Test items and test methods for smart antennas-EEWORLD

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1. Introduction
　　The research on smart antenna technology began in the 1960s. The initial research object was radar antenna array, with the main purpose of improving the performance of radar and the ability of electronic countermeasures. With the development of mobile communications and the gradual deepening of research on mobile communication radio wave propagation, networking technology, antenna theory, etc., the processing capacity of digital signal processing chips has been continuously improved, and it has become possible to form antenna beams in the baseband using digital technology. In the 1990s, array processing technology was introduced into the field of mobile communications, and a new research hotspot, smart antenna, was soon formed. Among them, China has successfully introduced smart antenna technology in the TD-SCDMA technology with independent intellectual property rights. To some extent, it can be said that smart antenna is one of the key features that distinguish 3G from 2G systems. Smart antenna
　　uses digital signal processing technology to generate spatial directional beams, so that the main beam of the antenna tracks the direction of arrival of the user signal, and the side lobe or null is aligned with the direction of arrival of the interference signal. It uses the orthogonality of multiple antenna units in space and the difference in the transmission direction of the signal to distinguish the signals with the same frequency or time slot and the same code channel, and maximize the use of limited channel resources. It has unique advantages in improving the communication quality of the system, alleviating the contradiction between the growing development of wireless communication services and the shortage of spectrum resources, reducing the overall cost of the system and improving system management.
　　Since smart antennas have so many benefits, as the commercialization of TD-SCDMA systems is getting closer,
smart antenna technology, as one of the key technologies of TD-SCDMA systems, has also received more and more attention. Therefore, the test method of smart antennas is also very important.
　　2. Classification of smart antennas
　　Smart antennas can be divided into omnidirectional smart antenna arrays and directional smart antenna arrays according to their types.
　　For directional smart antenna arrays, the following three types of test parameters are included.
(1) Circuit parameters. Including the preset value of the vertical electrical downtilt angle, the vertical electrical downtilt angle accuracy, and the vertical mechanical downtilt range; input impedance, standing wave ratio of each unit port, isolation of adjacent unit ports, and continuous wave power capacity of each port. (2
) Calibration parameters. Including the coupling degree from the calibration port to each unit port, the maximum amplitude deviation from the calibration port to each unit port, the maximum phase deviation from the calibration port to each unit port, the standing wave ratio of the calibration port, and the coupling directivity of the calibration channel.
(3) Performance parameters. Including active input return loss of each unit port, vertical half-power beam width, vertical upper first sidelobe suppression and lower first zero filling; unit beam horizontal plane half-power beam width, gain, front-to-back ratio, cross-polarization ratio (axial) and cross-polarization ratio (within ±60°); service beam horizontal plane half-power beam width, boresight gain, horizontal plane sidelobe level and front-to-back ratio, broadcast beam horizontal plane half-power beam width, boresight gain, level drop at boresight gain Φ=±60°, and level fluctuation within half-power beam width.
　　For omnidirectional smart antenna arrays, they can also be divided into three categories of test parameters.
(1) Circuit parameters. Including vertical plane electrical downtilt preset value, vertical plane electrical downtilt accuracy; input impedance, standing wave ratio of each unit port, isolation of adjacent unit ports, and continuous wave power capacity of each port.
(2) Calibration parameters. Including coupling degree from calibration port to each unit port, maximum amplitude deviation from calibration port to each unit port, maximum phase deviation from calibration port to each unit port, standing wave ratio of calibration port, and coupling directivity of calibration channel.
(3) Performance parameters. Including active input return loss of each unit port, vertical half-power beam width, vertical upper first sidelobe suppression and lower first zero filling; unit beam horizontal half-power beam width, gain, front-to-back ratio, cross-polarization ratio (axial) and cross-polarization ratio (within ±60°); service beam horizontal half-power beam width, boresight gain, horizontal sidelobe level, broadcast beam boresight gain and pattern circularity.
　　3. Test items and test methods of smart antennas
　　The following introduces the test items that are different from ordinary antennas.
　　First, smart antennas have an additional calibration port compared to ordinary antennas, mainly to dynamically calibrate the consistency of the amplitude and phase of each unit port. The accuracy of calibration directly determines the application effect of smart antennas. Therefore, corresponding requirements are put forward for the maximum amplitude deviation from the calibration port to each unit port and the maximum phase deviation from the calibration port to each unit port. During the test, the calibration port and each feed port form a calibration channel. The phase/amplitude error is measured for any port. The maximum deviation between all measured values at the same frequency point is taken to obtain this indicator.
　　The measurement diagram of the calibration circuit parameters is shown in Figure 1.

