Although the spatial focusing property of the conventional time reversal approach facilitates multiuser communications, there always is residual crosstalk between users. A recent paper [Kim and Shin, J. Acoust. Soc. Am. 115, 600-606 (2004)] proposed an adaptive active time reversal approach for simultaneous multiple focusing with minimal interference. This letter applies the adaptive approach to passive time reversal, multiuser communications for additional suppression of crosstalk among users. Experimental data at 3.5 kHz with a 1-kHz bandwidth demonstrate as much as 6.5-dB improvement per user in terms of output signal-to-noise ratio for three-user communications over a 20-km range in 120-m deep shallow water.
Shallow-water acoustic communication channel, referred to as a multipath-limited channel, produces inter-symbol interference that poses a significant obstacle to reliable communication. Accordingly, signal-to-multipath ratio (SMR), rather than signal-to-noise ratio (SNR), becomes an important factor affecting communication performance. However, it is difficult to estimate SMR from measured communication data, especially at higher frequency (>10 kHz) because many arrivals scattered from rough ocean boundaries produce a significant intrapath time spreading, which acts as random noise in communication. In this paper, the energy fraction of the channel impulse response existing in one symbol duration is proposed as a parameter for estimating the quality of shallow-water communication channels. This parameter is compared with the bit-error-rate performance for data acquired in shallow water off the south coast of Korea, where the water depth is 45 m and the bottom consists of sandy clay sediment. The results imply that the energy fraction in one symbol duration may be used as a parameter for describing shallow-water communication channels and applied to the quick decision of a symbol or bit rate in a shallow-water field for reliable underwater communication.
As Arctic warming accelerates, the underwater acoustic environment in the Arctic Ocean is rapidly changing. We present the first results of passive acoustic monitoring in the marginal ice zone of the East Siberian Sea (ESS). A high sea ice concentration (SIC) and seasonal variations in ice cover make the ESS an ideal region to verify how ambient sound levels respond to natural physical processes and anthropogenic activities during summer. Our observations show that the sound level in the ESS exhibits a strong negative correlation with SIC, and the sound level in September, which was higher than that in other months, was 16 dB higher than the annual average. This increase resulted from geophony and anthrophony with the reduction in the SIC, and sound level increased by 13 dB without anthrophony. Our results indicate that ambient sound level in the Arctic Ocean may increase as climate change accelerates sea ice melting.
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