Robust and fast frequency offset estimation for OFDM based satellite communication

A pilot-aided Orthogonal Frequency Division Multiplexing (OFDM) frequency offset estimator designed for satellite communication system is proposed in the paper. The estimator focuses on the acquisition of the integer part of carrier frequency offset and the acquisition range is as large as the whole signal bandwidth. Making full use of the phase difference between received pilot and local referential pilot, a fast estimation is obtained. Compared with existing method, our method can also work well even in SNR as low as 0dB. Simulations verify the good performance of our method.     

OFDM系统一种精确的频率误差估计

OFDM在对抗多径方面是一种有效的宽带数字通信技术，但是由于频率误差的存在，使得OFDM(正交频分复用)子载波之间的正交性遭到破坏，系统在频率选择性衰落信道中的传输能力大大下降。本文提出了一种精确的频率误差估计法，可以很好地恢复频率同步。该方法的主要特点是以FFT为界分别在时域和频域内进行频率误差估计，并在时域中进行频率误差校正。     

基于OFDM无线通信系统的线性回归频偏估计算法

为了适应 60 GHz新型 WiFi 对频偏同步的要求, 提出了一种基于 IEEE802.11ad - 2012 修正案系统 的 线性回归频偏估计 (LROE)算法 。 算法将接收到的前导序列与本地序列进行互相关运 算,然后提取 互相关 值的 相位角进行线性回归计算 , 估计出频偏值。 从理论 推导了算法在高斯信道和多径信道下 的 抗噪声 和 抗多径 性能。 在 多径 信道仿真中, 本算法 的频偏 估计精度优于传统自相关算法 3个 数量级, 满足了系统 对高精度频偏估计的要求。 在全系统仿真中,对比研究了本算法和自相关算法对系统 误比特率(BER)性能 的影响, 并与无频偏对照组仿真进行对比。 LROE 算法的 BER 明显优于自相关算法。 仿真研究表明, LROE 算法基 本消除了频偏对系统BER的影响, 其性能 达到了无频偏 系统 的效果 ,达到了 60GHz 系统对频偏同步的 要求。     

OFDM通信系统中的一种最大Doppler频移估计算法

在分析了已有各种Doppler估计算法的基础上，根据功率谱估计经典算法——窗函数法的原理，直接利用接收到的频域中连续导频信号，进行最大Doppler频移估计。该算法同时适合于单载波和多载波系统。仿真结果均表明：在单径和多径条件下，该算法均可对正交频分复用（OFDM）传输系统中的最大Doppler频移获得较精确的估计结果。该算法十分适合于OFDM移动通信系统。     

OFDM系统中一种新型盲载波频偏估计

正交频分复用(OFDM)是一种能够有效地抗频率选择性衰落的技术。由于载波频偏的存在会引起载波间干扰,导致系统性能的严重下降。在分析载波频偏对系统影响的基础上,提出了一种联合译码的新型盲频偏估计算法。该算法只需要一个OFDM符号,将BICM译码的硬判决反馈信息作为虚拟训练序列,利用该反馈信号与接收信号的相关性进行频偏估计。通过BICM迭代译码,更新译码反馈信息,提高频偏估计精度。仿真结果表明,提出的算法经过3次迭代,可以有效地进行频偏估计。     

A frequency offset estimation algorithm for OFDM systems

A new fine carrier frequency offset estimation algorithm in Orthogonal Frequency Division Multiplexing （OFDM） system is proposed. The correlation item is the training sequence instead of the received signal in the new algorithm. Simulation results show that the performance of the new algorithm is 4dB-9dB better than that of Schmidl＇s algorithm. Coarse frequency offset estimation is also discussed in this paper, which is the improvement of Zhang＇s method. The estimation range using the improvement method is twice as that using the Zhang＇s method. Based on the hardware of the receiver and the improved algorithm, a method using Fast Fourier Transform （FFT） is proposed to implement the coarse frequency estimation. The chip area of OFDM system can be reduced by using the proposed method.     

