# 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
At the receiver end, the subcarriers are separated using Fast Fourier Transform (FFT) to retrieve the original data bits. This process enables high data rates to be transmitted over a communication channel while reducing the effect of ISI and multipath fading. The core concepts of OFDM signal and the correlation between the frequency and time domains are depicted in Figure 3. In the frequency domain, adjacent subcarriers or tones are modulated with individual complex data. The subcarriers are transformed into OFDM symbols in the time domain via an Inverse FFT process. To prevent inter-symbol interference at the receiver due to multipath delay spread in the radio channel, guard intervals are inserted between each symbol in the time domain. Multiple symbols are combined to form the final OFDM burst signal. At the receiver end, an FFT is conducted on the OFDM symbols to retrieve the original data bits. Typically, the preamble and postamble waveforms are linear frequency modulated (LFM) or hyperbolic frequency modulated (HFM). These assist in detecting data, its synchronization, Doppler scaling, and estimation of the channel, while a few methods do not need a postamble. Researchers in [67] assembled the preamble. This is performed to match each set of parallel correlates to a distinct Doppler scaling factor keeping in view the dilation or compression of the waveform. This assembly performs the detection of signal and estimation of the Doppler scale. The peak-to-average power ratio (PAPR) problem in OFDM for UWAC is caused by the superposition of multiple subcarriers which can result in high peaks in the amplitude of the transmitted signal. These high peaks can lead to signal distortion, particularly in nonlinear UWA channels, and can result in a higher bit error rate (BER) [68; 69]. The PAPR problem in UWA OFDM is exacerbated by the narrow bandwidth of the UWA channel which restricts the use of standard PAPR reduction techniques, such as selected mapping (SLM) and partial transmit sequence (PTS). Moreover, the limited computational resources available on UWA modems make it challenging to implement complex PAPR reduction algorithms. The PAPR problem in MIMO-OFDM signals transmitted through underwater channels is caused by the superposition of multiple subcarriers in multiple transmit antennas. As a result, the peaks of the transmitted signal can be very high, leading to nonlinear distortion in the acoustic channel and a higher BER. The PAPR problem in MIMO-OFDM signals transmitted through underwater channels is more challenging to address than in single-input single-output (SISO) OFDM systems due to the additional complexity of multiple antennas. The use of MIMO-OFDM increases the number of subcarriers that need to be processed, and the computational complexity of PAPR reduction algorithms increases as the number of transmit antennas increases [70]. OFDM as a multicarrier modulation is most effective if the noise has been extended over a large part of the available bandwidth. It transfers signals across multiple orthogonal subcarriers concurrently and functions robustly in strict multipath scenarios thus attaining higher spectral efficiency. OFDM is considered the best alternative to a single carrier broadband modulation scheme in UWCs to attain transmission with high data rates [71; 72; 73; 74; 75]. This method has demonstrated its affectivity for considering the multipath delay spread eliminating the need for complex time-domain equalizers, because of its robustness for narrowband interference and frequency selective fading. Moreover, in comparison to the wideband carriers, every subcarrier deals with flat fading only instead of frequency selective fading if the number of subcarriers is sufficiently large. Only one or two of the complete group of subcarriers will be affected by the narrowband interference. Motion-provoked Doppler distortion is the foremost concern in relating OFDM to underwater channels because in a wideband acoustic signal, it generates a non-uniform frequency offset [71; 72; 73; 74; 75].