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  • 1. Antenna System Introduction Course Objectives: ·Understand antenna principles ·Understand technical parameters of antenna ·Grasp antenna selection methods
  • 2. i Contents 1 Antenna Overview...................................................................................................................................... 1 1.1 Antenna Development Overview ...................................................................................................... 1 1.2 Antenna Radiation Principles............................................................................................................ 2 1.2.1 Electromagnetic Wave Radiation of Electric Dipoles ............................................................ 2 1.2.2 Half-Wave Doublet ................................................................................................................ 3 1.3 Antenna Structure and Types............................................................................................................. 4 1.3.1 Directional Panel Dipole-Array Antenna ............................................................................... 4 1.3.2 Omni-Directional Series-Feed Dipole Antenna ..................................................................... 7 2 Antenna Technical Parameters ................................................................................................................. 9 2.1 Antenna Gain..................................................................................................................................... 9 2.2 Radiation Pattern............................................................................................................................. 10 2.3 Lobe Width...................................................................................................................................... 11 2.3.1 Horizontal Lobe Width......................................................................................................... 11 2.3.2 Vertical Lobe Width ............................................................................................................. 12 2.4 Frequency Band .............................................................................................................................. 12 2.5 Polarization Mode........................................................................................................................... 13 2.6 Downtilt Mode ................................................................................................................................ 14 2.7 Antenna Front-to-Back Ratio.......................................................................................................... 15 2.8 Antenna Input Impedance Zin......................................................................................................... 15 2.9 Antenna VSWR............................................................................................................................... 16 2.10 Side Lobe Suppression and Null Fill............................................................................................. 17 2.11 Third-Order Intermodulation......................................................................................................... 18 2.12 Inter-Port Isolation ........................................................................................................................ 18
  • 3. ii 3 Antenna Engineering Parameters ...........................................................................................................19 3.1 Antenna Azimuth .............................................................................................................................19 3.2 Antenna Height................................................................................................................................20 3.3 Antenna Downtilt.............................................................................................................................22 4 Antenna Categories...................................................................................................................................25 5 Antenna Application Scenarios................................................................................................................31 5.1 Dense Urban Area............................................................................................................................31 5.2 Common Urban Area (Towns).........................................................................................................31 5.3 Suburb Area and Country ................................................................................................................32 5.4 Railways and Highways ..................................................................................................................32 5.5 Scenic Spots.....................................................................................................................................33 6 Antenna Selection .....................................................................................................................................35 6.1 Antenna Selection in Urban Area.....................................................................................................35 6.2 Antenna Selection in Suburb Area and Countryside........................................................................35 6.3 Antenna Selection in Railway/Highway Coverage Area .................................................................36 6.4 Antenna Selection in Mountain Coverage Area...............................................................................37 7 Antenna Installation and Debugging ......................................................................................................39 7.1 Pole Antenna Installation.................................................................................................................39 7.1.1 Keeping Pole Vertical ...........................................................................................................39 7.1.2 Lightning Protection .............................................................................................................39 7.1.3 Diversity Reception ..............................................................................................................40 7.1.4 Antenna Isolation..................................................................................................................41 7.2 Antenna Installation at Iron Tower ..................................................................................................41 7.3 Summary..........................................................................................................................................42
  • 4. 1 1 Antenna Overview This chapter introduces antenna radiation principles, types, technical development, and development trend. 1.1 Antenna Development Overview In cellular mobile communication system, antenna functions as the converter between the communication equipment circuit signal and the radiating electromagnetic wave. The cellular mobile communication requires reliable communication between base station and MS, which puts special requirement on antenna. The RF signal power which is outputted by the radio transmitter is sent to antenna through feeder cable, and then radiated in the form of electromagnetic wave by antenna. The electromagnetic wave is received (only a very small part of the power) by antenna after arriving at the destination, and then is sent to the radio receiver through feeder cable. Therefore, antenna is an important radio device for sending and receiving the electromagnetic wave. System communication performance is affected by many factors including antenna gain, coverage direction, beam, available driving power, antenna configuration, and polarization mode. Technology and market situations of Chinese antenna enterprises According to statistics, the market share of domestic antenna products accounts for only 20% of the total antenna market shares in China in the following fields: · Civil base station antenna, in which technologies of mobile communication, spread spectrum, and microwave communication are applied · Intelligent antenna · Bluetooth antenna Till the first half of 2002, there were over 100 domestic antenna enterprises in China. However, according to the total amount of production and sales, only a few enterprises owned 200 plus employees and 30 million plus RMB operating income. Advantages of international antenna enterprises
  • 5. GSM Antenna system 2 International antenna enterprises own rich funds, enjoy well-known brands, and have plenty of human resources and advanced technologies. Many of them boast a history of more than 50 years and have an annual operating income of more than 2 billion dollars. After China joined World Trade Organization (WTO), many famous international antenna enterprises have made investment in China to establish factories, which put a huge pressure on Chinese antenna enterprises. Antenna industry development trend In 1897, Marconi invented the antenna, realizing radio communication for the first time. During the past 100 plus years, many countries attached great importance to this field due to military applications such as Radar. At present, the antenna design is developed towards the trend of broad-band, multifunction, and high-integrity. Various types of antenna such as dual-polarized antenna, electrical downtilt antenna, and multi-band multiplexing antenna are being put into commercialization. Also, great improvement has been made in intelligent antenna technology. Through the past 20 years of development, the technological gap between domestic antenna products and international antenna products are being shortened. The advantages of domestic antenna products include appropriate price, good service, and satisfying practical requirement for communication construction. 1.2 Antenna Radiation Principles The antenna performs conversion between circuit signals of the radio station and electromagnetic wave in the air. The ways to improve the radiating electromagnetic wave’s efficiency and intensity is related to microwave technology. This section introduces the radiation principles of the most commonly used antenna dipoles. 1.2.1 Electromagnetic Wave Radiation of Electric Dipoles If the distance between two electric points is far shorter than the wavelength λ, then the two electric points are called electric dipoles. When there is AC current flowing in the wire, the electromagnetic wave radiation is generated. The radiation intensity is relevant to the wire length and shape. As shown in Figure 1.2-1, if the distance between two wires is very short, then the electric field is restricted between the two wires, and the radiation is weak. After expanding the distance between the two wires, the electric field is distributed to the surrounding space, and the radiation is intensified.
