Fade Duration Research Articles

Rain-rate duration statistics over a five-year period: a tool for assessing slant path fade durations

A rain gauge network of 10 tipping bucket rain gauges on the Mid-Atlantic coast of the United States has been in continuous operation since June 1, 1986. Rain-rate distributions and estimated slant path fade distributions at 20 and 30 GHz covering the first five-year period have been derived from the gauge network measurements and published data of Goldhirsh, Krichevsky and Gebo (see ibid., vol.40, no.11, p.1408, 1992). In this article, we present rain-rate time duration statistics. The conversion of rain-rate duration statistics derived from in situ measurements to slant path fade duration statistics is complicated because of the vertical and lateral inhomogeneity of the rain. A benchmark set of fade duration statistics at 20 and 30 GHz for a vertical path is derived from the rain-rate duration statistics employing Crane's (1980) global model. These results may be used by investigators for comparison with and/or conversion to slant path fade duration statistics. Such statistics are important for better assessing optimal coding procedures over defined bandwidths. >

Results from the Virginia Tech propagation experiment using the Olympus satellite 12, 20 and 30 GHz beacons

A comprehensive set of propagation experiments was performed using the Olympus satellite 12, 20, and 30 GHz beacons. This set of experiments is unique in North America because of simultaneous reception of signals spanning the Ku- and Ka-bands from the same orbital slot, which permits direct inference of the frequency behavior of signal variations. The elevation angle from the receiving site in Blacksburg, VA, to the satellite was 14 degrees. Beacon, radiometric, and weather data for one year were analyzed. The statistical results for rain rate, beacon attenuation, attenuation ratios, radiometrically derived attenuation, fade duration and fade slope are presented. They are important to the design of Ku- and Ka-band satellite communication systems. The beacon attenuation results include cumulative statistics for attenuation with respect to free space and with respect to clear air. Attenuation ratio data are presented using attenuation with respect to clear air to focus on rain effects. Instantaneous attenuation ratios computed from instantaneous beacon levels were found to be nearly identical to statistical attenuation ratios obtained from cumulative attenuation statistics at each frequency.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Statistical characterization of Rician multipath effects in a mobile-to-mobile communication channel

A statistical model is developed for a narrowband mobile-to-mobile channel taking into consideration Rician scattering near receiving and transmitting antennas both individually and concomitantly. From the proposed channel model we obtain the probability density function of the received signal envelope, the time correlation function and RF spectrum of the received signal, and level crossing rates and average fade durations. We briefly discuss the impact of these parameters on communication networks supporting an intelligent vehicle highway system (IVHS).

Empirical models for rain fade time on Ku- and Ka-band satellite links

Rain attenuation data from the OLYMPUS satellite beacon measurements are used to investigate fade time in the Ku and Ka bands. Using statistical procedures, an empirical model is developed which predicts fade time as a function of attenuation level, frequency of operation, and fade duration interval. Total annual fade times are predicted over a frequency span of 12-30 GHz for attenuation levels in the 3-18 dB range and for fade duration intervals of 30-60 s, 60-120 s, 2-5 min, and 5-20 min. The predicted fade times are in good agreement with the measured values. An alternate model, described by two simple relationships in two different ranges of attenuation level, is also presented by simplifying the original single-equation model. The simplified model accounts for fades associated with stratiform rain and thunderstorms separately.

Three years of C band signal measurements for overwater, line‐of‐sight links in the mid‐Atlantic coast: 1. Fade statistics

Three years of fade statistics are described for two overwater, line‐of‐sight propagation linksalong the mid‐Atlantic coast of the United States operating at 4.7 GHz. Single‐terminal, joint fade, and fade duration statistics derived from time series of received signal losses due to refractive and diffractive effects are described for combined‐year, annual, and monthly cases. In particular, year‐to‐year and month‐to‐month variabilities in the statistics and also the efficacy of employing space diversity for the two links to mitigate fade margin requirements are examined. Sustained deep fade (SDF) events due to severe subrefraction during the 3‐year period tended to dominate the statistics. Statistics have been culled in terms of contiguous month groupings during which SDF events occurred (November–July) and did not occur (August–October) over the 3‐year period. Although the year‐to‐year variability in the annual fade statistics was relatively small, the year‐to‐year variability in the monthly statistics was large, especially during the April–July period, which separated the periods during which deep subrefractive fades were maximum and the periods during which they were nonexistent.

