of cellular structure and the coverage area

and goal of a cell are the foundation for a good network, so they must be carefully

considered during network planning.

4.3.1 Antenna Tilt design

The following factors must be considered in antenna tilt design:

Antenna height

Azimuth angle

Antenna gain

Vertical half power angle

Expected coverage area

For the base stations distributed in urban areas, when the antenna has no tilt angle or

the angle is very small, the serving area of each cell is determined by the antennaheight, azimuth angle, antenna gain, transmit power, landforms and ground objects.

In this case, the coverage radius can be calculated by the commonly-used

propagation module formulas, such as Okumura-Hata and COST231.When the tilt

angle of the antenna is large, the coverage radius cannot be calculated out because

the angle is not considered in the previous formulas. If accurate propagation module

and digital map are provided, however, the coverage radius can be calculated out by

planning software. In this case, the antenna vertical half power angle and tilt angle

helps to calculate the coverage radius directly based on the triangle geometry formula

as follows:






If the needed coverage radius is D (m), the antenna height is H (m), the tilt angle is

, and the vertical half power angle is , the relationship between the beams of the

major antenna lobe and the ground is shown in Figure 1.1.

Figure 1.1 Relationship between the beams of the major antenna lobe and the

ground

As shown in this figure, when the antenna tilt is 0 degrees, the beams of the major

antenna lobe, or the major energy, radiate horizontally; when the antenna tilt angle is

, the extension line of the major lobe and the ground will ultimately intersect at one

point (point A). Because a beam width is present in the vertical direction for the

antenna, intense radiation is present in the area form point A to point B.

According to the technical performance of the antenna, the antenna gain decreases

slowly within half power angle, but it decreases sharply beyond the half power angle,

especially for the upper lobe. Therefore, when the antenna tilt angle is considered,

the scope between the extension line of the half power angle to intersection point

(point B) can be taken as the actual coverage area of the antenna.

Based on the previous analysis and the principles of triangle geometry, the

relationship between the antenna height, tilt angle and coverage distance can be

obtained as follows:

= arctanarctanarctanarctanarctanarctan (H/D) + /2

This formula can calculate the coverage distance after the adjustment for tilt angle.

Actual results of on-site optimization projects show that this formula is of great

significance. However, the application of this formula meets limited conditions. It can

be applied when the tilt angle is 1.5 times greater than the half power angle, and the






distance (D) must be less than the distance calculated by the previous formula when

no tilt angle is present. For the width of vertical beams in the previous formula, it is

provided in the specific antenna technical indexes.

Figure 1.2 shows the relationship between coverage distance and antenna tilt angle

when the vertical beam width of the antenna is 17 degrees. (The antenna height is 40

meters.)

Figure 1.2 Relationship between coverage distance and tilt angle (The width of the

vertical beam is 17 degrees, and the antenna height is 40 meters.)






Figure 1.3 shows the relationship between coverage distance and antenna tilt angle

when the vertical beam width of the antenna is 6.5 degrees. (The antenna height is

40 meters.)

Figure 1.3 Relationship between coverage distance and tilt angle (The width of the

vertical beam is 17 degrees, and the antenna height is 40 meters.)

The previous two figures shows the relationship between the coverage distance and

the width of the antenna vertical beams when antenna height and tilt angle are

certain. The smaller the width of the vertical beam, the shorter the coverage distance

is. Therefore, if the cross coverage are effectively controlled, the antennas with

smaller vertical beam width and with the zero point filling function should be selected

during the planning phase. In this case, the cross interference can be controlled and

the indoor coverage near the base station.

However, if the vertical beam width grows smaller, the horizontal lobe will grow wider

or the antenna gain will grows larger. In this case, new cross interference is caused

and the cross coverage area between neighbor cells is too large. Therefore, the

antennas of medium gain are often selected in urban areas. For example, if the

antenna of 65 degrees and 15dBi is selected for a GSM 900MHz base station, the

vertical beam width is about 13 degrees to 15 degrees.






Note:

The adjustment of the tilt angle can serve to control the cross coverage and to

improve the indoor coverage near the base station, but the coverage far from the

base station will get worse.

4.3.2 Application

For the purpose of application and necessary overlaps of adjacent cells, the distance

(D) from the base station in populated urban areas to the target coverage area can be

simplified as the designed cell radius (R). The antenna height (H) refers to the relative

height from the base station and target coverage area. This textbook introduces the

application of antenna tilt planning in the areas similar to plains.

Antenna tilt can be divided into mechanical tilt and electrical tilt, and their effect on

coverage is almost the same. Because electrical tilt antennas are expensive,

mechanical antennas are more often used. Emulation shows that if the mechanical tilt

is greater than 10 degrees, the lobes are distorted, which will cause unexpected

interference against other cells. Therefore, it is better to keep the mechanic tilt within

10 degrees.

If only for the convenience of controlling network quality, the adaptation of the

electrical adjustment antenna will win more advantages. Because the electricaladjustment antennas are expensive, electrical antennas with a certain preset tilt angle

(for example, 6 degrees to 7 degrees) are more often used in actual networking.

When the network needs to be expanded and optimized, the electrical tilt antenna

and the mechanical tilt antenna work together to set the tilt angles greater than 10

degrees.

