Browsing by Author "Oliveira, Anabela"
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- Approximating fractional derivatives through the generalized meanPublication . Machado, J. A. Tenreiro; Galhano, Alexandra; Oliveira, Anabela; Tar, József K.This paper addresses the calculation of fractional order expressions through rational fractions. The article starts by analyzing the techniques adopted in the continuous to discrete time conversion. The problem is re-evaluated in an optimization perspective by tacking advantage of the degree of freedom provided by the generalized mean formula. The results demonstrate the superior performance of the new algorithm.
- Modeling LoRa Communications in Estuaries for IoT Environmental Monitoring SystemsPublication . Gutiérrez Gaitán, Miguel; D'Orey, Pedro; Cecílio, José; Rodrigues, Marta; Santos, Pedro Miguel; Pinto, Luis; Oliveira, Anabela; Casimiro, António; Almeida, LuisLow-power wide-area networks are extending beyond the conventional terrestrial domain. Coastal zones, rivers, wetlands, among others, are nowadays common deployment settings for Internet-of-Things nodes where communication technologies such as LoRa are becoming popular. In this article, we investigate large-scale fading dynamics of LoRa line-of-sight links deployed over an estuary with characteristic intertidal zones, considering both shore-to-shore and shore-to-vessel communications. We propose a novel methodology for path loss prediction which captures i) spatial, ii) temporal and iii) physical features of the RF signal interaction with the environmental dynamics, integrating those features into the two-ray propagation model. To this purpose, we resort to precise hydrodynamic modeling of the estuary, including the specific terrain profile (bathymetry) at the reflection point. These aspects are key to accounting for a reflecting surface of varying altitude and permittivity as a function of the tide. Experimental measurements using LoRa devices operating in the 868~MHz band show major trends on the received signal power in agreement with the methodology's predictions.
- Optimal approximation of fractional derivatives through discrete-time fractions using genetic algorithmsPublication . Machado, J. A. Tenreiro; Galhano, Alexandra; Oliveira, Anabela; Tar, József K.This study addresses the optimization of rational fraction approximations for the discrete-time calculation of fractional derivatives. The article starts by analyzing the standard techniques based on Taylor series and Padé expansions. In a second phase the paper re-evaluates the problem in an optimization perspective by tacking advantage of the flexibility of the genetic algorithms.