Basic Principles and Design Specifications of the Antenna in Mobile Communications-20020904-A-2.0

Document No. Product Version

Confidentiality

V2.00

Wireless Network System Radio Frequency Research Department Huawei Technologies Co., Ltd. Product Name: M900/1800 Total Pages: 37

Basic Principles and Design Specifications of Antenna in Mobile

Communications

(Revised edition, for internal use only)

Prepared by Ai Ming Date 2001/09/08 Reviewed by Date yyyy/mm/dd Reviewed by Date yyyy/mm/dd Approved by Date yyyy/mm/dd

Huawei Technologies Co., Ltd.

All rights reserved.

Basic Principles and Design Specifications of Antenna in Mobile Communications 001

Revision Record

Date Revised version

Description Author

1,999 1.00 Complete the first draft. Ai Ming 2000/11/9 1.00 Transfer the draft to the network planning

technical support team Network planning technical support team

2001/09/8 2.00 Revise the draft. Ai Ming

♦ Note: 1) The basic concepts of middle feed and bottom feeder of omni antenna are added; Sections 3.10 through 3.13 are added 2) Correct the errors in some figures. (2001-09-08)

2006-06-01 All rights reserved.

Table of Contents

1 Overview......................................................................................................................................... 5 1.1 Antennas............................................................................................................................... 5 1.2 Development Trends of BS Antenna .................................................................................... 7 1.3 Design Concepts of BS Antenna .......................................................................................... 8

2 Basic Technologies....................................................................................................................... 9 2.1 BTS Antenna......................................................................................................................... 9 2.2 System Requirements and Antenna Technologies ............................................................ 12 2.3 Types of Antennas.............................................................................................................. 14 2.4 Design of Shaped-beam Antenna....................................................................................... 18

2.4.1 Fan Beam Antenna .................................................................................................. 18 2.4.2 Vertical Shaped-beam Antenna ............................................................................... 23 2.4.3 Beam Tilt .................................................................................................................. 24

2.5 BS Diversity Antenna.......................................................................................................... 25 2.6 Passive Inter-modulation of Base Station Antenna ............................................................ 30

2.6.1 Relationship between PIM and Receiving-transmitting Frequency ......................... 30 2.6.2 PIM Generator and Suppression Technology.......................................................... 31

3 Major Index Requirement for BS Antenna Design ................................................................... 32 3.1 VSWR of BS Antenna......................................................................................................... 32 3.2 Gain (dBi)............................................................................................................................ 32 3.3 Half Power Beam Width (HPBW) ....................................................................................... 33 3.4 Front-to-Back Ratio (F/B).................................................................................................... 34 3.5 Isolation between Ports ...................................................................................................... 34 3.6 Polarization ......................................................................................................................... 34 3.7 Power Capacity................................................................................................................... 34 3.8 Zero Stuffing ....................................................................................................................... 34 3.9 Upper Side Lobe Suppression............................................................................................ 35 3.10 Beam Downtilt................................................................................................................... 35 3.11 Two-band Dual Polarization Antenna ............................................................................... 35 3.12 Two-band Dual Polarization Duplex Antenna................................................................... 35 3.13 Grounding system............................................................................................................. 36 3.14 Antenna Input Connector.................................................................................................. 36 3.15 Passive Inter-Modulation (PIM) ........................................................................................ 36 3.16 Dimensions ....................................................................................................................... 37 3.17 Weight............................................................................................................................... 37 3.18 Wind Load......................................................................................................................... 37 3.19 Working Temperature ....................................................................................................... 37 3.20 Humidity ............................................................................................................................ 37 3.21 Lightning Protection .......................................................................................................... 37 3.22 3-Proof Capability ............................................................................................................. 37


Basic Principles and Design Specifications of Antenna in Mobile Communications

(Second Edition)

Key words: Mobile communications, antenna gain, design specifications

Abstract: Base station antenna is a bridge between user terminal and the Base

Station Controller (BSC). It is widely applied in the cellular mobile

telecommunications and ETS wireless telecommunication systems. This

document presents the history of antenna development, the basic antenna

technologies, and the major technical indices. Readers are expected to have an

overall understanding about the antenna of BSs in the mobile

telecommunications. The impacts of antenna lobe, antenna downtilt (mechanical

and electronic), isolation on the cell coverage and frequency reuse are also

mentioned in this document.

Abbreviation List:

Reference list

Name Author Document No.

Release date Available place or channel for

reference Mobile Antenna System Manual Translated by Yang Kezhong and Jin

Shuhua 1997

Cellular Mobile Telecommunications --- Design of BTS Antenna Feeder System

Xu Yubo 1998 Mobile Telecommunications Engineering Lu Errui, Shun Rushi, etc. Microstrip Antenna Theory and Engineering

Zhang Jun, Liu Kecheng, etc. 1998 Cellular Mobile Communication Engineering Design

A. Marrola Telecommunication Engineering Design Manual ---- Mobile Telecommunications

Beijing Design Institute of Post and Telecommunications Department


1 Overview

1.1 Antenna

With the rapid development of China’s economy, great changes have taken place in the communications industry. Today, propelled by the technologies and the economic benefits, communications industry has become one of the largest industries in China. Major telecommunication organizations are restructured to accommodate to the rapid development of this industry. Along with the advancement of communications industry towards the information economy, communication is now become the key to the sustainable development of various sectors of economy.

The development of the mobile telecommunication is even more remarkable. Nowadays, people are no longer contented to process the information flow in fixed places. Mobile telecommunications are in great demand when people are traveling or on vacation. In China, the significant change of mobile telecommunications is evident. Various types of mobile phones are everywhere bringing information about politics, economy, culture, and life to people. The largest GSM network in China now provides services for its over 20 million subscribers. The wireless access development is also widely adopted to ensure the communications in rural and remote areas.

