一种基于APPA的码辅助载波同步算法

在低信噪比环境下,小的频偏和相偏的存在会使Turbo编码系统的译码性能恶化,所以必须结合迭代译码系统,对信号的残留频偏和相差进行估计。该文提出一种改进的残留频偏载波相位估计算法后验概率辅助(APPA)相位估计,该算法将译码器输出的外信息用于辅助迭代的相位估计。环路滤波器将相位误差信号转换成控制信号,控制数控振荡器的输出,这样可以得到待估计的相位误差。仿真表明,在极低信噪比下(比如,SNR-7.8 dB)该算法在同时存在频偏和相偏的时候能正常工作,其性能非常接近理想同步条件下的性能。     

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.     

TDMA信号的宽范围载波同步译码联合迭代处理

针对时分多址(Time Division Multiple Access,TDMA)信号提出了宽范围的载波同步译码联合迭代处理的方案。该方案主要分为三个部分,首先利用TDMA信号的同步码进行数据辅助同步,对信号中的大频偏和相偏进行粗估计和纠正;然后利用线性调频Z变换(Chirp-Z Transform,CZT)算法针对整突发帧进行非数据辅助同步,精确估计和纠正信号的剩余频偏;最后通过利用译码器输出的软信息进行编码辅助同步,多次迭代得到剩余相偏的精确估计,最终实现载波的精确同步并输出译码结果。实验结果表明,所提方案具有同步捕获范围宽、参数估计精度高、译码输出误比特率低等优点,在低信噪比环境下也能表现出优良性能。     

极低信噪比环境下的载波同步算法研究

在极低信噪比环境下,需要使用纠错能力强的编码比如Turbo码,但Turbo码系统的缺点是要求近乎理想的载波同步。然而,在极低信噪比环境下,Turbo编码系统的同步是具有挑战性的工作。文章提出了一种基于最大均方软输出的扫频和面向判决载波同步相结合的载波同步算法。仿真结果表明,该算法能捕获较大的频偏和相偏,且在极低信噪比环境下,仍能正常工作,其性能非常接近理想同步条件下的性能。     

一种改进载波相位恢复算法

为了解决载波相位恢复算法频偏估计范围小、估计精度低、计算复杂度高等问题,提出了一种基于Q次方的极性环与极性判决(Polar Decision,PD)算法相结合的Q次方极性判决(Q-th Power Polarity Decision,QPD)算法.首先对判决导向(Decision Directed,DD)算法、PD算法...     

基于判决反馈的OFDM载波频率同步环路

对OFDM系统中一种基于接收时域信号重构的载波频率盲估计算法进行了进一步的研究,分析了在这种方法中影响估计精度的4个方面的因素。为了减少载波频率估计的剩余误差,通过在频率补偿时增加反馈因子,提出了一种基于判决反馈的载波频率同步环路结构。通过调整同步环路的环路增益,可以获得更好的频率跟踪性能。如果信噪比达到9dB,即使存在较大的频率漂移,剩余频偏也被限制在子载波间隔的1%以内。     

适用于机载卫星通信的低信噪比载波恢复算法

随着卫星通信技术的不断发展,对低信噪比下的解调技术提出了越来越高的要求,尤其是针对机载平台下的卫星通信系统,需要在极低信噪比条件下进行解调。采用了一种基于载波相位内插的低信噪比解调算法,利用FFT进行频偏估计,然后利用相邻2个帧头的相位估计结果进行内插,得到帧内各符号的载波相位信息,实现载波恢复。仿真和结果表明,帧头长度为32时可以获得较好的解调性能,解调门限附近载波恢复导致的解调损失小于0.3 dB。     

一种扩展载波环频率捕获范围的方法

在MPSK数字载波恢复中,可以通过锁频锁相环捕获大范围频率偏移。低信噪比下,锁相环的频率捕获范围较小,而工程中锁频环剩余频偏通常较大,因而需要扩展锁相环的频率捕获范围。描述了频差估计以及COSTAS环中相位检测技术的实现方法,分析了COSTAS环的相位检测特性,并根据这种算法的鉴相特性提出了一种扩展频率捕获范围的载波恢复方法。以QPSK为例,通过MATLAB仿真了这种技术在低信噪比情况下对频率偏移的检测与跟踪性能,仿真结果表明,提出的方法适合低信噪比下大范围的频率捕获。     

