Browsing by Subject "Relaxation processes"

Now showing 1 - 15 of 15

Open Access
Compressive sampling and adaptive multipath estimation
(IEEE, 2010) Pilancı, Mert; Arıkan, Orhan
In many signal processing problems such as channel estimation and equalization, the problem reduces to a linear system of equations. In this proceeding we formulate and investigate linear equations systems with sparse perturbations on the coefficient matrix. In a large class of matrices, it is possible to recover the unknowns exactly even if all the data, including the coefficient matrix and observation vector is corrupted. For this aim, we propose an optimization problem and derive its convex relaxation. The numerical results agree with the previous theoretical findings of the authors. The technique is applied to adaptive multipath estimation in cognitive radios and a significant performance improvement is obtained. The fact that rapidly varying channels are sparse in delay and doppler domain enables our technique to maintain reliable communication even far from the channel training intervals. ©2010 IEEE.
Open Access
Dynamical screening effects in hot-electron scattering from electron-hole plasma and LO-phonon modes in quantum wires
(Elsevier, 1996) Bennett, C. R.; Tanatar, Bilal; Constantinou, N. C.
We present a fully dynamical and finite temperature study of the hot-electron momentum relaxation rate and the power loss in a coupled system of electron-hole plasma and bulk LO-phonons in a quantum wire structure. Interactions of the scattered electron with neutral plasma components and phonons are treated on an equal footing within the random-phase approximation. Coupled mode effects substantially change the transport properties of the system at low temperatures. Particularly, the "plasmon-like" and "LO-phonon-like" excitations yield comparable rates which, as a consequence of the singular nature of the ID density of states, can be large at the threshold. This is in contrast to room temperature results where only the LO-phonon mode contributes significantly to the rate. The density and temperature dependence of the power loss reveals that dynamical screening effects are important, and energy-momentum conservation cannot be satisfied above a certain density for a given initial energy.
Open Access
Electronic structure, insulator-metal transition and superconductivity in K-ET2X salts
(1998) Ivanov V.A.; Ugolkova, E.A.; Zhuravlev, M.Ye.; Hakioǧlu, T.
The electronic structure and superconductivity of layered organic materials based on the bis(ethylenedithio)tetrathiafulvalene molecule (BEDT-TTF, hereafter ET) with essential intra-ET correlations of electrons are analysed. Taking into account the Fermi surface topology, the superconducting electronic density of states (DOS) is calculated for a realistic model of K-ET2X salts. A d-symmetry of the superconducting order parameter is obtained and a relation is found between its nodes on the Fermi surface and the superconducting phase characteristics. The results are in agreement with the measured non-activated temperature dependences of the superconducting specific heat and NMR relaxation rate of central 13C atoms in ET. © 1998 John Wiley & Sons, Ltd.
Open Access
Finite temperature studies of Te adsorption on Si(0 0 1)
(Elsevier, 2002) Sen, P.; Çıracı, Salim; Batra, I. P.; Grein, C. H.; Sivananthan, S.
We perform first principles density functional calculations to investigate the adsorption of Te on the Si(0 0 1) surface from low coverage up to a monolayer coverage. At low coverage, a Te atom is adsorbed on top of the Si surface dimer bond. At higher coverages, Te atoms adsorption causes the Si-Si dimer bond to break, lifting the (2 × 1) reconstruction. We find no evidence of the Te-Te dimer bond formation as a possible source of the (2 × 1) reconstruction at a monolayer coverage. Finite temperature ab initio molecular dynamics calculations show that Te covered Si(0 0 1) surfaces do not have any definitive reconstruction. Vibrations of the bridged Te atoms in the strongly anharmonic potentials prevent the reconstruction structure from attaining any permanent, two-dimensional periodic geometry. This explains why experiments attempting to find a definite model for the reconstruction reached conflicting conclusions. © 2002 Elsevier Science B.V. All rights reserved.
Open Access
FIR filter design by iterative convex relaxations with rank refinement
(IEEE, 2014) Dedeoğlu, Mehmet; Alp, Yaşar Kemal; Arıkan, Orhan
Finite impulse response (FIR) filters have been a primary topic of digital signal processing since their inception. Although FIR filter design is an old problem, with the developments of fast convex solvers, convex modelling approach for FIR filter design has become an active research topic. In this work, we propose a new method based on convex programming for designing FIR filters with the desired frequency characteristics. FIR filter design problem, which is modelled as a non-convex quadratically constrained quadratic program (QCQP), is transformed to a semidefinite program (SDP). By relaxing the constraints, a convex programming problem, which we call RSDP(Relaxed Semidefinite Program), is obtained. Due to the relaxation, solution to the RSDPs fails to be rank-1. Typically used rank-1 approximations to the obtained RSDP solution does not satisfy the constraints. To overcome this issue, an iterative algorithm is proposed, which provides a sequence of solutions that converge to a rank-1 matrix. Conducted experiments and comparisons demonstrate that proposed method successfully designs FIR filters with highly flexible frequency characteristics.
