The presence of the O and X modes in the HF skywave propagation has previously been investigated experimentally for their role in providing polarization diversity and improving channel capacity when a MIMO structure is employed. A mathematical treatment of MIMO channel modelling and capacity improvement has also been reported but limited to NVIS links only. This paper reports the mathematical derivation of the HF 2×2 polarized MIMO channels when cross-dipoles, i.e., a pair of orthogonally polarized horizontally-oriented dipoles, are used at both ends to examine the cross-polarization property and the capacity improvement factor (CIF), with the transmitter-receiver range being near enough to allow single-hop paths only but also distant enough to make the trajectory oblique. Results on the cross-polarization property suggest that the power contributions of the waves from the two transmitters that arrive at each receiver are equal. In addition, the signal from any one of the transmitters is received by the two receivers with unbalanced powers that depend on the phase shift difference between the O and X paths. It is also observed that on average the MIMO channels with oblique paths have lower capacity than the NVIS MIMO channels due to the reduced orthogonality between polarizations of antennas in the dual-antenna system at each end. The above hypotheses are confirmed through ray-tracing simulation and field measurement over a 575 km HF radio link.
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