According to the previous analysis and in combination with the common antenna

height (25 meters to 50 meters), the reference tilt angles can be provided for the cells

whose radius is 250 meters, 500 meters, 800 meters, and 1000 meters in populated

urban areas. The case is the same for other situations.

Table 1.1 lists the reference tilt angles for antennas in populated urban areas.

Table 1.1 Reference tilt angles for antennas in populated urban areas

Antenna model Vertical half

power angle

Cell radius

R(m)

Antenna

height (m)

Tilt angle

(degree)

65 degrees, a gain of 15 dBi 15 200 25 15
















According to the table, when the cell radius is small, the coverage area cannot be

effectively controlled even through mechanically tilting the antenna. In this case, the

coverage area can be controlled through lowering the antenna height only. If it is hard

for the antenna height to be lowered, the antenna electrical tilt together with the

antenna mechanical tilt must be used.

The previous methods for calculating tilt angles are mainly applicable for the densebase station networking with the distance within 1200 meters (that is, R = 800 meters)

between stations.

When the distance from the base station to the coverage target is greater than 800

meters, large area coverage is still being emphasized. In this case, it is unnecessary

for you to consider the effect of the vertical half power angle when estimating the

antenna tilt angle. Generally, the tilt angle now is 1 degree to 4 degrees. In special

cases, such as the base station has already been installed at a high position, the tilt

angle may also be large.

However, because the environment around the base station is rather complicated, the

reflection caused the nearby mountains, water surface, huge glass walls has an effect

on antenna tilt angle. The reflection of this kind will easily cause unexpected

interference against the neighbor frequencies and time dispersion effect. In addition,

the shadow effect caused by building roofs, front dense buildings and mountains must

be also considered. In actual networking, however, geographic environment, such as

the barrier of high buildings and mountains, around the base station can be used to

control coverage area.

When a network is implemented in a populated urban area, the major lobe of the






4.4 Antenna Selection

The antenna selection is a very important part in a mobile communication network.

The antenna must be selected according to the actual conditions, such as coverage

requirement, traffic volume, interference, and the quality of service of the network. A

proper antenna can enlarge coverage area, reduce interference, and improve the

quality of service.

Because antenna selection is closely related to coverage requirement, the antenna

application environment can be divided into four types according to landforms or

traffic distribution. They are: urban area, suburban area, rural area, and highroad.

4.4.1 Problems Present in Antenna Selection

This section introduces the problem present in antenna application from the following

perspectives:

The antenna is selected only based on the covered traffic distribution, but little

consideration is given to the relationship between landforms and antenna

directional diagram. For example, if all antennas used in a network are of the

same type, when the antenna is installed at a high position, the phenomenon of

"blind under tower” will be present because the width of the beams in vertical

plane is narrow.

Too large antenna mechanical tilt angle results in the distortion of the directional

diagram. In this case, coverage problem or interference problem will occur.

Emulation shows that the restrictions on tilt angles must vary in accordance with

the antennas with different gains.

Too much attention is focused on the high gain performance of the antenna but

little attention is given to its drawbacks. As a result, the gains of almost all the

antennas used in a network are quite high. A high gain antenna has many

drawbacks, including large size, great weight, high side lobe, deep zero lobe,

and narrow vertical beams. No consideration is given to the difference between the vertical polarization

antenna and dual polarization antenna in terms of application. The dual

polarization antenna is selected from the perspective of installation.

4.4.2 Principles for Selecting Base Station Antenna in Urban Areas

Base stations are densely distributed in urban areas. Therefore, it is required that the

coverage area of each base station is as small as possible so as to reduce cross

coverage and interference among base stations, and enhance frequency reuse rate






as well. In this case, an antenna must meet the following requirements in principle.

I. Selection of antenna horizontal half power beam width

Because a large number of base stations are distributed in urban areas, overlapping

coverage and frequency interference rises as serious problems in a network. To

reduce the overlapping areas of neighbor sectors and the interference between base

stations, you can set the beam width of the antenna horizontal half power to a smaller

value. Generally, antennas whose horizontal half power beam width is 65° are

selected, but antennas whose horizontal half power beam width is above 90° are not

selected.

II. Selection of antenna gain

The base stations in urban areas are not required to cover a large area, so the

antennas with medium gain are recommended. Thus the antenna vertical beam can

be wider, which can improve the coverage quality within the areas to be covered. In

addition, the size and weight of the antenna with medium gain are small, which is

helpful for installing the antenna and reducing cost. According to present antenna

specifications, antennas with a gain of 15dBi (900MHz) and 15-18 dBi (1800 MHz)

are recommended in urban areas.

For the base stations on the outskirt of a city, if it is required to cover a large distance,

you can select the antennas with higher gains, such as 17dBi and 18dBi.

In principle, when designing base station coverage in urban areas, you should select

the antennas with a fixed electrical tilt angle. The degrees of the electrical tilt angle

can be set according to actual conditions (the recommended value is 6° to 9°).

In urban areas, to enhance frequency reuse rate and reduce cross interference, you

can select the shaped-beamed antenna with the first upper side lobe suppressed and

the lower side lobe filled.

Because space restriction is present in the antenna installation in urban areas, the

dual polarization antenna is recommended. And it is better to select the antenna with

a smaller size when the electrical specifications of the antennas are the same or

nearly the same.

