## 4 Modulation Techniques for Underwater VLC
### Basic Modulation Techniques  
Of all the modulation techniques, Binary Phase Shift Keying (BPSK) is the most basic for binary data transmission. The BPSK method characterizes binary information by the use of two different phases--usually 0\({}^{\circ}\) and 180\({}^{\circ}\)--to encode \"0\" and \"1\". Its simplicity and resistance to noise in the communication link make it fit for low SNR communications. Noise resistance comes at a price: BPSK represents one of the few data rate techniques. Since each BPSK symbol only carries one bit of information, the achievable data rate is naturally comparatively low. It is this inherent simplicity of BPSK that turns out to be highly desirable when the most sensitive requirement is for robustness to noise, including but not limited to an underwater optical communication system where the channel noise might become very debilitating [37; 38].  
Quadrature Phase Shift Keying is a more advanced modulation scheme than BPSK because it encodes two bits per symbol rather than just one. In Quadrature Phase Shift Keying (QPSK), this is accomplished with the use of four different phases, each representing four different symbol states. This will consequently help give far better data throughput without sacrificing the efficiency of the modulation scheme. QPSK is used in systems where a compromise between noise immunity and data rate is needed. In particular, QPSK appears to be reasonably effective under moderately bad signal-to-noise ratio (SNR) circumstances, yielding an optimum trade-off between robustness and throughput [39; 40].  
Taking a step further from the concept of QPSK, Quadrature Amplitude Modulation (QAM) changes both phase and amplitude to encode multiple bits per symbol. QAM can achieve higher data rates than QPSK by changing the amplitude and phase of the carrier signal. For instance, 16-QAM and 64-QAM encode 4 and 6 bits per symbol, respectively, ref. [41]. These higher-order QAM techniques are especially useful for high-data-rate applications where bandwidth is not as limited. However, as the number of bits per symbol increases, the sensitivity to noise also increases. This makes higher-order QAM more susceptible to degradation under noisy conditions. For instance, 16-QAM and 64-QAM are very effective in a medium SNR environment, but their performance considerably degrades under low SNR conditions. In picking up the QAM modulation schemes in an underwater VLC system, much care must be considered in the balancing of data rate versus noise resilience [42].  
Indeed, various studies have shown that while much higher data rates are possible using higher-order QAM schemes, such as 16-QAM and 64-QAM, their robustness is inherently limited by the greater number of possible symbol states. In this regard, a trade-off exists between achievable data rate and noise immunity. The relevance of such a trade-off in underwater optical communication, wherein signal attenuation and noise from the environment may severely affect the quality of the communication link, cannot be overestimated. Of these, the adaptation in the modulation technique may mitigate some of the challenges. It switches the modulation technique dynamically between lower- and higher-order QAM based on the instant channel condition [43].  
Hence, the choice of modulation technique could be either BPSK, QPSK, or higher-order QAM, all of which depend upon the communication system's requirements in terms of SNR environment, required data rate, and error rates. If one considers the unique challenges and advantages of each modulation scheme, there is much potential to optimizethe performance of the VLC system for underwater applications, including AUVs and other deep-sea communication systems [44].