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**1. Introduction**
Massive MIMO (massive antenna) technology is one of the key technologies in 4.5G and 5G, and it has attracted significant attention from the global communication industry. China Mobile and SoftBank Japan have already launched TD-LTE Massive MIMO technology, while operators such as China Unicom, China Telecom, and Telkomsel have completed FDD Massive MIMO field tests. Massive MIMO was also selected as a core technology in the first phase of China’s 5G trials, with five vendors including Huawei, ZTE, and Ericsson participating in the testing. The 3GPP standard has supported Massive MIMO as an important feature since the R13 release.
Massive MIMO leverages a large number of array antennas—such as 64, 128, or 256—to enhance wireless data traffic and connection reliability. Compared to traditional single- or dual-polarized antennas and 4/8-channel antennas, this technology improves spectrum and energy efficiency through multiple dimensions like space, time, frequency, and polarization. Techniques such as 3D beam shaping and channel estimation allow for adaptive adjustment of each antenna element's phase and power, significantly improving beam pointing accuracy. This ensures that signal strength is concentrated on specific areas and user groups, enhancing signal quality and reducing self-interference and inter-cell interference, which ultimately improves the carrier-to-interference ratio for users.
Evaluating the performance of Massive MIMO technology remains a critical challenge. What test indicators and methods should be used? How can we measure this technology fairly and efficiently? These are questions that the current communication industry is actively addressing.
**2. Massive MIMO System Architecture**
The active antenna base station architecture supporting Massive MIMO typically consists of three main functional modules: a radio frequency (RF) transceiver unit array, a radio frequency distribution network (RDN), and a multi-antenna array.
The RF transceiver unit array includes both transmitting and receiving units. The transmitting unit processes the baseband input and provides a radio frequency output, which is then distributed to the antenna array via the RDN. The receiving unit performs the reverse operation. The RDN distributes the output signals to the corresponding antenna paths and elements and routes incoming signals back in the opposite direction.
In some cases, the RDN may involve a simple one-to-one mapping between the transmitting or receiving unit and the passive antenna array. In such scenarios, the RDN may exist as a logical entity rather than a physical one.
Antenna arrays can be implemented in various ways, including different polarizations and spatial separations. The physical placement of the RF transceiver unit array, RDN, and antenna array may vary depending on the design, even if they logically represent the same structure.
**3. Massive MIMO Test Technology**
**3.1 Evolution of Antenna Systems and Testing Challenges**
As antenna systems evolve toward modernization, especially with the development of 5G, the integrated active antenna system (AAS) has become the mainstream. The number of channels is increasing, and the connection mode between the radio unit (RU) and antenna is becoming simpler. With high integration, RF indicators are no longer limited to traditional conduction testing. Instead, over-the-air (OTA) testing is becoming a key trend, bringing new challenges to the testing process.
| Antenna System Type | Antenna System Features | Test Method | Supports Conduction Test? | Test Challenge |
|---------------------|--------------------------|-------------|---------------------------|----------------|
| RRU + Antenna | Separated components; independent design; defined RF performance at base station port | RRU and antenna tested separately | Yes | Mature, no major challenges |
| Integrated Active Antenna | Integrated design; non-standard interface; synchronized with RF module | Integrated + split test | Yes (non-standard interface) | Non-standard interfaces complicate testing; OTA required |
| Massive MIMO Antenna | Deep integration; large-scale antenna; 3GPP proposes OTA standards | Mainly machine testing | Depends on design | No external RF interface; relies heavily on OTA testing |
**3.2 Test Signal Modulation**
Active antennas operate under various service carrier states to ensure network coverage. To evaluate their performance, the test system must support amplitude and phase testing of service signals, particularly for wideband signals. Additionally, the pattern of the test signal needs to be defined and discussed.
**3.3 Antenna Beam Diversification**
In complex beam radiation scenarios, several key issues arise: evaluating beam direction accuracy, side lobe levels, beam width, selecting appropriate multi-beam test scenarios, and improving test efficiency for multi-beam antennas. Two-dimensional evaluation of radiation characteristics is also essential for accurate coverage assessment.
Test recommendations include evaluating active antennas, especially Massive MIMO, under two main planes and studying 3D radiation index requirements. Establishing test case sets for multi-beam performance under real service conditions is also crucial.
**3.4 High-Frequency Communication Antennas**
High-frequency (millimeter wave) coverage has always been a challenge for the industry, but Massive MIMO offers a promising solution. As part of 5G, millimeter waves provide higher capacity. However, higher frequencies mean shorter coverage distances. This disadvantage can be offset by increasing the number of antenna elements, which leads to higher costs. Therefore, reducing antenna costs is a key issue for 5G multi-antenna technology.
High-frequency Massive MIMO antennas have features such as high frequency, large bandwidth, and ultra-large-scale arrays. These require redefining high-frequency antenna radiation indices, supporting large-diameter UHF antenna testing, and enabling ultra-wideband signal testing.
**3.5 RF Indicator Test Over-the-Air (OTA)**
With the advancement of antenna integration, especially with Massive MIMO, testing RF indicators has become more complex. There is currently no clear technical approach, and 3GPP is still discussing the best way forward. One direction is OTA testing, but defining and measuring air interface performance remains a major challenge.
The 3GPP R13 standard defines EIRP and EIS, while R14 addresses other air interface indicators. This area is still under study, and there is no consensus on how to conduct OTA testing for these RF indicators.
**3.6 3D Beamforming Characteristics**
Traditional 2D beam analysis may not fully capture the complexity of 3D beamforming. As shown in the figure, conventional E and H plane cuts fail to reflect the full sidelobe distribution. Moreover, the service beam of a Massive MIMO antenna changes dynamically based on user location, making it nearly impossible to test all possible beam scenarios. Therefore, testing should focus on typical business scenarios.
Compared to traditional antennas, Massive MIMO may have narrower beams, and the accuracy of beam pointing directly affects network coverage. Evaluating beam pointing accuracy is therefore crucial. Additionally, the ability to split multiple beams per antenna array is an important metric for coverage performance, and the throughput achieved by users under these beams should also be evaluated.
**4. Conclusion**
As networks continue to evolve, antenna and RF modules will become more deeply integrated, and Massive MIMO active antennas will dominate future developments. Integrated testing and OTA testing are likely to be the future of testing practices.
Compared to traditional methods, the test indicators, evaluation systems, principles, and platforms for Massive MIMO face significant challenges. These may lead to unprecedented innovations in mobile communication antenna systems, requiring continuous exploration and development.