New technologies and new devices in mobile communications posed as great challenges for antenna designers. For example, however small the terminal may be, user would not accept the idea if the conventional antenna is attached to his portable mobile terminal. Therefore, the antenna designers have to develop miniature or even electronic antennas to keep up with the development of modern technologies.

In addition to small size, antenna designers have to seek more sophisticated elements to equip the antenna with even more powerful functions such as the diversity receiving capability, optional polarity features, and capacity to reduce the multi-path fading. The focal point of antenna design is shifted from its physical features (e.g. small-size, light-weight, etc.) to sophisticated electromagnetic structure, so that antenna can play a significant role on the radio channel.

Antenna design will involve the propagation features, local environments, system compositions and performance, Signal Noise Ratio (S/N), bandwidth features, antenna's own mechanical structure, feasibility of production method, and the convenience of installation. The type of the mobile communications also affects the antenna design. The antennas used for the terrestrial system, offshore system, air system, and satellite system differ a lot. In the cellular systems, the radiation pattern should conform to segmentation pattern to avoid interference. In the urban areas, diversity receiving function should be employed to offset the multipath fading.

Antennas of smaller size are required for the terminal mobile. In the design of portable devices (e.g. the mobile phone), the antenna and Radio Frequency (RF) front end circuit of transceiver should be integrated. Antenna unit and the equipment should be treated as an antenna system.

In a word, the antenna should be designed as an organic party of the whole system instead of an independent part. See Figure 1.

The design specifications described in this document only involve the base station antenna (BS antenna) in wireless communication systems.


小型化强度 Miniaturized strength 区域距离 Distance between

regions

衰落多路径 Fading multipath 机械结构 Mechanical structure

电气指标 Electrical index 分集技术 Diversity technology

人机界面 Man-machine interface 环境 Environment

传播 Propagation 衰落 Fading

干扰 Interference 人为故障 Man-induced failure

天线 Antenna 系统 System

频率复用 Frequency reuse 多信道连接 Multi-channel connection

能力 Capability 类型 Type

陆地 Terrestrial 海事 Marine

航空 Navigation 卫星 Satellite

个人化 Customization

Figure 1 Integration of antenna and other systems



1.2 Development Trends of BS Antenna

BS antenna is the bridge between user terminal and the Base Station Controller (BSC). It is widely applied in the cellular mobile telecommunications and ETS wireless telecommunication systems.

The advancement of telecommunication technologies will definitely bring about the radical change of antenna. For the mobile communication system in1970s, the omni antennas or angle reflector antennas sufficed because the number of subscribers was not large. A few carriers and BSs can sufficiently cover a city and satisfy the demands of mobile telecommunications in a city.

However, mobile terminals are in great demand with the development of economy. Old BSs can no longer meet the demands. Moreover, new types of antennas are required as a result of the development of digital cellular technologies, so as to improve the multipath fading, area planning and frequency reuse of the multi-channel networks. The flat type antenna was widely adopted in the GSM digital cellular system due to its features of low section, light structure, easy installation and outstanding electronic performance.

From the mid1980s to the late 1990s, the unipolar antenna was used. As three antennas were needed for one sector (see Figure 2), and a cell was usually divided into three sectors, altogether nine antennas were needed for one cell. The large number of antennas brought great difficulties to the construction and installation of the base station. Under such a circumstance, the duplex polarization antenna technologies came into being. See Figure 3.

单极化天线 Unipolar antenna 主接收 Main receiving

发射 Transmitting 分集接收 Diversity receiving

Figure 2 Configuration of unipolar antenna in one sector



双极化天线 Bipolar antenna 主接收 Main receiving

发射 Transmitting 分集接收 Diversity receiving

双工装置 Duplex device

Figure 3 Two configurations of bipolar antenna in one sector

With channels and new BSs added, the cellular network should be adjusted and optimized, which demands new types of BS antennas such as adaptive antennas and intelligent antennas (The design specifications of these types will not be covered in this document).

1.3 Design Concepts of BS Antenna

As the number of mobile communication users is increasing, the frequency allocated to the mobile communication has been gradually raised from 30MHz to 50MHz, 150MHz, 250MHz, 450MHz, 800MHz and 1800MHz. The design of antennas has also been changed accordingly.

The design of antennas primarily relies on some mathematical methods and Computer Aided Design (CAD). The up-to-date method is Finite Difference of Time Domain (FDTD), which allows the radiation structure to be of any shape and to be made up of multiple layers of different materials. The BS antennas are usually divided into directional antennas and omni antennas.

The BS antennas used in High Frequency (HF) and Very High Frequency (VHF) and the omni antennas used for Ultra High Frequency (UHF) are of the line-shaped type, which are usually analyzed and designed by the moments method. The directional antennas used for UHF are normally the linear element antenna or paster-driven flat type antenna.

These types of antennas can be analyzed and designed by using the element method and Geometry Theory of Diffraction (GTD hybrid method). In fact, the latter type of antennas can be simulated by the HFSS software of HP and Ansoft. HFSS can be used to easily obtain the electrical specifications of this type of antennas, and then the best design can be worked out.



The BS antenna is a kind of open field-effect radiator, which involves sophisticated field analysis and numerical analysis. However, pure and time-consuming theoretical analysis is not desired as antenna is intended for practical use. Designers should accumulate experience and take the advantage of simulation software to work out the antenna design efficiently.

As previously mentioned, the design of antenna should take the system compatibility into consideration. System design and antenna design are closely related. A component (functional module) may be a high performance one when viewed independently. However, it may not be the best choice from a system point of view.

