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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
- Impact of the antenna directivity on path loss for different propagation environmentsPublication . Azevedo, Joaquim Amândio; Santos, Filipe Edgar; Sousa, Tony Andres; Agrela, Jenny ManuelaLog-normal is a very common propagation model used to estimate the path loss introduced by a given environment. However, in most cases the parameters of the model are antenna specific. In this work, it is shown that the path loss exponent is highly dependent on the antenna pair used in the communication system. Large errors are introduced in the prediction of the path loss if the antennas are different from those used to obtain the parameters of the propagation model. An improved model is developed for the 2.4 GHz industrial, scientific and medical band to estimate the path loss exponent for different antenna directivities knowing the path loss exponent of a reference antenna. The model uses the antenna beamwidth and the antenna gain to obtain a parameter related to the path loss.