一种OFDM定时同步与载波频偏估计算法

本文提出一个有效的利用周期性前导字的OFDM定时同步与频偏估计算法.算法分为两个步骤,首先根据前导字的特殊结构对Schmidl&Cox算法进行改进,将Schmidl&Cox算法中平台的检测变成单个峰值的检测,并由单峰的位置和复值可同时准确地估计符号定时和分数倍频偏;然后提出"相位差分序列",利用该序列的相关性对整数倍频偏进行估计.文中针对AWGN信道和SUI信道对算法进行了仿真,结果表明本文算法较其它算法具有更好的估计性能和鲁棒性.     

一种改进的适用于分组传输的OFDM频偏估计方法

提出了一种改进的基于训练符号的频偏估计方法.该方法通过综合利用训练符号和循环前缀的信息得到时域的细频偏估计,通过峰值检测得到频域的粗频偏估计.仿真结果表明,在多径衰落信道下,在频偏估计范围达到整个信号带宽的同时,本算法的估计精度也有了明显的提高,并且当信噪比达到12dB时,可以使剩余频偏限制在子载波间隔的2％以内.     

Carrier frequency offset estimation for OFDM systems using subcarriers

Carrier frequency offset (CFO) in orthogonal frequency-division multiplexing(OFDM) systems, which can induce the loss of orthogonality among subcarriersand result in significant performance degradation, is critical to be estimatedand compensated for. In this paper, a suboptimal scheme is proposed to estimatethe CFO using subcarriers. In the proposed scheme, the CFO is dividedinto the fractional part and the integer part, with the fractional CFO estimatedfollowed by the integer CFO estimation. Compared with previous work using subcarriers and exhaustive search, the implementation complexity of theproposed scheme is significantly lower due to the fact that the main componentrequired is a simple correlator. Furthermore, with only one trainingOFDM symbol and proper subcarrier allocation, the estimation range ofthe proposed scheme can be the inverse of the sampling duration. In contrast,conventional schemes with the same estimation range and complexity require two OFDM symbols. To achieve good performance,it will be shown that the subcarrier allocation should be based uponsome specific binary sequences such as the proposed extended m-sequences. It willalso be shown by simulations that the same subcarrier allocation criterionshould be employed for optimal maximum-likelihood CFO estimation.     

Efficient time domain threshold for sparse channel estimation in OFDM system

A novel efficient time domain threshold based sparse channel estimation technique is proposed for orthogonal frequency division multiplexing (OFDM) systems. The proposed method aims to realize effective channel estimation without prior knowledge of channel statistics and noise standard deviation within a comparatively wide range of sparsity. Firstly, classical least squares (LS) method is used to get an initial channel impulse response (CIR) estimate. Then, an effective threshold, estimated from the noise coefficients of the initial estimated CIR, is proposed. Finally, the obtained threshold is used to select the most significant taps. Theoretical analysis and simulation results show that the proposed method achieves better performance in both BER (bit error rate) and NMSE (normalized mean square error) than the compared methods has good spectral efficiency and moderate computational complexity.     

A novel frequency offset estimation method for OFDM systems with large estimation range

In this paper, we proposed a novel frequency synchronization method which has larger estimation range than conventional method proposed by Tufvesson. A two steps frequency offset estimation method is performed in the new method. In the first step the received signal is correlated with local training sequence to eliminate the influence of training sequence which makes the estimation range of frequency offset is independent of period of the repeated data in the training sequence. The received signal is correlated with its delay in the second step to obtain the fine estimation of frequency offset. Precise estimation can be achieved in the proposed method than Tufvesson's method. The period of the repeated data in the training sequence determines the estimation range of the frequency offset in the conventional method, while in the new method they are irrelevant. Tens of subcarriers spacing can be estimated by the new method. The new method can be used in other cases as long as a known sequence is transmitted, which will introduce valuable flexibility in training sequence design. The validity of the algorithm is verified in AWGN channel and multipath fading channel.     

多用户交织OFDM频偏估计

同步技术是OFDM的关键技术之一。提出一种针对多用户交织OFDM载频偏移估计的算法。首先分析上行链路中交织OFDM的信号结构，利用其周期性质导出特定的数据矩阵，实现频偏估计。这种估计方法不需要利用任何训练序列或导频信号，提高了系统频带利用率。仿真表明，该估计算法在信噪比大于零的情况下都能获得较好的频偏估计。     

Extending the threshold and frequency range for phase-basedfrequency estimation

Standard phase-based frequency estimation has a threshold that is high, is frequency dependent, and does not decrease with increasing signal length. These problems are solved by processing the signal with a highly overlapped filter bank before applying phase-based frequency estimation. By exploiting decimation, a closed-form matrix inversion, and cascades of simple filter banks, the computational complexity is kept low     

Maximum likelihood method for integer frequency offset estimation of OFDM systems

A novel method for integer frequency offset estimation of OFDM systems is derived, which is based on the maximum likelihood (ML) technique and exploits the differential information between two consecutive blocks of OFDM data symbols in the frequency domain. The reason why the proposed ML method has better performance than the conventional method is analysed.     