  • 6. 1 0BAntenna Overview Figure 1.2-1 Relation between Dipole Angle and Electromagnetic Radiation Intensity The following two points should be noticed: · When the wire length is far shorter than the wavelength λ, the radiation is weak. · When the wire length increases to be similar to the wavelength λ, the current in the wire increases greatly and forms strong radiation. 1.2.2 Half-Wave Doublet The doublet is a classical and most commonly used antenna. A single half-wave doublet can be used independently or used as the feed of parabolic antenna. Also, multiple half-wave doublets can be combined to form the antenna array. Note: Doublet is also called balanced dipole somewhere in this document. Dipole of which the two arms have the same length is called doublet. For half-wave doublet, each arm is 1/4 λ long and the total length is 1/2 λ, as shown in Figure 1.2-2. Wavelength 1/2λ Wavelen WavelengthWavelength 1/2λ Wavelengthgth Figure 1.2-2 Half-Wave Doublet Besides, there is also a heterogeneous half-wave doublet, which can be considered as folding the full-wave doublet into a narrow and long rectangle in which the two points of the full-wave doublet are overlapped. The narrow and long rectangle is called folded dipole, as shown in Figure 1.2-3. 3
  • 7. GSM Antenna system Note: The folded dipole’s length is also 1/2 λ, that is why it is also called half-wave folded dipole. Figure 1.2-3 Half-Wave Folded Dipole 1.3 Antenna Structure and Types 1.3.1 Directional Panel Dipole-Array Antenna The directional panel antenna is the most commonly used antenna in base station applications. It enjoys the following advantages: · High antenna gain · Good sector pattern · Small back lobe · Easy control of vertical-plane pattern depression angle · Reliable encapsulation · Long service life Figure 1.3-1 shows the antenna appearance. Figure 1.3-1 Directional Panel Antenna Appearance 4
  • 8. 1 0BAntenna Overview 1.3.1.1 Panel Antenna High Gain Formation Figure 1.3-2 Arranging Multiple Half-Wave Dipoles to Form a Vertical Linear Array Figure 1.3-3 Adding Reflection Panel at One Side of Linear Array to Realize Horizontal Orientation At present, the directional antenna design mainly adopts the panel dipole array structure. The following two types of dipoles are often used: · Balanced dipole 5
  • 9. GSM Antenna system · Microstrip dipole 1.3.1.2 Balanced Dipole In a standard half-wave doublet, an additional dipole is added to reduce the dipole’s height above the ground, which reduces the antenna’s thickness. Figure 1.3-4 Combining Multiple Half-Wave Dipoles to Form Directional Panel Antenna 1.3.1.3 Microstrip Dipole The microstrip dipole is a variation of the half-wave dipole. It radiates according to the 1/4 λ transmission line principle. Figure 1.3-5 Combing Multiple Microstrip Dipoles to Form Directional Panel Antenna 6
  • 10. 1 0BAntenna Overview 1.3.1.4 Antenna Dipole Array Structure Figure 1.3-6 Dipole Array Structures of Panel Antenna 1.3.2 Omni-Directional Series-Feed Dipole Antenna The omni-directional antenna realizes radiation gain combination and enhancement through the multi-half-wave-dipole series-feed mode. 7
  • 11. GSM Antenna system Figure 1.3-7 Omni-Directional Antenna Series-Feed Dipole Structure 8
  • 12. 2 Antenna Technical Parameters This chapter explains concepts of antenna technical parameters and their applications in network design. 9 2.1 Antenna Gain Antenna gain is an important parameter for antenna system design. Its definition is related to the half-wave dipole or the full-wave antenna. For omni-directional radiator, it is assumed that the radiation power is same in all directions. The antenna gain in a direction is equal to the field intensity it generates divided by the intensity generated by the omni-directional radiator in this direction. The unit of antenna gain is dBd or dBi. dBi represents the reference value of field intensity in the direction with the maximum radiation, relative to the omni-directional radiator (as shown in Figure 2.1-1 (middle)). For half-wave dipole (as shown in Figure 2.1-1 (left)), the antenna gain is represented by dBd. There is a fixed difference between dBi and dBd (as shown in Figure 2.1-1 (right)), that is, 0 dBd = 2.15 dBi. Figure 2.1-1 Reference for dBi and dBd At present, the range of antenna gain is between 0 dBi ~ 20 dBi. For indoor micro-cell coverage, the antenna gain is usually between 0 dBi ~ 8 dBi. For outdoor base stations, applications mainly involve the omni-directional antenna (with a gain of 9 dBi) and the directional antenna (with a gain of 18 dBi). Antenna with a gain of 20 dBi and comparatively narrow beam is often applied in highway coverage in areas with broad land and sparse population.
