Channel estimation strategy for LPWA transmission at low SNR: application to Turbo-FSK

International audienceTurbo Frequency Shift Keying has been considered as a promising physical layer for low power wide-area network connectivity. Because of its constant envelope amplitude and the efficiency of its iterative receiver performance close to Shannon's limit can be achieved. However, results published so far in the literature for the waveform have assumed perfect channel estimation or Signal-to-noise (SNR) levels that are higher than the SNR levels considered for these applications. This paper analyzes a channel estimation strategy based on a specifically adapted pilot sequence. Simulations have been performed to evaluate the performance of the proposed approach. Performance loss induced by imperfect channel estimation algorithms is estimated

Optimisation des codes LDPC pour les communications OFDM

Nous présentons dans cet article une nouvelle méthode d'optimisation des codes correcteurs d'erreurs pour les transmissions OFDM codées sur canal sélectif en fréquence, basée sur l'utilisation des codes LDPC irréguliers. Dans un premier temps, nous introduisons une nouvelle paramétrisation des codes LDPC irréguliers que nous appelons profil d'irrégularité. Ensuite, nous proposons d'utiliser cette paramétrisation pour optimiser les codes LDPC pour les communications OFDM en utilisant comme critère la minimisation du seuil de convergence du code. L'optimisation de ce nouveau critère est ensuite faite par approximation Gaussienne. Des simulations illustrent les performances de notre approche comparativement à celle d'un code optimisé pour canal BABG

Optimisation des codes LDPC pour les communications multi-porteuses

Un des inconvénients des transmissions OFDM est lié à l'émission de symboles à travers des sous-bandes fortement atténuées. En effet, ces symboles sont intrinsèquement affectés par un taux d'erreur binaire important nécessitant l'utilisation d'un codage canal performant (COFDM). Les codes correcteurs d'erreur que nous avons utilisés sont les codes LDPC. Nous en effectuons une large présentation incluant les notations et les outils algorithmiques indispensables. Afin de tenir compte de la non-stationnarité du canal OFDM sélectif en fréquence, nous avons alors généralisé la paramétrisation de ceux-ci en introduisant la notion de profil d'irrégularité. La chaîne de transmission COFDM (avec et sans allocation de bits) a été ensuite présentée, ainsi que les principales caractéristiques des canaux OFDM sélectifs en fréquence. La capacité de Shannon d'un tel système pour des entrées MAQ a été dérivée.Nous avons ensuite étudié le comportement asymptotique par évolution de densité des codes LDPC lors de transmissions OFDM à travers un canal sélectif en fréquence. Pour ce faire, nous montrons qu'il est possible de vérifier les hypothèses nécessaires à ce type d'analyse (symétrie du canal, indépendance des messages). Afin de pouvoir optimiser la structure des codes LDPC pour l'OFDM, une approximation Gaussienne de l'évolution de densité a alors été dérivée. Deux critères d'optimisation ont ensuite été introduis, l'un permettant d'obtenir le code présentant les meilleures performances asymptotiques (minimisation du seuil), et un critère original, mieux adapté aux hypothèses pratiques, basé sur la minimisation de la probabilité d'erreur sur les bits d'information.The major drawback of OFDM transmissions is that some symbols might be subject to strong attenuations. Hence, these symbols are potentially affected by an important bit error rate which involves the use of channel coding. The channel codes used are LDPC codes for which we have presented the main notations and a decoding algorithm. Considering the nostationarity of the OFDM frequency selective channel, we introduced a more general description of LDPC codes that we call irregularity profile. The COFDM communication system is then presented (without and with bit allocation) as well as the main characteristics of OFDM frequency selective channels. The channel capacity of this OFDM system with discrete MAQ input has been derived.We have then studied the asymptotic behaviour of LDPC codes with density evolution for OFDM transmissions through a frequency selective channel. In order to make possible this analysis, we have shown that the necessary conditions (messages independence and channel symmetry) are checked for the OFDM channel. A Gaussian approximation of the density evolution has then been presented and used for the optimization of LDPC codes. Two optimization criteria have been introduced, one based on the minimization of the LDPC decoding threshold and another one based on the minimization of the bit error probability at a given SNR. The later criterion has shown better performances for practical applications.REIMS-BU Sciences (514542101) / SudocSudocFranceF

Collective perception messages: new low complexity fusion and V2X connectivity analysis

Connected vehicles are equipped with sensors that can detect surrounding objects. However, vehicles perception is limited by sensors range or by the presence of other obstacles. Collective perception can improve vehicles perception by allowing them to exchange information about detected obstacles. In this context, vehicles rely on on-board sensors in order to generate Collective Perception Messages (CPM) that are exchanged by means of LTE-V2X connectivity. In order to reveal hidden obstacles and obtain a coherent visualization about the environment, CPM fusion is then crucial. In this work, we propose a novel low complexity fusion algorithm for CPM. Moreover, we evaluate the impact of LTE-V2X connectivity performance, specially in terms of packets loss, on the fusion. Simulation results in a smart junction demonstrate the relevance and efficiency of our algorithm in terms of obstacles detection capabilities

Turbo-FSK, a physical layer for LPWA: Synchronization and Channel estimation

International audience—Turbo Frequency Shift Keying (Turbo-FSK) has been considered as a promising physical layer for low power wide area applications. Its constant envelope at the transmitter combined with performance close to the Shannon's limit enable to achieve a high energy efficiency. However, results published so far in the literature for this waveform have assumed perfect synchronization and channel estimation. This paper, presents a synchronization and channel estimation approach based on a specifically built preamble and adapted to the performance of the new modulation. Simulations have been performed for both time and frequency synchronization as well as channel estimation. Less than 1 dB degradation in comparison to perfect detection is achieved for the most severe types of channels

