# A Survey on Physical Layer Techniques and Challenges in Underwater Communication Systems
## 2 UWAC Physical Layer Techniques  
The frequency portion of the communication signals is substantially restricted inside its real frequency zone since the UWAC is primarily linear. On the contrary, there might be a great variation in the amplitude and phase of the complex signal. As a result of this discovery, frequency-shift keying (FSK) has been selected as the noncoherent technique for primary systems of the UWAC [31]. The incoherent communication systems mostly produce comparatively lower data rates. However, it was thought in the 1980s that it would not be possible to have coherent communication in the UWA channels. In the 1990s, the development of numerous coherent systems was observed. In the last two decades, the phase-coherent UWAC systems had progressed considerably. The coherent communications are categorized as purely coherent, i.e., phase-shift keying (PSK), and differentially coherent, i.e., differential phase-shift keying (DPSK), or centered on the carrier synchronization technique. However, differentially coherent approaches use an easier synchronization method. They experience a functioning penalty in comparison to purely coherent procedures [32]. The coherent UWAC was made possible through the use of a decision feedback equalizer (DFE) and phase-locked loop (PLL) in the UWA [33]. In [33], a coherent communication system with integrated channel synchronization and equalization was developed and experimentally demonstrated. In addition, the authors in [34] developed a quadrature phase-shift keying (QPSK) high-rate communication system with adjustable equalization. It was demonstrated that a high-rate strategy worked effectively and efficiently in a medium-range shallow water channel. A high-rate scheme functioned well in a medium-range shallow channel of the water. This resulted from improved channel tracking using the equalizer and performing more regular channel sampling. Spread spectrum (SS) schemes offer robust communication in many selective frequency-fading surroundings. SS communication has been anticipated to perform well in such type of channel due to the frequency selective fading of the channel for UWAs. In [35], the authors studied the functioning of the direct sequence spread spectrum (DSSS) and the frequency hopping spread spectrum (FHSS). Both the FHSS and DSSS performed well in experimental scenarios. The DSSS showed an improved raw performance in comparison to the FHSS. Nevertheless, the ease of FHSS and its inherent near-far resistance made it suitable in some conditions. Despite the provision of robust performance, the SS schemes \begin{table}
\begin{tabular}{c c c c} \hline
**Author(s)** & **Technique** & **Strength** & **Weakness** \\ \hline \multirow{4}{*}{[PERSON], [PERSON] [27]} & \multirow{4}{*}{UOWC} & Underwater optical wireless & \\  & & communication recent advances and & \\  & & discuss the channel characterization, & \\  & & modulation schemes, coding techniques & \\  & & Focus on underwater wireless optical & \\  & & communication concerning system & \\  & & transmitters and receivers, advanced & \\  & & modulation formats, and & \\  & & underwater channel. & \\  & & Provide a survey on the challenges, & \\  & & advances, and prospects of underwater & \\ N. [PERSON] et al. [29] & UOWN & optical wireless networks in Physical, & \\  & & Data link, Network, Transport, & \\  & & Application, Localization, and their & \\  & & impact on them & \\ \hline \end{tabular}
\end{table} 
Table 2: _Cont._do not deliver an efficient bandwidth, except in the case of multi-user systems [36]. In bandwidth-limited channels for the UWA, the main drawback is the lower bandwidth efficiency of the SS systems. In the 1950s and 1960s, the concept of transmitting parallel data using frequency division multiplexing (FDM) was presented. In 1971, it was identified that the discrete Fourier transform (DFT) is an easy method to achieve FDM [37]. This helped to develop an effective OFDM system in the 1980s and 1990s.