WRAN系统中基于自适应内插的信道估计算法

提出了一种基于自适应复系数内插的信道估计算法,改善了无线区域网络（WRAN）系统对抗动态多径时延的能力。WRAN是第一个采用认知无线电技术改善频谱效率的宽带接入标准,系统在下行链路中采用了正交频分复用（OFDM）调制技术,而信道估计技术对于采用相干解调的OFDM系统十分重要。传统的OFDM信道频域响应（CFR）估计算法通常采用实系数频域内插的方式,在对抗WRAN系统长多径时延信道时,不能有效地工作。该文在研究实系数FIR内插变换域响应的基础上,提出了一种复系数内插算法。为了同时适用于短时延信道,提出了一种低复杂度、自适应匹配信道最大多径时延的算法。通过仿真,验证了该算法能够对抗更大的多径时延,提高信道估计的精度,改善系统误码性能。      

基于Vogler模型的短波OFDM性能仿真

OFDM(正交多载波调制)是一种在无线环境下具有较高频谱利用率的多载波调制技术,它具有较强的对抗多径干扰和消除符号间干扰的能力.基于Vogler模型出发,根据实测的短波信道参数数据进行了OFDM系统仿真,同时,用导频辅助的方法对信道进行估计,并进行信道补偿,克服了短波信道的多径衰落和多普勒频移等因素的影响,有效降低了系统的误码率,提高了整个系统的性能.     

基于OFDM技术的短波电台系统研究

OFDM（正交频分复用）技术是一种多载波数字调制技术,在无线多径衰落信道下具有较好的传输特性,文中设计短波电台系统采用了OFDM技术来对克服多径衰落带来的干扰,并且给出了系统的设计方案。利用Simulink建模,并在瑞利衰落信道和高斯信道下进行了仿真,结果显示出性能上有较大的改进。     

新一代宽带无线局域网系统中的OFDM技术研究

无线局域网是一种高速的无线数据传输系统,其调制技术的选择非常关键。OFDM(正交频分复用)是一种无线环境下的高速传输技术,适合在多径传播和多普勒频移的无线移动信道中传输高速数据。他能有效地对抗多径效应,消除符号间干扰,对抗频率选择性衰落,而且信道利用率高。文中对OFDM技术在新一代宽带无线局域网无线接入系统中的应用进行了分析和研究。     

OFDM系统仿真与分析

正交频分复用(OFDM)技术在高速数据传输中得到了广泛的应用,尤其在无线接入和移动通信中应用前景非常广泛.无线衰落信道使得通信系统在传输过程中出现符号间干扰和信道干扰,严重影响信道质量,而OFDM是一种多载波教字调制技术,在无线衰落信道下具有较好的传输特性.运用Matlab构建OFDM系统,并加入自适应功率和比特分配算法.通过具有高斯白噪声和频率选择性信道,观察在不同信噪比条件下系统误码率是如何变化的.给出仿真结果,从而分析出系统在不同调制方式下的性能.仿真结果表明,OFDM系统能抗多径衰落,而且具有良好的误码性能.     

一种基于OFDM的多载波通信系统的设计与仿真

OFDM(正交频分复用)是一种无线环境下的高速传输技术,它对由于无线信道的多径传播引起的ISI(符号间干扰)有很强的抵抗能力,已经引起了人们的广泛关注.文中首先介绍了OFDM技术的应用、发展以及基本原理,然后在计算机上用MATLAB对其性能进行了仿真,仿真中将OFDM与传统的单载波调制方式--16-QAM(正交幅度调制)对抗多径干扰的性能进行了比较,结果表明OFDM技术更适合于在有多径干扰存在的无线信道中传输.     

Raleigh多径信道下自适应OFDM系统性能分析

在Raleigh信道下对自适应调制结合空频分组码的OFDM方案进行性能分析.针对多径传播引起的快衰落问题,在简化为广义稳态非相关散射多径信道的基础上给出改进的Raleigh信道模型,并提出了一种基于信道状态信息的自适应SFBC-OFDM方案,该方法运用了Chow自适应比特分配算法及Alamouti分集算法,并给出自适应OFDM方案与之比较.仿真结果表明,在有信道信息延时的两种条件下增益分别高出了1.3～14 dB和3～17.5 dB.系统显示出较好的对抗多径影响的性能.     

