Optimal subarray size for spatial smoothing

We consider the popular spatial smoothing technique and show via the covariance matrix eigenvalue analysis that the simple suboptimal rule for choosing of the subarray size exists in a practically important situation of two coherent equipower closely spaced sources. This rule has been derived by maximizing the distance between the signal subspace and the noise subspace eigenvalues of spatially smoothed covariance matrix and it does not require any a priori information about the signal source parameters. >