Orthogonal chirp division multiplexing (OCDM) has recently become an appealing scheme for high-speed communications. However, due to the spreading feature of OCDM systems in both time and frequency domains, the channel estimation for OCDM systems with carrier frequency offset (CFO) is challenging. In this letter, we deal with a cyclic-prefix (CP) OCDM system. The CFO is first compensated by making use of the CP. Sequentially, a channel estimator based on superimposed pilot subchirps is proposed. The optimality of the pilot design in minimizing the mean squared error of the channel estimation is proved. The proposed approach is compared with the state-of-the-art and demonstrates superior performance.

Orthogonal chirp division multiplexing (OCDM) has been proposed as a novel modulation scheme to take advantage of its double-spreading feature in both the time and frequency domains. It shows more resilient against different impairments. However, it is mainly investigated under time-invariant channels. In this paper, we propose a channel estimation method based on pilot chirps for OCDM transmissions under time-varying multipath channels. Relying on a basis expansion model (BEM), the impact of OCDM modulation and demodulation on time-varying multipath channels is thoroughly analyzed. Furthermore, the pilot chirp assisted OCDM (pca-OCDM) is designed to minimize the minimum mean square error (MMSE) of the proposed MMSE channel estimator. The proposed channel estimation approach is compared with the state-of-the-art and demonstrates superior performance.

Underwater acoustic channel (UAC) is featured as fast time-varying characteristic, and challenges the transmission designs. To countermine the time variation effect, we propose an adaptive design for orthogonal frequency division multiplexing (OFDM) transmission systems by utilizing the long-term stability of the second-order statistics of the channel state information (CSI). We derive the analytical expression of signal-to-interference-plus-noise-ratio (SINR) at each subcarrier to reach the target error performance based on the statistical information of the CSI. Thereafter, a new adaptive coding and bit-power loading algorithm with low computational complexity is proposed to pursuit the highest achievable bit rate with fixed error rate. The validity of SINR calculations and the effectiveness of the proposed adaptive algorithm are demonstrated under various conditions, wherein both simulated and measured channels have been tested.

Autonomous underwater vehicles (AUVs) equipped with acoustic modems are currently one of the important means of obtaining underwater environment data such as sounds and images. They rely on high-speed underwater acoustic (UWA) communication technology for data transmission tasks. Orthogonal chirp division multiplexing (OCDM) is a multicarrier method based on chirp spread spectrum (CSS) which has better reliability in frequency selective fading channels than orthogonal frequency division multiplexing (OFDM). In this work, we combine index modulation (IM) and propose an OCDM-IM system that provides faster data rates when it activates the same number of subchirps as OCDM. On the other hand, it activates fewer subchirps when the data rate is the same, so it has better anti-interference capability. The simulation results under a measured UWA channel and random channels and the experimental results show that OCDM-IM has a better bit-error rate (BER) performance than that of OCDM and OFDM-IM systems when the subchirps are not fully loaded. In particular, when a small number of subchirps are activated, OCDM-IM is able to provide up to 2x bit rate with more than 5 dB improvement in BER performance. Our simulation and experimental results also show that IM has much better reliability than constellation mapping when Doppler effect is considered. Therefore, OCDM-IM is more suitable for the communication of AUVs.

Due to the limited bandwidth and severe noise interference in underwater acoustic channels, underwater image transmission typically requires a coding scheme with a high compression ratio and strong robustness. However, existing semantic communication methods are predominantly designed for terrestrial wireless scenarios and do not fully consider the unique characteristics of underwater images and channel features. To address these problems, we propose an efficient semantic communication scheme of underwater images guided by physical priors. Specifically, we design a semantic encoder-decoder with a hybrid architecture of CNN and Swin Transformer for efficient semantic feature extraction and reconstruction. Meanwhile, we design an underwater physical prior fusion module, which leverages physical prior knowledge from underwater imaging to optimize the encoding process, preserving and enhancing essential semantic features. Additionally, a noise-aware training strategy is proposed to guide the network in learning the noise characteristics of measured underwater acoustic channels, improving its robustness in intricate marine conditions. Extensive experiments verify that our method outperforms existing schemes regarding transmission efficiency and image quality.

Heart rate is a crucial physiological indicator for fish, but current measurement methods are often invasive or require delicate manipulation. In this study, we introduced two non-invasive and easy-to-operate methods based on photoplethysmography, namely reflectance-type photoplethysmography (PPG) and remote photoplethysmography (rPPG), which we applied to the large yellow croaker (Larimichthys crocea). PPG showed perfect synchronization with electrocardiogram (ECG), with a Pearson's correlation coefficient of 0.99999. For rPPG, the results showed good agreement with ECG. Under active provision of green light, the Pearson's correlation coefficient was 0.966, surpassing the value of 0.947 under natural light. Additionally, the root mean square error was 0.810, which was lower than the value of 1.30 under natural light, indicating not only that the rPPG method had relatively high accuracy but also that green light may have the potential to further improve its accuracy.