The survey and layout of BTS is the basis of construction for radio mobile network. It has the following functions:
Showing the system design of the network planning
Determining the structure of future network
Determining the quality of network operation
Thus, the proper survey and layout of BTS can ensure the installation, maintenance, and network planning
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Foreword
The survey and layout of BTS is the basis of
construction for radio mobile network. It has the following
functions:
Showing the system design of the network planning
Determining the structure of future network
Determining the quality of network operation
Thus, the proper survey and layout of BTS can ensure
the installation, maintenance, and network planning.
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Purpose
•Service flow of BTS survey
•Selection of site address
•Design of antenna
After learning this course, you should be
familiar with:
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Service Description
Survey task
Optical test
− Surrounding architecture environment and
nature environment
Spectrum measurement
− Electromagnetic environment
Site check
− Installation condition of antenna and
equipment
− Power supply and transmission supply
Survey purpose
Collect the site information required by the networking planning and
environment description. Check whether the site meets the requirements of
construction.
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Service Description
According to width of band, determine frequency multiplex mode
According to capacity forecast, traffic distribution, and coverage
requirement, estimate the required quantity of BTSs.
Confirm the position of BTS in theory.
Perform BTS survey
Design parameters of cell
Network hierarchy structure, transmit power, antenna system,
including the type, height, direction, and down tilt of antenna
Position of BTS survey in the network planning
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Radio network
estimation
report
Site
information
Whether
there is a site
Obtaining
spare sites
Site survey
Whether
conforming site
condition
Survey
report
Whether meeting
requirements of
site
Obtaining site;
selecting radius
Electromagnetic
environment
test
Yes
No
Yes
No
Yes
No
Service Flow
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Be familiar with the engineering and collect the materials that is
related to the project :
Engineering document
Background material
Network status
Map
Configuration list
Preparation
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Prepare the following tools that are
available:
Digital camera
GPS satellite receiver
Compass
Ruler
Laptop
Preparation
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Preparation
Survey preparation coordination meeting
Before survey, call in all the relative personnel to hold the survey
preparation coordination meeting. The main contents of this meeting are
listed as follows:
Testing electromagnetic background
Arranging survey and coordination personnel
Preparing vehicle and equipment
Making survey plan and confirming survey route
Offering the primary schemes of transmission
and power
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Site Selection
After completing preparation and knowing about the coverage
requirements, start to select site.
When confirming the site, consider the following information:
Original network condition
Population distribution and local custom
City structure and town distribution
Main streets and traffic flow
Mountainous region, lake, river, and coastline
Long-term development trend
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Principles of Selecting Site
Choosing sites in the
heavy-traffic region and
concentrated subscriber
region
Population distribution
Traffic distribution
Subscriber flow direction
Principle
Surrounding environment
Signal propagation quality
Trying not to choose sites near
mountain, radar, broadcasting
station, forest, and electrical
factory
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The specific principles are described as follows:
Construct BTS on the ideal position of a cell. The offset
should not exceed the ¼ of coverage radius.
Under the condition of the BTS layout is not affected,
choose the existing equipment to reduce the construction
cost and period as possible .
To control the coverage range, reduce the difficulty of
engineering construction, and be convenient for
maintenance, do not choose the sites on the mountain that
is on the border of the city or suburb.
(the relative altitude to the city: beyond 100m ~ 300m)
Principles of Selecting Site
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Principles of Selecting Sites
Construct the BTS at a place where transport is fluent, supply of
mains is available, environment is safe, and there is a few filed.
Do not construct the BTS near the high-power radio transmit
station, radar station, and other interference sources.
Construct BTS at a place that is far away from the forest to avoid
the fast fading of received signal.
Consider the signal reflection and time dispersion when
constructing the sites in the mountainous area, precipitous shore,
dense lake area, highland city, and the environment with high
metal building.
When choosing site in the complex, realize the division of network
structure according to the height of building.
At the early stage of constructing network, because of a few of
BTSs, ensure good coverage in important areas.
