短波信道条件下OFDM系统通信效果研究

正交频分复用(OFDM)是一种多载波数字调制技术,它能有效克服信号传输中产生的多径干扰,提高频谱利用率.以Simul ink为平台构建OFDM通信系统仿真模型,并在频偏和多径条件下进行仿真,分析仿真结果可以看到,频偏和多径对系统的传输效果都有严重影响,其中频偏对通信系统的影响更为严重.     

基于高阶统计量的OFDM子载波调制识别算法北大核心

针对OFDM(正交频分复用)信号子载波调制方式识别的问题,提出了一种将高阶矩和高阶累积量相结合的联合识别算法。该算法首先运用基于高阶矩的特征量把MPSK(多进制相移键控)调制(M=2、4)、64QAM(六阶正交幅度调制)和16QAM(四阶正交幅度调制)区分开,然后再利用基于高阶累积量的特征量区分BPSK(二进制相移键控)调制和QPSK(四相相移键控)调制。从理论上进行了推导与分析,该算法对多径衰落与噪声干扰不敏感。计算机仿真结果表明,所提算法在多径信道条件下具有良好的识别性能。     

适用于高阶QAM解调的载波恢复算法及其实现验证

高阶正交幅度调制（QAM）系统中,随着QAM信号阶数的提高,频偏和相偏对系统解调性能的影响越敏感,其对同步性能的要求也越来越高。因此,必须在接收端对系统中的频偏和相偏进行更精确的补偿,使得接收端与发送端的载波信号达到同频同相,来提高解调系统的性能。本文首先简单介绍了两种常用两种面向判决的载波相位恢复算法,并利用MATLAB／Simulink为该算法搭建了系统仿真模型进行分析,然后给出了具体的实现方法并对其性能进行了分析。仿真结果表明,面向判决的载波恢复算法实现简单,稳态时抖动较小,非常适用于高阶QAM解调系统。     

短波正交频分复用(OFDM)传输性能研究

短波通信作为一种重要的通信方式一直受到信道带宽的限制,加之传输信道的不稳定性致使其传输效果不佳,如何有效的提高频带利用率、改善传输性能是短波通信的重要研究内容。正交频分复用(OFDM)是一种多载波数字调制技术,在提高频谱利用率、克服多径方面发挥了重要作用。文章以Simulink为平台构建短波OFDM通信系统仿真模型,并在频偏和多径条件下进行仿真,分析仿真结果可以看到,频偏和多径对系统的传输效果都有影响,其中频偏对通信系统的影响较为严重。     

适合高阶QAM的载波恢复算法

为解决DVB—C系统中多电平调制信号(如64，128，256QAM)的频偏捕获，描述了一种极性判决鉴相的算法，该算法可捕获高阶QAM信号的频偏达200kHz。对于一般的频偏较小的QAM信号可用传统的DD算法纠正。这样对收到的带有较大频偏和噪声、部分多径的基带信号就可先用极性判决捕获大频偏，稳定之后再用全星座判决纠正剩下的小频偏。     

基于多门限极性判决的QAM载波恢复算法

正交幅度调制(QAM)是现代通信中最为常用的一种调制方法,其载波恢复是决定接收机性能的关键因素之一.从提高收敛速度和减小锁定误差的角度出发,提出了一种用于QAM解调的多门限极性判决载波恢复算法,该算法充分利用QAM星座图中对角线上锁定误差为零的信号点,增加了用于提取相位误差的符号集.分析和仿真表明,与传统的面向判决载波恢复算法相比,该算法不仅保留了极性判决载波恢复算法频偏搜索范围大的优点,且能进一步提高收敛速度.     

一种高阶QAM联合载波恢复算法的研究

文章在载波恢复的初始环节讨论了一种基于非线性变换的载波频偏盲估计算法,通过分析估计出了最小均方误差,提出了一种适用于中、高信噪比(SNR)下的简化算法,之后再用锁相环(PLL)技术跟踪细微的频偏和相位偏移.仿真结果证实了该算法的可信性和有效性,从而获得了一种简易的对高阶正交幅度调制(QAM)的载波恢复方法.     

一种可靠的OFDM系统时间和频率同步方案

在无线信道中,由于多径和频偏的存在,影响了OFDM系统中同步的准确性.结合中国移动多媒体广播(CMMB)信道帧结构、调制及其信道,提出了一种适用于多径衰落信道、高频偏的OFDM系统时间和载波频率同步方案,能准确地完成帧同步和频偏估计.     

