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An intrinsic issue of large-array beamforming and signal processing in random-inhomogeneous environments is known to be considerable degradation of signal coherence in space domain. Such a scenario arises in many applications, and array operations in underwater sound channels with emphasis on long-range sound propagation is one of the most typical cases. Another key issue is here the multimode propagation of the desired signal (and, generally, noise interference) so the spatial spectrum of the received field consists of numerous harmonics even for a single point source. In this paper, our focus is to demonstrate numerically the multimode signal coherence degradation at the large-array input, both for horizontal and vertical arrays in an underwater channel, and to predict the coherence effects on the output performance, array beampattern and gain included. The vertical array operation is emphasized to be the most interesting problem here due to the specific features of ambient noise in real oceanic channels. An essential conclusion concerns the fact that not only the signal coherence but signal localization in mode domain as is compared with the ambient noise affect together the array output performance.

antenna array; array beamforming; coherence length; underwater sound channel; cross-modal correlations; modal noise; array gain