Underwater acoustic LoRa communication. Synchronization and detection in doppler-affected multipath links

In underwater acoustic communication supporting data transmission over long distances, link robustness is affected by several propagation effects making the choice of signal modulation scheme a crucial issue to be addressed. To this aim, chirp spread spectrum techniques represent effective candidates to meet the requested communication reliability. Recently, preliminary studies have investigated the potential of Long Range (LoRa) modulation, suitably adapted from the radio-frequency to the acoustic domain, highlighting that Doppler shift is the primary impairment to tackle for the achievement of good performance. In this direction, we present a mechanism for Doppler estimation based on adaptive matched filtering, the accuracy of which allows synchronization at symbol level to be performed with high reliability. Furthermore, we explore the effectiveness of different detection schemes over typical underwater propagation scenarios characterized by multipath and Doppler. In this regard, an improved version of geometric detection, originally developed for terrestrial LoRa systems, is proposed to increase the robustness of underwater links typically suffering from multipath and Doppler. Numerical results demonstrate that the joint use of the considered Doppler estimation, synchronization and detection mechanisms contributes to achievement of very low bit error rates even at low signal-to-noise ratio conditions.