Massive MIMO (massive multiple-input multiple-output) technology has become a cornerstone of 4.5G and 5G networks, attracting widespread attention from the global telecommunications industry. China Mobile and SoftBank Japan have already deployed TD-LTE Massive MIMO solutions, while operators such as China Unicom, China Telecom, and Telkomsel have conducted field trials of FDD Massive MIMO. It was also a key technology in the first phase of China’s 5G trials, with major vendors like Huawei, ZTE, and Ericsson participating. Since the R13 release, 3GPP has officially included Massive MIMO as an essential feature. This technology leverages large-scale antenna arrays—such as 64, 128, or 256 elements—to significantly enhance wireless data capacity and connection reliability. Compared to earlier single- or dual-polarized antennas, or 4- or 8-channel systems, Massive MIMO improves spectral and energy efficiency through multi-dimensional optimization, including space, time, frequency, and polarization domains. Advanced 3D beamforming and channel estimation techniques allow for precise adjustment of each antenna element's phase and power, improving beam accuracy and focusing signal strength on specific users or areas. This helps reduce self-interference and inter-cell interference, ultimately boosting the user signal-to-interference ratio. However, evaluating Massive MIMO performance remains a challenge. Determining the right test metrics, methods, and fair and efficient measurement approaches is a critical issue that the industry continues to explore.
The architecture of a Massive MIMO system typically includes three main components: the RF transceiver unit array, the RF distribution network (RDN), and the antenna array. The RF transceiver unit handles both transmission and reception, converting baseband signals into RF outputs. These outputs are then distributed via the RDN to the appropriate antenna elements. The RDN may act as a logical entity, especially when there is a one-to-one mapping between transceivers and passive antenna elements. The antenna array can be configured in various ways, such as using different polarizations or spatial separations. The physical layout of these components may differ from their logical representation, depending on the implementation.
Testing Massive MIMO systems presents unique challenges, especially with the shift toward integrated active antenna systems (AAS). As 5G evolves, traditional conduction tests are no longer sufficient, and over-the-air (OTA) testing is becoming increasingly important. OTA testing allows for more realistic evaluation of antenna performance, but it also introduces new complexities, such as handling wideband signals and accurately measuring beam characteristics.
In terms of test signal modulation, the system must support amplitude and phase testing of high-bandwidth signals. Additionally, standardized test patterns need to be defined to ensure consistency across different systems. Beamforming diversity is another critical aspect, particularly in scenarios where multiple beams are used simultaneously. Evaluating beam direction, side lobe levels, and beam width requires advanced testing methodologies. Real-world service conditions further complicate the process, making it necessary to define representative test cases.
High-frequency millimeter-wave bands offer significant capacity for 5G, but they come with coverage limitations. Massive MIMO can help overcome this by increasing the number of antenna elements, which compensates for the reduced range at higher frequencies. However, this also increases costs, so optimizing antenna design is crucial. Testing high-frequency Massive MIMO systems involves redefining radiation indices, supporting large-diameter antenna testing, and ensuring instruments can handle ultra-wideband signals.
Air interface testing for RF indicators is still under development. While 3GPP has defined some parameters like EIRP and EIS, many aspects remain unclear. In-band and out-of-band performance evaluations pose distinct challenges, especially when dealing with wideband signals.
3D beamforming adds another layer of complexity, as traditional 2D measurements may not fully capture beam behavior. Real-time adjustments based on user movement make it difficult to test all possible scenarios. Therefore, selecting typical use cases is essential for effective evaluation.
In summary, as network technologies continue to advance, Massive MIMO will play a central role in future communication systems. Integrated testing and air interface testing are likely to become standard practices. The evolution of test methods, platforms, and evaluation criteria will require innovative approaches, offering new opportunities for research and development in the field.
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