Joint Time-Domain Tracking of Channel and Frequency Offsets for MIMO OFDM Systems

In this paper we address the problem of joint channel and frequency offset estimation and tracking in multiple-input multiple-output (MIMO) OFDM systems for mobile users. The proposed method stems from extended Kalman filtering and is suitable for time-frequency-space selective channels. Separate offset for each MIMO channel branch is considered because of the mobility and rich scattering. The channel taps and the frequency offsets are estimated in time-domain while the equalization is performed in frequency domain. Simulation results demonstrate that the proposed method tracks time-varying channels and frequency offsets with high fidelity. Realistic channel models are used in mobile scenarios. The proposed time-domain approach has improved performance and robustness in comparison to purely frequency domain processing. Computational complexity is lower as well.     

Blind Estimation of Carrier Frequency Offset and DC Offset for OFDM Systems

In this letter, a blind joint carrier frequency offset and DC offset (DCO) estimator for OFDM systems is proposed. By exploiting the underlying subspace of the received OFDM signals after the time-domain-average based coarse DCO cancellation, the proposed estimator can achieve excellent performance, which is demonstrated by simulations.     

Fast Frequency Offset Acquisition and Accurate Tracking in OFDM Systems

1　IntroductionOrthogonalFrequencyDivisionMultiplexing(OFDM )isaneffectivetransmissionschemetocombatmultipathfading[1～ 2 ,1 7] .Byinsertingaguardintervalbetweensymbolblockscalledcyclicprefix,theInter SymbolInterference (ISI)canbemitigated .OFDMisadoptedasthemodulationschemeforaDigitalAudioBroadcasting (DAB)sys tem[3] andAsymmetryDigitalSubscriberLoop(ADSL) [4] andisalso proposedastheterrestrialHDTVtransportinEurope[5] .OFDMsystemisverysensitivetocarrierfre quencyoffset.Manymet…     

Joint Blind Adaptive Carrier Frequency Offset Estimation and Channel Shortening

Multicarrier modulation has the advantage of robustness to multipath, so long as the delay spread of the multipath is less than the guard interval between successive transmitted blocks. However, it has the disadvantage of sensitivity to a carrier frequency offset (CFO), which disrupts the orthogonality of the subcarriers. If a CFO is present, it must be estimated and then corrected. In addition, if the channel delay spread exceeds the guard interval, this must be accounted for as well, usually by a channel shortening equalizer. Hitherto, CFO estimation and channel shortening have been addressed individually. However, this leads to a circular set of assumptions, since standard CFO estimators perform poorly when the channel is longer than the guard interval, and standard channel shorteners assume that CFO estimation has already been perfectly performed. In this paper, we investigate a blind, adaptive method for jointly performing CFO estimation and channel shortening.     

Joint Frequency Offset and Channel Estimation for OFDM Systems Using Pilot Symbols and Virtual Carriers

We consider joint estimation of carrier frequency offset and channel impulse response (CIR) for orthogonal frequency division multiplexing (OFDM) systems with pilot symbols and virtual subcarriers (VCs). We derive the receive-signal correlation structure due to the pilots and VCs, give the evidence of joint multivariate Gaussian distribution of the received samples, and derive an approximate maximum likelihood (ML) frequency offset estimator. We also derive the asymptotic mean-square error (MSE) and an approximate Cramer-Rao bound (CRB) and establish the asymptotic unbiasedness. Without pilots, in high signal-to-noise ratio, our estimator is equivalent to Liu and Tureli's estimator with N_v virtual carriers. When the pilot number (Np) is greater than the channel length L, our estimator acts as a subspace-based estimator with N_v + N_p - L virtual carriers. A decision-directed joint ML estimator is derived to iteratively update the estimates of frequency offset, data symbols and CIR. The optimal pilot and virtual carrier placement strategies are also discussed. The resulting decision-directed joint estimator performs within 0.8 dB of the ideal case even when the frequency offset is as large as 20%     

OFDM分数倍频偏与码元定时的联合估计

Smidle和Cox（SC）算法是OFDM系统中频偏和码元定时估计的经典算法。基于SC的导频设置,利用最大似然（ML）准则,提出一种分数倍频偏（FFO）和码元定时联合估计算法,通过计算机仿真比较可以看出,文中提出的联合估计算法的FFO估计性能与SC算法相同,而码元定时估计方法的性能明显优于SC算法。     

Joint Carrier Frequency Offset and Channel Estimation for Amplify-and-Forward Cooperative OFDM System

In this paper, we develop a new joint carrier frequency offset (CFO) and channel estimation algorithm for Amplify-and-Forward (AF) cooperative (CO)-OFDM systems. An expanded Basis Expansion Model (E-BEM) is proposed for time variant multipath channels. Based on the E-BEM, a joint OFDM channel estimation and synchronization method is derived. We also apply the space-frequency-block-code (SFBC) to AF CO-OFDM systems. Simulation results reveal that the proposed joint algorithm works well, leading to considerable gains of overall performance.     