A new method for the prediction of fade duration statistics in satellite links above 10 GHz

AbstractA model for the statistical prediction of fade durations, applicable both to scintillation and rain‐induced effects, is presented in this paper. It assumes that durations longer than about 1 minute are lognormally distributed whereas shorter durations follow a power‐law distribution. The model is assessed using data contained in the CCIR data bank and from measurements performed with the SIRIO and Olympus satellites. The model is tested using the same data, and the results give multiplicative errors in the range from 0.6 to 1.8 (averages) and from 0.3 to 3.0 (r.m.s.) in the estimation of the fade durations exceeded for fixed probability.

Statistical properties of mobile-to-mobile land communication channels

Statistical properties of mobile-to-mobile land communication channels have been developed. In particular, the level-crossing rate and duration of fades of the envelope, the probability distribution of random FM, the expected number of crossings of the random phase and random FM of the channel, and the power spectrum of random FM of the channel have been considered.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Space diversity and fade duration statistics at C-band for two overwater, line-of-sight propagation paths in the mid-Atlantic coast

The authors characterize space diversity and fade duration statistics at 4.7 GHz for two line-of-sight (LOS) overwater links in the mid-Atlantic coast of the United States. Single-terminal and joint probabilities are generated for both annual and monthly cases for fade and fade duration distributions over the year period from June 1, 1989 to May 31, 1990. The fade and duration distributions during the months November-June were heavily biased by sustained deep fade events. These events were caused by persistent and severe subrefraction, which generally existed simultaneously over both sites. Although the height geometries are dissimilar (13.7 m and 45.5 m), these events mitigated the effectiveness of space diversity. During periods in February and March 1990, for example, fading continuously exceeded 20 dB (relative to the free-space power) at both sites for a duration in excess of 24 h. Space diversity does however appear effective during those months in which fading were predominantly caused by ducting (July-October) and when sustained deep fade events were generally not present. >

Effect of multipath fading on millimetre wave propagation: a field study

Analysis of clear air fading encountered on millimetric wave and infrared radio links are presented. The analysis is based on data obtained during a four-year field study in the city of Riyadh, Saudi Arabia. The region can be con- sidered as a typical arid climate where the rate of evaporation is higher than the rate of precipi- tation. Maximum rain rate is of the order of 30 mm/h for 0.001% of the year. A brief descrip- tion of the experimental setup is presented, together with the results of measuring clear air fading experienced by the radio links. Statistical characterisation of fading is given for both the millimetric wave links operating near 40 GHz and the infrared radio link at a wavelength of 0.88 pm. It is shown that fading is dominated by multipath, having a Rayleigh amplitude distribution and an occurrence factor similar to the microwave band. Fades were highly correlated for the two links sharing the same path and separated in frequency by 1%. Multipath fading was also measured on the infrared link. Even at the short hop length at 0.75 km, the occurrence factor was about 1%. Time duration of fades are also analysed, and fade durations were essentially exponentially distrib- uted. Small fades, however, have normal duration distribution.

Fade statistics and propagation events at C band for two overwater, line‐of‐sight propagation paths over a 1‐year period

We examine signal fading statistics over a 1‐year period corresponding to two overwater, line‐of‐sight, propagation links in the mid‐Atlantic coast of the United States. These links are composed of a transmitter on a tower at Parramore Island, Virginia, operating at 4.7 GHz, sending simultaneous CW signals to two receiver systems located on a lighthouse and a lookout tower on Assateague Beach, Virginia, at distances of 44 and 39 km, respectively. The receiving sites are separated by approximately 5 km. Cumulative fade distributions corresponding to yearly, monthly, and diurnal time scales were derived. The annual distributions showed fades in excess of 49 and 54 dB during approximately 7.7 hours of the year, for the lighthouse and lookout tower links, respectively. Fade duration statistics corresponding to sustained attenuation events were also derived. These events, which were arbitrarily defined as having fades relative to free space powers in excess of 20 dB for durations of 2 hours or more, are believed to be generally due to extreme subrefractive conditions. Analysis of synoptic weather conditions and nearby rawinsonde data during four sustained deep fading periods showed atmospheric conditions consistent with extreme subrefraction, where the refractivity height profiles had positive lapse rates. A measured refractivity profile, having a positive lapse rate, was injected into a propagation model based on an approximation to the scalar Helmholtz wave equation, and fade levels in the vicinity of the antenna locations were calculated. Although the refractivity profile was measured during an earlier period in the vicinity of the links, it was obtained over the water and showed an extreme positive lapse rate in refractivity, indicating that such profiles do in fact occur. Furthermore, the calculated fades were noted to have the same range of values as measured during the sustained deep fade events, showing consistency with the hypothesis that severe subrefraction may be the cause of these events.

