Página 1 dos resultados de 5223 itens digitais encontrados em 0.025 segundos

Ferramenta de simulação computacional de canal de propagação em ambiente celular baseado em modelos geométricos estatísticos.; Communication channel simulation tool based on geometrical and statistical model of macro cell environments.

Castilho, Sergio Duque
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Dissertação de Mestrado Formato: application/pdf
Publicado em 29/09/2006 PT
Relevância na Pesquisa
45.96%
Este trabalho apresenta uma ferramenta de simulação computacional de canal de propagação para ambiente macro-celular baseada em modelos geométricos e no modelo estatístico apresentado no relatório COST 259 DCM. Para a implementação desta ferramenta é realizada, inicialmente, uma abordagem dos principais modelos de predição de perda por propagação, utilizados atualmente, assim como, um estudo dos modelos geométricos que fornecem as informações de distribuição de potência temporal e angular para diferentes tipos de distribuições estatísticas de espalhadores. A modelagem geométrica utiliza grupos independentes no qual os espalhadores são distribuídos com uma densidade Gaussiana. A utilização desta distribuição Gaussiana leva a distribuições de atraso e ângulo de chegada mais próximas dos resultados de medições do que o usando distribuição uniforme. A base geométrica define o conceito direcional e temporal. A base estatística define o número de grupo de espalhadores adicionais e suas localizações, quando estes existiram. Efeitos como: direção e potência de chegada de cada grupo de espalhadores, a presença ou não de visada direta entre transmissor e receptor a medida que a estação móvel percorre uma célula e a variação da polarização cruzada foram implementados nesse simulador. Desta forma...

IEEE 802.11p : análise de desempenho de redes sem fio veiculares

Basso, Diego Osmarin
Fonte: Universidade Federal do Rio Grande do Sul Publicador: Universidade Federal do Rio Grande do Sul
Tipo: Trabalho de Conclusão de Curso Formato: application/pdf
POR
Relevância na Pesquisa
45.91%
A norma de comunicação sem fio para ambientes veiculares IEEE 802.11p foi desenvolvida baseada na norma IEEE 802.11a, projetada para ambientes indoor. Porém, como os ambientes veiculares são mais complexos e degradam mais a qualidade do sinal, a quantidade de bits por pacote que pode ser transmitida se torna muito menor para a norma IEEE 802.11p. Este projeto realiza um estudo da quantidade de bits por pacote que se consegue transmitir em ambientes veiculares, para uma dada taxa de perdas, utilizando diferentes técnicas de estimativa do canal. Primeiramente este trabalho apresenta a descrição dos transmissor e receptor utilizados pelas normas IEEE 802.11a e IEEE 802.11p. Em seguida são mostrados modelos de canais veiculares e suas peculiaridades são discutidas. Na sequência são abordadas técnicas para a estimativa dinâmica do canal, visando melhorar a qualidade da transmissão. Finalmente o simulador utilizado é descrito e os resultados gerados são discutidos.; The wireless communications for vehicular networks IEEE 802.11p standard was developed based on the indoor IEEE 802.11a standard. However, vehicular environments are more complex and affect more the transmitted signal, implying in a lower quantity of bits per packet. This work studies the quantity of bit per packet which is transmissible for a given a packet error rate and for some channel estimates schemes. First...

MIMO-OFDM channel estimation: a pilot sequence design for time-domain processing

Ribeiro, C.; Gameiro, A.
Fonte: Universidade de Aveiro Publicador: Universidade de Aveiro
Tipo: Conferência ou Objeto de Conferência
ENG
Relevância na Pesquisa
45.83%
A pilot sequence design for MIMO-OFDM systems is investigated where all transmit antennas share the same sub-carriers to convey pilot symbols. The pilot sequences are endowed with phase-shifting properties, granting the receiver the possibility of extracting the impulse responses for all channels directly from the antennas’ time-domain received signals, without co-channel interference. A linear processing of the timedomain samples is all that is needed to get the channel impulse responses, resulting in an algorithm with very low computational load. The feasibility of the investigated method is substantiated by system simulation using indoor and outdoor broadband wireless channel models.

Low-complexity channel estimation for beyond 3G systems

Marques, P.; Gameiro, A.
Fonte: IEEE Publicador: IEEE
Tipo: Conferência ou Objeto de Conferência
ENG
Relevância na Pesquisa
45.85%
A robust minimum mean squared error (MMSE) channel estimation algorithm that does not rely on a priori knowledge of the channel statistics gives good results with MC-CDMA systems. The robust MMSE estimator takes advantages of the correlation between all Np pilot-subcarriers and requires one Np×Np matrix inversion for each pilot symbol of the burst structure. This complexity can be large depending on the number of pilot subcarriers in the system. The paper presents and analyses two low-complexity suboptimal approximations of the robust MMSE channel estimator. A complexity versus performance degradation comparison is done. The performance is presented in terms of mean square error for a 1024 tone MC-CDMA system over ETSI BRAN mobile channel models.

