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4 test methods for 5G base station antenna OTA
Source:Originally | Author:Calio Huang of C&T RF Antennas Inc | Publish time: 2019-05-03 | 860 Views | Share:


The Abstract

The research method of 5G base station antenna OTA test research on the large-scale MIMO active antenna OTA test method of the 5G base station. In this paper, the necessity of an integrated OTA test for 5G base station antenna is analyzed. Different OTA test schemes such as far field, compact field, multi-probe near field and single probe near field are introduced. The advantages and disadvantages of each test scheme are tested through the actual test. The comparative analysis points out the problems faced by the current 5G base station antenna OTA test and proposes a solution.

[Keywords] 5G; base station antenna; OTA test

1. Introduction

5G mobile communication technology can meet people's needs for fast-growing mobile communication services such as high speed, large capacity, high reliability, and low latency. The large-scale MIMO active antenna technology as one of the key technologies of 5G mobile communication can greatly improve the spectrum utilization efficiency through spatial multiplexing, and can greatly improve the communication system capacity by combining the new coding technology.  And the communication rate. Therefore, the large-scale MIMO active antenna technology is a commonly used technology in 5G mobile communication base stations, but it is followed by the problem of how to test 5G base station antennas.

For a traditional base station, the antenna and the RRU (Radio Remote Unite) are separated from each other. They are connected by RF cables, which are relatively independent and have no performance impact. Their respective performances can be independently tested. Carry out an inspection. The radiation performance test of the antenna can be done in the microwave darkroom through far-field or near-field methods. The far-field or near-field test of the passive antenna is a mature test method widely used in testing antenna performance. The RRU's RF specifications can be measured in the laboratory by conduction.

Referring to the traditional base station test mode, it is easy to propose a scheme of splitting the active antenna system into a passive antenna array and an RRU to perform antenna radiation performance test and RF conduction test respectively. In fact, according to laboratory testing experience, the beamforming pattern measured by "passive antenna array + power division network + signal source" is integrated with the 5G base station active antenna integrated OTA (Over the Air) test. The results are not consistent. The RF performance conduction test results of the "RRU+ Coupling Board" also differ from the RF radiation indicators measured by the integrated OTA. The reason is that for a 5G base station antenna, the antenna is integrated with the RRU. On the one hand, interference factors such as electromagnetic coupling and active standing wave cannot be completely eliminated; on the other hand, the calibration and amplitude and phase weighting of the active antenna pass through the respective RF channels. The combination of a series of active devices is quite different from the way in which the passive antenna array performs amplitude-weighting through a passive power division network. Therefore, for a 5G base station using massive MIMO active antenna technology, the integrated OTA test mode can effectively reflect its performance indicators. Especially in the millimeter wave band, the frequency band is higher, the device size is smaller, the electromagnetic interference problem is more prominent, and the split test will be very difficult, and only the integrated OTA test solution can be adopted.

The OTA test specification for all RF performance indicators of 5G base stations has been written in the 3GPP 5G new air interface protocol frozen in December 2017, which means that the integrated OTA test of 5G base station antennas will become the main solution for 5G base station hardware performance testing. However, the current OTA test of RF indicators still faces many difficulties. In this paper, the OTA test method of the large-scale active antenna system is deeply studied, and the test is carried out in different fields such as far field, compact field, multi-probe spherical near field and single probe near field. The advantages and disadvantages of each test scheme are compared. The analysis presented the problems faced and the corresponding solutions.

2. 5G base station antenna OTA test solution

The radiated performance of an antenna is typically tested in an OTA manner in its near-field or far-field region. The boundary between the near field and the far field of the antenna radiation is: the spherical wavefront emitted by the source antenna reaches the center of the antenna to be measured and the wave path difference is λ/16. The judgment based on the distance is d=2D 2 /λ, where d is the distance between the probe point and the antenna under test, D is the aperture of the antenna under test, and λ is the wavelength of the electromagnetic wave emitted by the antenna under test.

According to this, the antenna test is divided into two categories: far-field test and near field test, and different test plans will lead to differences in test results. Here are a few classic active antenna OTA test solutions.

(1) Far-field test plan

The far field test is the most direct test method. When the test distance is far enough, the incident wave approximates the plane wave on the receiving surface. Figure 1 shows the far-field test system. The device under test can be rotated 360° in the vertical and horizontal planes. The test probe is fixed in position and can be rotated and rotated. The test system can test the beamforming pattern of the 5G base station antenna and EIRP (Effective Isotropic Radiated Power), EVM (Error Vector Magnitude), occupied bandwidth, and EIS (Effective Isotropic Sensitive). Omnidirectional sensitivity and other RF radiation indicators.

 

Figure1. Far-field test system

(2) Compact field test plan

The compact field test is a far-field test method that uses a mirror or lens to convert a spherical wave from a feed at a focus into a plane wave to achieve far-field testing in a finite physical space. Figure2 shows a parabolic single mirror compact field test system that can test the beamforming pattern of 5G base station antennas and EIRP, EVM, occupied bandwidth, ACLR (Adjacent Channel Leakage Power Ration), Radiofrequency radiation indicators such as EIS and ACS (Adjacent Channel Selectivity).