OFDM系统中基于符号间相位差分调制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OFDM系统频偏估计中所产生的模糊度问题提出了一种基于符号间相位差分调制的频偏估计模糊度校正算法。它通过在相邻OFDM符号中相同子载波间进行相位差分调制来校正频偏估计模糊度。文中从理论上给出并证明了算法具有唯 可辨识性的充分条件,并分析比较了算法的复杂度和定时误差对算法性能的影响。理论分析和仿真结果表明,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具有运算量小和对定时误差不敏感的优点。     

基于高阶循环累积量的OFDM-弹性光网络信号调制格式识别

将高阶循环累积量引入基于正交频分复用(OFDM)的弹性光网络(EON)的调制格式识别中。通 过计算OFDM信号不同调制格式(BPSK、QPSK、16QAM和64QAM)的高阶循环累积量,给 出了OFDM-EON信号不同调制格式的识别阈值区间,并采用仿真方法验证了该区间的 识别准确率。结果表明,利用所提出的识别阈值区间,对波特率为28G的OFDM-EON信号(各 子载波随机设置调制格式)进行调制格式识别,在光信噪比(OSNR)为34dB时,子载波调制格式的识别准确率可达到100%。     

RLS-Based Joint Estimation and Tracking of Channel Response, Sampling, and Carrier Frequency Offsets for OFDM

This paper proposes a pilot-aided joint channel estimation and synchronization scheme for burst-mode orthogonal frequency division multiplexing (OFDM) systems. Based on the received signal samples containing pilot tones in the frequency domain, a cost function that includes the carrier frequency offset (CFO), sampling clock frequency offset (SFO) and channel impulse response (CIR) coefficients is formulated and used to develop an accompanying recursive least-squares (RLS) estimation and tracking algorithm. By estimating the channel CIR coefficients instead of the channel frequency response in the frequency domain, the proposed scheme eliminates the need for an IFFT block while reducing the number of parameters to be estimated, leading to lower complexity without sacrificing performance and convergence speed. Furthermore, a simple maximum-likelihood (ML) scheme based on the use of two long training symbols (in the preamble) is developed for the coarse estimation of the initial CFO and SFO to suppress dominant ICI effects introduced by CFO and SFO and to enhance the performance and convergence of the fine RLS estimation and tracking. Simulation results demonstrate that, over large ranges of CFO and SFO values, the proposed pilot-aided joint channel estimation and synchronization scheme provides a receiver performance that is remarkably close to the ideal case of perfect channel estimation and synchronization in both AWGN and Rayleigh multipath fading channels.      

Low-complexity block double-differential design for OFDM with carrier frequency offset

Carrier frequency offset (CFO) estimation for orthogonal frequency-division multiplexing (OFDM) has caught attention as OFDM systems have become widely adopted in recent years. In this paper, we design a novel double-differential (DD) codec with low computational complexity. Our design bypasses CFO and channel estimation, and is easy to implement at both transmitter and receiver. It also guarantees full multipath diversity, and reduces the peak-to-average power ratio from the number of subcarriers to the channel order. In addition, it is robust to CFO drifting. The closed form of the performance for our design is derived for OFDM transmissions over frequency-selective channels with CFO. Thorough simulation results corroborate our claims.     

BER Analysis of OFDM Systems Impaired by DC Offset and Carrier Frequency Offset in Multipath Fading Channels

We perform bit-error-rate (BER) analysis of orthogonal frequency division multiplexing (OFDM) systems impaired by both direct current (DC) offset and carrier frequency offset (CFO) in multipath Rayleigh fading channels. Since the performance of OFDM systems is sensitive to the CFO, it is necessary to estimate and correct the CFO at the receiver. The existence of DC offset degrades the performance of CFO estimator and results in large residual CFO after compensation. Moreover, the process of CFO compensation spreads the DC offset energy and causes DC offset interference to all subcarriers. By deriving the BER formula for OFDM systems employing binary phase-shift keying modulation, the dependency of BER on the DC offset, CFO, and estimated CFO is accurately quantified. Simulation results validate the correctness of our theoretical analysis.     

