## 4 Modulation Techniques for Underwater VLC
### Hybrid Modulation Techniques  
In this respect, hybrid modulation techniques are currently under research in UVLC systems due to the special challenges involved in the underwater environment. Hybrid modulation methods aim at an increase in data rate, higher reliability, and resistance to environmental factors such as turbulence, scattering, and fading by combining the advantages of various modulation schemes [37; 38]. A good example of hybrid modulation includes the combination of QAM with OFDM: QAM ensures a high data-carrying capacity since each symbol can carry several bits due to the different amplitudes and phases of the carrier that it is going to take, while OFDM offers robustness against frequency-selective fading, splitting the signal into multiple orthogonal subcarriers. In summary, hybrid QAM-OFDM systems effectively leverage the advantages for efficient bandwidth usage and enhanced performance in underwater channels experiencing serious multipath, as in [39; 40]. Some of the most recent implementations involving QAM-OFDM in UVLC demonstrate considerable enhancements to data throughput with robust BER performance under challenging conditions, such as [41]. Another new idea is the development of a hybrid modulation scheme using Quadrature Phase Shift Keying and Amplitude Phase Shift Keying for UVLC systems. This hybrid modulation scheme leverages the high spectral efficiency of QPSK and the excellent nonlinear channel performance of APSK. Such a combination has been able to yield much better robustness and efficiency in turbulent underwater channels where traditional single modulation usually struggles. Experimental results show that the proposed hybrid technique can maintain high data rates at low BER even for severe channel impairments, as shown in [42]. To further alleviate the impacts of underwater turbulence and fading, most hybrid modulation systems adopt internal and external modulation techniques. Although internal modulators optimize the encoding process against noise, an external modulator compensates for channel impairments based on dynamic signal characteristics changes according to real-time channel feedback. These dual-modulator architectures offer a significant improvement in system reliability, mainly within fast-varying underwater environments as well [45; 46]. Further, with the aim of a detailed description of ocean turbulence, sophisticated statistical models were considered, including the log-normal distribution. By embedding such models within hybrid modulation systems, methods have been developed to anticipate and compensate for channel degradations. For instance, the hybrid systems proposed, which rely on statistical modeling combined with sophisticated modulator architectures, ensure outstanding improvements in metrics of performance, such as BER and Q-factor, and thus are extremely effective for UVLC applications [43; 47]. Such systems are particularly suited for high-speed communication in scenarios that require real-time adaptability, such as underwater exploration and communication with AUVs [44]. Table 2 shows the comparison of modulation techniques for UVLC systems. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Modulation Techniques** & **Key Features** & **Advantages** & **Limitations** & **Performance Metrics** & **Citation** \\ \hline BPSK & \begin{tabular}{c} Binary phase modulation, \\ single-bit encoding \\ per symbol \\ \end{tabular}  & \begin{tabular}{c} High noise resilience, simple \\ implementation \\ \end{tabular}  & \begin{tabular}{c} Low spectral \\ efficiency, limited \\ data rates \\ \end{tabular}  & \begin{tabular}{c} Effective in low \\ SNR \\ environments; \\ robust BER \\ \end{tabular}  & [37; 38] \\ \hline QPSK & \begin{tabular}{c} Encodes 2 bits per symbol; uses \\ phase variations \\ \end{tabular}  & \begin{tabular}{c} Spectrally efficient, \\ robust in moderate \\ turbulence \\ \end{tabular}  & \ \begin{tabular}{c} Susceptible to \\ noise in severe \\ conditions \\ \end{tabular}  &