Vassilis Dalakas

A Novel Efficient Cluster-Based MLSE Equalizer for Satellite Communication Channels with

In satellites, nonlinear amplifiers used near saturation severely distort the transmitted signal and cause difficulties in its reception. Nevertheless, the nonlinearities introduced by memoryless bandpass amplifiers preserve the symmetries of the-ary quadrature amplitude modulation (-QAM) constellation. In this paper, a cluster-based sequence equalizer (CBSE) that takes advantage of these symmetries is presented. The proposed equalizer exhibits enhanced performance compared to other techniques, including the conventional linear transversal equalizer, Volterra equalizers, and RBF network equalizers. Moreover, this gain in performance is obtained at a substantially lower computational cost.

Model-based system engineering using SysML: Deriving executable simulation models with QVT

Systems Modeling Language (SysML) is used to define hierarchical system models in model-based engineering (MBE). Although SysML may effectively serve the description of complex systems, it can not effectively support all model-based engineering activities. For example, system validation is usually performed via simulation. In this case, SysML system models should be transformed to domain-specific models, e.g. executable simulation models suitable for specific simulation tools. This paper identifies the key issues for efficient SysML model simulation, utilizing Model Driven Architecture (MDA) concepts. The generation of executable simulation code from SysML system models is considered as a model transformation from the SysML meta-model to the simulation meta-model. Since SysML meta-model is defined using Meta-Object Facility (MOF), the definition of MOF simulation meta-models and the utilization of the Query/View/Transformation (QVT) language for model transformations are analytically discussed. The presented approach is not restricted in a specific simulation framework or type. However, in this paper, the experience obtained from a case study on discrete event simulation is evaluated and the conditions that favor the selection of specific simulation frameworks are identified.

A SysML Profile for Classical DEVS Simulators

Discrete event simulation specification (DEVS) is a formalism facilitating hierarchical and modular description of the models executed using DEVS simulators. Lack of standardized, easy-to-use interface enabling simulation practitioners to define their models is an important drawback, since in most cases DEVS models are defined as C++ or Java programs based on existing simulator-specific DEVS libraries. Standard MDA concepts can be applied for the construction of DEVS models executed in different programming environments. DEVS models can be defined using DEVSML, a platform-independent, XML-based format. SysML is proposed as a standardized, graphical representation language of DEVS models stored in DEVSML, consequently transformed into executable code for existing DEVS Simulators, as DEVSJava and DEVSim++. The first step toward this endeavor, is the formal definition of the DEVS SysML profile proposed in this paper.

A Comparative Study Between SC-FDMA and OFDMA Schemes for Satellite Uplinks

This paper presents a detailed comparative study of two single-carrier frequency-division multiple access (SC-FDMA) schemes, namely localized FDMA scheme (LFDMA) and interleaved FDMA scheme (IFDMA), versus orthogonal FDMA scheme (OFDMA), for a satellite uplink. The air-interface of the latter is based on the digital video broadcasting (DVB) family of standards. Considering two state-of-the-art high power amplifiers (HPAs), operating in the K- and S-bands, the performance of synchronous and asynchronous LFDMA, IFDMA and OFDMA is evaluated in a multi-user environment. Systematic comparison results show that although for synchronous reception IFDMA outperforms the other two schemes, for asynchronous reception it is the most sensitive to degradation caused by inter-block interference (IBI). Furthermore, due to its relatively large envelope fluctuations, OFDMA is the most sensitive scheme to non-linear distortion. Although for synchronous reception LFDMA shows only slightly inferior performance as compared to IFDMA, it outperforms the other two schemes for the asynchronous reception considered, especially for increased IBI distortion.

An integrated framework for automated simulation of SysML models using DEVS

System models are constructed to design, study, and understand complex systems. According to the systems modeling language (SysML) that is a standard for model-based system engineering, all engineering activities should be performed using a common model. To validate complex system models defined in SysML, simulation is usually employed. There are numerous efforts to simulate SysML models using different simulation methods and tools. However, the efficient support of automated generation of executable simulation code is still an issue tangled by the research community. This paper introduces DEVSys, an integrated framework for utilizing existing SysML models and automatically producing executable discrete event simulation code, according to model driven architecture (MDA) concepts. Although this approach is not simulation-specific, discrete event system specification (DEVS) was employed, due to the similarities between SysML and DEVS, mainly in system structure description, and the mature, yet ongoing research on expressing executable DEVS models in a simulator-neutral manner. DEVSys framework elements include (a) a SysML profile for DEVS, enabling integration of simulation capabilities into SysML models, (b) a meta-model for DEVS, allowing the utilization of MDA concepts and tools, (c) a transformation of SysML models to DEVS models, using a standard model transformation language as query/view/transform (QVT), and (d) the generation of DEVS executable code for a DEVS simulation environment with an extensible markup language (XML) interface. The definition and implementation of DEVSys elements, as well as the process for its application are demonstrated and discussed, with the aid of a simple working example.

