Contents I. Introduction
Polarization diversity, early applied to HF, radar, and imaging systems, has demonstrated its potential for improving the capacity of wireless communications systems, despite some disappointing premature predictions [1]. The improvement is typically granted by an additional decorrelated channel provided by a polarization state made orthogonal to the existing one, usually at the transmitting end . A randomly orientated linearly-polarized antenna is also typically used at the receiver for evaluating polarization diversity. Consequently, the cross-polarization discrimination (XPD) factor is the usual evaluating parameter, with low correlation coefficients being achieved even in NLOS situations [2]. Due to the significant difference in mean received signal level between copolarized and cross-polarized branches when one polarization is transmitted, considerably more attention has been paid to spatial diversity. The (de)coupling effect between different polarizations is a complex mechanism to be simulated, which has also limited true polarization diversity research. Yet, since at least horizontal and vertical separation distances are required for efficient spatial outdoor diversity in practice, polarization diversity has recently gained attraction. The use of vector antennas which can respond to more than one component and/or polarization states of the EM field through colocation instead of more voluminous spatially separated arrays can provide equivalent channel capacity increase [3]. The slanted polarization diversity combination was found to perform just as good as the spatial polarization diversity with two elements, and commercial systems like GSM and UMTS soon switched to this new technique. Some combinations of two-branch polarization and spatial diversity have also been reported [4]. Recently, a triaxial combination of polarization and pattern diversity has also been proposed [5], with some contradicting results.
