# A Survey on Physical Layer Techniques and Challenges in Underwater Communication Systems
## 2 UWAC Physical Layer Techniques
### MIMO-OFDM  
Underwater acoustic MIMO-OFDM is a technique that combines MIMO technology with OFDM to improve the performance of UWC systems. The MIMO technique involves the use of multiple transmit and receive antennas to transmit and receive multiple data streams simultaneously, while OFDM divides the available frequency spectrum into multiple subcarriers that are transmitted in parallel [95]. The use of MIMO-OFDM in UWC  
Figure 11: The frame structure of the received signal (**a**) conventional TDS-OFDM (**b**) The DPN-TDS-OFDM (**c**) TDS-OFDM based on CS [94]. Figure 10: CP-OFDM, ZP-OFDM, and TDS-OFDM signal structure (**a**) time domain (**b**) frequency domain [94]. has several advantages. One of the main advantages is its ability to combat the effects of multipath propagation which can cause significant signal distortion and degradation. By using multiple antennas and OFDM, MIMO-OFDM can take advantage of the frequency diversity and spatial diversity of the underwater channel, which can help to mitigate the impact of multipath fading which is caused by sound waves reflecting off objects in the water, and improve the overall system performance. MIMO-OFDM can also provide higher data rates and increased robustness compared to other UWC techniques, such as single-input single-output (SISO) and frequency-shift keying (FSK). However, the use of MIMO-OFDM in UWC also presents several challenges, such as the need for accurate channel estimation, synchronization, and power allocation as well as the impact of underwater noise and interference. Despite these challenges, there is ongoing research and development in UWA MIMO-OFDM, including the exploration of new optimization techniques, modulation and coding schemes, signal detection, and channel estimation algorithms [96; 97]. The ultimate goal is to achieve reliable and high-speed communication in underwater environments which has significant applications in areas such as underwater exploration, ocean monitoring, and underwater sensor networks. Overall, MIMO-OFDM is a promising technique for improving the performance of UWAC systems, and there is ongoing research exploring its potential applications and optimization techniques. Generalized Frequency Division Multiplexing (GFDM)  
GFDM is a promising modulation technique for UWAC. UWAC is a challenging task due to the unique characteristics of the underwater channel, such as multipath propagation, fading, and noise. GFDM has been developed to overcome these challenges and provide better performance compared to conventional OFDM [98]. In GFDM, the available bandwidth is divided into several subcarriers, and the information symbols are transmitted in parallel over the subcarriers. The subcarriers are orthogonal to each other and are modulated using a pulse-shaping filter. The pulse-shaping filter is designed to provide good spectral containment and minimize ICI [99]. Figure 12 depicts the transceiver of GFDM for UWAC. One of the key benefits of GFDM is its spectral efficiency. GFDM allows the transmission of multiple symbols in parallel over the subcarriers which results in a higher spectral efficiency compared to conventional OFDM. Additionally, GFDM provides good robustness to time and frequency synchronization errors. The orthogonality of the subcarriers ensures that the receiver can correctly demodulate the symbols even in the presence of synchronization errors. Another benefit of GFDM is its ability to mitigate the impact of ISI. In UWAC, ISI is a major issue due to multipath propagation and fading. GFDM reduces the impact of ISI by using a pulse-shaping filter that provides good spectral containment and reduces ICI [99; 100]. In conclusion, GFDM is a promising modulation technique for  
Figure 12: GFDM transceiver block diagram of UWA. UWAC that provides improved performance in terms of spectral efficiency, robustness in synchronization errors, and mitigation of ISI. Unfiltered Orthogonal Frequency Division Multiplexing  
OFDM is a popular modulation scheme for high-speed data communication over wireless channels.