OFDM系统中基于零子载波检测的频偏估计模糊度校正算法

OFDM系统的性能对频率偏移非常敏感。该文针对频偏估计中所产生的模糊度问题提出了一种基于零子载波检测的频偏估计模糊度校正算法。它利用OFDM符号中不传输信息的子载波进行频偏估计模糊度校正。文中分析了定时误差对算法性能的影响并分析比较了算法的复杂度。仿真结果表明,在性能相近的前提下,该文提出的频偏估计模糊度校正算法与传统的算法相比具有运算量小的优点。     

大频偏低信噪比条件下OFDM系统中的符号定时估计

符号定时估计是OFDM系统中的关键问题之一,该文就此问题提出了一种新的基于训练符号的定时估计算法。研究表明,新算法的性能基本不受载波频偏的影响,且具有很强的抗噪特性。通过采用这种算法可将定时误差缩小到只有抽样时间间隔的一半。并且,还可利用这个频域训练符号来进行载波频偏的估计,从而只需一个训练符号就能完成符号定时和载波频偏的联合捕获。     

一种优化的OFDM符号定时与频偏估计算法及其FPGA实现

符号定时与频偏估计是OFDM系统中的关键技术之一.本文分析了符号定时偏差与载波频偏对OFDM系统性能的影响,提出了一种优化的最大似然同步算法,并给出了在FPGA中的硬件实现方案,得到其仿真结果.传统的循环前缀符号同步算法,仅仅利用一个OFDM符号的循环前缀及其相应部分的相关性,其相关峰值不明显,同步效果不理想.本文提出一种充分利用连续多个OFDM符号同时进行相关功率运算,在同样信噪比的情况下,相对于单个OFDM符号进行同步估计的系统,符号定时与频偏估计的精度更高,误差更小.     

基于循环前缀的OFDM系统定时同步估计算法

正交频分复用(OFDM)是一种多载波高效调制技术,由于能够有效地对抗无线环境中多径信道引起的码间干扰(ISI),近些年来受到越来越广泛的关注.同步是一切数字通信系统能够有效工作的基础.由于OFDM相对于单载波通信系统对同步误差更加敏感,因此同步技术更是OFDM系统中最关键技术之一.文中定量分析了符号定时同步误差对系统性能的影响,并详细给出了基于循环前缀的符号定时同步估计算法及其性能分析.通过仿真对各基于循环前缀同步估计算法进行了比较,给出了算法的应用场三合.     

多径衰落信道下的OFDM时频联合同步算法

符号定时和载波频率偏差将严重影响OFDM系统的性能。基于具有重复结构的CAZAC序列,提出了一种仅利用一个CAZAC符号实现符号定时、整数倍频偏与小数倍频偏估计的同步算法。这种算法在完成粗定时同步的同时得到整数倍频偏估计,然后根据整数倍频偏估计值对定时估计进行修正,最后完成小数倍频偏估计。分析与仿真结果表明,该算法相对经典算法,不但提高了传输效率,还显著改善了OFDM系统的定时与频偏估计性能,相比CAZAC序列算法,提升了多径信道下的同步性能。     

利用CAZAC序列共轭对称性的OFDM时频同步算法

符号定时和载波频率偏差将严重影响OFDM系统的性能。基于具有重复结构的CAZAC序列,提出了一种仅利用一个CAZAC符号实现符号定时、整数倍频偏与小数倍频偏估计的同步算法。首先,所提算法在完成粗定时同步的同时得到整数倍频偏估计。然后根据整数倍频偏估计值对定时估计进行修正,最后完成小数倍频偏估计。分析与仿真结果表明,该算法相比于传统PARK算法,不仅完成了定时估计,而且得到了频偏估计;相对于经典SC算法,不但提高了传输效率,还显著改善了OFDM系统的定时与频偏估计性能;相比于利用CAZAC序列的同步算法,提高了算法的估计精度以及对多径信道的适应性。      