Analysis and evaluation of channel models: simulations of alamethicin.

Tieleman, D Peter; Hess, Berk; Sansom, Mark S P
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /11/2002 EN
Relevância na Pesquisa
45.93%
Alamethicin is an antimicrobial peptide that forms stable channels with well-defined conductance levels. We have used extended molecular dynamics simulations of alamethicin bundles consisting of 4, 5, 6, 7, and 8 helices in a palmitoyl-oleolyl-phosphatidylcholine bilayer to evaluate and analyze channel models and to link the models to the experimentally measured conductance levels. Our results suggest that four helices do not form a stable water-filled channel and might not even form a stable intermediate. The lowest measurable conductance level is likely to correspond to the pentamer. At higher aggregation numbers the bundles become less symmetrical. Water properties inside the different-sized bundles are similar. The hexamer is the most stable model with a stability comparable with simulations based on crystal structures. The simulation was extended from 4 to 20 ns or several times the mean passage time of an ion. Essential dynamics analyses were used to test the hypothesis that correlated motions of the helical bundles account for high-frequency noise observed in open channel measurements. In a 20-ns simulation of a hexameric alamethicin bundle, the main motions are those of individual helices, not of the bundle as a whole. A detailed comparison of simulations using different methods to treat long-range electrostatic interactions (a twin range cutoff...

Real Time Characterisation of the Mobile Multipath Channel

Teal, Paul D
Fonte: Universidade Nacional da Austrália Publicador: Universidade Nacional da Austrália
Tipo: Thesis (PhD); Doctor of Philosophy (PhD)
EN
Relevância na Pesquisa
46.03%
In this thesis a new approach for characterisation of digital mobile radio channels is investigated. The new approach is based on recognition of the fact that while the fading which is characteristic of the mobile radio channel is very rapid, the processes underlying this fading may vary much more slowly. The comparative stability of these underlying processes has not been exploited in system designs to date. ¶ Channel models are proposed which take account of the stability of the channel. Estimators for the parameters of the models are proposed, and their performance is analysed theoretically and by simulation and measurement. ¶ Bounds are derived for the extent to which the mobile channel can be predicted, and the critical factors which define these bounds are identified. ¶ Two main applications arise for these channel models. The first is the possibility of prediction of the overall system performance. This may be used to avoid channel fading (for instance by change of frequency), or compensate for it (by change of the signal rate or by power control). The second application is in channel equalisation. An equaliser based on a model which has parameters varying only very slowly can offer improved performance especially in the case of channels which appear to be varying so rapidly that the convergence rate of an equaliser based on the conventional model is not adequate. ¶ The first of these applications is explored...

Recovery of neuronal channel densities from calcium fluorescence

Raol, Jay
Fonte: Universidade Rice Publicador: Universidade Rice
ENG
Relevância na Pesquisa
45.9%
Neurons have the ability to dynamically adjust their own membrane channel densities to modulate the strength of communication with other neurons. This process is integral to such neuronal functions as spatial recognition and memory but has been difficult to measure experimentally. Historically, neuroscientists have used changes in voltage to infer changes in neuronal channel densities. However, voltage is difficult to measure away from the soma. Many important functions in the neuron, like synaptic integration, take place in the dendritic tree where traditional voltage measurements can not be taken. To interrogate the neuron in the dendrites, experimentalists have come to rely on calcium fluorescence based microscopy to infer qualitative information about voltage changes in the dendrites. In these experiments, intracellular calcium changes due to voltage depolarizations are recorded at spatially distributed sites on the dendrites through the binding of calcium to a fluorescent buffer. The recovery of channel densities can be posed as a parameter identification problem in a coupled nonlinear partial differential equation that relates the responses of calcium, the fluorescent buffer and voltage to neuronal stimulation. We convert temporally and spatially distributed fluorescence data into quantitative measurements of voltage sensitive channel densities by inverting slow time-scaled calcium data into fast time-scaled voltage data. Our approach is to solve four interrelated inverse problems corresponding to three different proposed experiments to go from calcium fluorescence to channel densities. In the first experiment...

