A novel frequency offset estimation method for OFDM systems with large estimation range

In this paper, we proposed a novel frequency synchronization method which has larger estimation range than conventional method proposed by Tufvesson. A two steps frequency offset estimation method is performed in the new method. In the first step the received signal is correlated with local training sequence to eliminate the influence of training sequence which makes the estimation range of frequency offset is independent of period of the repeated data in the training sequence. The received signal is correlated with its delay in the second step to obtain the fine estimation of frequency offset. Precise estimation can be achieved in the proposed method than Tufvesson's method. The period of the repeated data in the training sequence determines the estimation range of the frequency offset in the conventional method, while in the new method they are irrelevant. Tens of subcarriers spacing can be estimated by the new method. The new method can be used in other cases as long as a known sequence is transmitted, which will introduce valuable flexibility in training sequence design. The validity of the algorithm is verified in AWGN channel and multipath fading channel.     

A Method to Reduce Sampling Rate of the ADC in Feedback Channel for Wideband Digital Predistortion

This paper presents a new digital predistortion (DPD) solution for wideband signals with low feedback sampling rate. To reduce the minimum sampling rate of the analog-to-digital converter (ADC) for wideband digital predistortion, the proposed method uses a bandpass filter to form a narrowband signal before the ADC. Then, a deconvolution operation is performed to recover the original wideband signal from the ADC samples. The proposed method is evaluated with an international mobile telecommunication-advanced signal with 100 MHz bandwidth. The simulation results show that the recovered signal of the proposed method closely approximates to the original signal in the passband of the filter, and the mean square error of the deconvolution decreases as the signal-to-noise ratio increases. The proposed algorithm can reduce the sampling rate of the ADC from 1105.92 million samples per second (MSPS) to 368.64 MSPS, and improve the adjacent channel power ratio more than 20 dB, which is merely 5.6 dB less than the conventional DPD with 1105.92 MSPS sampling rate.     

热作模具修复层表面滚压加工研究进展

热作模具因工作环境恶劣,在服役过程中易出现失效,造成资源浪费和成本升高。热作模具修复工艺可以延长其使用寿命、提高生产效率,修复层需经加工才能在表面质量和尺寸精度上满足使用要求。滚压加工是一种经济高效的光整强化工艺,通过滚压工艺可以提高热作模具修复层力学性能和使用性能,对热作模具修复层滚压加工的研究逐渐成为当前的研究热点。介绍了热作模具常见的失效形式和性能要求,从修复预处理、修复方法、修复后处理三方面概述了热作模具修复工艺研究现状,综述了修复层滚压加工研究现状,分析了传统滚压存在的问题,对滚压工艺的发展和热作模具超声滚压研究进展进行了介绍。在此基础上,对热作模具修复层的超声滚压加工研究进行了展望。     

基于多传感器的机器人夹取系统研究

为解决由于内置传感器较单一导致的机器人抓取动作不灵活、作业精度低等问题,提出一种基于多传感器的机器人夹取系统。首先采用扩展卡尔曼滤波算法融合机器人内置传感器所测量的位置、速度和角度等信息,实现机器人自身位姿检测。在此前提下,利用外置传感器完成对目标物的自动识别与定位。为增强信息的可靠性和系统的分辨能力,并根据输出信号对机器人夹取动作进行运动学建模,设定夹取系统的硬件、软件环境及传输电路后完成系统设计全过程。实验结果证明：所提方法设计的机器人系统能够实现对夹取目标的精确对准,运动特性明显提高,作业误差有效减小。     

La2O3改性树脂基制动材料的摩擦学性能

通过定速摩擦试验、CHASE摩擦试验及磨损表面形貌观察等方法探讨La₂O₃含量对稀土La₂O₃改性树脂基制动材料的摩擦磨损性能、抗热衰退性能与恢复性能的影响。定速摩擦试验结果表明,制动材料中添加适量La₂O₃可有效提高其摩擦因数,降低其磨损率,同时还可增加其摩擦因数的稳定性;其中,添加20% La₂O₃试样的综合摩擦学性能为最优。CHASE摩擦试验结果表明,La₂O₃的加入可有效提高复合材料的抗热衰退性能与恢复性能。     

