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1.
We consider a space-time coded (STC) orthogonal frequency-division multiplexing (OFDM) system with multiple transmitter and receiver antennas over correlated frequency- and time-selective fading channels. It is shown that the product of the time-selectivity order and the frequency-selectivity order is a key parameter to characterize the outage capacity of the correlated fading channel. It is also observed that STCs with large effective lengths and ideal built-in interleavers are more effective in exploiting the natural diversity in multiple-antenna correlated fading channels. We then propose a low-density parity-check (LDPC)-code-based STC-OFDM system. Compared with the conventional space-time trellis code (STTC), the LDPC-based STC can significantly improve the system performance by exploiting both the spatial diversity and the selective-fading diversity in wireless channels. Compared with the previously proposed turbo-code-based STC scheme, LDPC-based STC exhibits lower receiver complexity and more flexible scalability. We also consider receiver design for LDPC-based STC-OFDM systems in unknown fast fading channels and propose a novel turbo receiver employing a maximum a posteriori expectation-maximization (MAP-EM) demodulator and a soft LDPC decoder, which can significantly reduce the error floor in fast fading channels with a modest computational complexity. With such a turbo receiver, the proposed LDPC-based STC-OFDM system is a promising solution to highly efficient data transmission over selective-fading mobile wireless channels 相似文献
2.
Inspired by the recent work [HHM03], we prove two stability results for compact Riemannian manifolds with nonzero parallel spinors. Our first result says that Ricci flat metrics which also admit nonzero parallel spinors are stable (in the direction of changes in conformal structures) as the critical points of the total scalar curvature functional. Our second result, which is a local version of the first one, shows that any metric of positive scalar curvature cannot lie too close to a metric with nonzero parallel spinor. We also prove a rigidity result for special holonomy metrics. In the case of SU(m) holonomy, the rigidity result implies that scalar flat deformations of Calabi-Yau metric must be Calabi-Yau. Finally we explore the connection with a positive mass theorem of [D03], which presents another approach to proving these stability and rigidity results. Dedicated to Jeff Cheeger for his sixtieth birthday 相似文献
3.
LILin LIUWen-li LUBin JUGuo-xian ZHANGYong-ming HAOYong-qin SUWei ZHONGJing-chang 《半导体光子学与技术》2004,10(3):179-181,193
Spectral and structural characteristics of distributed Bragg reflector (DBR) in vertical-cavity surface-emitting lasers were studied with photoluminescence and double- crystal X- ray diffraction measurement. The expected high quality epitaxial DBR structure was verified. In the X- ray double- crystal rocking curves of DBR the zeroth- order peak, the first and second order satellite peaks were measured. Splitting of diffraction peak appeared in the rocking curves was analyzed. The effects of introduced deep energy levels on the structural perfection and optical properties were discussed. 相似文献
4.
A reliable method is presented for the chiral separation of three basic drugs (mexiletine, chlorpheniramine and propranolol) with serum albumins (human and porcine, HSA and PSA) as chiral selectors by capillary electrophoresis in combination with the partial filling technique. Based on the systematic optimization of operation variables, the chiral separation of mexiletine, chlorpheniramine and propranolol was achieved in the pH 7.4 phosphate buffer by using HSA, PSA and PSA as selectors, respectively. The chiral recognition ability of HSA and PSA was compared. HSA and PSA show a different chiral recognition ability for each of the three drugs. In addition, the association constants between enantiomeric drugs and proteins were determined to be 2.00 and 3.80 x 10(2) M(-1) for mexiletine and HSA, 0.59 and 1.12 x 10(3) M(-1) for chlorpheniramine and PSA, and 0.87 and 1.42 x 10(3) M(-1) for propranolol and PSA. The method for the chiral separation and determination of association constants possesses the advantages of simple performance, effective avoiding of the interference of the UV detection from protein, and lowering of the reagent consumption. 相似文献
5.
Xiaodong Hu 《应用数学学报(英文版)》1993,9(2):97-103
Since the point-to-set maps were introduced by Zangwill[1] in the study of conceptual algorithms, various sufficient conditions for the algorithms to be of global convergence have been established. In this paper, the relations among all these conditions are illustrated by a unified approach; still more, unlike the sufficient conditions previously given in the literature, a new necessary condition is put forward at the end of the paper, so that it implies more applications. 相似文献
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8.
本文描述了用膜吸收法测量激光等离子体辐射温度空间分布的原理和方法给出了柱形缝靶轴向辐射温度随空间位置变化的特征,对测量结果进行了分析讨论。 相似文献
9.
稳态锁模产生4ps激光脉冲 总被引:1,自引:0,他引:1
首次利用GaAs光电导开关,控制Nd:YLF激光器腔内Q值,实现稳态锁模,获得脉宽和能量稳定性极高的4ps激光脉冲。 相似文献
10.
Xiaodong Xu Pingxin Song Guoqing Zhou Jun Xu Peizhen Deng 《Solid State Communications》2004,129(11):717-720
Cr4+:YbAG (Cr4+:Yb3Al5O12) crystal with a size up to Φ24 mm×30 mm was grown by the Czochralski method. In the absorption spectrum, there are two absorption bands at 939 and 969 nm, respectively, which are suitable for InGaAs diode laser pumping, and there is an absorption band at 1030 nm, which is suitable for passive Q-switched laser output at 1.03 μm. A broad emission spectrum from 970 to 1100 nm was exhibited from 940 nm wavelength pumping. This crystal is promising as a self-Q-switched laser crystal used for compact, efficient, highly stable, passive self-Q-switched thin chip solid-state lasers. 相似文献