Shallow underwater acoustic (UWA) channel exhibits rapid temporal variations, extensive multipath spreads, and severe frequency-dependent attenuations. So, high data rate communication with high spectral efficiency in this challenging medium requires efficient system design. Multiple-input multiple-output orthogonal frequency-division multiplexing (MIMO–OFDM) is a promising solution for reliable transmission over highly dispersive channels. In this paper, we study the equalization of shallow UWA channels when a MIMO–OFDM transmission scheme is used. We address simultaneously the long multipath spread and rapid temporal variations of the channel. These features lead to interblock interference (IBI) along with intercarrier interference (ICI), thereby degrading the system performance. We describe the underwater channel using a general basis expansion model (BEM), and propose time-domain block equalization techniques to jointly eliminate the IBI and ICI. The block equalizers are derived based on minimum mean-square error and zero-forcing criteria. We also develop a novel approach to design two time-domain per-tone equalizers, which minimize bit error rate or mean-square error in each subcarrier. We simulate a typical shallow UWA channel to demonstrate the desirable performance of the proposed equalization techniques in Rayleigh and Rician fading channels. 相似文献
Channel estimation is an essential part of many detection techniques proposed for data transmission over fading channels. For the frequency selective Rayleigh fading channel an autoregressive moving average representation is proposed based on the fading model parameters. The parameters of this representation are determined based on the fading channel characteristics, making it possible to employ the Kalman filter as the best estimator for the channel impulse response. For IS-136 formatted data transmission the Kalman filter is employed with the Viterbi algorithm in a Per-Survivor Processing (PSP) fashion and the ove rall bit error rate performance is shown to be superior to that of detection techniques using the RLS and LMS estimators. To allow more than one channel estimation per symbol interval, Per-Branch Processing (PBP) method is introduced as a general case of PSP and its effect on performance is evaluated. The sensitivity of performance to parameters such as fading model order and vehicle speed is also studied. 相似文献
In this paper, the energy efficiency (EE) of a decode and forward (DF) relay system is studied, where two sources communicate through a half-duplex relay node in one-way and two-way relaying strategies. Both the circuitry power and the transmission power of all nodes are taken into consideration. In addition, three different coding schemes for two-way DF relaying strategy with two phases and two-way DF relaying with three phases are considered. The aim is to maximize the EE of the system for a constant spectral efficiency (SE). For this purpose, the transmission time and the transmission power of each node are optimized. Simulations are used to compare the EE–SE curve of different DF strategies with one-way and two-way amplify and forward (AF) strategies and direct transmission (DT), to find the best energy efficient strategy in different SE conditions. Analytical and simulation results demonstrate that in low SE conditions, DF relaying strategies are more energy efficient compared to that of AF strategies and DT. However, in high SE conditions, the EE of two-way AF relaying and DT strategy outperform some of the DF relaying strategies. In simulations, the impact of different circuitry power and different channel conditions on the EE–SE curves are also investigated. 相似文献
In this study, the strain rate effects on transverse tensile and compressive properties of unidirectional Glass fiber reinforced polymeric composites are investigated. To demonstrate strain rate effects, the tensile and compressive composite specimens with identical configuration are fabricated and tested to failure in the transverse direction at quasi-static strain rate of approximately 0.001 s−1 and intermediate strain rates of 1–100 s−1. The tensile and compressive tests are performed using a servo-hydraulic testing apparatus equipped with strain rate increasing mechanisms. For performing the practical tests, a jig and a fixture and other test supplies are designed and manufactured. The performance of the test jig is evaluated and showed that it is adequate for composites testing under tension and compression loads. The effects of strain rate on mechanical properties (maximum strength, modulus, and strain to failure) are considered. The characteristic results for the transverse properties indicate that damage evolution is strain-rate-dependent for the examined material. Also, a strain-rate-dependent empirical material model associated with different regression constants is proposed based on the experimental results obtained to characterize the rate dependent behavior of Glass/Epoxy composite material. 相似文献