  • 13. GSM Antenna system 2.2 Radiation Pattern The base station radiation pattern includes the following two types: · Omni-directional radiation pattern. · Directional radiation pattern. Figure 2.2-1 Field Intensity Distribution for Omni-Directional Antenna and Directional Antenna As shown in Figure 2.2-1, the left figures are horizontal cross-section pattern and three-dimensional radiation pattern of the omni-directional antenna. The right figures are horizontal cross-section pattern and three-dimensional radiation pattern of the directional antenna. · Theoretically, the omni-directional antenna has the same radiation intensity in all directions in the same horizontal plane. It is suitable for the omni-directional cell. · As shown in Figure 2.2-1, the red part represents the metal reflection panel in the directional radome, which makes the antenna radiation in the horizontal plane directional. The directional antenna is suitable for sector coverage. 10
  • 14. 2 1BAntenna Technical Parameters 2.3 Lobe Width 2.3.1 Horizontal Lobe Width For omni-directional antenna, the horizontal lobe width is 360° (as shown in Figure 2.3-1 (right)). For directional antenna, the common horizontal lobe 3dB width can be 20°, 30°, 65°, 90°, 105°, 120°, and 180° (as shown in Figure 2.3-1 (left)). Figure 2.3-1 Antenna Horizontal Lobe 3dB Width · For antennas with 20º or 30º 3dB width, the antenna gain is large, and such antennas are often applied for coverage in long narrow area or highway. · Antennas with 65º 3dB width are often applied for three-sector coverage in dense urban area. · Antennas with 90º 3dB width are often applied for three-sector coverage in suburb area. · Antennas with 105º 3dB width are often applied for three-sector coverage in area with broad land and sparse population. Figure 2.3-2 shows three-sector coverage applications of antennas with different 3dB width. Figure 2.3-2 Three-Sector Coverage 11
  • 15. GSM Antenna system · Antennas with 120º or 180º 3dB width are often applied for coverage in sectors with special shapes. 2.3.2 Vertical Lobe Width Figure 2.3-3 Antenna Vertical Lobe 3 dB Width The width of antenna vertical lobe 3dB, which is often 10°, is closely related to the antenna gain and the horizontal lobe 3dB width. Usually, with the same antenna design technology and the same antenna gain, the wider the horizontal lobe is, the narrower the vertical lobe 3dB is. The narrow width of vertical lobe 3dB might cause many coverage holes. As shown in Figure 2.3-3, for the two non-downtilt antennas with the same height, the red one (with wide vertical lobe) has a coverage hole range of OX’’ while the blue one (with narrow vertical lobe) has a coverage hole range of OX. Therefore, in order to guarantee good coverage and avoid coverage hole, it is advised to select the antenna with wide vertical lobe 3dB width, on the premise of having the same antenna gain. 2.4 Frequency Band For base stations, the working band of the selected antenna must include the required band. · GSM 900 system: dual band antennas with working band of 890 MHz ~ 960 MHz, 870 MHz ~ 960 MHz, 807 MHz ~ 960 MHz, and 890 MHz ~ 1880 MHz can be selected. · CDMA 800 system: antennas with working band of 824 MHz ~ 894 MHz can be selected. 12
  • 16. 2 1BAntenna Technical Parameters · CDMA 1900 system: antennas with working band of 1850 MHz ~ 1990 MHz can be selected. In order to reduce the out-of-band interference, it is advised to select antenna of which the bandwidth just satisfies the band requirement. 2.5 Polarization Mode At the base station, antennas often adopt the linear polarization mode, as shown in Figure 2.5-1. Usually, the single-polarized antenna adopts vertical polarization mode and the dual-polarized antenna adopts ±45° dual-linear polarization mode. Figure 2.5-1 Common Polarization Modes for Antenna A dual-polarized antenna consists of two perpendicular polarized antennas which are encapsulated in the same radome as shown in Figure 2.5-2. It can reduce the number of antennas greatly, simplifies the antenna installation, and reduce the cost and occupied space of the antenna. Figure 2.5-2 Dual-Polarized Antennas 13
  • 17. GSM Antenna system 2.6 Downtilt Mode To avoid coverage hole near the base station and reduce the interference on other adjacent base stations, it is advised not to mount the antenna in very high places. Also, the downtilt mode is required. As shown in Figure 2.6-1, the antenna mounted in low place (the yellow one) has a coverage hole range of OX” while the antenna with downtilt (the green one) has a coverage hole range of OX’. The coverage hole range of both are smaller than that of the non-downtilt antenna mounted in high place (the blue one, the coverage hole range is OX). Figure 2.6-1 Antenna Downtilt Comparison There are many downtilt modes for antenna, including mechanical downtilt, fixed electrical downtilt, adjustable electrical downtilt, and remote-control adjustable electrical downtilt. For mechanical downtilt, the downtilt (often less than 10º) is set during antenna installation. When increasing the downtilt, the front of the coverage area becomes concaved and the two sides of the coverage area become flat, thus the antenna pattern becomes distorted, as shown in Figure 2.6-2. All these cause insufficient coverage in front of the antenna and enhance interference on base stations at the two sides. 14
  • 18. 2 1BAntenna Technical Parameters Figure 2.6-2 Antenna Downtilt Mode Comparison For electrical downtilt, the downtilt range is comparatively large (might be larger than 10º). When the downtilt is increased, the antenna pattern will not be distorted, and the antenna back lobe also becomes declined at the same time. Thus no interference will be generated on surrounding users. 2.7 Antenna Front-to-Back Ratio The antenna front-to-back ratio is relevant to the antenna reflection panel’s electrical size. A large electrical size guarantees good front-to-back ratio. For example, horizontal size of the horizontal lobe 3 dB antenna with 65º width is larger than that of the horizontal lobe 3 dB antenna with 90 º width. Usually, the front-to-back ratio of outdoor base station antenna should be larger than 25 dB. For micro-cell antenna, because its size is comparatively small, the front-to-back ratio range can be large. 2.8 Antenna Input Impedance Zin The antenna input impedance Zin is defined as the ratio of signal voltage to signal current at the antenna input port. Zin has two components: the resistance component Rin and the reactance component Xin, Zin = Rin + j Xin. The reactance component helps to reduce extracting the signal power from feeder cable by antenna, thus it is required to make the reactance component to be 0; in other words, to make the antenna’s input impedance to be the pure resistance. Actually, even for a well-designed antenna, its input impedance value still contains a small reactance component value. 15
