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Search Results: 1 - 10 of 1370 matches for " Thorsten HERFET "
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Beacon-driven Leader Based Protocol over a GE Channel for MAC Layer Multicast Error Control  [PDF]
Zhao LI, Thorsten HERFET
Int'l J. of Communications, Network and System Sciences (IJCNS) , 2008, DOI: 10.4236/ijcns.2008.12019
Abstract: In wireless networks current standard MAC layer protocols don’t provide any error correction scheme for broadcast/multicast. In this paper, we enhance a Leader Based Protocol (LBP) and propose a Beacon-driven Leader Based Protocol (BLBP) for the MAC layer multicast error control. To guarantee a very low Packet Loss Ratio (PLR) under strict delay constraints for video multicast over a Gilbert-Elliott (GE) channel, we analyze BLBP and compare it with LBP and different application layer multicast error control schemes via simulation experiments. Both the theoretical analysis and simulation results show that BLBP can correct nearly all the errors for all receivers in the MAC layer and is more efficient than LBP. BLBP is also more efficient than the application layer Automatic Repeat request (ARQ) scheme and the total multicast delay is much shorter. BLBP is very good for real-time multicast applications with strict delay constraints.
A Novel Adaptive Hybrid Error Correction Scheme for Wireless DVB Services  [PDF]
Guoping TAN, Thorsten HERFET
Int'l J. of Communications, Network and System Sciences (IJCNS) , 2008, DOI: 10.4236/ijcns.2008.12023
Abstract: Real-time applications usually not only have a certain Packet Loss Ratio (PLR) requirement but also can have strict delay constraints. In the past, we proposed a Hybrid Error Correction (HEC) scheme with Packet Repetition (PR) technique for guaranteeing a certain PLR requirement under strict delay constraints. Unfortunately, the HEC-PR scheme can only work efficiently in multicast scenarios with small group size and small link PLR. Our further studies show that better performance can be obtained by combining the HEC-PR scheme with other traditional HEC schemes such as Type I HARQ and Type II HARQ techniques. Based on this idea, in this paper, a novel Adaptive HEC (AHEC) scheme combining the HEC-PR scheme with Type I and Type II HARQ techniques is proposed to satisfy a certain PLR requirement for delay bounded multicast services. Furthermore, the performance of the AHEC scheme is optimized by choosing the scheme with the least needed redundancy information automatically among the three HEC schemes. Finally, by applying the AHEC scheme in a typical wireless DVB scenario, we analyze the performances of the AHEC scheme and compare it with the HEC-PR scheme and an Adaptive Forward Error Correction (AFEC) scheme. The results show that the proposed AHEC scheme outperforms both the AFEC scheme and the HEC-PR scheme.
Watermarks and Text Transformations in Visual Document Authentication
Igor Fischer,Thorsten Herfet
Journal of Computers , 2007, DOI: 10.4304/jcp.2.5.44-53
Abstract: Integrity of digital documents is a very important issue if they are to be legally binding. Common solutions, like digital signatures or message authentication codes, are based on cryptography and require computers or similar hardware to be produced. They can be trusted only as long as the employed hardware can be trusted. We present a complementary scheme for document authentication in untrusted environment. The scheme combines visual authentication, human- recognizable watermarks, and image transformations, and is suitable also for larger text document. Because it relies on visual cryptography, it requires practically no computational power on the receiver side. To prevent potential attackers from obtaining signatures without author’s knowledge, we propose using a simple challenge-response protocol.
Implant-related MRI artifacts of determined interbody test spacers: artifact calculations due to implant parameters in a porcine spine model  [PDF]
Thorsten Ernstberger
Health (Health) , 2009, DOI: 10.4236/health.2009.13035
Abstract: Aim: Intervertebral spacers for anterior spine fusion are made of different materials, which can affect the post-fusion MRI scans. Suscep- tibility artifacts specially for implants made of titanium alloys can decrease the image quality. This study focused on the influence of deter-mined implant parameters like shape and implant volume in MRI artifacting independent from se-lected MRI-sequences. Methods: In this study the post-implantation MRI scans of determined cuboids and cylinders were evaluated. All in-terbody test implants were made of titanium alloys. MRI scans were carried out by using T1 TSE sequences. The total artifact volume (TAV) of all examined implants were calculated for sta-tistical t-test correlation and implant volume (IV)/TAV-relation. Results: Considering all ex-amined test implants with an increasing implant size the TAV became significant larger (p<0,001) with simultaneous reduction of the respective IV/TAV-relation. According to an intergroup TAV- correlation for cylinders and cuboids with an equivalent implant volume the cylindric test im-plants demonstrated a significant smaller arti-fact range (p<0,05). Conclusions: Based on these results the MRI artifacts of larger test im-plants were more limited to the to the implant’s direct surroundings. In this connection for im- plants with identical material volumes a cylin- dric shape demonstrated more advantages con- sidering MRI artifacting than cubic forms.
