Enhancement of MARSALA Random Access with Coding Schemes, Power Distributions and Maximum Ratio Combining

Several random access (RA) techniques have been proposed recently for the satellite return link. The main objective of these techniques is to resolve packets collisions in order to enhance the limited throughput of traditional RA schemes. In this context, Multi-Replica Decoding using Correlation based Localisation(MARSALA) has been introduced and has shown good performance with DVB-RCS2 coding scheme and equi-powered transmissions. However, it has been shown in the literature that alternative coding schemes and packets power distributions can have a positive impact on RA performance. Therefore, in this paper, we investigate the behaviour of MARSALA with various coding schemes and various packet power distributions, then we propose a configuration for optimal performance. This paper also introduces the enhancement of MARSALA RA scheme by adding MRC to optimize replicas combination and study the impact on the throughput. We compare two different MRC techniques and we evaluate, via simulations, the gain achieved using MRC with different coding schemes and unbalanced packets. The simulation results demonstrate that the proposed enhancements to MARSALA show substantial performance gain, i.e. throughput achieved for a target Packet Loss Ratio (PLR)

Improving Synchronous Random Access Schemes for Satellite Communications

L’optimisation des communications par satellite devient un enjeu crucial pour fournir un accèsInternet aux zones blanches et/ou défavorisées et pour supporter des réseaux à grande échelle.Dans ce contexte, l’utilisation des techniques d’accès aléatoires sur le lien retour permetd’améliorer les performances de ces systèmes. Cependant, les techniques d’accès aléatoireclassiques comme ‘Aloha’ et ‘Slotted Aloha’ ne sont pas optimales pour la transmission dedonnées sur le lien retour. En effet, ces techniques présentent un taux élevé de pertes depaquets suite aux collisions. Par conséquent, des études récentes ont proposé de nouvellesméthodes d’accès aléatoire pour résoudre les collisions entre les paquets et ainsi, améliorerles performances. En particulier, ces méthodes se basent sur la redondance de l’informationet l’annulation successive des interférences. Dans ces systèmes, l’estimation de canal sur le lien retour est un problème difficile en raison du haut niveau de collisions de paquets. Dans une première contribution dans cette thèse, nous décrivons une technique améliorée d’estimation de canal pour les paquets en collision. Par ailleurs, nous analysons l’impact des erreurs résiduelles d’estimation de canal sur la performance des annulations successives des interférences. Même si les résultats obtenus sont encore légèrement inférieurs au cas de connaissance parfaite du canal, on observe une amélioration significative des performances par rapport aux algorithmes d’estimation de canal existants. Une autre contribution de cette thèse présente une méthode appelée ‘Multi-Replica Decoding using Correlation based Localisation’ (MARSALA). Celle-ci est une nouvelle technique de décodage pour la méthode d’accès aléatoire synchrone ‘Contention Résolution diversité Slotted Aloha’ (CRDSA), qui est basée sur les principe de réplication de paquets et d’annulation successive des interférences. Comparée aux méthodes d’accès aléatoire traditionnelles, CRDSA permet d’améliorer considérablement les performances. Toutefois, le débit offert par CRDSA peut être limité à cause des fortes collisions de paquets. L’utilisation deMARSALA par le récepteur permet d’améliorer les résultats en appliquant des techniques de corrélation temporelles pour localiser et combiner les répliques d’un paquet donné. Cette procédure aboutit à des gains en termes de débit et de taux d’erreurs paquets. Néanmoins, le gain offert par MARSALAest fortement dépendant de la synchronisation en temps et en phase des répliques d’un mêmepaquet. Dans cette thèse, nous détaillons le fonctionnement de MARSALA afin de corriger ladésynchronisation en temps et en phase entre les répliques. De plus, nous évaluons l’impactde la combinaison imparfaite des répliques sur les performances, en fournissant un modèle analytique ainsi que des résultats de simulation. En outre, plusieurs schémas d’optimisationde MARSALA sont proposés tels que le principe du ‘MaximumRatio Combining’, ou la transmissiondes paquets à des puissances différentes. Utilisées conjointement, ces différentespropositions permettent d’obtenir une amélioration très significative des performances. Enfin,nous montrons qu’en choisissant la configuration optimale pour MARSALA, le gain deperformance est considérablement amélioré.With the need to provide the Internet access to deprived areas and to cope with constantlyenlarging satellite networks, enhancing satellite communications becomes a crucial challenge.In this context, the use of Random Access (RA) techniques combined with dedicated accesson the satellite return link, can improve the system performance. However conventionalRA techniques like Aloha and Slotted Aloha suffer from a high packet loss rate caused bydestructive packet collisions. For this reason, those techniques are not well-suited for datatransmission in satellite communications. Therefore, researchers have been studying andproposing new RA techniques that can cope with packet collisions and decrease the packet lossratio. In particular, recent RA techniques involving information redundancy and successiveinterference cancellation, have shown some promising performance gains.With such methods that can function in high load regimes and resolve packets with high collisions,channel estimation is not an evident task. As a first contribution in this dissertation, wedescribe an improved channel estimation scheme for packets in collision in new RAmethodsin satellite communications. And we analyse the impact of residual channel estimation errorson the performance of interference cancellation. The results obtained show a performancedegradation compared to the perfect channel knowledge case, but provide a performanceenhancement compared to existing channel estimation algorithms. Another contribution of this thesis is presenting a method called Multi-Replica Decoding using Correlation based Localisation (MARSALA). MARSALA is a new decoding technique for a recent synchronous RAmethod called Contention Resolution Diversity Slotted Aloha (CRDSA). Based on packets replication and successive interference cancellation, CRDSA enables to significantly enhance the performance of legacy RA techniques. However, if CRDSA is unable to resolve additional packets due to high levels of collision, MARSALA is applied. At the receiver side, MARSALA takes advantage of correlation procedures to localise the replicas of a given packet, then combines the replicas in order to obtain a better Signal to Noise plus Interference Ratio. Nevertheless, the performance ofMARSALA is highly dependent on replicas synchronisation in timing and phase, otherwise replicas combination would not be constructive. In this dissertation, we describe an overall framework ofMARSALA including replicas timing and phase estimation and compensation, then channel estimation for theresulting signal. This dissertation also provides an analytical model for the performancedegradation of MARSALA due to imperfect replicas combination and channel estimation.In addition, several enhancement schemes forMARSALA are proposed likeMaximum RatioCombining, packets power unbalance, and various coding schemes. Finally, we show thatby choosing the optimal design configuration for MARSALA, the performance gain can besignificantly enhanced

