Ricerc@Sapienza

The last years have seen a growing interest in underwater acoustic communications because of its applications in marine research, oceanography, marine commercial operations, the offshore oil industry and defense. High-speed communication in the underwater acoustic channel has been challenging because of limited bandwidth, extended multipath, refractive properties of the medium, severe fading, rapid time variation and large Doppler shifts.

In underwater acoustic networks (UWANs) with bidirectional communication links, channel state information (CSI) plays an important role since having a good channel knowledge allows system performance enhancements. Two communicating nodes can use the information obtained by their respective channel estimation in order to adjust the transmission parameters to the channel characteristics. However forward channel estimation is generally different from the backward one. In this paper we investigate the acoustic channel reciprocity in a shallow water communication.

A new blind estimator of the sampling phase is proposed to support fractionally spaced equalization in underwater digital links employing pulse position modulation. Stemming from the relationship between the “spikiness” of the channel impulse response and the deviation from Gaussianity of the received signal, the sampling phase is estimated by exploiting non-Gaussianity measures offered by nonlinear statistics. In particular, the fourth order (kurtosis) and the first order nonlinear sample moments are considered and the resulting receiver performance is analyzed.

Bidirectional underwater acoustic communications usually request the transmission of some control information among nodes to govern the data flow. Control signaling, employed as frame reception validation, is also a peculiarity of the Automatic Repeat on reQuest (ARQ) schemes. An alternative solution to the error control problem is using forward error correction, even though high redundancy is needed to obtain the low error rate, hence leading to a sensible throughput reduction. Furthermore, no control on data integrity is allowed.