Adaptive turbo equalization for underwater acoustic communication

In this paper a multiband transceiver designed for underwater channels is presented. Multi-branch filtering at the receiver is used to leverage the diversity offered by a multi-scale multi-lag scenario. The multi-branch bank of filters is constructed by estimating scale and delay coefficients through an initial preamble composed by a maximum length sequence. An intelligent design of the pulse shaper at the transmitter and the receiver permits the reduction of the complexity of the equalization algorithm: the effective channel can be seen as a simple time-invariant finite impulse response filter possibly affected by a carrier frequency offset. Adaptive turbo equalization is chosen to deal with the high time-variability which characterizes many underwater channels and also to avoid the burden of estimating all parameters of the different paths. A phase locked loop and recursive least squares algorithm is implemented on each branch and for each subband which sweep the received sequence several times to refine decoding; in order to enhance the receiver performance the updating of the equalizer taps is obtained by making use of soft information provided as feedbacks from the turbo decoder. The proposed transceiver is tested on a realistic channel obtained by channel soundings performed in the Lyme Bay area, South England. Also comparisons with other existing turbo equalizer schemes are performed.