Characterization of MIMO Antennas and Terminals: Measurements in Reverberation Chambers

Doktorsavhandling, 2012

The reverberation chamber (RC) has drawn attention as a multipath emulator over the past decade for both passive and active over-the-air (OTA) tests. The overview of this thesis is given in Chapter 1. Although the main purpose of this thesis is characterization of MIMO (multiple-input multiple-output) terminals based on RC measurements, it is of importance to know under which channel condition the device has been tested. Parameters that are used to characterize the channel in a multipath environment are coherence bandwidth, delay spread, coherence time, Doppler spread, coherence distance and angular spread. In a normal RC, angular of arrival distribution is almost uniform. The corresponding coherence distances for different antennas can be derived at ease based on the a priori knowledge of the uniform angular distribution. Therefore, the main tasks of RC channel characterization are to determine coherence bandwidth, RMS delay spread, coherence time and Doppler spread. These studies are given in Chapter 2. For multi-port antennas used in MIMO systems, relevant characterizations are correlation, embedded radiation efficiency, diversity gain, and MIMO capacity, all of which can be measured in a RC. In order to compare the RC measurements with that of an anechoic chamber (AC), two methods for evaluations of AC measurement-based maximum ratio combining (MRC) diversity gain and MIMO capacity are presented correspondingly. After examining these two methods, they are applied, respectively, to a wideband multi-port antenna that is measured in both AC and RC. Comparisons show good agreements. Furthermore, a throughput measurement of a LTE (long term evolution) dongle is tested in the RC. A corresponding throughput model is presented. Simple as it is, this model can be used to predict the measurement results well. All of these are studied in Chapter 3. For both passive and active OTA tests, the measurement accuracy is of great importance. Previous RC measurement uncertainty works believed that the RC accuracy depends only on the independent sample number. This thesis, however, shows that the RC accuracy depends not only on the independent sample number, but also on the Rician K-factor, i.e. the power ratio of unstirred electromagnetic (EM) fields to the stirred ones, and that the K-factor represents a residual error in RC measurements. It is also proven on practical chambers that accuracy can be improved either by reducing the K-factor, or by introducing stirring methods that reduce it, such as platform and polarization stirring. This work is presented in Chapter 4. Chapter 5 concludes this thesis.