4.4.3 Principles for Selecting Base Station Antenna in Suburban Areas

Because the environment is suburban areas are largely different from that in urban

areas, antennas used in suburban areas can be selected according to the required

coverage area. Generally, in suburban areas, an antenna can be selected according






to the following principles:

The antennas whose horizontal half power beam width is 65° or 90° can be

selected according to actual conditions. If base stations are sparsely distributed,the antennas whose horizontal half power beam width is 90° is first considered.

If the base stations are densely distributed, the antennas are selected by

referring to the principles for selecting base station antenna in urban areas.

Omni antennas are not recommended for the purpose of smooth expansion in

the future.

4.4.4 Principles for Selecting Base Station Antenna in Rural Areas

In rural areas, traffic volume is small and base station are sparsely distributed, so

some base stations are required to cover a large area. In this case, the antennas are

selected based on the following principles:

Considering the construction cost, you are recommended to select an omni

antenna for the base stations whose coverage area is small and traffic volume is

low. However, because the gain of the omni antenna is low, the coverage of an

omni antenna is shorter than that of a directional antenna. When the base station

is required to cover a long distance, the directional antenna must be selected to

realize the coverage. Generally, a high gain vertical polarization antenna whose

horizontal half power beam width is 90° is recommended.

One point needs to be noted. That is, if the base station antenna is installed at ahigh position, but the area needs to be covered lies in a low location (the

depression angle is greater than 5°), when an omni antenna is used, the kind

with a preset tilt angle or with zero point filling function are recommended to

improve the coverage quality of this area. In this case, the phenomenon of “blind

under tower” and the signal fluctuation caused by uneven coverage can be

avoided.

4.4.5 Principles for Selecting Base Station Antenna along Highroads

The principles for selecting antennas along highroads are as follows:

For the base stations designed to cover the areas along railways and highroads,

a directional antenna with narrow beams can be selected.

For the base station designed to cover the highroads and the villages scattered

around the highroads, an omni antenna can be selected.

For the base station designed to cover highroads only, an 8-shaped antenna can

be selected, because the 8-shaped antenna help realize the highroad coverage

with only a few base stations.






For the base station designed to cover the highroads and the towns on both

sides of the highroads, the antenna whose horizontal half power beam width is

210° can be selected according to actual conditions. It is recommended to givethe priority to the 8-shaped antenna and the 210°antenna for highroads

coverage.

Figure 1.1 shows the application of a 210° antenna.

Figure 1.1 Application of a 210° antenna

Figure 1.2 shows the application of an 8-shaped antenna






Figure 1.2 Application of an 8-shaped antenna

4.4.6 Other Considerations for Antenna Selection

Apart from the basic principles for selecting the antenna in different places areprovided in the previous parts, other factors, such as system expansion and

equipment performance, must be considered for antenna selection.

Hereunder is an example:

If the 210° antenna and used to cover the highroads nearby a small town, and only a

cell is used to promise the coverage requirements, you should consider whether the

traffic of this area will increase in the future and whether to meet the expansion

requirements by adding carriers. Generally, once a carrier is added to the base

station, the combiner loss will increase, so the coverage distance will decrease after

the expansion. Therefore, when selecting an antenna, you should consider these

problems beforehand and work out a good plan for the selection of antenna gain and

base station type.

4.4.7 Antenna Selection Reference

Table 1.1 lists the antenna selection references.






Table 1.1 Antenna selection reference

Landform Station type Reference

Urban areas Directional

station

Generally, select the antennas with low

or medium gains and preset electrical tilt

angle depending on base station density.

An electrical adjustment antenna or

mechanical tilt angle can be selected.

Suburban

areas

Directional

station

Generally, select the antennas with high

gain; both electrical adjustment tilt

antenna and mechanical tilt antenna are

ok.

Plains &

Rural areas

Directional

station

Generally, select the 90° antennas; but

the best choice is the vertical signal

polarization antennas.

Directional

station

Select the antennas with zero point filling

first regardless of tilt angle.

Expressways Directional

station

First select the 8-shaped antennas, and

then consider using the power splitter of

0.5/0.5 configuration; it is preferred to

have zero point filling function.

Directional

station + Omni

station

First consider the 210° antennas, and

then consider using the directional

antenna and omni antenna together.

Mountain

areas

Omni station First consider the antennas with zero

point filling function, and then consider

the antennas with low gain; the antenna

tilt angel is considered last.

Directional

station

First consider the antennas with low gain

and wide vertical beams, and then

consider adding tile angle.






4.5 Combiner and Divider Unit

The functions of the combiner and divider unit include: Transmitting and receiving signals

Combining and filtering transmitting signals

Filtering receiving signals

Amplifying and dividing low noise

Providing feeding feeders for tower amplification

Therefore, the combiner and divider unit enables multiple transmitting signals and

receiving signals to share the same antenna unit.

4.5.1 Principles

The combiner and dividing unit has the following detection and alarm functions:

Standing wave detection

When it detects that the standing wave exceeds the preset threshold (1.5:1 or

2.5:1), it gives out alarm signals and indications, thus monitoring the feeder

status.

