Propagation studies in millimeter-wave link systems

The overall performance characteristics of ground-level millimeter-wave links are analyzed. Transmission reliability is examined on the basis of statistical data on pertinent meteorological variables and as a function of various link design parameters. Link systems operating under climatic conditions roughly characteristic of those prevailing along the mid-Atlantic coast of the United States are found to he limited most severely by attenuation due to rainfall. For a required degree of transmission reliability, trade-offs between the basic link design parameters are seen to lead to a certain range of hop distance and a corresponding range of transmitter power level. Detrimental effects due to reflection and diffraction by the terrain may in most cases be essentially avoided for short and medium hop distances. The discussion also includes considerations on diversity operation for increased transmission reliability and a brief comparison of the advantages of millimeter waves over both microwaves and optical wavelengths for links with short and medium hop lengths. Experimental work is reported on point-to-point transmission at 90.5 GHz (or 3.3 mm wavelength) carried out over a period of more than one year. Operation with antennas exposed to precipitation and wind has been found feasible. Measured transmission losses under a wide range of weather conditions were found to stay well within the limits predicted. The need for more research on short-term signal fluctuations ("scintillations") and propagation-time fluctuations is pointed out. The theoretical and experimental results have established a considerable degree of confidence in the feasibility of millimeter-wave link systems with short to medium hop distances and low power requirements.