# A Survey on Physical Layer Techniques and Challenges in Underwater Communication Systems
## 3 Modeling of UWA Channel Problems and Challenges
### Doppler Shift Estimation  
There are many issues when we implement the UWA multicarrier modulation. The main issues of these challenges are recharging capabilities, low power received, short communication range, high environmental noise, and Doppler shift. The Doppler shift can be considered a severe problem in acoustic UWCs. The UWA modulation is very sensitive for any moving node. The low propagation speed of the acoustic signal underwater causes a high Doppler effect.  
Doppler effects in the UWAC are instigated by the relative velocity of the receiver and transmitter or by movements of the medium for propagation medium [176]. The processing of signals received frequently generates added problems. For example, the UWACs with submarines directing at the speed of nearly 12 m/s, i.e., 25 knots cause the problem due to the huge Doppler shift that must be handled by a capable receiver. Expansion or compression of the signal which has been transmitted over the UWA channel results from the Doppler effects [177]. It is needed to eliminate the effect of these occurrences before any additional processing at the receiver, for instance, carrier recovery and timing. A receiver that executes channel equalization and optimal phase synchronization mutually has been recommended for communication systems underwater [33].  
Figure 18: Doppler shifts varying effects over OFDM subcarriers [175].  
[PERSON] proposed an algorithm to estimate the Doppler shift keeping in view the cross-correlation amongst sequences of training positioned at the start and end of data frames transmitted [176]. A coarse and precise phase is estimated from these training sequences. The precise estimation produces higher accuracy for the estimated Doppler frequency and shows a much smaller range of ambiguity comparable to course one. The researchers found that a suitable arrangement of the precise and coarse phases provides a better estimation of Doppler shift in the considered range of speed. The small carrier frequency of the UWA channel produces Doppler shifts dependent on frequency.  
Contingent to the ZP-OFDM/CP-OFDM null subcarrier, multicarrier permits Doppler compensation and the pilot subcarriers that are involved in the channel estimation, where an OFDM is inspected in the UWA channels of the wider band and non-uniform Doppler shifts [13]. For re-sampling, a method with two steps was suggested tracked by high-resolution steady compensation of the residual Doppler. Through this procedure, the non-uniform Doppler distortion is recompensed. Furthermore, a block-by-block receiver is considered appropriate for fast-varying channels. A technique was established to approximate the offset parameters in the OFDM transmission, relying on the pilot tones accessibility. An estimator for a hybrid channel was recommended later to view the offset that has been compensated [178]. To resolve the problems relevant to the estimation of the channel in the UWA system, the CS method can be used. Dependent on the existence of a sparse design for the treated signal and an over-complete dictionary with a non-orthogonal base set [179], a new class of channel estimator is suggested using the compressed sensing theory. It leads to an estimation of the sparse channel that is highly dependent on Doppler compensation. As a substitute for different compensation methods, the suggested index is designed, when the atoms shift in an over-complete dictionary, using models for Doppler shifts. This technique increases the MSE functioning with lesser computational complexity and reduces the cost of hardware. Moreover, this process has the added advantage of being less sensitive to the variety of Doppler rates.