排序方式: 共有15条查询结果,搜索用时 156 毫秒
1.
A soft-decision 8-DPSK modulation format is introducedin a concatenated coding scheme and the performance of the resultingsystem is evaluated over a slow Rayleigh fading HF ionospheric link inthe presence of Additive White Gaussian noise (AWGN). Well-known UngerboeckTCM techniques are used as inner codes and a Reed–Solomon blockcode as outer code. The coded/modulated signal is differentially encodedbefore transmission to combat random phase changes caused by the channel.Soft-decision demodulator's output is used as an input to a modifiedViterbi decoder that calculates the Euclidean distances of the receivedsignal from an 8-PSK constellation adapted to the signal's amplitudevariations. Block interleaving techniques are necessary to randomise longbursts of errors caused by the fading channel. Simulation results showthat significant coding gains are achieved with a minor bandwidth expansionover uncoded, diversity or other coded systems. Finally, theinteresting effects of interleaving on the performance of the proposedsystems are analysed. 相似文献
2.
Nikolaos Dimakis John Soldatos Lazaros Polymenakos Axel Bürkle Uwe Pfirrmann Gerhard Sutschet 《Autonomous Agents and Multi-Agent Systems》2010,21(1):36-68
Multi-agent software architectures have gained in popularity due to their beneficial behavior in designing and implementing
sophisticated applications. However, current approaches in implementing such architectures have led to application-specific,
non-scalable implementations which limit the reusablity and improvement of the whole architecture. Moreover, these attempts
lack features to enhance the user experience, thus slowing the adoption of the resulting services. In this paper we describe
a fully-fledged multi-agent architecture covering a large variety of preferred features including capabilities of ‘plugging’
ubiquitous services, servicing mobile users, interconnecting remote similar architectures and interfacing with advanced software
components such as knowledge bases. This framework exploits a wide-range of context-aware components making it essentially
context-aware, allowing for the development of ubiquitous context-aware human-centric services, which are the focus of our
research. To illustrate the flexibility of this architectural framework, we present four services which were built using this
architectural paradigm by different development teams and elaborate on their overall behavior. 相似文献
3.
Dimakis Nikolaos Soldatos John K. Polymenakos Lazaros Fleury Pascal Curí n Jan Kleindienst Jan 《Pervasive Computing, IEEE》2008,7(4):71-79
When designing context-aware applications for smart spaces, developers must provide interface specifications for perceptual components, which often come from multiple vendors. The authors outline a set of structuring principles that leverage such specifications. IBM has introduced two tools that facilitate the development of smart space applications. Chilix is a middleware library for bridging disaggregated systems, and SitCom is an integrated development environment enabling context modeling, and simulation and lifecycle management of perceptual components. Chilix provides an attractive lightweight alternative to legacy middleware for distributed computing. SitCom supports perceptual component developers, service developers and context-modeling experts across the development life cycle. 相似文献
4.
Dimakis A.G. Gohari A.A. Wainwright M.J. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2009,55(8):3479-3487
We investigate the structure of the polytope underlying the linear programming (LP) decoder introduced by Feldman, Karger, and Wainwright. We first show that for expander codes, every fractional pseudocodeword always has at least a constant fraction of nonintegral bits. We then prove that for expander codes, the active set of any fractional pseudocodeword is smaller by a constant fraction than that of any codeword. We further exploit these geometrical properties to devise an improved decoding algorithm with the same order of complexity as LP decoding that provably performs better. The method is very simple: it first applies ordinary LP decoding, and when it fails, it proceeds by guessing facets of the polytope, and then resolving the linear program on these facets. While the LP decoder succeeds only if the ML codeword has the highest likelihood over all pseudocodewords, we prove that the proposed algorithm, when applied to suitable expander codes, succeeds unless there exists a certain number of pseudocodewords, all adjacent to the ML codeword on the LP decoding polytope, and with higher likelihood than the ML codeword. We then describe an extended algorithm, still with polynomial complexity, that succeeds as long as there are at most polynomially many pseudocodewords above the ML codeword. 相似文献
5.
In this paper we present a low complexity algorithm based on the bubble search sorting method that can be used to generate Turbo code interleavers that fulfill several criteria like spreading (s-randomness), code matched criteria and even the odd–even property for Turbo Trellis Coded Modulation. Simulation results show that for \(s < \sqrt{N/2}\) the algorithm is extremely efficient for short to medium interleaver lengths. 相似文献
6.
E. Dimakis E. Iliopoulos M. Kayambaki K. Tsagaraki A. Kostopoulos G. Konstantinidis A. Georgakilas 《Journal of Electronic Materials》2007,36(4):373-378
The molecular beam epitaxy of In-face InN (0001) epilayers with optimized surface morphology, structural quality, and electrical
properties was investigated. Namely, compact InN epilayers with atomically flat surfaces, grown in a step-flow mode, were
obtained using stoichiometric fluxes of In and N and substrate temperatures in the range from 400°C to 435°C. Typical values
for the electron concentration and the Hall mobility at 300 K were 4.3 × 1018 cm−3 and 1210 cm2/Vs, respectively. The growth mode of InN during the very first stage of the nucleation was investigated analytically, and
it was found that the growth proceeds through nucleation and fast coalescence of two-dimensional (2-D)–like InN islands. The
preceding conditions were used to grow an InN/GaN quantum well (QW) heterostructure, which exhibited well-defined interfaces.
