Jean-Claude Dany

A new method for reducing the power consumption of portable handsets in TDMA mobile systems: conditional equalization

In time-division multiple-access (TDMA) systems, the autonomy of portable handsets is a major constraint. We propose a new method, conditional equalization, which minimizes the power consumption due to equalization by equalizing only when it is most needed. At each time slot, the receiver estimates the need for equalization with a deterministic criterion. The proposed criteria use thresholds, chosen to provide a compromise between loss in performance and percentage of saved equalization. The performance of our method is theoretically analyzed for two-path Rayleigh fading channels and simulated for the recommended COST 207 channels. Conditional equalization can ensure a degradation in the average signal-to-noise ratio (SNR), providing a fixed bit error rate (BER with an average SNR=9 dB with systematic equalization), of less than 0.5 dB while providing a gain of 43% spared equalization for bad urban (BU), 62% for hilly terrain (HT), 79% for typical urban (TU), and 98% for rural area (RA) models, respectively. Conditional equalization is shown to greatly improve the autonomy of the receiver by drastically reducing the power consumption due to equalization, without a noticeable loss in the performance.

Filtre correcteur de phase pour égaliseurs sous-optimaux

RésuméLes auteurs présentent une structure de récepteur destinée aux transmissions numériques à haut débit. De faÇon à assurer un bon compromis entre performance et complexité, le récepteur est composé ďun filtre transversal correcteur de phase, suivi par un égaliseur ne prenant que partiellement en compte la réponse impulsionnelle du canal. Les auteurs proposent une technique non itérative de détermination des coefficients du filtre, basée sur les propriétés du cepstre ďune réponse a phase minimale. Ses performances sont évaluées et illustrées par la simulation ďune chaîne de transmission comportant un canal de radiocommunication avec les mobiles avec trajets multiples, le filtre et un égaliseur dfse (decision feedback sequence estimator).AbstractWe propose a receiver structure intended for high bit rate transmissions. In order to ensure a good compromise between performance and complexity, the receiver is composed of a transversal phase- corrective filter followed by an equaliser, which takes only partially the channel impulse response into account. We propose a non-iterative technique to adjust the filter tap coefficients, based on the cepstral properties of a minimum phase response. The performance of the filter is evaluated and shown by simulating a transmission system including a multipath mobile radio channel, the filter and a dfse equaliser (decision feedback sequence estimator).

Scalable Multi-State Markovian Modeling of Time Series of Received Power from Measurements of Mobile Satellite Transmissions in the L-Band

In a context of mobile radio transmissions a statistical model of propagation channels is needed in order to evaluate the performance of the link. Many models involve an underlying Markov chain where the number of states and the associated probability law are a priori unknown. We propose a method to extract these parameters from measurements of instantaneous received power. It is applied to model the channel in the case of downlink transmissions in the L-band, from a non-geostationary satellite to a receiver on Earth in various terrestrial environments. The results are displayed for two categories of receivers, corresponding to vehicular and pedestrian users, and reveal difference between both models.

A new method for predicting the channel estimate influence on performance of TDMA mobile radio systems

Receivers for time division multiple access mobile radio systems usually consist of two functional blocks: A channel estimator and an equalizer. We present a new method for predicting the channel estimate influence on performance of such receivers operating over frequency selective time-varying Rayleigh fading channels. The method can be applied to optimize system parameters. It is shown that global receiver performance can be analytically expressed in terms of the performance characteristics of two blocks considered disjointly, i.e. the equalizer operating over a perfectly known channel and the channel estimator operating over a time-varying channel. The analysis leads to expressions of the error probabilities in a form that can conveniently be used in numerical work. The results obtained from Monte Carlo evaluation agree well with the analytical evaluation of the error probability derived from the proposed method with the benefit of the low complexity and rapidity for the latter. >

Phase-Correcting Filter for Sub-optimal Equalizers

We propose a receiver structure intended for high bit rate transmission over dispersive radio channels. To ensure a good compromise between performance and complexity, the receiver is composed of a transversal phase-correcting filter followed by an equalizer which takes only partially the channel impulse response into account. For such sub-optimal equalizers, we present a non-iterative algorithm to adjust the coefficients of the phase-correcting filter. This phase-correction technique is based on the cepstral properties of a minimum phase response. We evaluate the complexity of the algorithm and analyse its performances. Simulation results show that such a filter implemented together with a reduced-state Maximum Likelihood Sequence Estimator (MLSE) using decision feedback ensures a good compromise between receiver performance and complexity for high bit rate transmission in a multipath channel.

