# A Survey on Physical Layer Techniques and Challenges in Underwater Communication Systems
## 2 UWAC Physical Layer Techniques
### Multicarrier Modulation for the UWAC
#### 2.1.1 Orthogonal Frequency Division Multiplexing  
The OFDM is defined as a multicarrier communication modulation scheme and normally finds its applications in wireless networks, 4G mobile applications, digital television, audio broadcasting, etc. The developed methods effectively deal with problems related to the UWA channels. OFDM method is extensively implemented [64; 65; 66] due to its numerous benefits as discussed below. 1. Subcarrier signals' orthogonality allows easy transmission and generation of signals across a block of an Inverse Fast Fourier transform (IFFT). It eases separating receiver's transmitted data symbols across a fast block, eases equalization via scalar gain per subcarrier, and is easy to adopt for the MIMO channels. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sr. No.** & **Modulation Technologies** & **Merits** & **Demerits** \\ \hline \multirow{4}{*}{1.} & Orthogonal Frequency & Convert selective fading channel to & \\  & Division Multiplexing & flat fading channel, High utilization & Need long guard interval, Not effective in \\  & (OFDM) & for UWA channel, Simple & the long underwater communication \\  & & Equalization, Low complexity. & High computational complexity, High \\  & Multiple Input Multiple & The high data rate, Robustness, & Interference, Range limited, and Accurate \\  & (MIMO-OFDM) & High spectral efficiency, Scalability & channel estimation is critical \\  & Generalized Frequency & High spectral efficiency, Low out of & Require high synchronization, Sensitive to \\  & Division Multiplexing & band radiation, Good performance & frequency and timing offset, and High \\  & (GFDM) & over frequency selective channels & complexity due to a large number \\  & Unfiltered Orthogonal & Low power consumption, Not & Limited spectral efficiency, Poor band \\  & Frequency Division & filtering or equalization required, & emission, Sensitive to frequency offset \\  & Multiplexing (UF-OFDM) & Low complexity & Computational complexity, Nonlinear \\  & Continuous Phase Modulation & High energy efficiency, High data & distortion, Interference from other signals, \\  & OFDM (CPM-OFDM) & High spectral efficiency, No guard & Sensitive to carrier frequency offset (CFO) \\  & Filter Bank Multicarrier & High spectral efficiency, No guard & Complex transmitter and receiver design. \\  & Modulation (FBMC) & interval needed & Complex transmitter and receiver design \\  & Multiple Input Multiple & High spectral efficiency & with a high BER. \\  & Output (MIMO) & High energy efficiency can support & High BER with complex receiver design. \\  & Spatial Modulation & High energy efficiency can support & \\  & Technologies (SMTs) & long communication. & High BER with complex receiver design. \\  & Orthogonal Frequency & & \\
9. & Division Multiplexing Index & High energy and spectral efficiency & High BER with complex receiver design. \\  & Modulation (OFDM-IM) & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Merits and demerits of UWA multicarrier modulation technologies. 2. The existing bandwidth is partitioned into the largest collection of narrow sub-bands using narrowly spaced orthogonal subcarriers. 3. To improve the bandwidth efficiency and rate of transmission adaptive modulation systems may be applied to subcarrier bands. 4. The distinct OFDM symbol structure makes the carrier tasks simpler and helps in symbol synchronizations. Using an orthogonal partition method, OFDM divisions the channel into multiple narrowband parallel channels [59]. The partitioning in the OFDM is achieved using the Fourier transform that helps in transmitting the data on multiple equally orthogonal sinusoids. The orthogonality concept of subcarriers in OFDM is depicted in Figure 2. In OFDM, multiple subcarriers are used to transmit data simultaneously over a communication channel. These subcarriers are combined to create an OFDM symbol in the time domain using Inverse Fast Fourier Transform (IFFT). The OFDM symbol is then transmitted over the channel.