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Ramahi O.M.,University of Waterloo | Almoneef T.S.,University of Waterloo | Alshareef M.,University of Waterloo | Boybay M.S.,Antalya International University
Applied Physics Letters | Year: 2012

Metamaterials are typically made of an ensemble of electrically small resonators such as metallic loops. The fact that such particles resonate individually to generate a bulk material behavior having enhanced constitutive parameters is essentially indicative of these particles ability to collect energy. We show that such particles act as energy collectors when a resistive load is inserted within the particles gap. A proof of concept is provided using a 5.8 GHz field and a split-ring resonator (SRR) as the electromagnetic energy collecting cell. Numerical simulation for a 9 × 9 SRR array shows the effectiveness of an SRR array as an energy collector plate. © 2012 American Institute of Physics.

Tasdemir K.,Antalya International University
International Geoscience and Remote Sensing Symposium (IGARSS) | Year: 2013

Clustering has been a widely-used method for land cover identification using remote sensing images, thanks to its requirement of limited or no priori information. Among many methods, approximate spectral clustering, which depends on eigendecomposition of a similarity measure, has been popular due to its success and ability to extract arbitrarily-shaped clusters. The similarity measure, which is defined either based on distances or recently on density information, often underutilizes available information for accurate representation of dissimilarity. To address this challenge, a hybrid criterion merging density and distance information is proposed for approximate spectral clustering. Experimental results on remote-sensing images show that the hybrid similarity achieves accuracies greater than the accuracies obtained by the similarity solely based on distance or density. © 2013 IEEE.

Inaltekin H.,Antalya International University | Hanly S.V.,Macquarie University
IEEE Transactions on Information Theory | Year: 2012

This paper considers the optimum single cell power control maximizing the aggregate (uplink) communication rate of the cell when there are peak power constraints at mobile users, and a low-complexity data decoder (without successive decoding) at the base station. It is shown that the optimum power allocation is binary, which means that links are either on or off. By exploiting further structure of the optimum binary power allocation, a simple polynomial-time algorithm for finding the optimum transmission power allocation is proposed, together with a reduced complexity near-optimal heuristic algorithm. Sufficient conditions under which channel-state aware time division multiple access (TDMA) maximizes the aggregate communication rate are established. In a numerical study, we compare and contrast the performance achieved by the optimum binary power-control policy with other suboptimum policies and the throughput capacity achievable via successive decoding. It is observed that two dominant modes of communication arise, wideband or TDMA, and that successive decoding achieves better sum-rates only under near perfect interference cancellation efficiency. In this paper, we exploit the theory of majorization to obtain the aforementioned results. In the final part of this paper, we do so to solve power-control problems in the areas of femtocells and cognitive radio and find that, again, optimal solutions have a binary (or almost binary) character. © 1963-2012 IEEE.

Demirbas U.,Antalya International University
Optics Communications | Year: 2013

We present a numerical study, which investigates the potential of diode pumped Cr:LiCAF regenerative amplifiers in detail. Special attention has been given to relevant material properties of the gain media, like the Auger energy transfer upconversion (ETU) process, to utilize the full potential of the material and to develop guidelines in choosing optimal material properties like chromium doping concentration and length. Moreover, importance of pulsed pumping, rather than continuous-wave (cw) pumping, in obtaining higher small signal gain values is discussed in detail. Effects of pump power, cavity losses, excited-state absorption, seed pulse energy, optical damage and ETU on obtainable pulse energies will also be presented. The modeling results have shown that, Cr:LiCAF regenerative amplifiers pumped by two 675 nm state-of-the-art 1-W tapered diodes have the potential to produce 50-fs long pulses around 800 nm with 70 μJ pulse energy and 1.4 GW peak power at repetition rates up to 5 kHz. Moreover, a 20-W diode pumped Cr:LiCAF amplifier has the potential to produce pulse energies of 1.1 mJ and peak powers of 20 GW. Expected optical-to-optical conversion efficiencies of the systems are about 30%. These results demonstrate that, with ongoing progress in laser diode technology, low-cost and efficient Cr:Colquiriite amplifiers has the potential to replace the expensive Ti:Sapphire technology in the future. © 2013 Elsevier B.V.

Samarasinghe T.,Monash University | Inaltekin H.,Antalya International University | Evans J.S.,Monash University
IEEE Transactions on Information Theory | Year: 2013

This paper studies the structure of downlink sum-rate maximizing selective decentralized feedback policies for opportunistic beamforming under finite feedback constraints on the average number of mobile users feeding back. First, it is shown that any sum-rate maximizing selective decentralized feedback policy must be a threshold feedback policy. This result holds for all fading channel models with continuous distribution functions. Second, the resulting optimum threshold selection problem is analyzed in detail. This is a nonconvex optimization problem over finite-dimensional Euclidean spaces. By utilizing the theory of majorization, an underlying Schur-concave structure in the sum-rate function is identified, and the sufficient conditions for the optimality of homogenous threshold feedback policies are obtained. Applications of these results are illustrated for well-known fading channel models such as Rayleigh, Nakagami, and Rician fading channels. Rather surprisingly, it is shown that using the same threshold value at all mobile users is not always a rate-wise optimal feedback strategy, even for a network in which mobile users experience statistically the same channel conditions. For the Rayleigh fading channel model, on the other hand, homogenous threshold feedback policies are proven to be rate-wise optimal if multiple orthonormal data carrying beams are used to communicate with multiple mobile users simultaneously. © 1963-2012 IEEE.

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