In this paper, we proposed an embedded side information (SI) transmission scheme based on polar code construction for PAPR reduction using the PTS scheme in the underwater Acoustic (UWA) Orthogonal Frequency Division Multiplexing (OFDM) communication system. We use polar codes due to the ability of the arbitrarily designed code rate. Additionally, polar codes can be employed to establish a nested code structure consisting of multiple subsets. The SI bits can be embedded in a polar codeword by exploiting these features. Thus, the approach does not occupy existing data rates or cause additional loss in data transmission rates. At the same time, it embeds m-sequence into the polar code as an indicator vector for the blind SI detector, which makes the blind SI detector able to autonomously discriminate SI at the receiver. Simulation and tank experiment results indicate that the proposed embedded SI transmission scheme has the potential to significantly decrease the likelihood of whole-symbol error caused by SI errors. Meanwhile, the proposed PTS scheme eliminates the need to wait for the entire packet to be received before obtaining the SI, thereby preventing waste of data storage devices and ensuring real-time performance of the Underwater Acoustic Communication (UAC) OFDM system. This achieves symbol-level real-time calculation for the system.

In underwater acoustic (UWA) orthogonal frequency division multiplexing (OFDM) systems, the orthogonality among subcarriers is highly susceptible to Doppler-induced scaling, leading to severe inter-carrier interference (ICI). This paper proposes a coarse-to-fine Doppler estimation approach for coded orthogonal frequency division multiplexing (OFDM) systems operating in underwater acoustic (UWA) channels. The proposed method first employs the fractional Fourier transform (FRFT) to obtain an initial Doppler factor estimate from a linear frequency modulation (LFM) probe, exploiting the energy concentration property of chirp signals in the fractional domain. This coarse estimate then guides a refinement stage that leverages the cyclic prefix (CP) inherent to each OFDM symbol, enabling symbol-by-symbol Doppler tracking without waiting for the entire packet. As a result, the required memory and processing latency are substantially lower than with full-packet resampling or iterative gradient-descent alternatives. Numerical simulations conducted under both time-invariant and time-variant Doppler conditions demonstrate that the proposed scheme achieves a mean squared error (MSE) below 0.5% at signal-to-noise ratios (SNR) of 5 dB and above. Moreover, the bit error rate (BER) remains within 0.2 dB of an ideal Doppler-free system at a BER of 10⁻³. The combination of low storage demand, symbol-level operation, and robust performance makes the proposed method well-suited for real-time underwater acoustic communication.

In this paper, a partial transmit sequence (PTS) technique with a side information (SI) blind detector based on minimized error accumulation for PAPR reduction in the coded underwater acoustic (UWA) orthogonal frequency division multiplexing (OFDM) communication system is proposed. Due to the complexity of UWA channels, channel coding is inevitably employed to ensure accurate information transmission. However, the error correction capability of channel coding usually has a certain margin; the decoder will fail to provide the expected coding gain for the system when the number of error bits exceeds the error correction capability. Therefore, by utilizing the accumulation impact of bit errors inherent in channel coding, it can be regarded as an indicator vector for the SI blind detector, enabling autonomous identification of the weighted phase factor vector sequence index without prior information. Thus, there is no need to reserve the SI transmission symbols, as required by the conventional PTS (C-PTS) scheme, effectively reducing the probability of large-scale bit error caused by the wrong SI. The experimental results show that the BER performance of the system has seen an improvement of about one order of magnitude compared to C-PTS. Consequently, the proposed technique enhances system bandwidth utilization and communication efficiency, ensuring the real-time performance of the underwater acoustic communication (UAC) OFDM system.