"Research on OTA test method of 5g base station antenna" the OTA test method of large-scale MIMO active antenna of 5g base station is studied. This paper analyzes the necessity of integrated OTA test of 5g base station antenna, introduces different OTA test schemes such as far-field, compact field, multi probe near-field and single probe near-field, compares and analyzes the advantages and disadvantages of each test scheme through actual test, points out the problems faced by the current OTA test of 5g base station antenna, and puts forward the solutions.
1. Introduction
5g mobile communication technology can meet people's demand for fast-growing mobile communication services such as high-speed, large capacity, high reliability and low delay. As one of the key technologies of 5g mobile communication, large-scale MIMO active antenna technology can greatly improve the spectrum efficiency through spatial multiplexing. Combined with new coding technology, it can greatly improve the communication system capacity and communication rate. Therefore, large-scale MIMO active antenna technology is widely used in 5g mobile communication base stations, but then there is the problem of how to test the antenna of 5g base stations.
For traditional base stations, the antenna and RRU (radio remote unit) are separated from each other. They are connected through radio frequency cables, which are relatively independent and have no impact on each other's performance. Their respective performance can be verified through independent tests. The radiation performance test of antenna can be completed by far-field or near-field in microwave anechoic chamber. The far-field or near-field test of passive antenna is a mature test method widely used to test the performance of antenna. The RF index of RRU can be measured by conduction in the laboratory.
Referring to the traditional base station test method, it is easy to propose a scheme to split the active antenna system into passive antenna array and RRU for antenna radiation performance test and RF conduction test respectively. In fact, according to laboratory test experience, the beamforming pattern measured by "passive antenna array + power division network + signal source" is not consistent with the results of integrated OTA (over the air) test of active antenna of 5g base station“ The RF performance conduction test results of RRU + coupling plate are also different from the RF radiation indexes measured by integrated Ota. The reason is that for the 5g base station antenna, the antenna is integrated with the RRU. On the one hand, electromagnetic coupling, active standing wave and other interference factors cannot be completely eliminated; On the other hand, the calibration and amplitude and phase weighting of active antenna are completed through a series of active devices on each RF channel, which is very different from that of passive antenna array through passive power division network. Therefore, for 5g base stations using large-scale MIMO active antenna technology, the integrated OTA test method can effectively reflect its performance index. Especially in the millimeter wave band, the frequency band is higher, the equipment size is smaller, and the electromagnetic interference problem is more prominent. It will be very difficult to split the test, so we can only use the integrated OTA test scheme.
The new 3GPP 5g air interface protocol frozen in December 2017 has written OTA test specifications for all RF performance indicators of 5g base station, which means that the integrated OTA test of 5g base station antenna will become the main scheme for the hardware performance test of 5g base station. However, at present, the OTA test of RF index still faces many difficulties. In this paper, the OTA test methods of large-scale active antenna system are deeply studied. The tests are carried out in different fields, such as far field, compact field, multi probe spherical near field, single probe near field, etc. the advantages and disadvantages of each test scheme are compared and analyzed, and the problems and corresponding solutions are put forward.
2. OTA test scheme for 5g base station antenna
The radiation performance of the antenna is generally tested in the near-field or far-field by Ota. The boundary between near-field and far-field of antenna radiation is: the wave path difference between the spherical wavefront transmitted by the source antenna and the center and edge of the antenna under test is λ/ 16。 The judgment basis converted to distance is d = 2d2/ λ， Where, D is the distance between the detection point and the antenna under test, and D is the aperture of the antenna under test, λ Is the electromagnetic wave length transmitted by the antenna under test.
Therefore, antenna test can be divided into far-field test and near-field test, and different test schemes will lead to different test results. Several classic OTA test schemes for active antenna are introduced below.
(1) Far field test scheme
Far field test is a direct test method. When the test distance is far enough, the incident wave is similar to plane wave on the receiving surface. Figure 1 shows the far-field test system. The tested part can rotate 360 ° in the vertical and horizontal planes. The position of the test probe is fixed and can be polarized and rotated. The test system can test the beamforming pattern of 5g base station antenna and RF radiation indexes such as EIRP (effective isotropic radiated power), EVM (error vector magnitude), occupied bandwidth, EIS (effective isotropic sensitive).
(2) Compact field test scheme
Compact field test is a far-field test method. It can convert the spherical wave emitted by the feed at the focus into plane wave by using a mirror or lens, so as to realize the far-field test in a limited physical space. Figure 2 shows a paraboloid single mirror compact field test system, which can test the beamforming pattern of 5g base station antenna and RF radiation indicators such as EIRP, EVM, occupied bandwidth, ACLR (adjacent channel leakage power ratio), EIS, ACS (adjacent channel selectivity).
(3) Multi probe spherical near field test scheme
The near-field test is to collect the amplitude and phase information in the radiation near-field area of the antenna under test, and then convert the collected data into the far-field pattern through the near-field and far-field conversion algorithm. The multi probe spherical near-field test system is shown in Figure 3. A large number of probes are arranged along the circumference in the radiation near-field of the tested part. The measured part only needs to rotate 180 ° to collect the data of the whole radiation spherical surface. The system can test the beamforming pattern of 5g base station antenna in CW (continuous wave) mode.
