Massive MIMO wireless transmission test solution will become the key technology of 5G

Massive MIMO technology uses a large number (such as 64/128/256, etc.) of array antennas on the base transceiver station to achieve greater wireless data traffic and connection reliability. Compared to previous single-/dual-polarized antennas and 4/8-channel antennas, large-scale antenna technology can improve spectral and energy efficiency through different dimensions (airspace, time domain, frequency domain, polarization domain, etc.); 3D The shaping and channel estimation techniques can adaptively adjust the phase and power of each antenna element, significantly improve the beam pointing accuracy of the system, and concentrate the signal intensity on a specific target area and a specific user group, which can be significant while enhancing the user signal. Reducing self-interference in the cell and interference in neighboring cells is an excellent technique for improving the user signal to carrier-to-interference ratio.

How to evaluate Massive MIMO technology, what kind of test indicators and test methods are used, and how to measure Massive MIMO technology fairly and efficiently? This is also a great concern to current communication technology companies.

Massive MIMO System Architecture

The active antenna base station architecture supporting Massive MIMO is represented by three main functional modules: an RF transceiver unit array, an RF distribution network, and a multi-antenna array.

The RF transceiver unit array includes a plurality of transmitting units and receiving units. The transmitting unit obtains the baseband input and provides a radio frequency transmit output. The radio frequency transmit output is distributed to the antenna array through the radio frequency distribution network, and the receiving unit performs the opposite operation of the transmitting unit. The RDN distributes the output signals to the corresponding antenna paths and antenna elements and distributes the antenna's input signals in the opposite direction.

The RDN may include a simple one-to-one mapping between the transmitting unit (or receiving unit) and the passive antenna array. In this case, the RF distribution network will be a logical entity but not necessarily a physical entity.

The antenna array may include various implementations and configurations such as polarization, spatial separation, and the like.

The physical location of the RF transceiver unit array, RF distribution network, and antenna array may differ from the logical representation of the following diagram, depending on the implementation.

Massive MIMO Test Technology

1. Evolution of Antenna System Challenges Testing Technology

With the modernization of antenna systems, especially the evolution of 5G, the form of an integrated base station active antenna system (AAS) has gradually become mainstream, and the number of channels is increasing, and the active antenna connection method will also be simplified. RU and antennas Highly integrated, RF specifications are no longer confined to traditional RU conduction tests. OTA testing will become the direction of future test evolution, and it will also bring great testing challenges.

2. Modulation of test signals

The active antenna works under various service carrier conditions to achieve network coverage. In order to truly test the active antenna performance, the test system needs to have the following test capabilities: The test system needs to support amplitude and phase testing of service signals. In particular, there is a large bandwidth signal test; the pattern test signal pattern needs to be discussed and defined.

3. Antenna beam diversity

In the scenario where the antenna beam radiation characteristics tend to be complex: how to accurately evaluate the antenna service beam pointing accuracy, side lobes, lobe width, etc.; how to select a multi-beam test scenario; test efficiency problems of multi-beam antennas; how to pass for multiple beams Two-dimensional radiation characteristics to assess coverage performance.

Test proposal: Need to evaluate the index requirements of active antennas, especially Massive MIMO antennas, under two main planes. It is necessary to study and define the requirements for 3D radiation indicators; evaluate the performance of multi-beam radiation under real service signals and establish test case sets.

4. Communication antenna frequency band high frequency

High frequency (millimeter wave) coverage has always been a problem in the industry, and Massive MIMO can solve this problem well. As an extended band of 5G, it provides capacity guarantee.

In the case of an equal number of antenna elements, the higher the frequency, the shorter the coverage distance. High-frequency millimeter waves have a natural disadvantage in coverage. However, in theory, this can be compensated by increasing the number of antennas. As the frequency band rises, to achieve the same coverage distance, the number of antenna elements needs to be increased, which means that the cost of the antenna increases. Therefore, reducing antenna cost becomes one of the key issues of 5G multi-antenna technology.

The high-frequency Massive MIMO antenna is one of the key technologies for 5G evolution. Several key features are: high-frequency, large-bandwidth, ultra-large-scale array antennas.

These key features put forward new demands on the test: reanalysis and definition of the high-frequency antenna radiation index; test sites and instruments need to support the testing of large-diameter UHF antennas, especially the testing of OTA characteristics; test instruments need to support UHF, super Broadband signal testing.

5. RF index test air

With the development of antenna integration, especially the Massive MIMO antenna, the RF conduction RF index has radiation directionality and the number of channels is large. How to carry out the test of the radio frequency index is a huge challenge currently encountered. At present, there is no clear technical way. The 3GPP standard is also in the technical discussion. One of the current directions is to conduct air interface tests. However, how to define the air interface performance of these radio frequency indicators and how to conduct tests are all problems in the industry.

At present, radio frequency index air interface test, 3GPP R13 standard clearly defines EIRP and EIS. Other air interface indicators have been analyzed in the recent R14 standard of RAN program. At present, this part of the content is very complicated and all parties are studying it. There is no clear conclusion on how to conduct air interface tests on these RF indicators.

Currently, it is divided into two parts: In-band indicators – Currently, if the antenna performance is known, it can be evaluated by OTA's existing test solutions; Out-of-band indicators – The out-of-band performance of the antenna is unknown, and the out-of-band antenna has a very wide frequency Point-to-air testing is a huge challenge.

6.3D Beamforming Features

Compared with conventional antenna coverage, Massive MIMO antennas may have narrower service beams, and their pointing accuracy directly affects network coverage performance. Therefore, the accuracy test of the beam pointing of its service is particularly important.

The ability of each antenna array to split several beams also becomes an important indicator of the coverage performance of a Massive MIMO network. How much throughput can be achieved by users under these several beam coverages also needs to be part of the assessment.

to sum up

As the network continues to evolve, antennas and RF modules will be deeply integrated, and Massive MIMO active antennas will be the mainstream of future antenna development. Integration testing and air interface testing may become the evolution direction of future tests.

Compared with traditional antennas and RF test methods, test indicators and evaluation systems, test principles and methods, and test platforms are all subject to major challenges. These may be unprecedented major innovations in the mobile communication system's antenna feeder network, and they need to be explored.