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981.
982.
Equal channel angular pressing (ECAP) has been conducted on as-cast Mg-4.3 wt.%Zn-0.7 wt.%Y Mg alloy containing quasicrystal phase at a temperature of 623 K. After 8 ECAP passes, the grain size of the as-cast alloy is decreased from ∼ 120 to ∼ 3.5 μm, and the coarse eutectic quasicrystal phases are broken and dispersed in the alloy. Tensile testing has been performed on the ECAPed Mg-Zn-Y alloy at temperatures of 523 K and 623 K with initial strain rates from 1.5 × 10− 3 to 1.5 × 10− 4 s− 1. The ECAPed alloy exhibits a maximum elongation of about 600% when testing at 623 K using an initial strain rate of 1.5 × 10− 4 s− 1. Grain boundary sliding is considered to be the dominant deformation mechanism of the Mg-Zn-Y alloy in the temperature and strain-rate range investigated. 相似文献
983.
984.
信道信息隐藏中秘密信息的预处理研究 总被引:3,自引:0,他引:3
文章研究了基于信道编码的信息隐藏技术,提出了新的秘密信息预处理方法—采用RS码M公钥密码体制对秘密信息进行编码。理论分析和实际计算表明,该方法能够极大地提高秘密信息的安全性。 相似文献
985.
986.
Introducing multiple-input multiple-output (MIMO) relay channel could offer significant capacity gain.And it is of great importance to develop effective power allocation strategies to achieve power efficiency and improve channel capacity in amplify-and-forward relay system.This article investigates a two-hop MIMO relay system with multiple antennas in relay node (RN) and receiver (RX).Maximizing capacity with antenna selection (MCAS) and maximizing capacity with eigen-decomposition (MCED) schemes are proposed to efficiently allocate power among antennas in RN under first and second hop limited scenarios.The analysis and simulation results show that both MCED and MCAS can improve the channel capacity compared with uniform power allocation (UPA) scheme in most of the studied areas.The MCAS bears comparison with MCED with an acceptable capacity loss, but lowers the complexity by saving channel state information (CSI) feedback to the transmitter (TX).Moreover, when the RN is close to RX, the performance of UPA is also close to the upper bound as the performance of first hop is limited. 相似文献
987.
The orthogonal frequency division multiplexing (OFDM) transmission scheme is currently experiencing increased popularity due
to advances in very large scale integration technology. It is used for a variety of broadband systems such as asymmetric digital
subscriber lines, very-high-speed digital subscriber lines, digital video, and audio broadcasting, and wide local area network
standards such as IEEE 802.11a, IEEE 802.11 g, and ETSI Hiperlan/2. However, propagation impairments can cause severe degradation
in bit error rates (BER) for coherent detection. We derive a semi-analytical method to evaluate BER of a quadrature phase
shift keying (QPSK)-OFDM system in Nakagami, m < 1 fading and additive noise where pilot-assisted linear channel estimation and channel equalization are used. This allows
modeling of more severe fading environments than can be depicted by a Rayleigh distribution. Numerical simulations are used
to validate the proposed methods. The techniques developed can be applied to other channel estimation techniques, modulation
schemes and the performance evaluation of equalized single carrier narrowband systems. 相似文献
988.
Spectral Efficiency Evaluation for MRC Diversity Schemes Over Generalized Rician Fading Channels 总被引:1,自引:0,他引:1
Vidhyacharan Bhaskar 《International Journal of Wireless Information Networks》2007,14(3):209-223
In this paper, closed-form expressions for the capacities per unit bandwidth for generalized Rician fading channels are derived
for power and rate adaptation, constant transmit power, channel inversion with fixed rate, and truncated channel inversion
adaptation policies. The closed-form solutions are derived for the single antenna reception (without diversity combining)
and maximal-ratio combining (MRC) diversity cases. Truncated channel inversion adaptation policy is the best policy for the
single antenna reception case, while the channel inversion with fixed rate policy is the best policy for the MRC diversity
case. Constant transmit power policy provides the lowest spectral efficiency as compared to the other policies with and without
diversity. 相似文献
989.
Matthias Pätzold Bjørn Olav Hogstad Dongwoo Kim 《Wireless Personal Communications》2007,40(3):267-279
In this paper, we introduce a new technique for the design of high-performance Rayleigh fading channel simulators. The proposed
design method uses set partitioning – a technique, which plays a key role in the design of trellis-coded modulation schemes.
We show how set partitioning can be used to design multiple uncorrelated fading waveforms enabling the simulation of Rayleigh
fading channels. For the important case of isotropic scattering, we show that the sample average of the generated waveforms
results in a deterministic process, the autocorrelation function (ACF) of which tends to the zeroth-order Bessel function
of the first kind as the number of sample functions increases. The proposed procedure is completely deterministic. The comparison
with a stochastic procedure using Monte Carlo techniques will be made. A study of the performance shows clearly that the new
technique using set partitioning outperforms by far existing Monte Carlo methods. 相似文献
990.
提出一种使用非最大抽取有限冲激响应(Finite Impulse Response,FIR)多率滤波器组结构对信道进行盲估计的方法。该方法对未知信道的特性不敏感,对于零点位于单位圆上的信道及非最小相位信道都可以进行准确的估计。在两种典型信道下对该方法进行了仿真,验证了该方法的性能。 相似文献