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Selecting Antenna
Selecting Antenna
The selection for antenna is an important part of determining
the network quality.
According to the requirements of coverage and service
quality, traffic distribution, landform, coverage in the entire
network, and interference, to select the antenna.
According to landform or traffic distribution, the environment
of using antenna can be categorized into:
City, suburb, countryside, highway, mountainous area,
offing, tunnel, and indoor environment
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Principles of Selecting Antenna
Selecting Antenna for the BTS in the city
Choose the directional antenna with 60° ~ 65° half power angle.
Choose the antenna with about 15dBi gain.
Choose the antenna with 3° ~ 6°tilt angle.
Choose dual polarization antenna.
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Principles of Selecting Antenna
Selecting antenna for the BTS in the suburb
According to actual conditions, choose the directional antenna
with 65°or 90° half power angle.
Choose 15 ~ 18dBi middling or high gain antenna.
According to actual conditions, determine whether the tilt angle
should be used.
Dual polarization antenna and vertical polarization antenna are
both available.
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Principles of Selecting Antenna
Selecting antenna for the BTS in the countryside
According to actual conditions, choose 90°or 120° directional
antenna or omni-directional antenna.
Choose directional antenna with 16 ~ 18dBi gain.
Do not choose presetting tilt antenna. Choose null filling antenna
for the high BTS.
The vertical polarization antenna is recommended.
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Principles of Selecting Antenna
Selecting antenna for the BTS on the
highway
In general, choose narrow-beam and
high-gain directional antenna. According
to actual conditions, you also can choose
8-shaped antenna or omni-directional
antenna.
Because the BTS on the highway must
cover a long distance, do not choose the
presetting tilt antenna.
The vertical polarization antenna is
recommended.
The front-to-back ratio of the directional
antenna should not too great.
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Principles of Selecting Antenna
Selecting antenna for the BTS in the tunnel
Tunnel shorter than 2 km
Install the 10dB-12dB Yagi antenna/log-periodic
antenna/flat antenna in the jaws of tunnel to cover
a road that is less than 2km.
Tunnel longer than 2 km
Use leaky feeder, coaxial cable, and fiber
distribution system.
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Principles of Selecting Antenna
Selecting Indoor Antenna
Omni-directional antenna
Vertical polarization, 2dBi gain, cap-shaped or cup-shaped, small
size, convenient for installation, graceful
Directional antenna
Vertical polarization, 90°half power angle
7dBi gain
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AntennaHeight
Due to the installation space or cell planning, the heights of
antennas that are in different cells of a BTS can be different.
For the BTS in the city, the height of antenna is about 25m.
For the BTS in the suburb, the height of antenna is about 40m.
If an antenna, particularly the omni-directional antenna, is too high,
the coverage level of the place that nears to the antenna may be
lowered, that is, no signal under the power.
In addition, the too high antenna may lead to cross coverage and
intra-frequency/neighboring-frequency interferences that affect the
network quality.
Principles of Design for Antenna Height
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Principles of Design for Antenna Azimuth Angle
Azimuth Angle
Based on signal coverage, ensure that the azimuth angles of three sectors are the
same. Adjust the azimuth angles of antennas according to important coverage
objects on the border of city and suburb, in the traffic artery and isolated site of
suburb.
To enhance the signal strength and improve call quality, adjust the major lobe of
antenna to face to the heavy-traffic areas.
To control interference, adjust the major lobe of antenna to be apart from intra-
frequency cells.
The cross coverage areas of neighboring cells in the city should not exceed 10%.
The cross coverage areas of neighboring cells in the suburb and village should be
not too large. The azimuth angle of neighboring cells should not less than 90°.
To avoid cross coverage, do not make the major lobe of antenna face to the
straight streets in the crowded areas.
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Principles of Design for Tilt Angle
Tilt Angle
The beam tilt is a basic technology for improving Frequency Reuse
capability.
The antenna tilt technology is used to control coverage scope and
reduce the interferences in the system.
To reduce the interferences from co-channel cells and meet the
coverage requirements, set the tilt angles based on specific conditions.