Low noise amplifier fault alarm

If fault signals are taken from the supporting current of the low noise amplifier, it

generates alarm signals when the current exceeds a certain limit or no current is

generated.

Tower amplification alarm

When the tower amplifier is working, if fault signals are taken from the supporting

current of the tower amplifier, it generates alarm signals when the current

exceeds a certain limit or no current is generated.

Control function

It can control the power attenuation over the major reception path and the

diversity reception paths (the dynamic is 15 dB and step length is 1 dB); it can

drop out the feeder for the tower amplification configuration; and it can select the

feed current for different tower amplifiers.

Take Huawei equipment for example, it can provide three modules for the combiner

and divider unit. They are CDU, SCU, and EDU.

The schematic diagram of CDU is shown in Figure 1.1.






Figure 1.1 Schematic diagram of CDU

The schematic diagram of SCU is shown in Figure 1.2.






Figure 1.2 Schematic diagram of SCU

The schematic diagram of EDU is shown in Figure 1.3.






Figure 1.3 Schematic diagram of EDU

The loss of different combiner and divider unit varies, and it is configured based on

the configuration of actual station types. Theoretically, the insertion loss is 3dB for

each two-in-one combination; and the duplexer insertion loss is about 1dB.

4.5.2 Configuration of Combiner and Divider Unit

This section takes Huawei equipment as an example to explain the configuration of

various combiner and divider units. For details, see Table 1.1.






Table 1.1 Configuration of combiner and divider unit (take Huawei equipment as an

example)

Number of

carriers for

each cell

Normal

configuration

plan

Large

coverage

configuration

plan

Remark

7 – 8 TRXs 2 CDUs + 2

SCUs

- Large station; mainly

located in urban areas;

seldom applied to large

coverage.

5 - 6TRXs CDU + CDU+ SCU

CDU + CDU +SCU

Applicable to largecoverage configuration

plans; it works when

combined with Huawei

concentric circle

technology.

3 - 4TRXs CDU + SCU 2 CDUs -

1 - 2TRXs CDU EDU or 2

CDUs

Applicable to the sector

with no more than 2

carriers; it will be

replaced during system

expansion.

Note:

The large converge plan is not implemented through adding the number of antennas

and feeders to a cell. In actual networking, according to the coverage and capacity

requirements of different base stations and when the conditions of uplink anddownlink balance are met, you can perform the configuration flexibly and combine the

actual BSC software algorithms to enable the coverage quality to reach the best. For

example, you can adopt the configuration of {feeder + amplifier (40W, 60W, or 80W)},

and adopts the concentric circle control technology applicable to the situation when

the coverage of each carrier in a cell is inconsistent.






4.6 Tower Amplifier

In terms of technical principle, a tower amplifier is used to reduce the noise coefficientof the base station receiving system, thus improve the sensitivity of the base station

receiving system.

The contribution of the tower amplifier to uplinks is distinguished according to the low

noise amplifier performance of the tower amplifier itself but not according to its gain

only. If a tower amplifier is installed, the uplink and downlink must be modified and

calculated according to the methods for testing the tower amplifier sensitivity. In

addition, the sub band tower amplifier or the all band tower amplifier should be

selected according to different bands.

The tower amplifier indexes include band, gain, noise coefficient, insertion loss, and

so on.

4.5.2 I. 1Table 1.1 shows the principles of a triplex tower amplifier. This tower

amplifier transmits and receives signals using one feeder, and the bypass function is

present. (Automatic bypass is present when faults occur, and the receiving gain at

this time is about -2dB.)

Figure 1.1 Principles of a triplex tower amplifier






4.7 Feeder

The feeder selection is rather important for the whole design of a cellular system.Because the feeder is exposed to outdoors, it must stand the rough environments.

Both the foam and air can be pressed into the feeder as insulation medium. If the air

is used as the insulation medium, short circuit can easily occur, so the air is seldom

used.

I. Feeder selection

Two types of feeders are in common use, namely, 7/8" feeder and 5/4" feeder. They

are selected as follows:

For GSM 900MHz

If the required feeder length is shorter than 80 meters, use the 7/8" feeder;

otherwise use the 5/4" feeder.

For GSM 1800 MHz

If the required feeder length is shorter than 50 meters, use the 7/8" feeder;

otherwise use the 5/4" feeder.

II. Feeder technical indexes

Table 1.1 Technical indexes for feeders in common use

Feeder type 100-meter attenuation (dB) Standing

wave

(Any

length)

- 890MHz 1000MHz 1700MHz 2000MHz -

SYFY-50-22 (7/8") 4.03 - 5.87 6.46 1.15

LDF5-50A (7/8") 4.03 4.3 5.87 6.46 1.15

LDF6-50 (5/4") 2.98 3.17 4.31 4.77 1.15

M1474A (7/8") - 4.3 - 6.6 1.15

HFC22D-A (7/8") - 4.47 - 6.7 1.15

FSJ4-50B (1/2") 11.2 11.9 16.1 17.7 1.15

III. Feefer installation

The feeders to be selected must be the shortest and are easy for installation and

maintenance. The curvature of the feeder must comply with the specifications






required by the manufacturers. When the feeder enters the equipment room, its

external conductor must be well grounded regardless of the installation position of the

antenna.

