1. Overview of base station antenna
This chapter introduces the classification of base station antennas and the appearance of different types of antennas in mobile communication.
1.1 # base station antenna classification
360 DEG in the horizontal direction; It radiates uniformly, that is, it is usually called non directivity. In the vertical pattern, it is shown as a beam with a certain width. Generally, the smaller the lobe width, the greater the gain. Omnidirectional antenna is generally used in mobile communication system as the station type of suburban county and large area system, with large coverage.
In the horizontal pattern, it shows radiation in a certain angle range, that is, it is usually said to be directional. In the vertical pattern, it shows a beam with a certain width. Like the omnidirectional antenna, the smaller the lobe width, the greater the gain. Directional antenna is generally used in the station type of urban community system in mobile communication system, with small coverage, high user density and high frequency utilization.
Different types of base stations are established according to the networking requirements, and different types of antennas can be selected according to the needs of different types of base stations. The selection is based on the above technical parameters. For example, omnidirectional station adopts omnidirectional antenna with basically the same horizontal gain, while directional station adopts directional antenna with obvious change in horizontal gain. Generally, the horizontal beam width is 65 DEG in urban areas; In the suburbs, the horizontal beam width can be selected as 65 & deg 90° Or 120 & deg; According to the station configuration and local geographical environment, it is the most economical to select an omni-directional antenna that can achieve wide coverage in rural areas.
Refers to a mobile antenna that mechanically adjusts the tilt down angle. After the mechanical antenna is installed perpendicular to the ground, if the position of the support on the back of the antenna needs to be adjusted and the inclination of the antenna needs to be changed due to the requirements of network optimization. During the adjustment process, although the coverage distance in the direction of the main lobe of the antenna changes significantly, the amplitudes of the vertical and horizontal components of the antenna remain unchanged, so the antenna pattern is easy to deform.
The practice shows that the best dip angle of mechanical antenna is 1-deg- 5°； The dip angle is 5-deg- 10° When changing, the antenna pattern is slightly deformed, but changes little; The dip angle is 10 DEG- 15° When changing, the antenna pattern changes greatly; When the mechanical antenna tilts down 15 DEG; After, the shape of the antenna pattern changes greatly, from a pear without downward inclination to a spindle shape. At this time, although the coverage distance in the main lobe direction is significantly shortened, the whole antenna pattern is not in the sector of the base station, and the signals of the base station will also be received in the sectors of adjacent base stations, resulting in serious interference in the system. In addition, in daily maintenance, if the tilt angle of mechanical antenna is to be adjusted, the whole system shall be shut down and cannot be monitored while adjusting the tilt angle of antenna; It is very troublesome to adjust the downward inclination angle of the mechanical antenna. Generally, maintenance personnel need to climb to the antenna placement place for adjustment; The downward inclination angle of mechanical antenna is the theoretical value calculated by computer simulation analysis software, which has a certain deviation from the actual optimal downward inclination angle; The steps of adjusting the inclination angle of the mechanical antenna are 1 & deg;, The third-order intermodulation index is - 120dbc.
Electrically adjustable antenna:
A mobile antenna that uses electronics to adjust the tilt down angle. The principle of electronic downdip is to change the phase of the oscillator of the collinear array antenna, change the amplitude of the vertical component and horizontal component, and change the field strength of the composite component, so as to make the vertical pattern of the antenna downdip. As the field strength in each direction of the antenna increases and decreases at the same time, it is ensured that the antenna pattern changes little after changing the inclination angle, which shortens the coverage distance in the main lobe direction, and reduces the coverage area of the whole directional pattern in the service cell sector without interference. The practice shows that the dip angle of electrically tunable antenna is 1-deg- 5° When changing, the antenna pattern is roughly the same as that of the mechanical antenna; The dip angle is 5-deg- 10° When changing, the antenna pattern is slightly better than that of mechanical antenna; The dip angle is 10 DEG- 15° When changing, the antenna pattern changes more than that of mechanical antenna; When the mechanical antenna tilts down 15 DEG; After, the antenna pattern is significantly different from the mechanical antenna. At this time, the shape of the antenna pattern changes little, and the coverage distance in the main lobe direction is significantly shortened. The whole antenna pattern is in the sector of the base station. Increasing the downward tilt angle can reduce the sector coverage area without interference. Therefore, the use of electrically adjustable antenna can reduce call loss and interference. In addition, the electrically adjustable antenna allows the system to adjust the inclination angle under the vertical directivity map without stopping, monitor the adjustment effect in real time, and the step accuracy of adjusting the inclination angle is also high (0.1 & deg;), Therefore, the network can be finely adjusted; The third-order intermodulation index of electrically modulated antenna is - 150dbc, which is 30dbc different from that of mechanical antenna, which is conducive to eliminating adjacent frequency interference and stray interference.
