Shallow underwater communication with passive phase conjugation and iterative demodulation and decoding

We propose passive phase conjugation (PPC) with iterative demodulation and decoding for communication over the shallow underwater acoustic channel. PPC mitigates inter-symbol interference (ISI) caused by multipath in the channel. A softinput, soft-output (SISO) differential BPSK (DBPSK) demodulator and a SISO irregular repeat accumulate (IRA) channel decoder arranged in an iterative ‘turbo’ structure improve the bit error rate (BER) at higher symbol rates. A linear estimator tracks channel phase variations arising from Doppler effects and wave motion. The system was tested at Lake Pend Oreille in Idaho over a distance of 1.78 km in shallow water with BERs below 10−5 at 2,500 information bits per second.