Uso de técnicas de Network Coding y Multipath para la mejora de las comunicaciones sobre redes malladas inalámbricas

Gómez Fernández, David
Fonte: Universidade de Cantabria Publicador: Universidade de Cantabria
Tipo: Tese de Doutorado
SPA
Relevância na Pesquisa
45.85%
RESUMEN: Partiendo de los problemas que surgen de la combinación de redes inalámbricas y TCP, el objetivo principal de esta Tesis se sitúa en el estudio de diversas técnicas que buscan mejorar el rendimiento de TCP sobre entornos inalámbricos, utilizando dos aproximaciones diferentes: por un lado, se estudiará el comportamiento del protocolo MPTCP, que nace como una versión evolucionada de TCP, que posibilita la transferencia de información a través de varios subflujos de manera simultánea. Aprovechando el hecho de que los dispositivos portátiles incluyen de manera habitual múltiples interfaces, permitiéndoles la conexión a través de diferentes tecnologías, aparece la posibilidad de dividir el tráfico en varios subflujos, lo que permitiría mejorar las prestaciones del TCP tradicional, tanto en su rendimiento como en su robustez. Por otra parte, se utilizará una nueva filosofía en la operación y funcionalidad de los nodos intermedios, los cuales no sólo redirigirán la información sin más, sino que además tendrán la capacidad de procesarla y combinarla, en función de una serie de criterios y restricciones. A estas técnicas se las conoce en la literatura como NC. En concreto, se estudiarán dos soluciones de características opuestas: en primer lugar se combinarán los paquetes pertenecientes a diferentes flujos (NC inter-flujo); por otro lado...

Wireless channel models for the DTV frequency spectrum; Modelos de canal wireless en el espectro de la televisión digital

Hiniesto Bazán, María
Fonte: Universidade Carlos III de Madrid Publicador: Universidade Carlos III de Madrid
Tipo: info:eu-repo/semantics/bachelorThesis; info:eu-repo/semantics/masterThesis Formato: application/pdf; application/pdf
ENG
Relevância na Pesquisa
45.95%
The present project is a study about the parameters and methods associated with the spatial channel modelling that are common to the needs of the 3GPP and 3GPP2 organizations. We will work on physical parameters such as power delay profiles, angles spread, dependencies between parameters and a system evaluating methodology as the same way the IEEE 802.20 Working Group has done it before, but with an important difference; we’ll work with a different frequency in order to study the characteristics of the wireless channel models at low frequencies. We’ll study the behaviour for the DTV frequency band, seeing the benefits of allocating portions of the spectrum between 470 MHz and 862 MHz for broadband wireless services. Link level simulation alone will not be used for algorithm comparision because they reflect only one snapshot of the channel behaviour. This is one of the reasons link level simulations will be used only for calibration (the comparision of performance results from different implementations of a given algorithm) and we’ll also study the system level simulation for the final algorithm. In channel modelling we have to consider also different scenarios as a basic factor, because different test environments have different channel parameters. Urban and rural areas will not have the same channel characteristics; neither will tow different places within a building as we’ll see.

Statistical Properties of a Parametric Channel Model for Multiple Antenna Systems

Durrani, Salman; Bialkowski, Marek E; Latif, Saba
Fonte: Institute of Electrical and Electronics Engineers (IEEE Inc) Publicador: Institute of Electrical and Electronics Engineers (IEEE Inc)
Tipo: Conference paper
Relevância na Pesquisa
45.8%
Parametric channel models for Multiple Input Multiple Output (MIMO) systems have received much attention in recent years. This paper investigates the statistical properties of a parametric channel model for MIMO systems in an urban macro-cell environment.

An extended one-ring MIMO channel model

Zhang, Min; Smith, Peter; Shafi, M.
Fonte: Institute of Electrical and Electronics Engineers (IEEE Inc) Publicador: Institute of Electrical and Electronics Engineers (IEEE Inc)
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.96%
In this paper we develop a Multiple Input Multiple Output (MIMO) channel model and derive its spatial and temporal correlation properties. We present a generalized methodology to derive the spatial correlation when the angles of arrival (AoA) and angles o

Array independent MIMO channel models with analytical characteristics

Yao, Yuan; Zheng, Jianfeng; Feng, Zhenghe
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 21/04/2011
Relevância na Pesquisa
46.07%
The conventional analytical channel models for multiple-input multiple-output (MIMO) wireless radio channels are array dependent. In this paper, we present several array independent MIMO channel models that inherit the essence of analytical models. The key idea is to decompose the physical scattering channel into two parts using the manifold decomposition technique: one is the wavefield independent sampling matrices depending on the antenna arrays only; the other is the array independent physical channel that can be individually modeled in an analytical manner. Based on the framework, we firstly extend the conventional virtual channel representation (VCR), which is restricted to uniform linear arrays (ULAs) so far, to a general version applicable to arbitrary array configurations. Then, we present two array independent stochastic MIMO channel models based on the proposed new VCR as well as the Weichselberger model. These two models are good at angular power spectrum (APS) estimation and capacity prediction, respectively. Finally, the impact of array characteristics on channel capacity is separately investigated by studying the condition number of the array steering matrix at fixed angles, and the results agree well with existing conclusions. Numerical results are presented for model validation and comparison.; Comment: 30 pages...

