Blind channel estimation for multiple antenna OFDM system subject to unknown carrier frequency offset

The problem of channel estimation for multiple an- tenna orthogonal frequency division multiplexing （OFDM） systems subject to unknown carrier frequency offset （CFO） is addressed. Multiple signal classification （MUSIC）-Iike algorithm, which generally has been used for direction estimation or frequency estimation, is used for channel estimation in multiple antenna OFDM systems. A reduced dimensional （RD）-MUSIC based algorithm for channel estimation is proposed in multiple antenna OFDM systems with unknown CFO. The Cramer-Rao bound （CRB） of channel estimation in multiple antenna OFDM systems with unknown CFO is derived. The proposed algorithm has a superior performance of channel estimation compared with the Capon method and the least squares method.     

Blind CFO estimation algorithm for OFDM systems by using generalized precoding and trilinear model

This paper discusses the blind carrier frequency offset (CFO) estimation for orthogonal frequency division multiplexing (OFDM) systems by utilizing trilinear decomposition and generalized precoding. Firstly, the generalized precoding is employed to obtain multiple covariance matrices which are requisite for the trilinear model, and then a novel CFO estimation algorithm is proposed for the OFDM system. Compared with both the joint diagonalizer and estimation of signal parameters via rotational invariant technique (ESPRIT), the proposed algorithm enjoys a better CFO estimation performance. Furthermore, the proposed algorithm can work well without virtual carriers. Simulation results illustrate the performance of this algorithm.     

Pseudo-noise preamble based joint frame and frequency synchronization algorithm in OFDM communication systems

Frame and frequency synchronization are essential for orthogonal frequency division multiplexing （OFDM） systems. The frame offset owing to incorrect start point position of the fast Fourier transform （FFT） window, and the carrier frequency offset （CFO） due to Doppler frequency shift or the frequency mismatch between the transmitter and receiver oscil ators, can bring severe inter-symbol interference （ISI） and inter-carrier interference （ICI） for the OFDM system. Relying on the relatively good correlation charac-teristic of the pseudo-noise （PN） sequence, a joint frame offset and normalized CFO estimation algorithm based on PN preamble in time domain is developed to realize the frame and frequency synchronization in the OFDM system. By comparison, the perfor-mances of the traditional algorithm and the improved algorithm are simulated under different conditions. The results indicate that the PN preamble based algorithm both in frame offset estimation and CFO estimation is more accurate, resource-saving and robust even under poor channel condition, such as low signal-to-noise ratio （SNR） and large normalized CFO.     

突发OFDM系统的精确定时估计及性能仿真

首先从理论上分析了载波频偏对基于互相关的精确定时估计算法性能的影响,然后提出一种对频偏不敏感且与训练符号结构无关的精确定时算法,并利用仿真分析了不同信道条件下定时测度函数在精确定时位置的均值、方差分布特性。将该算法应用于IEEE 802.16d系统中进行仿真,结果表明该算法能够克服载波频偏影响,且定时精度高。     

Joint frequency offset tracking and PAPR reduction algorithm in OFDM systems

This paper presents an algorithm that aims to reduce the peak-to-average power ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) communication systems while maintaining frequency tracking. The algorithm achieves PAPR reduction by applying the complex conjugates of the data symbol obtained from the frequency domain to cancel the phase of the data symbol. A likelihood estimator is used to obtain the sub-carrier phase error due to the residual carrier frequency offset (RCFO) using the same complex conjugates as a pilot signal. Furthermore, a joint time and frequency domain multicarrier phase locked loop (MPLL) is developed to compensate additional frequency offset. Simulation results show that this algorithm is capable of reducing PAPR without impacting the frequency tracking performance.     

用于有虚载波OFDM系统的信道估计算法

提出了一种适用于有虚载波的正交频分复用系统的改进信道估计算法。由于传统的信道估计器无法估计虚载波处的信道传输函数,从而逆离散傅里叶变换导致信道能量泄漏,使得时域滤波方法不能直接应用。为了解决这个问题,利用信道冲激响应的有限性,推导了以最小二乘法(least square, LS)来估计虚载波处的信道传输函数。进一步观察信号子空间和噪声子空间中噪声的相关性,使用最大后验概率准则估计出信号子空间的噪声并消除其带来的估计误差。该算法不仅可在训练模式下应用,也可推广应用于基于判决反馈的跟踪模式。仿真结果表明,改进算法无误符号率平台效应,且其误符号率较传统LS算法有2 dB的信噪比增益。     

