Unique Word OFDM With Joint Time-Frequency Channel Estimation for Internet of Underwater Things

The acoustic-based Internet of Underwater Things (IoUT) is considered as one of the most challenging environments for communication because of issues in the underwater acoustic communication (UWAC) channel, such as multipath propagation and Doppler shift (DS). Accurately estimating these effects without sacrificing a significant portion of the bandwidth is extremely difficult, underscoring the need for robust and sophisticated techniques. In this article, we propose an acoustic-based unique word orthogonal frequency division multiplexing (UW-OFDM) scheme to enable communication between IoUT nodes over a doubly-selective channel. The proposed scheme employs a joint time-frequency channel estimation approach by leveraging the time-domain guard interval to identify the channel paths while utilizing only 3.1% of the frequency-domain subcarriers, compared to 25% in conventional methods, to track the channel path coefficients. The proposed method significantly enhances spectral efficiency while maintaining resilience to multipath propagation and DS impairments inherent in UWAC. Furthermore, the scheme eliminates interblock interference, which is critical in UWAC due to its distinctive propagation characteristics. We evaluate the performance of the proposed methods using both simulations and real-world experimental tests over a 300-m underwater channel. The results demonstrate that the proposed approach offers a reliable IoUT communication solution, achieving up to a 5 dB improvement in bit error rate and up to 17.56% higher subcarrier utilization compared to conventional schemes. In addition, the scheme exhibits strong robustness against DS effects with similar peak-to-average power ratio performance and a modest increase in computational complexity.