Multi-node processing for asymmetrical communications in underwater acoustic networks

Underwater acoustic communication networks have attracted attention for its applications in many areas such as mine warfare, rapid environmental assessment or search and rescue, where there is the need for conserted action of a group of observers/actuators. A common requirement in most of these applications is the need to make accessible to the global (terrestrial/aerial) user network large amounts of critical underwater collected data. This paper addresses this requirement by using multichannel nodes providing an asymmetrical point-topoint (P2P) connection where the upload link has a much higher data rate than the download link. The emphasis of this work is not so much on the final channel throughput but on establishing the optimal processing of spatially distributed multichannel nodes that serve as interface between the underwater nodes and the global/user network. The adopted strategy relies on passive time-reversal which can be viewed as a spatial pre-equalizer for each multichannel node. Then an intersymbol interference optimal combination of various nodes is divised for balancing poor communications of one or more nodes in the network. The method and techniques are theoretically derived and applied to real data acquired with a network of 2 surface buoys over an environmentally challenging area off the coast of Portugal in July 2007.FC

Probe timing optimization for time-reversal underwater communications

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

Fractional order capacitors

The performance of electrical devices, depending on the processes of the electrolytes, have been fully described and the physical origin of each parameter is well established. However, the influence of the irregularity of the electrodes was not a subject of study and only recently, this problem became relevant in the viewpoint of fractional calculus. This paper describes an electrolytic process in the perspective of fractional order capacitors. In this line of thought, are developed several experiments for measuring the electrical impedance of the devices. The results are analyzed through the frequency response, revealing capacitances of fractional order that can constitute an alternative to the classical integer order elements.N/

Modeling and simulation of walking robots with 3 dof legs

This paper describes a simulation model for a multilegged locomotion system with 3 dof legs and leg joint actuators having saturation. For that objective the robot prescribed motion is characterized in terms of several locomotion variables. Moreover, the robot body is divided into several segments in order to emulate the behavior of an animal spine. A non-linear spring-dashpot system models the foot-ground interaction, being its parameters computed from studies on soil mechanics. To conclude, the performance of the developed model is evaluated through a set of experiments while the robot leg joints are controlled using a proportional and derivative algorithm.N/

Research issues in biological inspired sensors for flying robots

Biological inspired robotics is an area experiencing an increasing research and development. In spite of all the recent engineering advances, robots still lack capabilities with respect to agility, adaptability, intelligent sensing, fault-tolerance, stealth, and utilization of in-situ resources for power when compared to biological organisms. The general premise of bio-inspired engineering is to distill the principles incorporated in successful, nature-tested mechanisms of selected features and functional behaviors that can be captured through biomechatronic designs and minimalist operation principles from nature success strategies. Based on these concepts, robotics researchers are interested in gaining an understanding of the sensory aspects that would be required to mimic nature design with engineering solutions. In this paper are analysed developments in this area and the research aspects that have to be further studied are discussed.N/

Acoutic-oceanographic buoy - An easily deployable, reconfigurable, and multifunctional acoustic-oceanographic system

The concept of an easy to use and easy to deploy ocean acoustic tomographic (OAT) system is presented. The system is composed of a network of buoys and a data inversion online processor. This study concerns the individual node of that network—the acoustic-oceanographic buoy (AOB)—the data inversion technique and the testing of the system at sea. The AOB is a lightweight surface buoy with a vertical array of acoustic and temperature sensors to be hand deployed in a free-drifting configuration from a small boat. The data are locally stored and transmitted online to a remote station for processing and monitoring. Data inversion is based on a broadband matched-field tomography technique where known and unknown parameters are simultaneously searched for (focalization). In situ recorded temperature data serve for algorithm initialization and calibration. The AOB was successfully deployed in several consecutive days during two rapid environmental assessment sea trials in 2003 (Mediterranean) and 2004 (Atlantic). Data collected at sea also show that the AOB can be reconfigured as a receiving array for underwater coherent communications in the band up to 15 kHz

Fractional order electrical impedance of fruits and vegetables

The idea of fractional calculus is not new. Fractional derivatives are almost as old as integer-order definition. In 1695 Leibniz discussed this problem with L’Hospital, but many other contributions are due to investigators such as Liouville, Abel, Heaviside and Riemann, that formalized the theory of the non-integer order systems. The area of fractional calculus has primarily been the domain of mathematicians, and only had the theoretical foundation. Nowadays, this concept is employed in physics, engineering, biology, economy and other scientific fields. In our work, we apply the concepts of fractional calculus and the theory of electrical impedance to botanical elements. The fractional order behaviour of these type of systems are studied and the relation with the electrical impedance is formulated.N/

Modeling arrival scattering due to surface roughness

Signal scattering due to surface roughness constitutes one of the most important modeling problems in underwater acoustics when dealing with signal processing at large frequencies (i.e. above 1 kHz). Such modeling requires, in particular, realistic predictions of the ocean surface, which is usually perturbed by the propagation of swells and wind driven surface waves. Most of experimental studies had been dealing with short-range propagation, although it still remains unclear the separated impact of periodic vs. stochastic surface roughness on the arrival structure of received signals. The main purpose of the discussion presented in this paper is to clarify such issues through the calculation of impulse responses at high frequencies, at short and large ranges, using a ray tracing acustic propagation model. The results of the simulations are expected to improve strategies of signal processing when scattering affects the received signal

Avaliação de híbridos interespecíficos de mandioca para a resistência a doenças.

Este trabalho objetivou a avaliação da incidência e severidade de doenças em híbridos interespecíficos de mandioca em diferentes localidades do Nordeste brasileiro

Underwater Acoustic simulations with a time variable acoustic propagation model

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