Experimental Investigation on Very-High-Rate Underwater Acoustic Communication

The demand for higher data rates and low-latency communication links has been increasing; hence, real-time very-high-rate underwater acoustic communication (UAC) has become more important. Previous coding modulation systems using decision-feedback equalization (DFE) receivers required period interleaving, which resulted in long throughput delay. To achieve low-delay real-time very-high-rate UAC, in this study, a two-stage DFE approach is proposed. In the first-stage, conventional DFE is employed to mitigate intersymbol interference, followed by the derivation of maximum ratio combining coefficients to enhance decoding accuracy through diversity combining. In the second stage, the low-delay decoding results are utilized as feedback for a noise-predictive DFE, effectively reducing the noise variance at the decoder input. In a horizontal UAC experiment at a depth of 630 m, using four-channel spatial multiplexing with 128 quadrature amplitude modulation (QAM) in the $100\pm f5\text{kHz}$ frequency band, a peak data rate of 350 kbps within a range of up to 200 m was achieved. Using 16-QAM, a peak data rate of 200 kbps within a range of up to 500 m was achieved.