IC Substrate - The test and measurement industry strives to solve the OTA test problem under 5G

In June 2018, 3GPP officially approved the R15 standard. Before the end of the same year, 5G commercial networks were quickly established in the United States (Verizon and AT&T) and South Korea (KT, LG UPlus, and SK Telecom). In 2019, more 5G networks will be launched in the entire telecommunications industry, and the focus will shift from LTE to 5G. As the 5G standard has not yet been fully determined, global base station and mobile phone manufacturers, wireless operators, and regulatory agencies must quickly gather together and reach an agreement on how to install, verify, and maintain 5G commercial networks. At this important time, this magazine interviewed nine leading companies in the test and measurement industry, summarizing the current challenges and solutions faced by OTA testing under 5G. These companies include Anritsu, EMITE, ETS-Lindgren, Keysight, MVG, National Instruments (NI), NSI-MI, Rohde & Schwarz (R&S), Boonton, Noisecom.

5G test challenges
Anritsu said that the primary problem is that there are fundamental differences between the test technologies used in 5G and LTE, such as millimeter wave frequencies, large-scale antenna arrays, beamforming, and dynamic physical layer properties, so mechanical methods are not feasible.

Countries around the world have adopted different frequency bands for 5G deployment. In addition to complying with 3GPP's 5G air interface (NR) standards, most of them also require compliance with local government regulations.

R&S mentioned in a recent article to this journal that 5G deployment depends on the performance of a highly integrated solution that integrates modems, RF front-ends and antennas. The current difficulty lies in how to open up new methods and provide new instruments for performance evaluation, because the number of RF test ports is decreasing, and beam control technology requires system-level testing. In this case, the performance parameters of the antenna and transceiver must be measured through OTA: effective isotropic radiated power (EIRP), total radiated power (TRP), effective isotropic sensitivity (EIS), total isotropic sensitivity (TIS), Error Vector Magnitude (EVM), Adjacent Channel Leakage Ratio (ACLR) and Spectrum Emission Mask (SEM) are some of the key indicators required.

R&S also pointed out that for this series of OTA evaluations, the key issue of measuring distance needs to be considered. We usually measure antenna characteristics in the far field (see Figure 1). If the far-field direct detection is adopted and the Fraunhofer distance criterion (R = 2D2/λ) is applied, the side length of the chamber required to test a large MIMO device under test with a radiation frequency of 2.4GHz and a size of 75 cm needs to be at least 9 meters . Even a smart phone with a length of only 15 cm and a transmission frequency of 43.5 GHz requires a test distance of 6.5 meters. This distance ensures that the device under test can be located in the quiet zone, that is, the area wrapped by the flow field of the impinging stream that is sufficiently uniform and approaching a plane wave with a phase difference of less than 22.5°.

One way to overcome the space constraints of the chamber is to use a reflector whose parabolic shape can project the incident spherical wavefront into a plane wave. This kind of reflector is widely used in millimeter wave OTA test equipment, called compact antenna test field

Anritsu claims that gated scanning can help measure EIRP well. With the help of gated scanning, users can determine which part of the 5G signal transmission process to measure. This is very important because 5G-NR signals can use 55 different TDD Tx/Rx ratios for time slot configuration within a 10ms frame. By selecting only specific subframes or symbols, users can ensure that only the radio frequency of the downlink is measured, which can more accurately reflect the radio frequency energy radiated into the atmosphere.

ETS-Lindgren and Anritsu also believe that significant changes are needed to carry out effective EMC testing of 5G equipment. Regulatory standards usually require the measurement of TRP to ensure that the radio transmission power will not be too large. At this time, the signal is sent by an isotropic transmitter that radiates energy uniformly in a certain sector of LTE, so it is easy to measure the total radio power and determine whether the energy in the air is within a safe range. ETS-Lindgren emphasized the difficulty of beamforming, as shown in Figure 3. Since the signal here is directional, we cannot easily measure the energy of any point, let alone know how much power is radiated into the atmosphere. Considering the side lobes and back lobes, the only way to measure TRP is to integrate the power into a 360° sphere wrapped around the antenna. Although this method is feasible, it takes time and consumables.

