4.1 Antenna height adjustment The antenna height is directly related to the coverage of the base station. Generally speaking, the signal coverage measured by our instrument is affected by two directional factors:
One is the longest distance that the direct wave emitted by the antenna can reach;
The second is that the signal strength at the location is sufficient to be captured by the instrument.
900MHz mobile communication is near-Earth line-of-sight communication. The longest distance (S) that can be reached by the direct wave transmitted by the antenna is directly related to the height of the transceiver antenna. The specific relationship can be simplified as follows:
S = 2R (H + h)
Among them: R-Earth radius, about 6370km;
The height of the center point of the H-base station antenna;
h- Antenna height of mobile phone or test instrument.
It can be seen that the longest distance that the base station wireless signal can reach (that is, the coverage area of ​​the base station) is determined by the height of the antenna.
In the early stage of construction of the GSM network, there are fewer sites. In order to ensure coverage, the base station antennas are generally erected higher. With the rapid development of mobile communications in recent years, the number of base station sites has increased significantly, and it has reached about 500m in the urban area as a station. In this case, we must reduce the coverage of the base station and the height of the antenna, otherwise it will seriously affect the quality of our network. Its impact mainly has the following aspects:
a. Traffic is uneven. If the antenna of the base station is too high, the coverage area of ​​the base station will be too large, resulting in a large amount of traffic of the base station. However, the neighboring base station has a small coverage and is covered by the base station. To play its due role, resulting in uneven traffic.
b. Interference in the system. If the antenna of the base station is too high, it will cause over-the-station wireless interference (mainly including co-channel interference and adjacent-channel interference), causing dropped calls, crosstalk, and large noise, etc., resulting in the deterioration of the quality of the entire wireless communication network.
c. The island effect. The island effect is a problem of base station coverage. When the base station is covered on a special surface such as a large water surface or mountainous area, due to the reflection of the water surface or mountain peaks, the base station has an "enclave" far away on the basis of the original coverage area unchanged. However, the neighboring base stations that have a handover relationship cannot be covered by the terrain, which results in no handover relationship between the "enclave" and the neighboring base station. The "enclave" therefore becomes an island, when the mobile phone is occupied When the signal in the "enclave" coverage area, it is easy to cause dropped calls because there is no switching relationship.
4.2 Adjustment of antenna pitch angle Adjustment of antenna pitch angle is a very important thing in network optimization. Choosing the proper pitch angle can make the ray between the antenna to the boundary of the cell and the ray from the antenna to the interfered cell be in the part of the antenna with the largest gain attenuation change in the vertical direction of the antenna, thereby causing the co-frequency and adjacent-channel interference of the interfered cell Minimize; In addition, choose the appropriate coverage area, make the actual coverage of the base station the same as the expected design range, and at the same time strengthen the signal strength of the coverage area.
In the current mobile communication network, due to the increase of base station sites, when we design urban base stations, they generally require coverage of about 500M, and according to the characteristics of mobile communication antennas, if the antenna does not have a certain If the pitch angle (or the pitch angle is too small), the coverage of the base station will be much greater than 500M, which will cause the actual coverage of the base station to be larger than the expected range, resulting in cross coverage between cells, adjacent The handover relationship is chaotic, and the frequency interference in the system is serious; on the other hand, if the pitch angle of the antenna is too large, it will cause the actual coverage of the base station to be smaller than the expected range, resulting in signal blind areas or weak areas between the cells, and it is easy to cause the antenna Changes in the shape of the pattern (such as changing from a pear shape to a spindle shape), which can cause serious interference within the system. Therefore, setting the pitch angle reasonably is the basic guarantee for ensuring the quality of the entire mobile communication network.
In general, the size of the pitch angle can be calculated by the following formula:
θ = arctg (h / R) ＋ A / 2
Among them: θ--the pitch angle of the antenna
h--the height of the antenna
R--the coverage radius of the cell
A-antenna vertical plane half power angle
The above formula is obtained when the direction of the main lobe of the antenna is aligned with the edge of the cell. In actual adjustment work, generally 1-2 degrees is added to the resulting pitch angle to make the signal more effective The ground is covered in this cell.
4.3 Adjustment of the antenna azimuth The adjustment of the antenna azimuth is very important for the network quality of mobile communications. On the one hand, the accurate azimuth angle can ensure that the actual coverage of the base station is as expected, and ensure the operation quality of the entire network; on the other hand, the azimuth angle can be adjusted appropriately according to the traffic volume or the specific conditions of the network. Optimize the existing mobile communication network.
According to the ideal cellular mobile communication model, at the junction of a cell, such signals are relatively complementary. Corresponding to this, in the current GSM system (mainly referring to ERICSSON equipment), the directional station is generally divided into three cells, namely:
Cell A: The azimuth angle is 0 degrees, and the antenna points to true north;
Cell B: The azimuth angle is 120 degrees, and the antenna points to the southeast;
Cell C: The azimuth angle is 240 degrees, and the antenna points to the southwest.
In GSM construction and planning, we generally install and adjust the azimuth of the antenna in strict accordance with the above regulations. This is also one of the important standards for antenna installation. If there is a deviation from the azimuth setting, it will easily lead to the actual coverage of the base station Inconsistent with the design, the coverage of the base station is unreasonable, which leads to some unexpected co-frequency and adjacent-frequency interference.