Decision-directed fine synchronization in OFDM systems

A new decision-directed (DD) synchronization scheme is proposed for joint estimation of carrier frequency offset (CFO) and sampling clock frequency offset (SFO) in orthogonal frequency-division multiplexing (OFDM) systems. By exploiting the hard decisions, we report accurate estimators of residual CFO and small SFO. The performance analysis and simulation results indicate that the proposed novel DD scheme achieves much better performance than the conventional pilot-based schemes in both additive white Gaussian noise and frequency-selective channels.     

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.     

Blind Frequency Synchronization in OFDM via Diagonality Criterion

In this paper, we address the problem of blind carrier frequency offset (CFO) estimation in orthogonal frequency-division multiplexing (OFDM) systems in the case of frequency-selective channels. CFO destroys the orthogonality between the carriers leading to nondiagonal signal covariance matrices in frequency domain. The proposed blind method enforces a diagonal structure by minimizing the power of nondiagonal elements. Hence, the orthogonality property inherent to OFDM transmission with cyclic prefix is restored. The method is blind since it does not require a priori knowledge of the transmitted data or the channel, and does not need any virtual subcarriers. A closed-form solution is derived, which leads to accurate and computationally efficient CFO estimation in multipath fading environments. Consistency of the estimator is proved and the convergence rate as a function of the sample size is analyzed as well. To assess the large sample performance, we derive the CramÉr–Rao bound (CRB) for the blind CFO estimation problem. The CRB is derived assuming a general Gaussian model for the OFDM signal, which may be applied to both circular and noncircular modulations. Finally, simulation results on CFO estimation are reported using a realistic channel model.     

Blind frequency-offset estimator for OFDM with general symbol constellation

An efficient feedforward blind carrier frequency offset (CFO) estimator for orthogonal frequency division multiplexing (OFDM) systems with general symbol constellation is presented. The proposed estimator requires only one OFDM symbol period to blindly and accurately estimate the CFO in Gaussian and multipath fading channels.     

Blind Feedforward Frequency Estimation for OFDM Signals Transmitted over Multipath Fading Channels

This paper provides an approximate closed form solution to the problem of maximum likelihood (ML) estimation of the carrier frequency offset (CFO) in an orthogonal frequency division multiplexing (OFDM) signal transmitted over a multipath fading channel. This results in a novel feedforward frequency synchronizer, requiring only an approximate statistical knowledge of the communication channel. The performance of the proposed algorithm is assessed by computer simulations and is compared with that provided by other synchronizers and with Cramer-Rao bounds.     

基于可变遗忘因子RLS在OFDM系统的载波频偏估计

在时变信道下正交频分复用（OFDM）系统中,通过导频辅助,提出基于可变遗忘因子RLS（VFF-RLS）的载波频偏（CFO）估计改进算法。针对传统RLS（CFF-RLS）算法中遗忘因子无法同时满足CFO估计收敛速度和收敛精度的缺陷,本文设计了线性变化遗忘因子(LFF)和非线性变化遗忘因子(NLFF) 两种可变遗忘因子方案来提升CFO估计性能。仿真结果显示：在低信噪比的情形下,基于VFF-RLS算法的CFO估计性能明显优于基于CFF-RLS算法的CFO估计性能。     

基于星座点特征的OFDM残留频偏跟踪算法

残留频偏会导致星座点发生旋转,而且这种旋转与时间有关,时间越长,旋转越大。若残留频偏长时间得不到校正,将会使星座点旋转到其他象限从而出现判决错误,使系统的性能急剧恶化。针对这个问题,提出了一种基于星座点特征的残留频偏跟踪算法。该算法要求在信道估计之后完成,并假定一个OFDM帧内的信道频率响应保持不变。该算法包括三个步骤,首先将每个OFDM符号的星座点划分为四个子集,然后对每个子集的均值求相位角获得相位误差,最后经环路滤波器输出一个校正量补偿时域频偏。由于残留频偏估计值的提取充分利用了所有数据子载波信息,并对每个子集进行了求均值处理,所以有效的改善了环路的输入信噪比。它适用于子载波为QPSK或QAM调制的OFDM系统,不同的子载波调制方式跟踪范围不同,最大跟踪范围为±0.05个子载波间隔。仿真表明:新算法性能要优于基于循环前缀的算法,与基于导频的跟踪算法性能相当。