Anritsu also pointed out that as the entire industry gradually unifies to the optimal installation and maintenance program, the next challenge will be to formulate the test process and determine the test equipment to ensure that it is as accurate, efficient and economical as possible. This requires test vendors to quickly respond to test requirements and prepare a new generation of hardware equipment to meet the challenges.

OTA test method
Keysight explained the test method in detail for us, and stated that the most important thing when formulating an OTA test plan is to fully understand the test object and the required test content, as well as the test methods applicable to different test cases. In the actual consumer market, modems, antennas, subsystems, and fully assembled end-user equipment will all be tested. The base station test will also be a similar process. From the R&D stage, to conformance and final equipment acceptance testing constitutes a typical test cycle.

Generally, testing can be divided into conformance testing and performance testing. If you want to release a new device, you must conduct a conformance test. It is a key requirement that requires us to connect the equipment to the wireless test system and complete the required 3GPP test content:

·RF transceiver performance-the lowest level of signal quality

· Demodulation-data throughput performance

·Radio Resource Management (RRM)-Initialization, Handover and Mobility

·Signaling--Upper layer signaling process

Keysight believes that modem chipsets, antennas, base stations and integrated devices require mixed application conduction and OTA testing. Most of the tests in frequency range 1 (FR1: 450 MHz to 7.125 GHz) will use the conduction scheme, and 3GPP has stipulated that all conformance tests in frequency range 2 (FR2: 24.25 to 52.6 GHz) adopt OTA test methods.

Keysight said that 3GPP has approved the following three OTA test methods so far:

· Direct far field method (DFF): The measurement antenna is placed in the far field. The far-field distance (Fraunhofer distance) starts at 2D2/λ, where D is the maximum diameter of the radiating element and λ is the wavelength. Reaching this distance means that the angular field distribution no longer changes. The direct far-field method can carry out the most comprehensive test and can measure multiple signals, but at the same time the millimeter wave frequency band will also cause the test field to be larger.

·Indirect far-field method (IFF): Create a far-field environment through physical conversion, generally using a parabolic reflector to collimate the probe antenna to transmit signals. This method is usually implemented by CATR. Although it can only be used to measure the arrival/departure angle of a single signal, the distance is much shorter and the path loss is smaller.

·Near-field to far-field method (NFTF): sample the phase and amplitude of the electric field in the radiation near-field area, and calculate the far-field pattern. This method is also only applicable to the measurement of a single LOS transceiver.

According to R&S, as of early January 2019, 3GPP has specified a series of transmitter and receiver tests in the TS38.521-3 standard. This standard is the normative regulation for NR user equipment wireless transmission and reception consistency, in which "-3" refers to the third part, which determines the working conditions of FR1, FR2 and LTE intercommunication, that is, NSA sub-6GHz and NSA millimeter wave frequency bands. Because the millimeter wave frequency band test needs to be realized through OTA, and construct the dark room to finish, so the difficulty is getting more and more serious. This means that the achievable measurement uncertainty (MU) and test tolerance (TT) will be much larger than the results of the sub-6GHz FR1 conduction test. 3GPP has been discussing how much MU and TT can be accepted under FR2. Until a definite conclusion is reached, it is not yet possible to establish a standard FR2 radio frequency conformance test.

Regarding the SA deployment situation, the corresponding first (sub-6 GHz) and second part (millimeter wave) of the 38.521 standard have been more detailed specifications, although the first batch of 5G NRs that will be deployed at the beginning of this year are NSA. In addition, the performance test standards (38.521-4) and RRM test requirements (38.533) under the NSA are not yet complete.

Table 1 is drawn by NSI-MI and summarizes the applicability of each test environment under different test types and antenna sizes, and uses colors to distinguish the quality of the solution. The consideration factors include signal-to-noise ratio, utility, and cost.
Following the release of the Field Master™ Pro MS2090A at the MWC Conference in Barcelona in February 2019, Anritsu launched the first portable 5G NR measurement instrument in the field, continuously covering sub-3 GHz, sub-6 GHz and millimeter wave frequency bands. . The R&D process of Field Master Pro MS2090A has been closely supported by a group of leading 5G base station manufacturers and was used to install the first commercial 5G NR network. A handheld device with such powerful functions is sure to have a big impact on the testing industry.