Joint Synchronization and Channel Estimation for OFDM Systems

1Introduction Orthogonal Frequency Division Multiplexing(OFDM) has become a popular technique for transmis-sion of signals over wireless channels . OFDMhas beenadoptedinseveral wireless standards such as Digital Au-dio Broadcasting ( DAB) , Digital Video Broadcasting(DVB-T) ,the IEEE 802 .11a[1]Local Area Network(LAN) standard and high performance LAN type 2(HIPERLAN/2)[2]standard. OFDMis also being pur-sued for Dedicated Short-Range Communications(DSRC) for road side to …     

OFDM系统中的同步偏移估计技术

在分析无线信道环境对OFDM系统影响的基础上,对目前该学术领域提出的多种同步算法进行了分析比较,探讨了数据辅助性估计的OFDM系统的同步过程,该过程包括符号同步、载波同步和采样率同步。该算法在时域上引入了一种更有效的导频方案,在此前提下,利用该PN序列的共轭对称性完成符号定时,然后通过一种基于PN的重复数据最大似然估计理论算法完成频偏估计。仿真结果证明,这种同步体制具有较好的同步精度。     

OFDM系统信道估计算法研究

在OFDM技术应用于移动通信领域时,其信道估计技术是一项关键技术。分析了级联的一维的信道估计方法,采用加窗的方式改进了基于FFT一维级联信道估计算法,在此基础上提出一种新的基于DCT（EIDCT）变换的信道估计算法。最后给出了改进算法的性能仿真,并对仿真结果进行分析。     

基于DCT的OFDM信道估计

在宽带移动环境下,由于多普勒频移以及多径的影响,信道是时变的。为了消除信道对传输信号的影响,需要对信道进行估计。给出了2种基于离散余弦变换(DCT)的信道估计方法,可以很好的降低频谱泄露造成的混叠失真,适用于非整数倍的抽样信道。其中一种性能较好,另一种实现更加方便。仿真结果表明,这两种算法的性能均优于基于DFT的信道估计算法。     

基于导频的光OFDM系统信道估计

为了克服光纤色散和模式的传输时延导致ICI设计了一种基于导频的信道估计器．并分析了LS和LMMSE算法估计原理,通过仿真实验比较了两种算法的误比特率和均方误差性能．实验结果表明LS与LMMSE在光OFDM系统中都有较好的作用但LMMSE算法性能要优于LS算法。     

基于导频的光OFDM系统信道估计

信道估计是光正交频分复用（OOFDM）系统关键技术之一,为了克服光纤色散和模式的传输时延所产生的载波间干扰（ICI）,设计了一种基于导频的信道估计器,通过分析最小二乘（LS）和线性最小均方误差（LMMSE）算法估计原理,仿真实验比较两种算法的误比特率和均方误差性能,实验结果表明,LS与LMMSE在OOFDM系统中都有较好的作用,但LMMSE算法性能要优于LS算法。     

OFDM系统在动态环境下的自适应信道估计

分析了OFDM通信系统信道估计采用二维自适应维纳滤波的情形，信道估计根据对不同的信道环境（多径时延，多普勒频移）的实时检测，自适应地选择适宜的滤波参数，动态地跟踪信道的变化特征。仿真分析得到了最优的滤波参数并且证明了在非完全匹配条件下的插值亦有很好的效果。     

OFDM系统中的盲信道估计

本文从OFDM信号的矩阵表示出发,分析比较了OFDM系统中现有的各种盲信道估计方法。OFDM盲信道估计方法分为两类,一类是统计型方法,它利用了发送信号和接收信号的统计特性;另一类是确定型方法,它利用了发送调制信号的固有特性。一般而言,统计型方法的计算量较小,但是估计精度不高且估计的实时性不好;而确定型方法的估计精度较高,实时性较好,但是其计算量较大。计算机仿真表明,这些盲信道估计方法的性能受信道参数尤其是多普勒频率影响很大,盲信道估计的实用化有待进一步研究。     

基于DFT的OFDM系统信道估计改进算法

基于DFT的信道估计算法计算复杂度比MMSE算法低,性能比LS算法好.但是传统的基于DFT的信道估计只消除了信道冲击响应估计中循环前缀长度之外的噪声,循环前缀长度内的噪声并没有得到抑制,因此算法性能还有提高的空间.本文提出了一种改进的基于DFT的信道估计算法.算法首先估计出噪声方差,然后利用噪声方差设定一个门限,通过此门限对循环前缀内的信道时域冲击响应值进行阈值,进一步消除噪声的干扰.仿真证明,本文的改进信道估计算法性能优于原算法.     

Adaptive Channel Estimation for Multiple Antenna OFDM Systems

We propose a simple adaptive algorithm for channel estimation of multiple antenna OFDM systems to provide competitive performance with low computational complexity and tracking ability. The derivation of the mean-squared error (MSE) for the proposed scheme is presented and the optimum selection of the parameter in the scheme is analyzed by utilizing the derived MSE expression.     

