E. Vilar

Optimum utilization of the channel capacity of a satellite link in the 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/sub 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. >

Spectral analysis of 49 years of rainfall rate and relation to fade dynamics

The nature of the rainfall rate R(t) as a transient random process is presented in the context of microwave attenuation and fade dynamics. Using 49 years of data recorded in Barcelona, the spectral densities of energy and power and the autocorrelation function are presented, together with their analytical model. Examples for specific rain events are also presented. Several regimes covering a wide range of intensities have been investigated, and it is found that the spectral densities all exhibit a similar form which is independent of the average regime of R considered. This leads to a mean frequency and a spectral width which appear to be universal constants and independent of the regime. Similar features are found for the autocorrelation function. The mathematical models and numerical values found are compared to the Maseng-Bakken model (1981) proposed in earlier work of fade dynamics, and the agreement is good. The method is proposed for modeling the denormalized autocorrelation, and hence the spectrum, using only the measured average rainfall rate during a period of interest. Finally, the average number of crossings per unit of time is modeled and compared to the experimental results, and the agreement is satisfactory. >

Measurement and Modeling of Amplitude Scintillations on Low-Elevation Earth-Space Paths and Impact on Communication Systems

Amplitude scintillations occurring on low-elevation earthspace paths can be large enough to significantly degrade the quality of satellite communications links at X -band and higher frequencies. This paper presents measurements of amplitude scintillations on low-elevation (7.1° and 8.9°) satellite downlinks at X -band. The statistical distributions of scintillation amplitude and rms intensity are approximated using the Moulsley-Vilar model [1]. An analytical approximation of this model is presented and is shown to be an easy means of predicting system outage time due to scintillations. Theoretical estimates of the average bit error rate for a digital satellite link subject to scintillation fading are presented for NCFSK and CPSK modulations. Other ways in which scintillations can affect a satellite communication link are discussed, and results are also presented showing the seasonal and diurnal variability of the scintillation intensity and long-term correlation with ground temperature.

An autoregressive approach to the identification of multipath ray parameters from field measurements

This study is concerned with using modern spectral analysis techniques to derive estimates of multipath ray parameters (delays and amplitudes) from measurements of selective fading over a wide band at microwave frequencies. The applicability of the general category of autoregressive methods is addressed, and the specific method adopted, the Prony algorithm, is described. An illustrative example in which this is applied to a typical field measurement is followed by an investigation into the capabilities and limitations of the algorithm. A statistical analysis of the method has been carried out using a database of laboratory measurements. The Prony algorithms ability to resolve delays to better than half the Fourier limit is shown, and its tendency to yield biased estimates is studied. >

Analysis of the time variant structure of microwave line-of-sight multipath phenomena

The authors report a novel technique, the SVDP (singular value decomposition prony) algorithm, which has demonstrated its superiority in identifying the multipath rays in line-of-sight microwave links. The technique has been applied to field data and it has proved itself to be a very useful tool in analyzing the time-variant structure of the actual physical mechanism, i.e. the evolution of the delay and amplitude of the multipath rays. The authors present the results of analysis of field measurements made at 11 GHz over a wide bandwidth of 400 MHz on the French CNET at Lannion, using a 50 km link. The major applications of this technique are: the modelling of a multipath channel using the least possible number of rays with the objective of minimizing the RMS error; the analysis of the time-variant structure of a channel and hence the design of adaptive equalizers; and the generation of the statistics of the time-domain multipath parameters, particularly the delay and amplitude distributions. >

Selective fading on LOS microwave links: classical and spread-spectrum measurement techniques

A survey is presented of techniques for measuring selective fading on line-of-sight (LOS) microwave links, particularly over a wide band. Classical pulse methods, including pulse compression, as well as the much used frequency-sweep approach are covered. Methods utilizing both frequency-hopping and direct-sequence spread-spectrum transmissions are also discussed. An analysis of this latter method (used by the US National Bureau of Standards and by the authors) is included and a relationship is established between the true channel transmittance and that estimated by this approach. The advantages and disadvantages of the various schemes are outlined with the aim of assisting an experimenter in the choice of technique in a given experimental context. >

A novel approach to the simultaneous measurement of phase and amplitude noise of oscillators

An approach for the simultaneous measurement of amplitude and phase noise in oscillators is presented. A description of the measurement system and the associated processing techniques is presented. The measurement system includes a conventional complex (I & Q) demodulator, a high speed sampler and a digital computer. The techniques are named time-domain phase unwrapping and real amplitude de-enveloping. They permit the reconstruction of the demodulated I and Q signals as a vector rotating at a convenient beat frequency randomized, in phase and magnitude, by noise. The problems encountered in conventional AM and PM detection can be overcome using the approaches described. As a result, direct and true amplitude and phase noise can be measured and characterized.<<ETX>>

Anomalous transhorizon propagation and meteorological processes of a multilink path

An event-based approach is developed and applied to 3 years of data of a four link transhorizon path over the English Channel (La Manche). The signal enhancements (events) observed on the chart recordings during anomalous propagation are selected and, according to their dynamical signatures, categorized into three main patterns of ducting, layer reflection, and high k factor diffraction. The statistical analysis carried out on this event basis gives the correlation between the signal levels of different links and the cross correlation between signal levels and local N index derived from surface meteorological monitoring. The results reveal the fundamental properties of the anomalous propagation regimes. Using case studies of several interesting events, the meteorological processes and resultant impact on transhorizon transmission are also explained. This extensive work contributes to a better understanding of anomalous propagation mechanisms present in transhorizon links.

Analysis and modeling of time intervals between rain rate exceedances 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. >

Extended variances and autoregressive/moving average algorithm for the measurement and synthesis of oscillator phase noise

Two related aspects are discussed of the recent research work carried out at Portsmouth Polytechnic in the UK regarding numerical and digital techniques for the measurement and synthesis of phase noise in oscillators. The first aspect concerns the introduction of a statistical quantity termed extended Hadamard variance to be applied in frequency stability analysis by making use of digital frequency counter measurements. The properties, the potential interest and the usefulness of this parameter are briefly demonstrated and applied to the study of a signal from a synthesizer that is frequency-modulated by white noise followed by the frequency counter and signal processing. It is shown that not only the Hadamard variance but also the Allan variance and the extended two-sample variance are sets of this variance. The second aspect refers to modern requirements for numerical simulation of random noise, emulating phase or frequency fluctuations. The authors describe the autoregressive integrated moving average technique used to generate various types of random noise, including radio propagation FM noise, and the Monte Carlo approach to transform from one distribution to another.<<ETX>>