On the Use of Experimental Drop Size Distributions From Different Instruments in Millimeter-Wave Propagation Studies

The attenuation produced by raindrops in millimeter-wave links relies on the Drop Size Distributions (DSD). However, empirical models dismiss this dependence and approximate the specific attenuation (γ (dB/km)) considering only the integrated rainfall rate (R (mm/h)). The use of empirical models is effective for relatively low frequencies but, as the frequency of interest becomes higher within the millimeter-wave range, empirical models lose accuracy. Moreover, the value of γ for a given R is expected to vary more than at lower frequencies, depending on the DSD. This paper aims to study this higher variability of γ, not incorporated in the current models, by calculating the specific attenuation γ from the experimental DSD measured by a vertical Doppler radar (MRR-2) and a Laser optical disdrometer (Thies Laser disdrometer) in periods of more than ten years. Both Ka-band and W-band have been considered for the calculations in order to understand the effect of increasing the frequency. Additionally, with the objective of evaluating the application of these techniques in propagation studies, the attenuation calculated from the DSD obtained from the above instruments has been compared to experimental values measured in a W-band terrestrial radio link at 75/85 GHz, yielding good results

Preprocessing and Assessment of Rain Drop Size Distributions Measured With a K-Band Doppler Radar and an Optical Disdrometer

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

Rain Attenuation of Millimeter Waves Investigated from Drop Size Distributions

Millimeter waves are expected to be among the main radio resources used in future 5G+ and 6G technologies. Rain attenuation is one of the most relevant propagation effects in these bands, compromising the maximum distance and/ or availability achievable at these frequencies. The scarcity of experimental measurements, especially above 40 GHz, can be mitigated by research based on detailed long-term meteorological data and physical models to predict rain attenuation. An investigation of this kind is presented in this paper, using a 14-year database of rain DSD gathered in Madrid, Spain, with a laser disdrometer and values of extinction cross-sections of drops with the normalized diameters of the disdrometer obtained with electromagnetic simulations for H and V polarization and a model for non-spherical raindrops. The results of the research are used to assess the predictions of the ITU-R model of specific attenuation versus rain rate, to analyze the impact of using this approach instead of the conventional use of that ITU-R model in the design of millimeter-wave links, and to validate, as far as possible, the procedures by the comparison with some available measurements from the same site

Characterization of Rain Attenuation in 80–200 GHz Radio Links Considering Non-Spherical Raindrops

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

Variability of Rain Attenuation in the 100-200 GHz Band Calculated from Experimental Drop Size Distributions

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

Characterization of Rain Attenuation in 80-200 GHz from Experimental Drop Size Distributions

The use of frequencies well into the EHF band (30-300 GHz) is a key element in 5G and beyond technologies. Rain attenuation is the major impairment affecting terrestrial radio links in this band. In the absence of enough experimental measurements, rain attenuation can be estimated by using physical models of radiowave scattering in rain drops combined with information about the rain Drop Size Distributions (DSD). In this work, the characterization of rain attenuation in the 80-200 GHz frequency range is carried out from a large database of twelve years of experimental DSD gathered in Madrid, Spain. Rain attenuation is estimated for each collected minute of rain using two approaches: a first one based on the assumption of raindrops as spherical and the application of the Mie theory, and a second one that uses a non-spherical raindrop model and electromagnetic simulations and is consequently more realistic. The main results show that the currently used ITU-R model generally underestimates rain specific attenuation; the influence of polarization becomes smaller and negligible as frequency approaches 200 GHz; and rain specific attenuation significantly varies due to the spread of the DSD. Moreover, a model for this variability of rain attenuation is also proposed