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。     

一种新的OFDM联合信道估计方法

OFDM（正交频分复用）作为一种高效可靠的传输方式,被广泛应用于无线通信系统中,近年来越来越受到人们的关注。文中主要研究基于导频的OFDM信道估计,在此基础上提出了一种采用最小二乘法的联合信道估计方法。这种方法能够将传送数据与导频结合起来进行更精确的信道估计,从而提高系统性能。仿真结果表明,文中方法优于经典的信道估计方法。     

基于特殊导频去噪的OFDM信道估计算法

主要研究了正交频分复用(OFDM)系统中基于导频辅助的信道估计算法,分析和比较了2种常用的算法,并提出了一种基于特殊导频序列去噪的信道估计算法。仿真结果表明:提出的基于特殊导频去噪的信道估计算法和LS算法相比,在没有增加LS算法运算复杂度的情况下,有效地抑制噪声的影响,提高了信道估计的准确性;和LMMSE算法相比,信道估计的准确性高于LMMSE算法。     

OFDM中基于导频的加窗FFT信道估计

正交频分复用（OFDM）是一种正交的多载波复用技术,分析了OFDM系统中基于导频的信道估计模型;基于导频的信道估计需要进行插值,为了降低插值算法的复杂度,研究了基于导频的FFT信道估计算法。由于直接FFT信道估计算法存在能量频谱泄露问题,提出了基于汉明窗函数的优化型加窗FFT信道估计的算法。实验仿真表明：在多径衰落信道下,该算法能够有效地提高OFDM系统的信道估计性能。     

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。     

OFDM系统中一种新的信道估计方法

在OFDM系统中,传统的基于导频的信道估计算法(如梳妆滤波器)都有一个共同的缺点,就是需要大量的导频符号,降低了系统的频谱效率。本文主要致力于研究新的信道估计算法,明确提出了将多径信道的时延估计和幅度估计分开的信道估计方案。对于时延估计,采用周期性插入块状导频估计时延。对于幅度估计,提出了一种全新的动态导频方式来估计多径的幅度,并在数学上证明了这种方法可以节省大量导频。仿真结果表明,本文提出的新算法和传统信道估计算法相比,可以节约至少百分之七十的导频,并且具有性能好,复杂度低的优点。     

光纤OFDM系统中的色散补偿技术研究

为了减小光纤的色度色散对光纤正交频分复用（OFDM）系统性能的影响,提出了将无线通信中的信道估计器引入光纤OFDM系统进行信道估计的色散补偿方法。在理论上分析了光纤的色度色散对于传输OFDM信号的影响,并在实验中采用梳状导频的形式,在每个OFDM码元的特定的子载波上插入导频,在接收端通过基于最小平方（LS）原则的LS估计器进行信道估计,得到了OFDM光纤传输的信道幅度响应和相位响应,通过使用LS估计器,直接调制的光OFDM信号在单模光纤中传输200km,误比特率低于10-6 ,功率代价小于2dB。结果表明,高频子载波较低频子载波更容易受到色散的影响,在光纤OFDM系统中引入信道估计器进行信道估计能够有效补偿由色度色散带来的相位偏移和幅度衰减。     

一种结合空时编码的导频辅助信道估计方法

多输入多输出-正交频分复用(MIMO-OFDM)作为一种高速率无线通信的有效方式,其信道估计技术具有非常重要的位置。将OFDM技术与基于发射分集的空时格码相结合,构造了一个基于空时格码的OFDM模型。基于MIMO-OFDM系统的导频设计准则及特性,提出了一种空时编码OFDM系统中基于导频的辅助信道估计方法。仿真实验结果表明信道估计的性能接近理想信道情况下的性能。     

基于OFDM的无线通信系统信道估计技术研究

OFDM是第四代移动通信系统的核心技术之一,信道估计是OFDM中急需解决的一个关键技术。首先介绍了OFDM技术的基本原理及其系统组成,重点研究了基于导频的OFDM信道估计技术,针对块状导频插入方式,讨论了LS（最小二乘）、MMSE（最小均方误差）和SVD（奇异值分解）算法,并通过仿真对其性能进行了比较。结果表明：MMSE和SVD算法比LS算法有更好的性能,但是LS算法的复杂度较低。     

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。     

Artificial Neural Network Channel Estimation Based on Levenberg-Marquardt for OFDM Systems

The many advantages responsible for the widespread application of orthogonal frequency division multiplexing (OFDM) systems are limited by the multipath fading. In OFDM systems, channel estimation is carried out by transmitting pilot symbols generally. In this paper, we propose an artificial neural network (ANN) channel estimation technique based on levenberg-marquardt training algorithm as an alternative to pilot based channel estimation technique for OFDM systems over Rayleigh fading channels. In proposed technique, there are no pilot symbols which added to OFDM. Therefore, this technique is more bandwidth efficient compared to pilot-based channel estimation techniques. Also, this technique is making full use of the learning property of neural network. By using this feature, there is no need of any matrix computation and the proposed technique is less complex than the pilot based techniques. Simulation results show that ANN based channel estimator gives better results compared to the pilot based channel estimator for OFDM systems over Rayleigh fading channel.     

Regression model based channel estimation for OFDM with multiple transmitters

Based on a nonlinear two-dimensional (2-D) regression model, we analyze the OFDM system performance with respect to the operation block size and pilot density. Specifically, we propose a regression model based channel estimation for the OFDM system with multiple transmit antennas. To obtain the proper estimation, we introduce the guidelines for designing pilot values and an expectation-maximization (EM) based solution with lower computation complexity.     

