Multirate digital filters for symbol timing synchronization insoftware defined radios

This paper describes the use of a polyphase filterbank to perform the interpolations required for symbol timing synchronization in a sampled-data receiver. The polyphase filterbank possesses advantages over architectures based on separate matched and interpolation filters. Interpolations are realized by filterbank index selection and a separate interpolating filter following the matched filter is not required. Maximum likelihood timing synchronization techniques can be easily incorporated into the polyphase filter bank in a natural way. An M-stage polyphase filterbank with input data sampled at approximately N samples/symbol can be used in a loop that operates at MN samples/symbol, N samples/symbol, or 1 sample/symbol. When operating at 1 sample/symbol, auxiliary control must also be included to adjust the clocking of data into the filter bank to account for small differences in the sample clock and N times the data clock. Examples are presented to illustrate loop performance and control     

一种基于插值和卡尔曼滤波的接收机钟差预测方法

针对多模多频接收机面临同时处理大量数据的压力,提出了一种基于插值和卡尔曼滤波的接收机钟差预测方法。插值方法分别用拉格朗日和三次样条,三次样条端点的一阶导数采用“差分法代替求导法”来确定。首先由插值方法得出每隔1s的卫星坐标、速度、钟差、频漂和伪距测量值,然后基于单星授时方法计算出静止接收机钟差,接着用卡尔曼滤波算法对接收机钟差和频漂进行预测,最后将预测的接收机钟差与加拿大空间参考系统( CSRS )提供的精密接收机钟差数据进行比较。结果表明,拉格朗日插值由于存在龙格效应,其接收机钟差的抖动幅度比三次样条略大,它们与CSRS钟差数据相比,均方根误差在3 ns之内。     

物理层网络编码的符号时钟估计

现有的关于物理层网络编码(PNC)的研究多建立在时钟已完全同步的基础上,对PNC的符号时钟同步研究较少。而实际上符号时钟在PNC中是必不可少的。针对这一问题,该文提出一种新的基于正交训练序列适用于双向中继信道PNC的符号时钟估计方法。该方法根据最大似然估计准则,运用基于离散傅里叶变换(DFT)的插值算法来估计时钟误差。仿真结果表明,所提出的DFT插值算法性能优越,在信噪比(SNR)大于10 dB的条件下,系统的均方误差(MSE)性能比经典优选采样点法提升1个数量级,并且非常逼近修正克拉美罗界(MCRB)。     

Optimal Farrow coefficients for symbol timing recovery

The Farrow structure provides an efficient way to implement interpolation filters. Such filters are often required in digital modems to allow symbol timing recovery. A method for generating Farrow filter coefficients that minimize the mean square error (MSE) at the symbol decision instants is presented. Minimizing the MSE at the symbol decision instants is nearly equivalent to minimizing the symbol error rate (SER) but is more mathematically convenient. The MSE at times other than the symbol decision instants does not affect the SER and is not considered in the optimization. Example coefficients are tabulated and performance is illustrated using SER results generated by computer simulation     

一种改进的GFDM时频同步算法

针对广义频分复用( GFDM)系统对符号定时同步要求较高的问题,提出了一种新的基于前缀码的同步算法。在接收端,在获取粗略定时信息的基础上,利用前缀码前后两部分的相位差实现载波频偏估计,并对接收序列的频率偏移进行纠正,然后通过纠正后序列与已知发射前缀的互相关函数实现精确的符号定时估计。由于该前缀码具有共轭对称的特性,使其避免了“平顶效应”的出现。结合5G中低时延高可靠场景,在频率选择性信道中对其进行仿真,并通过均方误差对其性能进行了评估。理论分析及仿真结果表明,该算法相对于原算法具有更好的定时同步性能和更低的复杂度,提升了GFDM系统的整体性能。     

Timing error detector design and analysis for orthogonal space-time block code receivers

A general framework for the design of low complexity timing error detectors (TEDs) for orthogonal space-time block code (OSTBC) receivers is proposed. Specifically, we derive sufficient conditions for a difference-of-threshold-crossings timing error estimate to be robust to channel fading. General expressions for the S-curve, estimation error variance and the signal-to-noise ratio are also obtained. As the designed detectors inherently depend on the properties of the OSTBC under consideration, we derive and evaluate the properties of TEDs for a number of known codes. Simulations are used to assess the system performance with the proposed timing detectors incorporated into the receiver timing loop operating in tracking mode. While the theoretical derivations assume a receiver with perfect channel state information and symbol decisions, simulation results include performance for pilot-symbol-based channel estimation and data symbol detection errors. For the case of frequency-flat Rayleigh fading and QPSK modulation, symbol-error-rate results show timing synchronization loss of less than 0.3 dB for practical timing offsets. In addition it is shown that the receiver is able to track timing drift with a normalized bandwidth of up to 0.001.     

