Multipath due to reflections of the sea surface, seabed, and obstacles, as well as inhomogeneity within the ocean, is an important characteristic of underwater acoustic channels. Mutual interference among multiple paths causes severe amplitude fading and frequency selective fading. The guard interval is an effective anti-multipath method, but an excessively long guard interval will reduce the data rate of multi-carrier underwater acoustic communication. In this paper, we propose a new antimultipath multi-carrier communication method based on orthogonal chirp division multiplexing (OCDM) that uses chirp signals for carrier modulation. As OCDM exploits the multipath components for diversity gain, the system robustness is improved. The new method also adds a data pick-based rake receiver for maintaining good communication performance, even at short guard intervals. We detail the implementation and parameter selection of the anti-multipath OCDM system and compare its performance with the traditional orthogonal frequency division multiplexing (OFDM) scheme using simulations; under a severe multipath simulation condition (the delay spread is longer than the guard interval), the anti-multipath OCDM achieves a bit error rate (BER) of 10-6, while the OFDM has a BER floor of 10-3. The simulation results verify the feasibility of the proposed method and the superiority of its anti-multipath performance.

Communication across the water–air interface is an open research problem. A common solution is to place a relay on the water–air interface, which transmits received underwater acoustic signals to shore via electromagnetic waves. However, this solution has limitations, such as surface relays easily floating away or underwater relays becoming resource-intensive due to frequent surfacing, which make water–air communications unstable and costly. In this article, we investigate a recently proposed cross-medium communication technique, translational acoustic radio frequency (TARF) communications, in which underwater nodes can directly communicate with airborne nodes via acoustic waves and millimeter waves. For the first time, we introduce a closed-form, end-to-end channel and present its experimental validation. The high-frequency TARF communication is achievable with a frequency of up to 4.4 kHz. In the experiment deployment, underwater acoustic signals traveled as pressure waves, which produced water surface waves when they impinged on the water–air boundary. Airborne radar was employed to measure and decode these water surface waves. Experimental results and theoretical analysis demonstrated that 1) the cross-medium communication channel is frequency selective and time invariant; 2) cross-medium communications with both single-carrier and multicarrier signals are feasible; and 3) high-frequency signals from 3.6 to 4.4 kHz can be detected, which was considered difficult previously. This study provides a theoretical and experimental basis for achieving cross-medium communications between deep-sea moving objects, such as autonomous underwater vehicles and airborne nodes.

With the rapid development of autonomous underwater vehicles (AUVs) and the continuous improvement of marine exploration requirements, AUV formation has emerged as a promising technique for performing underwater tasks with greater flexibility, adaptability, and scalability. To achieve AUV formation control, underwater acoustic communication networks (UACNs) are widely used to support information exchange among AUVs. Considering broadcast communications in fully connected mobile UACNs, existing medium access control (MAC) protocols face severe packet collision risk or low reuse efficiency. To improve the timeliness and reliability of the information exchange among AUVs, we develop an efficient scheduling-based adaptive broadcasting MAC (AB-MAC) protocol. The AB-MAC protocol is incompletely centralized, which can be reflected in two aspects. First, to ensure the consistency of the transmission schedule in the network, the AB-MAC protocol selects a control node for each frame to schedule the transmission of all nodes according to the updated network topology. The slot lengths and transmission sequences are changeable in each frame to avoid packet collisions and minimize frame lengths. Second, to further improve the efficiency and robustness of the protocol, all nodes adaptively adjust their transmission time by integrating the information contained in their previously received control packets and ordinary packets. The transmission scheduling problem is formulated as a combinatorial optimization problem, which can be solved by our proposed improved Genetic Algorithm. Numerical results demonstrate that the AB-MAC protocol not only performs well in terms of network throughput and average update interval but also has a high level of robustness even in low-quality underwater acoustic channels.

Underwater acoustic sensor networks (UASNs) have emerged as a viable networking approach due to their numerous aquatic applications in recent years. As a vital component of UASNs, routing protocols are essential for ensuring reliable data transmissions and extending the longevity of UASNs. Recently, several clustering-based routing protocols have been proposed to reduce energy consumption and overcome the resource constraints of deployed sensor nodes. However, they rarely consider the hot-spots problem and the sink node isolation problem in the multihop underwater sensor networks. In this paper, we propose a Q-learning-based hierarchical routing protocol with unequal clustering (QHUC) for determining an effective data forwarding path to extend the lifespan of UASNs. First, a hierarchical network structure is constructed for initialization. Then a combination of unequal clustering and the Q-learning algorithm is applied to the hierarchical structure to disperse the remaining energy more evenly throughout the network. With the use of the Q-learning algorithm, a global optimal CH and next-hop can be determined better than a greedy one. In addition, the Q-value that guarantees the optimal routing decisions can be computed without incurring any additional costs by combining the Q-learning algorithm with clustering. The simulation results show that the QHUC can achieve an efficient routing and prolong the network lifetime significantly.

Frequency-domain equalization (FDE) is a computationally efficient method to recover underwater acoustic single-carrier transmissions in a multipath environment. The FDE receivers often operate in a block-by-block manner, assuming that the channel is time invariant during a block duration. This assumption implies that traditional block-based FDE receivers experience performance degradation in fast time-varying channel conditions, since the interfrequency interference (IFI) is often not considered. Here, we focus on the IFI cancelation (IFIC) in single-carrier transmissions. The IFI formulations are developed for a blockwise FDE receiver, where the time reversal (TR) processing is incorporated to alleviate the interblock interference. Based on the formulations, the IFI is estimated by the time-varying impulse responses, which are obtained from an adaptive basis expansion model. A soft two-step IFIC strategy is then employed to address the IFI. In the first step, the main IFI is removed by the time-varying TR processing and explicit IFI cancelation. The residual IFI is suppressed by the frequency-domain decision feedback equalization (FD-DFE) in the second step. To further enhance the communication performance, we develop a soft-symbol-based iterative receiver, referred to as IFIC-FD-DFE. The impulse responses, explicit IFI, and residual IFI are updated in an iterative fashion, based on the soft symbol estimates. The proposed receiver has been tested using the at-sea measurements collected at the Gulf of Mexico in August 2017. Communication packets at four ranges at the carrier frequency of 160 kHz with a symbol rate of 32 kHz have been decoded. The effectiveness of the IFIC-FD-DFE receiver has been demonstrated via comparisons with several FDE schemes in the current literature.

Due to the power and transmission rate limitations, it is challenging to achieve underwater acoustic (UWA) communications over long distances from the unmanned underwater vehicle (UUV) to the shore station. As an alternative solution, the distributed acoustic sensing (DAS) underwater communication uses submarine optical cable to acquire and demodulate acoustic communication signals. There is little research on the channel and noise of DAS underwater communication. This paper analyzes the channel characteristics and the background noise of a DAS underwater communication system with armored optical cable based on the experimental measurements. Channel analysis shows that the envelope amplitudes of impulse responses follow the Burr distribution. The amplitudes get increased concentration at lower values as the sound source goes closer to the position right above the optical cable. Moreover, we observe that the received signals are influenced by the adjacent segments, leading to multiple arrivals within each acoustic bounce. Noise analysis shows that for frequencies lower than 1 kHz, unlike the Wenz noise spectrum primarily caused by shipping and wind, the −18 dB/octave decrease in power spectral density (PSD) of DAS noise is mainly caused by the equipment and cable themselves. The noise also exhibits full-band fluctuations. In underwater communications, full-band noise poses significant challenges for signal design. As far as we know, this article is the first report on the field experiment using a DAS underwater communication system equipped with armored optical cable, providing a reference for future underwater applications.