OFDM系统中信道编码对系统性能的影响

正交频分复用（OFDM）技术是下一代移动通信的核心技术之一。其最突出的优点是频谱利用率高和抗多径时延能力强,适合未来高速宽带无线通信的应用。OFDM技术通过将频率选择性多径衰落信道在频域内转换为平坦信道,减小了多径衰落的影响。根据OFDM基本原理特构建了一个OFDM通信系统仿真模型,分析了在不同调制方式、不同信道下信道编码对系统性能的影响。仿真结果表明,采用信道编码可有效的降低OFDM系统的误码率。     

OFDM系统基于信道多径时延检测的自适应信道估计

针对宽带正交频分复用(OFDM)系统,提出了一种基于信道多径时延估计的自适应信道估计算法.文章分析了无线信道的特性,随后利用导频频域相关性进行多径时延估计,提出通过动态跟踪信道时延估计参数自适应选择信道估计中适宜的导频数目的方法,同时实现循环前缀(CP)长度的实时调整.仿真结果表明,在时变环境下,本文算法可以有效获知当前信道时延参数的近似统计信息,实现对系统冗余信息的优化.     

自适应波束形成在OFDM系统中的应用

为了提高正交频分复用（OFDM）系统中传输数据的可靠性,对抗频率偏差带来的系统信噪比的下降,在OFDM系统接收端引入自适应波束形成。自适应波束形成实现了对期望信号方向的增益接收,同时对干扰信号方向进行有效的抑制。仿真结果表明,文中方法比单纯的OFDM系统具有更低的误码率,即使在有多普勒频移的多径信道下,也能获得更高的传输可靠性。从输入、输出信干噪比的曲线也可以看出,此方法是可行且收敛的。     

OFDM技术及在认知无线电中的应用

OFDM技术是一种无线环境下的高速多载波传输技术,抗频率选择性衰落或窄带干扰能力较强,同时又有很高的频谱利用率,适合在多径传播的无线移动信道中传输高速数据。论文分析了OFDM的基本原理,论述了其关键技术及优缺点。OFDM技术将成为未来无线通信中的核心技术,在认知无线电、4G移动通信和无线宽带接入等领域得到广泛的应用。     

Precoded OFDM with adaptive vector channel allocation for scalable video transmission over frequency-selective fading channels

Orthogonal frequency division multiplexing (OFDM) has been applied in broadband wireline and wireless systems for high data rate transmission where severe intersymbol interference (ISI) always occurs. The conventional OFDM system provides advantages through conversion of an ISI channel into ISI-free subchannels at multiple frequency bands. However, it may suffer from channel spectral s and heavy data rate overhead due to cyclic prefix insertion. Previously, a new OFDM framework, the precoded OFDM, has been proposed to mitigate the above two problems through precoding and conversion of an ISI channel into ISI-free vector channels. In this paper, we consider the application of the precoded OFDM system to efficient scalable video transmission. We propose to enhance the precoded OFDM system with adaptive vector channel allocation to provide stronger protection against errors to more important layers in the layered bit stream structure of scalable video. The more critical layers, or equivalently, the lower layers, are allocated vector channels of higher transmission quality. The channel quality is characterized by Frobenius norm metrics; based on channel estimation at the receiver. The channel allocation information is fed back periodically to the transmitter through a control channel. Simulation results have demonstrated the robustness of the proposed scheme to noise and fading inherent in wireless channels.     

Modulated Coded OFDM Systems with Special Precoder

Orthogonal frequency-division multiplexing (OFDM) has been considered as a strong candidate for next-generation wireless communication systems to achieve high rate data transmission in a mobile environment. However, the performance of OFDM systems may be degraded when inter-symbol interference (ISI) channels have spectral nulls, and the data rate overhead due to the insertion of cyclic prefix is high when ISI channels have many taps. Recently, the precoded OFDM systems and vector OFDM (VOFDM) systems were proposed to combat these two problems, respectively. We propose a novel modulated coded OFDM system with special precoder that is robust to spectral nulls and with reduced cyclic prefix length. The precoder can be easily formulated by channel coding and digital modulation. This precoding scheme uses the redundancy information introduced by channel coding (such as zeros inserted in precoded OFDM system). It is able to remove the spectral nulls of an ISI channel without knowing the ISI. Simulation results show that our proposed OFDM system performance is better than precoded OFDM and VOFDM system.     

