In this paper, a channel characterization of an RF link using circularly polarized antennas inside a mine is performed. The association of circular polarization with multiple-input-multiple-output (MIMO) radio technologies represents a powerful tool to improve the performance of an underground RF channel. The statistical parameters of the channel are derived from in-mine measurements at the 2.4 GHz band for both co-polarization (CP) and cross-polarization (XP) scenarios. Results show a remarkable improvement through the use of MIMO combined with circular polarization compared to the regular patch MIMO antenna system, in terms of channel capacity and path loss. This improvement increases significantly at the XP scenarios, reaching up to 18 bps/Hz for channel capacity and up to 21 dB for path loss. The RMS delay spread for a circularly polarized setup is generally higher than the linearly polarized MIMO patch setup due to surface roughness of the gallery. In the linear polarization case, a signal degradation of more than 15 dB at the XP case is observed compared to the CP scenario. This signal loss that is due to depolarization is somewhat mitigated by the surface roughness.Due to its superior and stable performance, MIMO combined with circular polarization is better suited than a regular MIMO patch system for in-mine uses, especially in the applications where the transmitter may change direction with respect to the receiver.
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