## 2 Literature Review  
Underwater acoustic communication is a challenging area due to the unique characteristics of underwater channels, including multipath propagation, high propagation delays, and the presence of ambient noise [7]. Due to its ability to effectively address the challenges mentioned, OFDM has emerged as a leading contender in the domain of multicarrier modulation [29; 39]. OFDM divides the transmission bandwidth into multiple orthogonal subcarriers, providing robustness against multipath fading and enabling efficient data transmission [40]. OFDM's effectiveness in combating the adverse effects of an underwater channel has been explored extensively. It offers advantages such as resistance to multipath fading and support for high data rates [40]. Research has focused on adapting traditional OFDM to an underwater channel's unique characteristics, including the use of frequency-domain equalization and adaptive modulation schemes [41; 42]. OFDM systems in underwater environments require robust channel estimation and equalization techniques due to time-varying and frequency-selective channels [43].  
In [44], the authors proposed a low-complexity scheme for PAPR reduction in underwater acoustic OFDM systems using XOR-ciphered BCH codes. The system enhances the reliability of communication through the use of error-correction codes and improves the PAPR of the system. In CPM-based modulators, correlation among the symbols is exploited through the continuity of phase from one symbol to another adding memory to the system [31]. Some of the recent works in the wireless communication domain [45; 46; 47] have focused on CPM modulators. The CPM modulator is a kind of finite-state machine that sends a continuous phase, constant envelope waveform to the channel based on the input stream and the internal state. Moreover, the authors in [24] evaluated a CPM-OFDM transceiver for the first time in the context of UWA channels. The traditional memoryless mapper was replaced with a CPM-based mapper and a Viterbi decoder was employed at the receiver side. Monte Carlo simulations over a realistic acoustic channel model suggested that the proposed setup can reliably communicate at distances of up to 1.5 KM without an equalizer and channel coding. Most of the previous implementations have been in the domain of radio frequency transmissions, such as [48], where the authors demonstrated the viability of an OFDM transceiver for aeronautical telematics using convolutionally encoded CPM. By exploiting space-time coding in a multiple input multiple output (MIMO) context, the CPM decoding complexity can be significantly reduced [49]. The authors in [50] had evaluated a CPM-based OFDM system for added a white Gaussian noise (AWGN) channel and proposed that the modulation index of 0.75 had superior performance as compared to the other modulation indexes used in their proposed system. The results were validated using a software defined radio system. According to a systematic review of the literature, no study has been conducted to date that examines the performance of coded CPM-OFDM for BER enhancement across UW channels. As a result, the proposed effort is the first of its type and adds considerably to the body of knowledge.