  • 19. GSM Antenna system The input impedance is relevant to the antenna structure, size, and working wavelength. Among basic antennas, the half-wave doublet is the most important, and its input impedance Zin = 73.1 + j 42.5 Ω. After making its length 3% ~ 5% shorter, its reactance component is made to 0, that is, the antenna’s input impedance is pure resistance, and the input impedance Zin = 73.1 Ω (nominal 75 Ω). The pure-resistance impedance of antenna is only applicable for dot frequency. Also, the input impedance of half-wave folded dipole is four times of half-wave doublet’s input impedance, that is, Zin = 280 Ω (nominal 300 Ω). However, impedance debugging can be performed for any antenna to make the input impedance’s imaginary part very small and the real part approximate 50 Ω. In this way, the antenna’s input impedance Zin = Rin = 50 Ω, which is necessary for antenna to have a good impedance matching with the feeder cable. 2.9 Antenna VSWR The antenna Voltage Standing Wave Ratio (VSWR) is the index which indicates the matching between antenna feeder and base station (transceiver). The VSWR is defined as: 0.1 min max ≥= U U VSWR Umax: anti-node voltage on the feeder cable. Umin: node voltage on the feeder cable. The process of VSWR generation is: The incident wave energy is not absorbed (radiated) completely when it is transmitted to the antenna input port B, the reflection wave is generated and superposed, which forms the VSWR. If the VSWR value is large, the reflection is also large, and the matching is worse. The lost energy and the manufacturing cost should be considered to decide whether a VSWR is appropriate. · VSWR > 1: It indicates that some input power is reflected back, which reduces the antenna radiation power. · Feeder cable loss is increased: The 7/8” cable loss is 4 dB / 100 m, which is measured when VSWR is 1 (perfectly matched). With the reflection power, the 16
  • 20. 2 1BAntenna Technical Parameters energy loss is increased, and the input power to antenna from the feeder cable is decreased. 2.10 Side Lobe Suppression and Null Fill Usually, the antenna is installed on the iron tower or the top of a building to cover the service area. Therefore, the upward side lobe on the vertical plane should be suppressed, especially the larger first side lobe, to avoid unnecessary energy wastage. Also, the null fill for the downward side lobe on the vertical plane should be implemented to make the null depth level lower in the antenna pattern of this area, and to improve coverage in areas near the base station. Figure 2.10-1 shows comparison before and after the null fill is performed. The X-coordinate represents the distance from the base station while the Y-coordinate represents the ground signal strength. Figure 2.10-1 Coverage Comparison before and after Null Fill Is Performed Antenna null-fill value = (the first downward null amplitude value / amplitude value in the maximum radiation direction)% 17
  • 21. GSM Antenna system 18 = 20 * log(the first downward null amplitude value / amplitude value in the maximum radiation direction)dB To ensure good coverage in the service area, antennas without side lobe suppression and null-fill functions can not be used. 2.11 Third-Order Intermodulation For most advanced international antenna products, the third-order intermodulation value reaches -150dBC@2×43dBm. However, for common antennas, this index’s value is only -130dBC@2×43dBm. It is related to the antenna design and connectors selected. Because the base station’s received signal is much weaker than the transmitted signal, the base station can not work normally once the intermodulation products of multi-band transmitting signals fall into the receiving frequency band. 2.12 Inter-Port Isolation For multi-port antenna, the isolation between different ports must be larger than 30 dB. For example, the isolation must be larger than 30 dB in the following cases: · Between the two polarized ports of dual-polarized antenna · Between the two frequency band ports of outdoor dual-band antenna · Between the four ports of dual-band dual-polarized antenna
  • 22. 19 3 Antenna Engineering Parameters This chapter introduces antenna engineering parameters and their influences on coverage, and the ways to improve the network performance by using these parameters. 3.1 Antenna Azimuth The electromagnetic field of antenna radiation is a graph which is distributed in fixed distance according to the angle coordinate. The graph is called antenna pattern. · The antenna pattern which is expressed by the radiation field intensity is called field intensity pattern. · The antenna pattern which is expressed by the power density is called power pattern. · The antenna pattern which is expressed by the phase is called phase pattern. The antenna pattern is a three-dimensional graph. It is often expressed in the form of directional patterns in two perpendicular main planes, which are called vertical directional pattern and horizontal directional pattern. There are two types of horizontal directional pattern: omni-directional-antenna-based and directional-antenna-based. Directional-antenna-based horizontal pattern also has many types, such as heart-shaped type and 8-shaped type. The directional characteristic of an antenna is caused by changes in the dipole array and the dipole feeding phase. Theoretically, it is similar to the optical interference effect, thus energy in some directions might increase while energy in some other directions might decrease, forming the lobe (or beam) and null. The lobe with the strongest energy is called the main lobe, the upward/downward lobe with the second strongest energy is called the first side lobe, and so on. Directional antenna has the back lobe. The antenna azimuth adjustment is very important for improving communication network quality. Accurate antenna azimuth guarantees normal network running and that the actual coverage is the same as the expected coverage. On the other hand, adjusting
