Antalya International University
Antalya, Turkey
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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.

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.

Boybay M.S.,Antalya International University | Ramahi O.M.,University of Waterloo
IEEE Microwave and Wireless Components Letters | Year: 2013

A microwave sensor for non-destructive measurement of dielectric thickness is presented. The sensor is a quasi-static resonator and based on complementary split ring resonator (CSRR) structure. When the CSRR structure is backed by a conductive medium covered with a dielectric layer the resonance frequency of the CSRR has a strong dependence on the thickness of the dielectric layer. Effect of the size of CSRR sensor on the sensitivity is analyzed numerically. For experimental verification, a CSRR sensor that operates in the 1.6 to 2.3 GHz band is fabricated and excited by a microstrip line. © 2001-2012 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.

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.

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.

Demirbas U.,Antalya International University | Baali I.,Antalya International University
Optics Letters | Year: 2015

We report significant average power and efficiency scaling of diode-pumped Cr:LiSAF lasers in continuous-wave (cw), cw frequency-doubled, and mode-locked regimes. Four single-emitter broad-area laser diodes around 660 nm were used as the pump source, which provided a total pump power of 7.2 W. To minimize thermal effects, a 20 mm long Cr:LiSAF sample with a relatively low Cr-concentration (0.8%) was used as the gain medium. In cw laser experiments, 2.4Wof output power, a slope efficiency of 50%, and a tuning range covering the 770-1110 nm region were achieved. Intracavity frequency doubling with beta-barium borate (BBO) crystals generated up to 1160 mW of blue power and a record tuning range in the 387-463 nm region.Whenmode locked with a saturable absorber mirror, the laser produced 195 fs pulses with 580 mW of average power around 820 nm at a 100.3 MHz repetition rate. The optical-to-optical conversion efficiency of the system was 33% in cw, 16% in cw frequency-doubled, and 8% in cw mode-locked regimes. © 2015 Optical Society of America.

Nekouei E.,University of Melbourne | Inaltekin H.,Antalya International University | Dey S.,University of Melbourne
IEEE Transactions on Signal Processing | Year: 2012

This paper derives tight ergodic sum-rate capacity scaling limits for cognitive radio secondary networks under five different communication environments (CoE) for two different network types when secondary users' (SUs) transmission powers are optimally allocated. The network types studied are power-interference limited (PIL) networks and interference limited (IL) networks. In PIL networks, SUs' transmissions are limited by both an average total power constraint and a constraint on the average interference that they cause to primary users (PUs). In IL networks, SUs' transmissions are only limited by an average interference constraint. The capacity scaling results in PIL networks are derived for three different CoEs in which secondary transmitter to secondary base station (STSB) channel gains are Rayleigh distributed while secondary transmitter to primary base station (STPB) channel gains are Rayleigh, Rician or Nakagami distributed. It is shown that secondary network capacity scales according to log log (N) in these three CoEs, where N is the number of SUs. In addition to these three CoEs, two more CoEs are also studied for IL networks: Rician or Nakagami distributed STSB channel gains and Rayleigh distributed STPB channel gains. It is shown that the secondary network capacity scales according to log (N) for all five CoEs in IL networks. This result implies exponential capacity gains in IL networks over PIL networks. The same capacity scaling results are shown to hold even for heterogeneous cognitive radio networks in which different SUs experience statistically different channel conditions. In some cases, our analysis leads to a new notion called effective number of users, which signifies the effective number of users contributing to multiuser diversity in cognitive radio networks. For example, effective number of users is given by K+1/e KN when STSB channel gains are Rayleigh distributed and STPB channel gains are Rician distributed with a Rician factor K. © 2011 IEEE.

Inaltekin H.,Antalya International University
IEEE Transactions on Signal Processing | Year: 2012

The problem of Gaussian approximation for the wireless multi-access interference distribution in large spatial wireless networks is addressed. First, a principled methodology is presented to establish rates of convergence of the multi-access interference distribution to a Gaussian distribution for general bounded and power-law decaying path-loss functions. The model is general enough to also include various random wireless channel dynamics such as fading and shadowing arising from multi-path propagation and obstacles existing in the communication environment. It is shown that the wireless multi-access interference distribution converges to the Gaussian distribution with the same mean and variance at a rate 1√λ, where λ > 0 is a parameter controlling the intensity of the planar (possibly non-stationary) Poisson point process generating node locations. An explicit expression for the scaling coefficient is obtained as a function of fading statistics and the path-loss function. Second, an extensive numerical and simulation study is performed to illustrate the accuracy of the derived Gaussian approximation bounds. A good statistical fit between the interference distribution and its Gaussian approximation is observed for moderate to high values of λ. © 2012 IEEE.

Kazan H.,Antalya International University
Nucleic acids research | Year: 2013

RBPmotif web server ( implements tools to identify binding preferences of RNA-binding proteins (RBPs). Given a set of sequences that are known to be bound and unbound by the RBP of interest, RBPmotif provides two types of analysis: (i) de novo motif finding when there is no a priori knowledge on RBP's binding preferences and (ii) analysis of structure preferences when there is a previously identified sequence motif for the RBP. De novo motif finding is performed with the previously published RNAcontext algorithm that learns discriminative motif models to identify both sequence and structure preferences. The results of this analysis include the inferred binding preferences of the RBP and the added predictive value of incorporating structure preferences. Second type of analysis investigates whether the instances of the previously identified sequence motif are enriched in a particular structure context in bound sequences, relative to its instances in unbound sequences. On completion, the results page shows the comparison of structure contexts of the motif instances between bound and unbound sequences and an assessment of statistical significance of detected preferences. In summary, RBPmotif web server enables the concurrent analysis of sequence and structure preferences of RBPs through a user-friendly interface.

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