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。     

基于EM算法的码辅助迭代载波同步的预估计

针对非贝叶斯迭代载波同步联合译码算法的收敛条件进行研究,以EM迭代估计模型为例,重点研究了多种频偏预估计算法的估计性能,并结合最大似然相位估计算法,仿真得到了使EM迭代估计器收敛的概率,分析无法收敛对系统译码性能的恶化程度,以及为满足最终误码性能所需要选择的预估计方案,并得到了一些有参考价值的结论.     

一种改进的Fitz频偏估计算法

针对通信接收中的载波同步问题,提出一种基于Fitz算法的改进载波频偏估计算法.分析了Fitz算法频偏估计方法中估计范围受限的原因,针对性地提出了改进方法,利用离散傅里叶变换(DFT)算法得到基准频偏,以此修正大间隔自相关函数估计频偏,保持了Fitz算法的估计精度优势,极大地扩展频偏估计范围,同时具有低的信噪比门限.最后给出了改进算法的实现步骤,并通过MATLAB仿真实验证明了算法的有效性.     

Carrier phase tracking from turbo and LDPC coded signals affected by a frequency offset

This contribution considers carrier phase estimation from coded signals, affected by a frequency offset at low operating signal-to-noise ratio (SNR). We derive a maximum likelihood (ML) based iterative code-aided feedback phase-tracker suited for receivers with iterative maximum-a-posteriori (MAP) detection. Simulations indicate that the proposed synchronizer is considerably more robust against frequency offsets than a feedforward synchronizer.     

Carrier phase and frequency estimation for pilot-symbol assisted transmission: bounds and algorithms

In this paper we consider the Cramer-Rao lower bound (CRB) for the joint estimation of the carrier phase and the frequency offset from a noisy linearly modulated burst signal containing random data symbols (DSs) as well as known pilot symbols (PSs). We point out that the CRB depends on the location of the PSs in the burst, the number of PSs, the number of DSs, the signal-to-noise ratio (SNR), and the data modulation scheme. Distributing the PSs symmetrically about the center of the burst and estimating the carrier phase in the center of the burst interval decouples the frequency and phase estimation, making the CRB for phase estimation independent of the specific location of the PSs. At low and moderate SNR, the CRBs for both phase and frequency estimation decrease as the fraction of the PSs in the burst increases. In addition, the CRB for frequency estimation decreases as the PSs are separated with more DSs. Numerical evaluation of the CRB indicates that the carrier phase and frequency of a "hybrid" burst (i.e., containing PSs and DSs) can be estimated more accurately when exploiting both the presence of the DSs and the a priori knowledge about the PSs, instead of using only the knowledge about the PSs (and ignoring the DSs), or considering all the received symbols (PSs and DSs) as unknown (and ignoring the knowledge about the PSs). Comparison of the CRB with the performance of existing carrier synchronizers shows that the iterative soft-decision-directed (sDD) estimator with data-aided (DA) initialization performs very closely to the CRB and provides a large improvement over the classical non-data-aided (NDA) estimator at lower SNR.     

Joint frame synchronization and frequency offset estimation in OFDM systems

A new joint frame synchronization and carrier frequency offset estimation scheme in orthogonal frequency division multiplexing (OFDM) systems is proposed in this paper, where both frame synchronization and carrier frequency offset estimation can be performed by using only ONE training symbol. Frame synchronization and carrier frequency offset acquisition are performed simultaneously in the proposed scheme. Reliable frame synchronization is obtained in the proposed scheme even in low SNR. The maximum carrier frequency offset acquisition range of the proposed scheme can be up to one half of the total signal bandwidth. The same training symbol can also be utilized to carrier frequency offset Fine Adjustment, which estimates the remaining carrier frequency offset after acquisition with higher accuracy. The performance comparison of the proposed Fine Adjustment algorithm and Schmidl's algorithm by using computer simulation illustrates and verify the superior performance of the proposed algorithm with regard to estimation accuracy.     