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OFDM定时同步算法

OFDM系统是一种多载波通信系统,对同步误差非常敏感,同步不准确在很大程度上降低了系统性能。在SC算法、Minn和Park算法的基础上,提出一种基于训练序列的改进的OFDM同步算法。新算法在频域上使用SC算法的训练符号,并通过对训练符号在时域乘以特殊的伪随机序列来消除定时同步中的峰值平台问题,利用训练序列在频域良好的自相关性估计整数倍频偏。仿真结果表明,与传统算法相比,改进算法在同步位置具有很尖锐的峰值,同步性能更好,符号定时更精确,载波频偏估计也有所提高。     

利用叠加导频技术进行OFDM系统同步的算法

主要讨论利用叠加导频技术对OFDM系统进行定时和频偏估计的算法。该算法利用线性叠加在OFDM符号上的PN序列估计定时和频率偏移。使用定时和频偏估计的方差来评价系统性能。通过在高斯和衰落两种信道中的仿真结果说明,此算法能够进行精确定时并且频偏估计范围较大。     

OFDM系统中同步问题的研究

OFDM系统的实现还存在着一些方面问题,如：同步问题尤其是频偏,峰值功率与平均功率比值过高,器件的非线性化等。OFDM系统对同步偏差敏感是本文讨论的重点,尤其是频偏。因此怎样获得准确的符号定时和载波频偏估计对OFDM系统性能至关重要。本文围绕OFDM同步过程中的符号定时和载波频率偏差问题进行描述,在分别介绍同步过程中捕获和跟踪两个阶段各自一些典型算法基础上,对一种基于保护间隔／循环前缀（PI／CP）的联合实现符号定时和载波同步最大似然估计（MLE）算法进行叙述并仿真。     

使用接收分集的确定性OFDM盲信道估计算法

本文针对无循环前缀的OFDM系统研究了非数据辅助的信道估计方法.借助于2个接收天线实现的信道分集,提出了一个确定性信道盲估计算法,并给出了信道可辨识性条件.该算法使用1个OFDM块便可在一个标量模糊因子的意义上确定信道冲激响应.算法计算量小,而且对输入信号的星座和统计特性没有约束.仿真结果证实了算法的有效性.     

MIMO radar OFDM chirp waveform diversity design with sparse modeling and joint optimization

In multiple-input multiple-output radar systems, the employed waveforms should have a large time-bandwidth product, good correlation property and satisfactory ambiguity function performance. This paper proposes a orthogonal frequency division multiplexing (OFDM) chirp waveform diversity design scheme with sparse modeling and joint optimization of the waveform code matrix and amplitude matrix. First, we formulate the waveform diversity design as a mathematical optimization problem with sparse modeling and optimally design a code matrix to select the hopping frequencies for each waveform. Thereafter, the waveform amplitudes are adaptively designed by minimizing the cross-correlation peaks with genetic algorithm. The proposed method can easily generate multiple pseudo-orthogonal waveforms with satisfactory correlation and ambiguity function performance. Additionally, the impacts of waveform number, hopping frequency interval and selectable frequency index are discussed. The superiority of the proposed OFDM chirp waveform design over existing methods is verified by extensive simulation results.     

Joint phase/amplitude estimation and symbol detection for wireless ICI self-cancellation coded OFDM systems

OFDM has been applied in the current wireless local-area networks and digital video broadcasting systems since it has the advantage over the conventional single-carrier modulation schemes when the frequency-selective fadings are present. Nevertheless, intercarrier-interference (ICI) due to Doppler frequency drift, phase offset, local oscillator frequency drift, and multipath fading will be a severe problem in OFDM systems. Previous ICI self-cancellation coding schemes can greatly reduce the ICI, but they are very sensitive to the phase ambiguity, which is due to the composite effect of the phase offset, the multipath fading and the local frequency drift. In this paper, the phase ambiguity and amplitude ambiguity problems in ICI self-cancellation coded OFDM receivers will be formulated and discussed. Then, a novel receiver which combines the current ICI self-cancellation coding techniques with a new expectation-maximization-based joint phase/amplitude estimation and symbol detection scheme is proposed. The outstanding performance of this proposed scheme is shown and compared with other existing methods at different noise levels through OFDM simulations.     