The design for tilt angle should be based on transmitter power, antenna
height, cell coverage, and radio environment.
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Tilt Angle
The antenna tilt is categorized into electrical mode and mechanical mode.
The angle of the electrical tilt is relative to the model of antenna. The angle
of the mechanical tilt is adjustable, but it is limited by installation accessory
and signal transportation. Often the angle should not exceed 15°.
The electrical tilt and mechanical tilt have surface radiations that differ with
the increases of angles.
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Installing Antenna
A. Installation Environment
The installation environment contains the ambient of antenna
and that of BTS.
For the ambient of antenna, check the isolation between
antennas and impacts of tower and roof on antennas.
For the ambient of BTS, check whether the high buildings that
are 500m away have impacts on signal transportation.
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2〉 10
12 ~ 10
0.50 ~ 2
GSM
1800M
3.5〉 30
310 ~ 30
21 ~ 10
0.50 ~ 1
GSM
900M
h ( m )D ( m )FREQ
Installing Antenna
When installing antenna, consider whether the ambient affects the
coverage cell. Keep the antenna away from barriers. If you need to
install antenna on the roof, try to install it on the edge.
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Installing Antenna
B. Antenna Isolation
The receiver and transmitter must have a certain isolation.
Isolation between antennas: Attenuation of signal from a
antenna port to another
GSM system: The isolation between two transmitter antennas
and the isolation between the transmitter antenna and the
receiver antenna should be not fewer than 30dB.
− Antenna vertical layout: Lv=28+40log(k/λ)(dB)
− Antenna horizontal layout: Lv=22+20log(d/λ)-(G1+G2)-
(S1+S2)(dB)
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C. Requirements for Space of Diversity
When you perform space diversity, the distance between two receiver
antennas should be 12λ to 18λ.
When an antenna is installed highly, the horizontal space of the diversity
antenna should be large. Usually the space of diversity antenna is 0.11
times of the height of the antenna.
To achieve the same effort, the vertical diversity space should be five to
six times of the horizontal diversity space.
To reduce the mutual effects of two antennas, the horizontal diversity
space should be more than 3m.
Installing Antenna
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Survey report
Survey Document
The standard survey report provides service for the subsequent
network planning and network optimization. It is the guarantee of
engineering quality and the basis of planning for network capacity.
The survey report consists of survey table and memo.
The survey table records the longitude and latitude of a BTS,
antenna design, and ambient.
Shoot the picture of BTS coverage area in the direction of north.
Get eight pictures every 45°.
The purpose of optical test: Check whether there are barriers in the surrounding of BTS, such as pile that may result in reflection of electrical wave.
The purpose of spectrum measurement: Know about whether the electromagnetic environments of the BTS and antenna surrounding are good.
The purpose of site check: Check whether the installation conditions of antenna and equipment, power supply, and nature environment are proper.
This chapter only describes the important parts of network planning. For the construction, refer to the relevant documents.
It is better to get the digital electrical map.
When using GPS to locate the longitude and latitude of the BTS, do not crowd around the GPS so that to improve the precision and reduce the measurement of location.
When using GPS to locate the longitude and latitude of the BTS in China, choose WGS-84 coordinate format.
During survey, record the conditions of surrounding environment, such as floor distribution, strong interference equipment, and co-site equipment. The preceding factors impact on signal propagation and determine the height of antenna and orientation angle. Thus, shoot the surrounding environment to confirm antenna parameters.
When using compass, keep it away from iron material to avoid magnetization.
If a tower is installed, check whether it is available and measure the distance between it and equipment room. (The distance is related to the type of feeder and the calculation of EiRP.)
The coordination meeting is the guarantee of survey.
When selecting site in the heavy-traffic region, consider how to handle capacity problem and improve network service grade.
To keep away from the external interference source, try not to choose the sites that are near to the radar, broadcasting station.
To avoid serious cross coverage that may result in a series of network problems, such as congestion, isolated island, interference, and no signal under the tower, try not to choose the site on the mountain.
