Precoding for Outage Probability Minimization on Block Fading Channels

The outage probability limit is a fundamental and achievable lower bound on the word error rate of coded communication systems affected by fading. This limit is mainly determined by two parameters: the diversity order and the coding gain. With linear precoding, full diversity on a block fading channel can be achieved without error-correcting code. However, the effect of precoding on the coding gain is not well known, mainly due to the complicated expression of the outage probability. Using a geometric approach, this paper establishes simple upper bounds on the outage probability, the minimization of which yields to precoding matrices that achieve very good performance. For discrete alphabets, it is shown that the combination of constellation expansion and precoding is sufficient to closely approach the minimum possible outage achieved by an i.i.d. Gaussian input distribution, thus essentially maximizing the coding gain.Comment: Submitted to Transactions on Information Theory on March 23, 201

Phase Precoded Compute-and-Forward with Partial Feedback

In this work, we propose phase precoding for the compute-and-forward (CoF) protocol. We derive the phase precoded computation rate and show that it is greater than the original computation rate of CoF protocol without precoder. To maximize the phase precoded computation rate, we need to 'jointly' find the optimum phase precoding matrix and the corresponding network equation coefficients. This is a mixed integer programming problem where the optimum precoders should be obtained at the transmitters and the network equation coefficients have to be computed at the relays. To solve this problem, we introduce phase precoded CoF with partial feedback. It is a quantized precoding system where the relay jointly computes both a quasi-optimal precoder from a finite codebook and the corresponding network equations. The index of the obtained phase precoder within the codebook will then be fedback to the transmitters. A "deep hole phase precoder" is presented as an example of such a scheme. We further simulate our scheme with a lattice code carved out of the Gosset lattice and show that significant coding gains can be obtained in terms of equation error performance.Comment: 5 Pages, 4 figures, submitted to ISIT 201

Comunità cristiane nell’islam arabo. La sfida del futuro

Preesistenti all'islamizzazione del Medio Oriente, le comunità cristiane costituiscono tuttora un importante elemento di pluralismo, sebbene oggi a rischio di sopravvivenza, nelle società arabe a maggioranza musulmana. Il volume presenta il ruolo storico delle comunità cristiane orientali nelle rispettive società arabe, la loro funzione di intermediazione culturale tra Oriente e Occidente e la discriminazione giuridica e sociale subìta con l'avvento dell'islam.- Indice #4- Introduzione, Andrea Pacini #10- Gli arabi cristiani: dalla questione d’Oriente alla recente geopolitica delle minoranze, Joseph Maïla #38- I cristiani arabi dell’Oriente: una prospettiva demografica, Philippe Fargues #64- Le comunità cristiane, soggetti attivi della società araba nel corso della storia, Samir Khalil Samir #84- Le chiese del Medio Oriente: origini e identità, tra radicamento nel passato e apertura al presente, Jean Corbon #110- Il diritto dello stato/nazione e lo status dei non musulmani in Egitto e in Siria, Bernard Botiveau #130- L’emigrazione degli arabi cristiani: dimensioni e cause dell’esodo, Bernard Sabella #148- La produzione culturale dei cristiani arabi oggi: espressione di identità nella società a maggioranza musulmana, Camille Hechaïmé #178- Le dinamiche politiche dei copti: rendere la comunità un protagonista atti, Dina El Khawaga #196- La posizione e il ruolo attuale dei copti nell’economia egiziana: tradizioni e specializzazioni, Adel A. Beshai #216- Le dinamiche dei cristiani libanesi: tra il paradigma delle ‘āmmiyyāt e il paradigma Hwayyek, Elizabeth Picard #226- Le comunità cristiane e la situazione economica e sociale in Libano, Boutros Labaki #250- Dinamiche comunitarie e sociopolitiche dei cristiani arabi in Giordania, in Israele e nei territori palestinesi autonomi, Andrea Pacini #290- I cristiani di Siria, Habib Moussalli #320- I cristiani in Iraq, Yūsuf Habbi #330- Il contributo delle comunità arabo-cristiane al futuro delle società arabe del Medio Oriente: alcune prospettive, Maurice Borrmans #342- I riti delle chiese orientali #356- Glossario #376- Riferimenti bibliografici #40

