Cross-Domain Equalization for Underwater Acoustic OCDM Systems with SSD

Orthogonal Chirp Division Multiplexing (OCDM) represents a compelling modulation alternative for Underwater Acoustic (UWA) communication systems, exhibiting notable Bit Error Rate (BER) improvements over Orthogonal Frequency Division Multiplexing (OFDM), particularly within frequencyselective channels. This work explores the application of Signal Space Diversity (SSD) in OCDM systems. Specifically, rotated and cyclically Q-delayed (RCQD) constellations are employed to augment the system's inherent diversity, thereby enhancing overall communication reliability. However, the equalization of SSD-augmented OCDM signals presents a formidable challenge due to the considerable expansion in the received constellation space. To overcome this, a novel cross-domain equalization technique is introduced. This technique involves an initial equalization of each OCDM symbol in the frequency domain, followed by a subsequent equalization in the Fresnel domain. Within the Fresnel domain, the correlation induced among OCDM symbols by the rotation and component interleaver is judiciously leveraged to achieve more effective mitigation of residual Inter-Symbol Interference (ISI). Experimental validation, including results from a real underwater channel, unequivocally demonstrates that the proposed receiver architecture substantially improves the resilience of the OCDM system to interference.