基于Chirp信号跳频调制的超宽带通信系统设计

目前在超宽带通信系统中,普遍采用的脉冲波形是高斯脉冲波形。用Chirp信号来代替高斯脉冲信号,由于正向和反向线性调频脉冲具有良好的自相关性以及Chirp信号可以用声表面波器件实现,具有系统易于实现,功耗低,复杂度低等特点,同时,跳频通信比较隐蔽难以被截获,具有良好的抗干扰能力。     

Residual Synchronization Error Elimination in OFDM Baseband Receivers

It is well known that an OFDM receiver is vulnerable to synchronization errors. Despite fine estimations used in the initial acquisition, there are still residual synchronization errors. Though these errors are very small, they severely degrade the bit error rate (BER) performance. In this paper, we propose a residual error elimination scheme for the digital OFDM baseband receiver aiming to improve the overall BER performance. Three improvements on existing schemes are made: a pilot‐aided recursive algorithm for joint estimation of the residual carrier frequency and sampling time offsets; a delay‐based timing error correction technique, which smoothly adjusts the incoming data stream without resampling disturbance; and a decision‐directed channel gain update algorithm based on recursive least‐squares criterion, which offers faster convergence and smaller error than the least‐mean‐squares algorithms. Simulation results show that the proposed scheme works well in the multipath channel, and its performance is close to that of an OFDM system with perfect synchronization parameters.     

A 60 GHz Medium Power Amplifier for Radio‐over‐Fiber System

We present the design and fabrication of a 60 GHz medium power amplifier monolithic microwave integrated circuit with excellent gain‐flatness for a 60 GHz radio‐over‐fiber system. The circuit has a 4‐stage structure using microstrip coupled lines instead of metal‐insulator‐metal capacitors for unconditional stability of the amplifier and yield enhancement. The gains of each stage of the amplifier are modified to provide broadband characteristics of input/output matching for the first and fourth stages and to achieve higher gains for the second and third stages to improve the gain‐flatness of the amplifier for wideband.     

Effect of Photo-Induced Periodic Gain Structure on Oscillation Power and Tunability in Polymer Dye Laser

Pumping dye-doped organic polymer by interference pattern of a frequency-doubled Nd:YAG 532 nm laser to induce a periodic gain-structure and confining it within a stable optical resonator, the oscillation output can be effectively enhanced in comparison with either that of uniform irradiation, i.e., pumping, or of distributed feedback (DFB) operation, and the oscillation wavelength becomes tunable by adjusting the period of the interference pattern. Using Rhodamine-B as a dopant over the range from 200 to 1000 ppm, the oscillation output was enhanced by - 10 dB, which was larger than the output of the uniform irradiation and was - 30% greater than the DFB scheme. Moreover, changing the period of the interference pattern by adjusting the setting angle of the Koster prism enables us to vary the oscillation wavelength around 600 nm.     

Sequencing games with repeated players

Two classes of one machine sequencing situations are considered in which each job corresponds to exactly one player but a player may have more than one job to be processed, so called RP(repeated player) sequencing situations. In max-RP sequencing situations it is assumed that each player’s cost function is linear with respect to the maximum completion time of his jobs, whereas in min-RP sequencing situations the cost functions are linear with respect to the minimum completion times. For both classes, following explicit procedures to go from the initial processing order to an optimal order for the coalition of all players, equal gain splitting rules are defined. It is shown that these rules lead to core elements of the associated RP sequencing games. Moreover, it is seen that min-RP sequencing games are convex. We thank two referees for their valuable suggestions for improvement. Financial support for P. Calleja has been given by the Ministerio de Educación y Ciencia and FEDER under grant SEJ2005-02443/ECON, and by the Generalitat de Catalunya through a BE grant from AGAUR and grant 2005SGR00984.     

The use of phase codes in ultrasound imaging: SNR gain and bandwidth requirements

Most of the literature on coded excitation describes the signal-to-noise ratio gain of a coded waveform in terms of the time-bandwidth product. We have shown that in the context of ultrasound imaging, the expression for the SNR gain provided by matched filtering a coded waveform, can be reduced to the total number of chips in the transmit signal. Hence, the SNR gain is independent of both the bandwidth and the duration of a single-chip. This concept is described in detail, clarifying this seeming contradiction. The impact of bandwidth and pulse duration on the SNR, SNR gain and axial resolution is described. Bandwidth requirements and the impact of regulatory peak-power limitations are also addressed.     

Extended Bose-Hubbard model with pair hopping on triangular lattice

We study systematically an extended Bose-Hubbard model on the triangular lattice by means of a meanfield method based on the Gutzwiller ansatz. Pair hopping terms are explicitly included and a three-body constraint is applied. The zero-temperature phase diagram and a variety of quantum phase transitions are investigated in great detail. In particular, we show the existence and the stability of the pair supersolid phase.     

光折变晶体中ps激光脉冲二波耦合增益正负转换现象

用二列脉宽为60ps的连续锁模Nd:YAG激光的倍频光在掺Ce的KNSBN晶体中进行二波耦合.当泵浦光脉冲迟于信号光脉冲到达晶体,或泵浦光脉冲虽先于信号光脉冲到达晶体.但时间提前量小于激光脉冲半高宽时.信号光的二波耦合平均增益是非负的;当泵浦光脉冲先于信号光脉冲到达晶体且时间提前量大于激光脉冲半高宽时.信号光的二波耦合平均增益可变为负的.对此给出物理解释.     

半导体单量子点增益与吸收特性的研究

采用密度矩阵的方法进行数值模拟计算紧束缚态的半导体单量子点的增益与吸收行为.其动力学数值结果显示,在某些不同时刻的增益谷和吸收峰的左右分布保持不变对称性,且有上下镜像反转的现象发生;在在长时间演化中发现该量子点系统的增益与吸收发生激烈的振荡而呈现出了量子光学中典型的崩塌与复苏的有趣物理现象.     

一种支持分级服务和最小带宽保证的改进TCP算法

本文提出了一种支持分级服务和最小带宽保证的改进TCP算法－DSAS－TCP，通过对TCP算中慢启动阈值和最小拥塞窗口的分级设置来使不同等级连接的平均吞吐量产生分级，并保证用户的最小带宽需求。通过理论分析和仿真实验验证了该算法的有效性。