高动态短时突发通信系统的载波同步技术

载波同步是数据解调中的重要环节之一。由于高动态短时突发通信系统的通信时间极短,在算法观测时间内可认为频率无变化,因此可对载波频偏进行一次校频,使之快速落入锁相环快捕带内,然后利用锁相环进行跟踪以完成载波同步。实践表明:利用一种改进的FFT校频技术对载波频偏进行一次校频,能使载波频偏快速落入锁相环快捕带内。最后分别对改进的FFT校频技术及锁相环进行了仿真分析,结果表明该方法在低信噪比下仍具有良好的性能。     

基于前导序列的突发通信载波同步算法

在突发通信中,载波同步技术是解调系统的核心和关键技术。给出了一种基于恒包络零自相关序列(CAZAC)作为前导的突发通信载波同步算法,该算法基于CAZAC序列零自相关特性,先用载波频差前向校正、载波相位前向校正进行载波初始同步捕捉,再用锁相环技术进行跟踪,能够在低信噪比下进行快速载波恢复。仿真结果表明,本算法的同步速度快,捕获的频率范围较宽。     

一种8PSK的载波同步新方法

低信噪比下的8PSK突发通信．在频偏较大的情况下载波同步难以达到快速锁定。本文对这种情况下的载波同步问题进行了研究,介绍了一种改进的快速载波同步方法。使之适用于低信噪比的工程应用环境。文中进行了这种改进算法的仿真和实现,证明该方法有效可行。     

TDMA卫星通信系统突发同步的性能研究

突发载波捕获和同步是TDMA系统的关键技术,使用数据序列辅助判决可有效缩短该处理过程需要的时间;在低接收信噪比下使用CAZAC序列的相关处理能够获得较高的突发捕获性能;比较了CAZAC序列和"1010"序列的载波和时钟同步误差估计性能,分析了它们在低信噪比门限时对信号快速捕获能力的影响。提出了2种序列组合应用实现载波快速同步的方案,完成信号捕获和载波估计功能。     

基于Turbo码迭代的低信噪比载波同步技术

针对低信噪比短突发通信系统的载波恢复问题,研究了一种基于迭代思想的载波相位估计算法.该算法首先利用导频进行初始的载波相位估计,然后再利用Turbo译码器输出的软信息进行载波相位细估计,进而实现有效的载波同步.仿真结果表明,该算法仅利用11个QPSK导频符号就能校正较大范围的相位偏移,进而达到理想的误比特性能.     

一种改进的MIMO OFDM系统导频设计方案

对于采用虚拟子载波的多输入多输出(MIMO)正交频分复用(OFDM)系统,传统最优的均匀分布导频序列在某些情况下是不可用的。为了获得此时的最优导频序列,该文基于最小化最小二乘(LS)信道估计均方误差(MSE)的准则提出了一种改进的导频设计方案。该方案能够快速有效地找到最优导频序列,或在最优导频序列不存在的情况下给出次优的导频序列。仿真结果显示了所提方法的有效性。     

一种基于低码率LDPC码的编码与导频联合辅助载波同步算法

针对深空通信系统中编码和导频辅助载波同步算法分别存在同步范围与精度受限的问题,该文提出一种基于编码与导频联合辅助的载波同步算法。首先按照最优导频放置方式设计了一种基于相关函数和的粗同步算法,其频率估计量的精度接近CRB界;然后分析了导频结构对算法估计精度与范围的影响机理;接着通过加入导频及频率积分器的方式对基于期望最大的细同步算法进行了改进;最后用码率为1/12的低码率LDPC-Hadamard码对算法进行了仿真验证。结果表明新算法可以大幅提高同步参数的估计范围与精度,在一定的导频长度下能达到理想同步。     

基于STBC-SCFDE系统的信道估计算法研究

针对频率选择性衰落信道下2发射天线的空时分组编码-单载波频域均衡(STBC-SCFDE)系统,分别设计了2种新颖的时域多天线导频序列和频域多天线导频序列,相应地提出了一种时域导频信道估计算法和一种频域导频信道估计算法,并分别进行了信道估计的MSE性能分析以及计算复杂度对比.仿真验证表明,相比传统经典多天线信道估计算法,所提2种信道算法在中低信噪比下均能实现良好的信道估计性能,为未来无线通信系统的信道估计开辟了新的解决途径.     

