This letter presents a deep insight on a real implementation of a train-to-wayside radio on subway tunnels that makes use of a 2 × 2 multiple-input-multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) setup. The main purpose of this letter is to study in detail the keyhole phenomenon of an MIMO-OFDM train-to-wayside communication system on a tunnel.
MIMO keyholes are studied in different tunnels sections, and capacity results are provided. Moreover, we introduce the first keyhole measurements on a railway tunnel. Finally, we follow a quantitative approach to estimate keyhole probabilities on each tunnel stretch and capacity outage curves.