Fade slope for four LOS links in Singapore: analyses and prediction

Three-year experimental data on rain attenuation is employed to analyse fade slopes for four terrestrial line of sight (LOS) systems in Singapore. The statistical results of fade slope are presented on both cumulative probability distributions during rain events and relationships with fade level for fade countermeasure. A fade slope occurrence prediction model is also provided for short links based on point rainfall rate     

Statistical analysis of rain fade slope

An analysis is made of the measured distributions of the fade slope of rain attenuation, conditional for attenuation values, measured at Eindhoven University of Technology from the satellite Olympus. It is found that the distribution is similar for positive and negative fade slopes and independent of frequency in the range from 12 to 30 GHz. A distribution model for the conditional distribution is found. The only parameter of the distribution is the standard deviation, which is found to be proportional to attenuation level and dependent on rain type, on the low-pass filter bandwidth and on the time interval used in the slope calculation. The observed relation between the standard deviation and attenuation is compared with results from other measurement sites. From this comparison it is found that the fade slope standard deviation is likely to depend on elevation angle and on climate, through its dependence on rain type.     

Analysis of rain fade slope for Ka and V-band satellite links in Southern England

The fade slope of rain attenuation at Sparsholt, U.K. has been measured for a link with the ITALSAT F1 and F2 satellites at frequencies of 18.7, 39.6, and 49.5 GHz and compared with the corresponding ITU-R model. The model fits the data well visually with a tendency to be less accurate with increasing frequency due to the assumption of constant scaling of equiprobable attenuations being less accurate at higher frequencies. The probability distribution used in the model fails a chi-squared test for all confidence levels when compared with the data. Conditional statistics of fade slope on a diurnal and seasonal basis are also analyzed and reveal that the autumn season has higher fade slopes than any other, which can in part be attributed to the higher ratio of stratiform to convective rain in the autumn season. Fade slope was found to have slightly higher values during midday.     

Frequency and attenuation dependent fade slope statistics

Results obtained from slant path propagation experiments carried out with the OLYMPUS satellite are reported. Concurrent attenuation measurements at 12.5, 20 and 30 GHz have been analyzed with regard to the rate of change of rain attenuation (fade slope). The results indicate that pronounced fade slopes occur mostly in the high attenuation range.<>     

Fade Slope Analysis of Ka-Band Earth-LEO Satellite Links Using a Synthetic Rain Field Model

Because the motion of a low Earth orbit (LEO) satellite across the sky causes the Earth-space path to very quickly pass through any rain cells in the vicinity, the degree of rain fading on such paths changes more rapidly and leads to steeper fade slopes than in the geostationary case. Because comprehensive measurement data have not yet been compiled for fading on LEO links in the Ka-band, we have used simulations based on Goldhirsh's method for determining the key parameters of the well-known EXCELL model of a horizontal rain structure from long-term global rain statistics to obtain plausible estimates of the fade slope distributions for selected scenarios. The results that we obtained for geostationary satellites closely match those observed at selected sites during the Advanced Communications Technology Satellite program. The results that we obtained for LEO satellites show how fade slopes will steepen as 1) the altitude of the satellite decreases; 2) the frequency band of operation increases; and 3) the average rain rate increases. Furthermore, they suggest that, at a given probability level, the fade slopes could be between two and ten times greater than those for geostationary satellites and that mobile terminals with a clear view of the sky will experience fade slopes that are similar to those encountered by fixed or transportable terminals. These results have important implications for the design of power control algorithms and other fade-mitigation techniques.     

Performance of synthetic storm technique in estimating fade dynamics in equatorial Malaysia

In this paper, the synthetic storm technique (SST) is applied to 1‐minute rainfall rate collected from a rain gauge to predict the fade dynamics of the signal in an earth‐to‐satellite link in an equatorial climate location without the needs of satellite beacon measurements. The obtained results are compared with the statistics calculated from measured rain attenuation and with the ITU recommendation model. As for fade duration, a good agreement with measurements has been found together with a significant improvement in terms of prediction errors with respect to the ITU‐R model. Synthetic storm technique is also able to predict fade slope statistics fairly well and has equivalent performance of ITU‐R model.     

