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。      

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。     

A robust timing synchronization design in OFDM systems?part II: high-mobility cases

In this paper we consider the design of a robust timing synchronization algorithm for pilot-aided OFDM systems in high-mobility fading environments. We first analyze the impact of both mobility and timing errors on the performance of a pilot-aided OFDM system for frequency selective fading channels, by deriving an expression for channel estimation error variance. The analysis will show that, even for high levels of mobility, a pilot-aided channel estimator is considerably sensitive to timing errors, due to the impact of rotations in different bases. We then show how this sensitivity can be utilized to design a robust timing synchronization algorithm for mobile OFDM systems, without relying on synchronization training information. Theoretical results are then confirmed by simulating the performance of an OFDM system in high delay and Doppler spread fading environments. Finally, we show how the proposed mathematical framework and algorithm can be used to address timing synchronization in the presence of a frequency offset as well. The analysis of this paper is the extension of the derivations of Part I [8], the accompanying paper on the design of a robust timing synchronizer for low-mobility OFDM systems.     

Low-Power and High-Accurate Synchronization for IEEE 802.16d Systems

Orthogonal frequency division multiplexing (OFDM) is a viable technology for high-speed data transmission by virtue of its spectral efficiency and robustness to multi-path fading. These advantages can be achieved only with good synchronization both in time and frequency. This paper proposes new efficient synchronization methods for an OFDM-based system, IEEE 802.16d. For the coarse time synchronization and the fractional carrier frequency offset (CFO) estimation, a disjoint architecture is proposed that performs auto-correlations separately to achieve more reliable frequency synchronization and to reduce overall hardware complexity and power consumption. In addition, for the fine symbol timing offset (STO) and the integer CFO, a new joint estimation method employing parallel cross-correlations between the received samples and the pre-rotated training sequences is proposed. Experimental results show significantly superior performance to the previous synchronization methods. A prototype synchronizer based on the proposed methods is designed with a 0.25- $mu$m CMOS process, which reduces power consumption by more than 60% compared to a conventional synchronizer.      

Carrier Frequency Synchronization and Tracking for OFDM Systems with Receive Antenna Diversity

Blind carrier frequency synchronization for orthogonal frequency division multiplexing (OFDM) systems with receive diversity is proposed in this paper. This method utilizes the fact that different receive branches contain uncorrelated observations about the same information bits. Due to receive diversity, our synchronizer can perform well without virtual subcarriers when channel state information (CSI) is known. In addition, we shall analyze the effects of CSI errors on the carrier frequency offset (CFO) estimation and show that the proposed algorithm is robust to CSI error. In particular, we derive the Cramer-Rao bound (CRB) from the mean square error (MSE) of the carrier frequency synchronizer. Computer simulation results show the performance improvement of our synchronizer to the single antenna systems and to the existing method.     

Joint frequency and timing recovery for MSK-type modulation

We investigate a novel nondata-aided method for jointly estimating timing and carrier frequency offset in MSK-type modulation. The algorithm has a feedforward structure and lends itself to a simple digital implementation. Its estimation accuracy depends on a design parameter that may be varied to trade performance for computational complexity. Setting the parameter to unity yields a synchronization scheme already known in the literature. Computer simulations are used to assess the synchronizer performance on AWGN Rayleigh fading channels with MSK and Gaussian MSK modulation     

基于频偏估计的无线传感器网络时间同步算法

针对无线传感器网络经典同步算法稳定性较差,时钟相偏和频偏联合校正算法存在高开销等问题,提出一种基于频偏估计的无线传感器网络时间同步算法（CSMS算法）。CSMS算法采用低开销相偏和频偏估计方法,提高了成对节点的同步精度和稳定性;结合分层和广播监听构建了同步策略,能够保证算法的稳定性和同步精度,实现了与邻居节点及根节点的同步,同时优化了同步总开销。实验结果表明,CSMS算法能够有效地平衡同步能耗、同步精度及同步稳定性。     

一种抗频偏的定时同步方法及其性能分析

针对长序列互相关定时同步性能受频偏影响的问题,提出了一种分段频偏补偿的定时同步算法.该算法在分段互相关的基础上,使用频偏补偿替代传统的绝对值操作,在减少载波频偏对同步峰值影响的同时不增加噪声均值.理论分析和仿真结果表明,改进方法定时同步性能优于传统分段方法,适用于低信噪比大频偏的无线通信系统.     

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.     

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。     

Robust and Accurate Frequency and Timing Synchronization Using Chirp Signals

We propose a new robust and accurate synchronization procedure using a training sequence composed of chirp signals. We use a new integer frequency estimation algorithm and propose a new combination of a known fractional frequency offset estimation algorithm and timing synchronization algorithm. The training sequence is composed of one up and two down chirp symbols, also known as Newman phases. The integer frequency offset estimation algorithm uses the effect of timing and frequency offsets on the matched filter outputs of the chirp signals. Autocorrelation and reversed autocorrelation are used to acquire the timing instant and the fractional frequency offset. We present the complete timing and frequency synchronization procedure and study the output signals of the autocorrelation and reversed autocorrelation algorithms. Finally, we check the performance of the synchronization procedure via Monte Carlo simulation in several multipath channels. Our algorithms are accurate and more robust compared to previously published state-of-the art algorithms.      

