Enabling Higher-Order Modulation for Underwater Backscatter Communication

Piezo-acoustic bacskcatter (PAB) is a recently-introduced ultra-low-power underwater communication technology. In contrast to traditional underwater acoustic modems, which need to generate power-consuming carrier signals, PAB nodes communicate by simply reflecting (i.e., backscattering) existing acoustic signals in the environment. This reflection-based approach enables them to communicate at net-zero power but also imposes significant constraints on their throughput and modulation schemes. We present PAB-QAM, the first underwater backscatter design capable of achieving higher-order modulation. PAB-QAM exploits the electro-mechanical coupling property of piezoelectric transducers to modulate their reflection coefficients. Specifically, by strategically employing reactive circuit components (inductors), we demonstrate how PAB-QAM nodes can modulate the phase and amplitude of acoustic reflections and realize higher-order and spectrally-efficient modulation schemes such as QAM. We designed and built a prototype of PAB-QAM and empirically evaluated it underwater. Our empirical evaluation demonstrates that PAB-QAM can double the throughput of underwater backscatter without requiring additional power, spectrum, or cost. Looking ahead, such increased throughput paves way for various subsea IoT applications in ocean exploration, underwater climate monitoring, and marine life sensing.