Fundamental limitations on the capacity of wireless links that use polarimetric antenna arrays

It was recently speculated that six-component polarimetric antennas $comprising six co-located electric and magnetic dipoles that generate the six components of electric and magnetic current or that measure the six components of the electric and magnetic field - could give six-fold improvements in capacity over single-component antennas in Rayleigh flat fading. Using an exact wave propagation formulation involving the plane-wave spectral decompositions of the transmitted and scattered fields, we show that the behavior of wireless links based on arrays of polarimetric antennas is qualitatively different than that of links based on single polarimetric antennas. The MIMO channel can be viewed rigorously in the wavenumber domain. For dense planar arrays, the use of polarimetric antennas yields at most a fourfold improvement in capacity, which can be realized by particular combinations of four of the six dipoles. Six-component elements, the expansion of the planar arrays into volume arrays, or spacing elements at intervals less than a half-wavelength only give additional logarithmic improvements in capacity.