Combination of PAPR reduction and linearization for the OFDM communication system

Orthogonal frequency division multiplexing (OFDM) has been widely used in many kinds of communication systems. However, OFDM signal has serious problem of high peak‐to‐average‐power ratio (PAPR) due to so many sub‐carriers. So, OFDM signal has very wide dynamic range. Therefore, the bit error rate (BER) performance may be degraded because of the nonlinear devices like the high power amplifier (HPA). Even if the linearization and large back‐off are used to compensate for the HPA nonlinearity, the power efficiency of the HPA is still very low since the PAPR is very high. Therefore, the PAPR reduction of the OFDM signal before the linearization would be more reasonable to improve the power efficiency and nonlinear compensation at the same time. In this paper, we propose a new combined method of SPW (sub‐block phase weighting) for PAPR reduction and linearization technique for the improvement of the power efficiency and for the nonlinear compensation of HPA. An updated SPW method is proposed to use a novel weighting factor multiplication of the complementary sequence characteristic and PAPR threshold technique. From the simulation results, it can be confirmed that BER performance is significantly improved and out‐of‐band spectrum radiations are much mitigated. Power efficiency of HPA can be enhanced since we can set small IBO (input back‐off) due to the PAPR reduction. The proposed system shows about 3 and 1 dB performance improvement than the LCP (linearized constant peak‐power)‐OFDM and LCP‐OFDM plus SPW at BER = 10⁻⁴. Copyright © 2010 John Wiley & Sons, Ltd.     

2007～2008年世界塑料工业进展

收集了2007年7月～2008年6月世界塑料工业的相关资料,介绍了2007～2008年国外塑料工业的发展情况,提供了世界塑料产量、消费量及全球各类树脂的需求量及产能情况.按通用热塑性树脂(聚乙烯、聚丙烯、聚苯乙烯、聚氯乙烯、ABS树脂)、工程塑料(尼龙、聚碳酸酯、聚甲醛、热塑性聚酯、聚苯醚)、特种工程塑料(聚苯·硫醚、液晶聚合物、聚醚醚酮)、通用热固性树脂(酚醛、聚氨酯、不饱和聚酯树脂、环氧树脂)不同品种的顺序,对树脂的产量、消费量、供需状况及合成工艺、产品应用开发、树脂品种的延伸及应用的进一步扩展等技术作了详细介绍.     

创新为世界高性能纤维带来勃勃生机

陈述了国内外近期聚丙烯腈基碳纤维、对位芳酰胺及其共聚纤维、超强聚乙烯纤维、玄武岩纤维、液晶聚芳酯纤维及沥青基碳纤维生产厂家的研发、创新和扩产情况,显示了这些高性能纤维对全球开发可再生新能源、节能减排和各种产品的更新换代有重要作用,因此在当前世界经济不景气的形势下,通过不断革新生产工艺、提高生产效率和产品质量、降低生产成本、开发新品种及发展下游复合材料等制品,仍可保持两位数的需求增长。中国的主要高性能纤维正在实现初步产业化,但有些性能档次较低、性能稳定性稍差,远未形成系列化产品,与国外同类产品相比差距较大,今后应进一步加大研发投入,通过不断创新发展自己的知识产权,才能逐步缩小差距,提高产品的国际竞争力。     

Iterative LCP solvers for non‐local loading–unloading conditions

This paper deals with the finite element analysis of a certain class of non‐local dissipative constitutive models, where the canonical pointwise backward‐Euler scheme cannot be employed for satisfying the loading–unloading conditions. In the presence of a non‐local dissipation, the admissibility conditions in a point depend on the inelastic strain increment of the surrounding points and can be cast as a linear complementarity problem (LCP) involving all Gauss points of the process zone. In order to actually solve the LCP, the use of iterative algorithms that can be easily embodied into existing FE codes is discussed. The performance of the proposed algorithms is tested in 1D and 2D examples for both elastoplastic and damaging materials. Copyright © 2003 John Wiley & Sons, Ltd.     

