Browsing by Subject "Analytical models"

Now showing 1 - 7 of 7

Open Access
Electromagnetic scattering solution of conducting strips in layered media using the fast multipole method
(Institute of Electrical and Electronics Engineers, 1996-08) Gürel, Levent; Aksun, M. I.
The fast multipole method (FMM) is applied to the solution of the electromagnetic scattering problems in layered media for the first time. This is achieved by using closed-form expressions for the spatial-domain Green's functions for layered media. Until now, the FMM has been limited to the homogeneous-medium problems. An integral equation based on the two-dimensional scalar Helmholtz equation is solved to compute the electromagnetic scattering from sample geometries of conducting strips in layered media in order to demonstrate the accuracy and the efficiency of the new method.
Open Access
Energy-Optimum throughput and carrier sensing rate in csma-based wireless networks
(IEEE, 2014) Koseoglu, M.; Karasan, E.
We propose a model for the energy consumption of a node as a function of its throughput in a wireless CSMA network. We first model a single-hop network, and then a multi-hop network. We show that operating the CSMA network at a high throughput is energy inefficient since unsuccessful carrier sensing attempts increase the energy consumption per transmitted bit. Operating the network at a low throughput also causes energy inefficiency because of increased sleeping duration. Achieving a balance between these two opposite operating regimes, we derive the energy-optimum carrier-sensing rate and the energy-optimum throughput which maximize the number of transmitted bits for a given energy budget. For the single-hop case, we show that the energy-optimum total throughput increases as the number of nodes sharing the channel increases. For the multi-hop case, we show that energy-optimum throughput decreases as the degree of the conflict graph corresponding to the network increases. For both cases, the energy-optimum throughput reduces as the power required for carrier-sensing increases. The energy-optimum throughput is also shown to be substantially lower than the maximum throughput and the gap increases as the degree of the conflict graph increases for multi-hop networks. © 2002-2012 IEEE.
Open Access
A novel queue-aware wireless link adaptation mechanism and its fixed-point analytical model
(SpringerOpen, 2015) Ozturk, O.; Akar, N.
A point-to-point (PTP) wireless link is studied that carries long-lived TCP flows and is controlled with active queue management (AQM). A cross-layer queue-aware adaptive modulation and coding (AMC)-based link adaptation (LA) mechanism is proposed for this wireless link to improve the TCP-level throughput relative to the case where AMC decisions are made based solely on the physical layer (PHY) parameters. The proposed simple-to-implement LA mechanism involves the use of an aggressive modulation and coding scheme (MCS) with high spectral efficiency and high block error rates when the queue occupancy exceeds a certain threshold, but otherwise a relatively conservative MCS with lower spectral efficiency and lower block error rates. A fixed-point analytical model is proposed to obtain the aggregate TCP throughput attained at this wireless link and the model is validated by ns-3 simulations. Numerical experimentation with the proposed analytical model applied to an IEEE 802.16-based wireless link demonstrates the effectiveness of the proposed queue-aware LA (QAWLA) mechanism in a wide variety of scenarios including cases where the channel information is imperfect. The impact of the choice of the queue occupancy threshold of QAWLA is extensively studied with respect to the choice of AQM parameters in order to provide engineering guidelines for the provisioning of the wireless link.
Open Access
On the numerical solution of Kronecker-based infinite level-dependent QBD processes
(2013) Baumann, H.; Dayar, T.; Orhan, M. C.; Sandmann, W.
Infinite level-dependent quasi-birth-and-death (LDQBD) processes can be used to model Markovian systems with countably infinite multidimensional state spaces. Recently it has been shown that sums of Kronecker products can be used to represent the nonzero blocks of the transition rate matrix underlying an LDQBD process for models from stochastic chemical kinetics. This paper extends the form of the transition rates used recently so that a larger class of models including those of call centers can be analyzed for their steady-state. The challenge in the matrix analytic solution then is to compute conditional expected sojourn time matrices of the LDQBD model under low memory and time requirements after truncating its countably infinite state space judiciously. Results of numerical experiments are presented using a Kronecker-based matrix-analytic solution on models with two or more countably infinite dimensions and rules of thumb regarding better implementations are derived. In doing this, a more recent approach that reduces memory requirements further by enabling the computation of steady-state expectations without having to obtain the steady-state distribution is also considered. © 2013 Elsevier B.V. All rights reserved.
Open Access
Radiation characteristics of a 2D parabolic reflector antenna excited by the H-polarized complex source
(IEEE, 2002-09) Oğuzer, T.; Nosich, A. I.; Altıntaş, Ayhan
The aim of this paper is to obtain accurate reference data for relatively large and realistic reflector antenna systems. We concentrate on a parabolic reflector antenna in the H-polarization case. The directive primary feed is modeled by the complex source point method and the relative accuracy of the results is verified. © 2002 IEEE.
Open Access
State aggregation-based model of asynchronous multi-fiber optical switching with shared wavelength converters
(Elsevier, 2013) Akar, N.; Raffaelli, C.; Savi, M.
This paper proposes new analytical models to study optical packet switching architectures with multi-fiber interfaces and shared wavelength converters. The multi-fiber extension of the recently proposed Shared-Per-Input-Wavelength (SPIW) scheme is compared against the multi-fiber Shared-Per-Node (SPN) scheme in terms of cost and performance for asynchronous traffic. In addition to using Markov chains and fixed-point iterations for modeling the mono-fiber case, a novel state aggregation technique is proposed to evaluate the packet loss in asynchronous multi-fiber scenario. The accuracy of the performance models is validated by comparison with simulations in a wide variety of scenarios with both balanced and imbalanced input traffic. The proposed analytical models are shown to remarkably capture the actual system behavior in all scenarios we tested. The adoption of multi-fiber interfaces is shown to achieve remarkable savings in the number of wavelength converters employed and their range. In addition, the SPIW solution allows to save, in particular conditions, a significant number of optical gates compared to the SPN solution. Indeed, SPIW allows, if properly dimensioned, potential complexity and cost reduction compared to SPN, while providing similar performance.
Open Access
Uçtan uca modelleme ile kesintisiz stereo video iletimi
(IEEE, 2008-04) Tan, A. Serdar; Aksay, A; Akar, G. B.; Arıkan, Erdal
Bu çalışmamızda, hata koruma yöntemi olarak oransız kodları (Raptor Kodları) kullanan kesintisiz bir stereo video iletim sistemi önermekteyiz. İlk olarak video kodlayıcının hızbozulum (HB) eğrisini ve kanal koruma kodunun performansını analitik olarak modelledik. Daha sonra, paket kayıplarından kaynaklanan stereo videodaki kalite bozulumunu kestirdik. Son olarak, analitik modelleri ve kestirilmiş paket kayıp bozulumlarını kullanarak uçtan uca bozulum enküçültmesi uyguladık ve en iyi video kodlayıcı bit hızlarını ve eşit olmayan hata koruması (EOHK) oranlarını elde ettik. Benzetim sonuçlarında, önerilen sistemin getirdiği önemli kalite kazanımları açıkça gözlenmektedir.