Feng Tong

Time reversal acoustic communication of mobile platform using doppler correction without resampling

With the rapidly increasing of different types of mobile platform in various marine missions, tremendous Doppler effects of mobile acoustic communication pose extreme difficulty for design of mobile acoustic communication receiver. To overcome this problem, the approach of resampling has been extensively used to address the scaling effect caused by Doppler. However, computational complexity as well as the high precision sampling rate conversion needed by accurately resampling, especially for multichannel systems, leads to additional overhead of the receiver. To facilitate no resampling Doppler correction, this paper adopts a time reversal mobile acoustic communication receiver employing a two-stage Doppler correction approach, which firstly performs bulk Doppler shift estimation as well as the initial channel estimation, and then addresses the Doppler distortion by periodic channel estimation based time reversal processor, that followed by a single channel adaptive decision feedback equalizer (DFE) to tackle the residual Doppler. Sea trial experiments carried out on moving platform associated with a Doppler of -16Hz are carried out to demonstrate the effectiveness of the proposed low complexity acoustic communication approach for mobile platform.

R&D of an spread spectrum acoustic communication modem with ranging capability

With development of ocean exploitation, maritime rights protection, and other fields of marine information acquiring and processing, integrated underwater acoustic communication and ranging are extensively needed in applications such as underwater networking and AUV navigation and control. Because the traditional underwater ranging system usually uses a single frequency pulse signal, a conflict exists between range ambiguity and ranging accuracy. Moreover, as underwater acoustic channels pose multiple difficulties such as multipath, time-space selectivity, frequency dependent noise, and Doppler shifts, high precision ranging is a challenging mission. Considering that the spread spectrum technology features high reliability, strong anti-interference and anti-multipath capabilities and low probability of detection (LPD) capability, this paper introduces the application of the m-sequence spread spectrum technology into the integrated underwater acoustic communication and ranging function to develop a spread spectrum acoustic communication modem with ranging capability. Experimental results obtained in a physical shallow water channel demonstrate the proposed systems effectiveness.

Experimental evaluation of NNCLMS sparse channel estimation for shallow water acoustic communication

Due to the sparsity that characterizes underwater acoustic (UWA) channels, in recent years interest has increased in designing of norm constraint sparse channel estimation algorithms for underwater acoustic communication. However, experimental evaluation and comparison is lacking regarding enhancing the performance of different norm constraint sparse channel estimation algorithms for UWA communication with a series of continuous experiments on a time scale of an hour. As shallow water acoustic channels pose significant difficulties such as multipath, time-space selectivity, frequency-dependent noise, and Doppler shift, this paper presents the experimental evaluation and comparison of our previously proposed non-uniform norm constraint least mean square (NNCLMS) algorithm with the classic 10-norm and 11-norm LMS algorithm in the context of a shallow water acoustic communication system. We provide and discuss experimental results obtained in shallow water channels in terms of the NNCLMS algorithms sparse exploitation performance and the classic norm constraint type sparse algorithms.

Joint temporal-sparse recovery approach for estimation of underwater acoustic MIMO channels

MIMO communication offers a potential solution for high speed underwater acoustic communication. However, the simultaneous presence of multipath and co-channel interference (Co-channel interference, CoI) poses serious difficulty for estimating acoustic MIMO channels. For MIMO channels with serous CoI, the performance gain achieved by exploiting the sparsity of a single acoustic channel is insufficient to meet the need of MIMO acoustic communication. In this paper, a temporal joint sparse recovery approach is proposed to exploit the sparse correlation between adjacent blocks and improve the performance of channel estimation. A joint sparse model under the framework of distributed compressed sensing (DCS) is adopted to derive a joint sparse recovery algorithm for estimating MIMO channels. Finally, underwater MIMO communication experimental results obtained in a shallow water channel are provided to demonstrate the effectiveness of the proposed method compared to the classic estimation methods.

Implementation and evaluation of the time reversal OFDM underwater acoustic speech communication system

Underwater speech communication plays an important role in many marine missions, such as oceanographic investigation, environmental monitoring, underwater rescuing and sightseeing. However, the difficulties of underwater acoustic channels such as multipath, time varying and Doppler shifting pose significant challenges to the design of high performance underwater acoustic speech communication. In this paper, time reversal time-frequency differential OFDM is proposed for underwater acoustic speech communication, which enables time-frequency domain focusing of multipath by multi-channel time reversal, as well as suppressing of the residual multipath with time-frequency differential OFDM. Finally, the quality of the sea trial speech communication is quantitatively evaluated via the PESQ (Perceptual evaluation of speech quality) method to demonstrate the effectiveness of the proposed method.

A sponge cover approach to reducing flow noise of hydrophone

When water flows through the measuring hydrophone and its cable quickly, pressure interference is produced. In this paper, sponge covering the hydrophone is used to isolate the impact of water flowing through the hydrophone and eliminate the burr noise in the signals. Results of the flow tests show that the sponge can effectively filter out the pressure interference caused by water flow, and of little attenuation to the signals of appropriate frequency. So the sponge is kind of a simple and efficient dome, and is able to improve the performance of hydrophone in daily underwater acoustic measurement.

The use of power cepstrum for multipath signal detection in underwater acoustic channel

The multipath effect is the primary interference factor among the underwater communication systems. Underwater acoustic channels are characterized as time-variant dispersive channels. Multipath propagation results in spreading of the transmitted signal in time, which leads to serious Signal amplitude attenuation and inter symbol interference (ISI). Here we describes a signal processing approach identified as “power cepstrum technique,” which has been found is efficient in recognizing wavelet arrival times and amplitudes, even in the present of noise. Thereby, a special signal, which has a short duration as well as a high power, is designed to detect the multipath effect of the channel.

Selective time reversal receiver for shallow water acoustic MIMO communications

MIMO systems is capable of achieving two to three spatially parallel communication channels in very shallow water to improve communication performance as well as capacity. For MIMO communications, simultaneous presence of multipath and co-channel interference poses substantial difficulties for the design of receiver. While the classic space-time multi-channel DFE receiver consisting of multiple DFE equalizers is limited by high computational complexity when multipath time delay is large, the low complexity time reversal receiver needs a large number of receivers (>10) to achieve satisfactory performance. This paper presents the design of a selective time reversal underwater acoustic MIMO communication receiver to accommodate the small receiver number of space-time DFE and the multipath focusing capability of time reversal by coupling the time reversal with multi-channel DFE. Sea trial experiments are carried out to demonstrate the superior performance of the proposed method.

Design of a small acoustic modem for dense and low-cost oceanographic monitoring

With increasing concern for the oceanic environmental problems caused by artificial pollution or natural disasters, there is a rapidly increasing requirement for dense and low cost monitoring of oceanographic parameters such as CO2, nutrition, O2, etc. The acoustic communication approach is able to wirelessly transfer sensor data from different monitoring locations to facilitate densely networked monitoring.

Lake trial of a multi-hop underwater acoustic network for ocean transect sampling

There is increasing interest in ocean transect sampling missions to do environmental (pollution, coral reef, seismic, ocean current, etc.) monitoring, oceanography parameter gathering (CO2, temperature, current, etc.), oceanographic investigation, and sea bottom resource exploitation. These have been previously performed by research vehicles sailing point by point or by sensors deployed first and then recovered. These traditional approaches have the following disadvantages: no real-time monitoring, no online system reconfiguration, no failure detection and limited storage capacity, among others.