基于多项式模型的时频二维OFDM信道估计算法

本文提出了一种内插导频的时频二维信道估计算法,用于无线移动信道下的正交频分复用(OFDM)系统。该算法基于梳状内插的导频信号模型,采用多项式模型对信道进行拟合,从而能够精确的估计出信道信息。与时频二维维纳滤波以及自适应判决反馈的算法相比,该算法不需要预先知道信道相关矩阵以及信噪比等信道信息,简单易于实现,并且能够有效的抑制信道估计中的噪声干扰。仿真结果表明,在衰落环境下,该算法的信道估计最小方差(MSE)性能与最小二乘(LS)估计最小方差性能相比有显著提高。     

DFT-S-GMC系统中均匀导频信道估计方法性能分析

针对我国提出的离散傅里叶(DFT)扩频广义多载波(DFT -S-GMC)系统,给出了使用均匀分布导频的信道估计方法.该方法首先采用最小二乘算法估计导频音的信道频响,然后内插或外推非导频音的信道频响,再利用信道频响DFT过采样特性,数据块特定用户数据子带信道频响可由与之对应的导频块内子载波处信道频响线性内插获得,并在时间维上线性内插出数据子带处的信道频响.最后,通过仿真比较分析了高低阶插值、内插外推方法以及导频密度对信道估计方法性能的影响.     

OTFS系统中基于最小多普勒间干扰的导频设计

正交时频空(OTFS)调制系统能够改进时变频选信道的错误概率,但是目前只是简单地采用矩形导频。文中在OTFS系统中一种基于冲激的信道估计模型的基础上,优化其导频图案。在固定导频开销的约束下,最小化多普勒间干扰,也即是最大化被估计的信道平均功率。经论证,该优化问题可以转化为一个经典的背包问题,并可以通过贪心算法求解。仿真结果表明,文中经过优化的导频图案与原矩形导频图案相比,相同的导频开销下,具有更低的均方误差和误码率。     

基于导频信号的LTE系统高移动性信道估计算法

在长期演进(Long Term Evolution, LTE)系统中,精确的信道估计是均衡和相干解调的前提。在高速移动信道下,由于LTE下行链路离散非均匀的导频模式,传统的时间方向信道估计方法性能较差。为了提高信道估计精度,首先利用离散傅里叶变换(Discrete Fourier Transform, DFT)的信道估计方法提高导频处的估计精度。然后根据LTE系统的导频结构,设计了两种时间方向信道估计方法:1) 当信道的统计特性未知时,把不等间隔的导频分布转换为等间隔的导频分布,并利用三阶样条内插估计出数据处的信道频域值;2) 当信道的统计特性已知时,基于最小均方误差准则,设计高性能的自适应内插器,以准确获得信道估计所需系数。仿真和分析表明,相对于传统的线性内插算法,这两种信道估计算法利用更多的导频信息,可大大提高信道估计的精度,同时提升系统的解调性能。      

BICM-OFDM系统中基于叠加导频的迭代信道估计

传统叠加导频算法应用在正交频分复用系统中存在信道估计精度差的缺点。针对比特交织编码调制（BICM）联合正交频分复用（OFDM）的系统（BICM—OFDM）,提出一种利用叠加导频的迭代信道估计算法。该算法充分利用了均衡器的反馈信息,分两步进行。首先,导频信号叠加在发送信号上,接收端采用一阶统计量进行初始信道估计;然后利用均衡器的硬判决反馈信息,在信道估计与均衡之间进行信息交换,通过迭代的方式提高信道估计精度。在短波宽带信道下的仿真结果表明,与传统叠加导频算法相比,新算法经过二次迭代可有效提高信道估计精度和系统误码率性能。     

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。     

小波包多载波系统中的LMS信道估计

为了提高正交小波包波分复用(OWPDM)系统抗多径衰落和多普勒频移的性能,提出了一种新的半盲信道估计算法。首先利用时域滤波原理滤去信道响应中的噪声,之后利用信号在时域补零相当于频域内插的原理来获得更精确的最小均方算法(LMS)估计器迭代初值。仿真结果表明,在插入更少导频数目的情况下,新算法估计性能优于最小二乘(LS)信道估计算法,在插入相同数目导频的情况下,与传统的LMS信道估计算法相比,新算法能使系统获得更好的误码性能。     

