# 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
In some receivers, turbo equalization is used if not all parity checks are satisfied on the initial attempt. In this method, information from satisfied parity checks is fed back to support the equalization process where they serve as added pilot tones. The duration of the OFDM block is given by \(T^{\prime}=T+T_{g}\), where, \(T\) defines the duration of the OFDM symbol and \(T_{g}\) denotes the guard interval between blocks. \(\Delta f=1/T\) is the frequency spacing. The \(xth\) subcarrier occurs at the frequency:  
\[f_{\kappa}=f_{c}+\frac{\kappa}{T_{D}}=f_{c}+\kappa\Delta f \tag{1}\]  
Figure 2.0  
where, \(f_{c}\) denotes the center frequency while K is presumed even. \(B=\frac{\kappa}{T}=\text{K}\Delta f\) is the bandwidth of the signal. Consider \(s[\kappa]\) depicting the \(\kappa th\) subcarrier information symbol. Figure 2.1 The receiving process of CP-OFDM and ZP-OFDM [75]. and \(s_{n}\) define the non-overlapping sets for active and null subcarriers. These subcarriers should satisfy the following relation :  
Figure 2.2  
The bandpass signal transmitted is given by:  
Figure 2.3  
where, \(g(t)\) denotes the pulse-shaping filter with Fourier transforms \(G(f)\). In this work, \(g(t)=1\), \(t\in[0,T]\) and \(g(t)=0\) otherwise. That is, \(g(t)\) is a rectangular window of duration \(T\). Time Domain Synchronous Orthogonal Frequency Division Multiplexing  
Authors in [80, 81] discussed a time-domain synchronous orthogonal frequency division multiplexing (TDS-OFDM) system for the UWAC with dual pseudorandom noise (DPN). Figure 9 shows the usage of a TDS-OFDM system for the UWA channel [81]. Due to fast channel deviation in each OFDM symbol, the regular functioning of a ZP-OFDM receiver is rigorously restricted by inter-carrier interference (ICI). Moreover, due to the small ratio of the carrier frequency to the bandwidth of the signal, the nature of the UWA channel is wideband [82, 83]. For the UWAC system, a cyclic shift keying spread spectrum OFDM technique is suggested in [84]. The objective of this research was to solve the problem related to low data rates of DSSS UWAC [85] and the one related to receivers' computational complexity for the M-array SS. Moreover, it also improved bit rates and the efficiency of bandwidth. This technique results in a low BER and high data rate in comparison to conventional approaches. The inter-block interference (IBI) between the training sequence and OFDM data blocks is the core difficulty in TDS-OFDM. Many attempts have been made to resolve this issue. The suggested approaches fall under two categories [86]. Classical iterative interference cancellation algorithm is the primary approach [87] and its extension involves procedures without altering the frame structure of the TDS-OFDM system [88]. However, only trivial progress can be attained with these techniques [86], specifically in UWA channels with a long delay. The other category depends on the alteration of the TDS OFDM frame structure for substantial cancellation of interference [86]. For instance, a noteworthy reduction in interference can be attained within OFDM data blocks  
Figure 2.4 OFDM receiver for UWACs [75]. due to the scattering of redundant frequency-domain pilots in the unique word OFDM (UW-OFDM) [89] to produce the training sequence (TS) in the time domain. The interference caused by OFDM data blocks to the TS sequence cannot be eliminated, but the DPN-OFDM system provides a promising solution[90]. This method adds two repeated guard intervals to each TDS-OFDM symbol to prevent interference from OFDM data blocks to subsequent PN sequences. Therefore, DPN-OFDM is now under extensive general research, and the implementation of hardware to assess digital terrestrial multimedia broadcast (DTMB) standards is being conducted [86; 90; 91]. However, the use of duplicated guard intervals can significantly reduce the system's spectrum and energy efficiency, particularly in UWA channels with significant tap delay.