A novel efficient cluster-based MLSE equalizer for satellite communication channels with M-QAM signaling

In satellites, nonlinear amplifiers used near saturation severely distort the transmitted signal and cause difficulties in its reception. Nevertheless, the nonlinearities introduced by memoryless bandpass amplifiers preserve the symmetries of the-ary quadrature amplitude modulation (-QAM) constellation. In this paper, a cluster-based sequence equalizer (CBSE) that takes advantage of these symmetries is presented. The proposed equalizer exhibits enhanced performance compared to other techniques, including the conventional linear transversal equalizer, Volterra equalizers, and RBF network equalizers. Moreover, this gain in performance is obtained at a substantially lower computational cost.

Integrating simulation capabilities into SysML for enterprise information system design

Performance requirements play a significant role in the design of large-scale systems, such as enterprise information systems. Systems Modeling Language (SysML), proposed by Object Management Group (OMG) for system engineering, provides for requirements specification, though a verification method for quantitative requirements as performance ones is lacking. In the information systems domain, performance requirements are usually verified using simulation. To integrate simulation capabilities into SysML the authors have proposed the concept of the Evaluation View, a discrete diagram to specify enterprise information system architecture under evaluation and the conditions under which performance requirements should be verified. A corresponding SysML profile, called Enterprise Information System (EIS) profile, has been defined. In this paper we present an approach that provides (a) the automated transformation of SysML-based EIS models defined in the Evaluation View to executable simulation code for Discrete Event System Specification (DEVS) simulation environments and (b) the incorporation of simulation results into the original EIS SysML models to enable the verification of corresponding performance requirements.

Simulating SysML transportation models

Model-based Systems Engineering (MBSE) promises efficient and effective systems development, by providing integrated system model views and streamlining the transition between different development activities. For instance, system testing with simulation should be provided as a simple facility for the performance evaluation of complex systems. Transportation systems are complex and their behavior is determined by dynamic factors. However, research efforts define simulation models for transportation systems, using custom or simulation-specific notation. Additionally, model-based approaches for transportation systems emphasize peripheral issues, such as safety conditions and environmental impact. In this work, a framework that enables seamless performance evaluation of Railway Transportation System (RTS) models via simulation is proposed. The de facto standard for MBSE modeling, Systems Modeling Language (SysML), is selected as the modeling infrastructure, while other standards, like Query/View/Transformation (QVT), are used for the generation of executable simulation models. The latter can be simulated in Discrete Event System Specification (DEVS) simulators and the simulation results are returned in the RTS SysML model. Additionally, the application of the framework in the public RTS of Athens and the obtained simulation results are presented.

Simulating simulation-agnostic SysML models for enterprise information systems via DEVS

Abstract Systems Modeling Language (SysML) is used as the modeling infrastructure in systems engineering, especially for complex systems design, independently of the system domain. Simulation is a common method to perform system model verification, during the systems development process. However, simulation code generation and execution is not integrated within the system design activity, as it is facilitated by SysML. It is either conducted as an external activity, after system design, or it affects the system design environment and practices, according to specific simulators requirements. This paper presents how existing, simulation-agnostic SysML models from the domain of Enterprise Information System (EISs), can be transformed to executable simulation code and in addition how the simulation results can be incorporated into the source SysML model through the exploitation of Model Driven Architecture (MDA) principles and techniques. To this end, several tools and technologies are utilized, while the verification process is triggered and finalized via the system modeling environment. Adoption of MDA provides a solid, high-level infrastructure and tool availability to the proposed approach.

BICMC and TD Comparative Performance Study of 16-APSK Signal Variants for DVB-S2 Systems

A comparative performance study, in terms of Bit Interleaved Coded Modulation Capacity (BICMC) and Total Degradation (TD), between Amplitude Phase Shift Keying (APSK) signal variants for the 2nd generation Digital Video Broadcasting via Satellite (DVB-S2) system is presented. Motivated by the observation that the presence of more signal points in the outer ring of 16-APSK signals appears to improve the performance in a non-linear (NL) channel, we investigate the performance of 2-14 and 3-13 APSK signals with two novel bit-to-symbol mappings as alternatives to the 4-12 APSK, which is currently proposed as the modulation scheme for the DVB-S2 standard. Performance evaluation results are obtained for an air-interface based on this standard, which includes a Non-Linear (NL) High Power Amplifier (HPA), predistortion, raised cosine Nyquist filters and Low-Density Parity-Check (LDPC) codes. Performance results have shown that the proposed 3-13 APSK signal not only achieves the best BICMC performance with higher gains occurring at high signal-to-noise ratios, but also outperforms, in terms of TD performance, all the other considered 16-ary APSK signal variants.