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Based on a planar and compact antenna, extensive measurements are conducted to investigate the behavior of the colocated tri-polarized multiple-input multiple-output (MIMO) system working at 3.5 GHz. Several scenarios with different multipath distributions have been considered, including a reverberation chamber (RC) and also some line-of-sight (LOS) and obstructed LOS (OLOS) real-world indoor environments. Many channel parameters of the MIMO system are analyzed based on the measured data, including cross-polarization discrimination (XPD), Ricean K-factor, correlation coefficient (CC), MIMO channel capacity gain (CG), channel eigenvalues, and ellipticity statistic (ES). The results show that for a multi-polarized MIMO system, the effects of polarization coupling and transition depend strongly on the polarimetric scattering characteristic of the environments. Furthermore, in all the scenarios tested, three non-zero eigenvalues of the MIMO channel matrix are obtained, which indicates that three independent subchannels can be supported by the tri-polarized MIMO system. INDEX TERMS Tri-polarized antenna, multiple-input multiple-output (MIMO), cross-polarization discrimination (XPD), Ricean K-factor, MIMO channel capacity gain (CG), measurement.

In this paper, a co-located tri-polarized loop (TPL) antenna is proposed and a co-located tri-polarized dipole (TPD) antenna is also designed for multiple-input multiple-output (MIMO) performance comparison. Both antennas have satisfactory performances of impedance bandwidth and isolation. The channel measurements are conducted in a non-lossy reverberation chamber (RC), a lossy RC, a corridor and an office room. For both antennas, the measured average correlation coefficients (CCs) in the non-lossy RC are the smallest, close to 0.1, in the lossy RC are the largest, 0.3-0.5, and in the two indoor scenarios are slightly larger than that in the non-lossy RC. Except for the signal correlation, the relative path loss (RPL) should also be included to fairly compare the channel capacities. If the RPL is considered, a strong dependence of the channel capacity on the propagation properties can be seen. For both antennas, the channel capacities in the two indoor scenarios are 20%-30% smaller than that in the non-lossy RC. Interestingly, both antennas have similar condition numbers (CNs) in all the four scenarios, whose PDFs are close to that of the independent identically distributed (i.i.d) complex Gaussian channel, which reveals that both antennas can obtain nearly three independent parallel subchannels and good multiplexing performance.Index Terms-Tri-polarized loop (TPL), tri-polarized dipole (TPD), multiple-input multiple-output (MIMO), measurement. 0018-926X (c)

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