Statistical approximation based fine frequency synchronization for OFDM systems

The paper proposes a novel approach for fine frequency synchronization of OFDM syn- chronization systems in multi-path channels. Maximum Likelihood (ML) function of frequency offsets including integral and decimal parts in frequency domain is developed according to the law of great number to eliminate the noise impact of the signal. When the timing delay close to the actual time, the proposed function produces a deep valley indicating frequency offset when large Valley-Square- Error (VSE) appears. Coarse timing offset can also be detected when function’s Valley-Square-Error (VSE) is maximized. Simulation results shows that the proposed algorithm gives very robust estimation of frequency offset, and a coarse timing offset estimation.     

Multi h CPM信号无数据辅助的反馈定时同步算法

针对Multi h CPM信号前馈定时同步不适用于连续传输模式通信系统问题,文中提出了一种无数据辅助的反馈定时同步算法。该算法基于最大似然准则推导出定时误差检测信号,利用误差检测信号形成闭环控制结构对接收信号进行跟踪定时,并与修正的克拉美 罗界进行比较。仿真结果表明,该算法能跟踪定时偏移,且具有较好的跟踪性能,适合连续模式传输的通信系统。     

一种改进的LTE系统下行同步算法

LTE(Long Term Evolution)系统中,作为小区搜索的第一步,符号定时算法需要具备很强的鲁棒性。为了进一步增强现有符号同步算法的抗频偏性能,提出了一种优化的符号同步算法。该算法一方面保持已有分段相关同步算法的优点,同时考虑结合预频偏处理来进行符号定时与粗频偏的联合估计,再将多个相关运算集做叠加处理。理论分析和仿真实验结果表明,在多径信道下,所提算法在完成符号定时和粗频偏估计的同时,大大提高了系统抗频偏性能。     

一种新的定时和频偏联合估计算法

提出了一种新的正交频分复用(OFDM)系统定时和频率同步联合算法.该算法仅需要一个训练符号就可以完成定时和频偏估计,额外开销量小,定时和频率同步的实现均在时域进行,简化了同步实现的复杂度.由于使用自相关处理,定时同步对频率偏差具有较强的鲁棒性.与以往的定时同步算法相比,该算法避免了定时平台和多峰值的出现,即使在非常低的信噪比下,其定时测度都是一个尖脉冲.仿真结果表明,该算法能快速地实现定时和频率同步.     

Frame synchronization for coded systems over AWGN channels

For the additive white Gaussian noise channel, we consider the problem of frame synchronization for coded systems. We present an algorithm that takes advantage of soft information provided by a soft decoder to produce an enhanced estimate of the frame boundary. To reduce complexity, a companion algorithm is introduced that is a hybrid of the optimal uncoded frame synchronizer introduced by Massey and the list synchronizer introduced by Robertson. The high-complexity coded maximum-likelihood frame synchronizer used by Robertson will accordingly be replaced by our algorithm, which operates on decoder-provided soft decisions. The algorithm begins by obtaining a list of high-probability starting positions via the log-likelihood function of the optimal uncoded frame synchronizer. Then, a test /spl delta/ is used to decide if the decision of the optimal uncoded frame synchronizer is sufficient, or whether list synchronization is required. If the test chooses in favor of using the optimal uncoded synchronizer, the estimate is obtained with relative ease. Otherwise, list synchronization is performed, and statistics provided by the decoder are used to resolve the frame boundary. Monte Carlo simulations demonstrate that the frame-synchronization-error rate (the probability of the synchronizer making an error) achieves the lower bound for signal-to-noise ratio values exceeding 1 dB.     

一种OFDM系统的频差补偿同步方法

一般的系统同步要求以定时同步为先，然后进行频偏的估计，但大频差偏移往往影响到了系统的符号粗同步定时。针对大频差下信号难以捕获的问题，文中提出预设频差补偿的同步技术。理论分析和仿真结果表明，所采用的同步方法，可以比较准确地完成系统的符号定时同步。     

一种卫星SCMA系统接收机同步算法

SCMA技术通过资源过载能够实现大规模用户的并发接入,非常适合物联网应用,但是SCMA解调性能受定时误差和载波频差影响较大。对此,文中通过基于导频的两阶段频差估计算法和ML相位偏差估计算法,实现大频差下的高精度频、相偏估计;通过SCMA和LDPC联合迭代,利用发送数据实现高精度定时误差估计。仿真结果表明,在AWGN信道下,载波同步算法采用优化的帧结构后可以获得距理想性能不到0.25dB的优异性能,定时同步算法在1次迭代下获得了距理想性能仅0.2dB的性能,在3次迭代下获得了与理想性能一致的优异性能,能适应大频差、高动态的卫星信道环境。     

An early-late timing recovery scheme for filter-bank-based multicarrier transmission

This paper examines a decision-directed timing recovery scheme for filter-bank-based multicarrier transmission. The symbol synchronizer is obtained from a maximum-likelihood approach and uses the digital equalization filters. The algorithm is first derived, the performances of the scheme (timing offset and timing jitter) are then computed, and computational results are provided for high bit-rate transmission over copper wires. Finally, different improvements are discussed.     

突发OFDM系统中的快速符号定时同步

提出了一种在突发OFDM系统中,基于改进前导序列的快速粗符号定时方法。与传统算法比较,该算法不受频率偏移的影响,适合于频偏捕获之前的符号定时同步。运算复杂度降低,适合于突发通信中的快速同步。提高了定时精度,克服了传统算法符号定时模糊的缺点,所以特别适合于突发模式下频偏捕获之前的快速粗符号定时同步。