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- Sleeping ZigBee networks at the application layerPublication . Azevedo, Joaquim; Rodrigues, Maurício; Santos, Filipe; Aguiar, LuísZigBee/IEEE 802.15.4 is one of the most used standards for low-power applications. However, full function devices must be always active to route data in a mesh network. The objective of this work is to implement a sleeping technique at the application layer that enables sleep mode for all nodes of a ZigBee network. A time synchronisation mechanism to deal with the clock drift of the sensor nodes was developed. The technique also enables the recovery of lost messages. A large network is organised into smaller groups to reduce latency and packet collisions. The active interval of each node is dynamically adapted to the network operation to optimise the energy consumption. The proposed technique was applied to a real testbed and the increase in energy efficiency was evaluated. The results demonstrated energy savings of about 95% for networks containing up to 20 nodes per group and wake up periods longer than 2 min
- Small scale wind energy harvesting with maximum power trackingPublication . Azevedo, Joaquim; Mendonça, FábioIt is well-known that energy harvesting from wind can be used to power remote monitoring systems. There are several studies that use wind energy in small-scale systems, mainly with wind turbine vertical axis. However, there are very few studies with actual implementations of small wind turbines. This paper compares the performance of horizontal and vertical axis wind turbines for energy harvesting on wireless sensor network applications. The problem with the use of wind energy is that most of the time the wind speed is very low, especially at urban areas. Therefore, this work includes a study on the wind speed distribution in an urban environment and proposes a controller to maximize the energy transfer to the storage systems. The generated power is evaluated by simulation and experimentally for different load and wind conditions. The results demonstrate the increase in efficiency of wind generators that use maximum power transfer tracking, even at low wind speeds.
- Implementation of a hydric energy harvesting system for wireless sensor networksPublication . Santos, Filipe; Azevedo, Joaquim
- HF spectrum occupancy and antennasPublication . Casimiro, António; Azevedo, Joaquim; Economou, Lefteris; Haralambous, Haris; Tulunay, Ersin; Tulunay, Yurdanur; Bahadirlar, Yildirim; Türk, A. Serdar; Warrington, E. MichaelThis paper deals with the research made during the COST 296 action in the WG2, WP 2.3 in the antennas and HF spectrum management fields, focusing the Mitigation of Ionospheric Effects on Radio Systems as the subject of this COST action.