Dual polarization antenna:
Dual polarization antenna is a new antenna technology, which combines + 45 DEG; And -45° Two antennas with orthogonal polarization directions work in the transceiver duplex mode at the same time, so its most prominent advantage is to save the number of antennas of a single directional base station; In general, the directional base station (three sectors) of LTE digital mobile communication network uses 9 antennas, and each sector uses 3 antennas (spatial diversity, one transmitter and two receivers). If dual polarization antenna is used, each sector only needs 1 antenna; At the same time, in the dual polarization antenna, & plusmn; 45° The orthogonality of polarization can guarantee + 45 DEG; And -45° The isolation between two antennas meets the requirements of intermodulation on the isolation between antennas (≥ 30dB), so the spatial spacing between dual polarization antennas only needs 20-30cm; In addition, dual polarization antenna has the advantages of electrically adjustable antenna. The use of dual polarization antenna in mobile communication network can reduce call loss, reduce interference and improve the service quality of the whole network. If the dual polarization antenna is used, because the dual polarization antenna does not have high requirements for erection and installation, there is no need for land acquisition and tower construction. It only needs to erect an iron column with a diameter of 20cm and fix the dual polarization antenna on the iron column according to the corresponding coverage direction, so as to save infrastructure investment, make the layout of the base station more reasonable and make the selection of the base station site easier.
For the selection of antenna, the mobile antenna suitable for the local mobile network shall be selected according to the actual situation of network coverage, traffic, interference and network service quality:
· In high traffic areas with dense base stations, dual polarization antenna and electric modulation antenna should be used as much as possible;
· Traditional mechanical antennas can be used in areas where the traffic is not high, where the base stations are not dense and where only coverage is required.
1.2 internal structure and types of mobile communication base station antenna
1.2.1 Directional plate dipole array antenna
Plate directional antenna is the most widely used kind of very important base station antenna. The antenna has the advantages of high gain, good sector pattern, small back lobe, convenient depression angle control of vertical pattern, reliable sealing performance and long service life. The outline of the antenna is shown in the figure below:
Figure 1-1} outline diagram of plate directional antenna
184.108.40.206 Formation of high gain plate antenna
Fig. 1-2 ， multiple half wave oscillators are arranged into a vertically placed linear array
Fig. 1-3 ， the principle of horizontal orientation by adding a reflector on one side of the linear array (taking the vertical array of two and a half wave oscillators with reflector as an example)
At present, the base station directional antenna design of antenna manufacturers basically adopts the plate type vibrator array structure. There are the following two types of vibrators, as described in the following two sections.
220.127.116.11 Symmetrical oscillator
Standard half wave symmetric array (add an additional oscillator to reduce the height of the oscillator from the ground and reduce the thickness of the antenna)
Fig. 1-4 directional plate antenna composed of multiple half wave oscillators
18.104.22.168 Microstrip oscillator
The deformation of half wave oscillator forms radiation by using the principle of 1 / 4 wavelength transmission line:
Figure 1-5 directional plate antenna composed of multiple microstrip oscillators
1.2.2 Omnidirectional series fed dipole antenna
The omnidirectional antenna adopts multiple half wave oscillators in series to realize the synthesis and enhancement of radiation gain.