Monojet versus rest of the world I: t-channel Models

Papucci, Michele; Vichi, Alessandro; Zurek, Kathryn M.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 10/02/2014
Relevância na Pesquisa
45.93%
Monojet searches using Effective Field Theory (EFT) operators are usually interpreted as a robust and model independent constraint on direct detection (DD) scattering cross-sections. At the same time, a mediator particle must be present to produce the dark matter (DM) at the LHC. This mediator particle may be produced on shell, so that direct searches for the mediating particle can constrain the effective operator being applied to monojet constraints. In this first paper, we do a case study on t-channel models in monojet searches, where the (Standard Model singlet) DM is pair produced via a t-channel mediating particle, whose supersymmetric analogue is the squark. We compare monojet constraints to direct constraints on single or pair production of the mediator from multi-jets plus missing energy searches and we identify the regions where the latter dominate over the former. We show that computing bounds using supersymmetric simplified models and in the narrow width approximation, as done in previous work in the literature, misses important quantitative effects. We perform a full event simulation and statistical analysis, and we compute the effects of both on- and off-shell production of the mediating particle, showing that for both the monojet and multi-jets plus missing energy searches...

The performance evaluation of IEEE 802.16 physical layer in the basis of bit error rate considering reference channel models

Ferdousi, Arifa; Enam, Farhana; Khan, Sadeque Reza
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 25/12/2013
Relevância na Pesquisa
45.85%
Fixed Broadband Wireless Access is a promising technology which can offer high speed data rate from transmitting end to customer end which can offer high speed text, voice, and video data. IEEE 802.16 WirelessMAN is a standard that specifies medium access control layer and a set of PHY layer to fixed and mobile BWA in broad range of frequencies and it supports equipment manufacturers due to its robust performance in multipath environment. Consequently WiMAX forum has adopted this version to develop the network world wide. In this paper the performance of IEEE 802.16 OFDM PHY Layer has been investigated by using the simulation model in Matlab. The Stanford University Interim (SUI) channel models are selected for the performance evaluation of this standard. The Ideal Channel estimation is considered in this work and the performance evaluation is observed in the basis of BER.; Comment: International Journal on Cybernetics & Informatics (IJCI) Vol.2, No.4, August 2013

Maximum Entropy MIMO Wireless Channel Models

Guillaud, M.; Debbah, M.; Moustakas, A. L.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 20/12/2006
Relevância na Pesquisa
46.03%
In this contribution, models of wireless channels are derived from the maximum entropy principle, for several cases where only limited information about the propagation environment is available. First, analytical models are derived for the cases where certain parameters (channel energy, average energy, spatial correlation matrix) are known deterministically. Frequently, these parameters are unknown (typically because the received energy or the spatial correlation varies with the user position), but still known to represent meaningful system characteristics. In these cases, analytical channel models are derived by assigning entropy-maximizing distributions to these parameters, and marginalizing them out. For the MIMO case with spatial correlation, we show that the distribution of the covariance matrices is conveniently handled through its eigenvalues. The entropy-maximizing distribution of the covariance matrix is shown to be a Wishart distribution. Furthermore, the corresponding probability density function of the channel matrix is shown to be described analytically by a function of the channel Frobenius norm. This technique can provide channel models incorporating the effect of shadow fading and spatial correlation between antennas without the need to assume explicit values for these parameters. The results are compared in terms of mutual information to the classical i.i.d. Gaussian model.; Comment: Submitted to IEEE Trans. Information Theory Dec. 2006; 16 pages...