UFMC系统中的载波频率同步技术

针对UFMC系统对频率偏差敏感的问题,提出一种适合用于UFMC 系统的频率同步算法。为了保留UFMC系统的良好特性、降低UFMC中滤波器的设计复杂度,该算法设计了一种子带间正交的导频序列,并采用非线性最小二乘（NLS）法进行CFO估计。通过序列构造和计算该构造序列与估计信号的相关性对算法进一步改进,使得低信噪比下的估计性能得到改善。理论分析和仿真表明,在高斯和瑞利衰落信道下,子带间正交导频序列的误码率、CFO性能均优于全1导频序列;在低信噪比环境下,改进算法的CFO性能优于NLS算法。     

大频偏低信噪比条件下QPSK载波环的设计

在介绍锁相环原理的基础上,针对大频偏和低信噪比条件对锁相环参数进行修正,并分析了传统判决反馈环载波跟踪方法的缺点。提出一种在大频偏和低信噪比条件下FFT频偏估计辅助锁相环的设计方法。仿真结果表明该环路在极低信噪比下仍可以快速捕获。将该方法应用于工程中,并与单锁相环载波同步系统的锁定时间进行比较,实测结果证明了分析的正确性。     

基于前向结构的高动态突发信号载波同步算法

为实现高动态突发通信系统载波快速同步,提出了一种新的基于前向结构的突发信号载波同步算法,采用多级处理方案分级实现多普勒变化率、多普勒频偏及载波相位的估计及恢复.理论分析及数值仿真表明,在高动态低信噪比条件下,新算法的载波同步信噪比损失小于1 dB,且对多普勒变化率及信号电平参数不敏感,实现复杂度低,能够满足高实时性应用需求.     

Pilot-Assisted Carrier Frequency Offset Estimation for OFDM systems with Multiple Preambles Over Fast Fading Channels

This paper proposes a carrier frequency offset (CFO) estimation scheme for OFDM systems over fast fading channels. In the proposed scheme, exploiting the multiple preambles with the identical values, we estimate the CFO over fast fading channels. In particular, we improve the performance of the CFO estimator by applying the overlapped windows to preambles. Through simulations, we validate the proposed estimation scheme by showing the effectiveness of the proposed estimator.     

非相干扩频捕获的匹配滤波器参数分析

在直接序列扩频传输系统中,接收端通常利用匹配滤波器实现扩频信号的初始捕获,扩频码同步时匹配滤波器的输出信噪比是决定捕获性能的一个重要因素。针对非相干直接序列扩频信号,通过理论推导和仿真验证得到了匹配滤波器输出信噪比与载波频差、扩频比以及匹配滤波器参数之间的关系,该结论可以为匹配滤波器的设计提供依据。     

Design of a preamble signal for synchronization in ultra‐wideband noncoherent energy detection receivers

We present an effective preamble design scheme that can improve synchronization performance in ultra‐wideband noncoherent energy detection receivers. Focusing on the effect of the nonuniform energy allocation of a preamble in energy detection‐based synchronization, we propose a preamble energy allocation scheme. This scheme determines the energy distribution of a preamble by using a constrained optimization method. Through the proposed scheme, we promise that we can provide a generalized solution to design preambles for energy detection‐based synchronization, in that it is possible to design the power distribution of preamble pattern in various shapes according to the system's specification. In addition, we also suggest two preamble design scheme that finally decides the practical shape of a preamble from the determined energy distribution of a preamble. By providing preamble design examples, we show that preambles based on nonuniform energy distribution provide improved synchronization performance without any additional increase in receiver complexity in ultra‐wideband channel environments. Copyright © 2011 John Wiley & Sons, Ltd.     