非数据辅助的OFDM系统联合同步算法设计

研究了正交频分复用(orthogonal frequency division multiplexing, OFDM)系统中非数据辅助的同步问题,通过对OFDM时域信号延时相关函数进行分析,设计了一种非数据辅助且具有较低复杂度的联合采样频率、载波频率及符号定时同步算法。算法根据延时相关函数以OFDM符号率为周期的周期特性,首先从延时相关函数的模中提取OFDM符号的周期标记,通过对收端和发端在相同时间内的采样点数之比进行采样频率偏差(sampling frequency offset, SFO)估计;然后通过对延时相关函数的模进行分段平均,从中提取去除循环前缀(cyclicprefix,CP)的开始时刻,从而进行符号定时同步和载波频率偏差(carrier frequency offset, CFO)估计。仿真结果表明,算法性能受累积时间长度和CP长度影响较大,而受OFDM基带调制方式影响较小;与其他同类文献相比,本文的联合同步算法具有较低复杂度和较好的性能。     

基于内插结合MMSE均衡消除OFDM系统相位噪声的算法

针对正交频分复用（orthogonal frequency division multiplexing, OFDM)系统对相位噪声的敏感特性,提出一种基于内插结合最小均方误差(minimum mean square error, MMSE)均衡消除OFDM系统相位噪声的算法,通过在发射端的IFFT之前,对符号进行零子载波的内插,可有效地降低OFDM系统相位噪声造成的子载波间干扰（inter carrier interference, ICI）,提高信干比。通过MMSE均衡方式,可消除OFDM系统相位噪声造成的公共相位误差（common phase error, CPE）。仿真结果表明,本算法可以消除CPE引起的整个信号星座的旋转,同时大大降低ICI引起的信号星座点的云状发散,OFDM系统的相位噪声在高斯信道和多径信道下都能够得到有效抑制,且复杂度低、实现简单,具备良好的实用价值。     

基于波达方向矩阵的MIMO-OFDM系统载频偏移估计

MIMO-OFDM系统中设置虚拟子载波后,时域接收符号向量与经频域信道系数加权的频域发射符号向量之间通过一个范德蒙矩阵相互联系,且该矩阵的列向量的结构决定于载频偏移量。基于此,提出了一种采用波达矩阵方法、利用范德蒙矩阵的移不变结构完成载频偏移估计的闭式频偏估计方法。该方法综合地利用了多收发天线的空域分集效应,降低了信道零点对频偏估计性能的影响,有效地提高了估计的均方误差性能。仿真结果验证了不同天线配置情况下所提出算法的性能。     

Joint timing synchronization and frequency offset acquisition algorithm for MIMO OFDM systems

For multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) sys-tems, a joint timing synchronization and frequency offset acquisition algorithm based on fractional Fourier transform (FRFT) is proposed. The linear frequency modulation signals superimposed on the data signals are used as the training signals. By performing FRFT on the receiver signals and searching the peak value of the FRFT results, the receiver can realize timing synchronization and frequency offset acquisition simultaneously. Compared with the existing methods, the proposed algorithm can provide better timing synchronization performance and larger frequency offset acquisition range even under multi-path channels with low signal to noise ratio. Theoretical analysis and simulation results prove this point.     

基于频偏补偿的TDS-OFDM系统同步方法

针对TDS-OFDM系统中载波频偏对基于PN序列自相关同步算法的影响,提出采用双滑动窗互相关的时间标尺函数来进行符号同步,利用该函数可以得到很好的相关峰值且不受频率偏移的影响。载波频偏由多径信道下各主要能量径的相关峰值估计得到,该估计算法可以得到较为准确的估计值和较大的估计范围。在典型的城市信道条件下对算法进行了仿真,结果表明：在较大的频偏条件下,本文方法得到了准确的符号同步和较好的频偏估计。     

正交频分复用系统中相位噪声的影响与抑制

在分析本振相位噪声对正交频分复用(orthogonal frequency division multiplexing,OFDM)系统性能影响的基础上,建立了一种新的相噪频域模型,并基于该模型提出了加入相噪估计导频来跟踪相噪的方法。由于导频与数据子载波之间有空子载波作为保护间隔,该方法可以估计出相噪的高阶频谱分量,从而有效地抑制相噪引入的共同相位误差(common phase error,CPE)和载波间干扰(inter-carrier interference,ICI)。仿真结果表明方法的有效性。     