  • 23. GSM Antenna system 20 the antenna azimuth according to the traffic and actual network situation can better optimize the current mobile communication network. According to the ideal cellular mobile communication model, signals at the boundary of a cell are mutually complementary. In current GSM systems (here, mainly refers to ERICSSON equipment), a directional site usually has three cells: · Cell A: Azimuth is 0º, the antenna points to the north. · Cell B: Azimuth is 120º, the antenna points to the southeast. · Cell C: Azimuth is 240º, the antenna points to the southwest. In GSM planning and construction, antenna azimuth is strictly set according to the above specification during antenna installation. Inaccurate antenna azimuth setting might cause the actual coverage different from the design, which might result in unexpected co-frequency interference and adjacent-frequency interference. In practical GSM network applications, the actual coverage might be different from that expected in the ideal model. It is often caused by signal refraction or signal reflection due to buildings, water, or mountains, which results in signal strength differing in different area. In such cases, we can adjust the antenna azimuth to ensure signal strength in different areas to optimize the network. Besides, the traffic in cells corresponding to different antennas might be different due to different population density. We can also adjust the antenna azimuth to equalize the traffic. In normal cases, it is not advised to perform antenna azimuth adjustment, which might cause intra-system interference. However, in some particular cases, such as at an emergency meeting or a large social gathering where traffic might be heavy in some cells. Temporarily adjust the antenna azimuth to equalize traffic and optimize the network. Antenna azimuth adjustment can also be performed in some coverage holes or areas with weak signals to optimize the network performance, and field intensity test can also be performed at the same time to guarantee the network quality. 3.2 Antenna Height The signal power received by the receiver is related to the following two types of factors: · Parameters at the transmitting end and the receiving end
  • 24. 3 2BAntenna Engineering Parameters · Interference due to landform and obstacles Parameters at the transmitting end and the receiving end include: · Transmission power · Antenna gain · Feeder cable loss · Antenna height · Working band · Distance between the transmitter and the receiver All propagation models are related to the height of transmitting antenna and the receiving antenna. Therefore, the antenna height has an important influence on the path loss. The coverage distance from the transmitting end to the receiving end is as follows: ( ) ( ) 4 1 4 1 2 1 4 1 ][ αLGGhh P P trtr r t ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ≈ th rG tG αL D rP Receiving power tP Transmission power rh Antenna height at the receiving end Antenna height at the transmitting end Receiving antenna gain Transmitting antenna gain Path loss correction factor With fixed transmitter and receiver parameters, the coverage area is in direct proportion to the antenna height and antenna gain. During the early GSM network construction, there are not so many sites, and antennas are installed in high places in order to have good coverage. With the rapid development of mobile communication, the number of sites increases greatly, there are about one site every 500 m in urban area. Therefore, the coverage area of a site must be reduced 21
  • 25. GSM Antenna system 22 by lowering the antenna; otherwise, the network quality will become worse, with the following negative influences: · Traffic is not equalized. If the antenna is too high, then the site’s coverage area will become too large. It causes heavy traffic at this site; meanwhile, the adjacent site’s coverage area becomes small and has low traffic. In other words, the traffic is not equalized. · Intra-system interference If the antenna is too high, then cross-site radio interference will be generated, including co-frequency interference and adjacent-frequency interference. It causes problems such as call drop, cross talk, and noise, and the entire communication network quality degrades. · Isolated island effect The isolated island effect is relevant to site coverage. If a site covers water area or mountainous area, then, on the premise of the original coverage area not being changed, the remote part of the coverage area becomes an isolated island due to reflection by water or mountain. Moreover, adjacent sites, to which handover could be performed before, now have no handover relationships with the site. When a handset occupies signal in the isolated island, call drop often occurs due to no handover relationship. 3.3 Antenna Downtilt Through adjusting the antenna main lobe’s downtilt to a certain angle, the power level of adjacent site can be reduced, which then reduces the interference. The antenna downtilt is related to the following parameters: · Antenna height · Coverage radius · Antenna vertical lobe · Electrical downtilt With the same coverage radius, the higher the antenna is, the larger the antenna downtilt is. In other words, with the same antenna height, the smaller the coverage
  • 26. 3 2BAntenna Engineering Parameters 23 radius is, the larger the antenna downtilt is. Interference tends to exist in urban area with many sites. In order to make most energy radiate in the coverage area and reduce interference on adjacent cells, it is required to make the half-power point on the antenna main lobe to aim at the coverage area edge. The calculation formula of antenna downtilt is as follows: α = arctg (2H/L) × (180/π) + (β/2) – γe In suburb, country, road, or sea area, to make the coverage as remote as possible, reduce the initial downtilt, and make the point on antenna main lobe with the maximum gain to aim at the coverage area edge. The calculation formula of antenna downtilt is as follows: α = arctg (H/L) × (180/π) + (β/2) – γ e In the above two formulas: α: The initial mechanical downtilt of antenna, the unit is degree (º). H: Valid height of the site. It is the difference between the average antenna height and the average ground elevation of surrounding coverage area, the unit is meter (m). L: Distance between antenna and the edge of coverage area, the unit is meter (m). β: Vertical lobe width of the antenna, the unit is degree (º). γ e: Electrical downtilt of the antenna, the unit is degree (º).