Wavelet-Based Semiblind Channel Estimation for Ultrawideband OFDM Systems

Ultrawideband (UWB) communications involve very sparse channels, because the bandwidth increase results in a better time resolution. This property is used in this paper to propose an efficient algorithm that jointly estimates the channel and the transmitted symbols. More precisely, this paper introduces an expectation-maximization (EM) algorithm within a wavelet-domain Bayesian framework for semiblind channel estimation of multiband orthogonal frequency division multiplexing based UWB communications. A prior distribution is chosen for the wavelet coefficients of the unknown channel impulse response to model a sparseness property of the wavelet representation. This prior yields, in maximum a posteriori estimation, a thresholding rule within the EM algorithm. We particularly focus on reducing the number of estimated parameters by iteratively discarding “insignificant” wavelet coefficients from the estimation process. Simulation results using UWB channels that were issued from both models and measurements show that, under sparseness conditions, the proposed algorithm outperforms pilot-based channel estimation in terms of the mean square error (MSE) and bit error rate (BER). Moreover, the estimation accuracy is improved, whereas the computational complexity is reduced compared with traditional semiblind methods.      

OFDM系统中的信道估计方法分析

对信道估计有利于消除信道对OFDM系统性能的影响。本文探讨了OFDM系统中信道估计的多种方法,重点对MMSE(最小均方误差)、LS(最小平方)、PSAM(导频辅助调制)和盲的信道估计方法及它们的特点进行了分析和比较。     

Combined Time Synchronization and Channel Estimation for OFDM Based Multi-relay Networks

In this paper a new combined time and channel estimation method for OFDM based multi-relay networks is proposed. Based on the channel statistics knowledge at the receiver, a maximum a posteriori estimate (MAP) of the channel impulse response is derived. Based on the channel estimate, we discern for the timing error. Computer simulations are presented to corroborate our analytical results.     

OFDM系统信道估计方法研究

分析了现有实用OFDM系统的信道估计方法及其硬件结构,并根据信道的统计特征给出了一种改进的门限滤波方法与其对应的硬件结构.在提高性能的同时,这种方法复杂度并不高,且不需要任何已知的信道统计特性.通过实验验证,这种新方法(尤其是二维的方法)明显优于传统的基于FFT的方法和最小二乘方法.     

一种用于实际OFDM系统的次优信道估计算法

基于对实际系统中信道冲激响应(CIR)泄漏问题的详细分析,提出了一种次优的时域信道估计算法。该算法克服了CIR泄漏的影响,因此能够很好地适应实际系统中的非整数采样多径衰落信道。该算法不需要预先知道信道统计特性,还具有低复杂度的特点。与已有信道估计算法进行了仿真比较,结果证明该算法极大地降低了信道估计MSE值,同时也不存在“地板效应”。     

OFDM系统中基于变换域信道估计方法的改进

在分析信道传输函数频谱特性的基础上,提出了一种改进的变换域信道估计方法,该方法通过改进低通滤波器的传输函数,有效地减小了符号间干扰和加性高斯白噪声对信道估计精确度的影响.仿真结果证明,在多普勒频移较大的情况下,该方法仍具有较小的误码率.     

Improved Channel Estimation Based on Parametric Channel Approximation Modeling for OFDM Systems

Orthogonal frequency division multiplexing (OFDM) together with high order modulation scheme requires accurate channel estimation to perform coherent demodulation. In this paper, improved channel estimation methods based on a parametric channel approximation model are proposed for the OFDM system using pilot subcarriers. This channel model is called fraction taps channel approximation (FTCA) model, which is defined as a finite impulse response (FIR) on some definitive delay taps that have a fraction tap delay spacing relative to the sampling interval. Then, based on the FTCA channel model, the minimum mean square error (MMSE) and least square (LS) estimators are derived. Simulations over non-sample-spaced channels prove that the use of the FTCA channel model can effectively eliminate the problem of multi-path delay estimation and reduce the signal subspace dimension of the channel correlation matrix, where the full-rank estimators using pilot subcarriers can be adopted, and consequently, improve the channel estimation performance.     

快速时变信道下的OFDM信道估计方法

为解决正交频分复用(OFDM)系统在快速时变信道下引入的子载波间干扰(ICI)导致性能急剧下降的问题,提出一种基于带状矩阵的数据辅助型信道估计算法。通过分析由多普勒频偏引入的子载波间干扰矩阵,运用线性模型对信道冲击响应算法进行建模,对每个子载波设计带状补偿矩阵以最大程度消除ICI。数值仿真结果表明,该信道估计算法较最小均方误差(MMSE)算法复杂度大大减小,便于实现,满足快速时变信道下的估计精度需要。     

OFDM系统中基于导频插入的信道估计技术

介绍了OFDM通信系统中用导频插入来进行信道估计的技术。根据导频插入的方法,信道估计可以分为基于集中式导频插入的信道估计和基于间隔式导频插入的信道估计。在描述OFDM系统信道估计模型的基础上详细阐述这两种方法,并利用多径Rayleigh衰落信道比较它们的性能。     

Blind Channel Estimation for OFDM Systems via a Generalized Precoding

In this paper, we consider the problem of blind channel estimation for single-input-single-output (SISO) orthogonal frequency-division multiplexing (OFDM) system via second-order statistics only. Based on the assumption that the transmitted symbols are independent and identically distributed, we develop a simple blind channel estimation technique for OFDM systems by utilizing a generalized linear nonredundant block precoding. Instead of using partial information from the signal covariance matrix, as done in previous works where a specific precoder is designed and only one column of the signal covariance matrix is exploited, our work jointly considers all the information contained in the signal covariance matrix. Compared to the popular subspace-based blind channel estimation methods, the proposed algorithm is much more computationally efficient. A design criterion of the precoders by which the performance can be improved is provided, and the closed-form stochastic Crameacuter-Rao bound is derived. The numerical results clearly show the effectiveness of our proposed algorithm, as well as its improvement over the existing techniques