Take the printed paster antenna for example. It is less efficient than the common dipolar antenna. But due to its small cross section and the advantage of printing technology, it has helped turn a lot of new systems into reality. Its advantages are evident in the application of mobile telecommunication terminal, micro cellular, radar, and navigation equipment. Hence, antenna designer should especially take the following factors into consideration:

Regional structure: Determine the signal coverage area and the antenna direction. BS antenna: the antenna height, structure, installation, down tilt requirement of beams. Noise level: the thermal noise and ambient noise. Interference: the interference level, features, and co-channel interference and neighbor channel interference. Signal requirement: the best working frequency, bandwidth, cross interference, and frequency reuse. Cost of research, development, and processing. Reliability: the technical maintenance required, installation, and installation charges. Vulnerability: Rust and corrosion if the antenna is installed outdoors. User requirements

There are also some other factors that need to be considered.

The key point is designers should turn these factors into specific requirements of hardware design and then design the antenna according to these requirements.

2 Basic Technologies

2.1 BTS Antenna

BS is widely used in the GSM digital cellular communication system, ETS wireless access system and other terrestrial communication fields. For different fields, different types of antennas are used, and the design specifications also differ.

In the mobile communications, the BS serves the Mobile Station (MS). Generally speaking, it is fixed, though it also can be semi-fixed or vehicle-mounted. The semi-fixed BS refers to the BS whose location often changes, but communication service is not required when it is moving.

The vehicle-mounted BS is usually used in the vehicle dispatching center, which requires communication in mobile state. The document describes only the antennas of the fixed BSs.



Figure 4 shows the major considerations in BS antenna design. Though the antenna design belongs to electrical design in the narrow sense, it involves with many other fields. The most important is hardware technological conditions worked out according to requirements of the system design.

基站天线设计 BS antenna design 电气设计 Electrical design

机械设计 Mechanical design 于无线链路有

关的设计事项 Design related to radio link

单元和天线件设

计 Unit and antenna component design

区域特点 Regional feature

要求的 D/U Required D/U 有无分集 Diversity requirement

频率范围 Frequency range 单元 Unit

方向图的合成 Pattern synthesis 馈电电路 Feeder cabling

无源交调 Passive inter-modulation

风载荷设计 Wind load design

地震负载设计 Earthquake design 天线罩设计 Antenna mask design

结构件设计 Structure design 包装设计 Package design

Figure 4 Key issues in BS antenna design

To determine the hardware technical specifications, the electrical and mechanical performance should be compared, and tradeoff between performance and cost is necessary. In some cases, performance and cost are put in the first place, followed by the mechanical design of electricity. Figure 5 shows the procedures of antenna design.



基站天线设计方

法 BS antenna design 无线电链路预

估 Radio link estimation

设备结构接口 Equipment interface 硬件分析 Hardware analysis

成本预估 Cost estimation 分析数据 Analysis data

测量数据 Measurement data 各种算法 Algorithms

CAD 技术 CAM

系统要求 System requirement 频率/带宽 Frequency/bandwidth

信道/容量 Channel/capacity 业务范围 Service range

D/U 值 D/U value 成本 Cost

指标要求 Index requirement

增益 Gain 方向图 Pattern

极化特征 Polarization feature 机械性能 Mechanical feature

尺寸/总量 Dimension/weight

设计输出 Design output

结构布局 Structure layout 天线效率 Antenna efficiency

馈线网络 Feeder network 材料要求 Material requirement



电气参数 Electrical parameters 机械参数 Mechanical parameters

成本组成 Cost composition

Figure 5 BS antenna design procedures

In practice, it is of great importance to consider how to install the antenna after the antenna is assembled. It is because it may be much more expensive to install a BS antenna than to produce one. Therefore, not only production cost but also an antenna structure that allows easy installation should be taken into consideration.

2.2 System Requirements and Antenna Technologies

In the mobile communication system, the antenna helps establish the wireless transmission connections between the wireless telephones. To ensure the communication between the BS and the MSs within the service area, the energy of the radio waves should radiate as evenly as possible, and the gain of the antenna should be as high as possible.

As the width of the service area is definite, the gain cannot be raised by narrowing the horizontal beam width. However, the vertical linear array antenna can raise the antenna gain effectively. In the cellular system, the gain of the BS antenna is usually between 7 dBd and 15 dBd.

Multi-channel communication is commonly used to increase the communication capacity and improve frequency reuse ratio. This requires a wide band system with functions of combiner and divider. At present, the frequency band of the BS devices in China GSM cellular system is 890--960MHz. 890--915MHz is used for receiving signals, and 935-960MHz for sending signals. The antenna relative band width is required to be greater than 8%, and intra-band VSWR less than 1.5. When the antenna is receiving and sending signals, passive inter-modulation will result, which in turn increases cross interference.

With the rapid increase of the subscriber base, insufficiency of communication channels has become a problem for urban communications. To solve this problem, application of frequency reuse technology is strongly demanded. Though the cellular system can reuse frequency, the effectiveness of this technology relies on the radiation pattern of the BS antenna. The major-beam tilt and bean shaping technologies can improve the reuse of frequency effectively.

Non-stadia transmission is one of the most common features in the mobile communications, especially in the modern cities. The numerous high buildings in the city constitute a complicated radio transmission environment for the mobile subscribers and result in fading of radio transmission. The receiving electrical level is thus affected and in some cases may fluctuate for more than 30 dB.

If the system design should be based on the lowest receiving level, the equipment could be rather expensive. The diversity receiving technology can overcome the fading effectively. Though application of this technology needs more devices, it is the most cost-effective solution from the system point of view and is at the moment the most commonly-used technology to overcome fading.



