# Energy Harvesting for TDS-OFDM in NOMA-Based Underwater Communication Systems
## 1 Introduction  
The underwater acoustic (UWA) channel is considered one of the most challenging channels due to its significant multipath spread, limited available bandwidth, poor physical link quality, and time-varied channels caused by Doppler effects. Therefore, multicarrier modulation is presented as an attractive solution to improve the utilization of the limited channel bandwidth and mitigate the wide delay spread of the underwater acoustic (UWA) channel. However, a significant Doppler's effect presented in UWA communication leads to high inter-carrier interference (IC) [1, 2, 3] in underwater multicarrier modulation. The most attractive multicarrier modulation technique used in UWA communication is orthogonal frequency division multiplexing (OFDM) due to its high capability of dealing with the UWA long multipath spread channel using low complex time-domain equalization technique used in single carrier communication systems [1, 4, 5]. Four schemes of OFDM are introduced for the UWA channels based on the guard interval insertion way: (1) cyclic prefix OFDM (CP-OFDM) [4], where the last part of the symbols in the transmitted signal is added on ahead of the transmitted frame; (2) zero padding OFDM (ZP-OFDM) [6]in this scheme, where the CP part is replaced by zeros; (3) time-domain synchronous OFDM (TDS-OFDM) [7], where the guard interval is used to transmit a pseudorandom noise (PN) training sequence known to both the transmitter and receiver; (4) the OFDM multicarrier technique without any guard intervals [1]. Each one of these multicarriers has its pros and cons. The CP-OFDM uses long pilots and guard intervals, and both are wasted on the spectral and energy of the UWA communication system. On the other side, CP insertion leads to a circular convolution instead of a linear convolution between the transmitted signals and the communication channel. Hence, such circular convolution relaxes the equalization. The ZP-OFDM was the preferred multicarrier communication technique in the UWA systems as it saves the power used in the guard interval transmission spatially, as the UWA communication systems are battery-based. However, ZP-OFDM is still suffering from the wasted spectral efficiency in the guard interval and pilots' signals; plus, it suffers from the on-off hardware problem. Thanks to efficient synchronization and high energy/spectral efficiency, the TDS-OFDM system has been proposed for underwater communication, but it is suffering from the inter-block interference (IBI) problem due to the mutual interference between the training sequence used as a guard interval and OFDM data blocks. The no guard interval OFDM (NGI-OFDM) is an OFDM scheme without any guard intervals proposed for UWA communication to improve the channel utilization [1], but the NGI-OFDM scheme wastes high energy in the pilot's needs for equalization, as it wastes half of the signal energy used in pilots. Moreover, it cannot provide a reliable communication system for the underwater channels due to the Doppler estimation task without any guard interval becoming a complicated task dominating the complexity of the receiver [8]. Recently, the non-orthogonal multiple access (NOMA) was proposed for the fifth-generation (5G) mobile network to increase spectral efficiency [9]. The NOMA spectral efficiency is compatible with the existing and future wireless communication networks. The conventional NOMA is a power domain system under the principles of superposition coding (SC) and successive interference cancellation (SIC) at the transmitter and receiver sides, respectively [10]. Compared to the orthogonal multiple access (OMA), where the number of users is limited due to the exclusivity of orthogonal resources, NOMA allows the allocation of the same orthogonal resources to more than one user simultaneously. The NOMA scheme was proposed for underwater communication to improve the utilization of the limited UWA channel [11].