Figure 1-7 # structure and product form of series fed oscillator of omnidirectional antenna
2 4G LTE antenna types and comparative analysis
The main content of this section is to introduce the comparative analysis of coverage and traffic between LTE dual polarization antenna and single polarization antenna
2.1 Correlation analysis of LTE dual polarization antenna and single polarization antenna
The introduction of LTE multi antenna technology increases the freedom of spatial dimension for wireless resources, and puts forward new requirements for wireless channel model. A spatial channel model (SCM) is proposed in 3GPP tr 25.996. The model is suitable for systems with a bandwidth of 5 MHz and a carrier frequency of about 2 GHz. The maximum number of multipaths is 6. The LTE system requires that the wireless channel can support up to 20 MHz. Therefore, the SCME (SCM extension) model is used in the technical report 36.803 to expand the channel bandwidth to 20 MHz and the maximum number of multipaths to 9. Among them, the wireless transmission characteristics between ENB and UE are a time-varying function, which changes with the changes of antenna configuration, antenna azimuth, antenna correlation and scattering environment, as shown in the figure below.
Figure 2-1 schematic diagram of SCM angle parameters
Dual polarization antenna and single polarization antenna (antenna spacing 10) λ） The difference of wireless performance mainly depends on the correlation coefficient of base station antenna. When the correlation coefficient is 0, it indicates that the antennas are relatively independent and the correlation is low. When the correlation coefficient is 1, it indicates that there is a strong correlation between antennas. When the system adopts transmit diversity mode (such as SFBC), receive diversity and MIMO dual stream mode, the wireless performance of low correlation antenna is better than that of high correlation antenna. The following figure shows the SFBC performance simulation results with correlation coefficients of 0.25, 0.5, 0.6 and 1 respectively. From the simulation results, when the correlation coefficient is 0.25, its performance is basically not affected (compared with the correlation coefficient of 0). The performance of SFBC with correlation coefficients of 0.5 and 0.6 decreases by about 0.3dB to 0.4db.
Figure 2-2} comparison of SFBC link level performance with different correlation coefficients
China Mobile tested the correlation coefficients of different antenna configurations in July 2008. The specific correlation coefficients are shown in the table below:
Table 2-1 # correlation coefficients corresponding to different antenna configurations
Note: the correlation coefficients in the above table are obtained from the test of China Mobile in dense urban areas according to SCME model. In dense urban areas, this correlation coefficient is representative. However, it does not mean that this correlation coefficient can be mapped to a specific project, nor does it mean that this correlation coefficient is applicable to all models of dense urban areas.
2.2 performance comparison between LTE dual polarization antenna and single polarization antenna
LTE defines seven multi antenna transmission modes, including transmission diversity, precoding based MIMO, beamforming, etc. The seven transmission modes defined by LTE mainly consider that the transmission modes can be flexibly selected under different scenarios and different channel models. Generally, the bottlenecks and short boards of wireless mobile communication network performance are divided into the following three categories:
Power limited system:
Typical application scenario: the main purpose is to increase coverage and overcome fading, such as effective area, wide rural coverage, etc
Type of antenna technology used: transmit diversity, receive diversity
Performance gap: 10 λ The performance improvement of single polarization antenna is less than 5% than that of dual polarization antenna, and there is little difference between the two.
Interference limited system:
Typical application scenario: it is mainly used in dense urban areas with small station spacing. Interference is the main factor affecting network performance.
Type of antenna technology adopted: rank = 2 MIMO dual stream, rank = 1 MIMO single stream, rank adaptive
Performance gap: rank adaptive algorithm is obviously better than MIMO forced dual stream; At the same time, the performance of dual polarization antenna is 10 λ The performance of single polarization antenna is basically the same.
Bandwidth constrained system:
Typical application scenario: the channel condition (CQI) is relatively good, there is no continuous coverage between base stations, the station spacing of base stations is relatively large, and the number of users is relatively rare. For example, single cell coverage in the initial stage of the experimental network,
Adopted antenna technology type: rank = 2 MIMO dual stream
Performance comparison: 10 λ The performance of single polarization antenna is better than that of dual polarization antenna, and the performance is improved by about 20%
Table 2-2 conclusion of antenna application scenario
The above mainly analyzes the performance gap between single polarization antenna and dual polarization antenna. However, it should be noted that the dual polarization antenna has the advantages of convenient installation in engineering. Including the use of a single holding pole, unified adjustment of the downward inclination, easy to beautify the antenna, etc. At present, when it is more and more difficult to select and install wireless network site, the above advantages of dual polarization antenna are particularly important.