Figure 6 shows the relationship between the system requirements and the antenna technologies.

系统要求 System requirements 高电平均匀照射业务区 Even radiation of high level in the service area

抑制业务区以外的辐射

（频率复用技术） Suppression of radiation outside the service area (frequency reuse technology)

多信道宽频带 Multi-channel and wide frequency band

稳定的接受电平 Steady receiving level 降低延迟扩展 Reduction of delayed expansion

体积小，重量轻，抗风 Compact, light, and wind-proof

天线技术 Antenna technology

主波束倾斜，赋形波束

综合 Integration of beam downtilt and shaped-beam technologies

宽带天线单元，宽带匹

配网络 Broadband antenna unit and broadband matching network

分集接收 Diversity receiving 机械设计 Mechanical design

Figure 6 System requirement and antenna technology



2.3 Types of Antennas

The structure or type of the BS antenna is determined by the size and landform of the service area, and the number of cells and channels.

If the service area is within the limited range of angles on the horizontal plane, the plat type antenna is often used. The half power beam angles of horizontal plane include 33°, 60°, 90°, 120°, 180°, etc.

If the service area should be covered in all directions horizontally, the omni antenna is often used, which can only tilted vertically. In the early cellular system, the length of the antenna was determined by the gain required, and even excitation was usually adopted for the array antennas to achieve a higher gain.

Figure 7 is the diagram of the typical structure of the omni antenna.

(a) Middle-feed mode (b) Bottom feed mode

Figure 7 Omni antenna

For the middle-feed antenna (see note 1), if the beam downtilt technology is not applied, the maximum directivity in the direction of 0° without any tilting or declining in the whole working frequency band.

As to the bottom-feed antenna (see note 2), however, the monotone phase variation of every unit will cause the maximum beam directivity to change with the frequency, which affects the network coverage seriously. When the cells should be re-divided to achieve the effective reuse of frequency, the value of D/U is a consideration more important than antenna gain in BS antenna design. At present, the electrical or mechanical major-beam downtilt technology is commonly applied to the BS antenna design in cellular mobile communications system.



Experiments show that beam downtilt can reduce the co-channel interference by about 10dB, as shown in Figure 8. The network optimization experts have fully realized that the beam downtilt technology is the basic technology to increase frequency reuse because it can form appropriate array antenna radiation pattern to compress the side lobes beside the major beam, thus reducing the frequency reuse distance. Figure 9 shows how the BS antennas can be classified by functions and by features.

是理想的自由方向图假

设条件下的 Ideal pattern 计算方向图 Computed pattern

接收信号强度 Strength of received signal 倾角＝3oC Downtilt=3oC

高基站距离 Distance between high BTSs

Figure 8 Influence of beam downtilt to the frequency reuse



天线 Antenna 低旁瓣 Lower side lobe

D/U 值增加，上旁瓣被

抑制 The D/U can be increased to suppress the upper side lobe.

波束倾斜 Beam downtilt

零点填充 Zero stuffing 高电平 High level

业务区 Service area 干扰区 Interference area

Figure 9 Impact of side lobe on frequency reuse



基站天线 BS antenna 分集天线 Diversity antenna

单个天线 Single antenna 阵天线 Array antenna

单元天线 Unit antenna 水平面赋形波束 Shaped beam on horizontal plane

垂直面赋形波束 Shaped-beam on vertical plane

空间分集 Space diversity

极化分集 Polarization diversity 水平面波束控制 Beam control on horizontal plane

多波束 Multi-beam 均匀激励 Uniform excitement

倾斜波束 E/M Beam downtilt E/M 旁瓣控制 Side lobe control

零点填充 Zero stuffing 振子，微带贴片，寄生

微带贴片 Oscillator, micro-paster, parasitic micro-paster

微带，缝隙，角反射器

天线 Micro-strip, slot, corner-reflector antenna

Figure 10 Classification of BS antenna

Note 1: Middle feeder refers to the case that the feeder point of the coaxial array omni antenna is at the middle element. In this case, no matter how the frequency changes, the phase change of the upper and lower elements is symmetrical, i.e., the maximum gain of antenna is at 0° (non-downtilt design technology).

Note 2: Bottom feed refers to the case that the feed point of the coaxial array omni antenna is at the bottom of each element



(Bottom feed is different from the case that the power input point is at the bottom, because the power input point of the middle-feed antenna is also at the bottom. The difference between the middle-feed antenna and the bottom-feed antenna lies in the actual location of the feed point). In the case of bottom feed, the phase change from the lower to the upper elements is not symmetrical, i.e. the maximum directivity is related with the frequency.

2.4 Design of Shaped-beam Antenna

The shaped-beam technology can increase the space frequency reuse rate. In the cellular system, the BS antenna is required to radiate the lowest possible level to another cell using the same frequency, but the highest possible level to the poorly-cover area within the service area. The shaped-beam antenna falls into two types. One is horizontal shaped-beam radiation pattern, referred to as fan beam in engineering; another is vertical shaped-beam radiation patter, or cosecant beam.

In fact, the major-beam downtilt is not the shaped-beam technology in real sense, though they are used for similar purpose. This document only covers the design of shaped-beam antenna in the cellular system. For implementation of beam synthesis and numerical technique, please refer to the related documents.

2.4.1 Fan Beam Antenna

In the metropolitan cellular system, the horizontal beam of BS antenna is not omni-directional. The fan beam can effectively cover the service area and improve the reuse of frequency. The typical fan beam antenna is the corner-reflector antenna. It can adjust the beam width by controlling the angle of the reflector. Figure 11 shows the basic geometry of the corner-reflector antenna.



