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Abstract-In this paper, we deal with channel estimation for orthogonal frequency-division multiplexing (OFDM) systems. The channels are assumed to be time-varying (TV) and approximated by a basis expansion model (BEM). Due to the time-variation, the resulting channel matrix in the frequency domain is no longer diagonal, but approximately banded. Based on this observation, we propose novel channel estimators to combat both the noise and the out-of-band interference. In addition, the effect of a receiver window on channel estimation is also studied. Our claims are supported by simulation results, which are obtained considering Jakes' channels with fairly high Doppler spreads.Index Terms-Basis expansion model (BEM), orthogonal frequency-division multiplexing (OFDM), pilot-assisted modulation, time-varying (TV) channels.

The alloy-design strategy of combining multiple elements in near-equimolar ratios has shown great potential for producing exceptional engineering materials, often known as 'high-entropy alloys'. Understanding the elemental distribution, and, thus, the evolution of the configurational entropy during solidification, is undertaken in the present study using the Al 1.3 CoCrCuFeNi model alloy. Here we show that, even when the material undergoes elemental segregation, precipitation, chemical ordering and spinodal decomposition, a significant amount of disorder remains, due to the distributions of multiple elements in the major phases. The results suggest that the high-entropy alloy-design strategy may be applied to a wide range of complex materials, and should not be limited to the goal of creating single-phase solid solutions.

a b s t r a c tFatigue behavior of a cold-rolled two-phase Al 0.5 CoCrCuFeNi high-entropy alloy (HEA) was studied. Some specimens were fabricated, using commercial-purity raw materials, while others were manufactured with high-purity components. Scatter in the fatigue life of the commercial-purity samples was found in the stress vs. lifetime plot (S-N curve). However, the high-purity samples showed less scatter, and fatigue life is predictable using fatigue statistics. The fatigue property of the alloy is comparable with and may even outperform many commercial alloys. Fatigue cracking is promoted by shrinkage pores with a size of $5 lm, while mechanical nanotwinning was found to be the main deformation mechanism before crack-initiation and during crack propagation by transmission electron microscopy (TEM). Two orientations of dense nanotwins were found at the crack-initiation site, while less-dense nanotwins were found away from the crack initiation site. The nanotwinning behavior resulted in strengthening of the alloy and, consequently, high fatigue strength (383 ± 71 MPa). Moreover, statistical models were applied to predict fatigue life, suggesting that using improved fabrication processes and/or high-purity raw materials may enhance the fatigue behavior and scatter by reducing the number of fabrication microcracks and pores in the test samples.Published by Elsevier Ltd. on behalf of Acta Materialia Inc.

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In this paper, we report on molecular dynamics (MD), continuum (based on linear and nonlinear beam theories) and combined molecular dynamics/continuum simulation of carbon nanotube based nanoelectromechanical switches. As a prototype device, we study the pull-in voltage characteristics of a nanoelectromechanical switch made of a suspended single wall nanotube over a ground plane. The various simulations (MD, continuum and combined MD/continuum) have been performed accounting for the electrostatic and van der Waals forces between the nanotube and the ground plane. The results from the nonlinear continuum theory compared well with the results from MD, except, for cases, where nanotube buckling was observed. When buckling occurs, the electromechanical behavior of the switch is simulated by employing a combined MD/continuum approach. The combined MD/continuum approach is computationally more efficient compared to the MD simulation of the entire device. Static and dynamic pull-in, pull-in time and fundamental frequency analysis is presented for fixed-fixed and cantilever carbon nanotube switches.

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Secondary phases, either introduced by alloying or heat treatment, are commonly 31 present in most high-entropy alloys (HEAs). Understanding the formation of secondary 32 phases at high temperatures, and their effect on mechanical properties, is a critical issue 33 that is undertaken in the present study, using the Al x CoCrFeNi (x = 0.3, 0.5, and 0.7) as 34 a model alloy. The in-situ transmission-electron-microscopy (TEM) heating observation, 35 an atom-probe-tomography (APT) study for the reference starting materials (Al 0.3 and 36 Al 0.5 alloys), and thermodynamic calculations for all three alloys, are performed to 37 investigate (1) the aluminum effect on the secondary-phase fractions, (2) the 38 annealing-twinning formation in the face-centered-cubic (FCC) matrix, (3) the 39 strengthening effect of the secondary ordered body-centered-cubic (B2) phase, and (4) 40 the nucleation path of the secondary phase thoroughly. The present work will 41 substantially optimize the alloy design of HEAs and facilitate applications of HEAs to a 42 wide temperature range.