Une méthode simple de turbo estimation de source en décodage conjoint source-canal

- Le principe du décodage conjoint source-canal (DCSC) est d'utiliser des connaissances a priori sur la source afin de réduire les taux d'erreur au décodeur en exploitant la redondance résiduelle de la source compressée. Le plus souvent les auteurs supposent que les statistiques de la source sont parfaitement connues du décodeur. Nous proposons ici une méthode simple d'estimation de ces statistiques dans le cas de sources codées en longueur variable. Des simulations, réalisées avec un turbo code, illustrent la convergence de l'estimateur et la possibilité d'atteindre les limites de performance de cette méthode de DCSC

A Flexible Physical Layer for LPWA Applications: Simulations and Field Trials

International audienceFuture Low Power Wide Area (LPWA) systems are expected to provide support for a wider range of applications: faster throughput and performance for lower levels of latency is hence forecast for a similar battery lifetime. These contradictory requirements lead to consider a flexible physical layer operating for different modes from low data rate, low power consumption, long range to high data rate. This new flexible physical layer approach is presented in this paper: simulation performance is given and compared to field trial measurements using a software defined radio implementation. Finally, field measurement comparison to state of the art LoRa is performed. The paper demonstrates with these measurements some practical benefits of the new approach

Post-traitement OSD pour le décodage BP basé sur des ensembles de LLRs complémentaires

National audienceThis article deals with Ordered Statistics Decoding (OSD) applied to the soft outputs of the Belief Propagation (BP) algorithm. We first model the weighted sum of the a posteriori LLRs across BP decoding iterations into a neuron. The neuron is then trained with the focal loss to compute for each BP decoding failure a set of accumulated Log Likelihood Ratios (LLRs) suited for OSD post-processing. Then, we then propose a recursive selection procedure of LLR sets, for multiple OSD post-processing. This selection is carried out from the sets of a posteriori LLRs calculated at each BP iteration, and from the accumulated LLRs optimized for the OSD, based on their joint probabilities of failure with OSD post-processing. An OSD is then applied to each set of LLRs belonging to the selection. Our results show that this new decoding method provides an effective way to bridge the gap to maximum likelihood decoding for short Low Density Parity Check (LDPC) codes.Cet article traite du post-traitement par OSD (Ordered Statistics Decoding) appliqué aux sorties souples de l'algorithme BP (Belief Propagation). Notre approche consiste dans un premier temps à modéliser la somme pondérée des LLRs (Log Likelihood Ratios) a posteriori calculés au cours des itérations du BP en un seul neurone. Ce neurone est alors entraîné avec la fonction de coût focale afin d'obtenir pour chaque échec du BP un ensemble de LLRs accumulés qui soient adaptés au post-traitement par OSD. Nous proposons ensuite une procédure de sélection récursive d'ensembles de LLRs pour un post-traitement OSD multiple. Cette sélection est réalisée à partir des ensembles des LLRs a posteriori calculés à chaque itération du BP ainsi que des LLRs accumulés et optimisés pour l'OSD, selon leurs probabilités conjointes d'échec avec le post-traitement OSD. Un OSD est alors appliqué sur chaque ensemble de LLRs de la sélection. Nos résultats montrent que cette nouvelle méthode de décodage fournit un moyen efficace d'atteindre la performance du décodeur par maximum de vraisemblance pour des codes LDPC (Low Density Parity Check) courts

A Comparison of the V2X Communication Systems: ITS-G5 and C-V2X

International audienceVehicle to vehicle/infrastructure communication systems have a significant role to play in optimizing road traffic and improving road safety. In this context, two standards have emerged, namely ITS-G5 (IEEE 802.11p) and C-V2X (3GPP Release 14). The objective of this article is to compare both standards by evaluating the performance of both physical layers and associated MAC layers. The physical layer performance of a single link is first evaluated and used to derive performance in a loaded network where each user is scheduled by their respective MAC layer. Performance evaluation shows an advantage for the C-V2X for low levels of vehicles density while when the congestion increases the performance gap reduces until ITS-G5 eventually outperforms C-V2X. Finally, latency was also assessed for both communication systems

Sets of complementary LLRs to improve OSD post-processing of BP decoding

International audienceThis article deals with Ordered Statistics Decoding (OSD) applied to the soft outputs of the Belief Propagation (BP) algorithm. We first model the weighted sum of the a posteriori LLRs across BP decoding iterations into a neuron. The neuron is then trained with the focal loss to compute for each BP decoding failure a set of accumulated Log Likelihood Ratios (LLRs) suited for OSD post-processing. Then, we propose a recursive selection procedure of LLRs sets, for multiple OSD post-processing. This selection is carried out from the sets of a posteriori LLRs calculated at each BP iteration, and from the accumulated LLRs optimized for the OSD, based on their joint probabilities of failure with OSD post-processing. An OSD is then applied to each set of LLRs belonging to the selection. In addition, we propose to reduce the OSD post-processing decoding complexity without significantly degrading its performance. Our results show that this new decoding method provides an effective way to bridge the gap to maximum likelihood decoding for short and long Low Density Parity Check (LDPC) codes