# 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
Figures 4 and 5 illustrate the block diagram of a typical OFDM transmitter and receiver, respectively. The bank of oscillators revealed in Figures 4 and 5 has been replaced by IFFT and Fast Fourier transform (FFT) blocks which have been computationally effective and easy to implement in transmitter and receiver, respectively. Figure 3: Demonstration of OFDM data in the time and frequency domain. Figure 2: Orthogonality concept of subcarriers in OFDM. Cyclic Prefix Orthogonal Frequency Division Multiplexing  
CP-OFDM is a prevalent modulation technique that is used in UWAC. It is a variant of OFDM that includes a CP which is a copy of the last few samples of each symbol, added to the commencement of the OFDM symbol, as shown in Figure 6. This prefix is used to mitigate the ISI effect caused by multipath propagation which is a common issue in UWC channels. In a UWAC, to transmit the data/information, sound waves are used through the water. The sound waves are affected by several factors, such as reflections, refraction, absorption, and scattering, which can cause attenuation and distortion of the signal. Multipath propagation is a phenomenon where sound waves take multiple paths to reach the receiver, resulting in multiple versions of the same signal arriving at different times. This can cause ISI and reduce the SNR of the signal at the receiving end. Extracting the original information from the received signal will be difficult. CP-OFDM addresses this issue by adding a CP to each OFDM symbol. This prefix contains a copy of the last few samples of the symbol, which allows the receiver to effectively filter out the ISI caused by multipath propagation. The length of the cyclic prefix is chosen based on the expected delay spread of the channel, which is the difference in arrival time between the direct and reflected signals. By choosing an appropriate length for the CP, the receiver can effectively remove the ISI caused by multipath propagation which can improve the performance and reliability of the UWC system. When a transmitting signal is combined with the response of the channel impulse through linear convolution, it can be converted to circular convolution if a CP is added to the input sequence of the channel [59]. This technique is commonly used in CP-OFDM. The OFDM data block is processed using Fourier transform on the transmitter side, while the samples that are analogous to the CP are discarded on the receiver side. CP-OFDM is a popular modulation technique used in UWAC that addresses the issue of ISI caused by multipath propagation. However, several specific requirements and constraints must be considered when designing a UWAC system using CP-OFDM, such as limited bandwidth, interference, power consumption, Doppler shift, and propagation delay. Figure 4: Block diagram of OFDM transmitter [76]. Figure 5: Block diagram of OFDM receiver [76]. CP-OFDM is adapted for UWAC by taking into account the unique characteristics of the underwater environment and the communication channel. Following are some ways through which CP-OFDM is adapted for UWAC. _Cyclic Prefix Length:_ The length of the cyclic prefix is chosen based on the expected delay spread of the channel. In UWA channels, the delay spread can be relatively large due to multipath propagation, so longer cyclic prefixes are often used to ensure effective ISI mitigation. _Modulation Scheme:_ The modulation scheme used in CP-OFDM can be adapted to the specific requirements of UWA communication. For example, a lower-order modulation scheme may be used to reduce the impact of noise and interference, or a higher-order modulation scheme may be used to increase data throughput. _Channel Estimation:_ Channel estimation is an important part of CP-OFDM in UWAC. Since the channel characteristics can change rapidly in UWA environments, adaptive channel estimation algorithms can be used to continuously estimate the channel response and adjust the transmitted signal to optimize performance.