The optimal focusing subspace for coherent signal subspace processing

We introduce a technique to determine an optimal focusing frequency for the direction-of-arrival estimation of wideband signals using the coherent signal-subspace processing method. We minimize the subspace fitting error to select an optimal focusing frequency. Direct optimization of this criterion can be computationally complex-the complexity increases with the number of frequency samples. An alternative technique is introduced that performs nearly as well as the optimal method. This suboptimal technique is based on minimizing a tight bound to the error. The computational complexity of the suboptimal method is independent of the number of frequency samples. The simulation results show that the proposed method reduces both the bias of estimation and the resolution threshold signal-to-noise ratio (SNR).