Underwater acoustic (UWA) communications have attracted a lot of interest in recent years motivated by a wide range of applications including offshore oil field exploration and monitoring, oceanographic data collection, environmental monitoring, disaster prevention, and port security. Different signaling solutions have been developed to date including non-coherent communications, phase coherent systems, multi-input and multi-output solutions, time-reversal-based communication systems, and multi-carrier transmission approaches. This paper deviates from the traditional approaches to UWA communications and develops a scheme that exploits biomimetic signals. In the proposed scheme, a transmitter maps the information bits to the parameters of a biomimetic signal, which is transmitted over the channel. The receiver estimates the parameters of the received signal and demaps them back to bits to estimate the message. As exemplary biomimetic signals, analytical signal models with nonlinear instantaneous frequency are developed that match mammal sound signatures in the time-frequency plane are developed. Suitable receiver structures as well as performance analysis are provided for the proposed transmission scheme, and some results using data recorded during the Kauai Acomms MURI 2011 UWA communications experiment are presented.
The acoustic characteristics of underwater cylindrical Helmholtz resonator are analyzed theoretically. Based on the theories of electro-acoustic analogy, a low frequency lumped-parameter model of the Helmholtz resonator is constructed with due consideration of the effects of the elasticity and the radiation impedance of the resonator. To our knowledge, this is the first time such a complete model is constructed. The input impedance and the transfer function of the system are given by circuit analysis. The effects of parameter values of the resonator on the acoustic characteristics are studied by numerical method. Some useful conclusions are drawn. A small aluminum cylindrical Helmholtz resonator is measured in a standing-wave tube filled with water. Error analysis is made in detail. The experimental results are in agreement with the simulation results considering the effect of the piezoelectric hydrophone. The validity of the theoretical analysis is testified. This paper supplies a theoretical and experimental basis for the design of underwater cylindrical Helmholtz resonators, and is useful for the estimation of underwater acoustic performance of Helmholtz resonators of other shapes.
Nonlinear frequency-modulated signals have been effectively applied in multiple-user communication schemes. In this paper, we propose to implement a multiple-user communication scheme using hyperbolic frequency-modulated (HFM) signals for underwater acoustic communications. We derive constraints on the HFM parameters to optimally reduce multiple access interference (MAI) at the transmission side. Additional constraints on the frequency-modulation (FM) rate reduce the underwater channel effects of multipath and scaling. The proposed signaling scheme is compared to an HFM-based code-division multiple-access (HFM-CDMA) scheme to demonstrate improved error performance. MOTIVATION AND RELATION TO PRIOR WORKUnderwater acoustic (UWA) communications methods, such as orthogonal frequency-division multiplexing (OFDM) and code-division multiple access (CDMA), have been widelyused for multiple users [1][2][3][4]. As the UWA communications channel is highly time-varying, it can cause undesirable distortions, such as multipath and Doppler scaling, under certain conditions [5][6][7]. Existing UWA communications schemes do not fully exploit matching the UWA channel characteristics to signaling schemes and often have to compensate for that at the receiver. In [1], the distorted OFDM signal due to Doppler is compensated at the receiver using resampling, and any remaining Doppler is removed using inter-carrier interference reduction techniques. A multiple resampling OFDM receiver front-end is designed in [2] such that each resampling branch deals with a different Doppler factor. Adaptive multi-user detection techniques are applied in [3], but their assumed UWA channel model does not include multiple Doppler paths at the receiver. A spread spectrum hyperbolic frequency modulation scheme in [8] can potentially match the UWA channel but only assumes one Doppler scale path and does not compensate for multiple time delay paths that can distort any potential scale diversity. In [9], we design an HFM scheme to match the This work is funded by the National Science Foundation grant NSF-ECCS 1102357.UWA communications channel. The HFM scheme is integrated with the use of a discrete time-scale canonical model, which represents the received signal from a wideband channel as a linear superposition of discrete time shifts and Doppler scalings of the transmitted signal, weighted by the wideband spreading function (WSF) [10]. We demonstrate an inherent joint multipath-scale diversity using this scheme, and we combine it with direct sequence CDMA for use in multi-user UWA communications. The different users are distinguished by a unique pseudo-noise (PN) sequence [9]. As we demonstrate, the scheme can successfully resolve information bits at the receiver side, but the performance degrades as the number of users increases.A nonlinear frequency-modulated signaling scheme is developed in [11] by deriving constraints on the nonlinear phase function of the transmit signal to achieve orthogonality; the scheme, combined with frequency-hopping CDMA, is use...
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