BICM-OFDM中利用叠加导频的迭代信道估计

提出了一种利用叠加导频的迭代信道估计算法。将导频信号叠加在发送信号上,利用一阶统计量进行初始信道估计。利用比特交织编码调制（BICM）中迭代译码的硬判决反馈信息,进行迭代信道估计。该算法在不牺牲信息速率的前提下,有效提高了系统带宽利用率。在BICM-OFDM系统中的仿真结果表明,与传统叠加导频算法相比,文章提出的算法可有效提高信道估计精度和系统性能。     

移动WiMAX下行系统SVD- FFT内插信道估计

针对移动WiMAX下行系统中导频个数少且分配不均匀导致的边缘效应问题,提出了一种基于奇异值分解(SVD)的快速傅里叶变换(F下T)内插信道估计方法.该方法首先得到每个符号实际分配导频处的最小二乘信道频响估计,当前符号在相邻符号实际分配导频处的信道频响由相邻符号信道频响线性内插得到;然后,将上述信道估计值合并且按子载波索...     

MIMO-OFDM系统导频辅助的信道估计

以MIMO—OFDM系统为研究对象，通过设计正交导频进行导频辅助信道估计，对数据点的信道响应进行线性内插、三次样条插值和二次曲线拟合算法，并通过Matlab进行计算机仿真，对这些算法的BER性能进行比较。结果表明：随着信噪比（SNR）的增加，三次样条（Spline）内插和线性内插性能要远好于二次曲线拟合导频估计方法。     

两种插值信道估计方法的比较

在瑞利衰落信道中,使用时分复用的导频信号辅助的信道估计方案时,提出了在使用极大似然方法估计出导频符号的信道参数之后,再利用一个时隙上所有导频符号的Chebyshev多项式的数学插值估计数据符号信道参数的方法。同时比较了分段线性插值加递归的信道估计方法。仿真结果表明,使用Chebyshev多项式的数学插值估计信道参数的方法比使用线性插值加线性递归的信道估计方法能更好的改善系统的性能。     

Model-based channel estimation for OFDM signals in Rayleigh fading

This paper proposes a robust pilot-assisted channel estimation method for orthogonal frequency division multiplexing (OFDM) signals in Rayleigh fading. Our estimation method is based on nonlinear regression channel models. Unlike the linear minimum mean-squared error (LMMSE) channel estimate, the method proposed does not have to know or estimate channel statistics like the channel correlation matrix and the average signal-to-noise ratio (SNR) per bit. Numerical results indicate that the performance of the proposed channel estimator is very close to the theoretical bit error propagation lower bound that is obtained by a receiver with perfect channel response information     

分布式空频编码协同通信系统中基于导频辅助的频域信道估计算法

针对频率选择性衰落信道下的分布式空频编码协同通信系统,提出了基于导频辅助的各种频域信道估计算法,包括最小二乘、线性最小均方误差、低阶近似线性最小均方误差算法,并给出了相应的均方误差性能表达式.理论分析和仿真结果表明本文提出的信道估计算法在协同通信系统中具有很高的实用性和精确度.     

OFDM系统中基于分组导频的迭代信道估计算法

对OFDM系统中由于信道的时变性产生的载波间干扰进行了分析,并在此基础上提出了一种分组导频结构的DFT迭代信道估计算法。与原有的信道估计算法相比,此种算法不仅在一般的信道条件下具有良好的性能,而且更加适合快变信道条件下的OFDM系统。仿真结果表明:提出的OFDM信道估计算法不仅可以给出精度较高的信道信息,而且近似达到理想信道估计的性能。     

BER Analysis of Equalized OFDM Systems in Nakagami, <Emphasis Type="Italic">m</Emphasis> < 1 Fading