一种无需导频的适用于差分OFDM系统的符号与采样钟联合同步方法

该文提出了一种适用于OFDM系统的联合符号和采样钟同步校正方法,其中同步校正是在数字域通过改变对接收过采样信号的插值(interpolation)和抽取(decimation)实现的。这种方法在发送端相邻载波间采用差分QPSK调制,在接收端利用QPSK的差分解调信号获得同步误差信号,从而获得关于OFDM符号同步和采样钟同步调整的算法,其特点是无需专门的同步导频信号。所提出算法的同步性能在高斯白噪声信道和多径衰落信道均得到验证。     

Unbiased timing-error estimation in the presence of nonidealinterpolation

We propose a method to eliminate the bias term present in the timing-error estimator employed in digital receivers where the input signal is sampled by a fixed clock which is not synchronized to the transmitter clock. This bias error results from the nonideal interpolation that precedes the timing-error estimator. We show that it can be derived as a function of the previously estimated symbol timings. An unbiased timing-error estimate can then be obtained by subtracting this bias term from the output of the timing-error detector. Simulation results are included to show the performance improvement realizable by employing this method     

Frequency-Domain Carrier Frequency and Symbol Timing Offsets Estimation for Offset QAM Filter Bank Multicarrier Systems in Uplink of Multiple Access Networks

In this paper, we study carrier frequency offset (CFO) and symbol timing offset (STO) estimation in the uplink of multiuser offset QAM filter bank multicarrier (FBMC) communication systems. A low-complexity frequency-domain CFO estimator using periodical training sequence is proposed. Also a good estimate of STO is obtained by maximizing the energy of subcarriers subject to training sequence at the output of analysis filter bank. Furthermore we derive the theoretical MSE for the proposed CFO estimator. Computer simulations show that the derived MSE matches the simulated MSE closely. Compared with state-of-the-art time domain estimator, the proposed estimator achieves better performance with a lower computational complexity.     

A novel timing estimation method for OFDM systems

We present a novel timing offset estimation method for orthogonal frequency division multiplexing systems. The estimator proposed here is designed to avoid the ambiguity which occurs in Schmidl's (1997) timing offset estimation method. The performance of the proposed scheme is presented in terms of mean and mean-square error (MSE) obtained by simulations. The simulation results show that the proposed estimator has a significantly smaller MSE than the other estimators.     

基于MPSK/MQAM的符号率偏差辅助捕获方法

在非协作通信中,发送端的符号率对于接收机是未知的,接收机必须从接收数据中估计出此参数。为了校正符号率的估计偏差,本文提出的适用于MPSK/MQAM的符号定时恢复环路,通过锁定检测来调整环路带宽可以快速捕获0.5%的符号率偏差,并确保环路有好的稳态性能。此外本文提出了一种新的符号率偏差辅助捕获方法——搜捕,可以捕获10%的符号率偏差,这一点在非协作通信中具有重要意义。而且仿真结果表明,在不同的符号率偏差下环路收敛后的稳态性能几乎一样,在相同的误比特率下实时的解调损失与理论值相比小于1dB。      

Maximum Likelihood (ML) Symbol Timing Recovery (STR) Techniques for Reconfigurable PAM and QAM Modems

A fast symbol timing recovery (STR) scheme suitable for flexible-symbol-rate, burst-mode M-ary QAM (M-QAM) and PAM (M-PAM) demodulators used in broadband communications is proposed. The maximum- likelihood (ML) estimation is used to derive a feed-forward STR strategy to achieve fast and accurate derivation of the optimum timing instant with a very short training preamble for applications requiring short packet transmission. In addition, the signal sample at the optimum timing instant is reproduced by interpolation. Analysis to derive the ML-based timing estimation algorithm and STR structure is presented. Analytical models are developed to evaluate the effects of imperfect interpolation and channel additive white Gaussian noise (AWGN) on the timing estimation performance. The derived expressions can be used for optimum interpolation filter design and selection of key parameters to achieve the required mean squared timing error. Simulation results are in good agreement with the analytical prediction.     

一种数字调相接收系统定时恢复算法

定时恢复是数字接收机同步系统的关键技术。经典数字接收机为防止最佳采样点抽取发生偏移造成接收信号散点图发散,往往通过锁相环控制抽样器以实现最佳采样点的抽取,而在全数字接收机中,最佳采样点是通过对采样信号进行插值计算得到的。全数字相位调制接收机定时恢复环路主要由平方定时相位偏差估计、定时控制单元和牛顿内插滤波器三部分组成。仿真结果充分表明该环路能够很好的实现MPSK、MQAM信号的定时恢复。     

Bit-rate sensitivity of filter-bank-based VDSL transmission totiming errors

The performance degradation of filter-bank-based multicarrier transmission due to timing errors is investigated. The receiver is made of a fractionally spaced linear or decision-feedback equalizer designed for some sampling phase. The actual sampling phase is different and the impact of the difference on the performance is investigated. Sampling phase offset and jitter are considered. Besides, assuming the sampling phase error can be estimated the efficiency of various types of interpolation is investigated     

Timing synchronization using cross ambiguity function for MIMO OFDM systems with distributed antennas

In this letter, we propose a novel timing offset estimation method for multiple input multiple output (MIMO) orthogonal frequency division multiplexing (OFDM) systems with distributed antennas using cross ambiguity function (CAF). The proposed algorithm has an impulse-shaped timing metric which allowed it to reduce the uncertainty due to the timing metric plateau. Simulation results show that the mean square error (MSE) performance of the proposed method is significantly improved.     

