Modelling and validation of basic characteristics in underwater acoustic sweep-spread-carrier communications

The deployment of underwater acoustic modems was designed to investigate the various effects of underwater acoustic channel onto propagation of hydro-acoustic signals and thus onto performance of hydro-acoustic data link. Data represented sequences of QPSK signals spread over broad frequency band (18-34 kHz) using sweep-spread carrier (S2C) technology. Transmissions were conducted with data rates between 1 and 20 kbps. Underwater acoustic channels were of a horizontal geometry with water depths between 8 and 22 m. There were selected two transmission distances between 328 m and 437 m. Data obtained from the deployment included modem performance statistics for over 2 days, channel impulse response for each data packet transmitted, estimations of Doppler impairments of received signals, noise levels, and sound velocity profiles measured two times a day. Communication and channels statistics were collected for two-way transmissions (interrogations). By means of channel probing the transfer function of underwater acoustic environment was repeatedly recorded and directly compared with the result obtained using channel simulations. This provided the possibility of a “prediction” of the modem performance in a channel with given characteristics. Predictions of the experimental performance over time were obtained using underwater acoustic simulator based on the Bellhop model. Simulator was progressively updated with settings of actual environmental parameters including sound velocity profile, bottom profile and its acoustic properties, as well as surface and bottom roughness. Comparison with the measured statistics demonstrated a reasonable agreement. The simulated performance did not correlate well with measured data when considering only the short-term fluctuations. Nevertheless, the simulation demonstrated an effective method for obtaining predictions of modem performance with flexibility, what would allow in further steps of modem developments for the implementation of additional features.