PHITOM - Probabilistic High-Frequency Ocean Tomography for Underwater Communications and Navigation

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Publications

OFDM demodulation in underwater time-reversed shortned channels

Publication . Gomes, João; Silva, A.; Jesus, S. M.

This work addresses the problem of OFDM transmission in dispersive underwater channels where impulse responses lasting tens of miliseconds cannot be reliably handled by recently proposed methods due to limitations of channel estimation algorithms. The proposed approach relies on passive time reversal for multichannel combining of observed waveforms at an array of sensors prior to OFDM processing, which produces an equivalent channel with a shorter impulse response that can be handled much more easily. A method for tracking the narrowband residual phase variations of the channel after Doppler preprocessing is proposed. This is a variation of an existing technique that can improve the spectral efficiency of OFDM by reducing the need for pilot symbols. This work also examines techniques to handle sparse impulse responses and proposes a channel estimation method where an l1 norm is added to the standard least-squares cost function to transparently induce sparseness in the vector of channel coefficients. Algorithms are assessed using data collected during the UAB’07 experiment, which was conducted in Trondheim fjord, Norway, in September 2007. Data were transmitted with bandwidths of 1.5 and 4.5 kHz, and recorded at a range of about 800 m in a 16-hydrophone array. Significant multipath was observed over a period of at least 30 ms.

2008Conference object

Open Access

Probe timing optimization for time-reversal underwater communications

Publication . Silva, A.; Jesus, S. M.; Gomes, J. P.

Passive Time Reversal (pTR) is one of the variants of time reversal applicable to digital underwater communications. In passive time reversal a probe-signal is transmitted ahead of the data-signal in order to estimate the channel impulse response for later use as a replica signal in a time reversal mirror fashion. In practice the received probe-signal must be captured in a time-window and, after correlation with the transmitted probesignal, give a noisy estimation of the channel impulse response. Therefore, the output Signal to Noise Ratio (SNR), the Inter- Symbolic Interference (ISI) and the detection rate of passive time reversal will strongly depend on the starting time and duration of such time-window. Typically the beginning and the duration of that time-window should depend on the travel time and the dispersion of the acoustic channel. In this paper, the maximization of the pTR output SNR relative to the probe time-window is derived in closed form. It will be shown that the probe timing that gives the lower detection error rate can be predicted using closed form metrics for the pTR output SNR and ISI. Theoretical results are found to be in full agreement with simulations and with results obtained on experimental data taken during the INTIFANTE’00 sea trial.

2011-06Conference object

Open Access

Underwater Acoustic simulations with a time variable acoustic propagation model

Publication . Silva, A.; Rodríguez, O. C.; Zabel, F.; Huillery, J.; Jesus, S. M.

The Time Variable Acoustic Propagation Model (TV-APM) was developed to simulate underwater acoustic propagation in time-variable environments. Such environment variability induces a strong Doppler channel spread, which is an important factor to test and evaluate the performance of equalization algorithms. In current simulations, Doppler spread is usually included a posteriori in a stationary Acoustic Propagation Model (APM), and is designed for specific environmental parameters such as source-receiver range variability or surface motion. However, environmental variations affect Doppler spread in a complex manner, and an accurate TV-APM simulation for time varying channels, being performed at the same sampling rate as the transmitted signal, would require a large number of runs at high frequencies. A strategy in the current implementation of the TV-APM was developed to reduce the number of runs, while preserving the variable-channel Doppler spread. Simulations were done to draw a performance map for a given equalizer in a given environment and the results revealed that the TV-APM is a useful prediction tool of communication equalizers performance.

2010Conference object

Open Access

A post-detection maximum ratio combiner. Experimental assessment on high diversity underwater channels

Publication . Silva, A.; Jesus, S. M.

Maximum Ratio Combiner (MRC) is a diversity combining technique applicable to underwater communications when the data transmitted by a single projector is received by more than one antenna/node. Post Detection MRC (PD-MRC) performs a weightedsum over initial detected outputs of multiple nodes. PDMRC can be applied to noise-only or ISI-only channels with the weights computed from the signal to noise ratio or from the channel impulse response of each node, respectively. In this paper it is shown that in the presence of noisy-ISI channels the weighs can be computed from the detector output constellation of each node. A performance of the PD-MRC is evaluated using real data collected during RADAR’07 experiment. Results show that a gain is always attained using two nodes in the PDMRC as compared to the node having the best MSE. Moreover, the PD-MRC gain is higher when both nodes present a similar MSE.

2011Journal article

Open Access

Arrival-based equalizer for underwater communication systems

Publication . Siddiqui, Salman Ijaz; Silva, António João Freitas Gomes da

One of the challenges in the present underwater acoustic communication systems is to combat the underwater channel e ects which results in time and frequency spreading of the transmitted signal. The time spreading is caused by the multipath e ect while the frequency spreading is due to the time variability of the channel. The main purpose of this work is to address these problems and propose a possible solution to minimize these e ects and to improve the performance of the underwater communication system. The passive Time Reversal (pTR) equalizer has been used in underwater communications because of its time focusing property which minimizes the time spreading e ect of the underwater channel. In order to compensate for the frequency spreading e ect, an improved version of pTR was proposed in the literature, called Frequency shift passive time reversal (FSpTR). In order to understand the e ects of geometric variations on the acoustic signals, a Doppler based analysis technique, called Time Windowed Doppler Spectrum (TWDS), is proposed in this work. The principle of TWDS is to analyze the temporal variations of the Doppler spectrum of di erent arrivals received at a hydrophone. The results show that each arrival is a ected in a di erent manner by the same environmental variation. In this dissertation, an arrival-based equalizer is proposed to compensate for the environmental variations on each arrival. Due to complex multipath structure of the underwater channel, the arrivals are merged into one another in time and it is very di cult to separate them. The beamforming technique is used, in this work, to separate di erent wavefronts on the basis of angle of arrival. The arrival-based equalizer compensates for the environmental variations on each arrival separately using the FSpTR equalizer. The proposed equalizer is tested with the real data and the results shows that the proposed approach outperforms the conventional FSpTR equalizer and provides a mean MSE gain up to 3.5 dB.

2012Master thesis

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