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Wireless Communications and Mobile Computing
Volume 2017 (2017), Article ID 9068272, 7 pages
Research Article

Keyholes in MIMO-OFDM: Train-to-Wayside Communications in Railway Tunnels

1Teoría de la Señal y Comunicaciones Department, ETSIS Telecomunicación, Universidad Politécnica de Madrid (UPM), Campus Sur, Calle Nikola Tesla s/n, 28031 Madrid, Spain
2ETSI Telecomunicación, Universidad Politécnica de Madrid (UPM), Ciudad Universitaria, 28040 Madrid, Spain
3The Engineering Department of Metro de Madrid S.A., Madrid, Spain

Correspondence should be addressed to Juan Moreno García-Loygorri

Received 3 March 2017; Accepted 5 July 2017; Published 7 August 2017

Academic Editor: Yin Xuefeng

Copyright © 2017 Juan Moreno García-Loygorri et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


One of the key challenges in railway engineering is how to provide passengers an efficient, secure, and safe service. To achieve this, operators and stakeholders demand robust and high-capacity train-to-wayside broadband radio. Current radio technologies implement MIMO (Multiple-Input Multiple-Output) technology, whose use requires a good characterization of the propagation. In this paper, the results of an experimental campaign on MIMO propagation in subway tunnels are presented. The campaign makes use of an OFDM testbed at roughly 600 MHz, allowing the measurement of the channel transfer matrix under different conditions regarding polarization, antenna separation, tunnel cross section, power allocation algorithm, and so forth. Particular attention is paid to the probability of appearance of keyholes, which imply a severe degradation of the MIMO performance even when transmitter and receiver antennas are uncorrelated. The measurements are carried out using a train that travels at the normal operative velocities. As a result of the measurements, it has been found that the use of vertical polarization at both terminals is advantageous and that, on average but for a narrow margin, λ-spaced antennas outperform λ/2-spaced ones (90% capacities of 7.00 and 6.76 bps/Hz, resp.), although the latter show a lower probability of keyholes. However, keyhole probabilities are always below 2%, so their influence on the performance of the whole system is limited.