# Energy Harvesting for TDS-OFDM in NOMA-Based Underwater Communication Systems
## 6 Experimental Results and Discussion
### System Throughput Improvement  
The main motivation of the proposed TDS-OFDM-NOMA scheme is to improve the UWA communication system throughput using only the OFDM multicarrier schemes exclusive to the resource allocation as a communication system treatment with both good and poor channel conditions in the same way. While the NOMA scheme allows users to access the resource allocation regardless of their channel conditions, in the NOMA scheme, far and near users have the freedom to access all channel resources. Thus, the channel is utilized by users with both good and bad channel conditions. In this way, the limited frequency channel can be allocated at the same time to different users belonging to the same cell. TDS-OFDM-NOMA improves the overall system capacity. Using the TDS-OFDM scheme in the state of the conventional ZP-OFDM or CP-OFDM in the NOMA scheme improves the total system throughput. The TDS-OFDM improves the subcarrier utilization by avoiding a pilot signal in the transmitted \begin{table}
\begin{tabular}{l c} \hline \hline
**Parameter** & **Value** \\ \hline Transmitted Power (\(P\)s). & 10 Watts \\ \hline Transmission Period (\(T\)). & One second \\ \hline Energy Harvesting Coefficient (\(\eta\)). & 0.7 \\ \hline Power Allocation Coefficients, \(|p_{1}|^{2}\). & 0.6 \\ \hline Power Allocation Coefficients, \(|p_{2}|^{2}\). & 0.4 \\ \hline Carrier Frequency (\(f\)) & 15 kHz \\ \hline Channel Bandwidth (\(B\)) & 6 kHz \\ \hline Modulation order & 2-QAM \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simulation Parameters. \begin{table}
\begin{tabular}{l c c} \hline \hline
**Parameters** & **Near Users (B)** & **Far Users (A)** \\ \hline Surface Height (m) & \((100\sim 50)\) & \((50\sim 10)\) \\ \hline Tx Height (m) & 1000 & 1000 \\ \hline Rx Height (m) & \((100\sim 50)\) & \((50\sim 10)\) \\ \hline Channel Distances & \((10\sim 50)\) & \((50\sim 100)\) \\ \hline Frequency Range (KHz) & \((10\sim 15)\) & \((10\sim 15)\) \\ \hline Doppler Factor (v/c) & \(7\times 10^{-2}\) & \(7\times 10^{-2}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Parameters of the UWA channel model. Figure 6: The time evolution of the magnitude baseband impulse response for (**a**) near UUV at distance 100 m, (**b**) far UUV at distance 800 m.  
subcarrier. Since, in our work, the spectral efficiency analysis is affected by the UWA multipath fading distortion, AWGN, and ambient noise, the signal-to-noise distortion (\(SNDR\)) can be represented in mathematical form as:  
\[SNDR=\text{log}\bigg{(}\frac{P_{\text{s}}}{P_{N}+P_{D}}\bigg{)} \tag{42}\]  
where \(P_{\text{s}}\) is the signal power, while \(P_{N}\) and \(P_{D}\) denote the total underwater noise and the distortion power, respectively. In case there are two users only as we consider in the simulation of this paper, the achievable data rate of NU (\(R_{1}\)) and FU (\(R_{2}\)) in the presence of multipath distortion and underwater ambient noise can be explained as follows:  
\[R_{1}=\text{log}_{2}\left(1+\frac{P_{1}|h_{1}|^{2}}{P_{D}+\chi_{1}+\rho_{1}}\right) \tag{43}\]  
\[R_{2}=\text{log}_{2}\left(1+\frac{P_{2}|h_{2}|^{2}}{P_{1}|h_{2}|^{2}+P_{D}+ \chi_{2}+\rho_{2}}\right) \tag{44}\]  
where \(\chi_{1}\) and \(\chi_{2}\) represent the AWGN of the near and far users, respectively. Similarly, \(\rho_{1}\) and \(\rho_{2}\) are the ambient UWA channel noise of the NU and FU, respectively. \(P_{1}|h_{2}|^{2}\) represents the effect of the NU with a better channel condition and the FU with a poor channel condition. The total system rate of the proposed OFDM-NOMA UWA system can be written as:  
\[R=R_{1}+R_{2}=\text{log}_{2}\left(1+\frac{P_{1}|h_{1}|^{2}}{P_{D}+\chi_{1}+ \rho_{1}}\right)+\text{log}_{2}\left(1+\frac{P_{2}|h_{2}|^{2}}{P_{1}|h_{2}|^{ 2}+P_{D}+\chi_{2}+\rho_{2}}\right) \tag{45}\]  
In conventional CP-OFDM and ZP-OFDM, a group of subcarriers is used for pilot signal transmission; these subcarriers represent a wasted resource. In contrast, all the TDS-OFDM subcarriers are used for data transmission.