José Manuel Riera

A survey on future railway radio communications services: challenges and opportunities

Radio communications is one of the most disruptive technologies in railways, enabling a huge set of value-added services that greatly improve many aspects of railways, making them more efficient, safer, and profitable. Lately, some major technologies like ERTMS for high-speed railways and CBTC for subways have made possible a reduction of headway and increased safety never before seen in this field. The railway industry is now looking at wireless communications with great interest, and this can be seen in many projects around the world. Thus, railway radio communications is again a flourishing field, with a lot of research and many things to be done. This survey article explains both opportunities and challenges to be addressed by the railway sector in order to obtain all the possible benefits of the latest radio technologies.

Spatial distribution of cloud cover

Satellite systems and high-altitude platform systems working in Ka and V bands require the application of adaptive techniques in order to mitigate link degradations caused by atmospheric impairments such as those due to cloud cover. Among these techniques, resource sharing system techniques and site diversity need information on the spatial distribution of impairments caused by cloud cover, including the probability of simultaneous occurrence of adverse conditions in various regions. A study has been performed in Spain, within the framework of COST Action 280, to investigate the large-scale spatial distribution of cloud cover using synoptic meteorological data. Cloud cover distribution for individual sites and the spatial correlation properties for pairs of sites have been investigated. The geographical distributions of the values obtained from the statistical analysis have been represented in maps of contour lines using standard mapping procedures. Correlation parameters are expected to decrease with distance; however, the maps derived taking a given site as reference reveal a significant influence of climatic and geographic factors such as weather fronts, orography or the proximity to the sea. The statistics and maps obtained can be useful to optimize power sharing in multi-beam satellite systems, as suggested in this paper. Copyright

Atmospheric Attenuation in Wireless Communication Systems at Millimeter and THz Frequencies [Wireless Corner]

This paper intends to give an overview about atmospheric propagation effects affecting millimeter and terahertz (THz) communication systems. The main focus is on attenuation caused by atmospheric gases and liquid water droplets, either in the form of suspended particles into clouds or rain falling hydrometeors. Theoretical aspects about each of them are presented, emphasizing on those that deserve special attention as frequency increases. Statistics of attenuation estimated from meteorological data and some experimental results, as in the case of rain attenuation, obtained in Madrid, Spain, are presented throughout the paper, thus providing further insights about the phenomena discussed.

ATMOSPHERIC PROPAGATION AT 100 AND 300 GHz: ASSESSMENT OF A METHOD TO IDENTIFY RAINY CONDITIONS DURING RADIOSOUNDINGS

The in∞uence of atmospheric gases and tropospheric phenomena becomes more relevant at frequencies within the THz band (100GHz to 10THz), severely afiecting the propagation conditions. The use of radiosoundings in propagation studies is a well established measurement technique in order to collect information about the vertical structure of the atmosphere, from which gaseous and cloud attenuation can be estimated with the use of propagation models. However, some of these prediction models are not suitable to be used under rainy conditions. In the present study, a method to identify the presence of rainy conditions during radiosoundings is introduced, with the aim of flltering out these events from yearly statistics of predicted atmospheric attenuation. The detection procedure is based on the analysis of a set of parameters, some of them extracted from synoptical observations of weather (SYNOP reports) and other derived from radiosonde observations (RAOBs). The performance of the method has been evaluated under difierent climatic conditions, corresponding to three locations in Spain, where colocated rain gauge data were available. Rain events detected by the method have been compared with those precipitations identifled by the rain gauge. The pertinence

Experimental assessment of slant-path rain attenuation variability in the Ka-band

Summary This study is based on the results of a slant-path Ka-band propagation experiment carried out in Madrid, Spain, regarding rain attenuation, which is the main propagation impairment in this frequency band. The experimental and statistical results correspond to seven complete years of measurements, a period large enough to accomplish a comprehensive analysis in order to characterize the variability of rain rate and attenuation. It is shown that year-to-year variability is significant in temperate climates as Madrids. The aforementioned significance is also apparent with regards to seasonal, monthly, worst-month and hourly variability concerning rain attenuation, which are also discussed and related when possible to the variability of the rain phenomena, either represented by the total amounts of rainfall in the different periods or by rain rate statistics. Copyright