The orthogonal frequency division multiplexing (OFDM) transmission scheme is currently experiencing increased popularity due to advances in very large scale integration technology. It is used for a variety of broadband systems such as asymmetric digital subscriber lines, very-high-speed digital subscriber lines, digital video, and audio broadcasting, and wide local area network standards such as IEEE 802.11a, IEEE 802.11 g, and ETSI Hiperlan/2. However, propagation impairments can cause severe degradation in bit error rates (BER) for coherent detection. We derive a semi-analytical method to evaluate BER of a quadrature phase shift keying (QPSK)-OFDM system in Nakagami, m < 1 fading and additive noise where pilot-assisted linear channel estimation and channel equalization are used. This allows modeling of more severe fading environments than can be depicted by a Rayleigh distribution. Numerical simulations are used to validate the proposed methods. The techniques developed can be applied to other channel estimation techniques, modulation schemes and the performance evaluation of equalized single carrier narrowband systems.     

Optimal and mismatched detection of QAM signals in fast fading channels with imperfect channel estimation

In this paper, we derive an optimal detector for pilot-assisted transmission in Rayleigh fading channels with imperfect channel estimation. The classical approach is based on obtaining channel estimates and treating them as perfect in a minimum distance detector (this is called mismatched detector). The optimal detector jointly processes the received pilot and data symbols to recover the data. The optimal detector is specified for fast frequency-flat fading channels.We consider spline approximation of the channel gain time variations and compare the detection performance of different mismatched detectors with the optimal one. Further, we investigate the detection performance of an iterative receiver in a system transmitting turbo-encoded data, where a channel estimator provides either maximum likelihood estimates, minimum mean square error (MMSE) estimates or statistics for the optimal detector. Simulation results show that the optimal detector outperforms the mismatched detectors. However, the improvement in the detection performance compared to the mismatched detector with the MMSE channel estimates is modest.     

Performance analysis of equalized OFDM systems in Rayleigh fading

Channel estimation is usually needed to compensate for the amplitude and phase distortions associated with a received orthogonal frequency-division multiplexing (OFDM) waveform. This paper presents a systematic approach for analyzing the bit-error probability (BEP) of equalized OFDM signals in Rayleigh fading. Closed-form expressions for BEP performance of various signal constellations [phase-shift keying (PSK), differential phase-shift keying (DPSK), quaternary phase-shift keying (QPSK)] are provided for receivers that use a linear pilot-assisted channel estimate. We also derive the optimal linear channel estimates that yield the minimum BEP and show that some previous known results are special cases of our general formulae. The results obtained here can be applied to evaluate the performance of equalized single-carrier narrowband systems as well.     

Effect of imperfect channel estimation on transmit diversity in CDMA systems

In this paper, the effect of imperfect channel estimation on the transmit diversity based on space-time block coding for the downlink of a direct-sequence code-division multiple-access system is studied. Two transmit antenna and one receiving antennas are employed. However, the results of this paper can be extended to the system with more receiving antennas. Each channel is modeled as frequency-selective Rayleigh fading and the pair of channels corresponding to two transmit antennas are mutually independent. Both spatial diversity gain and multipath diversity gain are obtained in the system. The system performance is evaluated in terms of bit-error rate under the perfect and imperfect channel estimation. A pilot-assisted channel-estimation scheme with one common spreading code sequence is exploited. It is shown that the inaccurate channel estimates suffering from multiple access and multipath interference significantly degrade the system performance and can be effectively improved by use of a simple low-pass filter. The investigation of the power ratio of pilot to data channels illustrates that the base station should dynamically adjust the transmit power of the pilot channel according to the varying system configurations in order to achieve the best performance.     

基于特定导频OFDM信道估计的研究

在OFDM系统中,准确的信道估计是对抗瑞利衰落信道中信道干扰的关键。提出了一种对OFDM信道估计的新方法,利用CR Spline函数的插值方法,设计一个特定的导频结构,从而导出信道估计的CR Spline函数形式,用该函数近似信道衰落包络。实验结果表明,仅用少量导频就可以近似出信道特性,并具有良好的平滑性。计算机仿真证明了提出算法的有效性。