System identification techniques for adaptive signal processing

Many problems in adaptive filtering can be approached from the point of view of system identification. The close interconnection between these two disciplines is explored in some detail. This approach makes it possible to apply recursive parameter estimation algorithms to adaptive signal processing. Several examples are discussed including: adaptive line enhancement, generalized adaptive noise cancelling, adaptive deconvolution and adaptive TDOA estimation. It is shown how the recursive maximum likelihood algorithm can be used for both FIR and IIR filtering, and some preliminary results are presented. Several alternative algorithms are briefly discussed.This work was supported by the Office of Naval Research, Contract No. N00014-79-C-0743.     

Amplitude estimation of rectangular narrow-band radio pulse with unknown duration and initial phase

This study investigates algorithms for the estimation of amplitude of the radio signal with rectangular envelope and unknown duration and initial phase. The synthesis and analysis of quasi-likelihood and quasi-coherent estimation algorithm have been performed. This algorithm implies that the unknown duration and initial phase are replaced with certain expected values of these parameters. The loss in accuracy of amplitude estimation owing to the a priori lack of knowledge of the duration and the initial phase is analyzed. The quasi-likelihood noncoherent algorithm for amplitude estimation with the initial phase adaptation is synthesized and its statistical characteristics such as the estimate bias and variance are determined. The relationships of the loss in estimation accuracy owing to the a priori lack of knowledge of signal duration are derived. The maximum likelihood algorithm for amplitude estimation was synthesized and analyzed. This algorithm implies that the unknown duration and the initial phase are replaced with their maximum likelihood estimates. The gain in accuracy of the maximum likelihood estimate as compared to the quasi-likelihood ones was investigated. It is shown that a priori lack of knowledge of the signal duration does not affect asymptotically the accuracy of maximum likelihood estimate of amplitude at large values of signal-to-noise ratio.     

Analysis of synchronization algorithms with time-out control over networks with exponentially symmetric delays

We study the performance of a class of time-offset estimation algorithms for synchronization of master-slave nodes based on asynchronous transfer of timing cells when GPS is not used. We implement a synchronization control mechanism based on cell acknowledgment time-out (TO) with wait or no wait options. We analyze the mechanism reliability and performance parameters over symmetric links using an exponential cell delay variation model. We show that the maximum-likelihood offset estimator does not exist for the exponential likelihood function. We analytically provide RMS error result comparisons for five ad hoc offset estimation algorithms: the median round delay, the minimum round delay, the minimum link delay (MnLD), the median phase, and the average phase. We show that the MnLD algorithm achieves the best accuracy over symmetric links without having to impose a strict TO control, which substantially speeds up the algorithm. We also discuss an open-loop estimation updating mechanism based on standard clock models.     

非数据辅助的QAM信号载波频偏估计算法

研究了正交幅度调制（Quadrature Amplitude Modulated ,QAM）信号的非数据辅助载波频率偏移估计算法。针对一些算法主要适用于低阶QAM信号以及计算复杂度较高等问题,提出一种基于加权相位直方图的载波频偏估计算法。该算法通过分析零频偏下的相位概率密度函数与加权相位直方图的关系,采用多级估计方法,对该相位直方图二阶矩的峰值进行测量,从而得到频偏估计值。进一步地,本算法通过联合快速傅里叶变换（Fast Fourier Transform,FFT）算法实现多级估计过程,从而简化计算过程降低计算复杂度。仿真结果表明,信噪比较高时,该算法在无信噪比、星座图以及前导信息等辅助信息条件下同样具有较好的估计性能。      

Iterative detection of turbo-coded offset QPSK in the presence of frequency and clock offsets and AWGN

The key contribution of this paper is to develop transmitter and receiver algorithms in discrete-time for turbo-coded offset QPSK signals. The procedure for simulating a clock offset between the transmitter and receiver is described. Due to the use of up-sampling, matched filtering and a differential correlation approach at the receiver, the time required for detecting the start of frame (SoF) is just around 500 symbols, which is also the length of the preamble. The initial estimate of the SoF and the frequency offset, obtained using the differential correlation approach, is improved using an iterative process. A novel two-step maximum likelihood (ML) frequency offset estimation is proposed, which significantly reduces the complexity over the conventional ML estimation. The decision-directed carrier and timing recovery algorithms use simple first-order IIR filters to track the carrier phase and clock slip. The proposed synchronization and detection techniques perform effectively at an SNR per bit close to 1.5?dB, in the presence of a frequency offset as large as 30% of the symbol-rate and a clock offset of 25?ppm (parts per million). It is shown via simulations that the performance loss with respect to the bare turbo code is only about 0.5?dB, for a preamble length of 500 and a BER of 10^?7. The proposed techniques are well suited for software implementation.     

