Browsing by Subject "Mobility"

Now showing 1 - 11 of 11

Open Access
Coupled plasmon-LO-phonon modes in GaInAs quantum wires
(American Institute of Physics., 1996-06-20) Mutluay, N.; Tanatar, Bilal
We study the collective excitation modes of coupled quasi-one-dimensional electron gas and longitudinal-optical phonons in GaInAs quantum wires within the random-phase approximation. In contrast to the higher-dimensional systems, the plasmon-phonon coupling is found to be strong at all Linear carrier densities of interest. We calculate the oscillator strength of the numerically evaluated coupled modes and the Raman scattering intensity. The effect of phenomenological LO-phonon broadening on the collective excitation spectrum is also investigated. (C) 1996 American Institute of Physics.
Open Access
Democracy, visa-waivers, and international mobility
(SpringerOpen, 2024-11-01) Altundal, U.; Zarplı, Ömer
In examining visa-waiver agreements, previous studies have primarily focused on economic motivations and effects. In this article, we explore the potential political motivations and consequences. Particularly, we propose that there is a positive relationship between visa-waivers and democracy. We test this using a global dataset that records bilateral travel visa requirements for all countries between 1973 and 2013. We find support for our main hypothesis. We also examine the relationship between democracy, visa waivers and economic development. Contrary to our expectations, we find that democracy continues to exert influence on visa waivers even at high levels of income. Non-democracies fail to attain visa waivers even when they are wealthy. Finally, we explore whether visa waivers could have an impact on individuals’ attitudes towards democracy. Using survey data from 18 Latin American countries, we find suggestive evidence that that visa-waivers can have a positive impact on individuals’ democratic attitudes. Taken together, the findings suggest that democracy and visa-waivers might have a mutually reinforcing relationship.
Open Access
Dis-placed: space, settlement, and agency
(Intellect Ltd., 2021-07-01) Batuman, Bülent
This article introduces the special issue ‘Dis-placed’. Questioning the term ‘refugee’ as an identity marker and pointing at the problematic connotations it embodies, the article explores the spatial forms of refugee experience. The knowledge of space, as produced within disciplines such as geography, urban planning, and architecture, is deployed by states to limit the movements of forced migrants across and within national borders. In response, the article calls for social/spatial justice, arguing that this can only be achieved through the blurring of the boundaries between host and refugee identities. The contributions in this special issue present investigations on different facets of the spatiality of forced migration through various disciplinary approaches and methodologies. Taken together, they underline the importance of the link between space and refugee agency in tackling forced migration.
Open Access
An online adaptive cooperation scheme for spectrum sensing based on a second-order statistical method
(Institute of Electrical and Electronics Engineers, 2012) Yarkan S.; Töreyin, B. U.; Qaraqe, K. A.; Çetin, A. Enis
Spectrum sensing is one of the most important features of cognitive radio (CR) systems. Although spectrum sensing can be performed by a single CR, it is shown in the literature that cooperative techniques, including multiple CRs/sensors, improve the performance and reliability of spectrum sensing. Existing cooperation techniques usually assume a static communication scenario between the unknown source and sensors along with a fixed propagation environment class. In this paper, an online adaptive cooperation scheme is proposed for spectrum sensing to maintain the level of sensing reliability and performance under changing channel and environmental conditions. Each cooperating sensor analyzes second-order statistics of the received signal, which undergoes both correlated fast and slow fading. Autocorrelation estimation data from sensors are fused together by an adaptive weighted linear combination at the fusion center. Weight update operation is performed online through the use of orthogonal projection onto convex sets. Numerical results show that the performance of the proposed scheme is maintained for dynamically changing characteristics of the channel between an unknown source and sensors, even under different physical propagation environments. In addition, it is shown that the proposed cooperative scheme, which is based on second-order detectors, yields better results compared with the same fusion mechanism that is based on conventional energy detectors.
