OFDM系统导频信道估计算法的性能研究

研究基于块状导频的最小平方（LS）信道估计、线性最小均方误差（LMMSE）和低秩LMMSE信道估计算法,并进行了算法仿真,得出不同信道估计算法下正交频分复用（OFDM）系统误比特率（BER）和信噪比（SNR）之间的变化关系,以及在不同导频符号间隔下的误比特率曲线图。仿真结果表明：LMMSE、低秩LMMSE和LS算法的性能依次变差,且导频符号间隔对误比特率性能影响较大。     

快时变环境下OFDM系统中的信道估计

在OFDM系统中,怎样对快时变信道进行较为准确的估计是一个具有挑战性的课题。该文在利用信道基扩展模型的基础上,提出了一种适合于快时变环境下OFDM系统的信道估计方法,并且依据使估计的均方误差最小的准则,推导了相应最优的导频序列,包括最优的导频取值和最优的导频分布。可以证明,最优的导频序列是由一些相邻的等间隔等能量的子序列构成,每个子序列的导频之间满足一定的相位关系。仿真结果表明了所提估计算法在快时变环境下的有效性和采用所推导的最优导频序列进行估计的优越性。     

采用BICM-ID的PP-OFDM系统及后缀幅度优化

该文设计了一种用于正交频分复用(OFDM)系统的新型保护间隔信号-导频后缀(Pilot Postfix, PP),并构建了采用比特交织编码调制-迭代解码(BICM-ID)技术的PP-OFDM系统。PP由OFDM符号中的频域导频符号进行逆傅氏变换(IFFT)生成,在接收端可与OFDM符号中的导频相干合并,从而提高信道估计性能。该文给出了相应的信道估计、均衡和BICM-ID算法,并通过研究信道估值误差带来的接收信号有效信噪比变化,给出了PP幅度优化设计方法。仿真表明：选取适当的PP大小后,PP-OFDM比循环前缀/补零后缀-OFDM(CP/ZP-OFDM)的信道估计性能更好,误包率更低。     

基于导频的OFDM信道估计

从最小二乘和最小均方误差两种估计准则出发，结合OFDM多载波并行传输中符号时间长度大于信道冲散响应持续时间的特点，在块状和梳状导频图案下。导出了时域和频域的信道估计算法。最后采用广义平稳非相关散射假设下的多径时变瑞利衰落信道模型。对各个算法进行了仿真,比较了不同导频图案下备算法对系统误符号率的影响，分析结果表明。当导频数大于以采样间隔为量度的等效信道长度时。时域估计比频域估计精度更高;当导频数目小于等效长度时,最小均方估计优于最小二乘估计。     

OFDM系统中基于遗传算法的导频设计

在实际的OFDM系统中，虚载波的存在破坏了等间隔等功率导频的最优性。本文针对实际的OFDM系统，在给定导频功率的情况下，利用遗传算法对导频的位置进行优化，从而避免了穷尽搜索。同时为避免求逆运算，推导了最小二乘准则下信道估计均方误差的一个上界，并将其作为遗传算法的适应度函数。仿真结果表明采用本文所提算法设计的导频信号优于传统等间隔等功率的导频信号。     

一种OFDM系统中的盲信道估计算法

该文利用OFDM系统传输的信息符号为有限字符集和各子载波的相互独立特性,提出了用伪导频符号(PPS)进行信道的盲估计算法。与用于做信道估计的导频符号不同,PPS传输的是有用的数据,因此提高了系统的带宽利用率,PPS的平均功率增加3dB或6dB可有效抑制信道的加性高斯噪声。对信道盲估计的均方误差(MSE)和由此算法获得的信道的状态信息对无编码的OFDM系统进行解调的误比特率进行了仿真,结果表明提出的算法是有效的并具有很好的灵活性。     

基于导频的OFDM系统信道估计方法研究

正交频分复用(OFDM)技术采用多载波传输方式,成为了第四代移动通信核心技术。OFDM系统可采用基于导频的信道估计方法,提高信道传输的准确性。文章在matlab环境下对块状导频进行仿真,对最小乘二算法和最小均方误差估计算法进行了比较,经过实验得出,块状导频更适合于慢衰落信道。     

LTE下行信道估计算法研究

近年来,正交频分复用（OFDM）技术以其在高速数据传输中具有的良好的抗多径衰落的特性以及较高的频谱利用率等优点得到了广泛的应用。在OFDM通信系统中,信号在带宽小于信道相干带宽的多个正交子载波上发射和传输,以克服多径衰落信道所带来的信号频率选择性衰落。为了补偿各个子载波上的多径衰落,这就需要一个性能良好的信道估计方案。对LTE中基于导频的下行信道估计算法进行研究,首先在导频子载波处采用LS（最小二乘法）算法估计出信道参数,然后通过内插的方法在数据子载波处内插得到信道参数。频域内插主要采用拉格朗日内插或是LMMSE（线性最小均方误差）内插;时域内插采用拉格朗日内插。     