Open Access
High-energy electron relaxation and full-band electron dynamics in aluminium nitride
(Elsevier, 2002) Bulutay, Ceyhun; Ridley, B. K.; Zakhleniuk, N. A.
Material properties of AlN, particularly its wide band gap around 6 eV, warrant its operation in the high-field transport regimes reaching MV/cm fields. In this theoretical work, we examine the full-band scattering of conduction band electrons in AlN due to polar optical phonon (POP) emission, which is the main scattering channel at high fields. First, we obtain the band structure for the wurtzite phase of AlN using the empirical pseudopotential method. Scattering rates along the full length of several high-symmetry directions are computed efficiently through the Lehmann-Taut Brillouin zone integration technique. In order to shed light on the behaviour of the velocity-field characteristics at extremely high electric fields, in the order of a few MV/cm, we resort to an Esaki-Tsu estimation. Comparison of these results for AlN is made with our similar work on GaN. With typically more than 50% higher POP scattering rate compared to GaN, AlN has poorer high-field prospects. Availability of these data for AlN and GaN paves the way for practical assessment of the high-energy electron dynamics for the ternary alloy, AlGaN.
Open Access
Iterative technique for 3-D motion estimation in videophone applications
(IEEE, 1994-04) Bozdağı, Gözde; Tekalp, A. M.; Onural, Levent
In object based coding of facial images, the accuracy of motion and depth parameter estimates strongly affects the coding efficiency. We propose an improved algorithm based on stochastic relaxation for 3-D motion and depth estimation that converges to true motion and depth parameters even in the presence of 50% error in the initial depth estimates. The proposed method is compared with an existing algorithm (MBASIC) in case of different number of point correspondences. The simulation results show that the proposed method provides significantly better results than the MBASIC algorithm.
Open Access
Nanomechanics using an ultra-small amplitude AFM
(Cambridge University Press, 2001) Hoffmann, P. M.; Jeffery, S.; Oral, Ahmet; Grimble, R. A.; Özer, H. Özgür; Pethica, J. B.
A new type of AFM is presented which allows for direct measurements of nanomechanical properties in ultra-high vacuum and liquid environments. The AFM is also capable to atomic-scale imaging of force gradients. This is achieved by vibrating a stiff lever at very small amplitudes of less than 1 Å (peak-to-peak) at a sub-resonance amplitude. This linearizes the measurement and makes the interpretation of the data straight-forward. At the atomic scale, interaction force gradients are measured which are consistent with the observation of single atomic bonds. Also, atomic scale damping is observed which rapidly rises with the tip-sample separation. A mechanism is proposed to explain this damping in terms of atomic relaxation in the tip. We also present recent results in water where we were able to measure the mechanical response due to the molecular ordering of water close to an atomically flat surface.
Open Access
Noise-enhanced M-ary hypothesis-testing in the minimax framework
(IEEE, 2009-09) Bayram, Suat; Gezici, Sinan
In this study, the effects of adding independent noise to observations of a suboptimal detector are studied for M-ary hypothesis-testing problems according to the minimax criterion. It is shown that the optimal additional noise can be represented by a randomization of at most M signal values under certain conditions. In addition, a convex relaxation approach is proposed to obtain an accurate approximation to the noise probability distribution in polynomial time. Furthermore, sufficient conditions are presented to determine when additional noise can or cannot improve the performance of a given detector. Finally, a numerical example is presented. © 2009 IEEE.
Open Access
Phase-only beam synthesis by iterative semidefinite relaxations with rank refinement
(IEEE, 2013) Alp, Yaşar Kemal; Arıkan, Orhan; Bayri, A.