4.8 Distributed Antenna System

With the development of the mobile communication, users have higher requirements

on the quality of service. They hope they can make successful calls anywhere and

anytime. In some places, such as in large buildings, tunnels, underground railways,

where the environments are rather complicated, only outdoor base stations cannot

promise the coverage, which will result in blind spots and conversation break.

Moreover, in some areas where high buildings are densely distributed, the strong

signals from base stations will cause frequent MS handover (ping pong effect) and

remarkable interference. In this case, the conversation may be broken.

To solve the previous problems, you can adopt a distributed antenna system. With the

help of the distributed antenna system, the capacity of abundant cells can be

transferred to other cells; thus the system capacity is well allocated.

4.8.1 Composition Principle of a Distributed Antenna System

Figure 1.1 shows the composition principle of a distributed antenna system. In terms

of function, it is equal to one set of signal polarization antenna connected to a base

station.

Figure 1.1 Composition principle of a distributed antenna system






As shown in this figure, the downlink signal from the base station first enters the

distributed antenna system through an interface. Then the power splitter divides the

signal into several tributaries, and the end of each tributary connects to a miniantenna, which has a certain coverage capacity. When a signal is not strong enough,

the bilateral amplifier amplifies the antenna gain. On the contrary, the downlink

signals form each tributary enters the base station through the mini antenna, power

splitter, and bilateral amplifier.

In this system, signals can be transmitted and distributed by one of the following

mediums:

Coaxial cable and RF power splitter

Optical transmission link

Leaky feeder

4.8.2 Types of Distributed Antenna Systems

I. Coaxial cable

The coaxial cable distributed antenna system is often used for indoor coverage. The

design of this type is flexible, the cost is effective and the installation is convenient.

The attenuation of coaxial cable is relatively small, so the antenna selection depends

on coverage area and installation restrictions.

Figure 1.1 shows the coaxial cable distributed antenna system






Figure 1.1 Coaxial cable distributed antenna system

II. Leaky feeder

It is better to use leaky cable in narrow and long coverage areas, but load matching (it

can be either load or antenna) is required at the end of the leaky feeder.

Leaky feeder looks like the continuous transverse antenna, so its coverage mainly

depends on its route. The coverage of the leaky feeder is realized by the small

windows provided on coaxial cables, because the signals are radiated to the

coverage areas through these windows.

Figure 1.1 shows the leaky feeder distributed antenna system.

Figure 1.1 Leaky feeder distributed antenna system

Compared to other antenna systems, the leaky feeder antenna system has the

following advantages:

The possibility of signal shadow and barrier is small. For example, if a distributed

antenna is used in a complicated tunnel, the distance between the mobile station

and an antenna may be barred; thus the coverage cannot be promised.

The signal fluctuation is slight. If the leaky feeder is used, the signal fluctuation is

slight in complicated environment.

The multiple service coverage can be provided. The leaky feeder had a wide

working band, so several radio systems can share a leaky feeder. For example,

some radio systems, such as paging system, alarm system, broadcasting

system, and mobile phone system, are often used in tunnels, because these

systems can share a leaky feeder, the antenna installation is simplified greatly.

Compared with the coaxial cable, both the equipment cost and installation

expenditure of the leaky feeder are higher.






III. Optical fiber

The fiber distributed antenna system can replace the coaxial distributed antenna

system in some complicated environments where the transmission distance is large.

Generally, the fiber distributed antenna system is more applicable to the base station

designed to cover underground areas such as tunnels, because the space for RF

feeder installation is rather limited.

Figure 1.1 shows the fiber distributed antenna system. The fiber distributed antenna

systems provided by different carriers may be different.

Figure 1.1 Fiber distributed antenna system

The fiber distributed antenna system has the following advantages:

The number of feeders is small for indoor installation.

Thinner feeders are applicable.

Optical fibers can be used to reduce electromagnetic interference.

The design in complicated network is more flexible.

Compared with the coaxial feeder, the transmission loss of the fiber is lower in a short

distance, but the cost is more and it requires local power and automatic detection

equipment.

IV. Summary

The comparison of the previous three types of distributed antenna system is

summarized in Table 1.1.






Table 1.1 Comparison of distributed antenna systems

Type Advantage Disadvantage

Coaxial cable Flexible design

Cost effective

Realizable

Great loss

Leaky feeder Flexible design High cost

Optical fiber Low loss

Easily installed

High cost

Inflexible design

Power required

4.8.3 Key Technical Indexes for Antenna Components

I. Two-in-one combiner (3dB mixed bridge)

Table 1.1 lists the technical indexes for the combiner.

Table 1.1 Technical indexes for a two-in-one combiner

Index GSM900MHz GSM1800MHz

Working band 890 – 960MHz 1710 – 1880MHz

Port imbalance 0.25dB

Insertion loss 3.6dB

Port standing wave 1.5dB

Power capacity 20W

II. Equal power distributor

The equal power distributor distributes the energy of the base station to several

tributaries. For the purpose of simplifying project design, only two types of equal

power distributor is used in the system. For the technical indexes, see Table 1.1.