Channel modeling and simulation in satellite mobile communication systems

An analog model describing signal amplitude and phase variations on shadowed satellite mobile channels is proposed. A linear combination of log-normal, Rayleigh, and Rice models is used to describe signal variations over an area with constant environment attributes while an M-state Markov chain is applied to represent environment parameter variations. Channel parameters are evaluated from the experimental data and utilized to verify a simulation model. Results, presented in the form of signal waveforms, probability density functions, fade durations, and average bit and block error rates, show close agreement with measurements. >

Fade-durations derived from land-mobile-satellite measurements in Australia

Transmissions from the Japanese ETS-V geostationary satellite were measured at L-band (1.5 GHz) in a vehicle driving on roads of south-eastern Australia. The measurements were part of a program designed to characterize propagation effects due to roadside trees and terrain for mobile-satellite-service (MSS). It is shown that the cumulative distributions of fade and nonfade durations follow a lognormal and power law, respectively. At 1% probability, fades last 2-8 m, and nonfades 10-100 m, depending on the degree of shadowing. Phase fluctuations are generally small, allowing the channel characteristics to be estimated from levels only. >

Computer models for fading channels with applications to digital transmission

The authors describe computer models for Rayleigh, Rician, log-normal, and land mobile satellite fading channels. All computer models for the fading channels are based on the manipulation of a white Gaussian random process. This process is approximated by a sum of sinusoids with random phase angle. These models compare very well with analytical models in terms of their probability distribution of envelope and phase of the fading signal. For the land mobile satellite fading channel, results of level crossing rate and average fade duration are given. These results show that the computer models can provide a good coarse estimate of the time statistic of the faded signal. Also, for the land mobile satellite fading channel, the results show that a 3-pole Butterworth shaping filter should be used with the model. An example of the application of the land mobile satellite fading channel model to predict the performance of a differential phase-shift keying signal is described. >

Carotid Artery Disease Assessed by Color Doppler Flow Imaging: Correlation with Standard Doppler Sonography and Angiography

Carotid artery disease was assessed in 180 patients by means of color Doppler flow imaging. Color Doppler findings in 360 carotid arteries were compared with the results of standard Doppler sonography, and color Doppler findings in 60 bifurcations were compared with the results of intraarterial angiography. The sensitivity of color Doppler for the detection of carotid disease was 100% when compared with angiography. The accuracy of color Doppler in classifying minor (40-60%), moderate (61-80%), and severe (81-90%) stenosis ranged from 91.3% to 97.8% vs standard Doppler sonography, and from 91.7% to 95.8% vs angiography. Whereas all occlusions were identified correctly by both color Doppler and angiography, four pseudoocclusions of the carotid artery were misdiagnosed as occluded. Characteristic features providing reliable criteria of the degree of stenosis are (1) intensity, extent, and duration of color fading; (2) postprocessed systolic peak frequency; (3) plaque extent on serial sonograms; and (4) poststenotic flow patterns. Display of hemodynamic disturbances induced by less pronounced plaques showed highly variable patterns that could not be anticipated from the plaque morphology alone. Thus, color Doppler preserves the advantages of standard Doppler and duplex sonography but provides additional information about otherwise anechoic necrotic and thrombotic material that often causes cerebral embolisms. With atherogenesis, repair mechanisms may be sustained or progression be stopped by reducing the risk factors and instituting medical treatment; thus, the application of this noninvasive technique is important.

SIRIO-OTS 12 GHz orbital diversity experiment at Fucino

When rain occurs at a site, orbital diversity (OD) may be an alternative diversity scheme for future satellite communications above 10 GHz. The SIRIO-OTS OD attenuation measurements at 12 GHz with an aperture angle of 27.8 degrees at Fucino show that the statistical gain obtainable is 20-25% of the OTS attenuation in decibels (if OTS is assumed as the satellite of the normal mode and SIRIO as OD resource, or spare link), or 30-40% of the SIRIO attenuation (if SIRIO is assumed as the satellite of normal mode, and OTS as OD resource). At 12 GHz these percentages represent a gain of a few decibels but at higher frequencies, such as 20/30 GHz, the gain may be multiplied by 2.5 to 5, giving noticeable power reductions. The instantaneous gain is also analyzed. The fundamental aspects of balancing an unbalanced diversity configuration are shown by applying the theory to the unbalanced SIRIO-OTS OD. Fade durations of the joint link show the same characteristics as the single-path ones: few long fades and many short fades. More important, the intervals of time in which the resource is used are long; this fact is favorable to the diversity configuration since switchover inefficiency is lowered. >

Carotid artery disease assessed by color Doppler flow imaging: correlation with standard Doppler sonography and angiography.

