Energy Collaboration for Non-Homogeneous Energy Harvesting in Cooperative Wireless Sensor Networks

In wireless sensor networks, energy harvesting is developed as an effective way to solve energy drain problem of nodes which powered by battery, just like that cooperative communication is used to improve transmission reliability. In some energy harvesting networks, the nodes' energy harvested from ambient environment is disequilibrium. For example, nodes exposed to sunlight collect more solar energy than those shaded. Impoverished energy will reduce transmission reliability because some nodes have not enough energy to send data or are forced to decrease transmission power to save energy. In this paper, energy collaboration is taken into consideration for non-homogeneous energy harvesting in cooperative wireless sensor networks, where all sensor nodes harvest solar energy and then store it in rechargeable battery. And an energy cooperative protocol is proposed for energy inhomogeneity to enhance transmission reliability among relay nodes through radio frequency, where the nodes with abundant energy share excess power to other nodes which undertake the forwarding task but store insufficient energy. By exploiting ACK/NACK frame fed back from the destination node and No Enough Energy (NEE) frame advertised by the forwarding relays in the active set, each candidate node forms its energy supply set when all of them are lack of energy for transmitting. For the relay selection, three strategies are presented for choosing the pair of the best forwarding relay and the optimal energy supply relay, which are respectively named Best Channel Strategy, Nearest Distance Strategy and Minimum Energy Sharing Strategy. Furthermore, the outage probability of cooperative transmission under energy collaboration is derived. Simulation results show that energy collaboration for cooperative wireless sensor networks can significantly reduce the outage probability of the system and improve transmission reliability of the network.