Comparison of measured rain attenuation in the 10.982‐GHz band with predictions and sensitivity analysis

The campaign to collect rain attenuation data on terrestrial links had commenced in Malaysian tropical climates for almost two decades. The terrestrial data so far collected have been greatly utilized to derive useful statistics for various microwave applications, such as frequency scaling, rain rate conversion factor, 1‐min rain rate contour maps, wet antenna losses, and fade slope duration analysis. However, there is still severe scarcity of rain attenuation data on earth–space links in Malaysia. The results of the 2‐year measurement (January 2009–December 2010) of rain rates and rain‐induced attenuation in vertically polarized signals propagating at 10.982 GHz have been presented in this paper. The rain attenuation over the link path was measured at Islamic International University Malaysia and compared with ITU‐R P.618‐10 and Crane global models in this paper. The test results show that the two prediction models seem inadequate for predicting rain attenuation in the Ku‐band in Malaysia. Sensitivity analysis performed on measured data also reveals that the sensitivity variables depend on rain rate. Copyright © 2014 John Wiley & Sons, Ltd.     

Analysis and modeling of time intervals between rain rateexceedances in the context of fade dynamics

Using a 49 year point rainfall rate databank the authors present the analysis and statistical modeling of the interval of time between exceedances of rainfall rate thresholds within the context of rain fade dynamics and system availability. The study distinguishes between successive exceedances occurring at different rain events. The global study of both return periods or intervals is also presented together with a comparison with the statistics of durations of exceedances. A comparative study is presented with published results, both on rain and on fade after conversion to point rain rate, together with the dependence of the annual average number of return periods upon the rainfall rate thresholds     

Results from the Virginia Tech propagation experiment using theOlympus 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     

The Use of Resource Sharing and Coding to Increase the Capacity of Digital Satellites

Resource sharing is a technique which can improve the circuit availability of digital satellites operating at frequencies above 10 GHz, without requiring large fade margins. In this paper, the resource sharing concept is generalized by fully exploiting the available clear-air carrier-to-noise ratio of the satellite link to achieve very high transmission capacity while maintaining low rain outage. During clear-air conditions, convolutional codes with a large channel signaling alphabet are employed to permit a high rate of information transfer. When the fade depth exceeds the built-in fade margin, the signaling alphabet is reduced and enough time slots are borrowed from a resource sharing reserved pool to maintain the data rate at the fade site. It is shown that this approach greatly diminishes the impact of rain attenuation on satellite communications. Effective utilized capacities exceeding 85 percent of that possible if it never rains are feasible, and the increase in capacity compared to a system not using resourcesharing protection is typically a factor of 3-10.     

Rain rate statistics and fade distributions at 20 and 30 GHzderived from a network of rain gauges in the mid-Atlantic coast over afive year period

The authors examine five years of rain rate and modeled slant path attenuation distributions at 20 and 30 GHz derived from a network of ten tipping bucket rain gauges located on the mid-Atlantic coast of the US in the vicinity of Wallops Island, VA. Distributions are derived from the variable integration time data from 1-min averages or rain rate time series. It is demonstrated that for realistic fade margins at 20 GHz and above, the variable integration time results are adequate to estimate slant path attenuations using models which require 1-min averages. An accurate empirical formula is developed to convert the variable integration time rain rates to 1-min averages. Rain rate and fade distributions corresponding to the overall network average, yearly network average, and site distributions averaged over the five-year period are derived. Significant differences in the distributions are noted for the year-to-year (temporal variability) and the worst year site-to-site (spatial variability) cases     

Optimum utilization of the channel capacity of a satellite link inthe presence of amplitude scintillations and rain attenuation

By considering the global fading process on the link caused by rain attenuation and amplitude scintillations, particularly at K_a band, it is possible to derive a long-term statistical model of the satellite channel capacity. The four-parameter distribution, which combines amplitude scintillations and rain fade within an up/down link system, is presented. Also presented are the degradation (and improvement) of bit error rate (BER) in the presence of amplitude scintillations, thus complementing the flat fade effect due to rain only. By implementation of adaptive communication systems, a more efficient channel capacity utilization is possible. The concepts and the use of novel analytical expressions combining a log-normal model of rain fade with a Moulsley-Vilar distribution for scintillations are illustrated. These are then applied to a very-small-aperture terminal (VSAT) example of a 29/19-GHz digital communications link through the Olympus satellite using M-ary phase shift keying (PSK) modulation schemes     

Advanced Communications Technology Satellite (ACTS) fadecompensation protocol impact on very small-aperture terminal bit errorrate performance

The Advanced Communications Technology Satellite (ACTS) communications system operates at Ka band. ACTS uses an adaptive rain fade compensation protocol to reduce the impact of signal attenuation resulting from propagation effects. The purpose of this paper is to present the results of an analysis characterizing the improvement in VSAT performance provided by this protocol. The metric for performance is VSAT bit error rate (BER) availability. The acceptable availability defined by communication system design specifications is 99.5% for a BER of 5E-7 or better. VSAT BER availabilities with and without rain fade compensation are presented. A comparison shows the improvement in BER availability realized with rain fade compensation. Results are presented for an eight-month period and for 24 months spread over a three-year period. The two time periods represent two different configurations of the fade compensation protocol     

