Rain attenuation in millimeter-wave links depends on the Drop Size Distributions (DSD) of the raindrops. Empirical models disregard this dependence and estimate the specific attenuation using only the integrated rainfall rate (R (mm/h)). This approach is valid for lower frequencies but it progressively losses accuracy as the frequency of interest becomes higher within the millimeter-wave range. Both the characterization of rainfall phenomena and the prediction of rain attenuation can be improved with the knowledge of DSD, which, in turn, depend on the type of rain event (stratiform or convective) and the R. In this paper, long-term DSD measurements from a vertical Doppler radar (MRR-2) and a laser optical disdrometer (Thies Laser disdrometer) are used to obtain, classify and compare the statistics of DSD in Madrid in periods of more than ten years. The process to obtain the DSD from these advanced instruments is analyzed in detail, providing recommendations about the calibration of the radar data and the most appropriate particle filtering to apply on the Laser disdrometer data.

The attenuation produced by rain can be derived from experimental Drop Size Distributions (DSD) using physical models of scattering in particles (Mie and its Rayleigh approximation). As the frequency increases within the mm-wave bands, the specific attenuation becomes more dependent on the DSD, whereas attenuation is mainly determined by the rain rate R in lower frequency bands. As is well-known, Mie scattering becomes dominant in the mm-wave band instead of Rayleigh scattering, which is the main extinction mechanism in cm-wave frequencies. In this document, long-term DSD measurements from an optical Laser disdrometer available in Madrid, Spain, were used to estimate the specific attenuation produced by rain. A very long period of twelve years has been used for the analysis of rain attenuation in the 100-200 GHz band. The results compare well on average with the ITU-R specific attenuation model of Rec. P.838-3, but they show a significant variability.

The use of EHF (30-300 GHz) in terrestrial radio links is one key element in future 5G and beyond 5G technologies. Rain attenuation is the major impairment affecting radio links operating at this band. Due to the lack of experimental measurements for the EHF band, one approach is to estimate rain attenuation by using information about the rain DSD. In previous works, rain attenuation and its variability have been estimated up to 200 GHz by applying the Mie Theory to experimental DSD, considering spherical raindrops. This work goes a step further and provides the derivation of rain attenuation using a non-spherical drop model. To that aim, full-wave electromagnetic software is used to derive the extinction cross section of non-spherical drops excited with horizontal and vertical polarizations. The results show that at such high frequencies the differences between polarizations are not large, but the variability of rain attenuation is not negligible.

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