双选择性信道中OFDM信号的均衡

正交频分复用（OFDM）技术是一种多载波调制方式，由于他易于实现并且能够有效对抗频率选择性衰落，因此受到了人们极大的重视，他将数据在相互正交的子载波上并行发送，同时在数据的前端引入循环前缀，可以有效地对抗频率选择性信道，消除符号间的干扰，实现数据的高速传输。但是当他在频率选择性和时间选择性信道条件下，也就是双选择性信道条件传输时，子载波之间的正交性会遭到破坏。文章讨论了双选择性信道条件下，在时域采用自适应均衡器来消除信道时变带来的载波间干扰，保证各个子载波的正交性。仿真结果表明，信号可以被正确的恢复。     

一种针对双衰落信道OFDM的新均衡算法

在OFDM系统中，子载波间的正交性是保证OFDM性能的重要保障。针对双选择性衰落信道下的OFDM系统，该文在分析载波间干扰（ICI）的基础上，提出了一种采用频域迭代消除ICI的均衡算法。分析和仿真结果表明此方法能有效地保证载波间的正交性和改善了OFDM系统的误码率（BER）性能。     

PSA-OFDM系统在时变衰落信道中的ICI消除

针对移动信道的时变衰落特性引起的载波间干扰(ICI)对无线OFDM系统性能的严重影响,介绍研究了一种利用循环前缀(CP)和导频信号来消除ICI的方法.仿真结果表明,通过消除时变衰落信道中ICI的影响,有效地提高了系统的性能.     

OFDM系统中时变信道下信道估计方法的研究

为减少OFDM系统中子信道间干扰(ICI)的影响,分析了分段频域均衡方法、近似分段线性的相邻符号ICI消除方法和卡尔曼滤波方法等几种快衰落环境下的信道估计方法的优劣,在此基础上提出了一种基于梳状导频的卡尔曼滤波信道估计方法.对滤波的结果,利用信道的频域相关函数和估计误差的协方差矩阵,在频域进行最小均方误差意义上的优化.仿真结果表明,该方法具有良好的估计性能.     

移动OFDM系统中的ICI消除算法研究

在宽带OFDM系统中,高速移动引起的多普勒效应导致载波间干扰(InterCarrier Interference, ICI)。该文在充分利用ICI分布特性的基础上,使用部分信道信息推导出两种新的低复杂度MMSE和DFE均衡算法。分析及仿真表明,新算法能够灵活可控地实现接收机性能与复杂度的有效折衷。在计算开销近似相同的条件下,提出的DFE均衡方案有效消除了传统接收机差错性能的地板效应(error floor),从而获得了性能改善。     

OFDM系统中成形脉冲抑制ICI方法研究

OFDM系统面临的一个主要问题是对频率偏移的敏感性,频率偏移将导致子载波间干扰(ICI)的产生,从而大大降低系统性能.本文主要分析频偏存在时脉冲成形与子载波间干扰的关系,给出各种成形脉冲性能的评价标准,并提出在此标准下性能最佳的成形脉冲.     

OFDM系统中成形脉冲抑制ICI方法研究

OFDM系统面临的一个主要问题是对频率偏移的敏感性,频率偏移将导致子载波间干扰（ICI）的产生,从而大大降低系统性能。本文主要分析频偏存在时脉冲成形与子载波间干扰的关系,给出各种成形脉冲性能的评价标准,并提出在此标准下性能最佳的成形脉冲。     

非理想载波同步下OFDM快衰落信道的稀疏与感知

针对稀疏导频 OFDM 系统,提出非理想载波同步 OFDM 快衰落信道稀疏表示与感知方法。首先提出了一种稀疏化信道核向量的广义信道冲激响应矩阵稀疏化表示方法;推导了稀疏导频快衰落信道估计压缩感知模型;利用OMP算法重构稀疏化信道核向量与广义信道冲激响应矩阵,从而完成非理想载波同步下的 OFDM 快衰落信道估计。仿真结果表明,该算法能有效降低非理想载波同步的稀疏导频 OFDM 系统的误码率。     