Blind estimation of OFDM carrier frequency offset via oversampling

Blind deterministic estimation of the orthogonal frequency division multiplexing (OFDM) frequency offset via oversampling is proposed in this paper. This method utilizes the intrinsic phase shift of neighboring sample points incurred by the frequency offset that is common among all subcarriers. The proposed method is data efficient - it requires only a single OFDM symbol to achieve reliable estimation, hence making it more suitable to systems with stringent delay requirement and mobility-induced channel variation. The proposed scheme is devised to perfectly retrieve frequency offset in the absence of noise. Quite remarkably, we show that in the presence of channel noise, this intuitive scheme is indeed the maximum likelihood estimate of the carrier frequency offset. The possible presence of virtual carriers are also accommodated in the system model, and some interesting observations are obtained. The Cramer-Rao lower bound is derived for the oversampling-based signal model, and we show through numerical simulation that the proposed algorithm is efficient. Practical issues such as identifiability, the front-end filter bandwidth, and the possible presence of correlated noises are also carefully addressed.     

Propagator based closed-form blind carrier frequency offset estimation for OFDM systems

Most of the blind Orthogonal Frequency Division Multiplexing （OFDM） Carrier Frequency Offset （CFO） estimators necessitate large number of samples to ensure the estimation accuracy. However, the number of samples can not be selected too large because of the carrier offset drift. In this letter, a new closed form algorithm for blind OFDM CFO estimation in frequency-selective channel is proposed. This method utilizes the propagator obtained from data matrix and the diagonal loading technique, thus it has better performance even only using one or two OFDM blocks. Furthermore, the range of the CFO estimation which can be handled is overall transmission spectral. Simulation results confirm its effectiveness.     

叠加训练序列OFDM系统的联合频偏与信道估计

频率偏移导致用于叠加训练序列系统中的一阶信道估计方法失效.文中提出一种利用叠加训练序列进行OFDM系统的联合频偏和信道估计的方案.在简单介绍所建立的OFDM系统后,文中利用训练序列的周期性推导出频偏特征向量,并根据阵列信号求子空间的原理,利用接收信号矩阵的奇异值分解求出上述向量所在的线性子空间,通过相关匹配求出频率偏移,最后获得时域信道参数.理论分析表明频偏估计范围反比于训练序列周期,因此算法具有估计范围大的优点,且适用于所有叠加周期训练序列系统.仿真分析表明了算法的有效性.     

一种高效的OFDM载波频偏盲估计方法

针对正交频分复用（OFDM）系统,提出了一种低复杂度、高性能的载波频偏盲估计 方法。该方法充分利用载波频偏估计代价函数在一定区间内的单谷特性,采用黄金分割法搜 索载波频偏估计代价函数极值,从而大大降低其计算复杂度。最后对该方法和Tureli′s MU SIC-like(TML)载波频偏估计方法在加性高斯白噪声信道和多径信道环境下进行了仿真,结 果表明, 在同等条件下该方法的计算复杂度大大低于TML载波频偏估计方法。     

A combined timing and frequency synchronization and channel estimation for OFDM

This letter addresses training-signal-based combined timing and frequency synchronization and channel estimation for orthogonal frequency-division multiplexing systems. The proposed scheme consists of two stages. At the first stage, coarse timing and frequency-offset estimates are obtained. Based on these estimates, a (coarse) channel response estimate is obtained. The timing and frequency-offset estimates at the second stage are obtained by maximum-likelihood (ML) realization based on a sliding observation vector. Then ML channel estimation is performed. A means of complexity reduction by an adaptive scheme is also presented. The simulation results show that the proposed combined approach performs quite well, and circumvents the problem of mismatch among individual synchronization tasks.