Integrated Multimode Adaptive CSS Modulation Based on OCDM-IM for UWA Communication

Underwater acoustic (UWA) channels exhibit significant variation across different environments, making single-mode modulation schemes inadequate for reliable data transmission. In this work, we adopt orthogonal chirp division multiplexing with index modulation to construct an adaptive modulation framework that integrates multiple chirp spread spectrum-based modulation modes. Based on this framework, we implement an integrated multimode UWA modem on a field-programmable gate array, and propose using effective signal-to-noise ratio and activated signal-to-noise ratio as channel metrics for mode switching. Experimental results show that the proposed system maintains stable transmission success rate and bit rate under varying channel conditions and transmit power levels.