连续光焦度透镜的设计研究

本文介绍了用能量守恒原理设计连续光信度透镜的方法并分析其特点，提出三种可行的结构形式，给出了设计实例及模拟结果。     

Effect of mesogenic crosslinking units on the mesomorphic phases of cholesteric liquid crystalline polymers

A series of cyclosiloxane‐based cholesteric liquid crystalline polymers were synthesized from cholest‐5‐en‐3‐ol (3β)‐4‐(2‐propenyloxy)benzoate and a nematic crosslinking monomer biphenyl‐4,4′‐diol bis[4‐(2‐propenyloxy)‐benzoate], containing mesogenic crosslinking contents among 0–7.8 wt %. Swelling experiments of the elastomers were employed to determine the effective crosslink density (M_c). With increase of mesogenic crosslinking component in the polymers, the isotropic temperature decreases, but both the temperature of glass transition and melting point change irregularly. All the polymers have high thermal stability and percentage weight loss occurred at 460°C decreases with increase of crosslinking units. The intensity of X‐ray diffraction peak at 2θ ≈ 17° decreases with increase of mesogenic crosslinking units, suggesting that the order between two neighbor liquid crystalline molecules disturbed by the mesogenic crosslinking agents. The maximum reflection bands shift slightly to long wavelength and become broad, indicating that the helical structure is partially disrupted because of both the constraint of chemical crosslinking agents and the different mesogenic units. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1161–1168, 2007     

Influence of the monomer feeding sequence on the structure and properties of thermotropic liquid‐crystalline poly(ester imide)s

A series of poly(ester imide)s mainly derived from N,N′‐hexane‐1,6‐diylbistrimellitimides, 4,4′‐dihydroxybenzophenone, and p‐hydroxybenzoic acid were synthesized by a direct polycondensation method in benzenesulfonyl chloride, N,N′‐dimethylformamide, and pyridine with different monomer feeding sequences. The molecular structures and properties of the resultant poly(ester imide)s were characterized with NMR, IR spectrometry, polarized light microscopy, wide‐angle X‐ray diffraction, differential scanning calorimetry, and thermogravimetric analysis. The results showed that the monomer feeding sequences had a great effect on the sequential structure of the molecular chains of the copolymers and consequently on their liquid‐crystalline (LC) properties, fiber‐forming capability, and other properties. Thus, it is probable that one could obtain an LC poly(ester imide) with given properties by controlling the monomer feeding sequence during the polycondensation process. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

液晶材料改性聚四氟乙烯的开发与应用

综述了液晶材料改性聚四氟乙烯(PTFE)的性能特点及国内外的研究应用情况,并对该类产品(特别是聚苯酯改性聚四氟乙烯制品)的市场情况进行分析和预测.介绍了液晶材料改性聚四氟乙烯的作用机理、成型工艺、配比选择、产品特点等.     

Influence of type of compatibilizer on the rheological and mechanical behavior of LCP/TP blends under different stationary and nonstationary shear conditions

The addition of small amounts of liquid‐crystalline polymers to thermoplastics leads to the formation of in situ–reinforced materials, with improved processability and mechanical properties. Nevertheless, the lack of adhesion between the thermoplastic and the liquid‐crystalline polymer often occurs, thus requiring the use of compatibilizers. In this case, the results of several previous works show that there is an improvement of strength, usually accompanied by a decrease of toughness and, thus, the interest of LCP/TP blends for industrial applications will certainly increase if both strength and toughness are obtained. Additionally, the emphasis of previous studies has been on the evaluation of the properties of the blend under stationary conditions and not under non‐stationary ones, which are, in fact, those most relevant to processing sequences. Thus, the present work focuses on the influence of type of compatibilizer on the mechanical and rheological properties of polypropylene/LCP blends under nonstationary conditions. In terms of mechanical properties, the traditional increase of tensile strength was obtained for all compatibilizers, which was essentially due to the formation, during processing, of thinner and longer fibrils of LCP dispersed in the matrix than those observed for the noncompatibilized blends. Additionally, an improvement of the impact strength and flexural modulus was also observed for the blend in which a compatibilizer with an elastomeric nature was used. Rheologically, the experiments most sensitive to the structure were those performed in transient shear, with an increase of the transient stress (in the form of an overshoot) of different magnitudes being observed for the different compatibilizers. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 694–703, 2005     

Thermotropic liquid‐crystalline polymers: Synthesis,characterization, and properties of poly(azomethine esters)

A series of poly(azomethine ester) copolymers were synthesized by the solution polycondensation method with different diamines. The synthesized polymers were characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry (DSC), hot‐stage polarized microscopy, wide‐angle X‐ray diffraction, and solution viscosity. All polymers showed good thermal stability. The thermotropic liquid‐crystalline properties were examined by DSC and by microscopic observations. Except for one, all of the polymers showed nematic liquid‐crystalline behavior. The effects of temperature on crystallinity and the substituent on solubility, thermal stability, melting temperature, and viscosity were also studied. The voluminosity and shape factor were also computed from the viscosity data. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 153–160, 2003