基于离散傅立叶变换扩频的广义多载波系统收发机频域简化实现方法

针对基于离散傅立叶变换扩频的广义多载波(DFT-S-GMC)系统的收发机时域实现方法复杂度较高的问题,提出了一种频域等效收发机实现方法;利用滤波器组各子带频率响应能量比原型滤波器的时域冲击响应能量集中的特性,提出了一种基于该频域实现方法的收发机简化实现方法和结构.仿真和分析表明,在多径信道下,采用频域简化实现结构的收发机与现有基于时域实现结构的收发机的误码率(BER)性能几乎相同,但与现有时域实现方法相比,采用频域简化实现方法,DFT-S-GMC系统发射机的实现复杂度可显著降低.     

钻柱声传输信号多载波调制激励分析

在随钻声遥测技术中,周期性钻柱结构产生的多重回波极易造成严重的码间干扰和较高误码率,为此,根据钻柱信道的多径传输特性,基于短钻杆条件下多载波传输的实验分析,建立了多节钻杆与管箍的周期性信道有限差分模型。考虑地面噪声和信道内多径回波干扰,利用最小均方自适应均衡,基于“4钻杆—3管箍”信道结构进行了多载波调制性能仿真分析。仿真结果表明,与单载波PSK（Phase-Shift-Keying）调制相比,OFDM（Orthogonal Frequency Division Multiplexing）多载波调制可使误码率平均降低约50%;在非周期性结构下其传输性能受信道变化敏感,但相同条件下仍优于PSK调制,且通过子载波相位补偿可有效改善误码率、提高传输性能。     

水声OFDM系统的载波间干扰抵消

本文分析了水声信道下OFDM系统的载波间干扰以及对子信道载干比的影响,并在接收端加入时变滤波器来消除载波干扰.仿真表明该方案可以明显提高子信道的载干比.     

802.20协议物理层关键技术OFDM和Flash-OFDM

就802.20协议物理层目前采用的OFDM/OFDMA技术和Flash-OFDM技术进行比较.OFDMA是一种多载波调制技术,它的原理是无线通信信道划分为多个窄带子信道同时传送,在每个了信道上使用一个子载波进行调制.Flash-OFOM技术采用FDD双工方式,上下行链路是数百个子信道组成的宽带载波,传输数据时给每个用户分配了信道.每个子信道采用了自适应调制和先进的编码技术,其频谱利用率比CDMA2000系统高3倍.Flash-OFDM在时间上以跳频方式使用OFDM的子载波,以实现信号扩频.它还有频率分集能力,减小了同一小区内的用户间干扰,同时具有OFDM和跳频扩频技术的优点,因而相邻节点可以使用相同频率的子载波.Flash-OFDM为解决小区间干扰,采用了功率控制,用户只发射能有效通信的功率.     

IEEE802.16e系统信道估计算法研究

针对IEEE802.16e系统下行链路的导频分配模式,提出了一种适用于IEEE802.16e系统下行链路的信道估计方法。该方法采用两个一维信道估计器的级联。首先在时域进行滤波,根据最近原则将前一个符号导频处的信道响应作为下一个符号相同位置子载波处的信道响应,从而得到梳状导频结构;其次在频域进行滤波,采用最大似然算法估计出所有子载波上的信道响应。车速60km/h和车速120km/h信道下的仿真结果表明,采用该信道估计方法可降低系统的误码率,提高高速移动环境下系统的性能。     

一种时变移动信道仿真方法

针对广义平稳非相干散射(WSSUS)多径时变信道,提出一种离散延迟-离散多普勒频率的"频移滤波器组"模型,并以此为基础提出一种时变多径信道仿真方法.与常用方法相比,该方法参数配置和实现简单,并且能清楚地反映信道的多径-多普勒结构.     

协作分集OFDM系统的混合排序子载波配对算法

考虑到信源子载波和中继子载波的配对能在频率选择信道环境下提高协作分集系统的性能,对协作分集正交频分复用(OFDM)系统的子载波配对进行了研究,推导了2种特殊情况下的最佳子载波配对方法,即顺序配对和逆序配对,在此基础上,提出了一种基于混合排序的子载波配对算法.该算法根据信源-中继链路、信源-目标链路和中继-目标链路上的每个子载波的信噪比状态,自适应地选择顺序配对或逆序配对.仿真结果表明,提出的混合排序子载波配对算法能显著提高系统的平均可取得速率,性能明显优于顺序配对和逆序配对.     

