# A Survey on Physical Layer Techniques and Challenges in Underwater Communication Systems
## 1 Introduction
\begin{table}
\begin{tabular}{c c} \hline \hline
**Abbreviations** & **Full Forms** \\ \hline DL-CIM-SS & Deep Learning Code Index Modulation Spread Spectrum \\ DPN & Dual Pseudorandom Noise \\ DPSK & Differential Phase Shift Keying \\ DSSS & Direct Sequence Spread Spectrum \\ DTMB & Digital Terrestrial Multimedia Broadcast \\ DTTB & Digital Television Terrestrial Broadcasting \\ EFGSM & Enhanced Fully Generalized Spatial Modulation \\ EW-RLS & Exponentially Weighted Recursive Least Square \\ FBMC & Filter bank Multicarrier \\ FDM & Frequency Division Multiplexing \\ FFT & Fast Fourier Transform \\ FGSM & Fully Generalized Spatial Modulation \\ FHSS & Frequency Hopping Spread Spectrum \\ FMT & Filtered Multi-tone Modulation \\ FSK & Frequency-Shift Keying \\ GFDM & Generalized Frequency Division Multiplexing \\ GQSM & Generalized Quadrature Spatial Modulation \\ GQSSK & Generalized Space Shift Keying \\ GSM & Generalized Spatial Modulation \\ GSSK & Generalized Space Shift Keying \\ HFM & Hyperbolic Frequency Modulation \\ IBI & Inter Block Interference \\ ICI & Inter Carrier Interference \\ IFFT & Inverse Fast Fourier Transform \\ IM-OFDM-SS & Index Modulation with Orthogonal Frequency Division \\  & Multiplexing Spread Spectrum \\ IoUT & Internet of Underwater Thing \\ ISI & Inter Symbol Interference \\ KSP & Known Symbol Padding \\ LFM & Linear Frequency Modulation \\ LPF & Low Pass Filter \\ MI & Magnetic Induction \\ MIMO & Multiple Input Multiple Output \\ ML & Maximum Likelihood \\ MRC & Maximal Ratio Combining \\ OFDM & Orthogonal Frequency Division Multiplexing \\ OFDM-IM & Orthogonal Frequency Division Multiplexing Index Modulation \\ PAPR & Peak to Average Power Ratio \\ P-CSI & Perfect Channel State Information \\ PLL & Phase Locked Loop \\ PSD & Power Spectral Density \\ PSK & Phase Shift Keying \\ QAM & Quadrature Amplitude Modulation \\ QPSK & Quadrature Phase Shift Keying \\ QSM & Quadrature Spatial Modulation \\ QSSK & Quadrature Space Shift Keying \\ RAP & Reliable Acoustic Path \\ RF & Radio Frequency \\ RLS & Recursive Least Square \\ SM & Spatial Modulation \\ SMT & Spatial Modulation Technology \\ SNR & Signal-to-Noise Ratio \\ SS & Spread Spectrum \\ SSK & Space Shift Keying \\ TDS-OFDM & Time-Domain Synchronization Orthogonal Frequency \\  & Division Multiplexing \\ TRM & Time Reversal Mirroring \\ TS & Time Sequences \\ UF-OFDM & Unfiltered Orthogonal Frequency Division Multiplexing \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._There are many survey papers addressing UWC, such as in [21; 22; 23; 24], but unfortunately, these papers address the UWWSN, without focusing on the UWA physical layer technologies and underwater channel problems. Other survey papers focus on the routing protocols in UWWSNs [23; 25]. There are other survey papers addressing UWC which focuses on optical communications in ocean environments [26; 27; 28; 29]. A short literature review has been mentioned in Table 2. According to the authors' knowledge, there is a shortage of survey papers focusing on UWAC for physical layer techniques, channel problems, challenges, and modern research areas. The available literature does not provide holistic coverage of the topic related to UWAC physical layer technologies and channel challenges. Additionally, the available literature focuses on the multicarrier modulation in UWAC, such as [30], which proposes and analyzes the use of multicarrier communication techniques, specifically orthogonal frequency division multiplexing (OFDM) and filter bank multicarrier (FBMC), for UWAC. This article discusses the characteristics of the UWA channel, such as multipath propagation, Doppler shift, and ambient noise, and explains that multicarrier communication can be used to mitigate these effects and improve the performance of UWC systems.