Volume 126, Issue 1, July 2009
Index of content:
- ACOUSTIC SIGNAL PROCESSING 
Interference suppression for code-division multiple-access communications in an underwater acoustic channel126(2009); http://dx.doi.org/10.1121/1.3147484View Description Hide Description
In a code-division multiple-access communications network, the signal from a nearby user often creates a strong interference for the signal from a distant user. This is known as the near-far problem. Power control of source levels is ineffective in an underwater acoustic channel due to the slow sound speed. Interference rejection based on code orthogonality is ineffective using matched-filter processing due to the fact that multipath arrivals effectively destroy the code orthogonality and that the signal arrival times between different users are not synchronized. An algorithm, called hyperspace cancellation by coordinate zeroing, is used in this paper to remove/suppress interference. Using a fast Walsh–Hadamard transform (FWHT) based on the interferer’s code sequence, the interference signal is enhanced and removed by coordinate zeroing. The residual signal is transformed back using an inverse FWHT. The filtered data, with the interference signal largely removed, are processed using the desired signal code sequence. Two methods previously developed for direct-sequence spread-spectrum communications in an underwater channel are used to extract the transmitted symbols. Low bit error rate is found with the at-sea data for signal-to-interference ratio as low as −8 to −11 dB.
126(2009); http://dx.doi.org/10.1121/1.3129382View Description Hide Description
The proposed model derives time-frequency maps to estimate perceived alterations due to reverberation in stereo audio signals reproduced in rooms. These alterations relate to monaural masking due to reverberant decay, derived via a computational auditory masking model and to inter-channel cues for the formation of the spatial position of the aural objects, derived via an inter-channel cue mapping module. The maps illustrate in detail the varying nature of the perceptually-relevant alterations due to room reverberation. Quantitative metrics are also introduced which were found to be proportional to reverberation interference, to room-reverberation time and to depend on the specific audio signal. A statistical approach classifies room response properties via their histogram distributions. Corresponding distributions were also applied to the proposed signal-dependent perceptual maps. Such distributions were found to be useful for interpreting the perceived alterations with different kinds of signals, such as music or speech.