12-GHz fade durations and intervals in the tropics

Direct-to-home very small aperture terminals systems operating at frequencies above 10 GHz in tropical climates are subjected to many fade occurrences due to heavy rain. In addition to annual and worst month cumulative rainfall rate and rain fade exceedances, statistics of seasonal and diurnal variations, together with average event length and separation, are needed to give the detailed insights for system design. This paper presents seasonal fade duration statistics and fade interval characteristics for a wet, tropical location in Lae, Papua New Guinea (PNG), measured over four years using satellite beacon and collocated radiometer receivers.     

Fade slope statistics for three 12-GHz satellite beacon links inBrazil

The fade slope is defined as the rate of increase or decrease of attenuation over time. The possible correlation between the statistics of fade slope values and attenuation levels is investigated. Results show a direct positive correlation between the mean values of fade slope and the attenuation levels. The distributions of fade slope values present a degree of skewness toward the negative values that makes the curves depart from the Gaussian distribution observed by other authors     

Predictive methods for rain attenuation using radar and in-situ measurements tested against the 28-GHz Comstar beacon signal

A program to measure the rain attenuation of the Comstar beacon signal at 28.56 GHz has been in continuous operation since March of 1977 at Wallops Island, VA. During the summer of 1977 simultaneous radar and disdrometer measurements at the site were also made and used for predicting path attenuation. The best-fit values ofaandbof the relationk = aZ^{b}were deduced for each rain period from the raindrop size measurements, wherekis the attenuation coefficient [dB/km] andZis the reflectivity factor [mm⁶/m³]. The measuredk-Zrelations and the simultaneous radar reflectivity measurements along the beacon path were injected into a computer program for estimating the path attenuation. Predicted attenuations, when compared with the directly measured ones, show generally good correlation on a case-by-case basis and very good agreement statistically after an empirical calibration adjustment is applied to the radar data. A method was also tested for predicting fade statistics at another frequency (e.g., 19 GHz) using simultaneous rain rate and fade distributions (28 GHz) in conjunction with disdrometer data. The predicted distributions showed good agreement with radar-predicted levels. The results demonstrate the utility of using radar in conjunction with disdrometer and rain gauge measurements for predicting fade events, long-term fade distributions, and establishing predictive criteria associated with earth-satellite telecommunications.     

Rain fades on low elevation angle Earth-satellite paths:comparative assessment of the Austin, Texas, 11.2 GHz experiment

Only a few years of rain attenuation data for Earth-space paths are available for low elevation angle paths, and these show a consistent tendency by the CCIR fade prediction model to underpredict rain fades. The authors contribute to the database four years of 11.2 GHz measurements, taken on a 5.8° elevation path in Austin, Texas, USA and assess higher than predicted fade results with respect to earlier measurements and the CCIR model. At the fiducial 0.01 percent of time, the rain fall rate was 73 mm/h and the attenuation exceeded the 25 dB measurement fade margin. It is found that the monthly variability of rain fall rates and decibel fades follows a normal distribution. Durations of rainfall and fades are classified into events, episodes, interevent gaps (longer than 4.2 h), and interepisode gaps (shorter than 4.2 h). Rainfall interevent gaps and event durations, as well as fade event, episode, and gap durations are log-normally distributed     

Rain attenuation successive fade durations and time intervals between fades in a satellite-Earth link

Long-term attenuation data at 11.6 GHz, obtained in a Sirio link, are analysed to provide information on the joint statistics between successive fade durations within a rain event (intrafade) and between different rain events (interfade). The results show that successive fade durations and the interfade or intrafade intervals are approximately statistically independent. Within the same rain event, interfades and fade durations longer than 10 s are statistically identical. These data may be important for planning adaptive systems and for devising prediction models of the dynamic behaviour of rain attenuation.     

A three-site comparison of fade-duration measurements

Important elements in the design of Ka-band communication systems are the duration of fade and nonfade events at a particular fade depth. The choice of modulation and forward error correcting codes will depend on both the fading time dynamics and annual fade occurrence statistics. This paper examines the fade duration measured at locations in three of the Advanced Communications Technology Satellite (ACTS) propagation experimental regions, namely, Florida (rain zone N of the radiocommunications sector of the International Telecommunications Union [ITU-R] model), New Mexico State University (ITU-R rain zone M), and Alaska (ITU-R rain zone C). Within each region, measures of the underlying uniformity are described and an interregional comparison examines the ability to scale the local results