基于子载波权衡的多用户OFDMA资源分配算法

在多用户OFDMA资源分配中提出一种基于子载波权衡的资源分配算法.资源分配过程中,在对公平性最差的用户进行子载波分配时,进行子载波权衡.根据权衡后子载波信道增益的变化情况,进行子载波分配.通过子载波权衡,将待分配的子载波分配给权衡用户;同时对于公平性最差的用户,从已分配给权衡用户的子载波中获得一个权衡子载波;使得权衡后分配的子载波较权衡之前获得更高的信道增益.仿真结果表明,算法在保证用户公平性的同时,提升了系统的频谱利用率.     

VMIMO-OFDM系统精频偏估计算法的研究

在对多种可应用于VMIMO-OFDM系统的频偏估计方法的性能和效率进行分析的基础上,针对基于VMIMO-OFDM的无线传感器网络中的精频偏估计问题,提出了一种在频域进行频偏估计的算法.同时,针对基于VMIMO-OFDM的无线传感器网络应用于瑞利衰落严重的环境时不同的导频子载波上频偏估计的误差差异较大的问题,提出了一种基于信道统计信息(CSI)加权的精频偏估计算法,该方法对各个子载波上的频偏估计结果进行加权平均,可使估计误差最小化.理论分析和仿真结果均表明,新提出的算法在高斯信道和多径信道中均有更优的性能.     

OFDM系统在时变信道下的迭代干扰抵消算法

为克服正交频分复用(OFDM)系统中因时变多径衰落信道破坏子载波的正交性引起载波间干扰(ICI),从而导致系统性能下降的问题,提出了一种基于最小均方误差(MMSE)估计和迭代算法相结合的迭代干扰抵消算法.为了消除一般MMSE均衡方法将载波间干扰当作加性高斯白噪声(AWGN)所带来的误差,该算法先使用MMSE方法对传输信号进行初值估计,再通过迭代干扰抵消和MMSE均衡相结合,来消除由时变信道所带来的载波间干扰.多次仿真表明,在时变多径衰落信道中使用本算法能够有效地消除载波间干扰,使系统性能得到很好的改善.     

Alternate video broadcasting scheme for popular videos

A video segment broadcasting scheme can reduce server and network bandwidth by periodically broadcasting popular videos that are most likely demanded by clients, instead of responding to each client requests. When video segments broadcast on a channel, in general, alternate broadcasting schemes periodically transmit all segments on a given channel with the same transmission period, which reduces the transmission efficiency of stream channels and requires sufficient client storage space for video segments broadcast on simultaneous channels. The author proposed a novel alternate video broadcast scheme, a delayed buffering broadcast that requires lower server bandwidth and client buffer space compared with those of previous approaches. In addition, the study provides an analytical analysis of the scheme, including a lower bound on the video segment transmission rate for any alternate broadcast scheme. It also derives an upper bound on its storage requirements from the client side. Using the performance study of the proposed scheme and simulation results, the author establishes that the proposed scheme uses fewer server channels and storage resources than previously reported alternate video broadcast schemes for any given client waiting time.     

Transmitter pre-filtering for the downlink of a time-division-duplex/direct sequencecode division multiple access system over time-varying multipath fading channels

In this paper, we investigate an optimised transmitter pre-filtering technique for downlink time-division-duplex (TDD) code division multiple access (CDMA) communications, which employs the conventional matched filter (MF) detector at the mobile receivers. The proposed pre-filtering technique eliminates the multiple-access interference and intersymbol interference (MAI/ISI) effects by applying a very simple transmission scheme that combines a signal transformation with a cyclic prefix strategy under a power constraint condition. Two constrained pre-filtering transformations are suggested depending on the information required at the mobile unit. An open-loop transmitter pre-filtering is first formulated; however, this solution does not consider the properties of the noise at the mobile receiver. A second solution is then presented via a closed-loop transmitter pre-filtering that includes an optimum gain for a given transmit and noise power. Some associated issues such as system efficiency, computational complexity and channel estimation errors are also addressed. Simulation results show that the proposed transmitter pre-filtering scheme can be used to increase the system performance and capacity. In addition, its performance is compared with another similar transmit pre-processing scheme in order to evaluate the performance improvement by the proposed algorithm.     

An Efficient Faint Object Spectrograph

A faint object spectrograph is described suitable for use with a solid-state detector array. High resolution and low light loss are achieved over a wide spectral range using a cemented assembly of transmission grating, cross-dispersing prism and the corrector plate of a wide aperture camera. Constructional data and performance figures are given.     

