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In this paper, we propose a communications receiver architecture composed of a time-reversal approach to simplify the shallow underwater acoustic channel and to suppress multiuser interference followed by an iterative, successive interference cancellation (SIC) scheme to cancel the remaining interference. The SIC process is the multiuser interference equivalent of the decision-directed, feedback equalizer for intersymbol interference channels. The interference experienced by a particular user is estimated using previously decoded symbols from competing messages. Then, the interference estimate is optimally scaled, synchronized, and subtracted from the received signal. This process is repeated for all users until a stationary point is reached. Data collected during the Focused Acoustic Fields 2005 experiment (FAF05) are used to demonstrate the performance of the interference cancellation scheme. Previous experimental results showed that 3 users separated vertically by about 12 m can transmit simultaneously to a receiving array at a range of 20 km in 120 m deep water with an aggregate data rate of 6 kb/s. Each user transmits 16-QAM symbols at a rate of 500 symbols per second over a 1 kHz bandwidth centered at 3.5 kHz. As expected, the combination of iterative SIC with passive time-reversal is shown to increase significantly the output SINR over time-reversal alone. The output SINR is increased by as much as 6 dB when in the presence of strong interference. This translates to a significant bit error reduction from 664/46800 bits in error (bit error rate of 1.42×10 2) to 37/46800 bits in error (bit error rate of 7.91×10~4). |
