Robust Underwater Telemetry With Adaptive Turbo Multiband Equalization

In this paper, a multicarrier modulation scheme is presented for acoustic communication at low signal-to-noise ratios (SNRs). User bits are put through a rate 1/3 turbo encoder and interleaved with periodic training bits. A maximal-length sequence is prefixed for signal detection and equalizer convergence, and the resulting bit stream is simultaneously modulated onto multiple phase-shift keyed carriers. Since each subband carries the same symbol sequence, the baseband ensemble is amenable to multichannel equalization. An adaptive multiband equalizer is thus constructed for joint equalization and despreading of the frequency bands. Iterative equalization using soft information from the turbo decoder further enhances the receiver performance. A reduction in complexity is achieved by equipping the subbands with separate recursive least squares tap updates, yet guided by a common error signal. The proposed algorithms are tested on acoustic data from the Baltic Sea, using eight subbands of 460 Hz each, at an effective data rate of 75 b/s. Robust receiver operation is demonstrated at overall receive SNRs down to - 12 dB in three different channels, which corresponds to an SNR per bit Eb/N ₀=+ 5 dB.