On the basic principle and test method of TD-SCDMA smart antenna
1 Introduction
TD-SCDMA, one of the third-generation mobile communication system standards, uses two of the most critical technologies, namely smart antenna technology and joint detection technology. Among them, the role of smart antennas for the system mainly includes:
(1) Through the maximum ratio combining of multiple antenna channels and array signal processing, the receiving sensitivity is significantly improved;
(2) The beamforming algorithm makes the base station highly directional for the reception and transmission of different users, so the interference between users can be well isolated in space;
(3) The spatial isolation of the interference between users by beamforming significantly increases the capacity of CDMA. Combined with the joint detection technology, TD-SCDMA can achieve full code channel configuration;
(4) The beam shaping algorithm can achieve flexible adjustment of the broadcast beam width, which allows the adjustment of the cell broadcast coverage of the TD-SCDMA network optimization process through the software algorithm (the broadcast beam of the conventional base station antenna is fixed and immutable) , If you want to adjust the coverage area, you must replace the antenna), which significantly improves the efficiency of network optimization;
(5) By beamforming the antenna array, the downlink signal can be aimed at one (or several users at different positions) users, which is equivalent to increasing the effective transmit power (EIRP) of the transmitter.
The CDMA system uses a high-power linear power amplifier, which is relatively expensive; the TD system using smart antenna technology can use multiple low-power amplifiers, thereby reducing manufacturing costs.
2 Basic working mechanism
According to the implementation of beamforming and the current application situation, smart antennas can usually be divided into multi-beam smart antennas and adaptive smart antennas.
The multi-beam smart antenna adopts the quasi-dynamic pre-multi-beam beam switching method, and uses multiple beams with different fixed directions to cover the entire cell. As the user moves in the cell, the base station selects the most suitable beam to enhance the strength of the received signal . The advantages of the multi-beam smart antenna are low complexity and high reliability, but the disadvantage is that it is greatly affected by parameters such as the antenna beam width, and its performance is worse than the adaptive smart antenna.
The adaptive smart antenna adopts the fully adaptive array automatic tracking method, and adjusts the weighting value of each antenna unit through different adaptations to form several adaptive beams and track several users at the same time, so as to maximize the current propagation environment. match. Adaptive smart antennas can achieve optimal performance in theory, but their implementation structure and algorithm complexity are significantly higher than multi-beam smart antennas.
The TD-SCDMA system uses an adaptive smart antenna array. The design of the antenna array unit, the downlink beamforming algorithm and the uplink DOA estimation are the core technologies of the smart antenna.
The realization principle of smart antenna array is similar to phased array antenna. Below we take the one-dimensional linear array phased array antenna as an example.
First, as the most basic one-dimensional beam scanning phased array antenna is a linear array arranged at equal intervals (see Figure 1), where the excitation phase of each radiation element of the array can be changed, that is, when the excitation phase of adjacent radiation elements When there is a specific equidistant series change, the array pattern is achieved by adjusting the amplitude and phase excitation of each column of antenna elements to achieve beam scanning.
Fig.1 The principle of one-dimensional linear array antenna beam scanning
When the maximum pointing direction of the beam deviates from the normal direction by θ0, the excitation wave path difference of each antenna port is: ФN = (N-1) 2πd sinθ0 / λ
Where: d is the distance between adjacent units, and λ is the wavelength of the antenna operating frequency
The smart (adaptive) antenna system is based on an array antenna and an adaptive signal processing algorithm. It can distinguish useful signals from multiple multipath signals and interfering signals, and automatically lock the maximum value of the main lobe to useful moving waves. In the signal direction, and automatically reduce the sidelobe level of the interference direction. The precise tracking capability and interference suppression capability of the smart antenna can enable several users in the same cell to use the same channel.
The working mechanism concept of the smart antenna system can be described using Figure 2 and Figure 3. In Figure 2, after the signals received by the N antenna radiating units are amplified by radio frequency, the complex weight coefficients of Wi are weighted in the baseband digital beamforming (DBF) network and superimposed and synthesized, and then enter the receiver, which is processed by DSP intelligent algorithm The device predicts the direction of the useful signal based on the amplitude / phase relationship of the incoming waves of N antenna radiating elements. The superimposed synthesis obtains the largest received signal. In Fig. 3, the DSP can adaptively generate suitable Wi complex weight coefficients according to the predicted useful signal direction and the predicted interference signal direction, and stimulate the radiation of each antenna element, thereby aligning the main lobe plate with the useful signal and setting the zero point Cancel.
Figure 2 Smart antenna uplink receiving principle
Figure 3 The principle of smart antenna downlink reception
A typical smart antenna array is shown in Figure 4. It has 9 ports in total, the middle port is the calibration port, and the remaining 8 ports are antenna ports. The function of the calibration port is used to correct the phase difference between each receiving (transmitting) channel of the smart antenna array in the actual application environment and each column of the antenna port surface, and the other eight ports are respectively connected to the transceiver channel of the base station.
Figure 4 Typical directional smart antenna array
3 Main test parameters and typical test methods
Because smart antenna testing is much more complicated than ordinary antennas, testing of smart antennas is also more complicated. Taking the smart antenna array shown in Figure 4 as an example, we can divide the measurement of the antenna into 2 categories: circuit parameter measurement and radiation parameter measurement.
The circuit parameters include: input impedance of each port, isolation of adjacent antenna unit ports, active reflection coefficient of each antenna port, and amplitude and phase consistency from the calibration port to each antenna unit.
