Pierre-Jean Bouvet

Capacity analysis of underwater acoustic MIMO communications

First introduced in the field of radio communications, Multi-Input Multi-Output (MIMO) principle consists of transmitting digital data from Nt transmitters to Nr receivers within the same frequency band. During the last decade, theoretical works and real-life experiments in wireless local area and cellular networks have demonstrated and confirmed that MIMO was a breakthrough in digital communications. Recently, MIMO principle has been applied to underwater acoustic communications (UAC) with promising results. However, only few work have been already done on the expected gain of MIMO over underwater channel. Main objective of our paper is on the one hand to quantify theoretically the MIMO gain in underwater acoustic channel by using Shannon capacity analysis and on the other hand to give guidelines to optimize MIMO underwater system with respect to capacity maximization.

Least Square and Trended Doppler Estimation in Fading Channel for High-Frequency Underwater Acoustic Communications

Hermes is an asymmetrical point-to-point underwater acoustic modem designed for short-range operations at very high bit rates in ports and shallow waters using broadband acoustic signaling (262-375 kHz). In exploring the possible conversion of Hermes into a multiple-input-multiple-output (MIMO) device, single-carrier phase-modulated spread-spectrum sequences were used for channel estimation and deconvolution purposes. It clearly appeared that the channel estimation and deconvolution routines were quite sensitive to rapid time changes in the acoustic channel impulse response (CIR), which usually reflects the presence of Doppler spread produced for the most part by moving boundaries and oscillating sensors. In this paper, the authors study the least square (LS) channel estimation routine ability to track the time-varying nature of the impulse response using broadband, single-carrier pseudonoise (PN) sequences transmitted by a single source and collected by a single receiver. In addition, the authors evaluate a trend estimation technique, based on the empirical modal decomposition (EMD) method applied to the LS estimate of the CIR. Simulated data produced with a Rayleigh channel model and experimental data collected in a marina are used. This paper shows that the channel estimation method can estimate the time-varying impulse response of the acoustic channel with a high resolution of both time and delay (down to 7 μs) at the expense of high computational requirements. In analyzing the time variation of the main and secondary echoes for a signal-to-noise ratio (SNR) of 32 dB, simulated results indicate that the root mean square error (RMSE) between theoretical and LS estimated response is 7.8% for the main path with an equivalent Doppler spread of 10 Hz and 15.7% for the second path with an equivalent Doppler spread of 15 Hz. Applying the trend estimation technique to the LS CIR greatly reduces this error, down to 2.9% for the main path and to 4.7% for the second path. The experimental data clearly show that the routine can closely track the time variations of the main echo and provide a meaningful estimate of the Doppler spread.

TS-MUWSN: Time Synchronization for Mobile Underwater Sensor Networks

Time synchronization is an important, yet challenging, problem in underwater sensor networks (UWSNs). This challenge can be attributed to: 1) messaging timestamping; 2) node mobility; and 3) Doppler scale effect. To mitigate these problems, we present an acoustic-based time-synchronization algorithm for UWSN, where we compare several message time-stamping algorithms in addition to different Doppler scale estimators. A synchronization system is based on a bidirectional message exchange between a reference node and a slave one, which has to be synchronized. Therefore, we take as reference the DA-Sync-like protocol (Liu et al., 2014), which takes into account nodes movement by using first-order kinematic equations, which refine Doppler scale factor estimation accuracy, and result in better synchronization performance. In our study, we propose to modify both time-stamping and Doppler scale estimation procedures. Besides simulation, we also perform real tests in controlled underwater communication in a water test tank and a shallow-water test in the Mediterranean Sea.

An analysis of MIMO-OFDM for shallow water acoustic communications

Multi-input multi-output (MIMO) techniques are currently extensively considered in underwater acoustic (UWA) communications to overcome the bandwidth limitation of under-sea channel. Associated with Orthogonal Frequency Division Multiplexing (OFDM) modulation, MIMO techniques provide substantial gain in spectral efficiency and fair robustness against frequency fading while keeping simple equalizer structure. In this paper, we study accurately the gain provided by MIMO-OFDM paradigm over Shallow Water Acoustic (SWA) channel by taking into account channel characteristics as well as OFDM parameters.