Special effect of epoxy resin E-44 on compatibility and mechanical properties of poly(butylene terephthalate)/polyamide-6 blends

An attempt was made to modify the properties of poly(butylene terephthalate) (PBT) by blending it with polyamide-6 (PA-6). Since PBT and PA-6 are incompatible, epoxy resin was used as a compatibilizer to form an alloy. Alloys of PBT and PA-6 with varying amounts (0–12%) of epoxy resin E-44 were prepared by melt blending. The notched Izod impact strength and flexural strength as a function of epoxy resin E-44 content were studied. Ultimate mechanical properties showed significant improvement on addition of epoxy resin E-44. The maximum increase of the notched Izod impact strength (≈600%) of PBT/PA-6 blends is obtained at 3% (weight) epoxy resin E-44 content. The impact fracture surfaces were studied using scanning electron microscopy (SEM): The SEM micrographs showed a noticeable change in the type of surface structure on adding epoxy resin E-44. DMTA also showed improved compatibility between PBT and PA-6 on adding epoxy resin E-44. DSC studies showed that the presence of epoxy resin E-44 hindered the crystallization of both PBT and PA-6 in the alloys. Wide-angle X-ray diffraction (WAXD) showed no obvious difference on crystallinity of PBT and PA-6 in the alloys with the presence of a small amount of epoxy resin E-44. © 1996 John Wiley & Sons, Inc.     

电路板复合材料微小孔加工技术

介绍树脂基复合材料印刷电路板的机械钻削和激光钻削微小孔加工技术,分析了影响钻削质量的因素以及加工过程中易出现的各种缺陷和改进措施。机械钻削微小孔时,轴向力和切削扭矩是导致各种加工缺陷的主要因素,低进给量、高主轴转速可明显提高钻削质量。激光钻削时,选择合适的激光功率及减少激光照射时间可提高钻削质量。     

PEEK基自润滑复合材料的摩擦学研究和应用

聚醚醚酮(PEEK)基自润滑复合材料具有摩擦因数低、耐磨性好等特点,可以在无润滑、高温、潮湿、污染、腐蚀等恶劣环境下使用。本文综述了对聚醚醚酮(PEEK)基复合材料摩擦学研究的新进展,讨论了纤维增强、无机填充、有机共混改性以及温度、对偶和介质、固体润滑剂等对PEEK基复合材料摩擦学性能的影响,介绍了PEEK基复合材料在人工关节假体材料方面的应用,指出PEEK摩擦复合材料今后的研究方向,以期扩大PEEK复合材料的摩擦学工程应用。     

碳纤维复合材料铣削加工研究进展

介绍碳纤维增强树脂基复合材料的特点、应用现状和加工难度,综述近10年碳纤维增强树脂基复合材料铣削机理(刀具磨损、切削力、表面质量、切屑形成机理)的研究进展,并讨论其将来的研究趋势。     

Estimation of carrier frequency offsets for MIMO systems with distributed transmit antennas

The problem of estimating the carrier frequency offsets in Multiple-Input Multiple-Output (MIMO) systems with distributed transmit antennas is addressed. It is supposed that the transmit antennas are distributed while the receive antennas are still centralized, and the general case where both the time delays and the frequency offsets are possibly different for each transmit antenna is considered. The channel is supposed to be frequency flat, and the macroscopic fading is also taken into consideration. A carrier frequency offset estimator based on Maximum Likelihood (ML) is proposed, which can separately estimate the frequency offset for each transmit antenna and exploit the spatial diversity. The Cramer-Rao Bound (CRB) for synchronous MIMO (i.e., the time delays for each transmit antenna are all equal) is also derived. Simulation results are given to illustrate the per- formance of the estimator and compare it with the CRB. It is shown that the estimator can provide satisfactory frequency offset estimates and its performance is close to the CRB for the Signal-to-Noise Ratio (SNR) below 20dB.