Table 1.1 Technical indexes for an equal power distributor

Index 1-to-2 1-to-4

Working band 800 – 2500MHz

Power distribution

ratio

1:1 1:1:1:1

Insertion loss 3.5dB 6.5dB

Port standing wave 1.5dB

Connector type N_Female

Note:

The insertion loss in this table includes distribution loss.

III. Power coupler

The coupler here refers to bilateral coupler, or unequal power distributor. The coupler

specifications must be selected according to actual project design. For the purposes

of even coverage and energy saving, as more base station signals as possible must

be distributed to each antenna as more as possible.

Table 1.1 lists the technical indexes for the 7dB coupler, 10dB coupler, and 15dB

coupler.

Table 1.1 Technical indexes for a power coupler

Indexes 7dB

coupler

10dB

coupler

15dB coupler

Working band 800 – 2500MHz

Coupling degree 7dB 10dB 15dB

Insertion loss 1.2dB 0.5dB 0.3dB

Port standing wave 1.5

Connector type N_Female






For the small-sized indoor coverage, the three types of couplers in this table can meet

the project design requirements. For large scale indoor converge, the truck amplifier

must be used.

IV. Indoor antenna

The antennas used in the distributed antenna system are required to have a low gain

and eye-catching appearance. When selecting an indoor antenna, you should select

the wideband antenna and consider whether it is possible for multiple systems to

share a distributed antenna system.

In tunnels, to enlarge coverage area as much as possible, you can use a directional

antenna along the tunnel direction. Two types of antennas are available. They are

bidirectional antenna and unidirectional antenna.

In tunnels, the advantage of the bidirectional antenna is that it can cover the distance

along the two directions of the tunnel as compared with the unidirectional antenna if

their gain is the same. If you intend to use two unidirectional antennas to cover the

distance along two directions, you should add a power splitter. Therefore, the

bidirectional antenna (8-shaped antenna) is recommended) because this kind of

antenna is cost effective and brings no extra loss.

V. Coaxial connector

Because the length of the feeders used for the indoor distributed antenna system is

determined according on-site requirement, coaxial connectors must be designed for

the feeders. The system uses two types of coaxial connectors.

Table 1.1 lists the technical indexes for a coaxial connector

Table 1.1 Technical indexes for a coaxial connector

Type N type N type

Description Coaxial connector-N type

connector-50Ω/right

angle/male-nut installation-

configured with SYV-50-7-1

Coaxial connector-N type

connector-50Ω/right

angle/female-configured with

7/8”LDF5-50A cable

Feeder

specification

SYV-50-7-1 7/8"

Impedance 50Ω

Standing 1.2






wave

VI. Leaky feeder

Hereunder a 7/8" leaky feeder is taken as an example to explain the technical indexes

for a leaky feeder. For details, see Table 1.1.

Table 1.1 Technical indexes for a leaky feeder

Index Description

Characteristic

impedance

50Ω

Attenuation

constant

900MHz: 0.051dB/m; 1800MHz: 0.076dB/m

Coupling loss 900MHz: 72dB; 1800MHz: 84dB; (the loss 2 meters away from

the coupling hole with an error of ±10dB)

Feeder structure

specification

7/8"

Type of supporting

connector

14040121

Fire-proof

performance

Be proof against flames and ultraviolet

VII. Coaxial cable (feeder)

When designing a distributed antenna system, you should use the feeder to connect

all the antenna components. Table 1.1 lists two specifications of feeders in common

use. For the SYV-50-7-1 specification, the cost is low and it is flexible, but the loss is

great; for the LDF5-50A-7/8" specification, the loss is less, but the cost is high and it

is inflexible. The former feeder is more applicable to the tributary connection from

power splitter to antenna, and the later feeder is more applicable to the trunk

connection from one power splitter to another power splitter.

Table 1.1 Technical indexes for a coaxial cable

Index SYV-50-7-1 LDF5-50A-7/8"






Characteristic

impedance

50Ω 50Ω

Attenuation constant 900MHz; 0.22dB/m

1800MHz; 0.31dB/m

1000MHz: 0.0446dB/m

2000MHz: 0.0659dB/m

Type of supporting N

connector

14040184 14040121

Flexibility Good Poor

VIII. Load

When the leaky feeder is used, a small antenna can be used as the load for its ends,

or the load can be directly used as a match. Table 1.1 lists the technical indexes for

the load.

Table 1.1 Technical indexes for load

Working band 0 to 2GHz

Characteristic impedance 50Ω

Port standing wave < 1.15

Connector type N_Male

Power capacity 2W

4.9 New Antenna Technology—Smart Antenna Overview

4.9.1 Smart antenna

With the rapid development of global communication services, the wireless mobile

communication as the major means of individual communication in the future attracts

people’s great attention. How to eliminate co-channel interference (CCI), multiple

access interference (MAI) and the effect of multi-path effect is a major consideration

on how to improve the performance of a wireless mobile communication system.

The smart antenna adopts the digital signal processing technologies, such as the

switched beam technology and adaptive spatial digital processing technology, to






produce space directional beams so that the antenna major beam can aim at the

direction where user signals arrive and the side lobes or zero lobes can aim at the

direction where interference signals arrive. In this case, the mobile used signals canbe efficiently used and the interference signals can be effectively removed and

suppressed.

The smart antenna is mandatory for TD-SCDMA; for WCDMA and CDAM2000, the

smart antenna is optional.