偶极子 Dipole 馈电路 Feeder circuit

几何机构 Geometric structure

H面方向图 Pattern on horizontal plane

Figure 11 Corner-reflector antenna

In the early cellular system, this type of antenna was commonly used to get the fan beam. However, it is now seldom used due to its defects such as less compact feeder network, large cross section, and complicated structure. Hence, this document will detail other types of fan beam antennas instead. These antennas are now commonly applied to the modern cellular system. See Figure 12-a, 12-b, and 12-c.



Figure 12 HFSS simulation instance

These units are called plat antenna units due to their thin cross section. When assembled with the appropriate antenna cover, it looks like a flat board. The formulas used for the design of these antenna units are rather complex and the hybrid method of MM and GTD is often resorted to. However, these methods are not suitable for an application engineer.

To solve this problem, two American companies, Ansoft and HP, released the High Frequency Simulation Software (HFSS) so that the answer to the electromagnetic field problem can be found out with basic antenna principles and experience about antenna on mind. Through the simulation of HFSS, flat antenna can change the values of width (W) and height (H) and thus can control the half-power beam width on the horizontal plane.

The half-wave dipole HFSS result can be controlled within the range of 55°-120° (It can be realized in terms of structure.). To obtain a beam width between 30° and 55°, two excitation sources should be placed in a certain interval on the horizontal direction of the flat.

Figure 13 is the HFSS simulation result of GSM 900MHz unipolar flat unit. Designers should be noted that the effect of antenna cover on the radiation performance should be taken into consideration.



2.4.2 Vertical Shaped-beam Antenna

As Figure 14 shows, the antenna fixed to a certain height covers a limited area on horizontal plane so that the receiving signal level is equal in each spot of the service area. To obtain shaped-beam on the vertical plane, multiple flat antennas are required to form an array on the vertical plane. Meanwhile, appropriate amplitude and phase feeding are required for each unit. The amplitude and phase control technology of feeding network is very important for the beam shaping on vertical plane. The more units there are, the more ideal shaped-beam can be obtained.

天线 Antenna 水平面 Horizontal plane

低旁瓣区 Lower side lobe 方向图 Pattern

业务区半径 Radius of service area

干扰区 Interference area

Figure 13 Shaped beam with low interference (vertical plane)



Figure 14 Pattern of antenna array with four antennas (horizontal)

Figure 15 Pattern of antenna array with four antennas (vertical)

HFSS can be firstly used to obtain the fan beam required. Its vertical pattern is

Fv( ).

Take the four-unit array for example. The amplitudes and phases of the units numbered from 1 to 4 are represented by A1, A2, A3, A4, ф1, ф2, ф3, and ф4 respectively. The following equation can be obtained:

f( ) = Ee−jkrr {A1e−jk( 3

2 dxCOS( )+ 1) + A2e−jk( 12 dxCOS( )+ 2)

+A3e−jk(− 12 dxCOS( )+ 3) + A4e−jk(− 3

2 dxCOS( )+ 4)} Fv( ) Change A1, A2, A3, A4, ф1, ф2, ф3, and ф4. With the help of computer, optimization can be done and the vertical shaped beam as shown in Figure 16 can be obtained. The figure clearly shows the first side lobe of the symmetrical radiation pattern is much higher.

After the shaping, the upper side lobe is obviously suppressed and is improved by 7 dB as compared with the symmetrical radiation pattern. The zero point of the lower side lobe is stuffed and the radiation level in the service area is improved.

2.4.3 Beam Tilt

The beam tilt technology aims to tilt the major beam so that the level of the radiation towards the frequency reuse area can be reduced. In this case, although the level of the carrier on the boarder of the area is reduced, the interference level declines much



more than the carrier level does. It is an advantage in the system design and is adopted worldwide by most of the cellular systems.

Figure 16

Figure (17-a) Maximum ratio combining of different correlation coefficients between channels

The beam tilt can also be realized through electric design. That is, the downtilt of beam can be achieved by adjusting the excitation coefficients, amplitude and phase. A set of antenna equipped with both electric downtilt and mechanic downtilt can be useful especially during the network optimization when the fixed electric downtilt is far from enough.

2.5 BS Diversity Antenna

BS diversity antenna has been widely applied in the cellular systems. It can reduce the fading when the two antennas are two wave lengths away from each other on horizontal plane. Although receiving diversity needs two or more ports, it can effectively reduce the fading. As a result, the BTS power is reduced and the transmission quality is improved.

In the mobile telecommunication, the signal received will be affected seriously in urban area with a lot of high buildings or forest with a lot trees. The fast fading is



caused by the reflection of fixed and mobile objects. Deep fading exists in a certain part of the wave length.

In a densely-populated area, the signals received by MS at any moment contain a lot of plane electromagnetic waves that are transmitted in parallel. The amplitudes, phases, and angles of these electromagnetic waves are random ones. Statistically, the amplitude and phase of one electromagnetic wave can be regarded as independent of other electromagnetic wave. All the signal components are synthesized into one complex standing wave, whose signal strength varies with the change of each component.

There could be a fading of 20-30dB in a distance of several vehicles and the existence of large amount of propagation path results in the multi-path symptom. This kind of fading is not only found in the MS receive signal, but also BTS receive signal. However, the multi-path fast fading disappears in the place which is ten wave lengths away. That is, the diversity receiving can improve the communication reliability without increasing the transmitter power or channel bandwidth.

The diversity receiving is based on one basic concept: when two or more samplings are made for a random process, these samplings are fading independently. The probability that all the samples are less than a fixed value is much lower than the probability that one sample is less than this value. Hence, the comprehensive sampling can help improve the performance of transceiver and the effect is much better than the single sampling.

