# Generalized Frequency Division Multiplexing-Based Low-Power Underwater Acoustic Image Transceiver
###### Abstract
The feasibility of the NOMA scheme in an underwater channel with distance-/frequency-dependent attenuation and frequency-dependent ambient noise was explained. In addition, the performance degradation of the sum-rate maximizing power allocation strategy for downlink underwater NOMA was investigated. NOMA is a possible multiple-access scheme for 6G mobile communication, and GFDM is a highly flexible NOMA [12]. [PERSON] et al. [13] evaluated the BER performance of OFDM, generalized frequency division multiplexing (GFDM), and frequency-shift keying (FSK) UWA modems over a UWA channel model. Simulation results show that UWA communication systems using GFDM have a better BER performance than those using OFDM for the same underwater transmission environments. The advanced wireless communication technology of the future aims to achieve high flexibility, high bandwidth, low latency, and high spectral and energy efficiency. GFDM, using filters with overlapping subcarriers to reduce the spectral broadening of the original signal, is a more flexible communication scheme to achieve the above aims. [PERSON] et al. [14] discussed a generalized digital multiple-carrier transceiver design concept, and adopted the channel bank multi-branch multicarrier design in the GFDM physical layer. [PERSON] et al. [15] demonstrated the BER and symbol error (SE) performance over different channel models. The modulation of independent time-frequency domain blocks was adopted in the GFDM scheme to achieve low latency. GFDM is an advanced physical layer, having high reliability, high energy efficiency, and robust throughput. GFDM provides highly flexible time-frequency block structures using inverse fast Fourier transform (IFFT) and multi-IFFT-symbol (subsymbols) methods for dynamic spectrum allocation. Each block structure had _MK_ samples with \(K\) subcarriers carrying \(M\) subsymbols. An entire block with multiple subsymbols adds a single cyclic prefix (CP), longer than the multipath channel impulse response, to combat multipath channel fading and achieve better spectral efficiency in the GFDM system. The subcarriers were filtered using a root-raised cosine (RRC) filter with circular shifts in the time and frequency domains to reduce out-of-band (OOB) emissions. The next-generation advanced communication techniques face different challenges when used in new applications. [PERSON] et al. [16] demonstrated that the OFDM scheme has difficulty fulfilling demanding requirements. Synchronization, channel estimation algorithms and MIMO techniques for GFDM were demonstrated in detail, and were found to be a suitable transmission method for advanced communication technologies of the future. However, compared to the OFDM approach, GFDM implementation requires more complex multiplications per data symbol to account for the overlapping subsymbols in the time-frequency domain. [PERSON] et al. [17] proposed a low-complexity and high-transmission data rate underwater acoustic OFDM technology. The acoustic OFDM system used differentially coherent detection with a small pilot overhead and experimental data transmitted over a 3-7 km shallow-water channel in the 10.5-15.5 kHz acoustic band. [PERSON] et al. [18] demonstrated design considerations and experimental results for an adaptive OFDM underwater modem. The adaptive binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), 8 phase-shift keying (8 PSK), 16-quadrature amplitude modulation (16-QAM), and power adjustment approach were integrated into the adaptive OFDM underwater communication scheme. [PERSON] et al. [19] conducted UWA communication tests using an OFDM-based data transmission method in a shallow water environment at Wdzydze Lake. The OFDM signal within a band of 5 kHz was transmitted in the frequency range between 27.5 to 32.5 kHz. The underwater OFDM-based system had a carrier frequency of 30 kHz, and used the binary phase-shift keying (BPSK) modulation strategy. [PERSON] et al.