One of the technologies seen as a key for optimizing the spectral efficiency of 5G networks is massive multiple-input, multiple-output (MIMO) antenna arrays. But before 5G standards are established and base stations are constructed, wireless network operators must determine an effective way to perform over-the-air (OTA) testing of massive MIMO base stations.
Testing massive MIMO arrays poses much more of a problem than the traditional characterization of an antenna’s radiation pattern in an anechoic chamber. Massive MIMO arrays consist of potentially hundreds of antennas in one location, such as a base station, to manage the use of frequencies and time slots for many simultaneous users. A great deal of theory has been presented to this point on the design of massive MIMO systems, but designing, building, and testing such arrays is another issue.
Traditional wireless base stations may have as many as eight separate antennas, and these can be tested individually through the cabled connection to signal generators and spectrum analyzers. The physical size of massive MIMO arrays, with typically 64 or 128 array elements, makes the challenge of performing OTA testing daunting just in providing a test facility that can accommodate the tens of wavelengths of propagated energy that must be measured simultaneously.
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For a test facility, such as an anechoic chamber, that is properly equipped to evaluate the performance of a 5G base station massive MIMO antenna array, for example, the number of OTA test antennas needed to characterize a device under test (DUT) must be equal or greater than the number of antenna array elements. Each array element is treated as an individual antenna and expected to perform as such for a designed frequency band of interest. Unfortunately, the costs of providing this many OTA antennas for this type of test facility would be extremely prohibitive. Add to those costs any additional hardware, such as power amplifiers, that might be needed in the testing of a massive MIMO array. Depending on the frequency/wavelength of the array testing, the test facility must also provide the physical dimensions to provide adequate distance between the DUT and the OTA antennas to represent realistic working conditions.
As with much of 5G, the strategies for developing and testing massive MIMO antenna arrays are still in their infancy. However, even based on theory, the concept of using multiple antenna elements in a time-and-frequency-coordinated array for a 5G base station makes sense in terms of achieving optimum spectral efficiency and making the most of the limited available bandwidth.
See: “A Step Toward 5G in 2020,” IEEE Antennas & Propagation Magazine, Vol. 59, No. 1, February 2017, p. 38.