Chapter 4-Application of Antenna Feeder System

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..................................................................................................12 4.2.5 Relationship between antenna lobe width and antenna gain................................17 4.3 Antenna Tilt Planning......................................................................................................18 4.3.1 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..............................................................................................31 4.5.1 Principles..............................................................................................................32 4.5.2 Configuration of Combiner and Divider Unit.........................................................35 4.6 Tower Amplifier................................................................................................................36 4.7 Feeder.............................................................................................................................37 4.8 Distributed Antenna System............................................................................................38 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..................................................43 4.9 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 Optimization Chapter 4 Application of Antenna Feeder System

Χ η α π τ ε ρ

4

For internal use only

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. 4.1 shows the composition of an antenna feeder system

Figure 1.1 Composition of an antenna feeder system

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4.2 Antenna Basics 4.2.1 Types Generally, antennas for mobile communication are passive. 4.2.1 lists the antenna types in terms of different division standards. Table 1.1 Antenna type and division standard Division standard

Type

Radiation direction

 

Structural feature

Polarization way

Omni-directional antenna Directional antenna



Linear antenna



Dish antenna



Cap antenna



Vertical polarization antenna (unipolarization antenna)



Cross polarization antenna (dual polarization antenna)

4.2.1 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

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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. 4.2.2 shows the principles of antenna radiation

Figure 1.3 Principles of antenna radiation As shown in 4.2.2 (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 4.2.2 (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. 11/30/2011

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As shown in 4.2.2 (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 4.2.2, an actual antenna consists of multiple dipoles.

Unit dipole

Unit dipole

Feeding network

Feeding network

Antenna connector

Antenna connector

Directional antenna

Feeding network

Omni-directional antenna

Figure 1.4 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 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 through 11/30/2011

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assembling 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: 

dBi = dBd + 2.15

Where, 

The dBi indicates the energy concentration capability of the antennas with directions (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”.

I shows the relationship between dBi and dBd.

Isotropic antenna Symmetrical dipole antenna Actual antenna

2.15d B

dBd dBi

Figure 1.5 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: 11/30/2011

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

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 plane antenna, 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. II shows a horizontal plane and a vertical plane of a directional antenna.

Figure 1.6 Directional diagram of the directional antenna (horizontal plane and vertical plane)

II list the parameters related to the antenna directional diagram. Table 6.1 Parameters related to the antenna directional diagram Parameter

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Description

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Zero power point lobe width Half power point lobe width


It refers to the included angle between the zero radiation directions on both sides of the maximum major lobe. 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. It refers to the ratio of the maximum major lobe to the maximum side lobe. —

Side lobe suppressi on ratio Front-toback 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 11/30/2011

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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. IV lists the antenna tilt type and related descriptions. Table 6.2 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

Z0

is 50 ohm. The return loss can also

indicate the match characteristic of the port, that is, R.L.(dB) = 20 lg ｜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

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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 neglected according 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 ratio should 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 11/30/2011

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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 garbage signals 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.

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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

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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 signal are 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

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

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 and received 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 11/30/2011

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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 1 S 1 (t ) + k 2 S 2 (t ) + ... + k q S 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 this textbook. 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 indicate this parameter, the relationship between is

=

h . D

For antennas placed horizontally, when

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 is used to

and the actual antenna height (h)

 ≤ 10, two signal can be irrelevant Page15 of 54



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 the vertical 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.

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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: G a ≈10 log

32400 θ ⋅β

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

β≈

half

power

angle

is

360°,

the

vertical

half

power

angle

is

32400 = 7.15 ° . 360 ⋅ 10 11 / 10

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. 4.2.5 shows the relationship of the antenna gain, vertical half power angle and horizontal half power angle.

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Figure 1.7 Relationship of the antenna gain, vertical half power angle and horizontal half power angle. As shown in 4.2.5, 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 (effective antenna 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 11/30/2011

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base station, antenna height, lobe width, direction, tilt angle, and EIRP together form a 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 antenna height, 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:

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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 4.3.1.

Figure 1.8 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:

 = arctan (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 11/30/2011

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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. 4.3.1 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 1.9 Relationship between coverage distance and tilt angle (The width of the vertical beam is 17 degrees, and the antenna height is 40 meters.)

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4.3.1 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 1.10 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.

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 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 electrical adjustment 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. 4.3.2 lists the reference tilt angles for antennas in populated urban areas. Table 10.1 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


15

200

25

13


15

250

30

14

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15

250

35

15


15

250

40

17


15

500

25

10


15

500

30

11


15

500

35

12


15

500

40

12


15

800

30

10


15

1000

30

2

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 dense base 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 antenna cannot be directly parallel to the street, or “waveguide” effect will easily occur. In this case, it is rather hard to prevent cross coverage. When a cell is required to cover the area above the antenna height, you can use a directional antenna and uplift this antenna. 11/30/2011

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When the antenna major lobe tilts, the effect of the antenna back lobes must be also considered. Generally, the front-to-back ratio of the antenna is about only 20dB, so the back lobes with strong signals will cause much interference against upper coverage. As a result, it is recommended that electrical tilt antennas should be used in populated urban areas, though down tilt is still present in back lobes. In addition, the effect of the upper side lobe must also be considered. Generally, the vertical half power angle of an omni antenna is vertically symmetric along the horizontal plane, so it can work well regardless that it is inversely installed or upwardly installed. In actual project implementation, however, the vertical directional diagram of a specific omni antenna must be considered. If electrical tilt is available, careful consideration must be given to inverse installation.

