## 3 Types of Underwater Communication Systems
### Applied Underwater Modulation Schemes
Unsupervised learning is an ML technique where algorithms learn patterns from unlabelled data without explicit guidance [5]. Unlike supervised learning, which requires labelled data to train a model, unsupervised learning algorithms discover hidden structures and relationships within the data itself [5]. Linear regression is a statistical method used to model the relationship between a dependent variable and one or more independent variables. Decision trees are tree-like models used to classify or predict outcomes. While decision trees make decisions based on a series of rules, linear regression assumes a linear relationship between the variables and aims to find the best-fitting line. DDSL focuses on finding sparse solutions to complex problems. It involves learning a model with a small number of non-zero parameters, leading to more interpretable and efficient models. SVMs are a powerful algorithm used for classification and regression tasks. They work by finding the optimal hyperplane that separates data points into different classes. NNss are inspired by the human brain and are composed of interconnected nodes called neurons. They are used for a wide range of tasks, including image recognition, natural language processing, and speech recognition. OFDM is a popular modulation scheme used in 4 th Generation (4G) technology due to its robustness against Inter-Symbol Interference (ISI) and its high spectral efficiency. These qualities make OFDM a suitable candidate for UWOC. [PERSON] et al. [61] have explored various OFDM-based techniques for UWOC, including (i) Direct Current Biased Optical Orthogonal Frequency-Division Multiplexing (DCO-OFDM) that excels in high bit rate transmission scenarios; (ii) Asymmetrically-Clipped Optical Orthogonal Frequency-Division Multiplexing (ACO-OFDM) offers a balance between performance and complexity; and (iii) Unipolar Orthogonal Frequency-Division Multiplexing (U-OFDM), which provides longer transmission range with lower power consumption. Another promising modulation technique for UWOC is Single-Carrier Frequency Division Multiple Access (SC-FDMA), which offers a lower peak-to-average power ratio [61]. By understanding the strengths and limitations of these modulation techniques, researchers can design efficient and reliable UWOC systems. Spread spectra are widely used modulation schemes both for civilian and military applications [34]. They are used as a mitigation technique to reduce acoustic source emissions that affect marine lifeforms by spreading the signal energy over a wide bandwidth; the spread spectrum operates at low SNR, reaching ranges greater than 10 km [34]. Bioacoustics studies have proven animal discomfort and bio-intrusion reduction provided by this method [34]. The simulations they performed resulted in transmission rates of 45 bps at \(-\)18 dB SNR and 140 bps at \(-\)12 dB SNR [34]. The white noise-like features, near-orthogonal band-limited Pseudo-Noise (PN) codes provided by M-ary Orthogonal Code Keying (M-OCK) and Binary Phase Shift Keying (BPSK) modulations of the spread spectrum have been demonstrated to possess a diminished impact on marine life compared to transmission that has tonal, chirp, or burst characteristics [34]. The spread spectrum approach also effectively overcomes the Doppler effect because it is equipped with an enhanced error correction mechanism, multipath ISI correction, while performing frame synchronisation, and it is able to combine multipath signals [34]. The spread spectrum method consists of spreading the signal over a large frequency band, resulting in increased resistance to natural interference, noise, and jamming or obstruction [104]. By lowering the signal per-Hz density due to the large bandwidth, the spread spectrum method prevents signal detection while limiting power influx density. Through the spread spectrum method, various signals can be combined over the same band, giving rise to multiple-access communications [104].