3 Application scenarios and selection of 4G LTE base station antenna
The main content of this section describes the classification of wireless network coverage area and corresponding antenna selection according to the actual network deployment scenario
3.1 Antenna selection of urban base station
Application environment features: the base stations are densely distributed, requiring a small coverage of a single base station. We hope to minimize the phenomenon of cross zone coverage, reduce the interference between base stations and improve the download rate.
Antenna selection principle:
Polarization mode selection: as it is difficult to select the base station site in the urban area and the antenna installation space is limited, it is recommended to select dual polarization antenna and broadband antenna;
Selection of pattern: in urban areas, it is mainly considered to improve frequency reuse, so directional antenna is generally selected;
Selection of half power beam width: in order to better control the coverage of the cell and suppress interference, the horizontal half power beam width of the urban antenna is 60 ~ 65°;
Selection of antenna gain: since the urban base station generally does not require a wide coverage distance, it is recommended to select the antenna with medium gain. It is recommended to select the antenna with 15-18dbi gain in urban area. If the microcellular antenna used for blind compensation in the urban area has a lower gain, the antenna can be selected;
Dip angle selection: because the antenna dip angle adjustment in urban area is relatively frequent, and some antennas need to set a large dip angle, while the mechanical dip angle is not conducive to interference control, it is recommended to select the preset dip angle antenna. An antenna with a fixed electrical downdip angle can be selected, and an electrically adjustable antenna can also be selected when the conditions are met.
3.2 Antenna selection of rural base stations in suburbs
Application environment features: the base stations are sparsely distributed, the traffic volume is small, the requirements for data services are relatively low, and the requirements for wide coverage. In some places, there is only one base station around, and coverage has become the most concerned object. At this time, the selection of antenna should be considered in combination with the area to be covered around the base station.
Antenna selection principle:
Pattern selection: if the base station is required to cover the surrounding area without obvious directionality and the traffic distribution around the base station is relatively scattered, omnidirectional base station coverage is recommended. At the same time, it should be noted that the coverage distance of the omnidirectional base station is not as far as that of the directional base station due to its small gain. At the same time, when installing the omni-directional antenna, pay attention to the influence of the tower on the coverage, and the antenna must be perpendicular to the ground plane. If the office has a longer coverage requirement for the coverage distance of the base station, it needs to use a directional antenna. Generally, the horizontal half power beam width of 90 & deg 105 °、 120 ° Directional antenna;
Selection of antenna gain: select antenna gain according to the coverage requirements. It is recommended to select directional antenna with higher gain (16-18dbi) or omnidirectional antenna with 9-11dbi in suburban and rural areas;
Selection of downward tilt mode: there are few downward tilt adjustments to the antenna in suburban and rural areas, and the adjustment range and characteristic requirements of downward tilt are not high. It is recommended to select mechanical downward tilt antenna; At the same time, when the antenna is more than 50m high and there are coverage requirements at the near end, the antenna filled with zero point can be preferentially selected to avoid the black problem under the tower.
3.3 antenna selection of highway coverage base station
Application environment characteristics:
In this environment, the traffic is low and users move at high speed. At this time, the focus is to solve the coverage problem. Generally speaking, what it wants to achieve is strip coverage, so the highway coverage mostly adopts two-way community; Omnidirectional community is also adopted in areas passing through towns and tourist spots; Another is to emphasize wide coverage. The type of antenna to be used shall be determined in combination with the selection of station site and station type. Different highway environments vary greatly. Generally speaking, there are relatively straight highways, such as expressways, railways, national highways, provincial highways, etc. it is recommended to build stations next to the highway, using S1 / 1 / 1 or S1 / 1 station type, equipped with high gain directional antenna to achieve coverage. There are winding roads, such as winding mountain roads, county-level self built mountain roads, etc. In combination with the rural coverage near the highway, stations should be built at high places.
In the initial planning for antenna selection, the high gain antenna with wide coverage distance shall be selected as far as possible for wide coverage.
