Armand J. Levy

Theory and measurement of the angle of arrival and time delay of UHF radiowaves using a ring array

The knowledge of directions of arrival (DOA) of waves is fundamental in understanding the physics of the mobile communications wide-band propagation channel. We first present a measurement setup that is able to accurately measure these DOA. It is based on the evaluation of the channel complex impulse response at regularly spaced locations on a circle. We theoretically investigate a new approach for deriving the DOA from this ring array. We then describe an experimental campaign, where such measurements were done on a building rooftop. We show that all the main paths are clearly retrieved and identified and that the ray theory is valid and applicable. The results emphasise the predominant role of UHF-wave specular reflection by walls in an actual urban environment.

An improved channel sounding technique applied to wideband mobile 900 MHz propagation measurements

Channel sounding experimental equipment to measure the wideband and variability characteristics of the 900 MHz mobile radio channel is presented. This equipment uses a modulated carrier, but no analog correlator. Various sources of nonlinearities may significantly degrade the dynamic range of estimated impulse response and, more seriously, introduce spurious signals undistinguishable from actual response components. The channel impulse responses are evaluated by a computerized postprocessing. A numerical technique for evaluating impulse responses, different from the classical correlation technique, is presented. Based on a regularized inversion and a windowing process, the signal processing algorithm is shown to produce responses with improved dynamics and resolution.<<ETX>>

A ray launching method for radio-mobile propagation in urban area

A computer code based on digitized terrain data was developed to estimate path losses and selectivity characteristics of the urban radio channel between a mobile located in a street and a base station on the roof of a high building. The principle of the program consists in a ray launching in projection on the horizontal plane supplemented by third-dimension considerations. Some simulated results are presented and compared with actual measured data. The computed mean impulse response is always in good agreement for the first delay paths. A significant discrepancy can arise for long delay paths, but realistic features are always observed.<<ETX>>

A statistical model for the simulation of multipath mobile propagation channel

A statistical representation model of the wideband radio propagation channel is proposed. This model is intended to be used in a software channel simulator. This approach is original with regard to more classical ray models because it does not try to identify the physical rays within the impulse responses, but only to reproduce, as faithfully as possible, the behavior of the channel, with the help of rays without any physical meaning. The proposed model is developed on the definition of a randomly varying transversal filter, with a delay between taps of 1/B, assuming input signals of limited bandwidth B.<<ETX>>

Signatures et modèles de propagation pour faisceaux hertziens numériques.

RésuméPour décrire un canal hertzien affecté d’évanouissements sélectifs, une méthode souvent utilisée consiste à construire un modèle de sa fonction de transfert. Cette modélisation comporte deux niveaux, choix d’un modèle de représentation (modèle mathématique de la fonction de transfert à petit nombre de paramètres), et construction d’un modèle statistique de propagation (loi de probabilité conjointe de ces paramètres). On peut utiliser un tel modèle statistique de propagation pour étudier la qualité d’une liaison dont les équipements sont spécifiés. La signature de l’équipement, pour la liaison considérée et dans le modèle de propagation choisi, joue alors un rle crucial. L’objet de cet article est de donner une vue d’ensemble d’une telle démarche. A cette fin, on rappelle d’abord la définition précise de la signature en insistant sur la nécessité d’avoir défini préalablement les équipements, le modèle de propagation et le critère de qualité. On établit ensuite la généralité de la notion de signature, qui s’applique à tout modèle. Divers exemples de signature sont donnés dans plusieurs modèles de propagation courants et leur emploi est discuté.In order to describe a microwave line-of-sight channel degraded by multipath fading, a usual technique consists in developing a model of its transfer function. This modelling includes two steps : the choice of a representation model (a mathematical parametered modelling function) and the evaluation of a statistical propagation model (the joint probability distribution of these parameters). Such a statistical propagation model can be used to study the quality of a link with specified equipment. In this context, it is shown that the notion of signature has an outstanding value. The object of this paper is to give an overview of this approach. The precise definition of the signature of an equipment is first recalled ; the necessity to define precisely the equipment, the propagation model and the quality criterion is underlined. The generality of the signature, which can apply to any model, is then established. Various examples of signatures in a few propagation models currently adopted are presented and their use is discussed.

Wide band characterization of the mobile-satellite propagation channel using a helicopter

The paper first presents an experimental set-up using a helicopter, which was realized to study the wide band and variable mobile satellite propagation channel. The first statistical characterizations stated are described. More specifically, the authors present cumulative distributions of narrow band and wide band excess path loss as a function of the elevation angle, as well as distribution functions of some frequency selectivity parameters.<<ETX>>

Impact of practical implementation errors on the capacity of cellular networks using adaptive antennas

Future third generation cellular systems (UMTS in Europe, IMT-2000 for ITU) will offer an increased range of services, and thus should accommodate much higher service bit rate than second generation. Unless the use of these new services is limited to very restricted area, like indoor office buildings, simple considerations on link budget and on spectral efficiency show that the use of smart antennas is mandatory in order to actually offer these high rates. This paper evaluates the capacity improvement of base station sites, allowed when using adaptive antennas (AA). It takes into account various errors that cannot be avoided from a practical point of view, like imperfect path loss evaluation and power control, or imperfect estimates of mobiles location. It concludes that the expected degradation is reasonable, and should not compromise the AA benefits.

An assessment of cell size versus receiver complexity using measured propagation data

The subject of the paper is an investigation to establish the minimum receiver complexity required when employing cells of different sizes. This was accomplished by using a GSM radio link simulation tool in conjunction with real channel data obtained by France Telecoms CNET in the centre of Paris, which was classified in terms of the distance between transmitter and receiver. The paper evaluates the trade-off between cell size and receiver complexity by simulating the performance of five alternative equalisers. In addition, the preprocessing operations necessary to import real channel sounding data into a radio link simulation are described in detail. It was found that when using a fixed height rooftop antenna, a simple 2 state receiver equaliser would only be acceptable if used in conjunction with additional multipath countermeasures. In terms of the current GSM system, the study shows that a 4 state equaliser can be used within about 1.2 km of the base station. Beyond this range, the standard GSM receiver is needed to provide the required performance.<<ETX>>

Performances en probabilité d'erreur des modulations à grand nombre d'états en période d'évanouissements sélectifs

AnalyseCet article présente une comparaison des modulations à grand nombre d’états, qui sont étudiées du point de vue de leur robustesse face aux phénomènes de trajets multiples. Afin d’aboutir à des résultats précis, cette étude a été menée en utilisant comme critère de qualité le taux d’erreur et non un critère indirect tel que l’ouverture de l’æil. A cette fin, une méthode d’évaluation du taux d’erreur a été développée, méthode qui se caractérise par sa souplesse et son faible coût de calcul. En particulier, elle a été appliquée à des modulations cohérentes courantes et à des modulations plus originales, à constellations hexagonales, dotées d’un bon coefficient de compacité.AbstractThe paper presents a comparison of high level modulation techniques and of their performance during multipath fading periods. In order to provide precise results, the author led this study using the error rate as quality criterion, thus avoiding the use of one of the other indirect criteria like eye opening. To this end, an error rate evaluation method was developped. The flexibility and low computing cost of this method allowed us to investigate current coherent modulation techniques and also, more original techniques with low « packing coefficient » such as hexagonal constellations.