Theory, Techniques and Validation of Over-the-Air Test Methods for Evaluating the Performance of MIMO User Equipment

Several over-the-air (OTA) test methods have been proposed to characterize the radiated performance of multiple input multiple output (MIMO) devices. Knowing that antenna gain and spatial correlation are important factors to system performance, it has become necessary to include OTA testing of MIMO antenna performance. This paper will discuss three predominant OTA test methods including the two-stage OTA method, the multiple test probe OTA method and the reverberation chamber method. In some test configurations, the probe antennas are connected to a wireless channel emulator, such as the Agilent N5106A PXB, to emulate real-world multipath environments. In addition, this paper will discuss two types of MIMO channel models, the correlation-based and geometry-based models, and show how antenna and spatial characteristics at the MIMO transmitter and receiver can be separated into independent terms. Separating antenna gains and spatial characteristics allow the practical implementation of accurate OTA measurement systems using commercially available test instrumentation. This paper will also show measurements of spatial correlation and system capacity taken from commercial MIMO devices, and comparisons will be made using several OTA methods. It is assumed that the reader is familiar with basic concepts of MIMO technologies and additional background can be found in the References section [1], [2], [3]. Additional information on the 3GPP MIMO channel models can also be found in References [4].