The function of synthesis is to correct the phase and delay of signals after the multi-path transmission, add up the input signal level vectors, and add the noise at random. So the signal-to-noise ratio after synthesis of channels is generally greater than that of the single receiving channel. As the chance of simultaneous fading of incoherent signals is slim, the system can be more reliable. Figure (17-b) shows how the correlation coefficient changes with the height of the antenna and the distance.

The structure of BS diversity antenna comes into three types: space diversity, pattern diversity, and polarity diversity. Space diversity is the most common one.

Relationship between space diversity antenna and related coefficients

To explain this relationship, one parameter is introduced, where = hbed ,hbe is

the effective height of the BS diversity receiving antenna and d is the distance between the BS diversity receiving antennas.

Figure 17-b displays the curve that illustrates the relationship between coherent of incidence angle ( ) and .

In the urban area, as there are a lot of scatterers along the propagation path between MS and BS, the coherent is much smaller than that in the suburb. The greater the coherent is, the higher diversity gain will be. When the coherent is greater than 0.7, the improvement of diversity gain is not so obvious than the case when is less than 0.7.

Figure 17-a shows that when the signal level is -10dB, the probability that the amplitude is lower than -10dB is 1.3% ( =0.7) or 0.52% ( =0.2). That is, when the coherent drops from 0.7 to 0.2 and the probability is improved by 0.8% only. When both the feasibility and cost are taken into consideration in practice, it is most appropriate to have less than 0.7. In this way, the BSs in urban area will have a better diversity gain.



Figure 17-b also shows that when the receive signal reaches , the coherent is affected substantially. When is equal to 0°, the coherent is the smallest and the diversity gain is the greatest. When is equal to 90°, the coherent is the greatest, while the diversity gain is the smallest. As the MS may move in any direction, i.e., could be any value within the range of 0°~90°, and the antenna will not be designed

based on the best ( =0°) case or worst case ( =90°), it is recommended to adopt the mean value 45° for and the distance between two receive antennas is determined by =45° and =0.7.

equaling to 45° and equaling to 0.7, With can be computed, i.e. 9. Table 1 shows the effective heights and inter-antenna spaces of the diversity antenna.

Table 2-1 Effective heights and inter-antenna spaces of the diversity antenna.( =45°, =0.7) Effective height of diversity antenna (m)

20 50 70 80 90 100

Space between antennas (m)

≥3.0 5.6 6.7 7.8 8.9 10 11.1

The following result can be obtained from the above data:

d = 0.11hbe [M]

The diversity gain is affected by the following factors: inter-antenna space, diversity combination technology, diversity tuple, and communication probability. When the duplex space diversity and maximum ratio combination are used, the relationship between diversity gain and coherent can be illustrated by Figure 17-a.

For example, if the probability that the amplitude is larger than the horizontal ordinate

is 90% and =0.7, the signal level is -4.6dB and the signal level of a single Rayleigh channel is -9.5dB. Thus, the gain of duplex space diversity is 4.9dB. The diversity gain corresponding to other probability can be obtained in the same way.

When the antenna is placed horizontally as shown in Figure 17-c, its isolation is determined by the antenna radiation pattern, the space, and gain.



辐射方向图 Radiation pattern 90o方向 90o

Figure 17 Horizontal placement of antenna

Generally, the fading introduced by Voltage Standing Wave Ratio (VSWR) is not included. If the gain of the transmit antenna on the maximum radio direction is Gat (dBi), the level of the side lobe at the direction of 90°is SLt, the gain of the receive antenna on the maximum radio direction is Gar(dBi), the level of the side lobe at the direction of 90° is SLr (dBp, against the major beam. The value is negative), the horizontal spacing is Dh, the inter-antenna isolation can be given by:

Adis = -22 - 20log (Dh/l) + (Gat + Gar) + (SLt + SLr) (dB) [negative]

If the antenna is omni antenna, SLr=SLt=0 (dB) and l in the equation is the working wave length. (regarded as far field). Generally the SL of 65° fan beam antenna is about -18dBp, that of 90° fan beam antenna is about -9dBp, and that of 120° fan beam antenna is about -7dBp. The actual value can be determined according to the antenna pattern.

Example 1: 65° fan antenna, Gat=Gar=15dBi, SLt=SLr=-18dBp,

f=915MHz,l=0.328m

Adis=-30 dB (This index must be met in GSM system.)

The following result can be obtained as per previous formula: Dh=1.25 l=0.41m

Example 2: Omni antenna, Gat=Gar=11dBi,SLt=SLr=0dBp, f=915MHz, l=0.328m

Adis=-30 dB (This index must be met in GSM system.)



The following result can be obtained as per previous formula: Dh=31.6 l=10.4m

When the antenna is placed vertically, the isolation between two antenna is approximately:

Adis=-28-40log(Dv/l)

垂直面内辐射方向图 Radiation pattern on vertical plane

90o方向 90o

Figure 18 Vertical placement of antenna

Figure 18 shows the pattern space antenna for a whole cell. It is composed of four sets of antennas, forming an angle of 90° with one another. They are used to achieve the 180° fan bean in the omni pattern and are placed separately. The internal between two omni antennas is 0. Thus, the difference of antenna receive power is caused by the difference of pattern.

When the distance between the 180° fan beam antennas is 6 wave lengths, the test shows that the correlation coefficient is less than 0.2.