Antenna selection principle:
Selection of pattern: in the base station aiming at covering the railway and highway, the directional antenna with narrow beam and high gain can be used. The antenna form can be flexibly selected according to the local topographic relief and turning of the road at the station;
For antenna gain selection, 17dbi-22dbi antenna is selected for directional antenna gain, and 11dbi is selected for omnidirectional antenna gain;
Selection of downdip mode: generally, there is no downdip angle for highway coverage. It is recommended to select a cheaper mechanical downdip antenna. When it is more than 50m and there are coverage requirements at the near end, the antenna with zero filling (greater than 15%) can be preferentially selected to solve the black problem under the tower;
Front to back ratio: since most users covered by the highway are fast-moving users, the front to back ratio of directional antenna should not be too high to ensure the normal switching.
3.4 Antenna selection of base station in mountainous area
Application environment features: in remote hilly and mountainous areas, the mountain block is serious, the propagation decline of radio waves is large, and it is difficult to cover. It usually has wide coverage. Scattered users are distributed within a wide coverage radius of the base station, and the traffic is small. The base station may be built at a suitable location on the top of the mountain, at the waist of the mountain, at the foot of the mountain, or in the mountain. It is necessary to distinguish different user distribution and terrain characteristics for base station location, selection and antenna selection. The following situations are common: station construction in basin type mountainous area, station construction on high mountain, station construction on hillside, station construction in ordinary mountainous area, etc.
Antenna selection principle:
Selection of pattern: the selection of pattern depends on the location, station type and surrounding coverage requirements of the base station. Omnidirectional antenna or directional antenna can be selected. For the base station built on the mountain, if the location to be covered is relatively low, the pattern with large vertical half power angle shall be selected to better meet the coverage requirements in the vertical direction;
Antenna gain selection: select medium antenna gain, omnidirectional antenna (9-11dbi) and directional antenna (15-18dbi) according to the distance of the area to be covered;
Inclination selection: when the station is built on the mountain and the place to be covered is at the foot of the mountain, the antenna with zero filling or preset inclination shall be selected. The size of the preset dip angle depends on the relative height between the base station and the place to be covered. The larger the relative height is, the larger the preset dip angle should be.
3.5 Summary of LTE base station antenna application scenarios
According to the above selection and the special situation of LTE, the recommended antenna selection principles are as follows:
Table 3-1 summary of antenna application scenarios
Generally, the site selection of LTE station uses the existing facilities, so whether there is enough space to install LTE antenna and whether the height meets the LTE planning is the biggest problem. Therefore, the technical parameters such as polarization mode, broadband antenna and dip angle mode adopted in the actual project need to be reasonably planned according to the actual situation after detailed investigation of the existing facilities. Due to the existence of MIMO technology in LTE, 2T2R and 4t4r are commonly used at present. Considering the construction cost and other factors, for 2T2R, dual polarization antenna is generally adopted; For 4t4r, generally, two dual polarization antennas are used, and the distance between antennas is 1-2 λ It corresponds to 2.6g, about 30-50cm.
4 5g massive MIMO AAU and application scenarios
The main content of this section briefly describes the antenna and application scenario selection of 5g AAU
4.1 5G Massive MIMO AAU
Multi input output (MIMO), which has been very mature in the 4G era, can effectively use multiple spatial channels between multiple antennas between transceiver systems to transmit multiple orthogonal data streams, so as to improve data throughput and communication stability without increasing communication bandwidth.
The massive MIMO technology developed from 4G to 5g is an upgraded version of MIMO technology. On the basis of limited time and frequency resources, hundreds of antenna units are used to serve dozens of mobile terminals at the same time, which further improves the data throughput and energy efficiency. The 5g communication frequency is high and the antenna size is shortened, so that more antennas can be plugged into the original space. Massive MIMO technology has laid the tone of communication technology in the 5g era, so the antenna has also become another explosive growth device following the RF front-end in the 5g era, in which the base station antenna accounts for 20% and the terminal antenna accounts for 80%.
The 5g base station is also called active antenna array unit (AAU) because it widely adopts the massive MIMO technology, the number of oscillators used in its internal integrated antenna is large, and the transceiver unit is integrated. Its internal structure schematic diagram and product form are shown in the figure below.
