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This paper considers the problem of beamforming in multiple-input multiple-output (MIMO) wireless systems. Assuming perfect channel state information at the receiver, the choice of the beamforming vector is made possible through a noiseless limited-rate feedback of one or more bits per coefficient to the transmitter. This paper proposes the use of beamforming codebooks based on quadrature amplitude modulation (QAM) and phase-shift keying (PSK) constellations, which essentially eliminates the need for storage of the codebook. We show that such codebooks perform arbitrarily close to the perfect feedback case as the constellation size increases, and that full diversity order is achieved. We demonstrate an equivalence between the beamforming codebook search problem with that of noncoherent sequence detection. Based on this we propose fast beamforming vector search algorithms. Monte-Carlo simulations are presented to show that the performance is comparable to the best known codebooks, and that the search complexity can be reduced by several orders of magnitude. Index Terms--Multiple-input multiple-output (MIMO), beamforming, limited feedback, vector quantization, wireless communications.

The lattice [A.sup.*.sub.n] is an important lattice because of its covering properties in low dimensions. Clarkson described an algorithm to compute the nearest lattice point in [A.sup.*.sub.n] that requires O(n log n) arithmetic operations. In this correspondence, we describe a new algorithm. While the complexity is still O(n log n), it is significantly simpler to describe and verify. In practice, we find that the new algorithm also runs faster. Index Terms--Channel coding, direction-of-arrival estimation, frequency estimation, lattice theory, nearest point algorithm, quantization, synchronization.

Separable least squares line search (SLS2-NEW) and lattice line search (LLS) are algorithms presented for solving the problem of estimating the period of a series of periodic events with noisy and sparse times of occurrence.

The case is considered in which a frequency-agile receiver (FAR) for electronic support (ES) attempts to intercept radar emissions over a wide search bandwidth. It was recently shown [1, 2] that a random strategy exists in which the expected intercept time can be made arbitrarily close to linear as a function of the scan period of the radar. Can a deterministic strategy be devised in which a similar linear relationship exists for the maximum intercept time? By applying the celebrated arithmetic results of van der Waerden [3] and Szemeredi [4], we show that no such strategy is possible.

Fundamental matrix estimation is a central problem in computer vision and forms the basis of tasks such as stereo imaging and structure from motion. Existing algorithms typically analyze the relative geometries of matched feature points identified in both projected views. Automated feature matching is itself a challenging problem. Results typically have a large number of false matches. Traditional fundamental matrix estimation methods are very sensitive to matching errors, which led naturally to the application of robust statistical estimation techniques to the problem. In this work, an entirely novel approach is proposed to the fundamental matrix estimation problem. Instead of analyzing the geometry of matched feature points, the problem is recast in the frequency domain through the use of Integral Projection, showing how this is a reasonable model for orthographic cameras. The problem now reduces to one of identifying matching lines in the frequency domain which, most importantly, requires no feature matching or correspondence information. Experimental results on both real and synthetic data are presented that demonstrate the algorithm is a practical technique for fundamental matrix estimation. The behavior of the proposed algorithm is additionally characterized with respect to input noise, feature counts, and other parameters of interest. Index Terms--Computer vision, epipolar geometry, fundamental matrix, robust estimation, projection-slice theorem, Radon transformation.

An interpretation of the maximum-likelihood estimator (MLE) and the Delogne-Kasa estimator (DKE) for circle-center and radius estimation in terms of convolution on an image, which is ideal in a certain sense, is presented and the relationship between the convolution with a phase-coded kernel (PCK) and the MLE is investigated. It is shown that the PCK is an approximate MLE (AMLE) and the AMLE method is compared to the MLE and the DKE in both real and synthetic digital images.

This correspondence considers block detection for blind wireless digital transmission. At high signal-to-noise ratio (SNR), block detection errors are primarily due to the received sequence having multiple possible decoded sequences with the same likelihood. We derive analytic expressions for the probability of detection ambiguity written in terms of a Dedekind zeta function, in the zero noise case with large constellations. Expressions are also provided for finite constellations, which can be evaluated efficiently, independent of the block length. Simulations demonstrate that the analytically derived error floors exist at high SNR. Index Terms--Blind detection, block detection, error floor, fading, noncoherent communication, number theory, sequence detection, wireless communications.

We consider a number of problems concerning the overlaps or coincidences of two periodic pulse trains. We show that the first intercept time of two pulse trains started in phase is a homogeneous Diophantine approximation problem which can be solved using the convergents of the simple continued fraction (s.c.f.) expansion of the ratio of their pulse repetition intervals (PRI's). We find that the intercept time for arbitrary starting phases is an inhomogeneous Diophantine approximation problem which can be solved in a similar manner. We give a recurrence equation to determine the times at which subsequent coincidences occur. We then demonstrate how the convergents of the s.c.f. expansion can be used to determine the probability of intercept of the two pulse trains after a specified time when one or both of the initial phases are random. Finally, we discuss how the probability of intercept varies as a function of the PRI's and its dependence on the Farey points. Index Terms - Pulse train analysis, probability of intercept, simple continued fractions, Farey series.

This investigation addresses the problem of stabilizing a class of imperfectly known nonlinear neutral-type time-delay systems. Invoking delay independent stability criteria and a deterministic methodology based on Lyapunov-Krasovskii functionals, a class of stabilizing controls is designed to ensure global uniform asymptotic stability of the zero state for the prescribed class of systems. An application of the results to a specific sub-class is provided with a simulation example included to illustrate the practical applicability. Index Terms--Feedback stabilization, neutral functional differential systems, uncertain systems.

A recent study on weighted linear predictor frequency estimator demonstrated the need for care in obtaining bounds to the variances of estimators. A new study was able to calculate an approximation to the variance of the estimator for fixed N and signal-to-noise ratio (SNR). Moreover, the new findings indicate that the relative error tended to zero as either N or the SNR increased.