Joint Maximum-Likelihood Estimation of Frequency Offset and Channel in MIMO Systems

This paper addresses the problem of frequency offsets and channel gains estimation for a Multiinput multi-output （MIMO） system in fiat-fading channels. The general case where the frequency offsets are possibly different for each transmit antenna is considered, The Maximum-likelihood （ML） estimation of the carrier frequency offset for each transmit antenna in a MIMO system is investigated in this paper. The exact solution to this estimation problem turns out to be too complex as it involves a search over a multi-dimensional domain. However, by making use of the Particle swarm optimization （PSO） method, we efficiently solve the above complex problem. So based on the PSO theory, a novel joint estimation algorithm of frequency offsets and channel gains is proposed. Simulation results show that the proposed algorithm has better performance as compared with the correlation-based estimation algorithm and asymptotically achieves the Cramer-Rao lower bound （CRLB）, which provides a new idea to solve the problem of joint frequency offsets and channel gains estimation for MIMO systems.     

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。     

A Numerical Technique for Blind Estimation of Carrier Frequency Offset in OFDM Systems

This paper proposes a numerical technique based on the Newton-Raphson method for blind maximum-likelihood estimation of carrier frequency offset in orthogonal frequency-division multiplexing systems. The proposed technique is characterized by low complexity and fast convergence while maintaining the estimation accuracy     

无线信道中基于代价函数的OFDM频偏跟踪算法

在OFDM系统中,频偏对系统的性能有很大的影响。本文推导出两种新的实际频偏和估计频偏的差值表达式, 在此基础上,提出了相应的基于代价函数的自适应频偏跟踪算法,并在多径瑞利衰落信道中针对多种时变频偏的情况进行了计算机仿真,获得了频偏跟踪曲线和均方误差曲线。仿真结果表明,由于算法的快速收敛特性,取得了较好的频偏跟踪性能。它们克服了非自适应方法和跟踪缓慢的自适应算法难以适应时变频偏的缺点。将跟踪到的频偏在接收端补偿以后, 可以减小子载波间串扰,降低系统解调后的误码率。     

OFDM系统信道估计与频偏跟踪联合算法

提出了一种信道估计与频偏跟踪的联合算法,该算法适宜于慢衰落信道。算法的基本 思想是利用慢衰落信道特性的慢变性,即相邻两帧间的信道条件变化小,用前一帧的信道特性作为该帧 的初始信道特性跟踪频偏,初步校正频偏之后再进行信道估计,估计后的信道特性再次用于校正频偏, 依次反复迭代。两次利用最大似然函数,经过几次迭代过程,逐步逼近频偏和信道冲击响应的实际值。 仿真结果表明该算法具有稳定好、精确度高等优点。     

Iterative Carrier Frequency Offset and Channel Estimation for Underwater Acoustic OFDM Systems

This paper presents a practical low-density paritycheck (LDPC) coded OFDM system designed for the underwater acoustic channel with its attendant sparse multipath channel and Doppler effects. The carrier frequency offset (CFO) and channel state information (CSI) are assumed unavailable to both to the transmitter and the receiver. Several different receiver structures are considered, all of which perform CFO/channel estimation, detection and decoding in an iterative manner. The convergence behavior of the iterative receivers and their asymptotic performance are evaluated using the extrinsic information transfer (EXIT) chart method. OFDM receiver performance is further evaluated through simulations and field tests in shallow water.     

RLS-Based Estimation and Tracking of Frequency Offset and Channel Coefficients in MIMO-OFDM Systems

In this paper, we investigate the problem of carrier frequency-offset (CFO) synchronization and channel estimation in multiple-input multiple-output orthogonal frequency-division multiplexing systems operating over unknown frequency-selective fading channels. We first propose a novel joint CFO and channel estimator, assuming time-domain training blocks are available. The proposed joint estimator consists of two recursive least-square (RLS) algorithms which iterate their estimated CFO and CIR values. We then derive a more precise pilot-aided RLS algorithm to estimate the residual frequency synchronization errors or track small CFO changes. With this, the accuracy of channel estimation is also enhanced. The analysis and simulation results show that, the proposed estimation and tracking scheme which is fully compatible with the existing standards is able to attain fast convergence, high stability, and ideal performances as compared with relevant Cramer–Rao bounds in all ranges of signal-to-noise ratio. Moreover, it can work well for wide tracking range up to ±0.5 of the subcarrier spacing.     