Carotid artery disease was assessed in 180 patients by means of color Doppler flow imaging. Color Doppler findings in 360 carotid arteries were compared with the results of standard Doppler sonography, and color Doppler findings in 60 bifurcations were compared with the results of intraarterial angiography. The sensitivity of color Doppler for the detection of carotid disease was 100% when compared with angiography. The accuracy of color Doppler in classifying minor (40-60%), moderate (61-80%), and severe (81-90%) stenosis ranged from 91.3% to 97.8% vs standard Doppler sonography, and from 91.7% to 95.8% vs angiography. Whereas all occlusions were identified correctly by both color Doppler and angiography, four pseudoocclusions of the carotid artery were misdiagnosed as occluded. Characteristic features providing reliable criteria of the degree of stenosis are (1) intensity, extent, and duration of color fading; (2) postprocessed systolic peak frequency; (3) plaque extent on serial sonograms; and (4) poststenotic flow patterns. Display of hemodynamic disturbances induced by less pronounced plaques showed highly variable patterns that could not be anticipated from the plaque morphology alone. Thus, color Doppler preserves the advantages of standard Doppler and duplex sonography but provides additional information about otherwise anechoic necrotic and thrombotic material that often causes cerebral embolisms. With atherogenesis, repair mechanisms may be sustained or progression be stopped by reducing the risk factors and instituting medical treatment; thus, the application of this noninvasive technique is important.

11·6 GHz rain attenuation measurements in Peru

AbstractThis paper presents the results of a Ku‐band propagation experiment conducted in a high‐rainfall‐rate region in Peru. Sky noise measurements were made with an 11·6 GHz radiometer over a period of 18 months. Simultaneous rainfall was also recorded with a tipping bucket rain‐gauge. The paper describes the experimental equipment, the characteristics of the measurement site, and the results obtained from the experiment in the form of long‐term distributions of rain rate and path attenuation. Duration statistics of attenuation are also presented.In the absence of an absolute calibration of the radiometer in terms of the path attenuation, selection of the best medium temperature to be used poses a problem. For comparison, sky noise to path attenuation conversion was done with two medium temperatures, 280 and 290 K. When compared with model predictions, 290 K seems to be more appropriate for the climate concerned. Both the attenuation and fade duration statistics are found to follow a lognormal distribution fairly closely.

Effects of correlated fading on level crossing rates and average fade durations with predetection diversity reception

General expressions for the level crossing rate (LCR) and average fade duration (AFD) are obtained for several diversity combining schemes employing two-branch predetection reception of correlated Rayleigh fading signals. These expressions are obtained from joint and conditional probability density functions (PDFs) of the received signals, and lead to a unified treatment. This simplified method contrasts with the characteristic function approach used in previous investigations. Numerical results are presented for a space-diversity system using horizontally spaced antennas at a mobile station. It is shown that while the angle between the antenna axis and the direction of vehicle motion does not appear in the cumulative distribution function (CDF) of the combined output signal envelopes, it affects the LCR and AFD when the two fading signals are correlated. When the two antennas parallel with the direction of vehicle motion are used, the LCR can be reduced below the value obtainable from signals which fade independently. When the two antennas are perpendicular to the direction of vehicle motion, the AFD is loosely dependent on the antenna spacing and, provided the antenna spacing is not too small, is approximately half that for the no-diversity case.

Level crossing rate and average fade duration for time diversity reception in Rayleigh fading conditions

Time diversity, in which the same data are transmitted several times, is attractive in digital land mobile radio and is simple to implement because only one antenna is required. Expressions for the level crossing rate (LCR) and average fade duration (AFD) are derived in this paper for a system in which each data symbol is transmitted twice (two-branch diversity). As far as LCR is concerned, the expected diversity advantages can be obtained for a data repetition period, normalised by the maximum Doppler frequency, of about 0.2. For a large data repetition period, the AFD is halved. Measured values of LCR and AFD, obtained from 900 MHz signals received at a base station site, are in good agreement with the predicted values.

Fade duration statistics of&amp;lt;tex&amp;gt;L&amp;lt;/tex&amp;gt;-band multipath fading due to sea surface reflection

Multipath fading in maritime satellite communications may be caused by a combination of sea surface reflection and tropospheric effects such as ducting and scintillations. Among them, multipath fading due to sea reflection is dominant when receiving satellite signals by use of a wide beamwidth antenna. In this paper, fade duration statistics of multipath fading due to sea surface reflection are analyzed using the data obtained by on-board experiments on the L -band (1.54 GHz). Results indicate that the mean value of fade duration and fade occurrence interval can be determined from the fading power spectrum with fairly good precision, and that the probability density function for a given time percentage, ranging from 50 to 99 percent, approximates well with the exponential distribution. Finally, based on the results obtained, a simple method for estimating the mean values of fade duration and fade occurrence interval is presented.