D. Cox

Delay Doppler characteristics of multipath propagation at 910 MHz in a suburban mobile radio environment

Statistical descriptions of the time delays and Doppler shifts associated with multipath propagation in a suburban mobile radio environment obtained from bandpass impulse response measurements are presented. The measuring equipment which has 0.1 mu s resolution in time delay and a data output bandwidth of less than 5 kHz is also described. For the first time small scale statistics of the multipath propagation for vehicle travel distances on the order of 30 m along streets are presented in the following forms: 1) average power-delay profiles made up of over 200 individual profiles, 2) cumulative distributions of signal amplitude at fixed delays, and 3) radio frequency Doppler spectra at fixed delays. Delay spreads for typical suburban streets are on the order of 0.25 mu s. Extreme cases have paths with significant amplitudes at excess delays of 5 to 7 mu s and the square root of the second central moment delay spreads up to about 2 mu s. Often the signal at fixed delays has a Rayleigh distributed amplitude but large departures from the Rayleigh distribution also occur. RF Doppler spectra at fixed delays indicate that some of the multipath is from one relatively discrete scattering center while at other delays several scattering centers distributed widely in angle are involved. The observed RF Doppler spectra are consistent with the cumulative amplitude distributions at the same delays.

Distributions of multipath delay spread and average excess delay for 910-MHz urban mobile radio paths

Distributions of average excess delay and delay spread for Gaussian wide-sense stationary uncorrelated scattering channels associated with 100 small scale areas at different locations within a 2 times 2.5 -km region of New York City are presented. For delay spread the mean is 1.3 mu s, the standard deviation is 0.6 mu s, the maximum value observed is 3frac{1}{2}mu s, and 10 percent of the areas exceeded 2frac{1}{2} mu s; for average excess delay the mean is 1.1 mu s, the standard deviation is 0.9 mu s, the maximum is 4 mu s, and 10 percent of the areas exceeded 2 mu s. The region is representative of the heavily built-up areas of many large cities in the United States.

Characteristics of rain and ice depolarization for a 19- and 28-GHz propagation path from a Comstar satellite

Relationships between rain and ice attenuation and depolarization for several incident polarizations have been determined experimentally for the first time. The relationships are based on measurements made using 19- and 28-GHz beacon transmissions from a Comstar satellite. Vertical and horizontal incident polarizations experience much less depolarization on the average than circular polarization or linear polarization incident at 45deg . The measurements confirm that the usual orientations of the major symmetry axes of nonspherical raindrops and ice crystals are nearly horizontal. Joint cumulative distributions of attenuation and depolarization for one year of continuous measurement at 19 and 28 GHz are also presented. These distributions are useful for determining joint attenuation and depolarization margins required to meet outage objectives of specific satellite communication systems. For example, the outage of a 19-GHz dual-polarized system received with a polarization angle 21deg from horizontal and having an attenuation margin of 20 dB would be depolarization-dominated unless the system could also tolerate at least -15-dB depolarization.

Phase and amplitude dispersion for Earth-satellite propagation in the 20- to 30-GHz frequency range

Amplitude and phase dispersion have been measured for over a year on a 19- and 28-GHz earth-space propagation path. In the experiment amplitude and phase differences were compared for a 28-GHz carrier with pm264 -MHz sidebands and a 19-GHz carrier, all transmitted from a COMSTAR satellite. No dispersion (frequency selective fading) was found of the type caused by multipath propagation with a large spread in time delay or by resonances in the propagation medium. The only frequency dependences evident were due to the bulk properties of water in rain. The conclusion from this investigation is that amplitude and phase dispersion should not pose a problem for wideband (on the order of 1 GHz) satellite communication systems operating at frequencies greater than 10 GHz with elevation angles from the earth terminals of greater than 15deg .

Ice depolarization statistics for 19-GHz satellite-to-earth propagation

Observations were made at 19 GHz of depolarization due to ice crystals along a satellite-earth path with a 38.6deg elevation angle. The one-year data base included sufficient information to determine depolarization for any incident polarization angle. Depolarization was often observed in the absence of significant rain-produced copolarized signal attenuation. This depolarization is caused by ice crystals whose symmetry axes, as observed in the plane perpendicular to the direction of propagation, were usually within 5deg of vertical and horizontal. Maximum depolarization was observed for 45deg linear or circular polarizations, and never exceeded -16 dB. For vertical and horizontal incident polarizations 8-10-dB lower maximum depolarization values were observed. Depolarization due to ice was also observed during most rain attenuation events. The unknown differential phase characteristics of rain-produced depolarization prevent further exact analysis of this ice depolarization.

Measured bounds on rain-scatter coupling between space-earth radio paths

Attempts were made to measure rain-scatter coupling for two satellite paths displaced in angle by 0.85deg . For rain attenuation up to 15 dB, upper bounds on coupling were -40 dB at 19 GHz and -45 dB at 28 GHz. These experimental bounds are limited by antenna sidelobe levels which are on the order of 20 dB greater than theoretical rain-scatter predictions.

Phase and amplitude measurements of transhorizon microwaves: Angular response patterns in elevation

Results from two 48-hour beam-swinging experiments in transhorizon microwave propagation are presented. A 3.2-GHz signal was transmitted over a 164-km path and received with a 12-element vertical antenna array (beamwidth 0.29deg elevation by 5deg azimuth). The beam was rapidly scanned in elevation. Experimental angular response patterns (antenna scans) averaged over about 3 min are compared with theoretical patterns computed from turbulent scattering theory. There is considerable variability in the experimental patterns for different time periods. The experimental patterns for different time periods are separated into three groups. Signal group 1 patterns resemble the response of the array to a point source. A propagation model based on partial reflection or refraction from a stratified atmospheric layer best describes these signal characteristics. Signal group 2 patterns can be described by a model predicting a smooth decrease in scattered power with scattering angle at a rate inversely proportional to the m th power of the angle with m between 4 and 10. A propagation model based on atmospheric turbulence can describe these signal characteristics only if it includes modifying factors to account for the difference in the exponent m observed at different time periods. Signal group 3 patterns are characterized by broadened maxima, by two or more maxima, or by a maximum which is significantly displaced above the horizon. These signals can be explained only by a model which contains a nonuniform scattering mechanism. A variation in signal characteristics as a function of time of day was evident in both experiments.

Antenna beamwidth independence of measured rain attenuation on a 28-GHz Earth-space path

Rain attenuation measured at 28 GHz on an earth-space path is independent of antenna beamwidth for beamwidths as small as 0.1deg and for attenuations up to 30 dB. The measurements imply that angle-of-arrival fluctuations are less than 0.02deg .