Geneviève Jourdain

Sensitivity of OFDM-CDMA Systems to Carrier Frequency Offset

This paper presents the impact of a carrier frequency offset on the performance of 2 dimensional spreading OFDM-CDMA systems. This is measured by the degradation of the Signal to Interference plus Noise Ratio (SINR) obtained after despreading and equalization. Using some properties of random matrix and free probability theories, a new expression of the SINR is derived. It is independent of the actual value of the spreading codes while still accounting for the orthogonality between codes. This model is validated by means of Monte-Carlo simulations. It is also exploited to compare the sensitivities of MC-CDMA and MC-DS-CDMA systems to carrier offset in a frequency selective channel. This work is carried out for zero forcing (ZF) and minimum mean square error (MMSE) equalizers.

On the system level prediction of joint time frequency spreading systems with carrier phase noise

Phase noise is a topic of theoretical and practical interest in electronic circuits. Although progress has been made in the characterization of its description, there are still considerable gaps in its effects especially on multi-carrier spreading systems. In this paper, we investigate the impact of a local oscillator phase noise on the multi-carrier 2 dimensional (2D) spreading systems based on a combination of orthogonal frequency division multiplexing (OFDM) and code division multiple access (CDMA) and known as OFDM-CDMA. The contribution of this paper is multifold. First, we use some properties of random matrix and free probability theory to give a simplified expression of signal to interference and noise ratio (SINR) obtained after equalization and despreading. This expression is independent of the actual value of the spreading codes and depends mainly on the complex amplitudes of estimated channel coefficients. Secondly, we use this expression to derive new weighting functions which are very interesting for the radio frequency (RF) engineers when they design the frequency synthesizer. Therefore, based on these asymptotic results, we adapt a new method to predict the bit error rate (BER) at the output of the channel decoder by using an effective SINR value. We show by simulations the validity of our models and that at a given BER, the required signal to noise ratio (SNR) may easily increase due to the carrier phase noise.

Sensitivity of multicarrier two-dimensional spreading schemes to synchronization errors

This paper presents the impact of synchronization errors on the performance of a downlink multicarrier two-dimensional spreading OFDM-CDMA system. This impact is measured by the degradation of the signal to interference and noise ratio (SINR) obtained after despreading and equalization. The contribution of this paper is twofold. First, we use some properties of random matrix and free probability theories to derive a new expression of the SINR. This expression is then independent of the actual value of the spreading codes while still accounting for the orthogonality between codes. This model is validated by means of Monte Carlo simulations. Secondly, the model is exploited to derive the SINR degradation of OFDM-CDMA systems due to synchronization errors which include a timing error, a carrier frequency offset, and a sampling frequency offset. It is also exploited to compare the sensitivities of MC-CDMA and MC-DS-CDMA systems to these errors in a frequency selective channel. This work is carried out for zero-forcing and minimum mean square error equalizers.

Chip-level Channel Equalization with Rake-like Structures for Multicode Downlink WCDMA Communications

In this paper, we propose a low complexity multi-code chip-level channel equalizer for the downlink UMTS FDD mode. Both block form and filter form implementations are considered. The proposed receiver can be of ZF or MMSE type, and consists of a linear equalizer followed by a bank of correlators corresponding to the spreading codes, and a decision device. It has a Rake-like structure and does not need the knowledge of the spreading codes to compute the equalizer filter coefficients. We show how to compute the equalization matrix at low complexity, and we give a measure of the upper-bound of the equalizer filter depth. Simulation results are given to corroborate the good performance of the equalizer in the high speed downlink packet access (HSDPA) scenario

False-Path Reduction in Downlink WCDMA Channel Estimation by Threshold Algorithm

In this paper, an iterative algorithm for multipath channel estimation is proposed for the downlink of the WCDMA systems. The estimation is performed by extracting the parameters of the channel impulse response. The channel impulse response is obtained by cross-correlation between the pilot symbols known at the receiver and the portion of the received signal which hold emitted pilot symbols. Channel estimation errors, and more particularly the false-path detection, damages the performances of coherent receiver. To avoid the false-path detection, we introduce a novel criterion which depend on signal-to-noise ratio and cross- correlation imperfections. The simulation results reveal optimum thresholds to have a better channel estimation.

Chip-level Channel Equalization with Automatic Depth Determination for Downlink WCDMA Systems

In this paper, a low complexity multicode MMSE chip-level channel equalizer for the downlink UMTS FDD mode is proposed. This equalizer has a Rake-like structure and does not need the knowledge of the spreading codes to compute the equalizer filter coefficients. Moreover, the equalizer depth is determined automatically using the knowledge of the channel impulse response and the noise variance. Thus, the structure of the equalizer changes as the channel conditions change and the equalizer can be reduced to a Rake receiver if the depth is equal to one chip (e.g., in low signal-to-noise ratio conditions and/or in weak frequency selective channels). This property is very important and can be used to optimize the power consumption, which is a major constraint that has to be accounted for in the user equipment design.