  • 27. 25 4 Antenna Categories There are various types of antennas, which are suitable for different situations. Antennas can be categorized according to different standards: · By usage Communication antenna, television antenna, radar antenna, and so on · By working band Short-wave antenna, super-short-wave antenna, microwave antenna, and so on · By appearance Linear antenna, panel antenna, and so on · By direction Omni-directional antenna, directional antenna, and so on At present, the working band, gain, and front-to-back ratio of various types of antennas are similar, and all satisfy the network performance requirement. This chapter mainly analyzes the above antennas according to antenna downtilt influence on the antenna pattern and the radio network. Omni-Directional Antenna For omni-directional antenna, the radiation is evenly distributed in 360° in the horizontal pattern. In the vertical pattern, the radiation is represented in the form of beam with certain width. Usually, the smaller the lobe’s width is, the larger the antenna gain is. The omni-directional antenna is often applied in sites which are designed for large cells in suburb, with large coverage area. Directional Antenna For directional antenna, the radiation is distributed within a certain range of angles in the horizontal pattern. In the vertical pattern, the radiation is represented in the form of beam with certain width. Similar to the omni-directional antenna, the smaller the lobe’s width is, the larger the antenna gain is. The directional antenna is often applied in sites which are designed for small cells in urban area, with small coverage area, high
  • 28. GSM Antenna system 26 subscriber density, and high frequency utilization ratio. Different types of sites are established according to different networking requirements, and different site types require different antenna types according to technical parameters mentioned previously. · For omni-directional site, the omni-directional antenna with the same gain at various horizontal directions is selected. · For directional site, the directional antenna with different gain at different horizontal directions is selected. · In urban area, the antenna with a horizontal beam width B of 65º is selected. · In suburb, the antenna with a horizontal beam width B of 65º, 90º, or 120º (according to actual site configuration and geographical environment) is selected. · In rural area, the omni-directional antenna that realizes large coverage area is preferred due to its economy. Mechanical Antenna For mechanical antenna, the antenna downtilt is adjusted mechanically. After the mechanical antenna is installed vertically on the ground, the downtilt can be changed by adjusting the bracket position on the rear side of antenna. During the process, the amplitude values of vertical component and horizontal component of the antenna do not change although the main lobe’s coverage changes. Therefore, the antenna pattern tends to be distorted. It is found through practical cases that: · The best downtilt for mechanical antenna is 1º ~ 5º. · When the downtilt changes between 5º ~ 10º, the antenna pattern is slightly distorted, but the distortion is not serious. · When the downtilt changes between 10º ~ 15º, the antenna pattern distortion is large. · When the downtilt reaches 15º, the shape of antenna pattern changes greatly (from the pear-shape to spindle-shape). The coverage distance in the main lobe direction becomes much shorter, and the entire antenna pattern is not within the
  • 29. 4 3BAntenna Categories 27 site’s sector. Therefore, signal of this site can also be received in the adjacent site’s sector, which causes serious intra-system interference. During the daily maintenance, the system should be powered off before adjusting the mechanical antenna downtilt, and system monitoring can not be performed during the antenna downtilt adjustment. Maintenance personnel should climb to the place where the mechanical antenna is installed and perform the downtilt adjustment. Moreover, the mechanical antenna downtilt is a theoretical value calculated through the emulation software, and has some difference from the best downtilt in practice. The step in mechanical antenna downtilt adjustment is 1º, and the third-order intermodulation value is -120 dBc. Electrical Antenna For electrical antenna, the antenna downtilt is adjusted through changing the phase of antenna dipoles in the same antenna array. During the process, the amplitude value of vertical component and horizontal component of antenna are changed, which causes the combined field intensity and the antenna downtilt to change. Because the field intensity in all directions of the antenna increases or decreases simultaneously, it guarantees that the antenna pattern does not change much after the antenna downtilt is changed. The coverage distance in the main lobe direction is shortened, and the coverage area within the sector of service cell is reduced without generating interference. It is found through practical cases that: · When the downtilt changes between 1º ~ 5º, the antenna pattern is similar to that of the mechanical antenna. · When the downtilt changes between 5º ~ 10º, the antenna pattern is improved compared with that of the mechanical antenna. · When the downtilt changes between 10º ~ 15º, the antenna pattern change is larger than that of the mechanical antenna. · When the downtilt reaches 15º, the antenna pattern differs greatly from that of the mechanical antenna. The shape of antenna pattern does not change much. The coverage distance in the main lobe direction becomes much shorter, and the entire antenna pattern is within the site’s sector. Increasing the downtilt helps to reduce the coverage area within the sector without causing interference.
  • 30. GSM Antenna system 28 Therefore, the electrical antenna can reduce the call loss and interference. Moreover, the electrical antenna allows downtilt adjustment during the system running, thus real-time monitoring can be performed for the downtilt adjustment. The step in electrical antenna downtilt adjustment is 0.1º, which guarantees precise adjustment. The third-order intermodulation value is -150 dBc, with a 30 dBc difference from the mechanical antenna. All these factors help to eliminate adjacent-frequency interference and spurious interference. Dual-Polarized Antenna A dual-polarized antenna contains two perpendicular polarized antennas (+45º and -45º) which simultaneously work in the transceiving duplex mode. It saves the number of antennas for single directional site. Generally, a directional site (three sectors) in GSM system requires nine antennas, each sector using three antennas (space diversity, one for transmitting and two for receiving). If the dual polarized antenna is adopted, each sector only uses one antenna. Meanwhile, the ±45º polarized perpendicularity ensures that the isolation between the +45º antenna and the -45º antenna satisfies the requirement for antenna intermodulation (≥ 30 dB). Therefore, the spatial distance between dual-polarized antennas is only 20 cm ~ 30 cm. The dual-polarized antenna also enjoys advantages of the electrical antenna, which can reduce the call loss and interference and improve the entire network service quality. The dual-polarized antenna installation does not require iron tower and only requires an iron column with a diameter of 20 cm. The antenna is fixed on the iron column according to coverage direction, saving the construction cost and making the site address selection much easier. Summary The antenna selection depends on actual conditions such as the network coverage, traffic, interference, and network service quality. · In areas with low traffic, the traditional mechanical antenna is preferred. · In areas with heavy traffic, the dual-polarized antenna and the electrical antenna are preferred. In areas with heavy traffic, the call loss rate is high and the interference is large. It is mainly due to the large mechanical antenna downtilt, which might cause the
  • 31. 4 3BAntenna Categories 29 distortion of antenna pattern. To solve this problem, the distance between sites must be shortened to increase the antenna downtilt. However, if the mechanical antenna is used, the antenna pattern becomes distorted when the downtilt is larger than 5º and the distortion becomes worse when the downtilt exceeds 10º. Thus it is advised to use the electrical antenna or dual-polarized antenna in areas with heavy traffic.
