Performance analysis of sweep-spread carrier (S2C) modulation for underwater communications

In the last decades, underwater acoustic communications have seen a growing development with a variety of research as well as commercial applications, dealing with channel distortions, multipath and Doppler effects typical of underwater channels. A very robust modulation able to guarantee connectivity in harsh environments (where spectral efficient solutions such as OFDM can be prevented) is the so called sweep-spread carrier (S2C) modulation, which is based on the usage of a linearly time-varying carrier. Underwater modems based on S2C modulations have been patented and successfully adopted in real-world deployments. In this paper, we analyze the performance of S2C both in simulation and infield experiments, based on our own S2C implementation. We undertake extensive simulation experiments, quantitatively measuring the impact of a variety of modulation parameters (such as the sweep duration and the number of coded symbols per sweep), and under different channel characteristics (depth, range, Doppler speed, etc.). Moreover, we test the performances of the S2C modulation at sea, obtaining good results also in shallow waters.