Laurent Castanet

HYCELL—A new hybrid model of the rain horizontal distribution for propagation studies: 1. Modeling of the rain cell

[1]xa0From radar observations of rain fields at midlatitudes, a new physical model of rain cells is proposed. It strives to describe optimally the rain rate horizontal distribution within rain cells down to 1 mm h−1. The approach is similar to that of the well-known EXCELL model. The mathematical definition of the model lies in the combination of a gaussian function and an exponential one, the cells having an elliptic horizontal cross section. Due to its hybrid structure, the new model has been named HYCELL. From a conceptual point of view, the gaussian component describes the convective-like high rain rate core of the cell, while the exponential component accounts for the surrounding stratiform-like low rain rate spreading down to 1 mm h−1. The modeling of a rain cell with HYCELL then requires the determination of seven parameters. The latter is obtained, cell by cell, by solving a set of five fit-forcing equations completed by two continuity equations. The fit-forcing equations involve radar parameters of integral nature which refer not only to the rain cell geometry (area, ellipticity) but also to the rain rate R distribution inside the cell (mean and root mean square values of R and gradient of R). Their analytical expressions are derived from the model definition, while their values are forced to be those derived from radar measurements. Using this method, thousands of rain cells identified from radar observations in the regions of Bordeaux (southwestern France) and Karlsruhe (southwestern Germany) have been modeled. Though both sites are at midlatitude, the climatic contexts differ: oceanic for Bordeaux and continental for Karlsruhe. Results of rain rate horizontal distribution modeling within cells using HYCELL and EXCELL are compared. It is then suggested that the HYCELL model is a new tool which deserves to be considered by system designers to compute propagation parameters.

Development and validation of time-series synthesizers of rain attenuation for Ka-band and Q/V-band satellite communication systems

The aim of this paper is to present recent developments in terms of propagation time-series synthesizers, carried out in the framework of the ESA study 16865/03/NL/EC Development of propagation models for telecommunication satellite systems (ONERA Final Report RF 4/07757/DEMR, 2004). The paper is composed of three parts: a review of the initial requirements related to propagation time series for system performance simulation, a description of a collection of rain attenuation time-series synthesizers and of their related input parameters, and a comparative analysis of the output characteristics of these time-series synthesizers as compared with experimental data collected during the OLYMPUS and ITALSAT propagation experiments.

HYCELL—A new hybrid model of the rain horizontal distribution for propagation studies: 2. Statistical modeling of the rain rate field

[1]xa0A methodology to simulate typical two-dimensional rain rate fields over an observation area Ao of a few tens up to a few hundreds of square kilometers (i.e., the scale of a satellite telecommunication beam or a terrestrial Broadband Wireless Access network) is proposed. The scenes generated account for the climatological characteristics intrinsic to the simulation area Ao. The methodology consists of the conglomeration of rain cells modeled by HYCELL and of two analytical expressions of the rain cell spatial density, both derived from the statistical distribution of the rain cell size. The scene generating requires, as an input parameter, the local Cumulative Distribution Function (CDF) of the rain rate, a meteorological data commonly available throughout the world. The rain rate field is then generated numerically, according to an iterative scheme, under the constraint of accurately reproducing the local CDF intrinsic to the simulation area Ao, and following rigorously the rain cell spatial density. All the potentialities of the HYCELL model are thus used in order to generate a two-dimensional scene having a mixed composition of hybrid, gaussian, and exponential cells accounting for the local climatological characteristics. Various scenes are then simulated throughout the world, showing the ability of the method to reproduce the local CDF, with a mean error, with respect to the rain rate distribution, smaller than 1.86%, whatever the location, that is, whatever the climatology. It is suggested that this statistical modeling of the rain rate field horizontal structure be used as a tool by system designers to evaluate, at any location of the world, diversity gain, terrestrial path attenuation, or slant path attenuation for different azimuth and elevation angle directions.

Weather radar data for site diversity predictions and evaluation of the impact of rain field advection

This paper presents the analysis on two weather radar data sets, collected at Spino dAdda (Italy) and Bordeaux (France), for the simulation and the performance evaluation of a site diversity system. Results from the two locations are compared and the impact of different factors such as the baseline and link orientation is assessed and related to the local climatologic characteristics. The results obtained are then compared with the model currently recommended by the ITU-R for the estimation of the site diversity performance. A linkage is then established between the preferable baseline orientation and the predominant direction of the rain field advection. The rain field displacement is finally shown to be well approximated by the wind speed and direction relative to the 700u2009hPa isobar extracted from the European Center for Medium-range Weather Forecast ERA-40 meteorological database. Copyright