Phone-based filter parameter optimization of filter and sum robust speech recognition using likelihood maximization

Because of noise and reverberation, accuracy of speech recognition systems decreases when the distance between talker and microphone increases. By the using of microphone arrays and appropriate filtering of received signals, the accuracy of recognizer can be increased. Many different methods for using microphone arrays have been proposed that can be classified into two main approaches: systems that perform in two independent stages of array processing and then recognition and systems that use array processing to generate a sequence of features which maximize the likelihood of generating the correct hypothesis in recognition phase. Following second approach, in this paper a new method for microphone array processing is proposed in which the parameters of array processing are adjusted in calibration phase based on phones used in language and maximum likelihood method. Optimized filter parameters are stored and used during recognition phase. A new modified Viterbi algorithm using optimal phone-based filter parameters is used for recognition phase. The proposed algorithm is analytically formulated and Persian language is used to find any improvement in speech recognition accuracy compared with results of delay and sum and utterance-based filter and sum algorithms. The results show 12.2% improvement in accuracy compared to utterance-based algorithm.     

Improved Cramer-Rao lower bounds for phase and frequency estimationwith M-PSK signals

We derive new Cramer-Rao lower bounds (CRBs) for the estimation of carrier phase and frequency offset from an unmodulated carrier or from a M-PSK data signal. The new CRBs are obtained formally from the exact likelihood function for the carrier phase and frequency offset, build from a block of received phase samples, and are applicable for a general alphabet size M. The bounds are compared with other previously obtained bounds (which also take into account the effect of random phase modulation) and with the performance of some popular feedforward algorithms for carrier phase and frequency offset estimation     

基于粒子群优化的MPSK信号频偏估计算法

针对传统多进制数字相位调制(MPSK)信号非数据辅助(NDA)频偏估计算法存在估计范围有限、估计方差较大、符号个数少时估计性能受限等问题,提出了基于粒子群优化的频偏估计方法。该算法以频偏估计的似然函数为目标函数,模拟群体智能搜索最优解。仿真结果表明,本算法无偏估计范围大,在符号数较少、信噪比较低时,估计方差接近克拉美罗下限(CRLB),性能优于经典的离散傅里叶变换(DFT)算法和Kay算法。     

平衰落信道下ML和MAP信道估计与导频符号长度的关系性能比较

信道估计是无线通信系统必须加以解决的关键技术之一，采用导频符号辅助的方法进行信道估计是目前各类无线通信系统常用的方法。本文针对平衰落信道提出了最大似然(ML)算法和最大后验概率(MAP)估计算法，给出了ML估计和MAP估计之间的关系，仿真了MAP估计和ML估计的方差与导频符号长度的关系，提出当导频符号长度的取值超过20个符号长度时，MAP信道估计明显优于ML信道估计。     

用于正弦波频率估计的修正Kay算法

Kay算法能够估计出采样点较少的正弦波频率,但低信噪比下估计性能不佳.针对此问题,提出了修正Kay算法.首先基于最大似然估计准则,推导了观测信号模值与相位的条件概率密度函数,进而重建了Kay算法的相位差噪声矢量协方差矩阵与权值矩阵.实验结果表明,修正算法能够有效估计正弦波信号频率,与Kay算法相比,抗噪性更强.     

Maximum likelihood synchronization for OFDM using a pilot symbol:algorithms

The design of a pilot symbol for orthogonal frequency-division multiplexing packet detection and synchronization is presented. Algorithms are described for packet detection and for synchronization using maximum likelihood estimation of channel frequency offset, phase offset for coherent detection, and subsample timing offset. The efficacy of the detection and synchronization algorithms is demonstrated experimentally     

Estimation of ultrawideband quasi-radio signal duration

Quasi-likelihood and maximum likelihood algorithms of duration estimation for ultra-wideband quasi-radio signal of arbitrary shape with unknown amplitude and initial phase, influenced by additive Gaussian white noise, are synthesized. It was considered that conditions of relatively narrow band of received signal are not satisfied and its duration can constitute only several periods or a fraction of period of harmonic oscillation. It is shown that the structure of the algorithm for duration estimation of ultra-wideband quasi-radio signal is significantly different from the structure of duration estimation algorithm for narrowband radio signal. Relative bias and variance are determined as the statistical characteristics of synthesized duration estimates. The influence of unknown amplitude and initial phase on the accuracy of duration estimation is investigated. Quantitative limits for relation of signal bandwidth to its center frequency are formulated, such that the classical solution of the problem of duration estimation for narrowband radio signal possesses the required accuracy.     

一种基于LMS自适应滤波的互相关时延估计优化算法

在多站时差定位系统中使用基于LMS自适应滤波的互相关法进行时延估计时,若采用固定步长因子则会在收敛速度和稳态失调之间存在较大矛盾,从而影响时延估计精度。针对这一问题,文中提出了一种基于分段变步长LMS自适应滤波和希尔伯特差值的互相关时延估计优化算法。该方法首先采用分段变步长LMS自适应滤波对信号进行滤波处理,然后将滤波后的信号作互相关运算,最后通过希尔伯特差值法锐化相关函数的峰值,进一步提高时延估计精度。在相同条件下,文中模拟分析了不同算法的时延估计精度。实验结果表明,新的优化算法时延估计精度更高。在不同信噪比下,新方法相较传统时延估计方法精度提高了2.2%以上,具有良好的抗噪声性能。     