Radiometric and Spatial Resolution Constraints in Millimeter-Wave Close-Range Passive Screener Systems

This paper presents a comparative study of the radiometric sensitivity and spatial resolution of three near-field (NF) passive screener systems: real aperture, 1-D synthetic aperture (SA), and 2-D SA radiometers are compared. The analytical expressions for the radiometric resolution, the number of required antennas, and the number of pixels in the image are derived taking into account the distortion produced by the NF geometry at nonboresight directions where the distortion is dominant. Based on the theoretical results, a performance comparison among the studied systems is carried out to show the advantages and drawbacks when using the radiometers in a close-range screening application. Moreover, the screener performance in a close-range environment is discussed from the results obtained in the aforementioned comparison.

Keyhole Estimation of an MIMO-OFDM Train-to-Wayside Communication System on Subway Tunnels

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.

An Application of IGS Zenith Tropospheric Delay Data to Propagation Studies: Validation of Radiometric Atmospheric Attenuation

The objective of this paper is to continue exploring the use of International GNSS Service (IGS) products as a tool to be used in Earth-space propagation experiments. With this purpose, 4 years of Zenith Tropospheric Delay data from IGS Villafranca station, in Madrid, Spain, have been processed with the aim to estimate the attenuation due to gases. These values are then used for the validation of a concurrent database of 4 years of radiometric measurements at 19.7 GHz, collected at Universidad Politécnica de Madrid (UPM) with the purpose of studying the attenuation effects of gases and clouds. The suggested validation procedure includes both an automatic routine and visual inspection. The validated measurements are statistically analyzed on a yearly and seasonal basis. Results point toward the usefulness of IGS data in propagation experiments as well as their integration in their processing routines.

Joint results of long-term earth-space propagation experiments at 20-GHz in Canada and Europe

Propagation effects such as rain or clouds attenuation cause deeper fades in the Ka-band than at lower frequencies. In this collaborative paper, the main results of four long-term Ka-band propagation campaigns are presented. The experiments are carried out in Ottawa, Canada; Aveiro, Portugal; Madrid, Spain; and Toulouse, France. Attenuation statistics are derived from satellite beacon data collected over 6 years at Aveiro, 5 years at Ottawa and Madrid and 2 years at Toulouse. Multi-year measurements allow the production of more stable statistics reflecting the long-term behavior of propagation phenomena and to investigate its year-to-year variability. The beacon signal data was monitored and collected on a continuous basis over the whole measurement period. After a brief introduction of the experiments, rain rate and excess attenuation results are discussed, first for a common measurement period and later for the whole database available. Seasonal attenuation statistics for Ottawa and Aveiro are compared. Finally, fade duration and fade slope statistics derived at three locations are presented and discussed.

Joint Statistics of Rain Attenuation at 20 GHz in Madrid and Aveiro

Rain attenuation is the most severe propagation impairment affecting radiowave signals in the Ka-band. The use of these frequencies for satellite communication systems is facilitated by the implementation of fade mitigation techniques (FMTs). Spatial correlation of rain rate and rain attenuation for distances of several hundred kilometers has been rarely characterized in experiments, however it is relevant for site diversity studies, with distances in this range, and for the design and operation of multibeam satellite systems with shared on-board radio resources. In this letter, experimental results regarding joint statistics of rain and rain attenuation are presented for Madrid, Spain, and Aveiro, Portugal, located 420 km apart. These results have been achieved from two years of simultaneous measurements of the Eutelsat HB-6 Ka-band beacon at 19.7 GHz. It is shown that high-attenuation events are independent, whereas there is some correlation for low-attenuation levels.