A Multi-Replica Decoding Technique for Contention Resolution Diversity Slotted Aloha

International audienceThis paper proposes a new method for data reception over a random access channel in a satellite communication system. The method is called Multi-replicA decoding using corRelation baSed locALisAtion (MARSALA). It uses the same transmission scheme as in Contention Resolution Diversity Slotted Aloha (CRDSA) where each user sends several replicas of the same packet over the frame. MARSALA is a new decoding technique that localises all the replicas of a packet using a correlation based method, then combines them to decode the data. With MARSALA, the system can achieve a normalized throughput higher than 1.2, resulting in a significant gain compared to CRDSA, while adding a relatively low implementation complexity at the receiver. We also highlight on the practical issues related to channel estimation and how to perform coherent signal combination in MARSALA

Effect of residual channel estimation errors in random access methods for satellite communications

International audienceIn recent random access methods used for satellite communications, collisions between packets are not considered as destructive. In fact, to deal with the collision problem, successive interference cancellation is performed at the receiver. Generally, it is assumed that the receiver has perfect knowledge of the interference. In practice, the interference term is affected by the transmission channel parameters, i.e., channel attenuation, timing offsets, frequency offsets and phase shifts, and needs to be accurately estimated and canceled to avoid performance degradation. In this paper, we study the performance of an enhanced channel estimation technique combining estimation using an autocorrelation based method and the Expectation-Maximization algorithm integrated in a joint estimation and decoding scheme. We evaluate the effect of residual estimation errors after successive interference cancellation. To validate our experimental results, we compare them to the Cramer-Rao lower bounds for the estimation of channel parameters in case of superimposed signals

Complexity analysis for recent ALOHA random access techniques in satellite communications

International audienceIn this paper we study the the complexity of packet localization at reception, for recent synchronous Random Access (RA) techniques based on the protocol ALOHA for satellite communications. The promising CRDSA (Contention Resolution Diversity Slotted ALOHA) offers better throughput, in comparison to the traditional slotted ALOHA protocols, thanks to the use of Successive Interference Cancellation (SIC) along with multireplica transmission. MARSALA (Multi-replicA decoding using corRelation baSed locALizAtion) is one of the many variants and enhancement schemes of CRDSA that have been proposed in the literature. It is applied to CRDSA each time a decoding deadlock situation is reached (when no packets can be retrieved by CRDSA). MARSALA first localizes the replicas of collided packets on a chosen reference time slot using correlations. Then it performs coherent signal combination of packet replicas prior to decoding. However, despite the good performance offered by MARSALA, its localization process adds a significant complexity to the receiver in terms of correlation operations. R-SPOTiT (Random Shared POsition Technique for Interfered random Transmissions) mitigates this complexity by introducing a shared information between the receiver and each of the transmitters, about all potential packets' locations on the frame, without any additional signaling overhead. We focus in this paper on the analysis of the total number of correlations which are needed to localize packets' replicas for both MARSALA and R-SPOTiT, with a single or with multiple Gold preambles. This should include preamble detection operations that are performed at CRDSA with a coarse and fine tracking. The results show that the most suitable system to use is the multi-preamble R-SPOTiT with two preambles