However, in the actual GSM network, on the one hand, due to the terrain, such as buildings, mountains, water, etc., it often causes signal refraction or reflection, resulting in a large difference between the actual coverage and the ideal model, resulting in some regional signals. Strong, the signal is weak in some areas. At this time, we can adjust the azimuth of the antenna according to the actual situation of the network to ensure the signal strength of the weak signal area and achieve the purpose of network optimization; on the other hand Because the actual population density is different, the traffic of the cell corresponding to each antenna is uneven. At this time, we can adjust the azimuth of the antenna to achieve the purpose of balancing traffic. Of course, in general, we do not agree to adjust the azimuth of the antenna, because this may cause a certain degree of interference within the system. However, under certain special circumstances, such as local emergency meetings or large-scale public events, the traffic volume of some communities is particularly concentrated. At this time, we can temporarily adjust the azimuth of the antenna to achieve balanced traffic and optimize the network. In addition, for some signal blind areas or weak areas in the suburbs, we can also optimize the network by adjusting the azimuth of the antenna. At this time, we should supplement the field strength test vehicle to test the surrounding signals to ensure the operation of the network. quality.
4.4 Optimization and adjustment of antenna position Due to changes in later engineering, traffic distribution, and wireless propagation environment, we have encountered some base stations in the optimization that are difficult to achieve local area coverage and base station utilization through the adjustment of antenna azimuth or inclination. For this, base station relocation is needed, in other words, the base station reselection process.
The following excerpts of some of our usual experiences in planning.
(1) The initial layout of the base station The layout of the base station is mainly restricted by three factors: field strength coverage, traffic density distribution, and station construction conditions. For large and medium-sized cities, the constraints of field strength coverage are already very small, mainly affected by traffic density The two factors of distribution and station construction are more restrictive. The density of the base station layout should correspond to the traffic density distribution.
However, at present, it is not possible to predict the traffic density by block in the urban areas of large and medium-sized cities.
(a) Prosperous business district;
(b) Concentrated areas of hotels, office buildings, and entertainment venues;
(c) Economic and technological development zones and residential areas;
(d) Industrial areas and cultural and educational areas; etc. for classification.
Generally speaking:
(a) (b) areas should be set up with the largest directional base station, such as 8/8/8 station type, the station spacing is 0.6 ~ 1.6km;
(c) Category areas should also be equipped with larger directional base stations, such as 6/6/6 station type or 4/4/4 station type, the distance between the base station and the station should be 1.6 ~ 3km;
(d) Generally, small-scale directional base stations can be set up in areas like category 2/2/2, with a station spacing of 3 to 5 km; if the base station is located on the edge of a city or near suburbs, and the station spacing is above 5 km, omnidirectional Base station.
In the above areas, stations are set up according to the requirements of even distribution of users. Suburban counties and major highways and railways can generally be equipped with omnidirectional or two-cell base stations, and the distance between stations is about 5km-20km.
Base station layout combined with local terrain and urban development planning:
a. The layout of the base station should be combined with the urban development plan and can be moderately advanced;
b. There should be base station coverage where there are important users;
c. Add a micro-cell station or increase the carrier frequency configuration in the "hot spot" area of ​​city traffic;
d. If it is necessary for large shopping malls hotels, subways, underground shopping malls, and stadiums to use micro-cellular or indoor distribution solutions;
e. Before the capacity of the base station is saturated, the GSM900 / 1800 dual-band solution can be considered.
(2) Site selection and survey After completing the initial layout of the base station, the network planning engineer should work with the construction unit and related engineering design units to select and survey the site according to the site layout. The urban site should be selected in the primary election until the homeowner basically agrees to use it as a base station. After the primary selection is completed, network planning engineers, engineering design units and construction units conduct on-site investigations to determine whether the site conditions meet the requirements for building the site, and determine the site plan. Finally, the construction unit and the owner will finalize the site. The location requirements are as follows:
--- Convenient transportation, reliable city power, environmental safety and small footprint.
--- When there are few stations in the initial stage of network construction, the selected site should ensure that important users and urban areas with high user density have good coverage.
--- Under the premise of not affecting the layout of the base station, the existing telecommunications hub, post and telecommunications office or microwave station should be selected as the site, and its equipment room, power supply and iron tower should be used.
--- Avoid setting up stations near high-power wireless transmitting stations, such as radar stations, TV stations, etc. If you want to set up stations, you should verify whether there is mutual interference, and take measures to prevent mutual interference.
--- Avoid setting up stations on high mountains. The alpine station has a large interference range, which affects frequency reuse. Setting up stations in high mountains in rural areas often fails to cover towns and villages in small basins.
--- Avoid setting up stations in the woods. To set up a station, keep the antenna above the tree top.
--- Among the base stations in urban areas, for the cellular area (R = 1 ～ 3km), the base station should be selected as the site that is higher than the average height of the building but lower than the highest building. The buildings of average height have stations and the surrounding buildings are well shielded.
--- Urban base stations should avoid high-rise buildings near the front of the antenna, causing obstacles or reflections to interfere with the same-frequency base stations behind them.
--- Avoid choosing sites where new buildings may affect the coverage area or co-channel interference in the future.
--- The base stations of the two network systems in the urban area should be co-located or close to the site selection.
--- Select the building with low renovation cost and low rent as the station site. If possible, the bureau, station computer room, and office building of this department should be selected as the site.
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