Figure 1 Schematic diagram of antenna calibration circuit measurement
The measurement steps are as follows:
(1) Install the antenna under test in free space or simulated free space that meets the measurement conditions;
(2) Perform system calibration according to the measurement system requirements;
(3) Connect the measurement system to the calibration port and the i-th feeding port of the antenna under test, and connect the remaining ports of the antenna under test to matching loads, and measure the transmission coefficient S (i, CAL) within the operating frequency range;
(4) Repeat step (3) to test the S (i, CAL) values of all ports.
After measuring the transmission coefficient S (i, CAL) from the calibration port CAL to multiple radiation ports i, calculate the modulus and phase angle of all tested S (i, CAL) values, draw all the modulus curves and phase angle curves in two graphs, compare and calculate the maximum modulus (i.e. amplitude) deviation and phase deviation.
　　Secondly, the active input return loss of each unit port is calculated.
　　The difference between active input return loss and ordinary return loss is that it is the return loss when there is an input signal at each unit port and beams in different directions are formed. Therefore, it is called active input return loss. The measurement diagram is shown in Figure 2.

Figure 2: Active return loss measurement diagram

The steps for indirect measurement of active input return loss are as follows:

1) Install the antenna under test in free space or simulated free space that meets the measurement conditions;
(2) Perform system calibration according to the measurement system requirements;
(3) Connect the measurement system to the i-th feeding port of the antenna under test, and connect the remaining ports of the antenna under test to matching loads. Measure the complex reflection coefficient Sii within the operating frequency range. The measured Sii reading is the self-reflection coefficient of the i-th feeding port;
(4) Connect the measurement system to the i-th and j-th feeding ports of the antenna under test, and connect the remaining ports of the antenna under test to matching loads. Measure the transmission coefficient Sij within the operating frequency range. The tested Sij reading is the transmission coefficient from the j-th feeding port to the i-th feeding port;
(5) Repeat steps (3) and (4) to measure the values of Sii and Sij of all ports;
(6) According to the matrix formula: =[S][a], the reflected signal bi corresponding to any amplitude/phase excitation ai can be calculated, and thus the complex reflection coefficient Γi=bi/ai of the i-th radiating port can be calculated. According to the complex reflection coefficient, the corresponding active input return loss of the i-th feeding port can be calculated as 20lg(Γi);
(7) Calculate the maximum value of the active return loss of all radiating ports;
(8) Repeat step (6) to give the amplitude/phase excitation ai with scanning angles of 0°, ±30°, ±45°, and ±55°, calculate the corresponding active return loss, and repeat step (7) to calculate the maximum value of all active return losses.

　　Third, smart antennas add the concepts of unit beams, broadcast beams, and service beams compared to other antennas.

　　A unit beam refers to
the radiation pattern emitted or received by any feed port in a smart antenna array when all other ports are connected to matching loads. For smart antennas, the unit beam index requirements are not much different from those of ordinary antennas, so they will not be introduced in detail here.

　　A broadcast beam refers to a radiation pattern with omnidirectional or sector coverage formed by applying specific amplitude and phase excitation to a smart antenna array.

　　For directional smart antennas, the broadcast beam can be divided into 30°, 65°, 90° and 100°, corresponding to the coverage requirements of different sectors. For omnidirectional smart antennas, the broadcast beam should cover 360°, so corresponding requirements are put forward for its circularity.

　　Different antenna manufacturers have different processes and designs, and the amplitude and phase weighting coefficients of broadcast beams are also different. Therefore, antenna manufacturers are required to provide corresponding amplitude and phase weighting coefficients for different broadcast beams.

　　The service beam refers to a directional pattern with arbitrary beam pointing scanning and high-gain narrow beam within the working angle domain formed by applying specific amplitude and phase excitation to the smart antenna array.

　　The first type of beam of a directional smart antenna refers to the beam obtained by inputting equal-amplitude and in-phase signals into the antenna port; the other type is the gain obtained when the excitation amplitude of each column unit is uniform and the excitation phase increases linearly (the differential phase is defined as, where: is the wavelength of the center frequency of the working frequency band, d is the horizontal spacing between adjacent columns, = 60°).