To reduce the influences on the radio signal, such as reflection, absorption, and time dispersion, try not to choose the sites near the lake and forest.
Meeting the requirements for the structure of cell is the standard of choosing site.
Use electrical map and city map to analyze and choose site.
The spare sites should be considered.
Consider the overall structure of the network, choose site according to the following factors:
Coverage
Anti-interference
Traffic balance
In fact, the operators need to negotiate about sites with customers. Select the sites that are in the ¼ area of the radius of cellular BTS.
Ensure that the calls in the government buildings, airports, railway stations, news centers, business halls of mobile communication corporations, grand hotels are good and avoid the overlapped coverage.
Actually before the engineers of the Network Planning Department perform BTS survey, the operators may have confirmed the sites.
Because the operators have different service development conditions, the selection principles may be different.
If the existing network is sound, the operators perform expansion capacity to handle traffic problems.
If the existing network is relatively integrated, the purpose of increase the capacity is to solve the traffic problem. Because the operators are more familiar with the network condition, the engineers of the Network Planning Department should communicate with the operators, and confirm the important coverage range. Then according to the range, choose the site.
If the existing network is not sound, the operators may only consider the cost and do not care about the network quality. In this condition, the engineers of the Network Planning Department should make enough preparations, negotiate with operators, and fill in the memos to record the sites that cannot meet the requirements of network designs.
If the selected antenna is proper, the coverage would be extended, the interference be reduced, and the service quality be improved.
If the selected antenna is improper, the network quality may be decreased.
This paper only focus on the environment of city, suburb, countryside and highway. For the others, please see the Guide to Selecting GSM Antenna.
Selecting antenna for the BTS in the city:
To control the coverage range and restrain interference, choose the directional antenna with not less than 90° half power angle and omnidirectional antenna.
Because the BTS in the city requires small coverage range, choose the middling gain antenna. At the same time, the volume and weight of antenna can be changed, which is convenient for installation and reduces cost.
Because the BTS in the city has strict coverage range and the tile angle is often large, choose the electrical tilt antenna to increase the adjustment range of tilt angle and control interference.
Because the installation space for antenna is limited, choose dual polarization antenna.
Selecting antenna for the BTS in the suburb
Because the suburb environment is similar to the city environment or countryside environment, refer to the suggestions on selecting antenna in city or the suggestion on selecting antenna in countryside according to actual conditions.
In view of the smooth upgrade in the future, do not choose Omni-directional antenna.
The presetting tilt angle of the antenna should be small.
The effect of using dual polarization antenna and that of using vertical polarization antenna are the same, so choose proper antenna according to the installation environment and cost.
The environment features of countryside are described as follows:
Sparse distribution of BTSs
Low traffic
Required for wide coverage
Selecting antenna for the BTS in the countryside:
If the BTS is required to cover the surrounding area without obvious direction and the traffic around the BTS is disperse, choose Omni-directional antenna.
If the BTS is required to cover remote area, use directional antenna with 90°, 105 °, or 120 °half power angle.
If the coverage area around the BTS takes on an obvious shape, choose terrain matching beam antenna, such as 210°antenna.
Because the BTS in the countryside must cover a wide area, the antenna gain should be high.
Because the presetting tilt angle antenna may affect the coverage area, do not use it.
The vertical polarization antenna has prefect transmission and receiving effect.
The environment features of the highway are described as follows:
Low traffic
Fast-moving mobile phone
High requirement for the coverage
Selecting antenna for the BTS on the highway:
The BTS on the highway is required to perform zonal coverage, so the bidirectional cell is configured. The BTS that is across town or tour spot, so the tri-directional or Omni-directional cell is configured.
According to the specific conditions of highway, select proper antennas.
The mobile phones often are on a move at a fast speed, the front-to-back ratio of antenna should not too great, otherwise the call drop may be caused.
The antenna that is used in the rural environment may be applicable.
Based on different conditions, confirm the heights of antennas:
In the urban area, the height of antenna can be 20m to 25m.
On the outskirts that is towards the suburb, the height of antenna can be 40m to 50m.
