Simultaneous correction of ghost and geometric distortion artifactsin EPI using a multiecho reference scan

A computationally efficient technique is described for the simultaneous removal of ghosting and geometrical distortion artifacts in echo-planar imaging (EPI) utilizing a multiecho, gradient-echo reference scan. Nyquist ghosts occur in EPI reconstructions because odd and even lines of k-space are acquired with opposite polarity, and experimental imperfections such as gradient eddy currents, imperfect pulse sequence timing, B0 field inhomogeneity, susceptibility, and chemical shift result in the even and odd lines of k-space being offset by different amounts relative to the true center of the acquisition window. Geometrical distortion occurs due to the limited bandwidth of the EPI images in the phase-encode direction. This distortion can be problematic when attempting to overlay an activation map from a functional magnetic resonance imaging experiment generated from EPI data on a high-resolution anatomical image. The method described here corrects for geometrical distortion related to B0 inhomogeneity, gradient eddy currents, radio-frequency pulse frequency offset, and chemical shift effect. The algorithm for removing ghost artifacts utilizes phase information in two dimensions and is, thus, more robust than conventional one-dimensional methods. An additional reference scan is required which takes approximately 2 min for a matrix size of 64 X 64 and a repetition time of 2 s. Results from a water phantom and a human brain at 3 T demonstrate the effectiveness of the method for removing ghosts and geometric distortion artifacts.     

Performance of the Viterbi algorithm for interleaved convolutionalcodes

This paper presents a maximum-likelihood decoder for error-burst channels with a very efficient implementation. In particular, the encoder is formed of an interleaved convolutional code with generator polynomials of the type [g⁽¹⁾(D^I), g⁽²⁾(D^I)] (for a rate 1/2 coder), where I may assume a very high integer value. The decoder consists of the Viterbi algorithm (VA) optimized for these sparse polynomials. For a given decoded bit-error probability, the required delay time and memory requirements of this approach are more inferior by far than those of the traditional method of interleaving. Moreover, for bursts of an average length less than 1/2, this method provides better performance than the dual-mode burst-error-correcting algorithm     

A generalized algorithm for evaluating distributed-programreliability

A one-step algorithm, GEAR (generalized evaluation algorithm for reliability), is introduced that computes the reliability of a distributed computing system (DCS), which usually consists of processing element, memory unit, input/output devices, data-files, and programs as its shared resources. The probability that a task or an application can be computed successfully by sharing the required resources on the DCS is termed the system reliability. Some of the important reliabilities defined using the above concept are discussed, including terminal-pair, computer-network, distributed-program, and distributed-system. GEAR is general enough to compute all four of these parameters, and does not require any prior knowledge about multiterminal connections for computing reliability expression. Many examples are included to illustrate the usefulness of GEAR for computing reliability measures of a DCS     

Kernel synthesis for generalized time-frequency distributions usingthe method of alternating projections onto convex sets

Cohen's generalized time-frequency distribution (GTFR) requires the choice of a two-dimensional kernel. The kernel directly affects many performance attributes of the GTFR such as time resolution, frequency resolution, realness, and conformity to time and frequency marginals. A number of different kernels may suffice for a given performance constraint (high-frequency resolution, for example). Interestingly, most sets of kernels satisfying commonly used performance constraints are convex. We describe a method whereby kernels can be designed that satisfy two or more of these constraints. If there exists a nonempty intersection among the constraint sets, then the theory of alternating projection onto convex sets (POCS) guarantees, convergence to a kernel that satisfies all of the constraints. If the constraints can be partitioned into two sets, each with a nonempty intersection, then POCS guarantees convergence to a kernel that satisfies the inconsistent constraints with minimum mean-square error. We apply kernels synthesized using POCS to the generation of some example GTFRs, and compare their performance to the spectrogram, Wigner distribution, and cone kernel GTFR     

基于OFDM的WLAN相位校正算法研究

相位校正对基于OFDM的高速数据传输系统至关重要.文章针对802.11a协议提出了一种使用导频进行相位校正的算法.该算法对接收导频样本进行最小二乘估计以获得相偏参数,算法综合考虑了抽样时钟偏移(SCO)和载波频偏(CFO)所引起的相位偏移.文章通过仿真比较了该算法与另一种相位校正算法在不同参数下的性能,结果证明在相偏主要由SCO和CFO引起的情况下该算法性能较好.     

A generalized Euclidean algorithm for multisequence shift-registersynthesis

The problem of finding a linear-feedback shift register of shortest length capable of generating prescribed multiple sequences is considered. A generalized Euclidean algorithm, which is based on a generalized polynomial division algorithm, is presented. A necessary and sufficient condition for the uniqueness of the solution is given. When the solution is not unique, the set of all possible solutions is also derived. It is shown that the algorithm can be applied to the decoding of many cyclic codes for which multiple syndrome sequences are available. When it is applied to the case of a single sequence, the algorithm reduces to that introduced by Y. Sugiyama et al. (Inf. Control, vol.27, p.87-9, Feb. 1975) in the decoding of BCH codes     

A method for decoding of generalized Goppa codes (Corresp.)