　　For the first beam of the omnidirectional smart antenna, according to the following formula:

Where i=1, 2, ...N, N=8 (for an 8-column array).

　　The amplitude and phase of the corresponding antenna port are calculated, and then excitation is performed to obtain the first beam, where is the center frequency of each working frequency band.

　　Taking gain measurement as an example, the testing of unit beam, service beam and broadcast beam can all adopt the test block diagram shown in Figure 3.

Figure 3 Antenna gain test diagram

The test conditions are as follows.
(1) The antenna under test and the source antenna have the same polarization mode.
(2) The measurement distance between the antenna under test and the source antenna should satisfy

Where: L - distance between source antenna and antenna under test (m);
D - maximum size of antenna under test (m);
d - maximum radiation size of source antenna (m);
—— wavelength of test frequency (m).
(3) The antenna under test should be installed in an area where the field strength is basically uniform. The field strength should be pre-detected in the effective antenna volume of the antenna under test using a half-wave dipole antenna. If the electric field variation exceeds 1.5 dB, the test field is considered unusable. In addition, the difference in field strength measured by the gain reference antenna on two orthogonal polarization planes should be less than 1 dB.
(4) Measurement equipment and instruments such as signal generators and receivers should have good stability, reliability, dynamic range and measurement accuracy to ensure the correctness of the measurement data. Measuring instruments should have metrological certificates and be within the calibration cycle.

　　Before the measurement begins, the antenna array amplitude-phase weighted feeding network corresponding to the measurement parameters should be prepared. While confirming its amplitude-phase weighted value, the modulus |Si,j| (dB) of the transmission coefficient from the total feeding input port to each array unit input port should be measured separately when the non-tested network unit is terminated with a matching load, and the formula is used:

(where N is the number of array unit feeding ports), calculate the insertion loss Ln of the antenna array weighted feeding network corresponding to the measured parameters.

　　When starting the measurement, the antenna under test and the gain reference antenna must be adjusted alternately in horizontal and elevation to ensure that each antenna is optimally pointed in the horizontal and elevation directions so that its received power level is the maximum.

　　The measurement steps are as follows.
(1) The gain reference antenna is aligned with the source antenna, and the gain reference antenna is connected to the receiver through switching. At this time, the receiver receives a power level of P1 (dBm).
(2) The antenna under test is connected to the receiver through a feed network with the required weighting value of the corresponding feed port, and then it is aligned with the source antenna through measurement adjustment. At this time, the receiver receives a power level of P2 (dBm).
(3) Repeat steps (1) and (2) until the repeatability of P1 and P2 measurements reaches an acceptable level.
(4) The gain G of the antenna under test at a certain frequency point is calculated according to the following formula:
G=G0+(P2-P1)+N Where:
G0 is the gain of the reference antenna (dBi);
N is the corrected value of the path loss from the receiver input to the output of the antenna under test and the gain reference antenna after taking into account the insertion loss Ln of the corresponding antenna array weighted feed network (dB).

(where N is the number of array unit feeding ports), calculate the insertion loss Ln of the antenna array weighted feeding network corresponding to the measured parameters.

5) Within the same operating frequency band, measure the high, medium and low frequency points and calculate the decibel average.
(6) According to the different gain definitions in the electrical performance requirements, set the amplitude and phase weighted values of each output port of the array feed network, first measure the corresponding insertion loss of the feed network, and then repeat steps (4) and (5) to perform corresponding gain tests respectively.

　　The performance criteria are:
for each working frequency band, the gains of the three frequency points of high, medium and low are tested. The average value should meet the requirements of the gain index, and the worst value of the gain of the three frequency points of high, medium and low cannot be less than the gain index of 1.0, otherwise it is judged as unqualified.

　　The measurement methods of the radiation pattern circularity (omnidirectional antenna), half-power beamwidth, front-to-back ratio, cross-polarization ratio and antenna electrical downtilt angle can also be carried out by referring to the gain test block diagram and test steps, which will not be introduced in detail here.

　　4. Summary

　　The complexity of smart antenna testing is much more complicated than that of ordinary antennas. Only by completing the above tests can the performance of smart antennas be comprehensively evaluated and the advantages of smart antennas be brought into play.

Reference address：Test items and test methods for smart antennas

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