  • 32. 5 Antenna Application Scenarios This chapter explains the coverage area types and radio environment with pictures of practical application scenarios. 5.1 Dense Urban Area Figure 5.1-1 shows the application scenarios in dense urban area. Figure 5.1-1 Dense Urban Area 5.2 Common Urban Area (Towns) Figure 5.2-1 shows the application scenarios in common urban area, such as towns. Figure 5.2-1 Common Urban Area (Towns) 31
  • 33. GSM Antenna system 5.3 Suburb Area and Country Figure 5.3-1 shows the application scenarios in suburb area and country. Figure 5.3-1 Suburb Area and Country 5.4 Railways and Highways Figure 5.4-1 shows the application scenarios in area with railways and highways. Figure 5.4-1 Railways and Highways 32
  • 34. 5 4BAntenna Application Scenarios 5.5 Scenic Spots Figure 5.5-1 shows the application scenarios in scenic spots. Figure 5.5-1 Scenic Spots 33
  • 35. 35 6 Antenna Selection This chapter introduces antenna selections for different coverage areas. 6.1 Antenna Selection in Urban Area In urban area, sites are densely distributed. The coverage area of a single site should be small to avoid the phenomenon of cross-area coverage, to reduce interference between sites and increase the frequency multiplexing ratio. Antenna selection rules · Polarization mode: The antenna installation space is limited in urban area, and it is advised to use the dual-polarized antenna in urban area. · Antenna pattern: In urban area, the frequency multiplexing ratio should be increased, and the directional antenna is often adopted. · Half-power beam width: To control the cell coverage area and to reduce interference, the horizontal half-power beam width of antenna in urban area is usually set between 60º ~ 65º. · Antenna gain: Because the site coverage area in urban area is usually not large, it is advised to use antenna with medium gain (15 dBi ~ 18 dBi). For micro-cell used to supplement coverage for coverage hole in urban area, antenna with lower gain can also be selected. · Antenna downtilt: In urban area, the antenna downtilt adjustment is more frequent compared with other areas. Moreover, the antenna downtilt is large in some cases, and the mechanical downtilt can not reduce interference effectively. It is advised to use antenna with preset downtilt, such as the antenna with fixed electrical downtilt. 6.2 Antenna Selection in Suburb Area and Countryside In suburb area and the countryside, sites are sparsely distributed, with low traffic. The coverage area should be broad in such areas.
  • 36. GSM Antenna system 36 Antenna selection rules · Antenna pattern: If it requires that the site covers surrounding area without direction specification and the traffic distribution is dispersed around the site, then it is advised to adopt the omni-directional site coverage. It should be noticed that the omni-directional site has small gain, thus it has shorter coverage distance than the directional site. Also, the tower’s influence on coverage should be considered during the omni-directional antenna installation, and the antenna must be perpendicular to the ground level. If it requires that the site coverage distance is long, then it is advised to use the directional antenna, and usually, the directional antenna with a horizontal half-power beam width of 90º, 105º, or 120º is recommended. · Antenna gain: The antenna gain is selected according to actual coverage requirement. In suburb area and the countryside, it is advised to adopt directional antenna with a higher gain (16 dBi ~ 18 dBi) or omni-directional antenna with a gain of 9 dBi ~ 11 dBi. · Antenna downtilt: The antenna downtilt adjustment is not frequent in suburb area and the countryside, and the requirement for adjustment range is not very strict. Therefore, the mechanical downtilt antenna is recommended. Also, if the antenna height is over 50 m and coverage is required for the near-end area, then the antenna with null-fill feature is preferred, to avoid coverage hole under the tower. 6.3 Antenna Selection in Railway/Highway Coverage Area In railway/highway coverage areas, the traffic is low and handsets move rapidly. Usually, the coverage area is strip-shaped, thus the bidirectional cell is often applied. However, the omni-directional cell is also adopted in places passing towns or scenic spots. The antenna type selected depends on the site address and site type. Because wide coverage is required in railway/highway coverage areas, the high-gain antenna that can realize wide coverage is often selected. Antenna selection rules · Antenna pattern: In railway/highway coverage areas, the narrow-beam high-gain
  • 37. 6 5BAntenna Selection 37 directional antenna can be used. · Antenna gain: For directional antenna, the antenna with a gain of 17 dBi ~ 22 dBi can be selected. For omni-directional antenna, the antenna with a gain of 11 dBi can be selected. · Antenna downtilt: In railway/highway coverage areas, the antenna downtilt is usually not set, and the cheap mechanical downtilt antenna is recommended. Also, if the antenna height is over 50 m and coverage is required for the near-end area, then the antenna with null-fill feature is preferred to avoid coverage hole under the tower. · Front-to-back ratio: Because most users in railway/highway coverage areas are rapidly moving users, the front-to-back ratio of directional antenna should not be set too large, to guarantee normal handover. 6.4 Antenna Selection in Mountain Coverage Area In remote mountainous area, the electric wave attenuation is large during propagation due to blocking of mountains, and it is difficult to realize coverage. Wide coverage is required in this area. Users are sparsely distributed in the large coverage area, with low traffic. The site is usually established on the mountaintop, mountainside, or at the foot of mountain. The site address, site type, and antenna type are selected according to actual user distribution and landform. Antenna selection rules · Antenna pattern: The antenna pattern is selected according to the site address, site type, and coverage requirement. Both omni-directional antenna and directional antenna can be used. For sites established on the mountain, if the coverage area is at comparatively lower place, select the antenna pattern with large vertical half-power angle to satisfy the coverage requirement for the vertical direction. · Antenna gain: The antenna with intermediate gain is selected for such area: omni-directional antenna (9 dBi ~ 11 dBi); directional antenna (15 dBi ~ 18 dBi). · Antenna downtilt: For sites on the mountain, if the coverage area is at the foot of mountain, select antenna with the null-fill feature or of which the downtilt can
  • 38. GSM Antenna system 38 be preset. The preset downtilt depends on the site’s relative height above the coverage area. The larger the relative height is, the larger the preset downtilt is.
