Performance Analysis of the LDPC‐Coded Hybrid <i>L</i> QAM‐MPPM for UWOC Systems

ABSTRACT This paper proposes a hybrid optical modulation scheme that integrates spectrally efficient quadrature amplitude modulation (QAM) and power‐efficient multipulse pulse position modulation (MPPM) for underwater wireless optical communication (UWOC) to achieve high spectral and energy efficiencies and combines it with LDPC codes to mitigate the adverse effects imposed by the underwater turbulence channel. Considering the effect of both direct‐current (DC) bias level and modulation index on signal‐to‐noise ratio (SNR), for the first time, an approximate closed‐form bit error rate (BER) expression for the hybrid L ‐QAM‐MPPM UWOC system over turbulence channels is derived and verified using Monte Carlo (MC) simulations. Subsequently, comparisons among the hybrid L‐ QAM‐MPPM, conventional MPPM and QAM systems with comparable spectral efficiency for exponentiated Weibull (EW)‐modelled turbulence channel indicate that hybrid modulation offers resilience to turbulence, although the BER in the additive white Gaussian noise (AWGN)‐only channel is higher. Finally, the effectiveness of LDPC codes in improving the performance of hybrid modulations is discussed, with coded 128‐QAM‐(12, 2) MPPM offering ∼6.5 and 5.8 dB code gains at a BER of in strong and moderate turbulence, respectively. These results demonstrate the promise of hybrid modulation with LDPC as a robust and efficient modulation scheme for UWOC systems.