Characterization of Terminal Antennas for Diversity and MIMO Systems by Theory, Simulations and Measurements in Reverberation Chamber

Doktorsavhandling, 2005

This thesis is based on the work of designing and building a small reverberation chamber, and to use this chamber for characterization of antennas on mobile terminals such as phones. The characterization is done in terms of radiation efficiency, diversity gain and maximum available capacity. The latter apply to multiport antennas for so-called MIMO (Multiple Input Multiple Output) systems. The reverberation chamber represents a scatter-rich multipath environment, which is similar to that appearing in real in- or out-door environments. A real multipath environment is a result of many interfering waves via different scatterers. We show that in reverberation chamber the angles of incidence of these waves are uniformly distributed over all directions in space, creating an isotropic multipath environment. We show that the measurement accuracy for small chambers can be significantly improved by rotating the antenna under test. This is demonstrated in the 900 MHz GSM band by measurements in a small reverberation chamber of dimensions 1.0m x 0.8m x 1.0m. We refer this new stirring method as platform stirring. The diversity gain was previously shown to be reduced due to correlation between the received signals on the two antenna ports, when the correlation was caused by mutual coupling between the two antenna elements. We show that the reduction in radiation efficiency due to mutual coupling cause a larger reduction of diversity gain. We define an effective diversity gain that accounts for the reduction of radiation efficiency due to mutual coupling. Furthermore, we show how the effective diversity gain can be measured straight forward in a reverberation chamber, and we show how well the measurements compare with results of analysis using both classical formulas and numerical simulations. The validation case is two parallel dipoles. A six-monopole circular antenna array for use in a MIMO system is considered. We show how to characterize this by using the classical embedded element patterns, that is found both by classical analytical formulas including mutual coupling and by solving the whole array by Method of Moments. The embedded element patterns include all information needed to calculate the radiation efficiency of each element, correlation and resulting diversity gain, as well as the average maximum capacity of the MIMO system when the array is located in a rich scattering environment. The calculated results agree with measurements in a reverberation chamber, representing a similar scattering environment. Such analysis using embedded element patterns and the verification by actual measurements was never reported before. Embedded element patterns of two parallel half-wavelength dipoles are calculated for various source impedances and dipole spacings. The maximum effective diversity gain is obtained when the source impedance is conjugate matched to the input impedance of the embedded dipole element. Then, there is no correlation due to the mutual coupling.