时频双选信道OFDM系统的ICI消除与均衡

在正交频分复用（OFDM）系统中,因高速移动造成的多普勒效应导致子载波间正交性的破坏并产生载波间干扰（ICI）。为了消除ICI,本文通过分析ICI分布特性及带状矩阵特性,提出了低复杂度的迭代干扰抵消算法和基于最小均方误差准则的排序串行干扰抵消(MMSE-OSIC)算法。仿真结果表明,与传统子载波间干扰频域均衡算法相比,新算法在计算复杂度和性能之间取得了良好的平衡,且MMSE-OSIC算法可以利用时变信道的时间分集特性在高信噪比情况下有效地消除“地板效应”。      

频率选择性衰落信道中的空时分组编码系统性能研究

空时分组码（STBC）由于具有较高的分集增益和简单的编译方法，从而得到普遍关注。但是它只适于频率平坦信道，而实际的信道多为频率选择性信道。丈中将考虑两种适用于频率选择性信道的空时分组码系统方案：OFDM-STBC和SC／FDE-STBC方案，即考虑空时分组码与正交频分复用或单载波频域均衡结合。通过对OFDM-STBC和SC／FDE-STBC系统性能仿真结果的比较和分析，表明SC／FDE-STBC方案要优于OFDM-STBC方案。此外，文中也给出一些细致的仿真性能的观察。     

Iterative estimation of doubly selective channels with ICI suppression for OFDM using KL-BEM

Channel estimation for orthogonnl frequency division multiplexing(OFDM)systems operating over doubly selective channels is complicated by the existence of intercarrier interference(ICI).This paper extends the idea of estimating time-variant channel impulse response(CIR)via time-averaged CIR.proposed by Hijazi,and proposes a low-complexity channel estimation scheme with ICI suppression based on Karhnnen-Loeve basis expansion model(KL-BEM),which is in a more general form.In the proposed scheme,the KL-BEM coefficients are obtained through their numerical relationship with the time-averaged CIR.The estimation performance is improved by adopting an expectation-maximization(EM)-based iterative structure combined with a low-complexity ICI equalizer and canceller.Theoretical analysis and simulation results show that the proposed scheme achieves superior performance over the original one with lower computational complexity.     

时变信道OFDM系统一种改进的连续ICI消除法

提出了一种改进的连续子信道间干扰(ICI)消除法,有效地消除OFDM信号在时变信道下产生的ICI。该方法检测出每一子信道上的数据后,立即估计并消除该数据对其他所有子信道产生的ICI,因此提高了下一子信道的检测精度。在Jakes模型产生的时变信道下进行仿真,结果表明,与传统的并行ICI消除法在迭代次数和运算量相同条件下,本算法具有较快的收敛性和较高的性能。     

Progressive iterative channel estimation in fast time-varying OFDM systems

Considering the joint channel estimation and data detection in time-varying orthogonal frequency division multiplexing （OFDM） and addressing transmission performance degradation induced by the severe inter-carrier interference （ICI） at very high speed, a new progressive iterative channel estimation scheme is proposed. To alleviate the error propagation of the inaccurate data due to ICI, the measurement subcarriers in the Kalman filter is designed to be extended from pilots subcarriers to all the subcarriers progressively through the iterations. Furthermore, in iteration process, the interference of the non-pilot data to the measurement subcarriers is considered to be part of noise in the modified Kalman filter, which improves the estimation accuracy. Simulation indicates that the proposed scheme improves the performance in fast time-varying situation.     

