2 Outdoor radio links in the millimeter wave band

At frequencies above 10 GHz, the transmitting rate is appreciably better but some natural factors, such as atmospheric gaseous and precipitation, strongly affect propagation. In particular, interactions between electromagnetic waves and hydrometeors, such as rain, hail and snow, produce attenuation by energy absorption in particles and energy scattering in all directions. Thus, the designers of high frequency communication systems require from the propagation specialists, the ability to be able to foresee these attenuation effects to determine the adequate fading margins and to ensure reliable predetermined signal levels for different weather conditions.

An overestimation of the propagation effects can result, on one hand, in a very expensive conception of system because of the high prices of the implemented devices to limit the disturbing effects of propagation and, on the other hand, on interference with other services. Besides, an underestimation of these propagation effects can lead to unreliable systems. So, the studies of propagation should be as accurate as possible to satisfy in the best conditions, in a statistical way, the quality criteria of the operational radio links.

For some years , important efforts have been initiated by the international community to update the radio climatic databases from which global predictions of propagation are established for terrestrial radio links in the millimeter wavelength band (30 “300 GHz). These works were the subject, among others, of CLIMPARA conferences, organised by the committee F of the URSI and the European project (COST) and Canadian project. Many countries combined their efforts to establish recommendations about the propagation aspects of the radioelectric waves in nonionised environments, under the aegis of the International Telecommunications Union for Radio communications (ITU-R). However, these techniques of global predictions are known to suffer from an important lack of observations of propagation in various parts of the world.

So, this explains why FTR&D decided to establish an experimental setup (Figure 13.1), near Belfort in France. This experiment should contribute to a detailed knowledge of the physical mechanisms of atmospheric and meteorological phenomena and also of their interactions with the electromagnetic waves. The purpose of the setup is to supply propagation data over a short range for which the weather conditions are almost constant.

Figure 13.1: Belfort experimental station set up to supply propagation data over short range

The experimental device consists of multifrequency transmission links over an 800 meters terrestrial path. The propagation path is horizontal, with direct visibility about 5 m above the ground. Four dynamic narrow band links operate at 30, 50, 60 and 94 GHz, with vertical polarisation. Meteorological equipment, set out all along the path, includes three tipping bucket rain gauges to measure rainfall rates and their equivalent for other hydrometeors, two disdrometers to measure raindrop size distributions, two identification sensors to characterise the kind of precipitation and a meteorological station to measure temperature, atmospheric pressure, hygrometry, wind speed and direction.

Investigation of the important radioelectrical and meteorological database led to development of attenuation propagation models due to the hydrometeors and thus to satisfy the new needs of the systems designers.