Lucile Sassatelli

Dynamic control of Coding in Delay Tolerant Networks

We study replication mechanisms that include Reed-Solomon type codes as well as network coding in order to improve the probability of successful delivery within a given time limit. We propose an analytical approach to compute these and study the effect of coding on the performance of the network while optimizing parameters that govern routing.

Nonbinary Hybrid LDPC Codes

In this paper, a new class of low-density parity-check (LDPC) codes, named hybrid LDPC codes, is introduced. Hybrid LDPC codes are characterized by an irregular connectivity profile and heterogeneous orders of the symbols in the codeword. It is shown in particular that the class of hybrid LDPC codes can be asymptotically characterized and optimized using density evolution (DE) framework, and a technique to maximize the minimum distance of the code is presented. Numerical assessment of hybrid LDPC code performances is provided, by comparing them to protograph-based and multiedge-type (MET) LDPC codes. Hybrid LDPC codes are shown to allow to achieve an interesting tradeoff between good error-floor performance and good waterfall region with nonbinary coding techniques.

Two-bit message passing decoders for LDPC codes over the binary symmetric channel

A class of two-bit message passing decoders for decoding column-weight-four LDPC codes over the binary symmetric channel is proposed. The thresholds for various decoders in this class are derived using density evolution. For a specific decoder, the sufficient conditions for correcting all error patterns with up to three errors are derived.

UEP concepts in modulation and coding

First unequal error protection (UEP) proposals date back to the 1960s (Masnick and Wolf; 1967), but now with the introduction of scalable video, UEP develops to a key concept for the transport of multimedia data. The paper presents an overview of some new approaches realizing UEP properties in physical transport, especially multicarrier modulation, or with LDPC and Turbo codes. For multicarrier modulation, UEP bit-loading together with hierarchical modulation is described allowing for an arbitrary number of classes, arbitrary SNR margins between the classes, and arbitrary number of bits per class. In Turbo coding, pruning, as a counterpart of puncturing is presented for flexible bit-rate adaptations, including tables with optimized pruning patterns. Bitand/ or check-irregular LDPC codes may be designed to provide UEP to its code bits. However, irregular degree distributions alone do not ensure UEP, and other necessary properties of the parity-check matrix for providing UEP are also pointed out. Pruning is also the means for constructing variable-rate LDPC codes for UEP, especially controlling the check-node profile.

Non-binary Hybrid LDPC Codes: structure, decoding and optimization

In this paper, we propose to study and optimize a very general class of LDPC codes whose variable nodes belong to finite sets with different orders. We named this class of codes hybrid LDPC codes. Although efficient optimization techniques exist for binary LDPC codes and more recently for non-binary LDFC codes, they both exhibit drawbacks due to different reasons. Our goal is to capitalize on the advantages of both families by building codes with binary (or small finite set order) and non-binary parts of their factor graph representation. The class of hybrid LDPC codes is obviously larger than existing types of codes, which gives more degrees of freedom to find good codes where the existing codes show their limits. We gave two examples where hybrid LDPC codes show their interest

Reliable Transport in Delay-Tolerant Networks With Opportunistic Routing

This paper tackles the issue of reliable transport in delay-tolerant mobile ad hoc networks (DTNs) that are operated by some opportunistic routing algorithm. We propose a reliable transport mechanism that relies on acknowledgements (ACKs) and coding at the source. The various versions of the problem depending on buffer management policies are formulated and a fluid model based on mean-field approximation is derived for the designed reliable transport mechanism. This model allows both the mean file completion time and the energy consumption to be expressed up to the delivery of the last ACK at the source. The accuracy of this model is assessed through numerical simulations and a detailed investigation of the impact of the system parameters on the performance is conducted. We eventually present a joint optimization of the mean completion delay with or without an energy constraint, to identify the optimal set of parameters to use.

Analysis of Non-binary Hybrid LDPC Codes

This paper is eligible for the student paper award. In this paper, we analyse asymptotically a new class of LDPC codes called non-binary hybrid LDPC codes, which has been recently introduced in L. Sassatelli and D. Declerq [2006]. We use density evolution techniques to derive a stability condition for hybrid LDPC codes, and prove their threshold behavior. We study this stability condition to conclude on asymptotic advantages of hybrid LDPC codes compared to their non-hybrid counterparts.

Estimating file-spread in delay tolerant networks under two-hop routing

We consider a Delay/Disruption Tolerant Network under two-hop routing. Our objective is to estimate and track the degree of spread of a message/file in the network. Indeed, having such real-time information is critical for on-line control of routing and energy expenditure. It also benefits the multi-casting application. With exponential inter-meeting times of mobile nodes: (i) for the estimation problem, we obtain exact expressions for the minimum mean-squared error (MMSE) estimator, and (ii) for the tracking problem, we first derive the diffusion approximations for the system dynamics and the measurements and then apply Kalman filtering. We also apply the solutions of the estimation and filtering problems to predict the time when a certain pre-defined fraction of nodes have received a copy of the message/file. Our analytical results are corroborated with extensive simulation results.

Understanding TCP cubic performance in the cloud: A mean-field approach

Cloud networking typically leads to scenarii where a large number of TCP connections share a common bottleneck link. In this paper, we focus on the case of TCP Cubic, which is the default TCP version in the Linux kernel. TCP Cubic is designed to better utilize high bandwidth-delay product path in an IP network. To do so, Cubic modifies the linear window growth function of legacy TCP standards, e.g., New Reno, to be a cubic function. Our objective in this work is to assess the performance of TCP Cubic in a cloud setting with a large number of long-lived TCP flows.We rely on a mean-field approach leading to a fluid model to analyze the performance of Cubic. After a careful validation of the model through comparisons with ns-2, we evaluate the efficiency and fairness of Cubic as compared to that of New Reno for a set of typical cloud networking scenarii.

Some UEP Concepts in Coding and Physical Transport

Unequal error protection is the key to future transport of multimedia data. The paper presents an overview of some new approaches realizing UEP properties in physical transport, especially multicarrier modulation, or with LDPC and turbo codes. For multicarrier modulation, a UEP bit-loading method is described allowing for an arbitrary number of classes, arbitrary SNR margins between the classes and arbitrary number of bits per class. In turbo coding, pruning, as a counterpart of puncturing is presented for more flexible bit-rate adaptations. Pruning is also the tool that leads to a construction approach for check-irregular LDPC codes with UEP properties.