# A Survey on Physical Layer Techniques and Challenges in Underwater Communication Systems
## 2 UWAC Physical Layer Techniques
### MIMO-OFDM
In the case of complex channels, OFDM is effective because of its ability to mitigate the effects of multipath propagation and Doppler shift. UF-OFDM is a variant of OFDM that does not use a pulse-shaping filter before transmission. This means that the signal occupies a wider bandwidth than conventional OFDM, but it also allows for a higher data rate. UF-OFDM is effective in mitigating the effects of ISI caused by multipath propagation. The wider bandwidth of UF-OFDM means that the delay spread of the channel is reduced, which reduces the effect of ISI. However, UF-OFDM also has some drawbacks. The wider bandwidth can lead to increased interference with other users of the UWA channel. Additionally, the lack of a pulse-shaping filter means that the signal has a higher PAPR, which can be problematic for some hardware implementations. Overall, UF-OFDM is a promising modulation scheme for communication, but its use should be carefully considered in the context of specific application and characteristics of the underwater channel. Continuous Phase Modulation Based OFDM (CPM-OFDM)  
UWAC is challenging due to the high attenuation, dispersion, and multipath propagation of sound waves in water. This results in severe signal distortion which makes it difficult to transmit and receive signals over long distances. To overcome these challenges, CPM-OFDM is used as a digital modulation technique. CPM-OFDM combines two key components: CPM and OFDM [101]. CPM is a modulation technique that maintains a constant amplitude but varies the phase of the carrier signal according to the input data. By continuously varying the phase of the carrier signal, the data is transmitted as changes in the frequency of the signal. This results in a constant envelope signal, which is more power-efficient than other modulation techniques that require varying amplitude. OFDM is a technique that divides the available bandwidth into multiple subcarriers, each carrying a different portion of the data. These subcarriers are orthogonal to each other, meaning they do not interfere with each other, even when transmitted simultaneously. This makes OFDM an efficient way to transmit data over a wide bandwidth. By combining CPM and OFDM, CPM-OFDM provides a powerful modulation technique for UWAC. The CPM component helps to maintain a constant envelope signal, which is critical in UWAC where the signal can experience severe attenuation and distortion. The OFDM component provides efficient transmission of data over a wide bandwidth, allowing for high data rates and reliable communication over long distances [101; 102]. In UWAC, CPM-OFDM can be used to mitigate the effects of multipath propagation, which causes signal distortion due to reflections, scattering, and refraction of sound waves. The OFDM component of CPM-OFDM allows for the use of multiple subcarriers, which can be selectively attenuated by the underwater environment. By selecting the subcarriers that experience the least amount of attenuation, the distortion caused by multipath propagation can be reduced. The CPM-OFDM transceiver block diagram for UWAC has been shown in Figure 13.  
In addition, the constant envelope nature of the CPM component of CPM-OFDM allows for the use of low-power amplifiers, which are critical in UWAC where power consumption is a major concern. The use of low-power amplifiers can also reduce the amount of noise generated by the transmitter, which is important in underwater environments where noise can be a major source of interference. Overall, CPM-OFDM is a powerful digital modulation technique for UWAC, offering high data rates, reliable communication over long distances, and resistance to the effects of multipath propagation, attenuation, and noise.