Designof a Quasi-Real-Time OCDM Underwater Communication System Based on a Software-Defined Architecture

Shallow-water underwater acoustic (UWA) communication is strongly affected by long-delay multipath propagation, time-varying Doppler distortion, and the computational burden of coherent demodulation. This paper presents a software-defined, quasi-real-time orthogonal chirp division multiplexing (OCDM) communication prototype implemented on a National Instruments compactRIO heterogeneous platform. The prototype maps previously developed multiplex resampling (MR) and Data Pick-Rake (DP-Rake) algorithms to a hardware–software processing chain, in which the FPGA executes sample-level physical-layer operations, including MR, DP-Rake windowing, OCDM demodulation, and equalization, while the ARM real-time controller performs frame-level control, FEC/CRC processing, and state-machine scheduling. The DMA-based RT–FPGA data exchange achieved a measured throughput of 80–100 MB/s. Under the maximum MR configuration, the FPGA physical-layer processing, DMA transfer, and ARM RT-layer processing required approximately 4.5 ms, 0.5 ms, and 5.0 ms per data block, respectively, resulting in a total receiver-side digital processing latency of approximately 10.0 ms. This corresponds to a 32.6 ms timing margin relative to the shortest acoustic data-block duration. The maximum FPGA resource utilization among the reported resource categories was 56.8%. The prototype was evaluated through replay-based channel emulation, Qiandao Lake mobility experiments, and Xiamen Outer Port sea trials. The replay-based results show that DP-Rake reception reduces the multipath-induced error floor under severe delay-spread conditions. In the Qiandao Lake experiments, dynamic MR branch selection achieved zero packet errors within the tested observation window while reducing the average number of active MR branches by 32.7% compared with the fixed-branch configuration. In the Xiamen Outer Port sea trials, an over 6 km shallow-water link with an approximately 12 ms delay spread was evaluated, and no packet errors were observed within the tested observation window when the measured SNR exceeded 7.9 dB. These results indicate that the implemented MR-DP-Rake OCDM prototype improves robustness over the OFDM-based baseline under the tested doubly spread UWA channel conditions. The main contribution of this work is the end-to-end software-defined implementation and field validation of MR-DP-Rake OCDM, rather than the proposal of new communication-theoretic algorithms.