5G OTA Measurements

Operational "How to" GuidesSummaryThis article is about Over-The-Air measurements for 5G. Are OTA tests required and what kind of tests should be performed ? ​​​​​​​How will OTA measurement solutions improve in future? DescriptionAre OTA tests required?   Yes they are. Current sub 6 GHz RF performance tests are mostly done using cables. That changes when you move to massive MIMO in sub 6 GHz or mmWave frequencies. At mmWave frequencies, beamforming antenna technologies are used to overcome higher path loss and signal propagation issues and to take advantage of spatial selectivity by using narrow signal beams. Phased-array antennas, such as those shown in figure 1, are typically highly integrated devices, with antenna elements bonded directly to ICs, making it difficult, if not impossible, to connect and probe. OTA enables test, but it introduces a more challenging ‘air interface’ between the component or device and the base station where the imperfections in the air interface need to be accounted for during test.    Figure 1. Example mmWave antenna arrays   What kind of OTA tests are required?   The types of measurements for 5G products vary throughout the development life cycle and are different for an UE versus a base station. During design and development, RF parametric tests such as transmitted power, transmit signal quality, and spurious emissions are done for radiated transmitter tests.  Base stations add tests such as occupied bandwidth and adjacent channel leakage ratio (ACLR), to name a few.  Beam-pattern measurements in 2D and 3D and beamsteering or null-steering performance tests are also done during R&D.  Conformance testing is also done to ensure the device meets 3GPP minimum requirements. These can be grouped into RF, demodulation, radio resource management (RRM), and signaling tests. The document attached below provides an explanation of these tests.   OTA tests are typically conducted in the radiated near-field or radiated far-field region of the antenna system under test. Measurements in the far-field are conceptually the simplest type of OTA measurement and an approved method identified by 3GPP.  A typical far-field anechoic chamber is shown in figure 2.  With the appropriate probing and test equipment, 2D and 3D beam patterns and RF parametric tests can be performed. The challenge is selecting a reasonable chamber that won’t take up your entire lab space. The length of a far-field chamber is roughly determined by 2D2/λ, where D is the diameter of the device being tested.  With this in mind, a 15-cm device at 28 GHz would therefore require a 4.2-m chamber as shown in figure 3. These chambers will be large and quite expensive.                                                                  Figure 2.  Far-field measurement   D (cm) Frequency (GHz) Near/far boundary (m) 5 28 0.5 10 28 1.9 15 28 4.2 20 28 7.5 25 28 11.7 30 28 16.8 Figure 3. How far is far-field?   An alternative for 5G RF tests that is being used by market leaders and is now being considered by 3GPP is the compact antenna test range (CATR). In Figure 4, a CATR uses a reflector so that it looks like the waveform is coming from a long way away. This seems to be a very promising direction for 5G OTA testing, and 5G market leaders are seeing this as a comprehensive and accurate test method.  However, that’s not the case for RRM test where there is no clear solution because of the many open issues due to the dynamic, multi-signal 3D environment with signal tracking and handovers.        Figure 4. Compact antenna test range (CATR)   How will OTA measurement solutions improve in future?  It’s going to take time to get these testing methods into the standards. In the meantime, the solutions coming from market leaders working directly with test vendors to enable OTA tests of the first 5G devices and basestations. These OTA test solutions are the ones to watch, as they will pave the way for the standards.See AlsoOTA Test for Millimeter-Wave 5G NR Devices and Systems