An AGC Applebaum array, which is a modified Applebaum array employing an automatic gain controller (AGC), is proposed. When the eigenvalues of the input covariance matrix of an array system are spread by orders of magnitude, conventional adaptive arrays are unable to remove all the interference signals quickly. The proposed array increases the cross-correlation between the low-power interference signals at the array input and output through the use of an AGC block in the feedback loop. As a result, the weight vector is adapted for the removal of both low-power and high-power interference signals. Computer simulations were performed to demonstrate that the proposed array can produce high output signal to interference plus noise ratio (SINR) with a fast convergence speed.

A sequentially and linearly constrained minimum variance beamformer (SLCMV), based on a split polarity transformation (SPT) and, called an SPT-SLCMV beamformer, is proposed to minimize the degree of freedom loss, steering error, and a desired signal elimination phenomenon under coherent interferences. The SPT-SLCMV beamformer reduces the degree of freedom loss, which is inevitable in the conventional SPT-LCMV beamformer, by successively applying sub-constraint matrices. Sub-constraint matrices are derived from a complete constraint matrix to remove the correlation between the desired signal and interferences. In addition, the SPT-SLCMV beamformer is combined with the iterative steering error correction method to reduce the steering error between the look direction of the beamformer and the incident angle of the desired signal. As a result, the proposed beamformer reduces the number of array elements while maintaining the performance of an exactly steered SPT-LCMV beamformer having sufficient array elements under coherent interferences.

An adaptive hybrid acquisition system is presented. The proposed system combines an antenna diversity technique and the cell-averaging constant false alarm rate (CA-CFAR) algorithm to acquire a pseudo-noise (PN) sequence for code-division multiple-access (CDMA) systems. Since conventional acquisition systems have a fixed threshold value, they are unable to adapt to varying mobile communications environments, resulting in a high false-alarm rate or low detection probability. Accordingly, an antenna diversity technique and adaptively varying threshold scheme using the CA-CFAR algorithm are applied to improve the detection performance. Based on deriving the detection probability, false alarm rate, miss detection probability, and mean acquisition time in Rayleigh fading channels, the performance of the proposed system is compared to that of a conventional system with a fixed threshold.