Multi-slot Coded ALOHA with Irregular Degree Distribution

This paper proposes an improvement of the random multiple access scheme for satellite communication named Multislot coded ALOHA (MuSCA). MuSCA is a generalization of Contention Resolution Diversity Slotted ALOHA (CRDSA). In this scheme, each user transmits several parts of a single codeword of an error correcting code instead of sending replicas. At the receiver level, the decoder collects all these parts and includes them in the decoding process even if they are interfered. In this paper, we show that a high throughput can be obtained by selecting variable code rates and user degrees according to a probability distribution. With an optimal irregular degree distribution, our system achieves a normalized throughput up to 1.43, resulting in a significant gain compared to CRDSA and MuSCA. The spectral efficiency and the implementation issues of the scheme are also analyzed.Comment: 6 pages, 8 figure

Toward a relational sociology of credit:An exploration of the French literature

This article aims to describe the sociological studies of credit developed in France over the past dozen years. These studies propose a specific method and approach to address credit, primarily understanding it as a result of social interactions embedded in organizational and legal structures, with consequences on inequalities, social stratification, and individuals’ life experiences. The article is divided into four parts: after an introduction presenting what can be called the French school of the sociology of credit, we present the ‘different voice’ of the French school of the sociology of credit, which analyses the credit market according to a relational approach. The third section examines the construction of social domination at the moment of credit assessment. We then focus on the demand side: borrowers are not atomized individuals but part of households and other local communities. Finally, the conclusion discusses how this French approach to credit may be useful outside of France

A Relationship and a Practice: On the French Sociology of Credit

This paper aims to describe the social studies of credit developed in France over the past dozen years. We argue that this French sociology of credit, mostly centered on France, can be useful for researchers analyzing other countries, with other institutional particularities, because it proposes a specific method and a specific way to raise questions: credit is mostly understood as a result of social interactions embedded in organizational and legal structures. French researchers also deeply analyze the consequences of the organization of the credit market for inequalities, social stratification, and people’s life experiences. The first part of the paper focuses on works that have examined credit as a social test, looking at the institutional, technical, and social frameworks of money lending. Then, credit is understood as a sociological experiment: how is it integrated into household economies? How do people use forms of credit? Finally, the third part concentrates on credit failure, when a bank loan becomes a debt. This aspect is mostly framed in French sociology as “over-indebtedness,” which is an administrative and a social category. Throughout the paper, we address credit as both a relationship and a practice. This approach is heuristic, as we seek to demonstrate, because it enables us to show that credit is a social and political issue

A Cooperative Network Coding Strategy for the Interference Relay Channel.

In this paper, we study an interference relay network with a satellite as relay. We propose a cooperative strategy based on physical layer network coding and superposition modulation decoding for uni-directional communications among users. The performance of our solution in terms of throughput is evaluated through capacity analysis and simulations that include practical constraints such as the lack of synchronization in time and frequency.We obtain a significant throughput gain compared to the classical time sharing case

Generic Approach for Hierarchical Modulation Performance Analysis: Application to DVB-SH

Broadcasting systems have to deal with channel diversity in order to offer the best rate to the users. Hierarchical modulation is a practical solution to provide several rates in function of the channel quality. Unfortunately the performance evaluation of such modulations requires time consuming simulations. We propose in this paper a novel approach based on the channel capacity to avoid these simulations. The method allows to study the performance in terms of spectrum efficiency of hierarchical and also classical modulations combined with error correcting codes. Our method will be applied to the DVB-SH standard which considers hierarchical modulation as an optional feature.Comment: To appear in WTS 2011 proceeding

Combining Adaptive Coding and Modulation With Hierarchical Modulation in Satcom Systems

We investigate the design of a broadcast system in order to maximize throughput. This task is usually challenging due to channel variability. Forty years ago, Cover introduced and compared two schemes: time sharing and superposition coding. Even if the second scheme was proved to be optimal for some channels, modern satellite communications systems such as DVB-SH and DVB-S2 rely mainly on a time sharing strategy to optimize the throughput. They consider hierarchical modulation, a practical implementation of superposition coding, but only for unequal error protection or backward compatibility purposes. In this article, we propose to combine time sharing and hierarchical modulation together and show how this scheme can improve the performance in terms of available rate. We introduce a hierarchical 16-APSK to boost the performance of the DVB-S2 standard. We also evaluate various strategies to group the receivers in pairs when using hierarchical modulation. Finally, we show in a realistic case, based on DVB-S2, that the combined scheme can provide throughput gains greater than 10% compared to the best time sharing strategy

Improving broadcast channel rate using hierarchical modulation

We investigate the design of a broadcast system where the aim is to maximise the throughput. This task is usually challenging due to the channel variability. Modern satellite communications systems such as DVB-SH and DVB-S2 mainly rely on time sharing strategy to optimize throughput. They consider hierarchical modulation but only for unequal error protection or backward compatibility purposes. We propose in this article to combine time sharing and hierarchical modulation together and show how this scheme can improve the performance in terms of available rate. We present the gain on a simple channel modeling the broadcasting area of a satellite. Our work is applied to the DVB-SH standard, which considers hierarchical modulation as an optional feature.Comment: 5 pages, submitte

Trade-off between spectrum efficiency and link unavailability for hierarchical modulation in DVB-S2 systems

Broadcasting systems have to deal with channel variability in order to offer the best spectrum efficiency to the receivers. However, the transmission parameters that optimize the spectrum efficiency generally leads to a large link unavailability. In this paper, we study the performance of hierarchical and nonhierarchical modulations in terms of spectrum efficiency and link unavailability for DVB-S2 systems. Our first contribution is the design of the hierarchical 16-APSK for the DVB-S2 standard. Then we introduce the link unavailability to compare the performance of hierarchical and non-hierarchical modulations in terms of spectrum efficiency and link unavailability. The results show that hierarchical modulation is a good alternative to nonhierarchical modulation for the DVB-S2 standard

Making hierarchical modulation more flexible

In a broadcast system using the hierarchical modulation, the system delivers several streams with different waveforms and required Signal-to-Noise Ratio (SNR), typically SD- TV and HD-TV. At the application layer, each stream is delivered with a particular rate. The physical layer must be defined in order to optimize the protection of each stream with respect to the double constraints of both the data rates and the SNR thresholds. We show in this paper that a standard like DVB-SH is not always well adapted to meet these system constraints in operational typical cases. After exposing the current limitations of a classical hierarchical modulation approach, we present two possible adaptations to address these operational requirements and offer more flexibility in hierarchical modulation design