Og 102 site survey and layout of bts issue1.5

Editor's Notes

• #6: 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.
• #10: It is better to get the digital electrical map.
• #11: 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.)
• #12: The coordination meeting is the guarantee of survey.
• #15: 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.
• #16: 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.
• #17: 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.
• #19: 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.
• #20: 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.
• #21: 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.
• #22: 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.
• #23: 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.
• #26: 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
• #27: 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.
• #28: 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.
• #29: 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.
• #30: 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.
• #31: 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.
• #32: 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.
• #33: 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.