The peculiarities of the climatic conditions in the Baltic Sea Region are reviewed. According to the values of rain rates measured in the Lithuanian Weather Stations with the 10minute's integration time, a relation between the rain rate and the annual precipitation has been derived. The model for one-minute rain rate calculation on the months starting from May up to September in Lithuania has been presented. The values of the electromagnetic waves attenuation due to the rain have been determined. The cloud attenuation has been computed by using the meteorological data measured at the ground level. The semi empirical method has been used. The values of the specific attenuation under conditions of cloud cover have been determined.

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Yearly, seasonal and daily variations of radio refractivity have been analyzed. The method proposed in the recommendation of International Telecommunication Union ITU has been used. The local meteorological data have been used in calculation of radio refractivity. The highest values of the radio refractivity have been observed in Klaipėda (in Seacoast) in the year 2009. In July, the values of the radio refractivity were highest in all localities investigated here and over all the time of the day in the year 2009. In the continental part of Lithuania (in Vilnius and Kaunas), analysis of radio refractivity has been made by using the meteorological data of longer period (starting from April 2005 up to July 2010). Five-year meteorological data collected in February, April, July and October have been used. It was obtained that the values of refractivity in the year 2010 are much higher than ones obtained in all the years of the period investigated here.

The increase of data traffic, a demand for high-speed reliable mobile networks and congested frequency bands raised both technological and regulatory challenges. Therefore, the fifth-generation mobile network (5G) is being developed. Recently, researchers have focused on a very promising terahertz (THz) band (frequencies from 100 GHz to 30 THz), which will allow fast transmission of huge amounts of data. However, transmission distance is limited due to atmospheric attenuation, as THz waves undergo significant absorption by water vapor and oxygen molecules in the atmosphere. Moreover, THz waves are very vulnerable by precipitation. Furthermore, the path of the propagating waves changes due to variations of the atmospheric refractive index. Nevertheless, the THz networks could be perfect candidates for fiber-to-THz bridges in difficult-to-access areas. The aim of this chapter is to present the possibilities and challenges of the THz networks from a point of view of atmospheric attenuation. The results show that simulations of the atmospheric attenuation using real-time data are a powerful tool that should complement technological basis, as it will help to foresee possible failures, extend transmission distance and improve reliability of the THz and other high-frequency broadband wireless networks.

In Vilnius, the daily and the seasonal variations of the radio refractivity values have been computed at the Earth surface in the years 2005-2010 and at the heights up to 1.2 km in November 2010. The International Telecommunications Union - Radiocommunications Sector (ITU-R) model was used for calculation of the radio refractive index according to the peculiarities of climatic conditions of Lithuania. As a result of obtained data analysis, the days with more variable meteorological parameters have been chosen for more detailed investigation. The radio refractivity gradient has been computed at the heights starting from the Earth surface up to 1.2 km above the ground, using the meteorological data measured at those heights. The variation of radio refractivity and its gradient are influenced by the changes in air's humidity and temperature in most part. Ill. 7, bibl. 17, tabl. 1 (in English; abstracts in English and Lithuanian).http://dx.doi.org/10.5755/j01.eee.111.5.349

The semi-empirical method for determination of the cloud attenuation was used. The cloud attenuation was determined by using the meteorological data measured at the ground level. It was assumed that the clouds would form at some height above the ground level when the conditions for vapour condensation would be present and the liquid water content in the air would be above zero at that height. The values of height have been determined by using the values of temperature at the ground level, the dew point temperature, and the temperature gradient. The calculation results show that known relation between the temperature and the cloud base height is not always suitable for Lithuanian climate conditions. According to the meteorological data measured in the weather stations, relation between the height and temperature within the cloud, recommended by International Telecommunication Union, was chosen. Only summer profile was suitable to use under conditions investigated here. The values of relative humidity and temperature at the ground level were used in calculations of liquid water content within the clouds. The values of specific attenuation under conditions of cloud cover were computed by using the obtained liquid water content values at frequencies starting from 10 GHz and up to 70 GHz.

Statistical peculiarities of terahertz (THz) wave attenuation in heavy rain conditions are evaluated. The expected extreme densification of the infrastructure and the application of highly directional beams of 5th generation (5G) and beyond 5G (B5G) wireless networks were taken into account. Calculations were performed emulating both drop size distributions of the real rain and the laboratory-controlled rain described in literature. Simulation results revealed that absorbance fluctuations of more than one percent would occur if THz waves and raindrops interact within the 100 m3volume. For much smaller volumes, short distances and narrow beams used for experiments with the laboratory-controlled rain, absorbance uncertainties could exceed the average absorbance value. A comparison of the simulation results at fixed average absorbance revealed that slightly lower uncertainties were expected in the case of a single raindrop size when compared to the Weibull distribution approximating the real rain. Nevertheless, in both cases the predicted deviations were substantially smaller than observed in the previously published experimental results. This fact predicts a new future application possibility for such laboratory-based experiments – they can be employed to predict the performance of wireless THz data transmission links when the resilience margin is required. Since much of the existing industrial test equipment is not designed to carry out calibrated over-the-air measurements of 5G/B5G wireless networks, such experiments can be employed to primarily predict the performance of data transmission links.