Adaptive interference suppression in multiuser wireless OFDMsystems using antenna arrays

This paper considers the problem of mitigating fading and interference in wireless orthogonal frequency division multiplexing (OFDM) multiple access communication systems. Applications include cellular mobile radio, wireless local loop, and wireless local area networks. The effect of interchannel interference (ICI) arising from time-selective fading and frequency offsets and co-channel interference (CCI) is analyzed. A loop-timing method that enables a synchronous uplink between multiple mobile transceivers and a base-station is described. Adaptive antenna arrays are utilized at the base for uplink reception, and optimum array combining based on the maximum SINR criterion is used for each subchannel over slowly time-varying channels. For operation over fast time-varying channels, a novel two-stage adaptive array architecture that incorporates combined spatial diversity and constraint-based beamforming is presented. While ICI alone is most effectively overcome by spatial diversity, combined beamforming and diversity are most effective to combat CCI in the presence of fading. The overall method is suitable for real-time implementation and can be used in conjunction with traditional coding schemes to increase the link-margin     

基于统计的OFDM符号定时同步

正交频分复用（OFDM）的主要思想是将信道分成若干正交子信道,将高速数据信号转换成并行的低速子数据流,调制在每个子信道上进行传输,每个信道上的信号可以看成平坦性衰落,可以消除符号间干扰。因此,OFDM技术具有很强的抗多径能力和很高的频谱利用率。但是由于OFDM采用了正交的子载波,对同步误差极其敏感。主要研究了OFDM系统的符号定时同步算法,并对现有的算法进行仿真和优化,最终通过硬件调试实现了算法的功能。     

Optimal OFDM design for time-frequency dispersive channels

Transmission over wireless channels is subject to time dispersion due to multipath propagation and to frequency dispersion due to the Doppler effect. Standard orthogonal frequency-division multiplexing (OFDM) systems, using a guard-time interval or cyclic prefix, combat intersymbol interference (ISI), but provide no protection against interchannel interference (ICI). This drawback has led to the introduction of pulse-shaping OFDM systems. We first present a general framework for pulse shape design. Our analysis shows that certain pulse shapes proposed in the literature are, in fact, optimal in a well-defined sense. Furthermore, our approach provides a simple way to adapt the pulse shape to varying channel conditions. We then show that (pulse-shaping) OFDM systems based on rectangular time-frequency lattices are not optimal for time- and frequency-dispersive wireless channels. This motivates the introduction of lattice-OFDM (LOFDM) systems which are based on general time-frequency lattices. Using results from sphere packing theory, we show how to design LOFDM systems (lattice and pulse shape) optimally for timeand frequency-dispersive channels in order to minimize the joint ISI/ICI. Our theoretical analysis is confirmed by numerical simulations, showing that LOFDM systems outperform traditional pulse-shaping OFDM systems with respect to robustness against ISI/ICI.     

OFDM系统均衡器研究与设计

无线OFDM通信系统中信道的时变特性非常明显,载波间正交性遭到破坏,出现载波间相互干扰（ICI）,给系统均衡带来了极大困难。基于等效数字基带模型下ICI分析,在现有的OFDM协议标准中,存在一些空闲载波。利用这些空闲子载波信息,研究了一种适用于时变信道OFDM系统的稀疏矩阵均衡器结构。该算法具有ICI消除与均衡效果良好,计算量适中,频谱利用率高的特点。     

A MIMO-OFDM prototype for next-generation wireless WANs

Coupled with a robust and efficient OFDM air interface, MIMO technologies lead to a very compelling high-speed data downlink solution for future wireless systems. This article presents Nortel Networks' MIMO-OFDM concept prototype and measured performance results. This prototype has been developed in the framework of a Nortel Networks system concept for 3G evolution systems and next-generation wide area wireless networks. The prototype is based on a shared access MIMO-OFDM physical layer in the downlink, supporting adaptive modulation and coding, with peak rates up to 37 Mb/s. The uplink is based on an enhanced UMTS WCDMA physical layer. Performance for the high-speed downlink has been measured under various emulated fading conditions. The measured performance illustrates the robustness of OFDM in frequency-selective channels and high-speed mobility channels, supporting speeds as high as 200 km/h. The prototype can also be used for over-the-air assessment of the technology.