An Enhanced SMLR algorithm for multipath channel estimation in downlink WCDMA

In this paper, a new algorithm for multipath channel estimation by means of the common pilot channel (CPICH) in downlink WCDMA-FDD is presented. The estimation is performed by using a single most likely replacement (SMLR) algorithm specifically derived for complex baseband signals where a Bernoulli-Gaussian (BG) model is introduced for the multipath channel. In the proposed algorithm, an interference cancelling technique is associated to the SMLR to enhance its performance. The proposed algorithm performs very well; it yields a satisfactory performance at a moderate extra computational cost compared to the SMLR

INTERPRÉTATIONS ET PERFORMANCES DU RÉCEPTEUR LINÉAIRE SUR LIAISON DESCENDANTE TD-CDMA ET EXTENSION MULTI-CAPTEUR

This paper deals with a td-cdma multi-user system downlink through a radio-mobile multipath channel, such as TDD-UMTS. We propose interpretations and equivalent models specific to such a multiple access mode, mostly via a frequency approach. The expressions and performances of the classic linear multi-user detectors are then reformulated. Furthermore, performance computation is applied to indoor and vehicular channel environments, showing the benefit of joint detection and diversity reception with two or three sensors at the mobile.RésuméCet article s’intéresse à la liaison descendante ďun système radiocommunication avec les mobiles multiutilisateur td-cdma soumis à des trajets multiples, tel le mode TDD de ľumts. Il propose des interprétations et des modèles équivalents spécifiques è ce mode ďaccès multiple, avec une approche le plus souvent fréquentielle. Les expressions et performances des récepteurs linéaires classiques sont alors reformulées. A titre ďillustration, le calcul de performances est appliqué aux modèles ďenvironnement º Intérieur » et º Véhicule », montrant le bénéfice de la détection conjointe ainsi que ľapport ďune réception avec 2 ou 3 capteurs sur le mobile.Cet article s’interesse a la liaison descendante ďun systeme radiocommunication avec les mobiles multiutilisateur td-cdma soumis a des trajets multiples, tel le mode TDD de ľumts. Il propose des interpretations et des modeles equivalents specifiques e ce mode ďacces multiple, avec une approche le plus souvent frequentielle. Les expressions et performances des recepteurs lineaires classiques sont alors reformulees. A titre ďillustration, le calcul de performances est applique aux modeles ďenvironnement o Interieur » et o Vehicule », montrant le benefice de la detection conjointe ainsi que ľapport ďune reception avec 2 ou 3 capteurs sur le mobile.

Filtrage dans le plan tempsfréquence. Caractérisation de signaux UBF et du milieu magnétosphérique

AnalyseLes auteurs s’intéressent à des signaux pour lesquels la représentation spectrale énergétique dans le plan tempsfréquence (donnée par exemple par un sonogramme) fait apparaître plusieurs motifs disjoints. Ils exposent une méthode qui permet d’isoler ces motifs. Le principe de la méthode est une démodulation à fréquence variable. Le pouvoir séparateur de ce traitement et ses propriétés sont testés sur des signaux synthétiques. Cette méthode est ensuite appliquée à des signauxUbfde typePc1.LesPc1sont caractérisés en première approximation par une montée en fréquence suivie de nombreux échos. La présente méthode permet d’isoler chaque écho et donc ensuite de comparer entre eux ces échos individuels. L’analyse interspectrale entre échos isolés et pris deux à deux, permet l’identification du milieu magnétosphérique à chaque passage de l’onde et évite donc les effets de moyennage des méthodes classiques. On peut en déduire l’évolution du milieu d’un passage au suivant.AbstractThe authors are concerned in studying some signals whose energetic frequency-time representation (given for example by a sonagram) exhibits several distinct patterns. A method is proposed in order to isolate each pattern from the others. The principle of the method is a variable frequency demodulation. The resolving power of this processing and its properties are evaluated with synthetic signals. Then this method is applied toUlfsignals ofPc1type. ThePc1are usually first approximated by a gliding tone with numerous echoes. The proposed method enables one to isolate the echoes and to compare them one to another. A characterization of the magnetospheric medium is made from an interspectral analysis between any two isolated echoes. It is then possible to avoid the averaging effects of classical methods. Thus one may follow the evolution of the propagating medium from one wave train to another.

État actuel de procédés d’étude et de description de systèmes et de canaux de transmission non stationnaires

RésuméOn rappelle les méthodes de caractérisation de canaux de transmission non stationnaires à l’aide du formalisme des filtres linéaires, aléatoires, à paramètres variables, et du modèle plus particulier qu’est le modèle réverbérant. On fait apparaître les liens existant entre tel ou tel modèle plus spécialisé. On donne les méthodes permettant d’estimer, en présence d’un milieu réel, les paramètres importants de ce milieu (élargissement temporel, élargissement fréquentiel) on insiste sur les implications des différents modèles de façon à guider le choix de la modélisation en présence d’un milieu réel.