Yang-Ho Choi

Alternating projection for maximum-likelihood source localization using eigendecomposition

Alternating projection allows us to make a maximum-likelihood estimation of arrival angles by one-dimensional search. In this letter, we propose an eigendecomposition-based alternating projection method that can greatly reduce computational complexity with no performance degradation as compared with the direct alternating projection. In the proposed method, initial estimation can be readily performed with MUSIC, which may lead to further reduction in the computational burden.

On conditions for the rank restoration in forward/backward spatial smoothing

The forward/backward spatial smoothing is an efficient technique for obtaining a modified covariance matrix of full rank in the presence of coherent sources. The condition for the full-rank restoration was presented in an earlier paper by Pillai and Kwon (1989). It, however, is incorrect. We present the exact condition. It is shown that the forward/backward spatial smoothing results in the full-rank restoration with probability 1.

ESPRIT-Based Coherent Source Localization With Forward and Backward Vectors

Novel methods are presented which estimate the direction of arrival (DOA) of coherent signals by efficiently exploiting forward and backward vectors. Some correlation vectors or the eigenvector corresponding to the largest eigenvalue of the covariance matrix is used as the forward vector, and the backward vector is then obtained by reversing the order of the complex conjugated elements of the forward one. The use of both forward and backward vectors allows us to find a signal subspace of larger dimension than in the existing method which obtains that from a Toeplitz matrix. The proposed methods with an increased effective array aperture resolve more signals and provide better DOA estimates than the Toeplitz based method.

Maximum likelihood estimation for angles of arrival of coherent signals using a coherency profile

This article addresses an ML estimation problem by introducing a coherency profile, which means the sizes of the coherent signal groups. We present the Cramer-Rao (CR) bounds for the estimation. An algorithm for implementing it is proposed based on the alternating maximization. Simulation results show that the performance improvement of the estimator over the conventional ML is significant, even with a small number of samples in low SNR.

Improved adaptive nulling of coherent interference without spatial smoothing

A new scheme for adaptive nulling in the presence of coherent interference is proposed that finds the weight vector using a nonsquare correlation matrix without spatial smoothing. Conventional schemes, which use square matrices, correspond to specific cases of the new method. It allows us to increase the dimension of the weight vector such that the nulling capability is enhanced. Theoretic performance analysis is made, showing that the output signal-to-interference ratio (SINR) of such a beamformer is proportional to the weight dimension for sidelobe interference. Simulation results confirm this.

Unified approach to Cramer-Rao bounds in direction estimation with known signal structures

When estimating the directions of signals impinging on a sensor array, the use of the information on signal structures leads to improvement in estimation performance. The Cramer-Rao bounds (CRBs), which are lower bounds on variances of estimation errors for any unbiased estimators, are different with signal structures exploited. A general formula for the CRB covariance matrix is presented which allows specific bounds with a known signal structure to be easily obtained. When the incident signals are of amplitude modulation, the CRB with the modulation information is first presented using the formula. The general formula provides easy analysis of the CRB for various signal structures on a unified basis.

Duvall-Structure-Based Fast Adaptive Beamforming for Coherent Interference Cancellation

We present a new adaptive beamforming method for coherent interference cancellation, which is based on the Duvall-structure to remove the desired signal component from the received signal. The proposed beamformer can suppress as many as M/2 coherent interferences for even M, where M is the number of array sensors, though the Duval beamformer can suppress a single coherent interference only. Simulation results show that it converges to the steady-state very quickly.

Fast communication: Adaptive nulling beamformer for rejection of coherent and noncoherent interferences

An adaptive beamforming method is presented which rejects coherent and noncoherent interferences by using an interference subspace, which is efficiently obtained from a submatrix of the covariance matrix of received signals. The use of the submatrix enables an increase in the effective array aperture size, as compared with the existing correlation-vector-based beamformer (CVB). As a result, the proposed beamformer can provide superior performance than conventional methods such as CVB and the weighted spatial smoothing (WSS).

Current status and development strategy of personal communications services of Korea Telecom

Korea Telecom (KT), Koreas main telephone operator, is planning to launch PCS service in 1998, and is developing PCS systems with local manufacturers. This article describes KTs PCS marketing strategies, PCS network implementation directions, and the current status of system developments. In early 1996, prototypes of each network element were completed, which was followed by testing and system integration. The PCS will not only open up a new market for KT; it will be a major revenue source. This KT-PCS will also be the basis for the next generation of wireless communications in the 2000s.