While underwater acoustic (UWA) communication offers a practical way to establish a wireless link with underwater vehicles, designing a UWA communication system onboard a small autonomous underwater vehicle (AUV) still poses significant challenges. As the adoption of the low-complexity, robust noncoherent communication technology is limited by low bandwidth efficiency and a low data rate, coherent UWA communication requires Doppler mitigation and channel equalization measures to achieve a relatively high data rate in a moving state. Due to the strict constraints of a small-scale AUV in terms of resources and energy consumption, it is not appropriate to use high-complexity Doppler/multipath compensation technology from the prospect of system implementation. In this paper, an efficient and low-complexity UWA differential binary phase-shift keying (DBPSK) system onboard a small AUV is proposed by simplifying the Doppler and multipath compensation. Specifically, for Doppler, the delay of the adjacent DBPSK symbols is calculated according to the Doppler estimate to facilitate delay-tuning Doppler correction. For multipath, low-complexity LMS channel equalization is incorporated with error correction coding to enable multipath mitigation. With a simple structure and low computational complexity, the proposed scheme facilitates the practical hardware implementation and system integration in the small AUV platform. The numerical simulations are conducted to assess the validity of the proposed scheme under different channel conditions and the effectiveness of the proposed scheme is further verified by two UWA communication field tests, which are performed at a practical shallow water sea and lake, respectively.

With the increasing demand for marine sensing information, more and more research has been investigated on underwater wireless sensor networks (UWSNs). Underwater acoustic communication (UWAC) MODEMs which are used to exchange marine sensing data have attracted wide attention, because they are the most important part of UWSNs. Orthogonal signal-division multiplexing (OSDM) is a compromise between single-carrier communication and multiple-carrier communication. The OSDM communication system is less sensitive to multipath and Doppler shift, and has a lower peak-to-average power ratio. The OSDM UWAC recently has been investigated in theoretical analysis, but as far as we know, there is extremely limited research on real-time OSDM UWAC MODEM design. Aiming at addressing this gap, in this paper, a real-time OSDM MODEM is researched and implemented for UWAC, including hardware and software. Specifically, the pre-amplifier, the bandpass filter, the power amplifier, and the signal processor are introduced. Moreover, detailed procedures for modulation and demodulation are also presented. In addition, to accommodate the different underwater acoustic channels and to get the balance between bit error rate and data rate, the parameters such as the number of zero vectors and pilot vectors are investigated. Our OSDM MODEM achieves error-free with the data rate of 4.35 kbps by using the minimum-mean-square-error equalizer and polar decoding under slow time-varying channels, and the communication range reaches larger than 5 km.

Due to its high spectral efficiency and robustness in multipath channels, orthogonal frequency division multiplexing (OFDM) has been considered one of the most promising coherent underwater acoustic (UWA) communication technologies. Channel estimation (CE) plays an important role in OFDM receivers for mitigating the negative impact of UWA channels, unfortunately, the performance of the conventional CE algorithms suffers from significant degradation under time-varying UWA channels due to the unavoidable mismatch between the CE and OFDM demodulation. As channel estimators are generally designed to optimize the signal-level cost functions. However, the results of which are used for bit-level OFDM demodulation. In this article, a minimum bit-error-ratio (BER) sparsity exploitation (MBSE) CE algorithm is proposed under the least-squares framework to address this mismatch. By tuning the sparsity exploitation parameters, i.e., threshold and diffuseness, in a novel manner, the proposed algorithm is designed to adapt to the time-varying UWA channels to improve OFDM demodulation. Specifically, while the sparsity exploitation threshold is obtained via minimum BER searching to determine the dominant multipath arrivals, the structural sparsity parameter, defined as multipath diffuseness, is symbol-wise updated to track the channel variations between the adjacent symbols. Numerical simulation and sea trial experiments verify the performance enhancement of the proposed algorithm in terms of output signal-to-noise ratio, CE mean-square error, and BER under artificial and physical time-varying UWA channels, respectively.

Underwater acoustic (UWA) communication provides an effective way for underwater devices to establish wireless links, which is crucial for the development of internet of underwater things (IoUT). Because non-coherent orthogonal frequency division multiplexing with M-ary frequency shift keying (OFDM-MFSK) adopts energy detection methods without channel estimation and channel equalization, it is not sensitive to the challenges posed by UWA channels, including low signal-to-noise ratio (SNR), large Doppler effect, and time-varying multipath propagation. As a result, it exhibits strong robustness for UWA communication. However, its spectral efficiency is severely limited. In this article, inspired by index modulation (IM) technology, OFDM-MFSK with IM systems are proposed. Specifically, two schemes are proposed: separate spectrum band OFDM-MFSK with IM (SOMI) and time spectrum band OFDM-MFSK with IM (TOMI). SOMI aims to overcome frequency-selective fading by leveraging the performance gain of multiple subbands, while TOMI is suitable for the channels with extremely limited bandwidth at the expense of time slots. To further enhance performance, a soft-decision algorithm, which is easily combined with channel decoding, is adopted for the proposed systems. The combination probability matrices are utilized to calculate the log-likelihood ratio for each bit. Finally, the efficiency of the proposed systems is demonstrated through simulation and sea trial experiments. In particular, by using a single receiving array, the proposed systems achieve error-free communication with a data rate of 2688 bps under an average SNR of 11.88 dB.