Polarity diversity antenna emerges along with the rapid development of cellular system. It integrates two orthogonal (0°/90° or +45°/-45°) polarity antennas and thus compactness is its most remarkable feature. However, the polarization feature of incidence angle is more likely to be vertical polarization and the average receive power of the port of 0°/90° dual polarization antenna differs a lot. Hence, the improvement of Signal Noise Ratio (S/N) is less obvious than other diversity measures do. But with the +45°/-45° dual-polarization antenna, the diversity gain equivalent to the one of space diversity antenna can be obtained.



2.6 Passive Inter-modulation of Base Station Antenna

Passive inter-modulation (PIM) is one of the major factors that generate co-channel interference. When the antenna works in duplex mode, PIM should be considered. In most of the cases, the PIM on transmit channel is caused by the non-linear feature of the metal heterojunction that is found between the antenna radiation unit and the feeder.

The co-channel interference is thus generated on the receiving branch. To enable the concurrent transmitting and receiving, the inter-modulation power should be lower than a standard value during the design and processing of antenna. For GSM cellular system, this standard value is around -103 dBm.

2.6.1 Relationship between PIM and Receiving-transmitting Frequency

Suppose the frequencies of two transmit carriers are respectively Fi and Fj, the (m + n)th modulation is:

mFi nfjF1M=

Where, m and n are positive odd numbers and is the frequency of the interference wave on the receiving band. The probability of the interference wave is decided by the space between transmit power and receive power and the value of (m + n).

F 1M

For example, the transmit frequency of GSM 900 MHz cellular system in China falls within the range from 935MHz to 960MHz and the receive frequency from 890 to 915MHz. The space between transmit power and receive power is 20MHz. Thus, the PIM is of 3-order. If no effective suppression measures are taken, serious interference will result. See Figure 19.



TX 产生 3 阶交调对 RX的干扰

TX generates 3-order inter-modulation that interferes RX.





Figure 19 High-order inter-modulation and interference

The relationship between the order of PIM and the power generated can be approximately illustrated by the formula: (m + n) × 10 dB. If the frequency space between transmit wave and receive wave is small, 5-order or 3-order PIM will generate interference and the level will be higher than the 7-order PIM by 20 or 40dB.

2.6.2 PIM Generator and Suppression Technology

Power generated by PIM is determined by the metal type and the structure of the connector. PIM is mainly generated on the antenna radiator, co-axial connector, welded joint and the contact surface that is likely to get rusted and corroded. By now, there is no definite answer to the relationship between PIM and the structure of the contact point.

With the rapid increase of mobile communication demand, a large number of BS antennas are in demand, especially the duplex antenna which is more cost-effective. The duplex antenna will be widely applied. Therefore, antenna designers should attach more and more importance to the development of PIM suppression technology.



Table 2-2 Table 2 Basic PIM suppression methods

PIM generator Suppression method: Radiator Use printed antenna to replace the oscillator unit Connector Increase the contact area and use silver plate Welded joint Reduce the number of welded joints and add solder to the

welded joint Rust and corrosive surface Coat the surface to prevent the oxidation Feeding network Try to use strip line or micro-strip line to replace the cable

3 Major Index Requirement for BS Antenna Design

3.1 VSWR of BS Antenna

For the BS antenna of mobile communication cellular system, the maximum value of VSWR should be less than or equal to 1.5:1. And this requirement should be met at the specified working frequency band and temperature range. If the input impedance of the antenna is ZA , the nominal characteristic impedance is , the reflection factor can be given by:

Z0

｜Г｜ = ｜ZA−Z0｜

｜ZA+Z0｜,VSWR = 1+｜Г｜

1−｜Г｜

Where is equal to 50 . The matching feature of the port can also be represented by the return loss:

Z0

R.L.(dB) = 20 ｜Г｜logloglog .

When VSWR is 1.5:1, the computed R.L. should be -13.98dB.

3.2 Gain (dBi)

The directivity characteristic of antenna can be depicted by the pattern. But generally the value is used to show the concentration degree of the electromagnetic energy radiated by antenna, i.e. directivity factor D. D is defined by the following formula:

D = SdS0｜P

∑d=P

∑0

When the thermal loss of the antenna is considered, the antenna efficiency A should be introduced. It is defined as follows:

A = PPA



P PAWhere, is the radiation power of antenna and is the input power of antenna.

When the radiation performances of the two antennas are compared, if their input powers remain unchanged, the antenna gain can be given by:

G = A×D

G = 10 (

(suppose the efficiency of unipole antenna is 100%).

As generally the gain is given in decibel (dB), the gain can also be given by

logloglog A×D) dBi (as compared with Isotropic antenna).

If the half-wave dipole is used as the reference antenna, the unit of gain is dBd and 0 dBd equals to 2.15 dBi (see Figure 20). Other units will not be used for the BS antenna. Please note that the BS antenna gain refers to the gain of the working frequency band unless otherwise specified.

实际天线 Actual antenna 半波偶极子天线 Half-wave dipole antenna

各向同性天线 Various like antennas

Figure 20 Relationship between dBi, dBd, ERP, and EIRP

3.3 Half Power Beam Width (HPBW)

As the BS antenna is generally erected vertically to the ground, the HPBWs of vertical plane and horizontal plane are often used to describe the HPBW of BS antenna. The range of HPBW should be given for the working frequency band, e.g. 65°±6°.For a directional antenna, the included angle between the two half-power points relative to the maximum radiant point is the half-power beam width.



3.4 Front-to-Back Ratio (F/B)

Front-to-Back Ratio (F/B) is an important factor measuring the suppression ability of the antenna backward beam. It is the level difference between the maximum beam and the side lobe within the range of 180°± 30° starting from 0o. It is a positive value in dB. F/B is associated with the antenna gain and the type of antenna and ranges from 18 to 45 dB.

