DD-OCQAM: A spectrally efficient modulation scheme for in-vivo ultrasonic sensors

Reliable ultrasonic communication is essential for biomedical sensors in implantable medical devices, targeted drug delivery, and real-time physiological monitoring. However, in-vivo ultrasonic channels experience severe attenuation, multipath propagation, and scattering due to biological tissue heterogeneity, which limit data rate and reliability. While orthogonal chirp division multiplexing (OCDM) offers robustness against multipath interference, its high bandwidth requirement restricts applicability in sensor-based systems. This paper proposes dual-dictionary orthogonal chirp and QAM modulation (DD-OCQAM), a multidimensional signaling scheme that jointly encodes information in chirp rate and QAM symbols to enhance spectral efficiency and resilience. A theoretical framework is developed for symbol-error-rate (SER) analysis, establishing analytical bounds that remain valid in low-SNR regimes. Monte Carlo simulations confirm the theoretical results and demonstrate consistent BER improvements over OFDM across AWGN, flat, and frequency-selective fading channels. Experimental validation using measured in-vivo ultrasonic data (Bos et al., 2019), including gelatin implant-to-implant scenarios, further verifies the scheme’s practicality and sensitivity to the chirp dictionary size N . These results demonstrate that DD-OCQAM achieves a favorable trade-off between processing gain and bandwidth usage, contributing to the advancement of next-generation ultrasonic biomedical communication networks.