一种应用于OFDM系统的MUSIC改进算法

根据OFDM信号在频域由多个子载波构成的特点,提出了适用于OFDM系统的阵列信号处理模型。并利用子载波的正交性,推导出一种可以应用于OFDM系统的MUSIC改进算法。该算法将MUSIC空间谱表示为各子载波空间谱的叠加,并可在不同信道特性下,选择不同的子载波数量,以在较小的运算量下达到较佳的性能。在此基础上,提出实用方案,并通过仿真验证了该算法的性能。     

Joint low‐complexity equalization and CFO estimation and compensation for UWA‐OFDM communication systems

Frequency synchronization has a great importance in preserving the performance of the underwater acoustic (UWA) orthogonal frequency division multiplexing (OFDM) systems. The carrier frequency offset (CFO) estimation can be blind or data‐aided. In this paper, the Zadoff‐Chu (ZC) sequences are used for OFDM synchronization in UWA communications, and they are compared with different data‐aided algorithms. We propose a low‐complexity algorithm for CFO estimation based on ZC sequences. Also, a joint equalization and CFO compensation scheme for UWA‐OFDM communication systems is presented. Simulation results demonstrate that the proposed CFO estimation algorithm allows estimation of the CFO accurately with a simple implementation in comparison with the traditional schemes. Also, the performance of the UWA‐OFDM system can be preserved in the presence of frequency offsets.     

The Diversity Gain and ICI Suppression for OFDM Systems Over Doubly Selective Channels

How to compensate the detrimental impact of intercarrier interference (ICI) on BER performance is the key issue of signal detection for orthogonal frequency division multiplexing (OFDM) systems in doubly selective channels. In this paper, we propose an optimal linear ICI suppression algorithm with successive detection based on oversampling at the receiver, by which the OFDM systems can benefit from doubly selective channels for achieving both the multipath diversity and Doppler diversity. To reduce the additional computational complexity caused by oversampling, a low complexity implementation method for this algorithm is also presented. Simulations results show that the proposed algorithm can enhance the BER performance with the increase of the Doppler spread and the oversampled factor.     

正交频分复用系统中抗线性调频干扰算法研究

针对基于离散傅立叶变换的正交频分复用系统(OFDM),提出了一种新的线性调频(LFM)干扰抑制算法。该算法分为两部分:干扰估计和干扰重构抑制。干扰估计时,首先采用离散傅立叶变换进行粗略估计,然后采用离散匹配傅立叶变换进行精确估计,整个过程均可共用傅立叶变换单元,有效地节省了系统资源。实验结果表明:该算法的干扰抑制性能良好;与现有的抗线性调频干扰算法相比,其运算复杂度大为降低,具有很好的工程实用性。     

Structure-Based Low Complexity MMSE Channel Estimator for OFDM Wireless Systems

Wireless communication systems utilizing orthogonal frequency division multiplexing (OFDM) transmissions are capable of delivering high data rates over multipath frequency selective channels. This paper deals with joint estimation/interpolation of wireless channel using pilot symbols transmitted concurrently with the data. We propose a low complexity, spectrally efficient minimum mean square error channel estimator which exploits the correlation structure of channel frequency response for reducing the complexity. Specifically, it is shown that if pilots are inserted appropriately across OFDM subcarriers, the proposed algorithm requires no matrix inversion, thereby significantly relieving the computational burden without deteriorating the performance. Moreover, the knowledge of channel correlation is also not required for the proposed estimator. Simulation results validate that the proposed technique outperforms existing low-complexity variants in terms of mean square error and computational complexity.

     

OFDM系统中ICI自消除算法的研究

OFDM系统对相位噪声极其敏感，会引入共同相位误差CPE（Common Phase Error）和载渡间干扰ICI（Intercarrier Interference），严重降低系统性能。对ICI自消除算法进行了研究，分析比较了数据取反（data—conversion）ICI自消除算法和数据共轭（data—conjugate）ICI自消除算法性能，并基于此提出了一种改进的取反相位噪声自消除算法。仿真结果表明，该算法实施起来非常简单，且可以有效的降低相位噪声的影响。