The greatest disadvantage of a traditional base station is that it wastes much radio

signal energy. Therefore, only a small amount of signal energy can reach the

receiving part. Moreover, apart from useful signals, the signals received by a base

station contain interference noises. For the smart antenna, however, it listens to users

signals more effectively and uses signal energy sent to them.Different from the traditional CDAM, FDMA, or CDMA, the smart antenna introduces

SCDMA, which is a four-dimensional multiple access. The SCDMA technology

enables a user to distinguish signals according to their space propagation paths

under same timeslot, same frequency or same address code.

The smart antenna is similar to a space filter, which can reduce the interferences

among user signals remarkably when it is controlled by the parallel antenna beams

aiming at different users. To be specific, the smart improves the performance of the

future mobile communication system in terms of the following aspects:

Enlarge coverage area Reduce interference and enhance system capacity

Enhance spectrum utilization ratio

Improve base station sensitivity

Reduce base station transmit power

Reduce electromagnetic environment pollution

The smart antenna is divided into two types: switched beam antenna and adaptive

antenna, between which the adaptive antenna is the major type.

For realization reasons, the smart antenna is mainly used at base station side.

I. Multiple-beam antenna

The multiple-beam antenna selects the beam that is directed towards a fixed

direction. The base station side selects the beams that can best improve signal

quality and suppress restriction to communicate with the mobile station. The beam

width is determined by the number of array elements.

When a user moves in a cell, the base station selects different beams so as to make

the signals received strongest. Because the user is not necessarily at the center of

the fixed beams, the received signal is poor when the user is at the beam edge. In






addition, because, the multiple-beam antenna often selects a beam according to the

maximum receiving power, it cannot adapt itself to adjust the zero suppression

interference of the beams. Therefore, the multiple-beam antenna always cannotreceive the best signals. Compared with the adaptive antenna, however, the multiple-

beam antenna has some advantages, such as its structure is simple and it needs not

judge the arriving direction of user signals.

Figure 1.1 shows the schematic diagram of a multiple-beam antenna.

Figure 1.1 Schematic diagram of a multiple-beam antenna

A multiple-beam antenna consists of a group of low gain antenna array elements,

BFW, and switched beam logical circuit.

II. Adaptive antenna

An adaptive antenna is an antenna array that can change the antenna directional

diagram dynamically according to noise, interference and multi-path effect. It can

monitor users and adjust itself to suppress the zero point interference of the beams

through changing the directional diagram so that it can receive the signal-interference

ratio to the maximum.

Figure 1.1 shows the schematic diagram of an adaptive antenna (for single user).






Figure 1.1 Schematic diagram of an adaptive antenna (for single user)

This figures shows the schematic diagram of an adaptive antenna when only one

user is present. When multiple users are present, the situation is far more

complicated. An adaptive antenna consists of the following four parts:

Antenna array

The number (N) and list way of antenna array elements is directly related to the

performance of the adaptive antenna.

Generally, the antenna arrays are listed in three ways: linear equal spacing (LES),

circle equal spacing (CES), and plane equal spacing (PES), among which the LES is

in common use.

The number of array elements is 8 or 16 in actual application.

Analog-to-digital conversion or digital-to-analog conversion

On downlinks, this part converts analog signals to digital signals; on uplinks, this part

converts digital signals to analog signals

BFW

Adaptive digital signal processor

An adaptive antenna can identify the arriving direction of user signals and form a

major beam in this direction. In Figure 1.1, W stands for weight vector, y (t) stands for

output. The y (t) is expressed by the following equality:






∑−

=

++=1

1

)()()()( M

j

T

j

T

k

T t nW t S W t S W t y

Where, Sk (t) is the arriving signals of the expected users from the direction θ (k); S j (t)

is the arriving interference signals of other users from the direction θ(j), with the total

number of M-1; n (t) is noise vector; and W is weighting vector.

The adaptive digital signal processor amplifies the expected signal and suppresses

the interference signals and noise signals through selecting a proper weighting vector.

There are two tasks leave for the smart antenna. One is identifying arriving direction

of user signals; the other is realizing the digital matrix. The representative algorithms

for the smart antenna to identify the direction of arrival (DOA) of the signals are Musicalgorithm, RSPRIP algorithm, and maximum likelihood algorithm. The adaptive

beams are shaped for obtaining the best weighting coefficient through adaptive

algorithms.

For which algorithm is selected, adaptive rules must be considered, and the rules in

common use include SINR, MMSE, minimum variance, and maximum likelihood. It

has been proved that the four rules can help the adaptive antenna to obtain the best

weighting coefficient value which has the same stale solution (Wiener solution).

The adaptive algorithms in common use are direct sampling covariance matrix

inversion algorithm (DMI), various minimum mean square algorithms (LMS), recursion

least square algorithm (RLS), and constant model algorithm (CMA).

4.9.2 Smart Antenna Application

I. Omni-directional beam and shaped beam

The functions of the smart antenna are realized through transmitting and receiving

shaped-beams adaptively. A smart antenna transmits and receives shaped-beams

based on the geometric structure of the base station and the user signals required

and received by the system.

