Cross polarization during precipitation on terrestrial links: A review

Abstract

This paper reviews recent theoretical and experimental research on cross polarization that occurs during precipitation on terrestrial microwave links. Consideration is first given to the physical mechanisms, such as raindrop canting, that cause precipitation depolarization. Coherent rain depolarization models are then discussed and recent investigations of incoherent depolarization effects summarized. Particular attention is given to semi-empirical models since they can be used most readily for statistical predictions in design applications. 'Model oriented' cross polarization measurements aimed at characterizing the physical properties of the depolarizing medium for use in model development are also given particular attention. Effective parameters of the raindrop canting angle distribution derived from these measurements are tabulated. Finally, some mention is made of the relative importance of rain depolarization and attenuation in causing outages on digital radio systems. INTRODUCTIONMany terrestrial analog radio systems now employ orthogonal linear polarizations on adjacent channels in order to minimize frequency separation and therefore increase capacity. Furthermore, an increasing number of digital radio systems that use orthogonal polarizations in the same frequency band are coming into operation. In both systems an adequate cross-polarization isolation (XPI) between these orthogonally polarized channels must be maintained. A major limiting factor to the reliability of operation, however, is the deterioration in XPI and consequent cross-polarization interference caused by propagation effects along the path. Some of these effects are now reasonably well understood, and practical propagation models for use in statistical predictions are being developed. This paper reviews recent theoretical and experimental research on the cross polarization that occurs on terrestrial paths as a result of precipitation. A companion paper [Olsen, 198 la] reviews corresponding research on cross polarization during clear-air conditions. It has long been recognized from theory [Oguchi, 1960] and experiment [Okamura et al., 1961] that rain attenuation is polarization dependent. Interest in precipitation-induced cross polarization was stimulated in the late 1960's by the realization thatthis could impose a limitation to systems utilizing dual polarizations. Since that time there has been an increasing effort, both theoretically and experimentally, to characterize the problem.Much of the emphasis in this review is on theoretical and experimental work that has contributed to a better understanding of the precipitation depolarization mechanisms and the development of propagation models. Particular emphasis is placed on semi-empirical models that can be readily used for statistical predictions in design applications. Although some consideration is given to the effects of cross polarization on particular types of radio systems, the review is less complete on this aspect.Since an extensive overall review of the most significant phenomen...

Drop Shapes, Model Comparisons, and Calculations of Polarimetric Radar Parameters in Rain

“…The shape information is also relevant in evaluating propagation effects on line-of-sight systems and earthspace communication links operating at microwave frequencies (Oguchi 1983;Olsen 1981;Allnutt 1989, chapters 4 and 5). Effects such as differential attenuation and rain-induced cross polarization, which are particularly relevant for systems employing orthogonal polarizations, are governed by the probable drop shapes and size distributions.…”

Cross polarization on line-of-sight links in a tropical location: effects of the variation in canting angle and rain dropsize distributions

“…Using small argument approximations based on an assumption by [6] that the propagation constants associated with the two characteristic polarizations propagated through a rain medium without depolarization are approximately equal, [2] simplified the semiempirical model of XPD prediction from which the theory is derived. The XPD and CPA are thus approximated by XPD (1) and CPA (2) where and represent the specific attenuations associated with the principal planes of the raindrop axes (major and minor axes of the raindrop) where (3) with and representing the differential attenuation and phase shift, respectively.…”

“…The XPD and CPA are thus approximated by XPD (1) and CPA (2) where and represent the specific attenuations associated with the principal planes of the raindrop axes (major and minor axes of the raindrop) where (3) with and representing the differential attenuation and phase shift, respectively. defines the effective canting angle is the polarization tilt angle ( for horizontal polarization and for the vertical polarization), is the effective standard deviation of the canting angle distribution, represents the path elevation angle ( for terrestrial propagation path), and is a measure of the pathlength through uniform rain.…”

“…Studies have, however, shown that the orientation of the raindrops varies along the path. The raindrop orientation tends to be approximately Gaussian distributed along a terrestrial path [2], [6]. Therefore, the previous studies tend to produce poor levels of cross-polarization isolation.…”

“…Both theory and experiments on terrestrial links, especially in the mid and high latitude regions, have shown that rain is the most important hydrometeor responsible for the reduction in the cross-polarization discrimination (XPD) on terrestrial path, especially for the high availability time requirements [2]. It is also clearly understood now that rain depolarization occurs due to: 1) the nonsphericity of the falling raindrops, thus making the specific attenuation polarization dependent and 2) the tendency for raindrops to align in a particular direction at a time (canting angle).…”