A channel model and estimation technique for MIMO underwater acoustic communications in ports and very shallow waters at very high frequencies

Hermes is an underwater acoustic modem that achieves very high-bit rate digital communications in ports and shallow waters. In its current form, this modem supports only Single-Input Single-Output communications. In this paper, the authors investigate the use of Multiple-Input-Multiple-Output (MIMO) technology. MIMO, applied to underwater acoustic (UWA) communications, has shown great promise in terms of improvements of data rate, flexibility and coverage area. The MIMO system developed here combines Hadamard training sequences and the traditional Hermes uplink message. Key to proper MIMO communications is an accurate channel least-square estimation and equalization. A MIMO equalization process consisting of a matrix-based linear filter optimized under the minimum mean square error (MMSE) criterion is presented here. The channel estimation technique is evaluated using simulated messages generated by a very shallow water Rician channel model. Simulation results indicate that a fading channel can be estimated with an RMSE of 0.191% on average at 75m under realistic conditions. In the sole presence of specular reflections, the mean value of the root mean square error (RMSE) is 94.59% before equalization and 32.26% after equalization. The variance of the RMSE is 32.014% before equalization and 12.77% after equalization, respectively. In the presence of sound scattering and using every echo, the mean value of the RMSE is 54.27% after equalization, vs. 120.55% before equalization.

MIMO underwater acoustic channel characterization based on a remotely operated experimental platform

In the framework of the development of a MIMO (Multi-input Multi-output) capable underwater acoustic modem, this paper describes on the one hand a remotely operated experimental platform at sea in real conditions able to transmit and receive real-time multi-streams signals and the other hand an analysis of the experimental channel capacity gain brought by the MIMO approach with respect to traditional single transmitter system. The original remotely operated generation/acquisition system provides an extensive characterization of the MIMO underwater acoustic channel for various transmission ranges and sea states. The capacity analysis extracted from real conditions channel estimates confirm that MIMO approach in experimental scenario leads to a substantial capacity gain with respect to single transmitter even by taking account signal overhead. As foreseen by theory, the MIMO capacity gain is also shown to be dependent from transmission range and transducers arrays configuration.

Underwater acoustic communication messaging time stamp applied to global time synchronization

This paper presents a new approach to time stamp messages in underwater acoustic communication. Message time stamping is a widely used method for synchronizing two clocks over a cabled or wireless communication, by exchanging time information in pilot messages. In this work we use a National Instruments Field-Programmable Gate Array (FPGA) for performing deterministic hardware time stamp of windowed acquired data, and then we apply a software detection of frame information inside this acquisition window. Frame time stamp in conjunction with global time reference enclosed in the message, provide enough information for achieving synchronization accuracy between two clocks below few microseconds.

On the analysis of orthogonal chirp division multiplexing for shallow water underwater acoustic communication

This paper describes the use of the novel Orthogonal Chirp Division Multiplex (OCDM) waveform for an end-to-end high data rate underwater acoustic transmission system. The described OCDM based transmission is evaluated over a realistic underwater acoustic (UWA) channel and compared fairly against the conventional orthogonal frequency division multiplex (OFDM). By efficiently exploiting multi-paths diversity of the channel, OCDM is shown to provide better robustness than OFDM especially in case of a underloaded configuration when only a subset of N chirp waveforms are modulated leading to an interesting trade-off between spectral efficiency and robustness against UWA channel impairments.

MIMO Underwater Acoustic Communications in Ports and Shallow Waters at Very High Frequency

Hermes is a Single-Input Single-Output (SISO) underwater acoustic modem that achieves very high-bit rate digital communications in ports and shallow waters. Here, the authors study the capability of Hermes to support Multiple-Input-Multiple-Output (MIMO) technology. A least-square channel estimation algorithm is used to evaluate multiple MIMO channel impulse responses at the receiver end. A deconvolution routine is used to separate the messages coming from different sources. This paper covers the performance of both the channel estimation and the MIMO deconvolution processes using either simulated data or field data. The MIMO equalization performance is measured by comparing three relative root mean-squared errors (RMSE), obtained by calculations between the source signal (a pseudo-noise sequence) and the corresponding received MIMO signal at various stages of the deconvolution process; prior to any interference removal, at the output of the Linear Equalization (LE) process and at the output of an interference cancellation process with complete a priori knowledge of the transmitted signal. Using the simulated data, the RMSE using LE is −20.5 dB (where 0 dB corresponds to 100% of relative error) while the lower bound value is −33.4 dB. Using experimental data, the LE performance is −3.3 dB and the lower bound RMSE value is −27 dB.

Underwater mobile target tracking with particle filter using an autonomous vehicle

This paper describes an underwater mobile target localization and tracking by using an autonomous surface vehicle for which the successive ranges between the target and the reference are the only information. In a dynamic system, such as range-only single-beacon underwater target tracking, a state-space model can be characterized, where the state vector may include position, and velocity of the mobile underwater target. Moreover, the range observations can come from a mobile autonomous vehicle, which is used as a moving landmark. Then, a nonlinear Bayesian filtering algorithm can be used to make extrapolations on the state vector from the observations, in order to obtain the target position at each instant of time. In this paper we consider the use of Particle Filter (PF) to perform such localization and tracking where its performance and characterization is studied under different scenarios.