# Energy Harvesting for TDS-OFDM in NOMA-Based Underwater Communication Systems
## 1 Introduction
In this paper, NOMA is adopted with OFDM and multiple users to allocate on a subcarrier at the same time to maximize the channel utilization. In this paper, the adoption of the NOMA-TDS-OFDM will be addressed and presented for underwater communication to obtain a reliable underwater communication system. Thanks to the simultaneous wireless information and power transfer (SWIPT), the RF signal was utilized in power transfer as well as decoding information [12] in 5G mobile networks based on the NOMA scheme. [PERSON] proposed the main idea of transmitting energy and information at the same time [12]. Based on the capacity-energy function, he provides a fundamental tradeoff performance for power transfer and simultaneous information. SWIPT behaviors in multi-input multiple-output (MIMO) were studied in [13; 14; 15]. In [13], the receivers are divided to be a group for decoding information, and the others are used for harvesting the energy from signals sent via a common transmitter. This idea was extended by considering the imperfect channel state information (CSI) at the transmitter side in [14]. Energy harvesting in MIMO relay systems was studied in [15] where different tradeoffs between the energy transfer and information rates were investigated. SWIPT for multiuser systems was studied in [16] with a receiver constraint for multiple access channels to achieve a maximum sum rate for harvesting relay in the multi-hop channel. SWIPT performance over a wireless communication network was studied in [16; 17]. The hybrid network which overlaps with an uplink cellular network deployed randomly and uses the beacon's power for mobile wirelessly recharge was addressed in [16]. Moreover, ref. [16] derived a tradeoff in network parameters under outage constraint. SWIPT in the cognitive radio networks was addressed in [17]. The nearby transmission of primary transmission is used for harvesting energy of the secondary user under outage constraints to maximize the secondary user system throughput. Unlike terrestrial communications, acoustic signals are used as wireless information transmission (WIT) instead of RF signals. The acoustic-based OFDM needs a long guard interval to avoid intercarrier interference (ICI) in addition to inserting a long pilot's signal to estimate the long-tap delay of the UWA channel. To save the battery power of the underwater nodes, recently researchers preferred the TDS-OFDM as a fit candidate for the underwater multicarrier modulation [7], especially after applying the compressive sensing for estimating the channel impulse response using a small free region of the guard interval unaffected by the inter-block interferences (IBIs) among the OFDM data blocks and guard interval training sequence [7]. In this paper, the corrupted part of the received PN sequence in the received NOMA-TDS-OFDM frame is used for energy harvesting. The long guard interval of the underwater TDS-OFDM helps in increasing the energy harvested using the proposed scheme. The time switching (TS) SWIPT (TS-SWIPT) [18] energy harvesting technique is applied for the proposed acoustic-based NOMA-TDS-OFDM underwater communication system. This paper can be summarized as follows:  
1. This paper presents the TDS-OFDM-NOMA over the UWA channel for the first time according to the authors' knowledge. TDS-OFDM-NOMA is proposed in this paper to improve energy and spectral efficiency. 2. A complete receiver architecture is proposed in this paper for the TDS-OFDM-NOMA. The proposed receiver architecture addresses the Doppler shift estimation and compensation problems plus the UWA channel estimation problem. 3. This paper proposes the time switching simultaneous wireless information and power transfer to harvest the energy of transmitted power over the guard interval in the TDS-OFDM-NOMA scheme. 4. We provide a detailed energy harvesting receiver architecture for the SWIPT in the proposed TDS-OFDM-NOMA. 5.