一种改进的高动态QPSK信号解调算法

正交相移键控(QPSK)调制解调方式广泛应用于深空通信、数字卫星通信等高速系统中,高动态QPSK信号具有较大的多普勒载波频偏,针对现有的解调算法对QPSK信号频偏估计范围小、精度较低等问题,提出了一种改进的高动态QPSK信号解调算法。该算法利用基于Zoom-FFT的Quinn频率估计算法对载波频偏进行估计,将频偏估计分为粗估计和精估计两个过程,对估计所得频偏进行补偿后利用Costas环完成载波同步。在MATLAB中对算法进行了仿真,仿真结果表明,该算法能够实现对高动态大频偏QPSK信号的有效捕获,与传统的解调方式相比,该算法对载波频偏的估计范围更大,且具有更高的频偏估计精度和更好的误码性能。     

突发通信中数据辅助频偏估计算法研究

分析了基于最大似然准则的3种典型数据辅助（DA）开环频偏估计算法：Kay算法、Fitz算法和L＆R算法。利用MPSK信号对上述算法进行了计算机仿真验证,仿真结果表明：Kay算法的鉴频范围大,而Fitz算法和L＆R算法的估计性能好。     

Feedforward symbol timing recovery technique using two samples per symbol

In this paper, a new feedforward symbol timing recovery technique using timing estimation followed by interpolation is presented for digital receivers with two samples/symbol or higher sampling rate. A few timing estimation algorithms are proposed to estimate the timing offset accurately. The basic algorithm uses only the in-phase (I) or quadrature (Q) signal for timing detection, which applies to a BPSK communication system. It is shown that the basic algorithm, when applied in quadrature modulation systems where both I and Q signals are available, can be modified slightly to yield an improved estimation precision. The mean and variance of the resulting timing estimate are analyzed and simulated, supporting a satisfactory estimation performance. It is also shown that by applying a postprocessing scheme, such as the Kalman filter, the variance can be further reduced, resulting in a smoothed timing estimate. Some of the issues concerning the implementation of the proposed technique are also addressed.     

Adaptive equalization of CPM signals transmitted over fast Rayleighflat-fading channels

A new algorithm for adapting the coefficients of an equalizer for continuous phase modulated data signals in a flat-fading environment is presented. The cost function to optimize is based on the maximum likelihood sequence estimation index for such signals and channel conditions. It is shown that this equalizer algorithm, called the maximum likelihood equalizer, involves the iterative computation of one of the eigenvectors of a matrix. An implementation is proposed, which combines iterative estimation procedures for QR decomposition, matrix eigenvalue tracking and channel prediction error. Simulation results are presented that demonstrate the ability of the algorithm to equalize the channel filtering effects in a fast fading environment, without requiring phase coherent carrier recovery     

Multi-h CPM信号的减少状态多符号差分非相干检测

针对多指数连续相位调制(Multi-h CPM)载波恢复困难且实现复杂度高的难题,提出了一种适用于Multi-h CPM信号的减少状态多符号差分非相干检测算法。该算法利用多符号差分信息进行分支度量计算的同时引入参考相位信号,实现了具有残留载波频偏校正的非相干序列检测,并且结合逐幸存处理技术,利用幸存路径上的假设判决信息减少了网格状态数。通过计算和搜索接收信号所有可能调制指数序列所对应的分支度量信息实现了调制指数的估计与同步。计算机仿真结果表明,与相干最大似然序列检测相比,该算法能够以较小的性能损失获得其实现复杂度的降低,而且当存在残留载波频偏时,该算法的检测性能也较为稳健。     

Blind Carrier Frequency Offset Estimation for OFDM System with Multiple Antennas using Multiple-invariance Properties

In this paper, we address the problem of carrier frequency offset (CFO) estimation for Orthogonal Frequency Division Multiplexing (OFDM) communications systems with multiple antennas. We reconstruct the received signal to form data model with multi-invariance property, and subsequently derive a multiple-invariance ESPRIT algorithm for CFO estimation. This algorithm has improved CFO estimation compared to ESPRIT method and maximum likelihood method. Simulation results illustrate performance of this algorithm.     

基于最大似然法的天波超视距雷达相位解污染算法

电离层解相位污染是天波超视距雷达信号处理的关键技术之一。由于模型的不准确性和电离层的复杂性,已有算法在污染较大时大多精度不高。该文提出一种基于最大似然法的相位解污染算法。该算法将信号建模为相位多项式,通过最大化似然函数来实现污染相位的估计。为了避免最大似然法中的矩阵求逆运算,该文进一步将最大似然问题转化为最小二乘问题,利用遗传算法求解相位系数。仿真结果表明,与传统算法比较,该文算法具有以下优点:相比HRR算法和CED算法,该文算法精度更高,校正后的信号频谱更加尖锐;在相位污染较大的情况下,该文算法仍具有较高的精度,有利于目标信息的提取;该文算法采用高阶多项式,避免分段处理和矩阵求逆,简化了运算。