# Energy Harvesting for TDS-OFDM in NOMA-Based Underwater Communication Systems
## 4 Energy Harvesting for TDS-OFDM-NOMA  
In general, the UWA channel is a long-tap delay channel, and a long guard interval is required between transmitted OFDM blocks to avoid intercarrier interference (ICI); plus, a long pilot signal is needed to estimate such a channel type. To save the battery power of the underwater nodes, recently researchers preferred the TDS-OFDM as a fit candidate for underwater multicarrier modulation, especially with compressive sensing-based channel estimation to estimate the UWA channel impulse response. The small free region of the guard interval unaffected by the IBI between the OFDM data blocks and the training sequence is used for channel estimation purposes. In Figure 4, the received signal frame structure of different TDS-OFDM schemes is shown. To the best of our knowledge, all research has been focused on removing mutual interference between the OFDM data blocks and the PN training sequence for better synchronization and channel estimation. In addition, no one is working on saving energy from the received PN sequence. In this paper, we plan to use the wasted power of corrupted parts in guard intervals for energy harvesting. This paper works on energy harvesting for the underwater node using SWIPT in the time switching energy harvesting mode. In this paper, the TS energy harvesting system is proposed to be used for the TDS-OFDM-NOMA system as shown in Figure 5. As shown in Figure 5, with the TS-mode, the first \(N\) data symbols, all the received power will be used for information decoding. As this OFDM subcarrier is carrying the data information, all signal power received is sent to  
Figure 4: Received signal frame structure. (**a**) Conventional TDS-OFDM, (**b**) DPN-TDS-OFDM, (**c**) CS based TDS-OFDM. the information receiver to achieve acceptable BER performance. For the residual OFDM frame, \(M\) PN symbols will be divided into two parts, the first part, the corrupted part \(L-1\); in this part, all signal power will be used for energy harvesting, as it is less information, and it highly corrupted by the OFDM data block. In the second part of the received guard interval (the remaining uncorrupted part), \(F=M-(L-1)\), all signals will be used for information decoding, and they will be used for channel estimation and equalization. Thus, the TS energy harvesting mode can be used in this paper as:  
\[\rho_{k}=\left\{ \begin{array}{c}0,\;k=1,\;\cdots\cdots,N\\ 1,\;k=N+1,\;\cdots\cdots,\;N+L-1\\ 0,\;k=N+L,\;\cdots\cdots,\;N+M\end{array} \right. \tag{30}\]  
In this way, the wasted power of the long guard interval (nearly 25% of OFDM transmitted power is wasted in guard interval) can be used in harvesting energy to increase the lifetime of the Internet of Underwater Thighs (IoUTs) nodes. The total energy harvested based on the proposed architecture can be written as:  
\[EH_{TDS}=\frac{T_{c}\eta Ps|h|^{2}}{2(1+b^{\alpha})} \tag{31}\]  
where \(T_{c}\) is the period of the corrupted PN sequence, and \(\eta\) is the energy harvesting coefficient. \(Ps\) is the sink node transmitted power; \(b\) is the distance between the transmitter and receiver; and \(\alpha\) is the underwater exponent path loss. As shown in Figure 5, the energy receiver will be dialed with the corrupted part of the received PN sequence. On the receiver side, we consider the case where the receiver is solely decoding the information and harvesting the energy from the received signal simultaneously. The received acoustic signal, \(y(t)\), is first converted to a complex baseband signal \(y_{b}(t)\) and then sampled and digitalized by an analog-to-digital converter for further decoding. The noise introduced by the in-band harmonic distortion introduced by the RF band to based band signal conversion \(n_{cov}(t)\) has \(n_{cov}\sim\mathcal{CN}\big{(}0,\sigma_{cov}^{2}\big{)}\). In our simulation and for simplicity, we assume an ideal ADC without any noise.