“Radiation Two-Stage (RTS)” MIMO OTA
As the core technology for wireless network expansion in the evolution of 4G standards, MIMO technology can effectively suppress channel fading, significantly improve system throughput and transmission distance under the same bandwidth and transmit power, and improve spectral efficiency and transmission reliability through space division multiplexing.
The actual transmission data rate of a MIMO communication system depends on many factors. In addition to the influence of the spatial propagation environment, the performance of the MIMO terminal, such as antenna performance, sensitivity distortion, etc., also has a decisive influence on the transmission rate. Therefore, the importance of OTA testing of MIMO terminals is self-evident. It is not only used by mobile operators as the basis for testing the performance of mobile terminals and issuing terminal network licenses, but also as an important technical means for terminal manufacturers in the process of R&D and quality control. In recent years, the US Wireless Communications and Internet Association (CTIA), 3GPP, and the European Organization for Cooperation in Science and Technology (COST) have been working on the standardization of MIMO OTA testing. The Radiated Two-Stage Method (RTS), jointly proposed by GTS and Keysight, has become one of the three standard methods proposed by 3GPP.
Different from the OTA test of the SISO system, the MIMO channel model must be introduced and implemented for the OTA test evaluation of the MIMO terminal. How to simulate the real wireless propagation environment so that the test and evaluation results of the OTA test system can reflect the actual effect in the real environment is the main technical problem of MIMO performance testing.
The three current standard methods are the multi-probe method (MPAC), the radiation two-stage method (RTS), and the reverberation chamber method with integrated channel emulator (RC+CE). In contrast, the radiometric two-stage method (RTS) is the only solution that can be implemented in traditional SISO OTA test chambers, which not only has low system cost, simple and convenient calibration and maintenance procedures, but also enables rich channel models. As long as an external 2-channel channel emulator and a 4G comprehensive tester are integrated, such as Keysight’s UXM, a pure software upgrade of the traditional SISO darkroom can be performed without building a new darkroom, which greatly saves the cost of system purchase and space use. Radiated Two-Stage (RTS) not only enables MIMO throughput testing, but also measures antenna radiation pattern information that is critical to the development and production of MIMO terminals. Due to its high speed and high flexibility, the radiation two-stage method (RTS) has become the best choice for engineers to verify the R&D design of MIMO terminals. In addition, the measurement of absolute radiated data throughput (a new mandatory MIMO metric added by 3GPP/CTIA) is based on the radiometric two-stage method (RTS), so there is no obstacle to the future standardization of the radiometric two-stage method (RTS).
The name of the radiation two-stage method (RTS) is only a representation of the physical process of the test. In the actual test, thanks to the design of the automated test software, it is completed in one go without interruption.
Introduction to Radiation Two-Stage (RTS)
In Radiated Two-Stage (RTS) testing, the first stage uses a conventional SISO anechoic chamber to measure the 2D or 3D antenna pattern of the DUT. In the second stage, the channel emulator integrated in the UXM combines the LTE signal generated by its base station emulator with the antenna pattern measured in the first stage and the selected LTE channel model. The downlink signal is received by the two antennas of the MIMO terminal through the air interface, enters the input port of the receiver, and then the uplink signal returns to the UXM to measure the throughput performance of the LTE device. There is no need for any large measurement chambers in the second stage, eliminating a major bottleneck for pre-qualification testing.
Channel Decoupler – Obtaining the Inverse Matrix of Spatial Transmission
It should be specially pointed out that in the channel emulation test of the second stage, the downlink two base station signals should theoretically be independently added to the two receiver input ports of the terminal. It can also be achieved by conducting connection, but it cannot meet the requirements of OTA to test the performance of the whole machine, such as the impact of sensitivity distortion on throughput. In the actual test, the method of the air interface must be used, that is, the two signals transmitted in space must be decoupled. The radiation two-stage method (RTS) achieves the required isolation level precisely by realizing decoupling. The general test adopts the independent patented technology Radiation Two-Stage Method (RTS) as the MIMO OTA test solution.
Advantages of air interface two-stage method
After the antenna radiation pattern information of the MIMO terminal is measured in the first stage, it is jointly imported into the channel simulator in combination with the selected MIMO channel model. In the second stage, the downlink signal passes through the GTS patented channel decoupler, and then is sent to the MIMO terminal through the air interface, avoiding the uncertainty caused by the cable conduction measurement, and distorting the sensitivity of the terminal antenna and self-interference. This is reflected in testing, further improving measurement accuracy and completeness.
Under the RTS scheme configuration, the two-stage measurement is carried out in the same microwave anechoic chamber, and the whole process is completed in one go. The two-stage test only needs to be completed in one step, which greatly reduces the difficulty of operation and improves the measurement efficiency. For R&D needs, the radiation two-stage method (RTS) can not only achieve accurate throughput measurement, but also provide a large amount of variable separation information and support the addition of arbitrary interference sources, helping R&D personnel to locate problems accurately and quickly.
Radiation two-stage method (RTS) has the characteristics of high precision, high flexibility and low cost, and is a MIMO OTA throughput test solution that can meet the requirements of certification and R&D at the same time.
Radiation Two-Stage Method (RTS) vs Multi-Probe Method (MPAC)
Consistent with the multi-probe method, the radiation two-stage method (RTS) can accurately measure the throughput of the whole machine in the air interface state in a stable time and space state. The difference is that the radiation two-stage method (RTS) adopts a more flexible, consistent and effective mathematical method to simulate all the channel models defined by 3GPP/CTIA. So it also has the following advantages:
- 2D and 3D channel models other than those defined by 3GPP can be simulated flexibly. Researchers can also flexibly access actual channel sampling information obtained from field measurements.
- Supports simulated drive tests in the lab.
- It greatly reduces the requirement on the number of channels of the channel emulator and reduces the cost of the instrument.
- Reusing existing SISO chambers greatly reduces measurement costs, as well as the complexity of system maintenance and calibration.
From Methods To Standards
The RTS method is a proprietary technology of GTS. From 2013, when the RTS method was first proposed, to today, a review of the nearly ten-year history of RTS.
In 2013, General Test proposed the RTS method and wrote it into 3GPP TR 37.977.
In 2015, the inverse matrix solution in the RTS method was automated and the RSRP and RSARP return errors were eliminated.
In 2016, an RTS-based diagnostic measurement scheme was proposed.
In 2017, the RTS method was certified as a 3GPP standard.
In 2018, the RTS method was written into 3GPP TS 37.544.
In 2019, the throughput rate model and the RTS efficient measurement method are proposed.
In 2020, the RTS method becomes a CTIA International Standard (draft).
In 2022, the RTS method is written into CTIA 4.0.0 standard.