MiXiM, PAWiS, and STEAM-Sim Integration - Combining Channel Models, Energy Awareness, and Real-life Application Code

Möstl, Georg; Springer, Andreas
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 11/09/2015
Relevância na Pesquisa
45.82%
After a decade of research in the field of wireless sensor networks (WSNs) there are still open issues. WSNs impose several severe requirements regarding energy consumption, processing capabilities, mobility, and robustness of wireless transmissions. Simulation has shown to be the most cost-efficient approach for evaluation of WSNs, thus a number of simulators are available. Unfortunately, these simulation environments typically consider WSNs from a special point of view. In this work we present the integration of three such specialized frameworks, namely MiXiM, PAWiS, and STEAM-Sim. This integration combines the strengths of the single frameworks such as realistic channel models, mobility patterns, accurate energy models, and inclusion of real-life application code. The result is a new simulation environment which enables a more general consideration of WSNs. We implemented and verified our proposed concept by means of static and mobile scenarios. As the presented results show, the combined framework gives the same results regarding the functionality and energy consumption as our "golden model". Therefore the system integration was successful and the framework is ready to be used by the community.; Comment: Published in: A. F\"orster...

Nucleon Resonances with Hidden Charm in Coupled-Channel Models

Wu, Jia-Jun; Lee, T. -S. H.; Zou, B. S.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 05/02/2012
Relevância na Pesquisa
45.82%
The model dependence of the predictions of nucleon resonances with hidden charm is investigated. We consider several coupled-channel models which are derived from relativistic quantum field theory by using (1) a unitary transformation method, and (2) the three-dimensional reductions of Bethe-Salpeter Equation. With the same vector meson exchange mechanism, we find that all models give very narrow molecular-like nucleon resonances with hidden charm in the mass range of 4.3 GeV $ < M_R < $ 4.5 GeV, in consistent with the previous predictions.; Comment: 17 pages, 3 figures

Microscopic Reversibility or Detailed Balance in Ion Channel Models

Nagy, I.; Tóth, J.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 20/09/2011
Relevância na Pesquisa
45.97%
Mass action type deterministic kinetic models of ion channels are usually constructed in such a way as to obey the principle of detailed balance (or, microscopic reversibility) for two reasons: first, the authors aspire to have models harmonizing with thermodynamics, second, the conditions to ensure detailed balance reduce the number of reaction rate coefficients to be measured. We investigate a series of ion channel models which are asserted to obey detailed balance, however, these models violate mass conservation and in their case only the necessary conditions (the so-called circuit conditions) are taken into account. We show that ion channel models have a very specific structure which makes the consequences true in spite of the imprecise arguments. First, we transform the models into mass conserving ones, second, we show that the full set of conditions ensuring detailed balance (formulated by Feinberg) leads to the same relations for the reaction rate constants in these special cases, both for the original models and the transformed ones.

Ion channel gating: insights via molecular simulations

Beckstein, Oliver; Biggin, Philip; Bond, Peter; Bright, Joanne Nicole; Domene, Carmen; Grottesi, Alessandro; Holyoake, John; Sansom, Mark S P
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.87%
Ion channels are gated, i.e. they can switch conformation between a closed and an open state. Molecular dynamics simulations may be used to study the conformational dynamics of ion channels and of simple channel models. Simulations on model nanopores reveal that a narrow (<4 Å) hydrophobic region can form a functionally closed gate in the channel and can be opened by either a small (∼1 Å) increase in pore radius or an increase in polarity. Modelling and simulation studies confirm the importance of hydrophobic gating in K channels, and support a model in which hinge-bending of the pore-lining M2 (or S6 in Kv channels) helices underlies channel gating. Simulations of a simple outer membrane protein, OmpA, indicate that a gate may also be formed by interactions of charged side chains within a pore, as is also the case in ClC channels.

Channel modelling for MIMO systems; Modelos de canal para sistemas MIMO

Mota, Susana de Jesus
Fonte: Universidade de Aveiro Publicador: Universidade de Aveiro
Tipo: Tese de Doutorado
ENG
Relevância na Pesquisa
55.99%
Systems equipped with multiple antennas at the transmitter and at the receiver, known as MIMO (Multiple Input Multiple Output) systems, offer higher capacities, allowing an efficient exploitation of the available spectrum and/or the employment of more demanding applications. It is well known that the radio channel is characterized by multipath propagation, a phenomenon deemed problematic and whose mitigation has been achieved through techniques such as diversity, beamforming or adaptive antennas. By exploring conveniently the spatial domain MIMO systems turn the characteristics of the multipath channel into an advantage and allow creating multiple parallel and independent virtual channels. However, the achievable benefits are constrained by the propagation channel’s characteristics, which may not always be ideal. This work focuses on the characterization of the MIMO radio channel. It begins with the presentation of the fundamental results from information theory that triggered the interest on these systems, including the discussion of some of their potential benefits and a review of the existing channel models for MIMO systems. The characterization of the MIMO channel developed in this work is based on experimental measurements of the double-directional channel. The measurement system is based on a vector network analyzer and a two-dimensional positioning platform...