Large‐scale modeling of rain fields from a rain cell deterministic model

[1]xa0A methodology to simulate two-dimensional rain rate fields at large scale (1000 × 1000 km2, the scale of a satellite telecommunication beam or a terrestrial fixed broadband wireless access network) is proposed. It relies on a rain rate field cellular decomposition. At small scale (∼20 × 20 km2), the rain field is split up into its macroscopic components, the rain cells, described by the Hybrid Cell (HYCELL) cellular model. At midscale (∼150 × 150 km2), the rain field results from the conglomeration of rain cells modeled by HYCELL. To account for the rain cell spatial distribution at midscale, the latter is modeled by a doubly aggregative isotropic random walk, the optimal parameterization of which is derived from radar observations at midscale. The extension of the simulation area from the midscale to the large scale (1000 × 1000 km2) requires the modeling of the weather frontal area. The latter is first modeled by a Gaussian field with anisotropic covariance function. The Gaussian field is then turned into a binary field, giving the large-scale locations over which it is raining. This transformation requires the definition of the rain occupation rate over large-scale areas. Its probability distribution is determined from observations by the French operational radar network ARAMIS. The coupling with the rain field modeling at midscale is immediate whenever the large-scale field is split up into midscale subareas. The rain field thus generated accounts for the local CDF at each point, defining a structure spatially correlated at small scale, midscale, and large scale. It is then suggested that this approach be used by system designers to evaluate diversity gain, terrestrial path attenuation, or slant path attenuation for different azimuth and elevation angle directions.

Statistical distribution of the fractional area affected by rain

[1]xa0The knowledge of the fraction of an area that is affected by rain (or fractional area) is of prime interest for hydrologic studies or for rainfall field modeling. Up to now, the statistical distribution of this parameter has been poorly studied. In the present paper, a model of the statistical distribution of the fraction of an area affected by rain over a given rainfall rate is proposed. It takes into account at the same time the size of the area and the local climatology. The analytic formulation of the distribution is established, considering that rainfall fields can be obtained from a nonlinear filtering of a Gaussian random field. As the analytic derivation of the distribution lies on some assumptions, the model accuracy is first evaluated from numerical simulations. It is then shown that the model reproduces accurately the distribution of fractional areas derived from radar observations of rain fields for various rain thresholds, sizes of area, and climatologies. A generic parameterization is then proposed for areas ranging from 100 × 100 to 300 × 300 km2.

On transmission modes subset selection in DVB-S2/RCS satellite systems

For satellite systems offering broadband and telecom services at Ka band frequencies and above Fade Mitigation Techniques (FMT), in particular Adaptive Modulation and Coding (ACM), are essential to improve the system capacity. One of the important aspects of ACM from a physical and MAC layer perspective is the choice of transmission modes to be used during the system operation that would achieve the optimal trade-off between the system complexity and performance. This paper deals with one such methodology of selecting the appropriate set of transmission modes (MODCOD subset) from a physical layer point of view for both DVB-S2 and extended DVB-RCS systems. Optimizing the achievable spectral efficiency with respect to implementation complexity has been the main focus of the proposed methodology. In particular, we show that subsets with 8 and 5 transmission modes provide best trade-off between the complexity of the ACM based system and the achieved average spectral efficiency for the forward and return link of the DVB-S2/RCS system respectively.

Performance of the Synthetic Storm Technique in a low elevation 5° slant path at 44.5 GHz in the French Pyrénées

We show that the Synthetic Storm Technique can be applied to slant paths of very low elevation angle, but with caution and some limitations. To this end, we have processed the rain attenuation measurements collected in the years 1999 and 2000, in a 5° slant path radio link at 44.5 GHz between the top of Pic du Midi (altitude 2865 m) and Lannemezan (altitude 600 m), a 28-km path length, in the French Pyrénées (experiment known as CELESTE), to obtain the exceedance probability distributions of rain attenuation and fade duration. Then we have compared the experimental distributions to those predicted by the Synthetic Storm Technique with strictly concurrent measurements of rain rate time series. We have found that if the rainy path length assumed in the Synthetic Storm Technique modeling is limited to 13~15 km, instead of 28 km, then, for a large attenuation range: (a) the probability distribution of rain attenuation is well reproduced; (b) the experimental probability distribution of fade duration is well reproduced for fade durations longer than about 10 minutes.

Short- And Mid-Term Prediction Techniques Of Propagation Conditions From Weather Radar Observations. Application to resource management in SATCOM systems

This paper discusses possible scenarios for inclusion of forecasting of rain fields in satellite communication system for the purposes of optimizing the use of layer 1 resources (EIRP, frequencies)

Hybrid Satellite-WiMAX architecture and access design for tropical areas

Tabatinga is a SATNEX initiative aimed at defining and designing a DVB-S2/RCS Wi-MAX system to provide access network services over the Amazonian region of Brazil at Ku band. This paper summarizes the work done on the physical and MAC layer aspects of that system.