It is shown how the theory of continued fractions for polynomials can be used for the decoding of a class of codes which contains Goppa codes.     

一种新的天线阵列位置误差校正算法

天线阵元的位置误差会影响天线阵元所接收到信号的相位.基于特征值分解的波达方向算法对信号的相位误差非常敏感,因此有必要对阵元的位置误差所带来的接收信号相位误差进行校正.提出了一种基于噪声子空间的最优化算法能有效的校正直线阵列的位置误差.与原方法相比,该算法的优化参数仅为原来的一半,提高了梯度优化算法的收敛速度.数值计算结果表明该算法是有效的.     

A quasi-Newton adaptive algorithm for generalized symmetriceigenvalue problem

We first recast the generalized symmetric eigenvalue problem, where the underlying matrix pencil consists of symmetric positive definite matrices, into an unconstrained minimization problem by constructing an appropriate cost function. We then extend it to the case of multiple eigen-vectors using an inflation technique. Based on this asymptotic formulation, we derive a quasi-Newton-based adaptive algorithm for estimating the required generalized eigen-vectors in the data case. The resulting algorithm is modular and parallel, and it is globally convergent with probability one. We also analyze the effect of inexact inflation on the convergence of this algorithm and that of inexact knowledge of one of the matrices (in the pencil) on the resulting eigenstructure. Simulation results demonstrate that the performance of this algorithm is almost identical to that of the rank-one updating algorithm of Karasalo (1986). Further, the performance of the proposed algorithm has been found to remain stable even over 1 million updates without suffering from any error accumulation problems     

A simple low-complexity algorithm for generalized spatial modulation

Generalized spatial modulation (GSM) is an extension of spatial modulation which is significant for the next generation communication systems. Optimal detection process for the GSM is the maximum-likelihood (ML) detection which jointly detects the antenna combinations and transmitted symbols. However, the receiver is much more complicated than SM due to inter-antenna interference and/or increased number of combinations. Therefore, the computational complexity of the ML detection grows with the number of transmit antennas and the signal constellation size. In this letter, we introduce a novel and simple detection algorithm which uses sub-optimal method based on the least squares solution to detect likely antenna combinations. Once the antenna indices are detected, ML detection is utilized to identify the transmitted symbols. For obtaining near-ML performance while keeping lower complexity than ML detection, sphere decoding is applied. Our proposed algorithm reduces the search complexity while achieving a near optimum solution. Computer simulation results show that the proposed algorithm performs close to the optimal (ML) detection resulting in a significant reduction of computational complexity.     

3(R)系统电波折射修正的实用方法

3R系统由一个雷达发射站和三个接收站组成。由于该系统没有距离测量信息,因此对3R系统的电波折射修正方法就不同于单脉冲雷达和干涉仪的修正方法。本文给出3R系统电波折射修正的实用方法。数据验证表明,此迭代算法收敛快。在仰角大于3°时不超过四次迭代就可以使精度达到10~(-8)量级。     

运用遗传算法结合FFT进行多单元失效阵列校准

提出了利用遗传算法（GA）结合快速傅立叶变换（FFT）方法来进行阵列失效的校准,通过引入傅立叶变换的变换域和角域的映射,在变换域中利用FFT计算个体阵列的阵因子,减少了GA评估个体的时间,从而大大提高了失效校准的速度。以一个-35分贝副瓣电平的32单元阵列为例,校准一单元失效和二单元失效的时间都减少了至少一个数量级,算法也可应用于两个以上单元失效的情况。     

A low-power asynchronous interleaved sampling algorithm for cochlear implants that encodes envelope and phase information

Cochlear implants currently fail to convey phase information, which is important for perceiving music, tonal languages, and for hearing in noisy environments. We propose a bio-inspired asynchronous interleaved sampling (AIS) algorithm that encodes both envelope and phase information, in a manner that may be suitable for delivery to cochlear implant users. Like standard continuous interleaved sampling (CIS) strategies, AIS naturally meets the interleaved-firing requirement, which is to stimulate only one electrode at a time, minimizing electrode interactions. The majority of interspike intervals are distributed over 1-4 ms, thus staying within the absolute refractory limit of neurons, and form a more natural, pseudostochastic pattern of firing due to complex channel interactions. Stronger channels are selected to fire more often but the strategy ensures that weaker channels are selected to fire in proportion to their signal strength as well. The resulting stimulation rates are considerably lower than those of most modern implants, saving power yet delivering higher potential performance. Correlations with original sounds were found to be significantly higher in AIS reconstructions than in signal reconstructions using only envelope information. Two perceptual tests on normal-hearing listeners verified that the reconstructed signals enabled better melody and speech recognition in noise than those processed using tone-excited envelope-vocoder simulations of cochlear implant processing. Thus, our strategy could potentially save power and improve hearing performance in cochlear implant users.     