Open Access
Optimization of transportation requirements in the deployment of military units
(2005) Akgün, İbrahim
We study the deployment planning problem (DPP) that may roughly be defined as the problem of the planning of the physical movement of military units, stationed at geographically dispersed locations, from their home bases to their designated destinations while obeying constraints on scheduling and routing issues as well as on the availability and use of various types of transportation assets that operate on a multimodal transportation network. The DPP is a large-scale real-world problem for which no analytical models are existent. In this study, we define the problem in detail and analyze it with respect to the academic literature. We propose three mixed integer programming models with the objectives of cost, lateness (the difference between the arrival time of a unit and its earliest allowable arrival time at its destination), and tardiness (the difference between the arrival time of a unit and its latest arrival time at its destination) minimization to solve the problem. The cost-minimization model minimizes total transportation cost of a deployment and is of use for investment decisions in transportation resources during peacetime and for deployment planning in cases where the operation is not imminent and there is enough time to do deliberate planning that takes costs into account. The lateness and tardiness minimization models are of min-max type and are of use when quick deployment is of utmost concern. The lateness minimization model is for cases when the given fleet of transportation assets is sufficient to deploy units within their allowable time windows and the tardiness minimization model is for cases when the given fleet is not sufficient. We propose a solution methodology for solving all three models. The solution methodology involves an effective use of relaxation and restriction that significantly speeds up a CPLEX-based branchand-bound. The solution times for intermediate sized problems are around one hour at maximum for cost and lateness minimization models and around two hours for the tardiness minimization model. Producing a suboptimal feasible solution based on trial and error methods for a problem of the same size takes about a week in the current practice in the Turkish Armed Forces. We also propose a heuristic that is essentially based on solving the models incrementally rather than at one step. Computational results show that the heuristic can be used to find good feasible solutions for the models. We conclude the study with comments on how to use the models in the realworld.
Open Access
Optimization of transportation requirements in the deployment of military units
(Elsevier, 2007) Akgün, İ.; Tansel, B. Ç.
We study the deployment planning problem (DPP) that may roughly be defined as the problem of the planning of the physical movement of military units, stationed at geographically dispersed locations, from their home bases to their designated destinations while obeying constraints on scheduling and routing issues as well as on the availability and use of various types of transportation assets that operate on a multimodal transportation network. The DPP is a large-scale real-world problem for which analytical models do not exist. We propose a model for solving the problem and develop a solution methodology which involves an effective use of relaxation and restriction that significantly speeds up a CPLEX-based branch-and-bound. The solution times for intermediate-sized problems are around 1 h at maximum, whereas it takes about a week in the Turkish Armed Forces to produce a suboptimal feasible solution based on trial-and-error methods. The proposed model can be used to evaluate and assess investment decisions in transportation infrastructure and transportation assets as well as to plan and execute cost-effective deployment operations at different levels of planning.
Open Access
Physical layer security over frequency selective fading channels
(2016-01) Ayhan, Kadir
The inherent open nature of the transmission medium makes security a challenging issue in wireless networks. Physical layer security, which is an alternative or a complement to the cryptographic approaches, exploits the differences between the physical properties of different channels in order to provide secrecy. The idea is to ensure that the received signal at an eavesdropper is degraded compared to that of the legitimate receiver in some sense which guarantees that the confidential messages cannot be recovered by an unintended receiver. Over the last decade, various researchers have studied fundamental limits of physical layer security under different wiretap channel models, including Gaussian and fading channels, and with different assumptions on the transmitter’s knowledge on the channel state information. In this thesis, we study physical layer security over frequency selective fading channels modelling certain wireless links. Specifically, we investigate optimal and suboptimal power allocation schemes across frequencies with perfect and partial channel state information at the transmitter with the objective of providing secrecy. We demonstrate that frequency selectivity allows for positive secrecy rates even though the eavesdropper’s channel is not a degraded version of the desired user’s channel. We also analyse the impact of user mobility and the resulting time variations in the wireless medium on the achievable secrecy rates. Furthermore, we consider quantized channel state information at the transmitter and evaluate the secrecy rate loss due to limited feedback from the legitimate receiver to the transmitter. Our results reveal that the partial channel state information at the transmitter can still be helpful in providing positive secrecy rates.
Open Access
Production of ‘abstract crisis’ and irregular human mobilities
(2019-07) Karayiğit, Hakkı Ozan
This thesis argues that political representation of space as being consisted of territorially bounded states leads to the impulsion to see an uncontrolled movement as risky, and eventually to call the event as crisis. By way of concentrating on three specific scholarly journals on migration movements, it aims to enlighten the question: who/what turns an event into crisis at global scale, and why? Through focusing on how migration literature explains what constitutes crisis, the thesis investigates how did the transition to calling migration a crisis take place and why. In pursuing the question, the thesis creates a theoretical framework by deconstructing IR space and human mobility with the help of Lefebvre and Cresswell. By asserting its own concept – ‘abstract crisis’, the thesis provides typology and taxonomy of the migration studies categorized under three crisis types.