Review on the Functional Determinants and Durability of Shape Memory Polymers
Thorsten Pretsch
Polymers , 2010, DOI: 10.3390/polym2030120
Abstract: Shape memory polymers (SMP) belong to the class of stimuli-responsive materials and have generated significant research interest. Their capability to retain an imposed, temporary shape and to recover the initial, permanent shape upon exposure to an external stimulus depends on the “functional determinants”, which in simplistic terms, can be divided into structural/morphological and processing/environmental factors. The primary aim of the first part of this review is to reflect the knowledge about these fundamental relationships. In a next step, recent advances in shape memory polymer composites are summarized. In contrast to earlier reviews, studies on the impairment of shape memory properties through various factors, such as aging, compression and hibernation, lubricants, UV light and thermo-mechanical cycling, are extensively reviewed. Apart from summarizing the state-of-the-art in SMP research, recent progress is commented.
Error Estimates of a Computational Method for Generalised Connecting Orbits
Thorsten Pampel
The Open Numerical Methods Journal , 2009, DOI: 10.2174/1876389801002010006]
Abstract: We provide error estimates for an approximation method to compute simultaneously solutions of two dynamical systems each with given asymptotic behaviour and both coupled only by conditions on initial values. The method applies to compute connecting orbits point to point, point to periodic and periodic to periodic as in the literature and in numerical applications. Since our set up is more general, we call solutions of our systems generalised connecting orbits and provide further applications like Skiba points in economic models or solutions with a discontinuity. By specifying the asymptotic rates our method also applies to the computation of solutions converging in a strongly stable manifold. The numerical analysis shows that the error decays exponentially with the length of the approximation intervals even in the strongly stable case and for periodic solutions. For orbits connecting hyperbolic equilibria this is in agreement with known results in the literature. In our method we select appropriate asymptotic boundary conditions which depend typically on parameters. In order to solve these types of boundary value problems we set up an iterative procedure which is called boundary corrector method.
The Physics probed by the P_T Dependence of the Nuclear Suppression Factor
Renk, Thorsten
High Energy Physics - Phenomenology , 2013,
Abstract: The nuclear suppression factor R_AA of single inclusive hadrons measured in ultrarelativistic heavy-ion collisions was the first observable to study jet quenching, i.e. the final state interaction of hard parton showers with the surrounding bulk matter. While its transverse momentum (P_T) dependence of R_AA observed at RHIC was weak and hence never decisive in constraining models, there is now a strong and non-trivial P_T dependence observed at the LHC. This has been a challenge for several models which worked well at RHIC kinematics. However, in more general terms it is also of importance to understand what physical properties of the hard process and the parton-medium interaction are reflected in R_AA(P_T). The results of the work presented here suggest that the two main effects underlying the P_T dependence are the Quantum-Chromodynamics scale evolution of the fragmentation function and the limited distance (set by the typical medium length scale) for which a shower evolves in the medium.
Soft-Collinear Effective Theory: Recent Results and Applications
Feldmann, Thorsten
High Energy Physics - Phenomenology , 2008,
Abstract: Soft-collinear effective theory (SCET) has become a standard tool to study the factorization of short- and long-distance effects in processes involving low-energetic (soft) particles and high-energetic/low-virtuality (collinear) modes. In this contribution I give a brief overview on recent results for inclusive and exclusive B decays and on applications in collider physics.
Photon Emission from a Medium-Modified Shower Evolution
Renk, Thorsten
High Energy Physics - Phenomenology , 2013,
Abstract: Photons from the interaction of a highly energetic jet with a thermal medium are an important contribution to the total photon yield measured in ultrarelativistic heavy-ion collisions and also an important probe to study the medium degrees of freedom. Previously this contribution has often been computed in the context of a leading parton energy loss approximation. In this work, jet-medium interaction photons are instead estimated using a medium-modified shower evolution model, where the energy degradation due to vacuum radiation prior to medium formation, the virtuality evolution of intermediate states and the photon emission from subleading shower partons is taken into account consistently. The results indicate that the leading parton energy loss approximation does not appear to work well for photon emission from jet-medium interaction.
The Phenomenology of Elastic Energy Loss
Renk, Thorsten
High Energy Physics - Phenomenology , 2007, DOI: 10.1103/PhysRevC.76.064905
Abstract: The unexpectedly strong suppression of high p_T heavy-quarks in heavy-ion collisions has given rise to the idea that partons propagating through a medium in addition to energy loss by induced radiation also undergo substantial energy loss due to elastic collisions. However, the precise magnitude of this elastic energy loss component is highly controversial. While it is for a parton inside a medium surprisingly difficult to define the difference between elastic and radiative processes rigorously, the main phenomenological difference is in the dependence of energy loss on in-medium pathlength: in a constant medium radiative energy loss is expected to grow quadratically with pathlength, elastic energy loss linearly. In this paper, we investigate a class of energy loss models with such a linear pathlength dependence and demonstrate that they are incompatible with measured data on hard hadronic back-to-back correlations where a substantial variation of pathlength is probed. This indicates that any elastic energy loss component has to be small.
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