## 4 Modulation Techniques for Underwater VLC
### Adaptive Modulation Techniques  
Adaptive modulation is a technique that varies the modulation scheme dynamically with real-time channel conditions, more precisely with the signal-to-noise ratio. It allows communication systems to maintain a very optimum balance between throughput and reliability, adapting to different environmental factors. In underwater optical communication systems, for instance, underwater VLC, channels are time-varying because of water turbulence, light scattering, and ambient noise. Adaptation in this respect ensures that the link is efficient and reliable even in bad conditions [37; 38].  
For low SNR scenarios, lower-order modulation schemes, like BPSK, are preferred in such an environment. BPSK has noise immunity but at a relatively lower data rate since one symbol carries just 1 bit. Such a modulation technique will be highly useful under conditions of high signal attenuation when the quality of a communication link is very poor, such as in the case of deep-sea applications, where the optical signal undergoes extreme scattering and absorption. With an increasing SNR, higher-order modulation formats can be employed that provide much higher data rates. For example, as the SNR increases, methods such as 256-QAM can be used to send more bits per symbol, greatly enhancing the throughput without losing that much reliability within the system [39; 40; 41].  
One main feature of AM is its modulation scheme, which changes dynamically based on the real-time SNR measurements taken from the channel. In any ordinary adaptive modulation system, the SNR range falls into distinct parts, and each range in turn corresponds to some modulation scheme. For example, an adaptive system may use a lower-order scheme such as BPSK or QPSK in low SNR conditions and use higher-order schemes like 64-QAM or 256-QAM at high values of SNR. This approach allows the maximum capacity of the system in data transmission to be utilized under variable channel conditions while also keeping the error rate within an acceptable limit [42].  
It is proved that adaptive modulation enhances the performance of the UVLC system by significantly reducing BER and maintaining high throughput in dynamic underwater conditions. In this case, the adaptive modulation scheme proposed in [45] is discussed: the signal-to-noise ratio divides into eight regions, each representing a different modulation technique. The results demonstrated that dynamic adjustment significantly reduced the bit error rate (BER) against a fixed modulation scheme at high data transmission rates. In situations where environmental conditions cause rapid changes in channel quality, adaptive modulation allows the system to react swiftly, minimizing packet loss and ensuring continuous communication [43].  
Moreover, adaptive modulation schemes can be further enhanced with channel state information (CSI) to make real-time decisions on the best modulation to use. It thus enables the system not only to adapt to changes in SNR but also to optimize the overall performance of the system by taking into consideration extra factors such as channel fading and interference [44]. In UVLC systems, underwater light channels may be influenced by many dynamic factors; hence, the ability to adapt to such changes is a key issue in maintaining reliable communication.