频率选择性衰落OFDM的半盲信道估计算法研究

从系统性能分析和设计的角度详细地研究了基于无线HIPERLAN2通信协议的OFDM(orthogonal frequency division multiplexing，正交频分多路复用技术)系统信道估计与均衡的一种半盲算法，提出了结合直接法和Cholesky分解法的切换盲算法。这种半盲算法综合了全盲算法得到的信息与已知导频符号，充分利用了原发信号的统计特性和OFDM帧结构中插入的导频符号，克服了全盲和导频训练序列存在的问题^[6,7]，且不需额外的带宽。仿真结果表明，在误比特率和收敛性方面，该算法比现有的主要三种全盲算法有更好的收敛和抗干扰特性。     

快衰落信道下OFDM系统的信道估计简化算法

在快衰落时变信道中，正交频分复用（OFDM：Orthogonal Frequency Division Multiplexing）系统子载波间的正交性会遭到破坏，由此引起的载波间干扰（ici：Inter-Carrier Interferellce）会对系统性能带来很大的影响。本文提出了一种利用时域线性模型的信道估计算法，利用随时间线性变化的模型描述信道。仿真结果表明，本算法能够有效地消除载波间干扰，降低系统的BER。与类似算法相比，当系统的BER性能相同时，算法复杂度大大的降低。     

Over-sampling basis expansion model aided channel estimation for OFDM systems with ICI

The rapid variation of channel can induce the intercarrier interference in orthogonal frequency-division multiplexing （OFDM） systems. Intercarrier interference will significantly increase the difficulty of OFDM channel estimation because too many channel coefficients need be estimated. In this article, a novel channel estimator is proposed to resolve the above problem. This estimator consists of two parts： the channel parameter estimation unit （CPEU）, which is used to estimate the number of channel taps and the multipath time delays, and the channel coefficient estimation unit （CCEU）, which is used to estimate the channel coefficients by using the estimated channel parameters provided by CPEU. In CCEU, the over-sampling basis expansion model is resorted to solve the problem that a large number of channel coefficients need to be estimated. Finally, simulation results are given to scale the performance of the proposed scheme.     

Kalman filter‐based channel estimation and ICI suppression for high‐mobility OFDM systems

The use of orthogonal frequency division multiplexing (OFDM) in frequency‐selective fading environments has been well explored. However, OFDM is more prone to time‐selective fading compared with single‐carrier systems. Rapid time variations destroy the subcarrier orthogonality and introduce inter‐carrier interference (ICI). Besides this, obtaining reliable channel estimates for receiver equalization is a non‐trivial task in rapidly fading systems. Our work addresses the problem of channel estimation and ICI suppression by viewing the system as a state‐space model. The Kalman filter is employed to estimate the channel; this is followed by a time‐domain ICI mitigation filter that maximizes the signal‐to‐interference plus noise ratio (SINR) at the receiver. This method is seen to provide good estimation performance apart from significant SINR gain with low training overhead. Suitable bounds on the performance of the system are described; bit error rate (BER) performance over a time‐invariant Rayleigh fading channel serves as the lower bound, whereas BER performance over a doubly selective system with ICI as the dominant impairment provides the upper bound. Copyright © 2008 John Wiley & Sons, Ltd.     

Reducing ICI effect in OFDM system using low‐complexity Kalman filter based on comb‐type pilots arrangement

In OFDM systems, time‐varying channels destroy orthogonality between subcarriers causing Inter‐Carrier Interference. To reduce this effect, a Kalman filter, as a benchmark, is used for channel estimation, based on comb‐type pilot arrangements of the OFDM system. An advantage of comb‐type pilot arrangements in channel estimation is the ability to track the variation of the channel caused by Doppler frequency. Kalman method has been proposed to estimate the channel frequency response (CFR) at the pilot locations, then CFR, at data subchannels, is obtained by means of interpolation between estimates at pilot locations. The low‐complexity Kalman method is introduced to reduce the complexity of the system while achieving the same BER/SNR. Different types of interpolations have been also compared such as Low‐pass, Spline‐cubic and Linear interpolation methods. The BER/SNR performance of BPSK modulation schemes are considered for time varying Rayleigh fading channels. Our results has shown that the low‐complexity Kalman estimation, used with the pilot arrangement and a suitable interpolation method, gives almost the same performance as that of the Kalman method specially for low SNR values and hence the effect of Doppler shift effect is controlled. Copyright © 2010 John Wiley & Sons, Ltd.