Reduced-complexity channel estimation and equalization for TDMA mobile radio systems

A non-iterative block processing algorithm for channel estimation is presented. This is suitable for time-division multiple access (TDMA) digital radio systems with a signal structure based on time-slots. Such channel estimator can be applied in conjunction with decision feedback equalizer, maximum likelihood sequence estimator (MLSE) or near-MLSE. A theoretical analysis of the estimator performances for slow and fast time-varying channels is presented. A procedure for determining the required training sequence length and the number of estimated coefficients in the channel impulse response is developed. Depending on channel characteristics, there is a particular length which yields optimum performances at the beginning of the data block. This length is analytically evaluated for constant amplitude zero-autocorrelation (CAZAC) sequences. Finally, the effects of channel estimation optimization are assessed by computer simulations.

New optimal hard decoding of convolutional codes with reduced complexity

It is well known that convolutional codes can be optimally decoded by the Viterbi algorithm (VA). We propose an optimal hard decoding technique where the VA is applied to identify the error vector rather than the information message. In this paper, we show that, with this type of decoding, the exhaustive computation of a vast majority of state-to-state iterations is unnecessary. Hence, under certain channel conditions, optimum performance is achievable with a sensible reduction in complexity.

A new importance sampling technique for the evaluation of performance in digital communication systems

Importance sampling simulation is an efficient tool for the evaluation of the probability of rare events and performs well while estimating low probability errors in digital communication systems. After a brief description of principalis techniques, we present a new technique which uses probability density functions of the « generalized Rayleigh tail » category. The good performance of this technique is demonstrated by computing the associated estimation variance and asymptotic confidence interval. Analytical and simulation results show that the proposed technique is robust with respect to parameter variations. The optimal parameters are also obtained for the minimised estimation variance. Finally, we present a random variable generation method adapted to our technique.RésuméLa simulation par la méthode ďéchantillonnage par importance est un moyen efficace pour évaluer la probabilite ďoccurrence des événements rares et importants; elle s’avére trés performante dans ľestimation de la probabilité ďerreur des systémes de télécommunication numérique lorsque cette probabilité est faible. Aprés avoir briévement presenté les différentes méthodes ďéchantillonnage par importance, une nouvelle version qui utilise des densités de probabilité du type « extrémité de Rayleigh généralisée » est exposée. Le calcul de la variance ďestimation et de ľintervalle de confiance asymptotique montre que cette méthode est plus performante que ďautres existantes. De plus, des résultats obtenus analytiquement et par simulation montrent sa robustesse vis-à-vis de la variation des paramétres. Sont obtenus également des paramétres optimaux au sens de la minimisation de la variance de ľestimateur et une méthode de génération de nombres aléatoires adaptée à cette technique est présentée.

Codage pour modulation d'amplitude en quadrature à grand nombre d'états avec partition à plusieurs niveaux

RésuméLes auteurs présentent une nouvelle technique de codage pour la modulation d’amplitude en quadrature (MAQ) à grand nombre d’états (MAQ-64 et MAQ-256) avec partition à plusieurs niveaux. Ils effectuent une partition de la constellation en quatre classes repérées par deux bits qui sont codés chacun par un même codé BCH. En ce qui concerne la redondance, la simplicité du décodage et les performances obtenues, ce système, employé avec un code BCH correcteur d’erreurs doubles, donne de meilleurs résultats que les systèmes actuellement connus (utilisant le code de Reed-Solomon (64,62), des codes de Lee-Nakamura ou de Hamming). Le problème de l’ambiguïté de phase est aisément résolu par codage différentiel préalable des deux bits codés et par l’emploi d’un code BCH transparent.AbstractWe present a new multilevel coding technique for the quadrature amplitude modulation with a high number of states (64-QAM and 256-QAM). The constellation is partitioned into four subsets, two bits coded with the samebch code selecting the label of each subset. We show that our system, with a double error correctingbch code, gives better results, considering redundancy, decoding simplicity and performance, than the systems known bx now (using the Reed-Solomon code (64,62), the Lee-Nakamura and Hamming codes). Preliminary differential encoding of the two coded bits and utilisation of a transparentbch code can solve the phase ambiguity problem in a simple way.