主用户链路深度衰落下的认知网络中继

针对主用户链路经历深度衰落而发生通信中断的问题,提出了一种认知网络对主用户进行"透明"中继的方案。在不改变主用户通信协议的前提下,该方案首先感知主用户的状态,以判断其是否需要中继服务。当主用户通信发生中断时,认知网络利用从用户的能量检测器选出一个最优的节点解码转发主用户信号。从中断概率角度证明了这种最优单节点中继具有与多节点中继相同的空间分集作用,能够提高主用户平均传输效率,有较大的中继信道容量。通过仿真分析,验证了其分集效果和传输效率的提升。     

Coded-orthogonal frequency division multiplexing in hybrid optical networks

Future Internet should be able to support a wide range of services containing large amount of multimedia over different network types at a high speed. The future optical networks will therefore be hybrid, composed of different single-mode fibre (SMF), multi-mode fibre (MMF) and free-space optical (FSO) links. In these networks, novel modulation and coding techniques are needed that are capable of dealing with different channel impairments, be it in SMF, MMF or FSO links. The authors propose a coded-modulation scheme suitable for use in hybrid FSO - fibre-optics networks. The proposed scheme is based on polarization-multiplexing and coded - orthogonal frequency division multiplexing (OFDM) with large girth quasi-cyclic low-density parity-check (LDPC) codes as channel codes. The proposed scheme is able to simultaneously deal with atmospheric turbulence, chromatic dispersion and polarisation mode dispersion (PMD). With a proper design for 16-quadrature amplitude modulation (QAM)-based polarisation-multiplexed coded-OFDM, the aggregate data rate of 100 Gb/s can be achieved for OFDM signal bandwidth of only 12.5 GHz, which represents a scheme compatible with 100 Gb/s per wavelength channel transmission and 100 Gb/s Ethernet.     

Adaptive pseudo-noise code acquisition scheme using automatic censoring for DS/SS communication in frequency-selective Rayleigh fading channel

An adaptive pseudo-noise code acquisition scheme based on automatic multipath signal cancellation for mobile communication systems in frequency-selective Rayleigh fading channel is proposed. The proposed system combines a serial strategy and an automatic censored cell-averaging constant false alarm rate (CFAR) detector based on ordered data variability. This system does not require any a priori information about the number of interferences caused by the presence of multipath signals in the reference channel. The mean acquisition time and the detection performances of the proposed processor are evaluated and compared with those of the conventional adaptive acquisition scheme based on fixed-censoring point detector. It is shown that the considered scheme outperforms significantly the conventional one. The effects of various channel parameters on the acquisition performance, namely the number of resolvable paths, the partial correlation length and the signal-to-noise ratio are also investigated.     

Real-Time Measurement of Microwave Parameters and EM Fields

A technique for making fixed-frequency or wide-band real-time measurements of reflection coefficients, transmission coefficients, and fields in open or closed structures at UHF and microwave frequency is given. The basic system uses two isolated channels, which originate from a common CW source, and one channel is single-sideband modulated. These two signals are then added and mixed in a (homodyne) detector, and the detected signal at the modulation frequency displays the desired amplitude and phase information in real time simultaneously and independently. The sources of inherent phase and amplitude errors are considered, including those due to the unwanted sidebands created by imperfect modulation. Operation on the linear portion of the characteristic curve of a balanced detector tends to reduce these errors and increase the dynamic operating range. The system sensitivity should approach -110 to -120 dBm, and dynamic operating ranges of the order of 100 to 110 dB are possible.     

Prediction of interference by modeling a radio interference meter

A method of predicting interference by modeling a radio-interference meter which satisfies the requirements of the CISPR is considered. The quasi-peak voltages of the radio interference is determined by analyzing the transmission of the interference signal through a channel consisting of a bandpass filter, a quasi-peak detector and an indicator instrument. The peak, mean and root mean square radio-interference voltages are calculated from the envelope at the output of the bandpass filter. Examples of the calculation of the radio-interference voltage for different forms of detectors in the measuring instrument are given.Translated from Izmeritel'naya Tekhnika, No. 6, pp. 44–47, June, 1994.     

线阵CCD在颜色测试中的应用

介绍了一种以线阵ＣＣＤ器件为多通道探测器的光源颜色快速测试系统。该系统采用平场凹面光栅为主器件构成多色仪分光系统，测量全过程由微型计算机控制，实现快速，自动的光源能谱及颜色测量。具体给出了系统测试的原理，总体结构，软件和硬件配置，并提供了系统定标方法和实验结果，该系统具有广阔应用前景。     

Single-walled carbon nanotube thin film emitter-detector integrated optoelectronic device

We use the suspended single-walled carbon nanotube (SWNT) thin film technology to assemble the first prototype of an integrated optoelectronic SWNT device, a SWNT optocoupler in which a SWNT emitter and a SWNT detector couple two electrical circuits by the transmission of a signal through the optical channel. Our experiments show that the integrated SWNT emitter/SWNT detector is an ideal couple in which the broadband wavelength character of the emission matches the broadband detection capabilities.