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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 11/30/2011

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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

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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 a high 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.

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


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 give the priority to the 8-shaped antenna and the 210°antenna for highroads coverage.

4.4.5 shows the application of a 210° antenna.

Figure 1.11 Application of a 210° antenna 4.4.5 shows the application of an 8-shaped antenna

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Figure 1.12 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 are provided 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 4.4.7 lists the antenna selection references.

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Table 12.1 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

Plains & Rural areas

Expressways

Mountain areas

Directional station

Generally, select the antennas with high gain; both electrical adjustment tilt antenna and mechanical tilt antenna are ok.

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.

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.

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. 11/30/2011

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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 4.5.1.

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Figure 1.13 Schematic diagram of CDU The schematic diagram of SCU is shown in 4.5.1.

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Figure 1.14 Schematic diagram of SCU The schematic diagram of EDU is shown in 4.5.1.

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Figure 1.15 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 4.5.2.

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Table 15.1 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 large coverage configuration plans; it works when combined with Huawei concentric circle technology.

3 - 4TRXs

CDU + SCU

2 CDUs

-

1 - 2TRXs

CDU

EDU CDUs

or

2

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 and downlink 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 coefficient of 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 11/30/2011

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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 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 1.16 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 11/30/2011

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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 16.1 Technical indexes for feeders in common use Feeder type

100-meter attenuation (dB)

-

890MHz

1000MHz

1700MHz

2000MHz

Standing wave (Any length) -

SYFY-50-22 (7/8") LDF5-50A (7/8") LDF6-50 (5/4") M1474A (7/8") HFC22D-A (7/8") FSJ4-50B (1/2")

4.03 4.03 2.98 11.2

4.3 3.17 4.3 4.47 11.9

5.87 5.87 4.31 16.1

6.46 6.46 4.77 6.6 6.7 17.7

1.15 1.15 1.15 1.15 1.15 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. 11/30/2011

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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 4.8.1 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 1.17 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 mini antenna, 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

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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. I shows the coaxial cable distributed antenna system

Figure 1.18 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. II shows the leaky feeder distributed antenna system.

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Figure 1.19 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. III shows the fiber distributed antenna system. The fiber distributed antenna systems provided by different carriers may be different.

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Figure 1.20 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 IV. Table 20.1 Comparison of distributed antenna systems Type

Advantage

Disadvantage

Coaxial cable

Flexible design Cost effective

Great loss

Realizable Leaky feeder

Flexible design

High cost

Optical fiber

Low loss Easily installed

High cost Inflexible design Power required

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4.8.3 Key Technical Indexes for Antenna Components I. Two-in-one combiner (3dB mixed bridge) I lists the technical indexes for the combiner. Table 20.2 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 II.

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Table 20.3 Technical indexes for an equal power distributor Index

1-to-2

Working band

800 – 2500MHz

Power ratio

1:1

1:1:1:1

Insertion loss

3.5dB

6.5dB

Port standing wave

1.5dB

Connector type

N_Female

distribution

1-to-4

 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. III lists the technical indexes for the 7dB coupler, 10dB coupler, and 15dB coupler. Table 20.4 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.

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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. V lists the technical indexes for a coaxial connector Table 20.5 Technical indexes for a coaxial connector Type

N type

N type

Description

Coaxial connector-N type connector-50Ω/right angle/male-nut installationconfigured 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 wave

1.2

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 VI. 11/30/2011

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Table 20.6 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. VII 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 20.7 Technical indexes for a coaxial cable Index

SYV-50-7-1

LDF5-50A-7/8"


50Ω

50Ω

Attenuation constant

900MHz; 0.22dB/m

1000MHz: 0.0446dB/m

1800MHz; 0.31dB/m

2000MHz: 0.0659dB/m

Type of supporting N connector

14040184

14040121

Flexibility

Good

Poor

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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. VIII lists the technical indexes for the load. Table 20.8 Technical indexes for load Working band

0 to 2GHz


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 can be 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.

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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 cannot receive the best signals. Compared with the adaptive antenna, however, the multiplebeam antenna has some advantages, such as its structure is simple and it needs not judge the arriving direction of user signals. I shows the schematic diagram of a multiple-beam antenna.

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Figure 1.21 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. II shows the schematic diagram of an adaptive antenna (for single user).

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Figure 1.22 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 II, W stands for weight vector, y (t) stands for output. The y (t) is expressed by the following equality: 11/30/2011

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M −1

y (t ) =W T S k (t ) + ∑ W T S j (t ) +W T n(t ) j =1

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 Music algorithm, 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. 11/30/2011

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I 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, the omni 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 1.23 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 11/30/2011

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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 and downlink 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 11/30/2011

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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. In this 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 because the 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.

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Chapter 4-Application of Antenna Feeder System

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