In a mobile communication system, the smart antenna adopts shaped-beams on the

uplink signals of each user, which serves to improve the performance of the system

directly. However, if the user is in receiving status and in idle mode, it is impossible for

the base station to know the location of the user. In this case, the base station

performs the transmission using the omni directional beams. For example, the

physical channels, such as the pilot channel, synchronization channel, broadcast

channel and paging channel are available for the omni directional beams.






Figure 1.1 shows a base station with the omni directional coverage, and the beams

transmitted on different channels are different. Therefore, the base station must

provide both the omni directional and directional shaped-beams. In this case, theomni directional channels requires much higher transmit power (the possible

maximum transmit power required by the omni directional channels is 101gN dB

higher than that required by dedicated channels, where N stands for the number of

antenna array elements). And this must be considered in system design.

Figure 1.1 Coverage area required by different channels

II. Shared downlink channel and discontinuous

In the mobile communication systems providing IP data services, the multiple user

shared downlink and uplink channels are designed and the discontinuous

transmission technologies are applied between base stations and user terminals.

For the base station using smart antennas, it cannot know the location of the user due

to the user movement. In this case, therefore, the base station can adopt the omni

directional beams only. In addition, the base station can also perform directional

transmission for each user by adding on more access process. Both the two methods

can be used because each method has advantages.

III. Smart antenna alignment

The real time and automatic alignment technology must be applied to the smart

antenna when it is in use. For actual base stations, however, the radio link of each

path cannot necessarily be the same and its performance changes with the time,

working level and environment. If real time and automatic alignment are not






performed, downlink shaped-beams will be seriously affected. In this case, the

advantages of the smart antenna are inapplicable, or even the communication failure

will occur.

IV. Frame structure and physical layer technolgy

The application of the smart antenna has no special requirement on the physical layer

technologies of a mobile communication system. Furthermore, the basic payer

technologies, such as modulation and demodulation, spectrum spreading,

interleaving, error correction, and data multiple connection, are completely the same.

If the smart antenna is used, however, the physical layer can be designed in a more

effective way. For example, in a TD-SCDMA system, if the synchronization CDMA

technology is adopted, the receiver can be simplified; if a specified uplink anddownlink pilot timeslots are designed for the physical layer timeslots, the interference

against cell search and random access is reduced. Therefore, all the previous

technologies enable the functions of the smart antenna to be fully played.

V. Combination of the smart antenna with other anti-interference

technologies

At present, the balance between the complexity and the real time realization

possibility of antenna algorithms must be emphasized. Therefore, the practical smart

antenna algorithms can neither handle the multi-path interference, whose delay

excels the width of one chip, nor overcome the channel deterioration caused by the

Doppler Effect.

When the multi-path effect is great, desirable results can be obtained only through

applying the anti-interference digital processing technologies to the smart antenna.

These digital signal processing technologies include joint detection, interference

cancellation, rake receiver. Currently, there are practical algorithms for the

combination of the smart antenna and the technologies of joint detection and

interference cancellation, but the algorithms for the combination of the smart antenna

and the rake receiver technology are still in research.

VI. Problems of beam shapping speed

Due to the mobility of user terminals, mobile communication is time-variable channel.

For the smart antenna, its received signals shape the uplink and downlink beams, so

the TDD period cannot be too long. For example, when a user terminal moves at the

speed of 100km/h higher, the Doppler shift is near to 200Hz, and the location change

reaches 28cm in 10ms. In this case, the location change will be greater than one






wavelength at 2GHz band, which will cause great error against shaping the downlink

beams. Therefore, the TDD period is expected to be reduced at least by half so that

the interval between transmission and reception can be controlled within 2-3ms. Inthis case, the smart antenna can work normally. If the terminals in this system are

required to move at a higher speed, the TDD uplink and downlink switching period

must be further reduced.

VII. Considerations for equipment complication

The performance of the smart antenna is improved with the increase of the number of

antenna array elements, but the greater the number of antenna array elements, the

more complicated the system is. In this case, the amount of the baseband digital

signals to be processed will increase geometrically. Nowadays, especially becausethe CDMA system is more in favor of wideband, the chip rate is already quite high,

stricter requirements are put forward to microelectronic technologies due to the

complication of baseband processing. As a result, the number of antenna array

elements will not be too great. Currently, the number ranges from 6 to 16.

In addition, the complication of the mobile communication environment causes other

problems for the smart antenna; such as the multi-path effect is great and message

sources generally outnumber antenna array elements.

Due to the characteristic of multiple sources and multiple paths, the followings must

be considered in the research and development of the smart antenna.

Thoroughly understand the mobile communication environment, especially the

space characteristics. To achieve this, you need to rebuild the models for mobile

communication and obtain great experimental results.

Based on the understanding of the characteristics of the mobile environments,

develop new algorithms. And these algorithms must seamlessly cooperate with

mobile communication systems and other radio technologies.

Research the cooperation between the smart antenna and the technologies,

such as power control, multiple user detection, synchronization technology, and

rake receiver technology.

These previous factors must be considered for the purpose of eliminating

interference, balancing, utilizing and improving system performance.

Chapter 4 of GSM RNP&RNO Application of Antenna Feeder System-20060327-A-1.0

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Transcript of Chapter 4 of GSM RNP&RNO Application of Antenna Feeder System-20060327-A-1.0