## 2 Physical Layer-Based Full-Duplex Underwater Acoustic Communication Systems
### OFDM Based UWAC Systems
#### 2.2.2 OFDM-Based IBFD Systems  
Researchers introduced the IBFD-OFDM-UWAC systems to allow maximum use of available bandwidth to combat the problem of insufficient bandwidth in previously proposed UWAC systems and thus showed significant results [20, 21]. Despite their numerous benefits, the OFDM-IBFD-UWAC systems suffer due to a PAPR caused by the use of OFDM as a modulation technique. Researchers have used the clip-off signaling technique before amplification to reduce the signal PAPR [23]. Furthermore, because of the FD communication path, IBFD-OFDM based UWAC systems suffer from the most severe SI and SLI from nearby transmitters, which reduces the efficiency of the IBFD-UWAC systems [41, 58], as shown in Figure 3. After a long time, the SLI appears and is exceedingly difficult to suppress. Reflections from the sea's top and bottom cause multiple reflection interference. In an IBFD-UWAC device, the clipped OFDM signal technique is very useful for estimatingthe SI. In recent years, a lot of work has been done to tackle self-interference in IBFD-UWAC systems, which is discussed below. * **HSIC**. By introducing a hybrid analog and digital cancellation mechanism to predict dynamic range saturation, a novel HSIC scheme has been created to eliminate self-loop and multiple-reflection interferences [20]. The analog SI cancellation is applied before the low-noise-amplifier (LNA) [41]. In addition to the analog to digital converter, the digital SI is used to estimate residual SI (ADC). The proposed scheme uses an IBFD communication framework on FD modems, which dramatically reduces SLI and multiple reflection interference (i.e., interference produced due to reflection from the sea surface and seafloor). * **TRDC**. Using time-reversal and soft-decision feedback turbo equalization, a novel TRDC automatic cancellation scheme for the elimination of ISI and RSI is proposed (SDFE) [22]. When compared to the previously used scheme, this proposed scheme had a substantial gain of 2.5 dB and demonstrated significant performance by achieving high spectral efficiency. * **ML-SC-IBFD**. A novel ML-SC-IBFD estimator-based SI channel estimation algorithm is proposed, that takes advantage of sparse constraints and OFDM modulation techniques while using both digital and analog cancellation schemes to suppress the SLI [23]. Experimental results showed that this proposed scheme for IBFD-UWAC demonstrated better BER performance, highly accurate SI estimation, and cancelation performance, as compared to the previous methods. Further, it executed a faster convergence rate than Non-parametric-Maximum-likelihood (NPML), and the lowest level of mean square error (MSE), than that of conventional methods. In contrast to previous methods, experimental results showed that this proposed scheme for IBFD-UWAC demonstrated better BER performance, highly accurate SI estimation, and cancelation performance. It also had a higher convergence rate than Non-parametric-Maximum-likelihood (NPML) and the lowest mean square error (MSE) than traditional methods. * **A-IBFD**. A new A-IBFD framework has been introduced in [24] to remove two major issues in IBFD-UWAC systems: asynchronous contact and non-linear distortion induced by the power amplifier (PA). In the proposed scheme, the researchers devised two cancellation schemes: analog and digital cancellation [59]. However, the authors primarily concentrated on digital cancellation because analog cancellation has an indirect effect on the output of the digital canceler technique's signal-to-interference-ratio (SIR). We discovered that by synchronizing the intended signal with the addition of a spread spectrum non-overlapping area, an over-parameterization algorithm is used to estimate the non-linear SI channel. The proposed scheme outperformed previous SI cancellation schemes in terms of BER, with the added benefit of faster sparse constraints than the Over-Parameterization based Recursive Least Squares (OPRLS).