[1] Attenuation of radio waves caused by precipitation, especially in the form of rain, is considered to be the limiting factor for new communication systems that will exploit the radio wave spectrum at frequencies higher than about 30 GHz. Over the last 40 years, much effort has gone into theoretical studies characterizing rain in terms of statistical drop size distributions (DSDs), the shapes and velocity dependence of raindrops, and the calculation of raindrop extinction cross sections. This paper focuses on specific data sets and data processing and different ways of viewing DSDs that may help in quantifying some of the important parameters in radio wave propagation from experimental data. Values for the coefficients k and a in the relationship for specific rain attenuation g = kR a are presented, together with rain-rate-dependent parameters for fits of DSDs to standard statistical distributions. These are based on data from Chilbolton, England, and from Singapore. The distributions measured at Chilbolton and Singapore are very different, which strongly suggests that drop size distributions differ under different climatic conditions. Comparisons are also presented of attenuations calculated with values of k and a determined from the DSDs and values found from logarithmic regression between simultaneous rain rate and attenuation measurements at 57, 97, 135, and 210 GHz at Chilbolton. This paper gives a strong indication that the International Telecommunication Union Radiocommunication Sector model for specific rain attenuation is inadequate at frequencies higher than about 70 GHz.
The use of millimeter radio communication systems has seen a remarkable increase in recent years. However, there remains a need for validation and improvement of millimeter propagation prediction methods for safe design and operation of the systems. At this high frequency range, propagation related degradation is primarily caused by rainfall. The goal is an improved method for predicting specific attenuation due to rainfall. A general method for establishing the relationship between rainfall rate and specific attenuation is developed. Drop size distributions (DSDs) measured by a Joss distrometer are analyzed, and the three climate classification parameters rain rate, DSD median, and DSD mode are used in the development of these new methods. These parameters are investigated as candidate classes to categorize drop size distributions. Categories of the classes are then established to improve the long-term DSD estimate. The typical long-term DSD is used to derive new prediction methods that are compared with observed attenuation. The influence of wind on the rainfall measurements is studied, and a corresponding correction is suggested, reducing the prediction error significantly. Testing of the three new methods using measurements at 40 and 60 GHz shows noticeable improvement over the method recommended by the International Telecommunication Union (ITU) using measurements at 60 GHz at deep attenuation.Index Terms-Drop size distributions (DSDs), millimeter wave attenuation, propagation prediction methods, rainfall rate.
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