In phased array antennas, by varying the complex element weights beam patterns with desired shapes can be synthesized and/or steered to desired directions. These complex weights can be implemented by using amplitude controllers and phase shifters at the system level. Since controlling the phase of an RF signal is much easier than controlling its power, many systems do not have an individual amplitude controller for each element. Hence, beamshaping and steering are to be achieved by varying only the element phases. In this work, a new approach is proposed for phase-only beam synthesis problem. In this approach, the phase-only beam synthesis is formulated as a non-convex quadratically constrained quadratic problem (QCQP). Then, it is relaxed to a convex semidefinite problem (SDP), which generally provides an undesired high rank solution. An iterative technique is developed to obtain a rank-1 solution to the relaxed convex SDP. Conducted experiments show that, proposedmethod can successfully synthesize beam shapes with desired characteristics and steering directions by using only the element phases. © 2013 EURASIP.
Open Access
Recovery of sparse perturbations in Least Squares problems
(IEEE, 2011) Pilanci, M.; Arıkan, Orhan
We show that the exact recovery of sparse perturbations on the coefficient matrix in overdetermined Least Squares problems is possible for a large class of perturbation structures. The well established theory of Compressed Sensing enables us to prove that if the perturbation structure is sufficiently incoherent, then exact or stable recovery can be achieved using linear programming. We derive sufficiency conditions for both exact and stable recovery using known results of ℓ 0/ℓ 1 equivalence. However the problem turns out to be more complicated than the usual setting used in various sparse reconstruction problems. We propose and solve an optimization criterion and its convex relaxation to recover the perturbation and the solution to the Least Squares problem simultaneously. Then we demonstrate with numerical examples that the proposed method is able to recover the perturbation and the unknown exactly with high probability. The performance of the proposed technique is compared in blind identification of sparse multipath channels. © 2011 IEEE.
Open Access
Stochastic signal design on the downlink of a multiuser communications system
(IEEE, 2012-06) Tutay, Mehmet Emin; Gezici, Sinan; Arıkan, Orhan
Stochastic signal design is studied for the downlink of a multiuser communications system. First, a formulation is proposed for the joint design of optimal stochastic signals. Then, an approximate formulation, which can get arbitrarily close to the optimal solution, is obtained based on convex relaxation. In addition, when the receivers employ symmetric signaling and sign detectors, it is shown that the maximum asymptotical improvement ratio is equal to the number of users, and the conditions under which that maximum asymptotical improvement ratio is achieved are presented. Numerical examples are provided to explain the theoretical results. © 2012 IEEE.
Open Access
Stochastic signaling under second and fourth moment constraints
(IEEE, 2010) Göken, Çağrı; Gezici, Sinan; Arıkan, Orhan
Stochastic signaling is investigated under second and fourth moment constraints for the detection of scalar-valued binary signals in additive noise channels. Sufficient conditions are derived to determine when the use of stochastic signals instead of deterministic ones can or cannot enhance the error performance of a given binary communications system. Also, a convex relaxation approach is employed to obtain approximate solutions of the optimal stochastic signaling problem. Finally, numerical examples are presented, and extensions of the results to M-ary communications systems and to other criteria than the average probability of error are discussed.
Open Access
Theoretical assessment of electronic transport in InN
(Elsevier, 2004) Bulutay, C.; Ridley, B. K.
Among the group-III nitrides, InN displays markedly unusual electronic transport characteristics due to its smaller effective mass, high peak velocity and high background electron concentration. First, a non-local empirical pseudopotential band structure of InN is obtained in the light of recent experimental and first-principles results. This is utilized within an ensemble Monte Carlo framework to illuminate the interesting transport properties. It is observed that InN has a peak velocity which is about 75% higher than that of GaN while at higher fields its saturation velocity is lower than that of GaN. Because of the strongly degenerate regime brought about by the high background electron concentration, the electron-electron interaction is also investigated, but its effect on the steady-state and transient velocity-field characteristics is shown to be negligible. Finally, hot phonon generation due to excessive polar optical phonon production in the electron scattering and relaxation processes is accounted for. The main findings are the appreciable reduction in the saturation drift velocity and the slower recovery from the velocity overshoot regime. The time evolution of the hot phonon distribution is analysed in detail and it is observed to be extremely anisotropic, predominantly along the electric force direction.
Open Access
Trapped interacting Bose gas in nonextensive statistical mechanics
(The American Physical Society, 2002) Tanatar, Bilal
We study the Bose-Einstein condensation (BEC) phenomenon in an interacting trapped Bose gas using the semiclassical two-fluid model and nonextensive statistical mechanics. The effects of nonextensivity characterized by a parameter are explored by calculating the temperature dependent thermodynamic properties, fraction of condensed particles, and density distributions of condensed and thermal components of the system. It is found that nonextensivity in the underlying statistical mechanics may have large effects on the BEC transition temperature.