Practical active antenna evaluation using the two-stage MIMO OTA measurement method

This paper discusses practical and cost-effective ways of evaluating the performance of active antenna array systems (AAS) using the two-stage MIMO OTA measurement method. The paper first introduces the concept of the two-stage method which is based on a first stage of antenna pattern measurement followed by a second stage of throughput measurement using a signal generated by convolving the device antenna pattern with the desired spatial channel model. The resulting signal is applied to the DUT either through a conducted or radiated connection. The radiated version of the two-stage method was recently approved by 3GPP in Technical Report 37.977 v12.0.0 [1]. The primary advantage of the two-stage method is that arbitrary 2D or 3D spatial channel models can be emulated without the need for the conventional multi-probe anechoic chamber approach which is significantly more complex and expensive. The extension of the two-stage method to active antenna systems is possible due to the unique ability of the two-stage method to provide intermediate antenna pattern measurements as part of the process. By measurement of a finite set of possible antenna pattern states the performance of the device for any orientation in the chosen spatial channel model can be evaluated for each antenna state and the optimum performance identified. The antenna state which is shown to provide the best performance can then be compared with the state chosen by the device when a known channel is applied through any particular state. To allow active antenna analysis to work, a proprietary feedback mechanism between the device and the test system is necessary to indicate the device's chosen antenna pattern to the test system. The described concepts apply to both mobile devices and base stations, and are particularly suited to the latter due to the size of the device which makes evaluation in a 2D or 3D anechoic chamber impractical.

I. Introduction to the Two-Stage MIMO OTA Test Method

This section will describe with reference to Figure 1 and Figure 2, the basic concept of the two-stage MIMO OTA test method as described in subclause 6.3.1.3 of [1]. The first stage is to acquire the DUT's antenna pattern. This may be done, depending on the test context, by simulation, design or measurement of the actual DUT antenna pattern. For evaluation of real devices it is possible to measure the antenna patterns in an anechoic chamber of sufficient size using network analysis techniques. Although conceptually simple, such techniques have the drawback that they require access to the device antenna where it connects into the device receiver. Making a connection at this point is problematic since the integrity of the device is altered due to the presence of the connecting cables and changes in the impedance seen by the antennas. To overcome these shortcomings the two-stage method employs, in its first stage, a novel non-intrusive antenna measurement method which does not require the connection of cables and consequential changes to the antenna patterns and impedance that would otherwise alter the measured antenna performance.