OFDM同步跟踪阶段的频偏估计算法

ＯＦＤＭ对栽频偏差是很敏感的，因此不仅在同步的捕获阶段，而且在同步的跟踪阶段，精确的频偏估计是很重要的。文献［３］提出了一种简化的相干判决反馈的迭代算法来估计同步跟踪阶段的载频偏差，对于１２８个子载波、ＱＰＳＫ调制的ＯＦＤＭ系统，在多径衰落信道下，当频偏小于子载波间隔的１５％时，频偏估计的标准方差小于１％，但是这个结果是在假设信道频率响应、定时偏差、载波相位等参数已知的条件下取得的，而当这些参数的估计存在误差时，由仿真结果可看出，频偏估计性能恶化了，因此，提出了采用差分判决反馈的频偏估计算法，虽然只能估计小于子载波间隔 ８％的频偏，但差分判决反馈算法不需要知道信道的参数，由于在同步跟踪阶段，频偏一般都较小，因此采用差分判决反馈算法可以很好地估计出同步跟踪阶段的频率偏差。     

Blind Integer Carrier Frequency Offset Estimation for OFDM Systems with Constant Modulus Signaling

This paper presents a blind integer carrier frequency offset (IFO) estimator for the constant modulus (CM) signaling based OFDM systems. The key idea behind this method is a power reallocation technique which modifies the power of data in some predefined subcarriers over two consecutive OFDM symbols, thus the IFO can be estimated in the receiver via analyzing the distribution of these subcarriers. Instead of the exhaustive search, a low-complexity algorithm is proposed. The selection criterions of these predefined subcarriers are carefully designed and more than two OFDM symbols are employed to improve the robustness of this method to the multipath effect. Furthermore, this method can be applicable in multiple-input multiple-output (MIMO) OFDM systems with the orthogonal space-time block coding (OSTBC).     

Joint Data Detection and Channel Tracking for OFDM Systems With Phase Noise

This paper addresses the problem of data detection in orthogonal frequency division multiplexing (OFDM) systems operating under a time-varying multipath fading channel. Optimal detection in such a scenario is infeasible, which makes the introduction of approximations necessary. The typical joint data-channel estimators are decision directed, that is, assume perfect past data decisions. However, their performance is subject to error propagation phenomena. The variational Bayes method is employed here, which approximates the joint data and channel distribution as a separable one, greatly simplifying the problem. The data detection part of the resulting algorithm provides soft data estimates that are used for channel tracking. The channel itself is modeled as an autoregressive process allowing for a Kalman-like tracking algorithm. According to the developed algorithm, both data and channel estimates are exchanged and updated in an iterative manner. The performance of the proposed algorithm is evaluated by simulations. Furthermore, since OFDM is extremely sensitive to the presence of phase noise, the algorithm is extended to operate under severe phase noise conditions, with moderate performance degradation.      

基于RLS算法的OFDM CFO与STO联合估计方法

针对OFDM载波频偏与采样时偏的联合估计问题,提出了一种基于递推最小二乘(RLS)算法的新方法.该估计方法采用递推计算方式,能方便实时地更新偏差量的估计值.仿真结果表明,这一方法与现有的同类方法相比,在相同的信道条件下具有更优的估计性能.     

Decision Feedback Frequency Offset Estimation and Tracking for General ICI Self-Cancellation Based OFDM Systems

Inter-carrier interference (ICI) self-cancellation schemes were often employed in many OFDM systems as a simple and effective approach to suppress ICI caused by carrier frequency errors. The same procedure, however, can not perform very well at high frequency offsets. We propose a simple decision feedback scheme based on the general ICI self-cancellation scheme to perform estimation and tracking of the carrier frequency offsets. A system with the scheme does not consume additional bandwidth since it used the same data symbols employed for ICI cancellation for the estimation. After an initial estimation is completed, the scheme switches to the tracking mode to carry out the estimation of deviations in the frequency offsets. Finally this fine-tuned estimate is applied to the ICI self-cancellation scheme concurrently for frequency offset correction and hence improved the system performance greatly. Simulation results showed that our scheme allowed up to 9% of random variations in the frequency offset. The effectiveness of our scheme is further verified by calculating the bit error rate performance of various OFDM receivers.