1536-1225 (c) 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LAWP.2016.2633718, IEEE Antennas and Wireless Propagation Letters P2016-05 1 Abstract— This paper describes a short-term rain attenuation predictor model for terrestrial radio links located in tropical area. The predictor is based on the fade-slope statistics that are modeled by a generalization of Van de Kamp (VDK) distribution for rain attenuation levels between 1 and 30 dB. The predictor can improve adaptive fade countermeasures as it predicts the rain attenuation in the next ten-seconds based on the current attenuation value. The model is applied to data collected in four links located in tropical region and its results show that the absolute values of prediction errors are under 2.9 dB for 99.9% of the predictions. Index Terms— Fading countermeasures, prediction model, rain attenuation, terrestrial links, tropical region. I. INTRODUCTION ADIO links operating at frequencies above about 10 GHz are subjected to severe impairments due to the propagation in the troposphere. Rain attenuation is the main cause of link unavailability and adaptive fade countermeasures are required to counteract rain effects, e.g. by increasing transmitted power or changing modulation coding scheme. Stochastic models of the rain attenuation process have been proposed to synthesize long term rain attenuation time series for terrestrial and satellite links in different regions [1]-[5]. These models reproduce successfully the stationary behavior of rain attenuation in terms of time percentage that an attenuation level is exceeded and depend on the link parameters and the climatologic characteristics of the regions where experimental data are collected. These models also aim to reproduce the dynamic characteristics of the rain attenuation. The synthesized time series can be used for developing and evaluating the performance of adaptive fade countermeasures since the rate of change of rain attenuation, referred as fade-slope, and duration of rain events are input parameters for these countermeasures. However, operation of the adaptive techniques also requires Manuscript received July 30, 2016. This manuscript is supported by Fapemig and the work is supported in part by CNPq under covenant 573939/2008-0 (INCT-CSF). F. J. A. Andrade and A. A. M. de Medeiros are with Federal University of Juiz de Fora, Rua Jose Lourenço Kelmer s/n, Juiz de Fora, MG, 36036- 900, Brazil. (e-mails: {fjaandrade, alvaro}@engenharia.ufjf.br). L. A. R. da Silva Mello is with Centre for Telecommunication Studies, Pontifical Catholic University of Rio de Janeiro, Rua Marquês de São Vicente 225, Ala Kennedy, Rio de Janeiro, RJ, 22453-900, Brazil. (e-mail: smello@cetuc.puc-rio.br). real-time prediction of the attenuation during the occurrence rain events. By analyzing beacon signals received from satellite links subjected to rain attenuation up to 22 dB at different frequencies in a temperate region, Van de Kamp [6] studied the fade slope properties and proposed a fade-slope conditional distribution model for given values of attenuation, assuming that the fade-slope standard deviation is linearly proportional to attenuation level in satellite signals. The Van de Kamp (VDK) distribution-based model was subsequently applied to predict rain attenuation in satellite links, using the relationship between the fade-slope standard deviation and attenuation level [7]. It performs successfully when experimental and predicted short-term attenuation are compared. Recently, experimental data from a satellite link located in a tropical region were used to propose a modification of the VDK model using a third- order polynomial to model the dependency between fade- slope standard deviation and attenuation level for attenuation values up to 10 dB [8]. In this letter, we present a predictor model for the attenuation in terrestrial links located in tropical area. Statistics of fade-slope were derived from experimental data collected in four convergent terrestrial links with different path lengths located in a tropical region. In our case, it is observed that the variance of the fade-slope, rather than its standard deviation, is linearly proportional to the attenuation level in the range from 1 to 30 dB. Furthermore, we propose a generalization of VDK distribution to model the fade-slope statistics. Based on that, a short-term predictor model for attenuation during rain events, that considers the current rain attenuation value, is proposed. II. EXPERIMENTAL DATA Experimental rain attenuation time series were obtained in a two year measurement campaign in line-of-sight terrestrial links located in the city of São Paulo, in the tropical region in Brazil. Data from four convergent links with different path lengths operating in vertical polarization at frequencies around 14.5 GHz were used to investigate the dependence of fade-slope statistics with the path length. Link characteristics are given in Table I and more detailed information about the measurement campaign can be found in [2], [3], [9]. In the experiment, the data sampling rate for Short-Term Rain Attenuation Predictor for Terrestrial Links in Tropical Area Fernando J. A. Andrade, Álvaro A. M. de Medeiros and Luiz A. R. da Silva Mello R