 

Figure2. Single mirror compact field test system

(3) Multi-probe spherical near-field test scheme

The near-field test acquires amplitude and phase information in the near-field region of the antenna under test and then converts the acquired data into a far-field pattern by a near-far field conversion algorithm. The multi-probe spherical near-field test system is shown in Figure3. A large number of probes are arranged circumferentially in the near field of the device under test, and the measured object only needs to be rotated by 180° to collect the data of the entire radiation sphere. The system can test the beamforming pattern of a 5G base station antenna in CW (Continuous Wave) mode.

 

Figure3. Multi-probe spherical near-field test system

(4) Single probe near field test system

The single-probe near-field test is less efficient than the multi-probe spherical near-field test, but its structure is simpler and requires less space. As shown in the small near-field test system shown in Figure 4, the device under test can be rotated in a horizontal plane, the probe can be rotated in a vertical plane, and the system can collect data of a radiation sphere with the cooperation of two rotating shafts. The system can test the beamforming pattern of the 5G base station antenna in CW mode, and can also test the RF radiation index in the service signal mode, but the processing of the test results needs further analysis.

 

Figure4. Single-probe near-field test system

3. Comparison of advantages and disadvantages of each test plan

The advantage of the far field test is that since the receiving antenna is larger than the far field criterion from the transmitting antenna, the electromagnetic wave is approximated to the plane wave when the transmitting antenna propagates to the receiving antenna, and the collected data does not need to be converted by the far field, and the test device can transmit the high power signal. Test modulated wideband signals, support multi-user testing, and more. The disadvantage is that because the test distance needs to be larger than the far field criterion, the test site has a large area and high construction cost. Taking an antenna with a diameter of 1 m and operating in the 3.5GHz band as an example, the far-field condition is calculated to be 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 about the problem of too much space loss. In addition, since the far field test generally has only one probe, a single test can only draw a section of the antenna radiation sphere. If you want to obtain the 3D pattern of the entire radiation sphere, you need to measure multiple times on different sections, test time. And the cost of testing has increased dramatically.

The advantages of the compact field test are: Significantly reduced site size compared to the far field, which greatly reduces site construction costs and measurement path loss. The test results are closest to direct far-field testing and can test CW waves and business signals. Thanks to the reduced path loss, it can measure more RF radiation than the far field solution. The disadvantage is: similar to the short-field test, the 3D pattern test is less efficient, and the other is the mirror cost and post-maintenance cost.

The advantages of the multi-probe spherical near-field test are: small footprint, single-test giving 3D pattern, high test efficiency, low space loss, and the pattern test results in CW mode are close to the far-field test results. The disadvantage is: the upper limit of the receiving power of the test system is low. When the 5G base station is tested for full power transmission, the receiving device must be pre-fadered; the measurement data needs post-processing for near-far field conversion; the near-far field conversion requires a reference phase. At present, the measurement results in the service signal mode are still unsatisfactory due to the problem of the reference phase.

The advantages of the single-probe near-field test are a small footprint, the low construction cost of the darkroom, simple structure of the turntable, easy installation and disassembly of the device under test, low space loss, and comparison of the test results and far-field test results in CW mode. Close. The disadvantage is: due to structural reasons, the data acquisition of the antenna back flap is incomplete; there is only one test probe, the efficiency of testing the 3D pattern is less than that of the multi-probe sphere; the collected data needs to be followed by near-far field conversion.

4. Problems and solutions

The current OTA test solution, whether it is a far-field solution or a near-field solution, can test the radiation pattern of a 5G base station antenna in CW mode. However, regarding the radiation performance test of radio frequency indicators, the current far-field scheme is limited by the large path loss, and only the parameters with high power levels such as EIRP, EVM, occupied bandwidth, and EIS can be tested. For downlink RF indicators with particularly low power levels, such as ACLR, switching time templates, and spurious emissions, it is difficult to test after a long distance test distance and attenuated to a lower noise level. When measuring the uplink indicator, the interference signal sent by the auxiliary signal source is attenuated by the path of the far field, and it is difficult to reach the power level required for the RF index test such as ACS, in-band blocking, and co-location blocking, 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, the method of taking the reference phase in the broadband service signal mode is still problematic, and the RF radiation test result is still far from the expected value.

Since the indicators required for test verification in the laboratory R&D test phase are comprehensive, the far field test method of compact or loss reduction should be adopted for this type of test. By shortening the far-field test distance, increasing the horn antenna gain, using low-loss RF cables, and shortening the RF line cabling distance within a certain range, the path loss can be greatly reduced, and the far-field scheme can be extended to test RF indexes such as ACLR and ACS. The path loss of the compact field itself is much smaller than that of the far field, and it can measure more RF targets than the far field. However, there are still some RF indicators that are particularly low due to their own power. How to reduce 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, space is small, and the typical index can be tested. The single-probe near-field test scheme is more suitable. As for the future 5G high-band test, due to the higher frequency and more serious loss, far-field testing will become less suitable, and conduction testing will be more difficult, requiring a combination of near-field testing inductive near-field testing. The far-field conversion algorithm requires a reference signal, which requires the equipment manufacturer and the measurement instrument manufacturer to solve the problem of taking reference signals from the device.

5. The conclusion

This paper studies the large-scale MIMO active antenna OTA test method for 5G base stations. Using the 5G base station equipment of the unit to study different OTA test schemes such as far field, compact field, multi-probe near field and single probe near field, the construction cost, test capability and test efficiency of each site were analyzed. The problems faced in the test and the corresponding solutions are proposed, which provide a reference for current and future 5G base station antenna OTA testing.