Availability improvement of free-space optical and microwave communications links — A diversity system

Free-space optical (FSO) and microwave telecommunication links have considerably adverse propagation characteristics. For radio waves that are operating in the millimeter wavelength one of the most harmful circumstantial factors is the attenuation caused by precipitation, especially by rain. However, for FSO links the main adverse effect is fog, being most responsible for the unavailability of FSO links. These adverse propagation characteristics allow good efficiency for a site diversity system that consists of parallel FSO and millimeterwave links. In order to study the propagation characteristics of these types of links a measurement system is operating at Budapest University of Technology and Economics, Department of Broadband Infocommunicatons and Electromagnetic Theory (BME-HVT). In this paper the efficiency of a parallel FSO-microwave site diversity system is presented on the basis of the results of the measurement system. The outage probabilities of the FSO and microwave links are compared to the outage probability of the diversity system. Moreover, the possibility of microwave transmitter power reduction is investigated as well in case of the application of the hybrid diversity system.