The classical Mie scattering theory together with the bistatic radar equation has been applied in this paper to quantify interference levels in broadband fixed wireless access (BFWA) for two different network topologies: mesh and cellular. The complete scattering analysis takes into account the realistic variations of rainfall along the propagation paths and inside the common volume. The interfering element is a rain cell model that follows closely the experimental characteristic dimension, decay and movement obtained in recent research by the authors. The signal-to-interference power ratios (S/I) for two different frequencies, 28 and 42 GHz, and several scattering angles have been calculated. They show that, when two beams cross each other, rain-induced interference must be taken into account for an efficient planning of radio communication systems operating at these millimeter-wave frequencies as well as for an adequate design and implementation of fade mitigation techniques.

The design of a microwave channel probing analyzer is presented and discussed. Special attention is paid to the characteristics of the transmission path to be measured; the choice of spread spectrum probing signal; the signal processing methods used; modulation, demodulation, and synchronization at the transmitter and receiver sites; the path model to be fitted to the measured data and finally, the impact of all of these issues upon the necessary hardware and software. Following a formulation of the channel-probing problem, transfer function regression techniques are reviewed to illustrate the advantages of periodic averaging over established correlation methods. The construction of an experimental sounder based on these methods is described. The equipment has been used to measure the time-variant frequency-selective channel dynamics of a short (158 km) troposcatter transhorizon link at a frequency of 11.647 GHz over a 31.25-MHz bandwidth. Off-line, nonlinear spectrum analysis, based on Prony's method and singular value decomposition, is used to estimate parameters of a high-resolution delay model that accurately reflects the underlying ray structure of the transmission path.

The authors describe the application of the singular value decomposition Prony (SVDP) algorithm to the analysis of the time-variant structure of a microwave line-of-sight multipath channel. The algorithm deduces the multipath ray delays and associated amplitudes from measurements made in the frequency domain. The capability of this algorithm to resolve closely spaced rays is demonstrated by analyzing both computer simulated data and field measurements. The authors present experimental results of the time-variant structure of the transmittance H(jw,t) and include the evolution in time of the amplitude, group delay, and the constituting rays thus giving direct insight into the underlying multipath mechanism of line-of-sight microwave links.>

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.>

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.>

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.>

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 algorithm's ability to resolve delays to better than half the Fourier limit is shown, and its tendency to yield biased estimates is studied.>

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.>

The joint statistical analysis of duration D and intensity R of rain exceedances relevant to the design of microwave communication systems is considered. The data analyzed are derived from point precipitation-rate observations recorded over a period of 49 years in the city of Barcelona, Spain, using a Jardi gauge. The concept of event and the duration of a single exceedance of a threshold R within that event is introduced. The analysis shows the parametric R dependence of the annual average number of exceedances and of the average duration.>

This paper is concerned with the statistical investigation of a massive data bank of 49 years of rainfall rateRcontinuously recorded in Barcelona using a rain-rate gauge with ten seconds response time. With radio communications in mind, the paper addresses and reviews in detail: 1) reliable statistical model forR, 2) number of years required to obtain a database from which to derive a reliableR-distribution, and 3) the CCIR worst-month concept. The research has shown that the generalized Paretoaexp(-\muR)/R^{b}gives nearly perfect fit for all ranges ofRfollowed closely by the gamma distribution, and the simpler square root (R^{1/2}) normal distribution gives excellent fit too. The log-normal distribution was unsatisfactory forR \geq 60mm/h. The spread of the yearly distribution ofP(R)is cube root normally distributed([P(R)]^{1/3})and between 7 and 10 years are required before a reliable average distributionP(R)can be obtained. The study of theP(R)return time in years is also presented. High resolution ofP(R)is presented looking at the evolution of the annualP(R)in terms of the hourly and monthly contributing parts revealing statistical features such as the location in time of rain rates above 50 mm/h. Finally, the study shows that the calendar month contribution toP(R)remains at all times well below the synthetic CCIR worst month and recommendations are then given about its use.

Amplitude scintillations occurring on low-elevation earthspace paths can be large enough to significantly degrade the quality of satellite communications links atX-band and higher frequencies. This paper presents measurements of amplitude scintillations on low-elevation (7.1° and 8.9°) satellite downlinks atX-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.