MU-MIMO-OFDM上行频偏与信道联合跟踪

本文提出了一种在MU-MIMO-OFDM （Multiple-User Multiple-Input-Multiple-Output Orthogonal-Frequency-Division-Multiplexing）上行链路中,联合跟踪残留频偏（Residual Carrier Frequency Offset,RCFO）和信道的算法.本算法采用EM （Expectation Maximization）方法求解该非线性问题,并使用变分推断来近似原来复杂的隐变量的后验概率.在估计RCFO时,考虑了信道估计误差的概率分布,从而降低了信道估计误差对跟踪性能的影响.仿真中,本算法达到了较高的跟踪精度,尤其是在高信噪比时没有误差平台问题.     

基于顺序蒙特卡罗方法的窄带MIMO信道盲跟踪

目前流行的空时处理技术的关键假设在于接收机已知MIMO信道状态信息,而MIMO信道是时变的,所以如何跟踪MIMO信道就变得十分重要。该文提出了一种基于顺序蒙特卡罗技术的窄带MIMO信道盲跟踪方法,仿真结果表明该方法能够很好地跟踪信道。     

OFDM频偏信道联合估计算法

针对OFDM系统中频偏和信道联合估计问题进行了研究。为了克服单独估计的缺陷,提高精度,提出了一种低复杂度的频偏信道联合估计算法。该算法首先通过采用正交性质的恒包络零相关序列,运用最大似然法估计信道,提取频偏估计值,然后对信道结果进行修正,使之更接近真实信道。同时给出了频偏估计的克拉美罗界。仿真结果表明,相比于最小二乘联合估计法,本文算法估计均方误差更小,具有更好的估计性能。      

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.     

OFDM系统中的全数字频偏跟踪环

频偏跟踪是影响到OFDM系统性能的关键技术之一。该文基于迟早门类型的锁相环,提出了一种新的适合于OFDM系统的频偏跟踪算法。研究表明,该算法既能在AWGN信道中也能在多径衰落信道中完成频偏跟踪工作。并且其低信噪比下的频偏跟踪性能比一般的频偏跟踪方法好几个数量级。如果系统对定时估计的要求不是特别高,该算法还能实现定时偏移和频率偏移的联合跟踪。     

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%     

A Blind Sequential Monte Carlo Detector for OFDM Systems in the Presence of Phase Noise, Multipath Fading, and Channel Order Uncertainty

In this paper, an efficient detector is developed to address the blind detection problem for an orthogonal- frequency-division-multiplexing (OFDM) system in the presence of phase noise and unknown multipath fading, even with channel order that is possibly not known and time varying. The proposed maximum a posteriori detector is a combination of the sequential Monte Carlo (SMC) method and the variance reduction strategy known as Rao-Blackwellization. Being blind, the developed detector, does not rely on pilot tones for the detection of the transmitted data. However, as in most work found in the literature, the aforementioned detector, which we call the RB-SMC detector, invokes the assumption of a fixed and known channel order, which may be a limitation in a number of scenarios. Therefore, to relax this assumption, we model channel order uncertainty via a first-order Markov process and subsequently introduce appropriate extensions to the RB-SMC detector, thereby proposing a novel algorithm called the E-RB-SMC detector. The performance of the novel SMC-based detectors are validated through computer simulations. It is shown that the proposed SMC-based detectors achieve near bound performance. In terms of convergence speed, the proposed E-RB-SMC detector also shows the smallest acquisition time amongst the considered algorithms.     

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.     

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。     

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.     

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。     

OFDM系统的对数似然比最大化盲频偏估计算法

为了消除短波OFDM通信中的频率偏移问题,对差分OFDM信号进行了分析,并对最大 后验概率（MAP）译码的性质进行了研究,提出了一种似然比最大化的迭代盲频偏估计 算法。由于该算法充分利用了软译码得到的似然比信息,因此可以实现最优信号检测。计算 机仿真证明了该算法的有效性。进行了短波OFDM信号通信实验,得到了实测数据。实测数据 检测结果表明,该算法的误码率性能随着迭代次数的增加而提高,且一发两收系统性能要比 单发单收系统性能至少提高2 dB。     

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。     

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.