TH预编码的超奈奎斯特系统残余干扰消除算法

超奈奎斯特(Faster-Than-Nyquist, FTN)技术能够有效提高传输系统的频谱效率，然而代价是引入了严重的码间串扰(Inter-Symbol Interference, ISI)。在基于Ungerboeck模型的FTN系统中的发送端使用TH预编码(Tomlinson-Harashima Precoding, THP)并不能完全有效消除FTN系统所引入的ISI,会存在残余ISI。对此，针对FTN系统中的残余干扰提出了两种接收端检测算法，分别是基于后向串行干扰消除(Backward Serial Interference Cancellation, BSIC)的无迭代检测算法和基于连续干扰消除(Successive Interference Cancellation, SIC)的Turbo迭代均衡改进算法，并通过蒙特卡洛方法寻找最优的扩展取模间隔。计算机数值仿真结果表明，在选定的扩展取模间隔下，两种接收端检测算法在FTN系统的加速因子为0.8和0.9时均能有效消除FTN技术所引入的ISI,其误码率趋近于奈奎斯特正交传输性能。     

FTN新型编码技术性能研究

最近几年来,随着4G技术在商业化运营中的推广,通信线路带宽进一步提升,但是它们依然无法满足无线用户的快速增加所带来的高系统容量需求。码元速率在提升通信系统传输速率方面中具有重要的意义。信息传输速率的大小直接影响到系统的容量,通信系统进行信息传输时,以超过奈奎斯特速率的方式进行信息传输将有助于进一步提升系统容量。文章以FTN(超奈奎斯特编码)新型编码技术为研究对象,对其性能进行了分析研究。实验结果显示FTN在编码、传输及解码方面都有助于提高通信系统容量。     

超奈奎斯特信号载波频偏估计的梯度下降算法

超奈奎斯特（Faster-than-Nyquist, FTN）传输技术是一种基于时域压缩的信号调制技术，具有更高的传输速率及频带利用率，但是却引入了无限长的码间串扰（inter-symbol interferences , ISIs），这对载波频偏估计提出了新的挑战。对于FTN信号的频偏估计，传统算法在门限和精度等方面都有所下降。本文将梯度下降算法应用到载波频偏估计中，通过迭代搜索的方式，获得对频偏的精确估计。估计过程中直接通过数据部分进行估计，不需要借助额外的导频序列。仿真结果表明，梯度下降法与传统算法在性能上相比有了较好的改善，虽然具有较多的迭代次数和运算量，但是却能够较好地适应FTN信号的特性。梯度下降法不仅在门限范围内更靠近克拉美罗界（Cramer-Rao bound，CRB），而且在整个频段上具有更稳定的性能。      

重叠滤波多音系统的频偏性能分析及优化

重叠滤波多音(O-FMT)是针对滤波多音(FMT)系统频谱利用率低的缺点而根据超奈奎斯特(FTN)概念引入子载波重叠得到的方案。鉴于O-FMT系统在频偏环境下的性能缺陷,理论分析了频偏对系统信号各部分的影响,比较了O-FMT与正交频分复用（OFDM）系统的抗频偏性能。针对频偏性能优化问题,提出了基于最佳线性无偏估计（BLUE）的改进盲估计算法,设计了基于频域滤波器系数的内嵌结构进行频偏补偿。仿真实验表明,O-FMT相比OFDM具有更好的抗频偏能力,改进的盲估计算法估计值精度更高,抗干扰、抗衰落能力更强,设计的频偏补偿结构可以使系统获得更好的性能。     

基于BMST编码的超Nyquist传输系统性能研究

超奈奎斯特(FTN)传输技术是一种能够有效提高频谱效率的非正交传输技术。非等功率分配的分层FTN结构可以通过调节各层的码率或功率进行多级译码,从而实现干扰抵消,进而有效降低码间串扰(ISI)所带来的译码复杂度。分组马尔可夫叠加传输(BMST)编码是一种码率灵活可变的长卷积码构造方法,且能够获得额外的编码增益。介绍和讨论了FTN和BMST的原理,提出了一种基于BMST编码的FTN传输方案。仿真结果表明,该传输系统能够显著提高频谱效率。     

SCM-OFDM系统中一种新型的盲频偏估计方法

该文提出了一种用于重叠码调制-正交频分复用(SCM-OFDM)系统中基于信号检测的频偏估计方法。这种估计方法首先利用SCM迭代检测器输出的判决符号重构接收信号,再利用最小均方误差准则对频偏进行估计。该方法不需要额外的训练序列或导频符号,是一种高效的盲频偏估计方法。此外,该文还提出了一种改进的频偏估计方法。在改进的方法中,接收机将频偏估计与信号检测进行联合迭代;这样在每一次迭代过程中,信号检测和频偏估计都需要执行一次,并且接收机在信号检测时可以利用上一次得到的频偏估计值进行频偏补偿。仿真结果表明,该文提出的基于信号检测的频偏估计方法能够高效并准确地估计出SCM-OFDM系统的频偏。     

