Chien-Chung Yeh

Performance of DMI and eigenspace-based beamformers

The performance of the direct matrix inversion (DMI) method for antenna arrays of arbitrary geometry is analyzed by asymptotic statistical techniques. The effects of eigenspace disturbance caused by finite samples on the output interference and noise powers are examined under the unit gain constraint in the direction of the desired signal. The results show that the performance of the DMI method is degraded mostly by the disturbed noise subspace. That suggests the use of an eigenspace-based beamformer in which the weight vector is computed by using the signal-plus-interference subspace component of the sample correlation matrix. Convergence properties of the eigenspace-based beamformer are evaluated for the cases in which the source number is known and in which it is overestimated. Theoretical analyses validated by computer simulations indicate that the eigenspace-based beamformer has faster convergence rate than the DMI method. >

Bearing estimations with mutual coupling present

Estimating angles of arrival of radiation sources using an antenna array with mutual coupling between elements is investigated. The presence of mutual coupling distorts phase vectors of radiation sources and the eigenstructure of the covariance matrix. Two approaches are proposed to counteract that distortion so that eigenstructure methods can perform well. The method described first eliminates the distortion of the signal component of the the covariance matrix and then applies spatial smoothing to decorrelate the coherence between sources. Computers simulation results show that this method successfully counteracts mutual coupling. >

A novel hybrid of adaptive array and equalizer for mobile communications

The combination of the adaptive array and equalizer (AE) has been developed for suppressing the cochannel interference and the intersymbol interference (ISI) in mobile communications. In this paper, a novel hybrid of the adaptive array and equalizer (NHAE) system is proposed to combat the problems of insufficient degrees of freedom and mainbeam multipath interferers. The NHAE utilizes a modified training sequence to adjust the weight vector of the array that leads the array to cancel only the cochannel interferers. The ISI which is caused by the multipath interferers and the transmission system is removed by the equalizer following the array. Therefore, the array in the NHAE may need a fewer number of the elements than the conventional array which cancels both the cochannel interferers and multipath interferers. Besides, the presence of the mainbeam multipath interferers, which may seriously degrade the performance of the AE, has much less effect on the NHAE since it is suppressed by the equalizer instead of by the array. Simulation results are presented to demonstrate the merits of the NHAE.

A covariance approximation method for near-field direction-finding using a uniform linear array

Most narrowband high-resolution bearing estimation algorithms were proposed based on the assumption of a far-field source environment. When this assumption is violated, i.e. some sources are in the near field, these algorithms generally provide unsatisfactory performances. A new algorithm is proposed to handle this near-field problem using a uniform linear array. The received signal covariance matrix for near-field sources is viewed as a corrupted measurement of the ideal signal covariance matrix for far-field sources located at the same bearings. Taking this viewpoint, the authors reconstruct a far-field approximation (FFA) covariance matrix from the received covariance matrix to reduce the corruptions. Therefore, existing high-resolution bearing estimation techniques can be directly applied to estimate the bearings of the near-field sources. The properties of the proposed approach are investigated in detail. Simulation results demonstrating the effectiveness of this approach are included. >

Coherent interference suppression by an antenna array of arbitrary geometry

An approach is investigated that can be applied to an array of arbitrary geometry for coherent and incoherent interference suppression. This approach consists of constructing a weight vector which minimizes the array output power subject to null constraint in the directions of the coherent interferers. In this way, the coherent interference is suppressed by the null constraint and the incoherent interference is suppressed by minimizing the array output. The approach requires the estimation of the angles of arrival of the coherent interferers. A simple bearing estimation method is described which can be applied to the scenario of one coherent interferer. The effect of error in bearing estimation on the array performance is discussed. The use of a high-order null constraint to counteract that error is discussed. >

Performance analysis of derivative constraint adaptive arrays with pointing errors

The performance of adaptive arrays with arbitrary order derivative constraints to avoid the signal nulling caused by the error in the steering angle is investigated. The generalized sidelobe canceller (GSC) is used to evaluate the bearing responses of adaptive arrays with pointing errors. Under the assumption of equispaced linear arrays, the Legendre polynomial is used to derive a closed-form formula for the sidelobe leakage factor, the ratio in which the signal leaks into the GSC sidelobe cancelling branch. Moreover, the effective beamwidth, the maximum range of pointing inaccuracy allowable to keep the signal loss in array output less than a given value, is derived for the GSC with a delay-and-sum quiescent beamformer. Such adaptive arrays can achieve the maximum output signal-to-noise ratio when the presteering is perfect, and the performance is independent of the location of the phase origin. Numerical results are presented to confirm the analysis. >

Bearing estimation without calibration for randomly perturbed arrays

The bearing estimation problem of linearly periodic arrays with the presence of sensor position errors is discussed. Conventional approaches for solving this problem generally consist of two steps: calibrating sensor locations and then performing bearing estimation based on the calibrated sensor locations. Approaches to carrying out bearing estimation based on the Toeplitz and eigenstructure reconstruction of the covariance matrix without the need for calibration are proposed. The Toeplitz approximation method (TAM) and a modification of it (MTAM) are used to reconstruct a matrix with Toeplitz structure. To further enhance the capabilities of the TAM and MTAM, an iterative algorithm incorporating with the TAM (ITAM) and the MTAM (IMTAM) is proposed to iteratively reconstruct both the Toeplitz and the desired eigenstructure from the observed covariance matrix. Computer simulations show that the MTAM is more effective than the TAM. Moreover, the iterative methods are superior to the noniterative ones at the price of computations. >

The effect of a finite-distance signal source on a far-field steering Applebaum array-two dimensional array case

Approximate expressions are obtained for the output signal-to-noise ratio (SNR) expressed in terms of the finite signal distance and signal direction for a finite-distance signal sources effect on the performance of a far-field steering two-dimensional Applebaum-type adaptive array. The expression is shown to be consistent with the actual simulated value. Using that expression, a simplified rule is obtained to determine the distance between the signal source and the array center at which the output SNR loss is given by a specific value. The SNR value so obtained varied with two-dimensional signal direction. The analysis for the case of an arbitrarily located array is presented, followed by the cases for rectangular, circular and elliptical arrays. It was found that this distance associated with the given SNR value is always less for a rectangular array than that of a linear array when the total number of array elements for both are equal and the performances of an elliptical array are similar to those of a circular array. >

On the coherent interference suppression using a spatially smoothing adaptive array

The behavior of a spatially smoothing adaptive array is examined. An expression for the weight vector is first derived. Using the array gain on the desired signal and the coherent interference is obtained. Then the expression for output signal-to-noise (SNR) is derived. It shows that the performance of the spatially smoothing array depends on the number of the subarrays, the angle separation, relative power and initial phase difference between the desired signal and the coherent interference. For good interference suppression it is found that the magnitude of the phase difference of the incident and interference signals must be greater than the beamwidths of both the subarray and the equivalent array. There is also a tradeoff between increasing the groups of subarrays and decreasing the number of elements in the subarrays. Computer simulation results are given that validate the analysis. >

The effect of a finite distance source on an Applebaum array

The effect of the finite distance of the signal source on the performance of a far-field steering Applebaum type adaptive array is examined. The output signal-to-noise ratio (SNR) is expressed in terms of 1) the distance between the signal and the array center, 2) the input SNR, 3) the element number and locations, and 4) the signal direction. From that expression a rule of thumb is drawn to determine the distance between the signal source and the array center at which the degradation of the output SNR is 1 dB. That distance is in general much larger than the far-field range of a conventional beam forming array.