A ray-tracing approach is used to generate quantitative information on the ranges of angles-of-arrival, delay times, and amplitudes associated with individual paths in a multipath propagation situation as expected on typical terrestrial microwave links. Some experimental evidence relating to, and confirming, the predicted values is considered.

Angles-of-arrival and delay times on terrestrial line-of-sight microwave links

An analytic approach tO the modeling of refractivity profiles, together with ray-tracing considerations, is discussed with a view to providing relevant information regarding path parameters in a multipath situation. Some points arising from rays launched from within an elevated duct are raised and discussed.

Raypath parameters in tropospheric multipath propagation

It is very difficult to relate the behavior of a multipath microwave channel to its physical description. Much information, however, can be gained from radioelectrical probing of the channel, and models can be built independent of the complex underlying physical situation. In this paper we review the various experimental techniques used to investigate the channel, their successes, and their limitations. We conclude by stressing the directions in which progress has to be made.

Experimental probing of multipath microwave channels

Ray equations are derived for microwave multipath propagation in a two-layer atmosphere, and the connection between the different solutions is demonstrated by numerical examples. Analytical expressions are developed to characterize the condition for multiple rays, the variation in angle-of-arrival (AOA), and the delay spread. The results suggest that more than three rays will be rare in practice and that the three-ray model (obtained from the zero-order rays defined in the paper) may provide a good representation of selective fading on microwave links.

Characterization of multipath parameters for line-of-sight microwave propagation

Microwave propagation experiments were performed on two separate line-of-sight paths across the Bay of Fundy on the east coast of Canada. The main objective was to investigate the mechanisms of anomalous propagation observed on such oversea paths. An interferometer which employs a wide frequency sweep of 1 GHz (from 9.5 to 10.5 GHz) was used. The parameters studied include the amplitude, angle-of-arrival (AOA), and relative delay time of the received rays. The behavior of these parameters during quiet and disturbed periods are compared and discussed in detail. Statistical results on the ray parameters and on the received signal level are also presented. Explanations for the various experimental observations are offered; these were arrived at by using a ray tracing method combined with refractivity profile modeling. It was found that the observed propagation anomalies result from interactions between specular reflection and atmospheric layer formations.

Microwave propagation on two line-of-sight oversea paths

A microwave system for direct measurement of angles of arrival and multipath delay times is described. Based on the utilization of interferometer and very wide frequency sweeping (1- GHz range) techniques, high accuracy measurements on these parameters are possible. Some experimental results are presented which suggest that movements in the angle of arrival of a single ray path (as opposed to multipath) may be responsible for much of the fading experienced on light-of-sight microwave links in southwestern Ontario.

T. Ueno

Papers

Tropospheric microwave propagation--An X band diagnostic system