In this paper, size-dependent dynamic stability of axially loaded functionally graded (FG) composite truncated conical microshells with magnetostrictive facesheets surrounded by nonlinear viscoelastic foundations including a two-parameter Winkler–Pasternak medium augmented via a Kelvin–Voigt viscoelastic approach is analyzed considering nonlinear cubic stiffness. To this purpose, von Karman-type kinematic nonlinearity along with modified couple stress theory of elasticity was applied to third-order shear deformation conical shell theory in the presence of magnetic permeability tensor and magnetic fluxes. The numerical technique of generalized differential quadrature (GDQ) was used for the solution of microstructural-dependent dynamic stability responses of FG composite truncated conical microshells. It was seen that moving from prebuckling to postbuckling domain somehow increased the significance of couple stress type of size dependency on frequency. In addition, within both prebuckling and postbuckling regimes, an increase of material gradient index decreased the importance of couple stress type of size dependency on the frequency of an axially loaded FG composite truncated conical microshell. Furthermore, it was revealed that by applying a positive magnetic field to an axially loaded truncated conical microshell with magnetostrictive facesheets, its frequency at a specific axial load value was increased in prebuckling domain and decreased in postbuckling domain. However, this pattern was reversed by applying a negative magnetic field.
In the present study, the formation of superhydrophobic (SHP) structure on the surface of Mg alloy was investigated by immersion in the CuCl2 and NiSO4 solutions following by soaking in the stearic acid (SA) solution. The results revealed the presence of some stearic acid bonds on the surface of Mg alloy. The contact angle of the surface after the process measured about 151.5°, which could be due to the presence of flake-like morphology and the adsorption of hydrophobic substances of SA. X-ray diffraction pattern showed the presence of NiO as the resistant phase against the diffusion of species. Besides, the values of noise and corrosion resistance regarding SHP Mg were at least three orders of magnitude higher than that of bare Mg alloy due to the formation of SHP structure. 相似文献
Multi-walled carbon nanotubes (MWCNTs) were used in the low-viscosity, thermosetting polyester epoxy/amine resin LY-5052 with high temperature resistance to fabricate MWCNT/epoxy composites. Tensile tests of the specimens were carried out to obtain mechanical properties of MWCNT/epoxy composites for various weight-percents (wt.%) of MWCNTs. Experimental results show that the Young’s modulus and the tensile strength of the composites can be significantly improved by adding a small percentage of MWCNT. A new form of the rule of mixtures, including an exponential shape function, length efficiency parameter, orientation efficiency factor and a waviness parameter, is proposed for a more accurate prediction of the mechanical properties of MWCNT-reinforced epoxy composites, for both low and high wt.% ranges. In order to verify the suitability of the model, the ensuing predictions are compared to the available experimental data in the literature. Results demonstrate a good predictability of the modified form over a wide range of tests. 相似文献
In this paper, two element multiple input–multiple output (MIMO) meander line antenna systems with improved isolation performance and compact size are proposed and fabricated in WLAN frequency band. To increase isolation among antenna elements, a novel metamaterial spiral S-shaped resonator is embedded between two radiating elements. The proposed resonator has planar configuration and miniaturized size and is capable of blocking electromagnetic propagation between antenna elements by exhibiting negative effective permeability in the desired frequency band. To illustrate and evaluate the design process, two design samples are fabricated and tested in WLAN frequency band and the agreement among measurement and simulation results approves the design method. In the frequency range of 2.38–2.48 GHz, some MIMO communication system requirements like total active reflection coefficient, envelope correlation coefficient and capacity loss are tested on design samples which show satisfactory results, so this method can be employed in designing array antennas for small mobile communication systems. The designed MIMO antenna systems separated by 13.8 mm (less than λ/9), has better than ??40 dB isolation coefficient and near zero correlation coefficient and capacity loss at the operating frequency (2.4 GHz).