The specific index requirement is determined by the network planning and optimization. At present, the F/B ratio of Huawei 900/1800 MHz directional antenna is 20-25dB.

3.5 Isolation between Ports

There are various types of multi-port antennas, such as dual polarization antenna, two-band dual polarization antenna, two-band dual polarization duplex antenna. When they work in duplex mode, the isolation between ports should be greater than 30dB.

3.6 Polarization

Polarization refers to the orientation of electric filed vector radiated by the antenna on the space. The linear polarization antenna is often adopted for the BS.

With the ground as the reference plane, if the electric field vector is vertical to the ground, it is called Vertical Polarization (VP).

If the electric field vector is parallel to the ground, it is called Horizontal Polarization (HP). The dual polarization antenna often adopts the +45° and -45° cross dual-line polarization.

3.7 Power Capacity

Power capacity here refers to the average power capacity. Antenna is composed of matching device, balancing device, phase-shifting device, and other coupling device.

The power it can bear is limited. Based on the actual maximum input power of BS antenna (Single carrier power is 20W.), if one antenna port can receive maximum 6 carriers, the antenna input power should be 120W. Thus the power capacity per antenna port should be greater than 200W when the temperature is 65°C.

3.8 Zero Stuffing

When shaped-beam design is adopted in the vertical surface of base station antennas, the first zero point of the lower side lobe need to be stuffed without any obvious depth, so as to make the radiant level more even within the service area.

Usually, if the zero depth is greater than -26dB in relation to major beam, this means the antenna has zero stuffing.



1)

2)

3)

3.9 Upper Side Lobe Suppression

To enhance the frequency reuse efficiency and reduce the co-channel interference to adjacent cells in a microcell cellular system, the base station antenna beam can be shaped based on some principles.

That is, those side lobes that aim at the interference area should be lowered as much as possible, and the D/U value (ratio of strengths of desired and undesired signals) should be increased, and the level of the upper side lobe should be less than -18dB.

There is no such a requirement for macrocell base station antennas

3.10 Beam Downtilt

Antenna downtilt needs to be adjusted to meet the coverage requirement or network optimization requirement. However, if the downtilt is adjusted mechanically, when the downtilt is adjusted by an angle of more than 8°, the horizontal beam width of base station antenna will loss its shape, which affects the sector coverage. At present, there are following types of beam downtilts:

Fixed beam electronic downtilt. By adjusting the amplitude and phase of radiator, the antenna major beam can deviate from the normal direction of the antenna array element for a certain angle, e.g. 3°, 6°, or 9°. When used together with mechanical downtilt, electronic downtilt allows an adjustable range of antenna downtilt angle of 18-20°. Manual-adjustable beam electronic downtilt. The adjustable phase-shifter can be adopted for the BS antenna, so that the direction of the main bean can be adjusted continuously within the range of 0-10° (not including the mechanical adjustment).The major suppliers of this type of antenna include HUBER-SUNNER and ALLEN DB. Remote-control beam electronic plunge angle. This type of base station antenna is equipped with micro servo system. The phase shifter can be controlled by the precision electric engine so as to remotely control the program. However, the addition of active control circuit degrades the reliability of antenna and complicates the lightening-proof problem. DELTEC (New Zealand) is one of the major suppliers of this type of antenna.

3.11 Two-band Dual Polarization Antenna

It is a new type of antenna that integrates the antennas of two bands, e.g., GSM/DCS, GSM/WCDMA and DCS/WCDMA. See figure 21.

3.12 Two-band Dual Polarization Duplex Antenna

To reduce the feeders, duplexer (in fact it is a filter combiner) is used to combine the two powers with the same polarization but different frequency into one. See Figure 21.



双频双极化天线 Two-band dual polarization antenna

双频双工双极化天线 Two-band dual polarization duplex antenna

GSM 天线单元 GSM antenna unit DCS 天线单元 DCS antenna unit

滤波合路器 Filtering combiner

Figure 21 Multi-port antenna

3.13 Grounding system

BS antenna is normally installed on a high position. To prevent the lightning strike, the DC resistance between inner and outer conductors of antenna port should be designed as 0.

3.14 Antenna Input Connector

To reduce the passive inter-modulation and ensure the RF connection, the input connector of antenna adopts 7/16DIN-Female. Before the antenna is used, the connector should be properly capped to avoid the oxidation and the intrusion of impurities.

3.15 Passive Inter-Modulation (PIM)

To reduce the noise resulting from the non-linearity of antenna, the PIM of antenna should be less than -103dBm (2x10W).



3.16 Dimensions

To facilitate the storage, transportation, installation, and ensure the security, so far as the electrical indices are satisfied, the size of antenna should be as small as possible.

3.17 Weight

To facilitate the storage, transportation, installation, and ensure the security, so far as the electrical indices are satisfied, the antenna should be as light as possible.

3.18 Wind Load

As BS antennas are usually installed on high buildings and towers, it is required that an antenna should work normally when the wind speed is 36m/s, and remain undamaged when it is 55m/s, especially in coastal areas where the wind is usually strong.

3.19 Working Temperature

A BS antenna should work normally when the environmental temperatures is between -40°C and +65°C.

3.20 Humidity

A BS antenna should work normally when the environmental relative humidity is between 0 and 98%.

3.21 Lightning Protection

Direct DC grounding is required for all the radio frequency input ports of a BS antenna.

3.22 3-Proof Capability

A base station antenna should have a 3-proof capability, namely, humidity-proof, salt fog-proof and mould-proof. A base station omni-antenna should allow upside-down installation, and should satisfy the 3-proof requirement as well.


Basic Principles and Design Specifications of the Antenna in Mobile Communications-20020904-A-2.0

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