Outage thresholds of LDPC codes over nonergodic block-fading channels

Comunicació presentada en la 16th European Wireless Conference 2010 (EW '10), celebrada els dies 12, 13, 14 i 15 d’abril de 2010 a Lucca (Itàlia), organitzada per l’IMT Institute for Advanced Studies.This paper derives approximations allowing the estimation of outage probability for standard irregular LDPC codes and full-diversity Root-LDPC codes used over nonergodic block-fading channels. Two separate approaches are discussed: a numerical approximation, obtained by curve fitting, for both code ensembles, and an analytical approximation for Root-LDPC codes, obtained under the assumption that the slope of the iterative threshold curve of a given code ensemble matches the slope of the outage capacity curve in the high-SNR regime.This work was supported by the European FP7 ICT-STREP DAVINCI project under contract INFSO-ICT-216203

Digital processing algorithms and architectures for UWB low cost communication system

Ce manuscrit traite des récepteur UWB à faible coût pour les hauts débits (100Mbps). Le système considéré émet des impulsions très courtes modulées en position (PPM), avec un simple détecteur d'énergie analogique à la réception. Pour surmonter les problèmes des débits très élevés, telles que l'interférence entre symboles et l'interférence intra-symbole, un égaliseur probabiliste est proposé. Cette étude a perrmis de mettre en œuvre un modèle de détection d'énergie adapté qui tire profit des niveaux d'interférences. Les simulations sont réalisées sur les canaux du 802.15.3a. Les résultats montrent que l'égaliseur probabiliste améliore les performances du récepteur pour un CSI parfait. Les paramètres du canal nécessaires à l'égalisation sont estimés à l'aide de l'algorithme EM. L'égaliseur probabiliste et l'algorithme EM sont exécutés conjointement dans la boucle itérative d'un décodeur canal afin de minimiser le BER. On a pu montré que l'algorithme EM est capable d'estimer les paramètres du canal pour un résultat similaire au CSI parfait. Une deuxième alternative d'estimation des paramètre du canal basée sur une nouvelle séquence d'apprentissage est proposée. Cette séquence permet d'avoir une estimation du canal avec une complexité moindre comparée à l'algorithme EM. En dernière partie, l'implantation matérielle de l'égaliseur probabiliste y est étudiée. Où, la fonction de vrassemblance du Chi2 est approchée par une fonction moins complexe. Ensuite, la complexité calculatoire est encore réduite en effectuant les calculs dans le domaine logarithmique. Les simulations en virgule fixe sont comparées au résultats obtenus avec la fonction non approchée en précision double.This thesis report deals with a low cost IR-UWB receiver for high-data rates (100Mbps). The system under consideration uses a PPM modulation with a simple analog energy detector. In ordeer to overcome the problems inherent to high-data rates, such as inter-symbol interference and intra-symbol interference, a probabilistic equalizer is proposed. On this study, we define a well matched energy model that takes advantage from the interference levels. COmputer simulations are performed on the channel models of IEEE 802.15.3a Task Group. Numerical results show that the energy equalizer improve the receiver performances if perfect CSI was assumed. Channel parameters necessary for equalization are estimated by the mean of the EM algorithm. The energy equalizer and the EM algorithm are jointly run into the loop of an iterative channel decoder in order to enhance the BER performance and achieve 100Mbps data rate. We show that the EM algorithm is capable of estimating the channel parameters as accurately as the perfect CSI. Besides, a new training sequence for channel estimation is proposed. This allows to obtain a very low complexity channel estimation with regards to the EM algorithm. Finally, the hardware implementation feasability of the probabilistic equalizer is studied. For this purpose, we apporximate and replace certain complex mathematical functions, such as the chi-squared likelihood function, by ones less complex. Then, the computational complexity is further reduced by performing the calculations in the logarithmic domain. Fixed-point data types simulations are carried out and compared to the results obtained from the reference floating point non-approximated functions.PARIS-Télécom ParisTech (751132302) / SudocSudocFranceF

Full diversity Random LDPC codes

The block fading (BF) channel is a useful model for various communication systems in urban environments. Full-diversity error-correcting codes are required to approach the physical limits of the performance on BF channels. Low-density parity-check (LDPC) codes are good error-correcting codes, but full-diversity standard random LDPC codes for the BF channel are not known yet. We design full-diversity random LDPC ensembles at infinite block length by optimizing the threshold in multiple points in the fading space, which takes into account the randomness of the fading. However, this is not sufficient to achieve full-diversity at finite block length, because of stopping sets. We therefore propose a method to generate full-diversity code instances at finite length so that stopping sets over information bits are avoided. The asymptotic and finite length word error rate performance is verified by means of density evolution and Monte Carlo simulations, respectively, confirming that full-diversity standard random LDPC codes exist and perform well