Schottky contacts were successfully fabricated using a 15-nm GaN barrier enhancement cap layer. Capacitance-voltage measurements
revealed the confinement of electrons within the InN QW and demonstrated the capability to modulate the electron density within
an InN channel. The sheet concentration of the confined electrons (1.5 × 1013 cm−2) is similar to the calculated sheet polarization charge concentration (1.3 × 1013 cm−2) at the InN/GaN interface. However, electrons may also originate from ionized donors with a density of 8 × 1018 cm−3 within the InN layer. 相似文献
7.
In this paper, we propose a nonlinear stochastic model for time-varying resonances where the instantaneous phase (and frequency) of a sinusoidal oscillation is allowed to vary proportionally to an /spl alpha/-stable self-similar stochastic processes. The main motivation of our work stems from previous experimental and theoretical evidence that speech resonances in fricative sounds can be modeled phenomenologically as AM-FM signals with randomly varying instantaneous frequencies and that several signal classes related to turbulent phenomena are self-similar 1/f processes. Our general approach is to model the instantaneous phase of an AM-FM resonance as a self-similar /spl alpha/-stable process. As a special case, this random phase model includes the class of random fractal signals known as fractional Brownian motion. We theoretically explore this random modulation model and analytically derive its autocorrelation and power spectrum. We also propose an algorithm to fit this model to arbitrary resonances with random phase modulation. Further, we apply the above ideas to real speech data and demonstrate that this model is suitable for resonances of fricative sounds. 相似文献
8.
Courcoubetis C.A. Dimakis A. Stamoulis G.D. 《Networking, IEEE/ACM Transactions on》2002,10(2):217-231
For a multiplexer fed by a large number of sources, we derive conditions under which a given subset of the sources can be substituted for a single source while preserving the buffer overflow probability and the dominant timescales of buffer overflows. This notion of traffic equivalence is stronger than simple effective bandwidth equality and depends on the multiplexing context. We propose several applications of the above traffic substitution conditions. First, we show that fractional Brownian motion as a single source substitute can effectively model a large number of multiplexed sources using information obtained purely from traffic traces; this has direct application to simple but accurate traffic generation. Second, we focus on dynamic (i.e., on-line) estimation of available capacity and buffer overflow probability. This requires the solution of a double optimization problem expressed in terms of functions whose values are obtained from time averages of the traffic traces over a large range of timescales. We show how to solve this problem on-line by reducing it to the calculation of a fixed-point equation that can be solved iteratively by combining traffic substitution using fractional Brownian motion with dynamic measurements of the actual traffic. We have validated this approach by extensive experimentation with large numbers of real traffic sources that are fed to a high bandwidth link, and comparing our on-line estimation of available capacity and the resulting dynamic call admission control with other existing approaches. The superior accuracy of our approach also suggests that taking the buffer size into account, as does our on-line algorithm, may be vital for achieving approximations of practical interest 相似文献
9.
I. K. Nissopoulos C. E. Dimakis S. S. Kouris 《International Journal of Wireless Information Networks》2000,7(3):167-175
The performance improvement of conventional TCM schemes over frequency nonselective slow Rayleigh fading channels, caused by a slight modification on the metric calculation of the Viterbi decoder, is investigated in this paper. A statistical analysis of the Euclidean distances at the input of the Viterbi decoder is carried out to explain this improvement. Furthermore, the performance improvement of two concatenated coding systems incorporating the above modification, under the same conditions, is studied. The performance of the TCM and the concatenated systems is evaluated by Monte Carlo simulation. The results show that the examined systems, using the proposed modification on the metric calculation, achieve BER performances comparable to other codes and systems, especially designed for fading channels. 相似文献
10.
Daskalakis C. Dimakis A.G. Karp R.M. Wainwright M.J. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2008,54(8):3565-3578
We initiate the probabilistic analysis of linear programming (LP) decoding of low-density parity-check (LDPC) codes. Specifically, we show that for a random LDPC code ensemble, the linear programming decoder of Feldman succeeds in correcting a constant fraction of errors with high probability. The fraction of correctable errors guaranteed by our analysis surpasses previous nonasymptotic results for LDPC codes, and in particular, exceeds the best previous finite-length result on LP decoding by a factor greater than ten. This improvement stems in part from our analysis of probabilistic bit-flipping channels, as opposed to adversarial channels. At the core of our analysis is a novel combinatorial characterization of LP decoding success, based on the notion of a flow on the Tanner graph of the code. An interesting by-product of our analysis is to establish the existence of ldquoprobabilistic expansionrdquo in random bipartite graphs, in which one requires only that almost every (as opposed to every) set of a certain size expands, for sets much larger than in the classical worst case setting. 相似文献