一种改进的高动态QPSK信号解调算法

正交相移键控(QPSK)调制解调方式广泛应用于深空通信、数字卫星通信等高速系统中,高动态QPSK信号具有较大的多普勒载波频偏,针对现有的解调算法对QPSK信号频偏估计范围小、精度较低等问题,提出了一种改进的高动态QPSK信号解调算法。该算法利用基于Zoom-FFT的Quinn频率估计算法对载波频偏进行估计,将频偏估计分为粗估计和精估计两个过程,对估计所得频偏进行补偿后利用Costas环完成载波同步。在MATLAB中对算法进行了仿真,仿真结果表明,该算法能够实现对高动态大频偏QPSK信号的有效捕获,与传统的解调方式相比,该算法对载波频偏的估计范围更大,且具有更高的频偏估计精度和更好的误码性能。     

一种用于全数字化QPSK解调的大频偏矫正算法

在全数字化QPSK解调器中,有时存在着相当大的相对载波频偏,导致接收机不能正常工作.本文提出了一种新的算法,该算法将对频偏的直接估计转换成对一个新序列的周期进行估计,达到对大频偏进行有效估计并矫正的目的,解决了这一难题.此算法计算量较小,统计特性好,已在用单片DSP TMS320C6201实现的多路QPSK信号的数字化解调系统中得到应用.     

基于欠奈奎斯特采样的超宽带信号总体最小二乘重建算法

该文针对超宽带无线通信中需要设计高速模数转换器的问题,提出了一种欠奈奎斯特采样方法,该方法所要求的采样率仅与信号新息率相关,低于奈奎斯特率1个数量级。基于欠采样得到的离散时间超宽带信号,从理论上推导出信号的傅里叶频谱表达式,由此给出了一种总体最小二乘参数估计算法,能够准确地估计出冲激串信号的幅度和时移;通过将估计出的冲激串信号与高斯单脉冲波形卷积,完成超宽带信号的波形重建。仿真和实验结果表明,该文算法能够准确地重建原始超宽带信号,且算法性能优于现有的零化滤波重建算法。     

一种改进的基于DS/CDMA信号共轭周期平稳特性的载波频偏盲估计方法

在DS／CDMA系统中,我们可以利用接收信号的共轭周期平稳特性估计载波频偏,但是用于估计频偏的样本个数的有限性降低了这类方法估计结果的准确性和稳定性。为此本文在这类方法的基础上提出了一种改进的载波频偏盲估计方法,该方法通过对接收信号预置一个已知的载波频偏,有效的消除了有限的样本个数对估计性能造成的影响。同时它是一种盲估计算法,不需要专门的训练序列,不会降低无线带宽利用率。仿真结果表明该方法能够很好的利用有限长度的样本数据快速准确的估计出系统载波频偏,它的估计性能较改进前的方法有明显的提高。     

一种改进的OFDM水声通信频偏估计算法

在OFDM水声通信中,由于OFDM系统对频率偏差的敏感性和水声信道的复杂性,载波频偏估计显得尤为重要。本文首先研究了Song和M engali两种频偏估计算法,提出了一种改进的反馈重复频偏估计算法,该算法不仅保持了高精度,增大了估计范围,而且运算复杂度也较小。还在理想条件下对这种改进算法与以上两种算法进行了性能分析比较。此外,还在多径衰落的水声信道下对Song算法和改进方法进行了仿真比较。     

A Robust SFO Estimation Scheme for OFDM-Based DRM with Non-Symmetric Distributed Pilots

This paper proposes a robust sampling frequency offset (SFO) estimation scheme in an orthogonal frequency division multiplexing based digital radio mondiale system with non-symmetrically distributed pilots. In particular, a post fast Fourier transform SFO estimation method is suggested to provide an unbiased estimate of SFO in the presence of frequency selectivity of the channel as well as non-symmetrically distribution of pilot symbols. In order to verify the performance of the proposed frequency-offset estimation scheme, a closed-form expression for the mean square error (MSE) is derived. We show via simulations that the proposed estimation scheme achieves a remarkable robustness against frequency selectivity fading and the theoretical MSE analysis is proven accurate.     