Open Access
Step-edge-induced resistance anisotropy in quasi-free-standing bilayer chemical vapor deposition graphene on SiC
(AIP Publishing, 2014) Ciuk, T.; Cakmakyapan, S.; Özbay, Ekmel; Caban, P.; Grodecki, K.; Krajewska, A.; Pasternak, I.; Szmidt, J.; Strupinski, W.
The transport properties of quasi-free-standing (QFS) bilayer graphene on SiC depend on a range of scattering mechanisms. Most of them are isotropic in nature. However, the SiC substrate morphology marked by a distinctive pattern of the terraces gives rise to an anisotropy in graphene's sheet resistance, which may be considered an additional scattering mechanism. At a technological level, the growth-preceding in situ etching of the SiC surface promotes step bunching which results in macro steps similar to 10 nm in height. In this report, we study the qualitative and quantitative effects of SiC steps edges on the resistance of epitaxial graphene grown by chemical vapor deposition. We experimentally determine the value of step edge resistivity in hydrogen-intercalated QFS-bilayer graphene to be similar to 190 Omega mu m for step height h(S) = 10 nm and provide proof that it cannot originate from mechanical deformation of graphene but is likely to arise from lowered carrier concentration in the step area. Our results are confronted with the previously reported values of the step edge resistivity in monolayer graphene over SiC atomic steps. In our analysis, we focus on large-scale, statistical properties to foster the scalable technology of industrial graphene for electronics and sensor applications. (C) 2014 AIP Publishing LLC.
Open Access
Two-dimensional 2γ-In2Se3 in bilayer-like coloring triangle lattice: Mechanical, electronic, transport, and photocatalytic properties
(2024-11-04) Gorkan, Taylan; Özbey, Doğukan Hazar; Kilic, Mehmet Emin; Durgun, Engin
The discovery of two-dimensional (2D) materials derived from non-van der Waals (vdW) bulk counterparts has opened up a new era, drawing attention to the crystals composed of asymmetrically bonded vertical exotic layers. In this respect, gamma -In2Se3, a promising material utilized in various applications, built with coloring triangle layers, emerges as a suitable candidate. Through first-principles calculations, we show that a novel 2D structure, the 2 gamma -In2Se3 monolayer, consisting of a bilayer-like coloring triangle lattice, can be exfoliated from bulk gamma -In2Se3 with minimal external energy. The formation of this exotic 2D lattice is facilitated by sp3 hybrid bonds. Comprehensive phonon dispersion and finite-temperature molecular dynamics analyses confirm the thermodynamic stability of the 2 gamma -In2Se3 monolayer. The material exhibits an anisotropic mechanical response due to missing bonds at lattice sites, making it suitable for flexible nanoelectronic devices. It possesses semiconductor characteristics with an indirect band gap in the visible region. Analysis of band edge positions and charge carrier mobility suggests that the 2 gamma -In2Se3 monolayer is highly efficient for photocatalytic water-splitting applications.
Open Access
User grouping in wireless networks with full duplex base stations and legacy mobile stations
(2018-08) Ünal, Deniz
Improving spectral efficiency is a key objective in next generation wireless networks. Recent advances in self-interference cancellation techniques made in-band full-duplex wireless communications possible. Unlike half-duplex systems which require orthogonal frequency or time resources to separate transmission and reception, in-band full-duplex radios utilize the channel bidirectionally and theoretically can double the ergodic capacity. However due to cost, power consumption and complexity constraints, mobile stations may not support this technology. In this work, operation of full-duplex base stations with legacy half-duplex mobile stations is considered. An inherent issue of this topology is the presence of signi cant inter-user interference between half-duplex mobile stations. In order to manage this at network level, an optimization problem is formulated for a cellular network topology. Solution methods and their corresponding sum throughput are compared with respect to the number of mobile stations. An analytic solution is presented to evaluate the throughput and full-duplex gains of random pairing method for the same scenario. Then the case of limited channel state information is evaluated and a learning strategy is introduced to extend the user pairing problem to a continuous case. Performance evaluation with 100 mobile stations show that the proposed learning strategy can reduce the overhead airtime more than 80%. A weighted random sequential algorithm which is integrated to the learning process is proposed, and its performance evaluation under random walk and random waypoint mobility cases are performed.