An Underwater Multi-User Communication System Using Photomultiplier-Tube-Based Photon Detector with Dead-Time Effect and Shot Noise

We investigate detection for practical photon-counting receivers in underwater optical wireless communication (UOWC) systems, where dead-time effects and multi-user interference (MUI) pose significant challenges. In this paper, we propose an iterative detection method for multi-user uplink UOWC with signal-dependent shot noise, finite sampling rates, and thermal noise. We design a sparse interleaver and construct a sparse factor graph dedicated to the low-photon regime of non-perfect photon-counting systems, improving noise resilience, mitigating MUI, and enhance detection performance. We also employ iterative multi-user detection (MUD) based on a robust maximum a posteriori probability framework to address the photomultiplier tube (PMT) dead-time effects. Simulation results show that the proposed method achieves up to 5 dB bit error rate (BER) gains over conventional photon-counting schemes and maintains robust performance under diverse channel and noise conditions.