## 4 Modulation Techniques for Underwater VLC
### Advanced Techniques: QPSK and APSK  
QPSK is one of the most widely used modulation techniques in underwater visible light communication (UVLC) systems, particularly due to its high bandwidth efficiency. In QPSK, two bits of data are encoded onto each symbol, effectively doubling the spectral efficiency compared to BPSK, which encodes only one bit per symbol. Despite the higher bit rate, QPSK is still robust to noise and interference, just like BPSK, and can be applied for UVLC systems where the quality of the communication link can be degraded by signal attenuation and scattering [37; 38].  
QPSK demonstrates excellent performance in water when the conditions of underwater turbulence are moderate. Different works have shown that QPSK can withstand such turbulence with a relatively low BER. This robustness is particularly important for the UVLC systems used in underwater exploration or in communicating with underwater autonomous vehicles, where signal degradation owing to environmental factors is inevitable [39; 40]. Under these conditions, QPSK has been shown to achieve reliable communication over significant distances with a BER well below the threshold for high-quality data transmission and therefore this technique is an ideal choice for many applications in UVLCs [41].  
Another advanced modulation technique, with combinations of both amplitude and phase modulations, includes Amplitude Phase Shift Keying. This hybrid technique has several merits over classical phase modulation schemes, especially in nonlinear communication channels, as normally experienced in underwater optical communication. The Amplitude Phase Shift Keying (APSK) is particularly useful under the nonlinearity conditions of a channel resulting from environmental factors such as scattering, absorption, and turbulence in UVLC systems. It provides a much stronger capability for the system to sustain high data rates at improved signal resilience against channel distortions by varying both the phase and amplitude of the signal [42].  
Recent research using APSK shows major improvements in performance concerning the data rate and BER. One of the notable studies reports that APSK, working with the Levin-Campello bit-loading algorithm, can achieve data rates up to 3.18 Gbps while keeping the BER below the threshold for high-speed communication [45]. Therefore, APSK is particularly suitable in applications of UVLCs where both high throughput and reliability are crucial. In combination, APSK with adaptive bit-loading schemes enables dynamic adjustments in modulation parameters according to real-time channel conditions for further performance optimization, hence assuring efficient communication even in challenging underwater environments [43].  
APSK has been shown to perform well in nonlinear channels compared to other modulation schemes under similar conditions, thus making it attractive for underwater communication systems that require high data rates and low latency. As UVLC systems continue to evolve and as demand for faster data transmission increases, APSK is poised to play a crucial role in enabling high-speed communication for a variety of underwater applications [44].