Radiation parameter tests include: the directional pattern and gain of each antenna unit, the directional pattern and gain of typical service beams; the directional pattern and gain of broadcast beams.
Since the circuit parameter index is a mandatory test index for smart antennas, let's focus on the circuit parameter test items and test methods of smart antennas. The circuit parameter test of an 8-unit single-polarized smart antenna array includes:
(1) The isolation of adjacent ports, that is, the characteristics of S12, S23, S34, S45, ... S78 (excluding the calibration port);
(2) The amplitude and phase consistency between the calibration port and each antenna unit, that is, the amplitude and phase characteristics of S01, S02, ..., S07, S08 (Mag | S01 |, | Mag | S02 |, Mag | S03 |, Mag | S04 | , Mag | S05 |, Mag | S06 |, Mag | S07 |, Mag | S08 |; Pha | S01 |, Pha | S02 |, Pha | S03 |, Pha | S04 |, Pha | S05 |, Pha | S06 | , Pha | S07 |, Pha | S08 |);
(3) The passive reflection coefficient (or passive return loss) of each antenna port, that is, the characteristics of S00, S11, S22, ..., S33, S88;
(4) The active reflection coefficient (or active return loss) of each antenna port, considering the mutual coupling between the units and the amplitude and phase excitation of each unit.
According to the following S-parameter excitation matrix model
(2.1)
The active reflection coefficient of each port can be derived as
(2.2)
When performing beam scanning, the source is phase-weighted. Typical values ​​for testing are given in a group:
(2.3)
For general antenna testing, a 2-port vector network analyzer can be used. The smart antenna has 8 antenna ports and one calibration port, and its test items and test complexity are much higher than ordinary antennas, so the general 2-port or 4-port vector network is difficult to meet its test requirements. However, in order to ensure the performance of smart antennas, the test items mentioned above are often items that must be tested during antenna development and production. Therefore, we need to find a fast and comprehensive measurement solution.
Rohde & Schwarz (R & S) ZVT is the industry's only 8-port vector network analyzer. It has built-in 4 independent sources, 16 independent receiving channels, and has extremely fast measurement speed, so it is the best choice for smart antenna and phased array antenna testing (see Figure 5). It can complete one S88 full matrix test at a time, which is impossible for 2-port and 4-port vector networks.
Figure 5 Testing the smart antenna with R & S's 8-port vectorial ZVT
(2.4)
For the first and second tests, R & S ZVT can be completed in one go.
For the third test (a total of 9 ports are required), the R & S ZVT can be completed in two steps (as shown in Figures 6 and 7), and combined with Trace Math (trajectory calculation, arbitrary calculation of multiple trajectories, To expand the measurement function) function, you can calculate and display the amplitude / phase consistency of each channel in real time (as shown in Figure 8).
Figure 6 R & S ZVT test for smart antenna amplitude and phase consistency (step 1)
Figure 7 R & S ZVT test for smart antenna amplitude and phase consistency (step 2)
Figure 8 Typical amplitude consistency test results (calibration port to each antenna port)
For the fourth test, with the help of the powerful Trace Math function of R & S ZVT, the θ in formula (2.3) can be programmed into the formula editor of ZVT, and the full matrix (2.4) measured by R & S ZVT can be displayed in real time. Typical measurement results of the active reflection coefficient of each port are shown in Figure 9 and Figure 10:
Figure 9 Active reflection coefficient of smart antenna port 1 (k * d * sinθ = π / 3 condition)
Among them: k = 2 * π / λ, d = the distance between adjacent antenna elements (these two items are constant);
θ is the scan angle of the synthesized beam of the smart antenna (this item is a variable)
Figure 10 Active reflection coefficient of smart antenna port 1 (k * d * sinθ = π / 5)
Among them: k = 2 * π / λ, d = the interval between adjacent antenna units (these two items are constant); θ is the scan angle of the smart antenna synthesized beam (this item is a variable).
It can be seen from Figure 9 and Figure 10 that the ZVT's 8 ports and the powerful Trace Math function can display the active reflection coefficient of each port at any scan angle in real time, which provides great opportunities for the development and production test of smart antenna systems. convenient.
4 Conclusion
Smart antennas are much more complex than ordinary antennas, and the performance evaluation of smart antenna systems is also more complicated. In the research and development and production stage, the smart antenna must be fully tested, so that its performance can be comprehensively evaluated, and the advantages of the smart antenna can be brought into full play. It is difficult to test smart antennas comprehensively and quickly using a general 2-port or 4-port vector network. R & S's ZVT has 8 ports and has a powerful Trace Math function, so it can meet the test requirements of smart antennas and can help antenna manufacturers to quickly and comprehensively test their smart antennas.
Attachment: Explanation of related terms
Polarization: refers to the trajectory of the electric field in space. When the trajectory of the electric field is a straight line, it is called linear polarization; when the trajectory of the electric field is a circle (or ellipse), it is called circular (or elliptical) polarization. Linear polarization is divided into vertical polarization (that is, the polarization direction is perpendicular to the ground) and horizontal polarization; circular polarization is further divided into left-hand circular polarization and right-hand circular polarization (using the right-hand rule).
Antenna gain: refers to the ratio of the radiated power of an antenna at a certain point in space relative to the radiated power of an ideal point source (no directional antenna, which actually does not exist) at that point.
Active reflection coefficient: For a multi-port antenna (or microwave device), the reflection coefficient of a certain port under the condition that several other related ports are excited.
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