(4) Single probe near field test system
The efficiency of single probe near-field test is lower than that of multi probe spherical near-field test, but its structure is simpler and requires less space. For the small near-field test system shown in Figure 4, the tested part can rotate in the horizontal plane and the probe can rotate in the vertical plane. The system can collect the data of a radiation sphere with the cooperation of two rotation axes. The system can test the beamforming pattern of 5g base station antenna in CW mode and RF radiation index in service signal mode, but the processing of test results needs further analysis.
Explain in detail the OTA test method for large-scale MIMO active antenna of 5g base station
3. Comparison of advantages and disadvantages of each test scheme
The advantages of far-field test are: because the distance between the receiving antenna and the transmitting antenna is greater than the far-field criterion, the electromagnetic wave propagates from the transmitting antenna to the receiving antenna is similar to a plane wave, and the collected data does not need near-field and far-field conversion. The test equipment can transmit high-power signals, test modulated wide-band signals, and support multi-user testing. The disadvantages are: because the test distance needs to be greater than the far-field criterion, the test site covers a large area and the construction cost is high. Taking the antenna with an aperture of 1 m and working in 3.5 GHz band as an example, the far-field condition is greater than 25 m according to the far-field criterion formula. The farther the test distance is, the closer the electromagnetic wave radiation is to the plane wave, but at the same time, it will bring the problem of too much space loss. In addition, since there is generally only one probe in the far-field test, only one section of the antenna radiation sphere can be drawn in a single test. If you want to obtain the 3D pattern of the whole radiation sphere, you need to measure multiple times on different sections, and the test time and cost will increase significantly.
The advantage of compact field test is that it greatly reduces the site size compared with the far field, which greatly reduces the site construction cost and measurement path loss. The test results are close to the direct far-field test, and can test CW wave and service signal. Thanks to the reduction of path loss, it can measure more RF radiation indicators than the far-field scheme. Disadvantages: similar to the disadvantages of far-field test, the test efficiency of 3D pattern is low, and the cost of mirror and later maintenance are high.
The advantages of multi probe spherical near-field test are: small floor area, 3D pattern can be given in a single test, high test efficiency and low space loss. The pattern test results in CW mode are close to the far-field test results. The disadvantages are: the upper limit of the receiving power of the test system is low. When the tested 5g base station transmits at full power, the test receiving equipment must be pre attenuator; The measured data needs post-processing for near-field and far-field conversion; The near-field and far-field conversion requires a reference phase. At present, due to the problem of reference phase, the measurement results in service signal mode are not satisfactory.
The advantages of single probe near-field test are: small occupied space, low construction cost of darkroom, simple turntable structure, convenient installation and disassembly of tested equipment, low space loss, and the test results of directional pattern under CW mode are close to those of far-field test. The disadvantages are: due to the structure, the antenna back lobe data acquisition is incomplete; With only one test probe, the efficiency of testing 3D pattern is less than that of multi probe spherical near field; The collected data needs to be converted into near-field and far-field.
4. Problems and Solutions
The current OTA test scheme, whether far-field scheme or near-field scheme, can test the radiation pattern of 5g base station antenna in CW mode. However, for the test of RF index radiation performance, the current far-field scheme is limited by large path loss, and can only test parameters with high power levels such as EIRP, EVM, occupied bandwidth and EIS. For downlink RF indicators with particularly low power level, such as ACLR, switching time template, spurious emission, etc., it attenuates to the noise level or even smaller after a long test distance in the far field, which is difficult to test. When measuring uplink indicators, the interference signal sent by the auxiliary signal source is difficult to reach the power level required for RF indicators such as ACS, in band blocking and common address blocking after far-field path attenuation, which also brings difficulties to the test. Although the path loss of the near-field test scheme is much lower than that of the far-field, there are still problems in the way of taking the reference phase in the broadband service signal mode, and there is still a big gap between the RF radiation test results and the expected value.
Since the indicators to be tested and verified in the laboratory R & D test stage are relatively comprehensive, the far-field test method of compact field or loss reduction should be adopted for this kind of test. By shortening the far-field test distance, increasing the horn antenna gain, using low loss RF cables and shortening the RF line wiring distance within a certain range, the path loss can be greatly reduced, so that the far-field scheme can expand the RF indicators such as ACLR and ACS. The path loss of the compact field itself is much smaller than that of the far field, and more RF indicators can be measured than that of the far field. However, there are still some RF indicators. Because the power itself is particularly low, how to reduce the path loss is not enough. At this stage, it can only be tested by conduction. For the production line test, the test cost is low, the efficiency is high, the occupied space is small, and the typical indexes can be tested. The single probe near-field test scheme is more suitable. As for the future 5g high-frequency band test, due to the higher frequency and more serious loss, the far-field test will become less suitable and the conduction test will be more difficult. The scheme of combining near-field test and inductive near-field test needs to be adopted, and the near-field and far-field conversion algorithm needs reference signal, which requires the cooperation of equipment manufacturers and measuring instrument manufacturers to solve the problem of taking reference signal from equipment.
5. Conclusion
This paper studies the OTA test method of large-scale MIMO active antenna in 5g base station. Using the 5g base station equipment of our unit, different OTA test schemes such as far-field, compact field, multi probe near-field and single probe near-field are studied through actual test. The construction cost, test capacity and test efficiency of each site are analyzed, and the problems faced in the test and corresponding solutions are put forward, which provides a reference for the current and future OTA test of 5g base station antenna.
Reprinted from viku electronic market network“
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