Capacity bounds for the CDMA system and a neural network : a moderate deviations approach

Löwe, Matthias; Vermet, Franck

ESAIM: Probability and Statistics, Tome 13 (2009), pp. 343-362.

Nous étudions deux systèmes basés sur des sommes de variables aléatoires de Bernoulli valant $\pm$ 1 avec égale probabilité et faiblement dépendantes. Nous montrons qu’une seule étape de la méthode de suppression d’interférences SD-PIC, utilisée dans la troisième génération de télécommunication mobile CDMA, permet déjà d’augmenter considérablement le nombre d’utilisateurs supporté par le système. Nous considérons également une variante du modèle neuronal de Hopfield. Nous montrons que cette variante, proposée par Amari et Yanai [2], admet une capacité de stockage supérieure au modèle original. Les deux situations conduisent à l’étude des déviations modérées d’une somme de variables aléatoires de Bernoulli faiblement corrélées. Nous montrons un principe de déviations modérées pour une telle somme convenablement normalisée.

We study two systems that are based on sums of weakly dependent Bernoulli random variables that take values $\pm$ 1 with equal probabilities. We show that already one step of the so-called soft decision parallel interference cancellation, used in the third generation of mobile telecommunication CDMA, is able to considerably increase the number of users such a system can host. We also consider a variant of the well-known Hopfield model of neural networks. We show that this variant proposed by Amari and Yanai [2] has a larger storage capacity than the original model. Both situations lead to the question of the moderate deviations behavior of a sum of weakly dependent Bernoulli random variables. We prove a moderate deviations principle for such a sum on the appropriate scale.

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