Ifiok Otung

Prediction of tropospheric amplitude scintillation on a satellite link

Semi-empirical models are presented for annual and worst-month distributions of scintillation fades, enhancements, peak-to-peak amplitude excursions, and intensity. The models utilize the well-known theoretical scaling of scintillation with signal frequency, antenna diameter, and path-elevation angle, and its empirical dependence on the wet term of atmospheric refractivity. Dependence on time percentage is obtained by detailed regression analysis of experimental scintillation data obtained at Sparsholt, UK (51.5850/spl deg/ N, 1.5033/spl deg/ W) over a one-year period using the Olympus satellite 19.7704 GHz beacon viewed at a nominal elevation of 28.74/spl deg/. Results are compared where possible with the ITU-R and Moulsley-Vilar scintillation models and with Olympus measurements in Germany.

Extracting scintillations from satellite beacon propagation data

Beacon measurements on a moderate elevation satellite link at 20 and 30 GHz during concurrent scintillation and rain attenuation are analyzed. The spectra of the two beacon signals are compared and scintillation amplitudes are extracted from the raw beacon data using a high-pass filter set at 14 different cutoff frequencies f/sub c/ ranging between 2-400 mHz. The effect on scintillation statistics of the choice of f/sub c/ and the dependence of this choice on path elevation angle are discussed.

Analysis of rain cell size distribution for application in site diversity

Communication systems operating at higher frequency bands suffer from severe attenuation due to rain, which is highly variable in time and space. However, the temporal and spatial inhomogeneity of rain fields can be exploited to improve the availability of the communication link. Knowledge of rain cell size distribution is relevant for the modeling of earth-space propagation in radio communication. To determine the spatial structure of rain cells, long term rain rate time series can be processed by applying the synthetic storm technique assuming some known value of storm translation speed. The underlying hypothesis is that rain patterns move along a line with a constant speed and that advection is the predominant mechanism to account for the spatial variability of rain-rate. The hypothesis holds when a statistical description of rain structure is required, rather than the exact space distribution of rain. Furthermore, as rainfall patterns move over a rain gauge, it is possible to estimate the horizontal extent of rain cells from the duration of various rain rate thresholds as recorded by the rain gauge if a mean advection velocity of rain cells is assumed. In this paper, a comparison is made between cell advection speed, Doppler speed measurements recorded in Chilbolton, England and local wind speed measurements at Brize Norton, some 63 km from the radar location. The distribution of rain cell diameters is determined by applying the rain cell translation velocity. In an alternative approach, the PPI (Plan Position Indicator) radar scans of different rain events were analysed to determine the size distribution of various rain-rate thresholds. The rain cell size distributions obtained using the two methods are compared, and the implications of these results on the configuration of site diversity in earth-space communication systems are discussed.

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

Statistical Prediction of Site Diversity Gain on Earth-Space Paths Based on Radar Measurements in the UK

This paper presents an empirical model for the statistical prediction of site diversity gain on earth space propagation paths. A modelling approach is described which incorporates 46 months of Nimrod rain radar data recorded in South England. The dual site diversity (SD) gain is developed as a function of several parameters, including time percentage of average year, site separation, baseline orientation, link frequency and path elevation angle. The model performs with an overall root mean square error of 1.87 dB computed over a wide variety of link configurations with elevation angles ranging from 10° to 50° and transmission frequencies from 16 to 50 GHz. The performance of the new model is checked against satellite beacon measurements as well as the empirical and physical models recommended by ITU-R.

Observed effects of cloud and wind on the intensity and spectrum of scintillation

This paper examines the observed effects of wind and cloud presence on the intensity and power spectrum of tropospheric scintillation, using propagation data recorded during a three-month period (May-July 1997) at Sparsholt from the ITALSAT satellite beacons at 39.59 and 49.49 GHz. The results show strong correlation between corner frequency and transverse wind speed, a weaker correlation with overall wind speed and a negligible correlation between scintillation intensity and transverse wind speed. Furthermore, it was found that the presence of cumulus clouds led on average to increased scintillation levels and a reduced correlation of the corner frequency with wind speed.

Modeling rainfall drop size distribution in southern England using a Gaussian Mixture Model

Understanding and modeling the rainfall drop size distribution is important in a number of applications, in particular predicting and mitigating attenuation of satellite signals in the millimeter band. Various standard statistical distributions have been proposed as suitable models, the first widely accepted being the exponential distribution. Subsequently, gamma and lognormal distributions have been shown to provide better rainfall rate computations. Some empirical studies have revealed bimodal distributions under some circumstances. A natural question to ask therefore is how often gamma and lognormal distributions fit the empirical data. In this paper we fit lognormal and gamma distributions to 1 min slices of rainfall drop size distributions taken from 7 year data from the Chilbolton Observatory in southern England. The chi-square goodness of fit of the models against the data is calculated, and it is found that failure to fit is greater than would normally be expected. This failure to fit is broken down and examined against seasonal variations, different rain rates, atmospheric temperature, and wind speed. Possible reasons for the lack of fit are explored, and alternative fits using models based on Gaussian Mixture Models are developed and found to be an improvement.

Evaluating Time Diversity Performance on an On-board Processing Satellite to Earth Station Downlink

A method for assessing the performance of time diversity (TD) along a single downlink path is proposed. The evaluation is done using TD gain, which is obtained from joint distribution of similar retransmitted rain attenuation time series separated by finite time delay. Statistics use to assess the performance of TD are obtained from analyses of measurements from the most recent British Atmospheric Data Centre (BADC) database. TD is presented in this paper as a suitable fade mitigation scheme for increasing link availability and improving the quality of service in broadcast systems. Prior to achieving a model of TD, analyses of propagation datasets (November 2005) and extraction of attenuation statistics have been carried out. These statistics are necessary for the modeling process and for the design of a satellite link between any two earth stations having a separation distance in excess of the mean rain cell size which is roughly 10 Km. Comparisons are made with ITU-R model and some useful conclusions deduced.

Propagation Effects in WiMAX Systems

WiMAX is a promising technology for providing broadband wireless access. The WiMAX links are significantly affected by propagation impairments. In the high frequency range of 2-66 GHz links these impairments can lead to signal outage. Using the ITU-R and other existing models we evaluate the effects of individual environmental factors on the signal and thereby calculate the overall degradation caused. From the losses obtained at various frequencies and distances we find the availability of the signal at various BER. These results show that there are improvements needed to overcome the signal outages.

Observed frequency scaling of amplitude scintillation at 20, 40, and 50 GHz

This paper presents an analysis of instantaneous frequency scaling of scintillation using propagation data recorded during a three month period (May-July 1997) at Sparsholt UK from the ITALSAT satellite beacons at frequencies 18.7, 39.6, and 49.5 GHz. Variations in the height of turbulence within reasonable limits were found to have a negligible effect on the scaling ratios. Furthermore, the exponent in the power law dependence of scintillation intensity on signal frequency was found to be on average 27% smaller than the theoretical value of 7/12 and to exhibit a slight diurnal effect. It is shown that this behavior can be partly accounted for by receiver thermal noise contribution to the measured signal variance. Ascribing the minimum observed short-term variance in each beacon to thermal noise and excluding this contribution yielded a higher exponent, which was nevertheless 15% below the theoretical value.