Fig. 4-1 structural schematic diagram of # massive MIMO AAU
Figure 4-2 # explosion diagram of product form of massive MIMO AAU
For AAU in < 6GHz band, communication equipment manufacturers generally use 192 oscillators. There are 12 rows in the horizontal direction and 8 rows of oscillators in the vertical direction, plus & plusmn; 45° Dual polarization, a total of 12x8x2 = 192 oscillators. Each group of three oscillators is called a pair of antennas, so the AAU has 192 / 3 = 64 antennas in total. If every six oscillators form an antenna, the AAU has 192 / 6 = 32 antennas.
Fig. 4-3 dipole diagram of 65tr and 32tr AAU antenna
AAU in < 6GHz band usually adopts all digital beamforming. It can be considered that the number of antennas, transmission channels and power amplifiers are the same. The number of arrays is an important factor of coverage. The more the number of arrays, the narrower the beam, the more concentrated the energy, the more the number of antennas and channels, the more the number of power amplifiers in the AAU, the greater the consumption of baseband resources, and the higher the cost of equipment.
4.2 5g signal fading model
3GPP tr 38.901 gives four scenarios: indoor hot office area (INH Office), urban micro street canyon, urban macro cellular (UMA) and rural macro cellular (RMA). Each scenario is divided into eight propagation models: non line of sight (NLOS) and line of sight (LOS). This paper selects the path loss model under the UMA Los / NLOS scenario of urban macro station. Among them, FC operating frequency (GHz), effective height of HBS base station antenna (m), effective height of hut mobile station antenna (m), horizontal distance between D2D base station and mobile station (m), and linear distance between D3D base station antenna and mobile station antenna (m).
According to the above table, the typical maximum allowable path loss (MAPL) of different channels of urban macro station can be calculated according to the following formula. It can be seen that the maximum allowable path loss difference between 5g NR 3.5GHz uplink and downlink reaches 13.65 dB, and the network coverage is limited by the uplink and the uplink Pusch channel.
Among them, PTX base station transmit power, lf feeder loss, GTX base station antenna gain, MF shadow fading and fast fading allowance, ML interference allowance, Grx mobile phone antenna gain, LP building penetration loss, LB human body loss, SRX mobile phone reception sensitivity
4.3 5GAAU application scenario
In dense urban areas, the complex wireless environment leads to the deterioration of interference, and the high-rise buildings lead to high requirements for vertical coverage and large demand for user capacity. 64tr equipment can provide better large-scale MIMO beamforming, realize high traffic multi-user MIMO transmission, and significantly improve the vertical coverage. In suburban and rural areas, MU-MIMO pairing success rate is reduced, and 64tr equipment can not give full play to its capacity advantages, so low configuration equipment can be used. The use of 32 antennas can meet the demand. For more remote areas, the requirements for capacity are not high, and the main solution is to solve the coverage problem. At this time, even massive MIMO is not used. Just use 8-port RRU to connect the antenna directly.
The following figure shows the scenario network coverage scheme proposed by an equipment manufacturer. Macro station is the most important product form. 64tr AAU solves the continuous high-capacity demand in 4G / 5G stage, and 32tr AAU with low configuration solves the demand for 4G / 5G low traffic area and low-cost network construction. The indoor distribution system products include 2tr and 4tr equipment. The existing network passive room subsystem or new construction are used to solve the indoor scenes with high value and high flow. In addition, the radio frequency unit (RRU) of the MicroStation base station 4tr is widely used in busy and hot scenes such as residential areas and pedestrian streets.
Figure 4-4 scenario network coverage solution of an equipment manufacturer
It can be seen from the calculation results in Section 4.2 that the uplink coverage of 5g NR in 3.5 GHz band is limited, which is mainly affected by the limited terminal power. Considering that the uplink and downlink service rate requirements are still asymmetric in a period of time after the launch of 5g in the future (the downlink rate requirements are much higher than the uplink), in order to maintain the existing network structure and reduce the network construction cost, This enables operators to quickly stack and deploy 5g on 4G existing website points, and the uplink and downlink decoupling (SUL) can be used to solve the problem of uplink coverage bottleneck. That is, the uplink uses the old LTE equipment. The bandwidth of the 1.8 GHz FDD is 5g NR to improve coverage, and the downlink deploys 5gnr in the 3.5 GHz band.
Figure 4-5 decoupling solutions for uplink and downlink using different frequency bands
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