The frequency domain preprocessing in the coherent signal-subspace method (CSM) for wide-band direction finding involves the alignment/focusing of the signal subspaces at different frequencies via transformations to obtain a coherent signal subspace. Time domain equivalents of focusing are derived based on the least squared error approach for general transformations and the Taylor series expansion approach for closed-form transformations. The preprocessor is realized by a multichannel digital FIR filter. For diagonal transformations, the problem reduces to implementing different delays at each sensor which in turn leads to computational simplicity. Various tapped delay line filters for realizing an arbitrary delay are proposed and compared. Simulation results reveal that indeed low-order filter structures in the time domain approach achieve similar performance to the frequency domain approach even at detection and resolution thresholds.
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