基于EM的低复杂度迭代载波同步算法

编码辅助载波同步算法充分利用译码软信息,可获得较理想的同步性能,但是目前这类算法存在复杂度高、处理时间长的问题。首先在推导迭代载波同步中后验均值与译码软信息关系的基础之上,通过选择性地进行译码判决反馈来减少不必要的计算点数,再根据前两次的迭代结果自适应地减小频率搜索区间长度,从而大幅降低同步参数估计的运算量。仿真结果表明,该算法能够有效工作在低信噪比环境下,可获得接近理想同步条件下的译码性能。     

Soft Decoding Assisted SNR Estimation Under Block Fading Channels for Orthogonal Modulations

The performance of the coded orthogonal modulation (OM) system under slow fading channels heavily depends on the estimation of the signal-to-noise ratio (SNR), including the fading amplitude and the noise spectral density. However, a relatively long packet of pilot symbols is often required to guarantee the accuracy of the SNR estimation, which makes it impractical in some situations. To address this problem, this paper proposes an iterative SNR estimation algorithm using the soft decoding information based on the expectation-maximization algorithm. In the proposed method, a joint iterative loop between the SNR estimator and decoder is performed, where the extrinsic information generated by the soft decoder is employed to enhance the estimation accuracy and the SNR estimated by the estimator is used to generate the soft information to the decoder. Also, no pilot symbols are needed to estimate the SNR in the proposed estimator. The Cramer–Rao lower bound (CRLB) of fully data-aided (FDA) estimation is derived to works as the final benchmark. The performance of the proposed algorithm is evaluated in terms of the normalized mean square errors (NMSEs) and the bit error rates (BERs) under block fading channels. Simulation results indicate that the NMSE of the proposed estimator reaches the CRLB of the FDA estimator and outperforms that of the approximate ML (ML-A) estimator proposed by Hassan et al. by 4.1 dB. The BER performance of coded OM system with the proposed estimation algorithm is close to the ideal case where the channel fading and the noise spectral density are known at the receiver.     

MPSK信号载波频率盲估计

该文针对调制自动识别中,MPSK信号调制相制和训练序列未知的情况,提出了一种利用定时同步信号的差分相位有效识别MPSK信号相制M,并精确估计其载波频偏的方法。该算法不仅克服了调制制式不明对频偏估计的不利影响,还通过引入线性相位展开和高阶时延相关,大大提高了大频偏、低信噪比下估计的性能,对调制自动识别中MPSK信号的澄清和参数估计起到关键作用。     

ML iterative soft-decision-directed (ML-ISDD): a carrier synchronization system for short packet turbo coded communication

Modern communication systems are required to provide services based on high data rates burst-mode packet-data transmission, capable of operating at very low SNR conditions. Turbo codes enable the operation at low SNR, close to the Shannon limit. However, carrier frequency and phase synchronization, needed for optimal coherent performance of the receiver, still remains a problem in low SNR and short bursts conditions. This paper proposes a new carrier synchronization method, the Maximum-Likelihood Iterative-Soft-Decision-Directed (ML-ISDD), which uses the turbo-decoder soft decisions to improve the carrier synchronization performance at low SNR values. The ML-ISDD method operates iteratively and jointly with the turbo decoder, enhancing both the turbo-decoder and the synchronization performance. The ML-ISDD method has been shown by simulation to significantly increase the allowed initial frequency and phase uncertainty region, thus allowing the use of very short training sequences for initial carrier synchronization.     

调频遥测信号载波频率的精确估计

传统调频遥测信号载波频率估计算法对输入信号降采样后直接进行快速傅里叶变换，实现方法虽然简单，但测量精度较差，无法适应高动态、低信噪比等复杂场景。为此，提出了一种调频遥测信号载波频率的精确估计算法。两并联补偿支路先分别采用正、负调频频率对输入信号进行频率预先补偿，低通滤波后完成降采样处理，削弱调频频率的频谱影响；频率搜索状态对采样数据进行载波多普勒变化率的频率补偿，经过快速傅里叶变换、非相干积分和频谱重心搜索完成频率解算，提高载波频率的检测性能。试验与分析表明，所提算法在高动态、低信噪比等复杂场景下可显著提高调频遥测信号载波频率的估测性能。     

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。