MIMO Over-the-Air Testing
Antenna efficiency, element spacing, polarization, spatial correlation, branch imbalance, and physical antenna placement are critical to the performance of MIMO devices, so testing with conducted signals offers little confidence. Nor is field testing a viable option, as it is neither repeatable nor accurate enough for many purposes. What is needed is a method of creating realistic Over-the-Air (OTA) signals and conditions in a controllable environment.
The wireless industry, via the Third-Generation Partnership Project (3GPP), COST2100, and the CTIA, are currently considering several possible solutions to this problem. In general, these boil down to three different approaches:
1. The reverberation chamber method – Several proposals have discussed a “stirred reverb chamber” approach, where reflective paddles are set in motion inside a reverb chamber.
2. Fader-based OTA – this approach uses an anechoic chamber, a series of wireless channel emulators (faders) and radiating antennas to provide an environment that is both realistic and repeatable.
3. “Virtual OTA” – sometimes called the “two-step approach”, where the first step is to measure the effects of antenna patterns in an anechoic chamber. In the second step, the effects are replicated by a channel emulator conducting signals to the receiver inputs and bypassing the actual device antennas. Unlike traditional conducted testing, the measured antennas are replicated in the test environment.
This article focuses on the second approach, why it appears to be the most viable immediate solution, and some of the theories and challenges behind this approach. In Part 1 of this article, we will focus on the OTA testing; Part 2 of the article (to be published in a subsequent issue) will focus on the anechoic chamber as a critical component of the test system.