Abstract
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 2×2 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.
Highlights
Communication between trains and wayside is essential today
The likelihood of a physical phenomenon that affects the performance of a MIMO system has been measured
The research described in the paper can be related to the use of OFDM systems, such as LTE, in the context of railway applications using frequencies below 1 GHz
Summary
Communication between trains and wayside is essential today. Services like CCTV, CBTC (Communications-Based Train Control), ERTMS (European Rail Traffic Management System), and push-to-talk radio telephony are a few of the most popular services in this area. LTE (and LTE-A as well) has been flagged to be the key technology in the field of railway communications [2], but many operators around the world have opted instead for IEEE 802.11-based solutions This is a very actual topic in the railway field, as shown by the first deliverable of the EU-funded Roll2Rail Project [3]. We provide more results on the keyhole effect and we map the influence of other parameters on the MIMO capacity In this short review of the bibliography, we will only mention a few of the most relevant references: a pioneer paper was centered on narrowband measurements [11] and another one focused on polarization effects in tunnels [12] and broadband measurements [13]; they lack the involvement of any train.
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