基于软件无线电的直接扩频序列接收机新方案

在低信噪比的低压电力线通信环境中,使用传统的扩频解调方法,不易提取同步载波。针对这一问题,提出一种基于软件无线电结构的直接扩频序列（Dsss）接收机方案。该方案以带通采样定理为依据,结合A／D转换器和数字低通滤波器,无需提取同步栽波即可完成已调信号的解调,并在基带完成信号的解扩。通过理论和仿真证明,该方案开销低,抗噪性能强,适合在恶劣的通信环境中使用。     

Estimating Clock Uncertainty for Efficient Duty-Cycling in Sensor Networks

Radio duty cycling has received significant attention in sensor networking literature, particularly in the form of protocols for medium access control and topology management. While many protocols have claimed to achieve significant duty-cycling benefits in theory and simulation, these benefits have often not translated into practice. The dominant factor that prevents the optimal usage of the radio in real deployment settings is time uncertainty between sensor nodes which results in overhead in the form of long packet preambles, guard bands, and excessive control packets for synchronization. This paper proposes an uncertainty-driven approach to duty-cycling, where a model of long-term clock drift is used to minimize the duty-cycling overhead. First, we use long-term empirical measurements to evaluate and analyze in-depth the interplay between three key parameters that influence long-term synchronization: synchronization rate, history of past synchronization beacons, and the estimation scheme. Second, we use this measurement-based study to design a rate-adaptive, energy-efficient long-term time synchronization algorithm that can adapt to changing clock drift and environmental conditions, while achieving application-specific precision with very high probability. Finally, we integrate our uncertainty-driven time synchronization scheme with the BMAC medium access control protocol, and demonstrate one to two orders of magnitude reduction in transmission energy consumption with negligible impact on packet loss rate.     

Distributed beamforming in wireless relay networks with quantized feedback

This paper is on quantized beamforming in wireless amplify-and-forward (AF) relay networks. We use the generalized Lloyd algorithm (GLA) to design the quantizer of the feedback information and specifically to optimize the bit error rate (BER) performance of the system. Achievable bounds for different performance measures are derived. First, we analytically show that a simple feedback scheme based on relay selection can achieve full diversity. Unlike the previous diversity analysis on the relay selection scheme, our analysis is not aided by any approximations or modified forwarding schemes. Then, for highrate feedback, we find an upper bound on the average signalto- noise ratio (SNR) loss. Using this result, we demonstrate that both the average SNR loss and the capacity loss decay at least exponentially with the number of feedback bits. In addition, we provide approximate upper and lower bounds on the BER, which can be calculated numerically.We observe that our designs can achieve both full diversity as well as high array gain with only a moderate number of feedback bits. Simulations also show that our approximate BER is a reliable estimation on the actual BER. We also generalize our analytical results to asynchronous networks, where perfect carrier level synchronization is not available among the relays.     

一种适用于极低信噪比环境的迭代同步方案

卫星通信、深空通信等应用要求接收机在很低信噪比下工作。近年来研究热点基于码辅助同步算法能够利用编码间依赖关系提高参数估计性能,以实现正确解调。本文以码辅助同步算法为基础,综合分析同步各环节参数估计需求,设计较短帧头的帧格式,提出了一种完整的迭代接收机结构。同时精心选择预估计算法,将定时偏差、载波频偏压缩到很窄范围。最后对码辅助算法迭代复杂度高的问题进行了优化。仿真表明,在极低信噪比下,该结构同步性能良好,复杂度低,传输效率高。     

一种低信噪比SMCC+系统快速同步算法

本文基于中国数字广播电视标准候选方案之一的同步多载波扩频(SMCC )系统,提出了一种低信噪比下高速(250km/h)移动环境的快速同步算法。该算法利用巴克码组码片重复而成的短PN序列产生同步信号,解决了常规同步算法中同步信号多个峰值易误判、门槛值难以选择、频偏估计范围不够大以及低信噪比下同步难等缺点。仿真证明,在-20dB信噪比以上的高速移动AWGN信道和瑞利多径衰落信道环境,与常规算法相比,该法时间同步性能更优,频偏估计范围更大。     

QPSK扩频调制信号载波跟踪环路设计

扩频接收基带通常需要载波跟踪环来完成本地载波与接收信号载波的同步,科斯塔斯环是常用的非相干载波相位跟踪环,具有较好的鉴相特性。本文基于常规的单路单载波解调的科斯塔斯环原理,对科斯塔斯环做了改进,提出了对双通道平衡QPSK扩频基带所用载波跟踪环科斯塔斯环的改进方法,并推导出环路误差鉴相信号,用Matlab进行了仿真实验,结果证明利用本环路可实现载波稳定跟踪。