Blind NLLS Carrier Frequency-Offset Estimation for QAM, PSK, and PAM Modulations: Performance at Low SNR

We address the problem of blind carrier frequency-offset (CFO) estimation in quadrature amplitude modulation, phase-shift keying, and pulse amplitude modulation communications systems. We study the performance of a standard CFO estimate, which consists of first raising the received signal to the$M$th power, where$M$is an integer depending on the type and size of the symbol constellation, and then applying the nonlinear least squares (NLLS) estimation approach. At low signal-to noise ratio (SNR), the NLLS method fails to provide an accurate CFO estimate because of the presence of outliers. In this letter, we derive an approximate closed-form expression for the outlier probability. This enables us to predict the mean-square error (MSE) on CFO estimation for all SNR values. For a given SNR, the new results also give insight into the minimum number of samples required in the CFO estimation procedure, in order to ensure that the MSE on estimation is not significantly affected by the outliers.     

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.     

OFDM传输分集中的频率同步

近年来结合OFDM的传输分集技术得到了普遍关注。可是OFDM的一个固有缺点是其对频率偏移非常敏感,因此如何对基于OFDM的传输分集系统进行有效的频偏估计和校正是一个值得探讨的问题。为此该文给出了一种基于导频的粗频偏估计方法和盲自适应细频偏估计算法,并通过仿真验证了所给方法的有效性。     

超奈奎斯特传输技术:面向6G的应用价值与挑战

超奈奎斯特（FTN）传输技术通过压缩发送符号时域/频域间隔,在一个符号周期内重叠发送多流数据,从而打破了奈奎斯特脉冲波形的正交性,实现了在有限带宽内传输更多数据的目的。首先,阐述了FTN的技术原理,梳理了其发展现状;其次,分析相关的关键性问题——明确了FTN容量与香农容量的关系,以及相关技术的内在联系;最后,讨论了 FTN在 6G通信中的应用可能性,提出了其应用价值和挑战：FTN 是一种可逼近香农容量的高谱效传输方案,但接收机复杂度过高可能成为其在 6G 应用中的关键阻碍。     

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.     

基于连续干扰消除的超奈奎斯特信号迭代接收技术

为了消除超奈奎斯特(FTN)调制技术引入的干扰,设计了基于连续干扰消除(SIC)的Tur-bo迭代均衡接收机.对于单载波FTN系统,首先建立了FTN信号等效模型,然后接收机根据等效模型和译码器输出的软信息重建干扰,并通过多次连续干扰消除和译码逐步消除,具有较低的复杂度.同样地,在多载波FTN系统中,根据译码器输出的软信息和成型脉冲的时频特性重建引入的干扰并通过多次迭代给予消除.仿真表明,在加速因子不是太小时,基于SIC-Turbo均衡的接收机能够有效消除干扰,可获得近似正交传输时的误码性能.     

基于双通道DFRFT互谱法的Chirp信号时延估计

针对脉冲Chirp类信号的时延估计问题,理论推导了基于离散分数阶Fourier变换的脉冲Chirp信号的特性,分析了当时延参量等效的分数阶Fourier域的频率大于采样率时,脉冲Chirp信号的分数阶Fourier域谱产生混叠,造成时延估计模糊的问题,并提出基于离散分数阶Fourier变换（DFRFT）双通道互谱法进行时延估计,给出两个通道采样率选取的原则及算法的性能分析,实验结果表明,在一定的采样率下,算法能够快速精确地估计脉冲Chirp信号的时延参数.     

A robust timing and frequency synchronization for OFDM systems

A robust symbol-timing and carrier-frequency synchronization scheme applicable to orthogonal frequency-division-multiplexing systems is presented. The proposed method is based on a training symbol specifically designed to have a steep rolloff timing metric. The proposed timing metric also provides a robust sync detection capability. Both time domain training and frequency domain (FD) training are investigated. For FD training, maintaining a low peak-to-average power ratio of the training symbol was taken into consideration. The channel estimation scheme based on the designed training symbol was also incorporated in the system in order to give both fine-timing and frequency-offset estimates. For fine frequency estimation, two approaches are presented. The first one is based on the suppression of the interference introduced in the frequency estimation process by the training symbol pattern in the context of multipath dispersive channels. The second one is based on the maximum likelihood principle and does not suffer from any interference. A new performance measure is introduced for timing estimation, which is based on the plot of signal to timing-error-induced average interference power ratio against the timing estimate shift. A simple approach for finding the optimal setting of the timing estimator is presented. Finally, the sync detection, timing estimation, frequency estimation, and bit-error-rate performance of the proposed method are presented in a multipath Rayleigh fading channel.