# A Survey on Physical Layer Techniques and Challenges in Underwater Communication Systems
## 2 UWAC Physical Layer Techniques
### MIMO-OFDM
#### 2.1.2 Filter Bank Multicarrier Modulation  
FBMC is suggested to deploy the OFDM scheme with long CP to combat the time dispersion in the UWA channel. Furthermore, OFDM data symbols are longer than CP length by at least four times and are utilized to attain high data transmission with efficiency in bandwidth. FBMC has deliberated the substitution of conventional OFDM schemes for spectrum pooling cognitive radio because of the small power leakage of the sideband [103]. This informs the OFDM that the channel is highly time-varying for each OFDM data symbol which might be unacceptable, and this results in the ICI of a significant level [104]. Over the doubly dispersive UWA channel, FBMC is the candidate which results in robust performance [104]. Twenty percent of every OFDM data symbol is allocated for the CP in the OFDM system, so it has a loss of transmission bandwidth efficiency. In the OFDM system, the length of CP must be equivalent to one-fourth of every FFT block length. Furthermore, at least the CP length must be considered equal to the length of the CIR; generally, the UWA channel is long because in the UWAC the length of the data symbol of the OFDM system is very long [104]. As a Nyquist filter, the prototype filter \(\rho(t)\) is intended to evade the ISI in FBMC systems [105; 106]. The proposed method in [107] creates an isotropic filter according to the following equation:  
\[\rho(t)=\sum_{\kappa=0}^{L}a_{\kappa}h_{\kappa}(t) \tag{6}\]  
where, \(h_{\kappa}(t)\), the set of Hermit's functions is defined as [104]:  
\[h_{n}(t)=\frac{1}{\left(2\pi\right)^{n/2}}e^{\pi t^{2}}\frac{d^{n}}{dt^{n}}e^{ -2\pi t^{2}} \tag{7}\]  
The author in [104] shows that the existence of the channel will generate \(A_{\rho}^{d}(\tau,v)\), i.e., a disturbing ambiguity function with smeared-out null points \(A_{\rho}^{d}(\tau,v)\). The robust prototype filters are created by relaxing the constraints on the null points of \(A_{\rho}(\tau,v)\), the ambiguity function. Each null point is substituted by a null region in the \((\tau,v)\) plane. Thus, one should choose to minimize the cost function of designing the \(\rho(t)\):  
\[\zeta=\int_{A_{0}}\big{|}A_{\rho}(\tau,v)-1\big{|}^{2}d\tau dv+\sum_{\kappa=1} ^{N}\gamma_{\kappa}\int_{A_{\kappa}}\big{|}A_{\rho}(\tau,v)\big{|}d\tau dv \tag{8}\]  
where, \(\gamma_{\kappa}\) are positive weighting factors set, \(A_{0}\) is the region around \((\tau,v)=(0,0)\) over which the peak of \(A_{\rho}(\tau,v)\) remains equal to one approximately, and \(A_{\kappa}\) are sets of null regions, where \(\kappa=1,2,...N\).  
The FBMC system class has been proposed for the UWAC [104]. This FBMC system was developed for robust performance in contrast to channel dispersion in frequency and  
Figure 13: Transceiver block diagram of CPM-OFDM for UWAC [101].  
time domain. When we compare the OFDM with the FBMC system, the FBMC system performance is much better in terms of bandwidth saving and high spectral efficiency than OFDM [104]. In the scenario of single-user communication, the computational complexity of OFDM is lower than FBMC but the bandwidth efficiency of FBMC is higher. FBMC can be used in definite MIMO setups while OFDM offers full flexibility in MIMO communications. The flexibility is similar to the OFDM and is only presented using filtered multi-tone modulation (FMT). Though both OFDM and FMT undergo a similar loss of bandwidth [108], FBMC modulation can feasibly replace OFDM in the UWAC and is considered a capable contender for systems of communication. Authors in [109] state the estimation of the channel through auxiliary and coding pilot methods for underwater FBMC systems. It is suggested that the multipath UWAC is considered with additive white Gaussian noise. FBMC for UWC has not yet been the emphasis of research and thus will require further investigation in the future.