High reliability direct-sequence spread spectrum for underwater acoustic communications

Many emerging underwater applications involve the wireless transmission of controlling signals and commands to autonomous underwater vehicles (AUVs) and underwater sensors. Such communication links often require high reliability with low complexity receivers and only a few hydrophones. In this paper, we propose a direct-sequence spread spectrum (DSSS) scheme to meet such a need. DSSS systems are recently introduced to underwater communications because of their capability of resolving multipath and enabling the collection of delay diversity and channel energy. Similar to these existing schemes, our proposed approach also has very low receiver complexity requiring only matched filter operation. However, different from them, we simultaneously transmit multiple symbols during each sequence period. Compared with existing underwater DSSS schemes, our proposed approach requires shorter channel coherence time and is thus more robust against moderate channel variation that is inevitable in underwater scenarios. In addition, our high reliability (HR-)DSSS scheme also facilitates higher and more flexible rates. More importantly, the high reliability and high data rate are achieved with negligible self- and co-channel interference. Besides simulations, our scheme is also tested in sea trials using QPSK modulation without any chip level equalization.