This paper analyses optimal subband beamforming performance mainly aimed at speech enhancement and acoustic echo suppression for personal communication devices, personal computers and wireless cellular telephones. The focus is on theoretical limits of finite impulse response (FIR) beamformers for spatially spread sources in the array near-field. Performance of the Wiener solution is compared to the direct maximization of the array gain for different lengths of the FIR filters and different source interference spreads. The evaluation is performed individually in subbands with constant increasing logarithmic bandwidth. Results show that the difference between the Wiener solution and the direct array gain maximization is less than 2 dB in the measure of Signal-to-Noise plus Interference Ratio (SNIR), for small interference spread. With increasing interference spread the difference in SNIR performance increases, in favor of the array gain maximization.

Performance of the Wiener solution is compared to the optimum signal-to-noise plus interference beamformer (SNIB) for different lengths of the FIR filters. The comparison includes different spatial spreading of the interference source. Results show that the difference in the measure of SNIR is small between the solutions in low frequency bands. It is also shown that the performance is close between the solutions when the spatial spread of the interference is small, i.e. the same size as the source. However, when the interference spread increases, the performance gain with the SNIB is significant, as much as 10~dB. By subdividing the fullband signals into constant increasing logarithmic bandwidth subbands, the number of FIR filter parameters needed is approximately 10-20 taps and it is nearly the same across the subbands.