A method of analysis for interleaved Reed-Solomon coding witherasure decoding on burst error channels

A practical definition of an erasure is presented and leads to new expressions for correct and incorrect decoding probabilities for Reed-Solomon codes, assuming an incomplete decoder. The main benefit of this approach is that, in contrast with the usual approach, one is able to analytically demonstrate the performance improvement provided by errors-and-erasures decoding relative to errors-only decoding. A technique is presented for evaluating the performance of Reed-Solomon codes for both types of decoding on an interleaved burst error channel such as is seen in digital magnetic tape recording. Several illustrative examples are included     

A generalized Lloyd-type algorithm for adaptive transform coder design

In this paper, we establish a probabilistic framework for adaptive transform coding that leads to a generalized Lloyd type algorithm for transform coder design. Transform coders are often constructed by concatenating an ad hoc choice of transform with suboptimal bit allocation and quantizer design. Instead, we start from a probabilistic latent variable model in the form of a mixture of constrained Gaussian mixtures. From this model, we derive an transform coder design algorithm, which integrates optimization of all transform coder parameters. An essential part this algorithm is our introduction of a new transform basis-the coding optimal transform-which, unlike commonly used transforms, minimizes compression distortion. Adaptive transform coders can be effective for compressing databases of related imagery since the high overhead associated with these coders can be amortized over the entire database. For this work, we performed compression experiments on a database of synthetic aperture radar images. Our results show that adaptive coders improve compressed signal-to-noise ratio (SNR) by approximately 0.5 dB compared with global coders. Coders that incorporated the coding optimal transform had the best SNRs on the images used to develop the coder. However, coders that incorporated the discrete cosine transform generalized better to new images.     

A generalized recursive algorithm for wave-scattering solutions intwo dimensions

A generalized recursive algorithm valid for both the E _z and H_z wave scattering of densely packed scatterers in two dimensions is derived. This is unlike previously derived recursive algorithms which have been found to be valid only for E_z polarized waves. In this generalized recursive algorithm, a scatterer is first divided into N subscatterers. The n-subscatterer solution is then used to solve the (n+n')-subscatterer solution. The computational complexity of such an algorithm is found to be of O (N²) in two dimensions while providing a solution valid for all angles of incidence. This is better than the method of moments with Gaussian elimination, which has an O(N³) complexity     

A generalized split-window algorithm for retrieving land-surfacetemperature from space

Proposes a generalized split-window method for retrieving land-surface temperature (LST) from AVHRR and MODIS data. Accurate radiative transfer simulations show that the coefficients in the split-window algorithm for LST must vary with the viewing angle, if the authors are to achieve a LST accuracy of about 1 K for the whole scan swath range (±55° from nadir) and for the ranges of surface temperature and atmospheric conditions over land, which are much wider than those over oceans. The authors obtain these coefficients from regression analysis of radiative transfer simulations, and they analyze sensitivity and error over wide ranges of surface temperature and emissivity and atmospheric water vapor abundance and temperature. Simulations show that when atmospheric water vapor increases and viewing angle is larger than 45°, it is necessary to optimize the split-window method by separating the ranges of the atmospheric water vapor, lower boundary temperature, and the surface temperature into tractable subranges. The atmospheric lower boundary temperature and (vertical) column water vapor values retrieved from HIRS/2 or MODIS atmospheric sounding channels can be used to determine the range for the optimum coefficients of the split-window method. This new algorithm not only retrieves land-surface temperature more accurately, but is also less sensitive to uncertainty in emissivity and to instrument quantization error     

A novel adaptive algorithm of CPE correction for OFDM systems

On the basis of the analysis of the effect of PHase Noise (PHN) and Common Phase Error (CPE) on Orthogonal Frequency Division Multiplexing (OFDM) systems, a cost function is constructed. By the cost function and the idea of Least-Mean-Square (LMS) adaptive algorithm, the adaptive algorithm for the correction of CPE is presented. The simulations have been performed to investigate the performance for tracking PHN and estimating CPE, the results show that the algorithm performs soundly.     

Generalized asynchronous time interleaved (G-ATI) sampling structure for ultra-wideband signal

Multidimensional Systems and Signal Processing - Based on synchronized time interleaved (STI)analog to digital conversion system, an alternative method for ultra-wideband signals sampling has been...