在OFDM系统中用导频符号进行信道估计的新方法

对信道的准确估计是提高相干解调的正交频分复用（OFDM）系统性能的关键，利用在OFDM系统中信道的频域响应是过采样的特性，提出了在频域内用均匀梳状分布的导频符号和导频符号数相同点的傅里叶变换相结合扒道估计方法，在16QAM OFDM系统中将其与带低通滤波器的Isi(sin(x)/x) 插值法，cubic插值法和Winner插值法进行了信道估计的均方误差和无编码的误比特率捻 真结果进行了比较，其结果表明本方法的估计性能优于si和cubic插值法，并接近优化的Win-ner插值法但其计算复杂度与Winner插值法相比却大大降低。     

宽带移动通信系统中基于梳状导频的信道估计

本文主要研究了OFDM系统中基于梳状导频的空时信道估计方法,并提出一种新的信道估计算法,即实现了空时二维信道估计。基于梳状导频的空时信道估计的算法分为两步:第一步是利用导频信号对导频位置的信道进行估计,第二步是信道插值,得到所有频域位置的信道信息。同时还研究了基于LS准则的信号估计以及基于线性插值的信道插值。     

一种有效的MIMO OFDM信道估计方法

传统的MIMO OFDM信道估计方法总是假设信道在由若干OFDM符号组成的每一帧的传输过程中保持不变,但这个假设在快变信道的条件下就不成立了.本文提出了一种适合快变信道的有效的估计方法.该方法通过在每个OFDM符号中设置若干训练子载波来跟踪快变的信道.首先讨论了在每个OFDM符号中所需的训练子载波的数目及其相应的位置,给出了在多发射天线多接收天线条件下的训练子载波的正交结构,然后在估计出训练子载波处的信道频率响应后.提出了一种不同于根据邻近训练子载波进行线性插值的以获得用于传输数据的子载波的信道频率响应的方法.仿真结果表明了我们所提出的方法的有效性.     

An Adaptive Channel Estimation Technique in MIMO OFDM Systems

In this paper, an adaptive channel estimation for MIMO OFDM is proposed. A set of pilot tones first are placed in each OFDM block, then the channel frequency response of these pilot tones are adaptively estimated by reeursive least squares （RLS） directly in frequency domain not in time domain. Then after the estimation of the channel frequency response of pilot tones, to obtain the channel frequency response of data tones, a new interpolation method based on DFT different from traditional linear interpolation method according to adjacent pilot tones is proposed. Simulation results show good performance of the technique.     

一种改进的MIMO OFDM系统信道估计算法

本文对MIMO OFDM系统中基于训练序列的信道估计问题进行了研究，针对信道冲击响应的最大抽头数大于每个OFDM符号中导频数的情况，提出一种有效的结合前后若干训练序列进行信道估计的算法和结合方式。仿真结果表明，在基于无线局域网（WLAN）中打包传送的MIMO OFDM系统里，本文的方法比采用块状训练序列的估计算法有着更小的归一化均方误差。     

Optimal pilot sequence design for channel estimation in MIMO OFDM systems

In orthogonal frequency division multiplexing (OFDM) systems, since virtual subcarriers are not used for transmission, approach of conventional uniformly placed pilot tones is not applicable in some situations. In this letter, based on nonuniform pilot tone placement, we derive optimal pilot sequences, which can achieve the minimum mean square error of the least squares estimate for multiple-input multiple-output (MIMO) OFDM systems. Simulation results demonstrate the effectiveness of the proposed approach.     

Performance Analysis of Analytic Interpolator for Comb-Type Pilot-Aided Channel Estimation in OFDM Systems Without ICI

Pilot-aided channel estimation in orthogonal frequency division multiplexing (OFDM) systems often needs an interpolation technique to construct the channel frequency response on nonpilot subcarriers. Many studies on interpolations have been published; however, the performance of interpolations such as cubic spline interpolation and linear interpolation is limited by either the number or the position of the pilot subcarriers. This paper proposes an analytic interpolator developed using a linear least square criterion to replace conventional interpolation techniques for comb-type pilot-added channel estimation in OFDM systems without inter-carrier interference. The performance evaluation of the analytic interpolator is performed both on the basis of theory and through simulation, and the results from both the analyses are found to be in good agreement. The simulation results show that the proposed method can effectively mitigate the ill-influences of limitations on the number and position of the pilot subcarriers and that it also outperforms the cubic spline interpolation and linear interpolation, especially for short or nonequidistant pilot patterns.     

