Due to the simultaneous existence of severe difficulties caused by multi-path, Doppler, and environmental noise caused by underwater acoustic channels, designing a stable and reliable underwater acoustic communication system (UWACS) is a challenging task. In addition to channel estimation and equalization, the technique of channel codes, which is capable of approaching Shannon’s capacity at a low encoding and decoding complexity, offers an efficient and reliable solution to improve the performance of the UWACS and thus draws extensive attention from the research community. By seeking a balance between communication efficiency and channel tolerance via multiple coding rate tuning, it has been recognized that the rate-compatible codes can achieve smooth rate-adaptive transmission under highly diverse underwater acoustic channel states. While there has been a substantial number of investigations on rate-compatible channel codes for the UWACS, in this paper, the research of rate-compatible channel codes has been summarized, reviewed, and compared, mainly focusing on the low-density parity-check (LDPC) and polar codes, which render flexibility in terms of code length and rate. Furthermore, typical research was evaluated and compared from the practical viewpoint of performance under real noise in a shallow water environment. Finally, problems encountered by the LDPC and polar codes for the UWACS were highlighted, and several future research issues were discussed.

The inherent sparsity intrinsic to the underwater acoustic (UWA) channel has been a driving force behind substantial efforts towards improving UWA communication, i.e., exploring the sparsity characteristic in crucial aspects like UWA channel estimation and UWA channel equalization. While extensive empirical evidence obtained from comprehensive sea trial observations supports the recognition that assuming sparsity as a fundamental attribute of the UWA channel is well-founded, the current evaluation measures employed to assess the sparsity of UWA channel primarily rely on subjective analysis, lacking a comprehensive analysis and evaluation of measurement indicators tailored specifically for quantifying sparsity in such channel. In this paper, the issue of measuring UWA channel sparsity is addressed from the perspective of real-world experimental observation. We evaluate three measures for analyzing UWA channel sparsity: the number of multipath components (MPCs), the Gini index, and the Ricean K factor. Although these measures have been used individually in studies related to radio or UWA wireless channel, a comprehensive comparison for quantifying the sparsity of UWA channel is yet to be established. To bridge this gap, we conduct a measurement and analysis of UWA channel sparsity in a shallow water acoustic communication scenario in the real world. By utilizing real-world data collected at a center frequency of 15.5 kHz in Xiamen Harbor, we assess the performance of the aforementioned measures in quantifying UWA channel sparsity with respect to marine environmental behaviors. Our findings demonstrate that these sparsity measures show consistent and reliable results in terms of UWA channel sparsity quantification, which also points to the Gini index as the most suitable sparsity indicator from a practical point of view. Moreover, the relationship between the UWA channel sparsity and marine environmental behaviors, i.e., wind, tide, and wave, is analyzed to validate our findings.

Originating from the concept of cognitive networks (CNs), which are becoming popular in wireless terrestrial communication scenarios, underwater acoustic cognitive networks (UACNs) are drawing more and more attention in the field of the Internet of Underwater Things (IoUT). However, as the implementation of cognitive mechanisms in underwater acoustic networks is different from that of wireless scenarios, it is impossible or difficult for traditional simulation platforms to carry out simulations of UACNs. There is a lack of specialized simulation tools in terms of UACNs. To enable the quantitative evaluation of the effectiveness and performance enhancement of a UACNs in an adverse underwater environment, a simulation platform of acoustic cognitive networks (SPACNet) was designed and investigated in this article. First, based on a state machine-based protocol programming framework, the SPACNet is capable of supporting the implementation of different state-transform types associated with cognitive networking protocols. Moreover, to facilitate the realization of cognitive function at comprehensive levels of signal, information, and link, an underwater acoustic channel model with an environmental parameter input is integrated in SPACNet to generate underwater environment-driven multiple-aspect behaviors. Moreover, a simplified collision model consisting of an environment factor, channel response, and node location is used to reduce the complexity of the simulation of UACNs signal reception. A simulation was carried out to verify the effectiveness of SPACNet in evaluating the cognitive capabilities of UACNs. Finally, a field UACNs experiment was performed to validate the general consistency between the conclusion obtained with the SPACNet-based simulation and that from the field test.