Adaptive spatial combining for passive time-reversed communications

Passive time reversal has aroused considerable interest in underwater communications as a computationally inexpensive means of mitigating the intersymbol interference introduced by the channel using a receiver array. In this paper the basic technique is extended by adaptively weighting sensor contributions to partially compensate for degraded focusing due to mismatch between the assumed and actual medium impulse responses. Two algorithms are proposed, one of which restores constructive interference between sensors, and the other one minimizes the output residual as in widely used equalization schemes. These are compared with plain time reversal and variants that employ postequalization and channel tracking. They are shown to improve the residual error and temporal stability of basic time reversal with very little added complexity. Results are presented for data collected in a passive time-reversal experiment that was conducted during the MREA’04 sea trial. In that experiment a single acoustic projector generated a 2/4-PSK phase-shift keyed stream at 200/400 baud, modulated at 3.6 kHz, and received at a range of about 2 km on a sparse vertical array with eight hydrophones. The data were found to exhibit significant Doppler scaling, and a resampling-based preprocessing method is also proposed here to compensate for that scaling

Joint passive time reversal and multichannel equalization for underwater communications

Time reversal has aroused considerable interest in communications applications as a means of partially compensating for the intersymbol interference introduced by the channel. This paper presents experimental results for a passive time reversal experiment conducted off the west coast of Portugal during the MREA’2004 mission. A single acoustic projector generated a 2/4-PSK stream at 200 and 400 baud, modulated around a carrier frequency of 3600 Hz. The signals were received at a range of about 2 Km from the source on a sparse vertical array with 8 hydrophones. We examine several strategies for demodulating the data, including equalization, time-reversal, and combinations of both techniques. This work emphasizes the use of low-complexity multichannel combining algorithms for tracking channel variations prior to generating a passively focused signal.This work was supported by Fundação para a Ciência e a Tecnologia (ISR/IST plurianual funding) through the POS– Conhecimento Program that includes FEDER fund

OFDM demodulation in underwater time-reversed shortned channels

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.FC

Passive time reversal probe-signal capture optimization for underwater communications

Passive Time Reversal (pTR) is an emerging technique for underwater communications where a channel probe is sent ahead of the data packet. The channel probe must be captured in a time window for post crosscorrelation with the data. The Signal to Noise Ratio (SNR) of pTR, and hence the detector error rate, will be dependent on the starting point and duration of this time window. Typically the beginning and length of the time window should depend on the time dispersion of the acoustic channel which, in turn, depends on the environment properties and on the experimental geometry. Heuristic reasoning would suggest that if a short time window fails to include all significant multipath it will result in imperfect focusing, while an overly long window will reduce the efficiency of the communication system by introducing additional noise in the pTR system. That problem calls for an optimization that has previously been addressed only heuristically. In order to bring the pTR capabilities to a practical modem the engineering problem of time window automatic optimisation must be solved, and that is the main purpose of our paper. An expression for the optimal pTR SNR time window length is presented and its validity confirmed with simulations and real data from the INTIFANTE’00 sea trial

Performance analysis of multichannel lattice equalization in coherent underwater communications

This work examines the numerical fixed-point performance of a new multichannel lattice RLS filtering algorithm using data from two underwater acoustic communication experiments. The algorithm may be an appealing choice for underwater equalization due to its robust numerical behavior and linear scaling of the computational complexity with filter order. Simple modifications to widely-used methods for carrier/timing synchronization and symbol slicing in transversal equalizers are proposed. Experimental results show that the algorithm is as accurate as the similarly array-based QR-RLS, tolerating word lengths as low as 16-20 bits with minor degradation relative to floating-point benchmarks. These features, coupled with a very modular and regular structure, are highly desirable in energyefficient hardware or embedded implementations.FC

Experimental results of geometric and geoacosutic parameter estimation using a vector sensor array

The objective of this paper is to present an overview of the work developed at SiPLAB, University of Algarve, with vector sensor data collected during Makai experiment 2005, in geometric and geoacoustic parameter estimation. During this experiment devoted to high frequency initiative, acoustic data were acquired by a four element vertical vector sensor array (VSA). A vector sensor is a directional sensor constituted by one omni directional pressure sensor and three velocity-meters, where both the acoustic pressure and the three particle velocity components are measured. The spatial filtering capabilities of the vector sensors are used to estimate the direction of arrival (DOA) of low and high frequency acoustic sources considering a single and a multiple sensor VSA. An inversion method based on Bartlett estimator is used for three dimensional localization of ship’s noise where the noise source is estimated in range and depth taking into accounts the azimuth given by DOA. Moreover, this method is applied to seabed parameters estimation like sediment compressional speed, density and compressional attenuation, contributing to improve the resolution of these parameters

Nanomaterials for reversion of multidrug resistance in cancer: a new hope for an old idea?

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Motivación docente, satisfacción en el trabajo y capital psicológico positivo: una revisión

Teacher motivation is vital for the educational system. For teachers to be motivated their work satisfaction and positive psychological capital are crucial. The state-of-the-art on teacher motivation requires a literature review regarding the studies that relate teacher motivation and the abovementioned constructs. In this paper, through electronic databases, the published studies between 1990-2014 about these issues were identified. A total of 43 studies were obtained. The main conclusions were the following: (a) 2012 was the year with more publications (n = 7; 16.28%); (b) quantitative methodologies (n = 40; 93.02%) and self-report questionnaires (n = 43; 89.59%) were the dominant methodology and instrument type; and (c) work satisfaction was the most studied concept with teacher motivation (n = 42; 97.67%). Our results underline the importance of work satisfaction on teacher motivation and emphasize the need to realize more studies on the relationship between teacher motivation and positive psychological capital. </jats:p