## 3 Types of Underwater Communication Systems
### Applied Underwater Modulation Schemes
The variability of the underwater channel causes a shift in the frequency profile of the target signal across the underwater channel. The effect is known as the Doppler effect, which can cause ISI to make the signal hard to demodulate, leading to degradation of the quality of the received signal. That is why mitigation techniques to overcome the Doppler effect are crucial to enable a quality communication system. In addition to the spread spectrum, methods such as AMC, synchronisation, channel encoding, and time-frequency synchronisation are used to mitigate the Doppler effect [5,81]. [PERSON] et al. [34] reported a resemblance between FSK and the M-OCK modulation based on bandwidth spreading and message duration. Identical to OFDM, COFDM implements orthogonal code in its modulation and is used as a digital multi-carrier modulation scheme, wherein Forward Error Correction (FEC) is applied before signal transmission [105]. The goal for applying FEC is to overcome errors during transmission [106]. Several other types of modulation are applied to underwater communications, such as Fully Generalised Spatial Modulation (FGSM) [107], Multiple-Input Multiple-Output/Orthogonal Frequency Division Multiplexing (MIMO-OFDM) [108], and Amplitude Shift Keying-OrthogonalFrequency Division Multiplexing (ASK-OFDM) [109]. ASK-OFDM is a modulation scheme that combines Amplitude Shift Keying (ASK) and OFDM. ASK-OFDM utilises ASK modulation on each subcarrier of an OFDM system. This allows for efficient use of bandwidth and improved spectral efficiency, and it is useful in scenarios with limited bandwidth and noise interference, such as the underwater channel. MIMO-OFDM as a modulation scheme has several benefits, namely (i) high data rate by exploiting spatial diversity and frequency diversity [108], (ii) improved reliability by offering more robustness to interference and fading [108], and (iii) with better spectral efficiency, it can efficiently utilise the available spectrum [108]. MIMO-OFDM find applications in modern wireless communication systems, such as Wireless Fidelity (Wi-Fi) (both 802.11n [110] and 802.11 ac standards [110]), cellular networks (4G and 5 th Generation (5G) cellular technologies) and wireless broadband (Worldwide Interoperability for Microwave Access (WiMAX) and Advanced Long-Term Evolution - Advanced (LTE-Advanced)). [PERSON] et al. [108] reported that although MIMO-OFDM has been used for over two decades in air-based wireless communication, it is a novelty in UWAC that necessitates improvements. Nevertheless, basic MIMO-OFDM systems can employ null and pilot carriers for Doppler and channel estimation in the underwater domain [108]. There are several challenges for implementing workable modulation schemes. Water enables variable propagation speed due to factors such as temperature, salinity, and pressure, which complicate accurate timing and synchronisation [5; 21; 81]. Propagation speed impacts the efficiency of communication protocols, especially those relying on quick feedback mechanisms [5; 21]. Acoustic signals are rapidly absorbed by water molecules and scattered into multipaths by particles in the water [45; 74; 81]. Both scattering and absorption limit the effective communication range. Higher frequencies attenuate more rapidly than lower frequencies, influencing the choice of modulation schemes [34; 61]. In the case of multipath propagation, signals reach the receiver via multiple paths, which can lead to ISI, distortion, and increased delay spread. When it comes to ISI, it can degrade the quality of received signals, making it difficult to recover the original data [5; 81]. Another challenge is the Doppler shift, or Doppler effect, which is a phenomenon observed where frequency shift occurs [5; 34; 81; 94]. The relative motion between the transmitter and receiver causes a Doppler shift in the received signal frequency. The Doppler effect can impact the performance of coherent modulation schemes.