On the robustness of the finite-length MMSE-DFE with respect to channel and second-order statistics estimation errors

The finite-length minimum mean square error decision-feedback equalizer (MMSE-DFE) is an efficient structure mitigating intersymbol interference (ISI) introduced by practically all communication channels at high-enough symbol rates. The filters constituting the MMSE-DFE, as well as related performance measures, can be computed by assuming perfect knowledge of the channel impulse response and the input and noise second-order statistics (SOS). In practice, we estimate the unknown quantities, and thus, inevitable estimation errors arise. We model the estimation errors as small perturbations, and we derive a second-order approximation to the excess MSE. Furthermore, we derive second-order approximations to the mean excess MSE in terms of the parameter estimation error covariance matrices and simple and informative bounds, revealing the factors that govern the behavior of MMSE-DFE under mismatch. Simulations confirm that the derived second-order approximations provide accurate estimates of the MMSE-DFE performance degradation due to mismatch.     

The effect of interpolation on the BER performance of narrowbandBPSK and (O)QPSK on Rician-fading channels

This paper deals with the bit error rate (BER) performance of a fully digitally implemented receiver performing coherent detection on a narrowband BPSK or (O)QPSK signal, transmitted over a slow non-frequency selective Rician fading channel. The considered digital receiver operates on samples of the received signal, taken by a fixed clock which is not synchronized to the transmitter clock. Signal samples needed for detection of the information sequence are obtained from interpolation between the available samples. In the case of nonideal interpolation a BER degradation occurs. By means of theoretical analysis, we demonstrate that even simple interpolators, operating at only a few samples per symbol, yield BER degradations that are limited to a small fraction of a dB. This paper is an extension of the work of Erup, Gardner and Harris (see ibid., vol.41, no.6, p.998-1008, 1993) where only the AWGN channel was considered and results were obtained by computer simulations     

基于FPGA的OFDM系统同步算法的研究

符号、载波偏移严重影响着正交频分复用(OFDM)接收系统的性能.本文在分析和研究同步估计算法的基础上,设计了一种OFDM同步系统实现方案,并对符号偏移,载波频偏和采样钟偏移的估计及补偿算法和具体电路实现进行了相应优化.并采用VirtexII系列xc2v1000 FPGA进行实现验证,结果表明该同步系统不仅能有效地实现OFDM系统符号和载波同步,而且可降低硬件资源的消耗.     

跳频/多载波频率分集/扩频多址无线通信系统的研究

本文提出了一种跳频／多载波频率分集／扩频多址（ＦＨ／ＭＣＦＤ／ＳＳＭＡ）无线通信系统,给出了ＦＨ／ＭＣＦＤ／ＳＳＭＡ系统的发送和接收模型,对判惟变量统计特性进行了分析,然后对峰窝系统反向链路在理想定时和信道估计条件下用户平均接收误码率进行了仿真。结果表明,ＦＨ／ＭＣＦＤ／ＳＳＭＡ蜂窝通信系统具有较好的抗多径衰落能力,同单载波ＦＨ／ＳＳＭＡ系统相比其误码性能和频谱效率有显著改善。     

Multiuser receivers for CDMA systems in Rayleigh fading channels

Multiuser demodulation in relatively fast fading channels is analyzed. The optimal maximum likelihood sequence detection (MLSD) receiver is derived and a general suboptimal receiver to approximate the MLSD is proposed. The performance of the parallel interference cancellation (PIC) and decorrelating receivers is compared. The PIC receiver is demonstrated to achieve better performance in known channels than the decorrelating receiver, but it is observed to be more sensitive to the channel coefficient estimation errors than the decorrelator. At high channel loads the PIC receiver suffers from bit error rate (BER) saturation, whereas the decorrelating receiver does not. The performance of data-aided (DA) and decision-directed (DD) multiuser channel estimation is also compared. DA channel estimation is shown to be more robust than DD channel estimation, which may suffer from BER saturation caused by hangups at high signal-to-noise ratios (SNRs). The impact of channel estimation filter impulse response on the BER is studied by comparing optimal and suboptimal channel estimation filters. The implementation complexity of the decorrelating and PIC receivers is compared in terms of required floating point operations and clock cycles in a practical communication scenario. It is observed that the PIC receiver is only moderately more complex to implement than the conventional matched filter bank receiver, whereas the decorrelating receiver is significantly more complex