Analysis and Design of Binary Message-Passing Decoders

Binary message-passing decoders for low-density parity-check (LDPC) codes are studied by using extrinsic information transfer (EXIT) charts. The channel delivers hard or soft decisions and the variable node decoder performs all computations in the L-value domain. A hard decision channel results in the well-know Gallager B algorithm, and increasing the output alphabet from hard decisions to two bits yields a gain of more than 1.0 dB in the required signal to noise ratio when using optimized codes. The code optimization requires adapting the mixing property of EXIT functions to the case of binary message-passing decoders. Finally, it is shown that errors on cycles consisting only of degree two and three variable nodes cannot be corrected and a necessary and sufficient condition for the existence of a cycle-free subgraph is derived.Comment: 8 pages, 6 figures, submitted to the IEEE Transactions on Communication

On receiver design for low density signature OFDM (LDS-OFDM)

Low density signature orthogonal frequency division multiplexing (LDS-OFDM) is an uplink multi-carrier multiple access scheme that uses low density signatures (LDS) for spreading the symbols in the frequency domain. In this paper, we introduce an effective receiver for the LDS-OFDM scheme. We propose a framework to analyze and design this iterative receiver using extrinsic information transfer (EXIT) charts. Furthermore, a turbo multi-user detector/decoder (MUDD) is proposed for the LDS-OFDM receiver. We show how the turbo MUDD is tuned using EXIT charts analysis. By tuning the turbo-style processing, the turbo MUDD can approach the performance of optimum MUDD with a smaller number of inner iterations. Using the suggested design guidelines in this paper, we show that the proposed structure brings about 2.3 dB performance improvement at a bit error rate (BER) equal to 10-5 over conventional LDS-OFDM while keeping the complexity affordable. Simulations for different scenarios also show that the LDS-OFDM outperforms similar well-known multiple access techniques such as multi-carrier code division multiple access (MC-CDMA) and group-orthogonal MC-CDMA

A framework for pathologies of message sequence charts

This is the post-print version of the final paper published in Information Software and Technology. The published article is available from the link below. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. Copyright @ 2012 Elsevier B.V.Context - It is known that a Message Sequence Chart (MSC) specification can contain different types of pathology. However, definitions of different types of pathology and the problems caused by pathologies are unclear, let alone the relationships between them. In this circumstance, it can be problematic for software engineers to accurately predict the possible problems that may exist in implementations of MSC specifications and to trace back to the design problems in MSC specifications from the observed problems of an implementation. Objective - We focus on generating a clearer view on MSC pathologies and building formal relationships between pathologies and the problems that they may cause. Method - By concentrating on the problems caused by pathologies, a categorisation of problems that a distributed system may suffer is first introduced. We investigate the different types of problems and map them to categories of pathologies. Thus, existing concepts related to pathology are refined and necessary concepts in the pathology framework are identified. Finally, we formally prove the relationships between the concepts in the framework. Results - A pathology framework is established as desired based on a restriction that considers problematic scenarios with a single undesirable event. In this framework, we define disjoint categories of both pathologies and the problems caused; the identified types of pathology are successfully mapped to the problems that they may cause. Conclusion - The framework achieved in this paper introduces taxonomies into and clarifies relationships between concepts in research on MSC pathologies. The taxonomies and relationships in the framework can help software engineers to predict problems and verify MSC specifications. The single undesirable event restriction not only enables a categorisation of pathological scenarios, but also has the potential practical benefit that a software engineer can concentrate on key problematic scenarios. This may make it easier to either remove pathologies from an MSC specification MM or test an implementation developed from MM for potential problems resulting from such pathologies

Translating Message Sequence Charts to other Process Languages Using Process Mining

Message Sequence Charts (MSCs) are often used by software analysts when discussing the behavior of a system with different stakeholders. Often such discussions lead to more complete behavioral models in the form of, e.g., Event-driven Process Chains (EPCs), Unified Modeling Language (UML), activity diagrams, Business Process Modeling Notation (BPMN) models, Petri nets, etc. Process mining on the other hand, deals with the problem of constructing complete behavioral models by analyzing event logs of information systems. In contrast to existing process mining techniques, where logs are assumed to only contain implicit information, the approach presented in this paper combines the explicit knowledge captured in individual MSCs and the techniques and tools available in the process mining domain. This combination allows us to discover high-quality process models. To constructively add to the existing work on process mining, our approach has been implemented in the process mining framework ProM (www.processmining.org)

Advanced Positioning and Location based services in 4G Mobile-IP Radio Access Networks

2004 IEEE 15TH IEEE International Symposium on personal, indoor and mobile radio communications (PIMRC 2004), Barcelona, Spain, 5-8 september 2004.This paper proposes an evolved architecture from 3G networks to provide basic and advanced positioning methods for location based services in mobile IPv6-based radio access networks. We start analyzing current status of location-based services (i.e. LBS or LCS) and architectures in 3G networks as well as state-of-the-art research on LBS and mobile Internet. Next we set the requirements the solution should fulfill. We continue proposing the evolved architecture for support of basic and advanced positioning methods, using MIPv6 and HMIPv6 as mobility scenario for the mobile IPv6 based RAN, describing element's functions and changes from current approaches as well as description of the dynamic behavior. We complete the proposal with a bandwidth analysis of the signaling, identifying issues when planning implementation of LCS services in the network