The isolated site should not exceed 70m.
For the network with 1×3 hopping technology, the heights of all the antennas in the city should be the same.
Notes:
Too high antenna may result in the following problems:
Difficult frequency planning
Interference
Isolated island effect
The network with 1×3 hopping technology has strict requirements for azimuth angles.
At the early stage of network construction, to enhance the signal strength, adjust the major lobe of antenna to face to the heavy-traffic areas.
But this adjustment may lower the coverage levels of other areas. Thus, confirm proper azimuth angles based on actual conditions.
When the tilt angle is too large, consider whether the front-to-back radio of the antenna is proper and ensure that the back lobe of the antenna have no interference to the back cells and the side lobe have no interference to the neighboring cells.
For the impacts of tilt angle on cell coverage, refer to the Guide to Planning and Adjusting Tilt Angle.
The beam tilt can be realized through electrical design, which is, changing the excitation factors of array unit, such as amplitude and phase.
The beam tilt can be also realized through mechanical adjustment.
If an antenna with both electrical tilt and mechanical tilt are practical, because only have the electrical tilt is not enough during network optimization.
At present, the space between GSM networks becomes short, so the cell coverage scopes in the city should be precise, particularly for the network with 1×3 (or 1×1) hopping technology. Often the tilt angle of antenna is large, so you need to choose the antenna with electrical tilt and mechanical tilt.
Install the directional antenna on the wall. The transmitting direction of antenna should be vertical to the wall. If you need to adjust azimuth angle, ensure that the angle between the transmitting direction and wall is more than 75°.
To obtain a proper coverage scope, ensure that there is no barrier in the 50m to 100m places around the antenna. For the 900M GSM system, the first Fresnel zone is about 5m. That is to say, the base of antenna should be higher 5m than the ambient.
Make use of the height of ambient building to obtain a proper coverage scope.
When installing antenna, consider whether the ambient affects the coverage cell.
If there are high barriers, such as buildings and mountains, the signal coverage of cells may be affected.
When installing antenna on the roof, try to install it on the edge, which can reduce or eliminate the effect on antenna.
When you cannot install antenna on the edge of antenna, try to install it highly. But you need to consider the bearing capacity of building and the pull of the antenna.
Do not consider the effect of tilt angle.
In the preceding table, the heights of antennas for the GSM900 and the GSM1800 are listed.
In the preceding formula of calculation for antenna isolation, the relevant parameters are described as follows:
λ: Wavelength of carrier
k: Vertical isolated distance
d: Horizontal isolated distance
G1: Gain of transmitter antenna on the maximum radiation direction (dBi)
G2: Gain of receiver antenna on the maximum radiation direction (dBi)
S1: Secondary lobe level of transmitter antenna at 90°(dBp)
S2: Secondary lobe level of receiver antenna at 90° (dBp)
In general, the S of the 65° antenna is -18dBp, the S of the 90° antenna is -9dBp, the S of the 120° antenna is -7dBp, and the S of the omni-directional antenna is 0 dBb.
About Diversity
To reduce multipath fading, split the receiving multipath signal into irrelevant signals, and then combine the signals by rule, so that the receiving signal is strongest, the Signal Noise Ratio increases, and the error rate is small.
The diversity technology is an efficient method of reducing multipath fading. In the horizontal, when there are 10 wavelengths between two antennas, the fading is reduced.
Diversity modes:
⑴Space diversity: Multiple antennas that have d space between each other are used to receive signals to realize diversity.
⑵Polarization diversity: Using the orthogonal condition of vertical polarization and horizontal polarization to perform diversity
⑶Time diversity: Code interleaving technology
⑷Frequency diversity: Frequency hopping technology
1λ=0.333m (900MHz); 1λ=0.167m (1800MHz)
When the actual height of diversity antenna is less than 30m, the space of diversity antennas should be less than 3m. Two diversity antennas are in each other's near fields, so the direction figures of antennas may be changed. To ensure that the change of the antenna direction is no more than 2dB, install the diversity antennas more than 3m.