Estimation au maximum aposteriori d'un canal avec évanouissements rapides pour la diversité de transmission spatio-temporelle d'ALAMOUTI : Performances et comparaison avec des techniques différentielles de transmission et de réception

Le schéma de diversité spatio-temporelle à deux antennes à l'émission proposé par ALAMOUTI effectue une détection au maximum de vraisemblance basée sur un traitement linéaire au niveau du récepteur. Quand le canal n'est pas connu -à l'émetteur et au récepteur- ce schéma nécessite une estimation des deux canaux de propagation vus par les deux antennes de transmission. Notre objectif est d'évaluer cette technique de diversité avec une estimation réaliste considérant un canal très variable dans le temps. Pour une estimation robuste du canal, nous proposons un algorithme, bloc par bloc, d'estimation au maximum a posteriori. La performance de cet algorithme est comparée à celles obtenues avec des techniques différentielles de transmission et de réception

Linear Approximation of the Exponential Map with Application to Simplified Detection in Noncoherent MIMO Systems

Nous étudions une approximation quadratique du Critère de Vraisemblance Généralisé (GLRT) pour pouvoir décoder efficacement une certaine classe de codes espace-temps unitaires pour le canal MIMO à évanouissement par bloc. L'approximation quadratique est dérivée à partir de la série de Taylor de la métrique du GLRT tronqué au deuxième ordre. Une expression analytique du récepteur GLRT approximé est ainsi dérivée. Ses performances et les pertes par rapport au cas sans approximation sont illustrées à travers des simulations

Codage spatio-temporel sans connaissance a priori du canal

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Efficient Offline Waveform Design Using Quincunx/Hexagonal Time-Frequency Lattices

Conventional orthogonal frequency division multiplexing (OFDM) may turn to be inappropriate for future wireless cellular systems services, because of extreme natural and artificial impairments they are expected to generate. Natural impairments result from higher Doppler and delay spreads, while artificial impairments result from multisource transmissions and synchronization relaxation for closed-loop signaling overhead reduction. These severe impairments induce a dramatic loss in orthogonality between subcarriers and OFDM symbols and lead to a strong increase in intercarrier interference (ICI) and intersymbol interference (ISI). To fight against these impairments, we propose here an optimization of the transmit/receive waveforms for filter-bank multicarrier (FBMC) systems, with hexagonal time-frequency (TF) lattices, operating over severe doubly dispersive channels. For this, we exploit the Ping-pong Optimized Pulse Shaping (POPS) paradigm, recently applied to rectangular TF lattices, to design waveforms maximizing the signal-to-interference-plus-noise ratio (SINR) for hexagonal TF lattices. We show that FBMC, with hexagonal lattices, offers a strong improvement in SINR with respect to conventional OFDM and an improvement of around 1 dB with respect to POPS-FBMC, with rectangular lattices. Furthermore, we show that hexagonal POPS-FBMC brings more robustness to frequency synchronization errors and offers a 10 dB reduction in out-of-band (OOB) emissions, with respect to rectangular POPS-FBMC

Sentence boundary detection for transcribed Tunisian Arabic

International audienceWe study, in this paper, the problem of detecting the sentence boundary in tran-scribed spoken Tunisian Arabic. We compare and contrast three different methods for detecting sentence bounda-ries in transcribed speech. The first method uses a set of handmade contex-tual patterns for identifying the limit of sentences. The second method aims to classify transcriptions words into four classes according to their position in a sentence. Both methods are based only on lexical and some prosodic information such as silent and filled pauses. Finally, we develop two techniques for mixing the results of the two proposed methods. We show that sentence boundary detec-tion system can improve the accuracy of a POS tagger system developed for tag-ging transcribed Tunisian Arabic

Les déterminants du conservatisme comptable conditionnel dans le contexte français

International audienceThis study is particularly aims to measure the level of accounting conservatism and to identify the factors related to the characteristics of the firm that may affect the conditional conservatism. Based on a sample of 86 French firms which belong to the SBF 120 index during the period extending from 2005 to 2014, we found that these companies are less conservative. We also demonstrated that small firms and companies with high leverage level, strong growth opportunities and a high market risk are more likely to be conservative.La présente étude a pour objet de mesurer le niveau du conservatisme comptable et de dégager les facteurs liés aux caractéristiques propres de l’entreprise qui influent le conservatisme conditionnel. Sur la base d'un échantillon composé de 86 sociétés françaises appartenant à l'indice SBF 120, pendant la période s'étalant entre 2005 et 2014, nous avons trouvé que ces sociétés sont moins conservatrices et que les sociétés de petite taille, ainsi que les sociétés ayant un niveau d'endettement élevé, de fortes opportunités de croissance et un risque de marché élevé sont plus susceptibles d'être conservatrices

Symbol MAP Detection Aided with First Channel Estimate for TDMA Systems Over a Frequency Selective Channel

Abstract—In this paper, we consider joint iterative equalization and channel estimation for TDMA systems over a frequency selective, quasi-static fading channel. We assume a slowly varying channel impulse response from one burst to the following one. We modify the maximum a posteriori (MAP) criterion by taking into account an apriorichannel estimate obtained from the burst preceding the one processed. Assuming that the receiver knows the channel statistics, we propose to use the EM (Expectation-Maximization) algorithm for a joint iterative equalization and channel estimation. During each iteration of the receiver, the EM algorithm reestimates the channel by using the a posteriori probabilities (APP) available at the output of the equalizer. The algorithm we propose has linear-time complexity per iteration. Simulations show that our MAP receiver reaches the performance of the MAP equalizer with perfect channel knowledge after a few iterations, for high SNR and for small values of normalized Doppler spread. They show also that our receiver outperforms the MAP equalizer that uses a channel estimate obtained by using a training sequence. I

{Generalized MAP: sequence detection for non ideal frequency selective channel knowledge}

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Game Theoretical Framework for Joint Channel Selection and Power Control in Hybrid NOMA

Non-Orthogonal Multiple Access (NOMA) is an interesting candidate to tackle the massive access challenges in Beyond 5G (B5G) systems. However, arranging Machine Type Devices (MTDs) into NOMA clusters, and allocating resources to these clusters is a non-trivial task. In this paper, we consider a Hybrid NOMA system where every NOMA cluster is allocated an orthogonal subcarrier, and propose a game theoretical framework based on a bi-level game in order to achieve joint channel selection and power allocation for MTDs. Indeed, the proposed game is composed of a non-cooperative Power Control (PC) game underlying a cooperative Hedonic game that enables MTDs to self-organize into coalitions. Furthermore, we propose two low-complexity algorithms that enable us to obtain a Nash-Stable partitions where MTDs decide autonomously the resource block and the transmit power to use in order to deliver their packets