MIMO-OFDM系统中的时频二维信道估计技术研究

本文主要针对M IMO-OFDM系统中的导频信道估计技术进行了阐述,首先对我们采用的仿真系统进行了简单的描述,接着较为详细的介绍了我们采用的时频二维信道估计技术的原理,在第三部分,对采用的导频的正交性进行了分析;最后利用仿真测试的结论,验证了算法的有效性和鲁棒性。     

基于虚导频和功控导频的OFDM信道估计

提出了一种基于虚导频和功控导频的OFDM信道估计方法。该方法通过选择可靠的解调数据符号作为虚导频，同时对导频采用功率控制，从而提高了信道估计的质量。计算机仿真结果显示，相对于传统方法，可以用较少的导频获得性能的改善。     

Spatial filtering for pilot-aided WCDMA systems: a semi-blind subspace approach

This paper proposes a spatial filtering technique for the reception of pilot-aided multirate multicode direct-sequence code division multiple access (DS/CDMA) systems such as wideband CDMA (WCDMA). These systems introduce a code-multiplexed pilot sequence that can be used for the estimation of the filter weights, but the presence of the traffic signal (transmitted at the same time as the pilot sequence) corrupts that estimation and degrades the performance of the filter significantly. This is caused by the fact that although the traffic and pilot signals are usually designed to be orthogonal, the frequency selectivity of the channel degrades this orthogonality at the receiving end. Here, we propose a semi-blind technique that eliminates the self-noise caused by the code-multiplexing of the pilot. We derive analytically the asymptotic performance of both the training-only and the semi-blind techniques and compare them with the actual simulated performance. It is shown, both analytically and via simulation, that high gains can be achieved with respect to training-only-based techniques.     

Channel estimation techniques based on pilot arrangement in OFDM systems

Channel estimation techniques for OFDM systems based on a pilot arrangement are investigated. Channel estimation based on a comb type pilot arrangement is studied through different algorithms for both estimating the channel at pilot frequencies and interpolating the channel. Channel estimation at pilot frequencies is based on LS and LMS methods while channel interpolation is done using linear interpolation, second order interpolation, low-pass interpolation, spline cubic interpolation, and time domain interpolation. Time-domain interpolation is obtained by passing to the time domain by means of IDFT (inverse discrete Fourier transform), zero padding and going back to the frequency domain by DFT (discrete Fourier transform). In addition, channel estimation based on a block type pilot arrangement is performed by sending pilots in every sub-channel and using this estimation for a specific number of following symbols. We have also implemented a decision feedback equalizer for all sub-channels followed by periodic block-type pilots. We have compared the performances of all schemes by measuring bit error rates with 16QAM, QPSK, DQPSK and BPSK as modulation schemes, and multipath Rayleigh fading and AR based fading channels as channel models.     

Firefly Algorithm for Joint Estimation of Frequency Offsets and Channel in OFDMA Uplink

This paper addresses the problem of joint maximum-likelihood estimation of carrier frequency offsets, and channel responses in an orthogonal frequency division multiple access (OFDMA) uplink, which is a computationally-intensive, multiparameter, multidimensional problem. We propose to apply a recently-developed meta-heuristic algorithm called the firefly algorithm (FA) for the estimation of required parameters. We also introduce a new initialization procedure for the FA algorithm based on the separability of cost functions, which drastically reduces its computational complexity. A separate pilot OFDMA symbol is transmitted prior to the data symbols to estimate the parameters. The proposed technique is shown to achieve excellent performance at reduced computational complexity, especially at low signal-to-noise ratio. Also, it can be applied to OFDMA systems with any type of carrier assignment schemes. The significant features of the proposed estimation method are substantiated through extensive computer simulation studies and are compared with an important classical estimation technique available for the problem, to highlight its superiority.     

Robust channel estimation and ISI cancellation for OFDM systems with suppressed features

A feature-suppressed orthogonal frequency-division multiplexing (OFDM) system and the corresponding channel estimation and intersymbol interference (ISI) mitigation techniques are investigated in this paper. Cyclic prefix (CP) and pilot tones, which are commonly used in civilian OFDM systems for ISI mitigation and channel estimation, create distinctive waveform features that can be easily used for synchronization and channel estimation purposes by intercepting receivers. As a result, CP and pilot tones are eliminated in the proposed feature suppressed OFDM system to reduce the interception probability. Instead, a set of specially designed OFDM symbols, driven by different pseudorandom sequences, are employed as preambles to avoid unique spectral signature. These preambles are inserted into the OFDM data symbol stream periodically and in a round-robin manner. In addition, a random frequency offset is introduced to each preamble to further mask the multicarrier signature. New challenges arising from these feature suppression efforts are studied, including robust channel estimation and demodulation techniques in the presence of frequency offset and severe interference. Based on our interference analysis, an iterative ISI and intercarrier interference (ICI) estimation-cancellation-based technique is proposed for both channel estimation and OFDM data demodulation. Our channel estimator performs joint frequency offset and channel impulse response estimation based on the maximum-likelihood (ML) principle. To reduce its complexity, we employ a number of techniques, which include approximation of the ML metrics, as well as fast Fourier transform pruning. The performances and feasibility of the proposed feature suppressed OFDM system and the channel estimator are analyzed and verified through numerical simulations.     

Channel estimation using pilot tones in OFDM systems

Channel estimation techniques using pilot tones in OFDM systems are investigated. Two time-domain channel estimation methods are proposed. Based on theoretical analysis and computer simulation, for a channel with multipath distortion and AWGN, the proposed time-domain channel estimation schemes perform better than the conventional frequency-domain channel estimation techniques