  • 39. 39 7 Antenna Installation and Debugging This chapter introduces methods for antenna installation and debugging. 7.1 Pole Antenna Installation 7.1.1 Keeping Pole Vertical If the pole is bent or slanting, it might directly influence the directional antenna downtilt precision and the omni-directional antenna receiving effect. Therefore, it should be ensured that the pole on which the antenna is installed is vertical. Use the plummet to check the antenna verticality and ensure that the omni-directional antenna is perpendicular to the ground. For directional antenna, the downtilt is measured with the downtilt tester, and the mechanical downtilt should include the slanting angle or bending angle of the pole. 7.1.2 Lightning Protection To protect the site (especially the antenna system) in mountainous area from lightning attack, the lightning protection design must be considered in antenna installation to ensure operation safety and normal system running. For a complete lightning protection device, the following factors should be considered: · Lightning rod design: to control the lightning attack point to avoid lightning attack at dangerous places. · Good grounding structure and appropriate grounding resistance value. · Good down lead. · Reliable equipotential bonding, to avoid high-voltage lightning attack. · Preventing from leading in lightning high-voltage surge. The RF antenna is installed within the 45º protection range of the lightning rod, which is connected with the down lead through reliable soldering. The down lead is made of 40 mm × 40 mm galvanized flat steel. The distance between the down-lead connection
  • 40. GSM Antenna system 40 point and the grounding inlead in the ground net should be not less than 10 m. 7.1.3 Diversity Reception In mobile communication, the multipath transmission causes rapid signal fading, and the fading signal level range can reach 30 dB, approximately 20 times per second. The antenna diversity technology can greatly reduce the signal fading extent and improve the link quality. The distance between antennas is decided based on the principle that branch signal fading of all antennas are uncorrelated or approximately uncorrelated. The signal independence is evaluated by the coefficient of correlation of branch signals. The coefficient of correlation of the receiving signal must be less than 0.7. Diversity distance for single-polarized antenna The distance between horizontal diversities is 20 λ, and the distance between vertical diversities is 15 λ. On the premise of not changing the distance between antennas, the correlation between antenna receiving signals can be reduced by increasing the antenna height. The gain of horizontal space diversity is about 3 dB ~ 5 dB, and the gain of vertical space diversity is about 2 dB ~ 4 dB. The horizontal space diversity performance is better than the vertical space diversity performance. In practical applications, the minimum distance between horizontal space diversities of two single-polarized antennas in the same sector must be larger than or equal to 10 λ. Table 7.1-1 Distance between Antenna Horizontal Diversities Distance between Horizontal Space Diversities Distance between Vertical Space DiversitiesWorking frequency Minimum Value Recommended Value Minimum Value Recommended Value 450 MHz 6.7 m 13 m 10 m 800 MHz 3.6 m 7 m 5.4 m 1.9 GHz 1.6 m 3 m 2.4 m 2 GHz 1.5 m 3 m 2.3 m Diversity of dual-polarized antenna For two antennas at the same place of which the polarization directions are perpendicular, the signal fading is mutually uncorrelated. The dual-polarized antenna uses this feature to realize diversity reception. In other words, after installing ±45º polarized antennas on the receiving-end antenna, polarization diversity can be realized
  • 41. 7 6BAntenna Installation and Debugging for the two ways of received signals of which the signal fading is mutually uncorrelated. The polarized diversity antenna obtains independent fading signal through the perpendicular polarized antenna, thus the space diversity is not required. For sites in urban area, it is difficult to install antennas that satisfy the space diversity distance requirement. Thus the polarization diversity becomes an important approach to realize diversity reception. It should be noticed that the distance between two single-polarized antennas is the vertical distance between two parallel lines in the antenna direction, not the direct distance between the two antennas. For dual-polarized antenna, the distance measurement is not required. 7.1.4 Antenna Isolation Antenna isolation within the same system demands that the distance between antennas in different sectors of the same system must be larger than 0.6 m. In practical applications, the 1-meter antenna pole arm is installed on the arm bracket, and the antenna is installed on the antenna pole, as shown in Figure 7.1-1. Figure 7.1-1 Three-Dimensional Diagram and Planform 7.2 Antenna Installation at Iron Tower In practical applications, the antenna is mounted on the arm over 1 m above the iron tower platform. The vertical distance between antennas on different platforms is larger than 1 m. The following cases should be noticed during antenna installation on the iron tower: 41
  • 42. GSM Antenna system 42 · Installing directional antenna at the iron tower To reduce the iron tower influence on the antenna pattern, make the distance between the directional antenna center and the iron tower to be λ/4 or 3/4 λ. It helps to get maximum directions. · Installing omni-directional antenna at the iron tower To reduce the iron tower influence on the antenna pattern, ensure that the iron tower is not the antenna reflector, and the minimum distance between antenna and anywhere of the iron tower must be larger than λ. · Multiple antennas sharing the same iron tower To reduce the coupling interaction and mutual influences between transceiving antennas of different networks, increase the isolation between antennas. It can be realized by increasing distance between antennas. Vertical installation is preferred in this case. 7.3 Summary Distance from the iron tower platform: > 1 m Distance between antennas: · Diversity reception antenna within the same cell: > 3 m · Omni-directional antenna (horizontal distance): > 4 m · Directional antenna (horizontal distance): > 2.5 m · Antennas at different platforms (vertical distance): > 1 m The transceiving antenna can not be arranged upside down except for special specification. The antenna should be arranged within the lightning rod protection range. Antenna azimuth (for directional antenna): · Sector 1: 60° north by east · Sector 2: south · Sector 3: 60° north by west
  • 43. 7 6BAntenna Installation and Debugging 43 The actual antenna downtilt should satisfy the engineering design requirement and the error must be less than 2°. Except for sites with antenna downtilt, the antenna verticality must be smaller than 2°.