时变衰落信道下OFDM信号参数融合估计

提出了一种基于代价函数实现短循环前缀正交频分复用(orthogonal frequency division multiplexing, OFDM)信号参数融合估计与识别的算法。该算法同时利用OFDM信号循环前缀产生的自相关和循环自相关特性,同步估计有用符号时间和符号周期。首先对OFDM信号的循环自相关特性进行了深入的分析,然后构造了代价函数并讨论了时变衰落信道条件下引入循环频率的数目对代价函数的影响,最后通过计算机仿真,验证了算法的有效性。     

Bayesian integer frequency offset estimator for MIMO-OFDM systems

Carrier frequency offset （CFO） in MIMO-OFDM systems can be decoupled into two parts： fraction frequency offset （FFO） and integer frequency offset （IFO）. The problem of IFO estimation is addressed and a new IFO estimator based on the Bayesian philosophy is proposed. Also, it is shown that the Bayesian IFO estimator is optimal among all the IFO estimators. Furthermore, the Bayesian estimator can take advantage of oversampling so that better performance can be obtained. Finally, numerical results show the optimality of the Bayesian estimator and validate the theoretical analysis.     

宽带OFDM系统中的简化滤波查表预失真算法

为了解决宽带正交频分复用（orthogonal frequency division multiplexing, OFDM)系统中放大器有记忆非线性特性给系统带来的失真问题,提出了一种简化的滤波查表(simplified filter look up table, S FLUT)预失真方法。该方法由一个无记忆预失真器子系统串联一个自适应滤波器子系统构成,能有效解决放大器带来的失真问题,同时采用简化的自适应更新结构,可以弥补原滤波查表法反馈支路结构复杂、收敛精度低的缺点。     

一种基于导频的OFDM系统信道估计方法

提出一种使用矩形导频图案的OFDM信道估计方法。接收端首先在频率方向估计信道,然后在时间方向插值。该方法仅需累加和少量乘法运算就能估计出信道冲激响应,复杂度低,实时性高。在未编码16QAM-OFDM系统中,通过仿真将提出的方法与线性、cubic和MMSE插值法进行均方误差和误比特率的比较。结果表明该方法性能优于前两者,接近后者。     

接收分集OFDM通信系统的盲信道估计

提出一种新的利用接收分集技术的OFDM通信系统盲信道估计方法.所提出的算法根据单入多出系统模型,可直接在一个接收数据块内估计出各个接收分支所对应的多径信道,且不要求接收信号的二阶统计量等相关统计信息.由于采用频域相除抵消技术,各分支的信道参数可分离估计,因此降低了计算复杂度.仿真分析表明该算法的有效性.     

多载波OFDM信号识别方法

无人机(unmanned aerial vehicle, UAV)技术发展迅速, 应用十分广泛, 用作UAV测控数传的正交频分复用(orthogonal frequency division multiplexing, OFDM)信号, 给民用无线频谱管理、军用非合作通信技术带来不小的挑战。基于此, 提出了一种基于包络相关谱的OFDM信号检测识别算法。主要利用OFDM信号的时域相关性, 通过计算信号的复包络相关谱, 并检测判断其相关谱的离散周期性, 实现了在非合作通信中OFDM信号的识别。相比基于高阶累积量的检测识别算法, 所提算法具有更好的信噪比适应能力, 而且不需要设定判别的经验阈值, 同时还能估计出子信道的带宽。仿真实验结果验证了所提方法的有效性。     

OQAM/OFDM系统中基于信道估计的原型滤波器设计

针对正交频分复用/交错正交幅度调制系统频率选择性信道,提出了基于信道估计的原型滤波器设计方案。就其导频频谱效率低的问题,提出了能够提高频谱效率的导频结构,并进行了信道估计。用信号与干扰加噪声比的倒数衡量原型滤波器对信道估计的影响并将其作为优化问题的约束条件之一,将最小化阻带能量作为目标函数,对原型滤波器的设计问题进行建模。为了加快收敛速度,保证获得全局最优解,利用基于块的分支定界法对原型滤波器的优化问题进行求解。仿真结果证明了所提方法的有效性。