Underwater Communication in the Baltic Sea Using Iterative Equalization

We introduce a multi-channel soft input/soft output receiver for underwater communication that performs joint iterative channel estimation, linear equalization, and decoding. The transmitted symbols were encoded using a turbo coded bit-sequence. Our method exploits the gain present in the turbo code through a feed-back of soft information from the decoder to the channel estimator and to the equalizer. The use of several receiving hydrophones makes the method more robust to frequency selective or low-SNR channels, or can improve reception in channels with very large delay spreads. We give examples of such channels measured in the Baltic Sea at two different sea trials during 2005 and 2006. Properties like time variations of the channel impulse response or channel Doppler spread are given, as well as decoding examples using our method. It is shown that the multi-channel version of the receiver achieve error free reception of 4000 coded symbols per second up to 60 km through channels featuring very large delay spreads. The maximum transmitter source level used was 180 m at Pa re dB 1 1 at the carrier frequency 12 kHz.