A time-domain estimation method of rapidly time-varying channels for OFDM-based LTE-R systems

This paper addresses the performance of fast doubly selective fading channel estimation combined with Inter-Carrier Interference (ICI) cancellation for Long Term Evolution (LTE) communication platform in the High Speed Railway (HSR) environment. We consider the Channel Impulse Response (CIR) coefficients with a critical Doppler frequency shift and multi-path fading that were taken from the WINNER II channel model and the D2a propagation scenario, where the conditions of HSR are analyzed. As multi-path fading increases and the channel varies in the order of the symbol period, we first propose a novel approach for designing a pilot symbol structure in the time domain. Then, we describe the deployment of the proposed pilot symbol structure to estimate the channel in the time domain. Channel information corresponding to the data positions is obtained by linear interpolation. In each OFDM symbol, the slope and the initial value for establishing an interpolation function are estimated to adapt to the time variation of the channel. An accurate estimate of channel state information is used for the purpose of ICI cancellation. The simulation results show that the channel estimated by our proposed method can follow the real channel well, even in a very high Doppler frequency. The estimation method in terms of Mean Squared Error (MSE) significantly outperforms the state-of-the-art methods. The combination of our channel estimator with several interference cancelers provides a considerably better system performance than that achieved when frequency channel estimation is used.     

Equalization attacks against OFDM: analysis and countermeasures

This paper introduces, investigates, and proposes power‐efficient equalization jamming attacks against the orthogonal frequency division multiplexing (OFDM) and possible countermeasures. Signals known a priori, called pilot tones, are employed in the conventional OFDM‐based systems to estimate the channel response and perform equalization. Attacks against these pilot tones can hamper equalization and degrade target's performance. This paper begins with noise‐based pilot tone jamming and moves onto pilot nulling attack. The paper embraces two mitigation strategies of randomizing the pilot tones. The paper presents mathematical model of effective noise per symbol and simulation results for OFDM systems with conventional and randomized pilot‐tones under such attacks. The paper concludes that the noise‐based pilot tone attacks are power efficient than conventional attacks, and the pilot nulling is capable of doing the most damage, but in the expense of additional channel state information. The paper also claims that the mitigation strategies offer a remedy, but works relatively poorly when no jammer is present. Copyright © 2015 John Wiley & Sons, Ltd.     

Channel Estimation for Pilot-Aided OFDM Systems in Single Frequency Network

In single frequency network (SFN), there exist some special long delay spread channels as well as conventional multipath channels. For pilot-aided orthogonal frequency division multiplex (OFDM) systems, channel estimation is usually accomplished by interpolation with pilots. However, few pilot sub-carriers exist within the coherent bandwidth of the long delay spread channels in SFN. In this case, conventional frequency domain interpolation methods cannot work properly. In a narrowband channel, this paper indicates that both the real and the imaginary parts of channel frequency response can be accurately approximated as a sine-wave with DC offset. For many practical pilot-aided OFDM systems, the bandwidth of the narrowband channel mentioned before is comparable with the interval between several adjacent pilot sub-carriers. Then this paper proposes a sine-wave based frequency domain interpolation method for the channel estimation of pilot-aided OFDM systems in SFN. As simulation results show, the proposed method performs well in the long delay spread channel, whereas the mismatched Wiener interpolation filter (WIF) estimates channel response inaccurately. Moreover, the proposed method gives accurate channel estimation in conventional multipath channels, especially for the systems which adopt high order modulations.     

Decision-directed maximum-likelihood estimation of OFDM framesynchronisation offset

The authors propose a method to estimate the synchronisation offset for orthogonal frequency division multiplexing (OFDM) frame alignment without resort to pilot tones. A decision-directed maximum-likelihood estimation of frame synchronisation offset is derived, and the performance of the proposed scheme is confirmed by computer simulation for QAM systems     

限幅OFDM系统中的判决辅助信道估计

在正交频分复用(OFDM)系统中，对基带时域信号限幅是通常采用的一种降低信号峰均比的方法，但是由限幅产生的非线性干扰使系统的信道估计和信号检测性能降低。本文针对OFDM系统的频域插值信道估计方案，分析了限幅对信道估计性能的影响，提出一种利用判决的信号辅助信道估计的方法，并分析了该方法为信道估计所能提供的最大信干比增益。理论分析和仿真结果表明这种方法能够提高信道估计的精度，从而提高信号检测的性能，降低接收机的误符号率。