## 3 Related Work  
Several strategies have been proposed to reduce energy consumption in WSNs, however only some of them are suitable to be used in a HWSN, as they take into account the node mobility and weak communication links among them. [PERSON] and [PERSON] present an interesting survey of energy efficient strategies to maintain the topology of mobile _ad hoc_ and sensor networks [21]. Several of these proposals could be eventually used to reduce energy consumption in the islands of a HWSN. These energy saving strategies can be classified in four categories: _energy efficient routing, topology control by tuning the node transmission power, scheduling the nodes sleeping state_, and _reducing the volume of transferred information_. Concerning the first category, [PERSON] and [PERSON] [22] present a survey of energy-efficient routing strategies for machine-to-machine networks. Some of them could also be used in HWSN. [PERSON] _et al_. [23] propose the Lifetime Prediction Routing protocol for mobile ad hoc networks, where each node tries to estimate its battery lifetime on the basis of its past activity. Hence, it is possible to increase the overall network lifetime by finding better routing solutions that take into account these predictions. Several other proposals have been reported in this category, for example [24, 25, 26]. These proposals either explore routes that include nodes with maximum residual energy, minimize the total end-to-end transmission energy for a message, or a (weighted) combination of both strategies. Most energy efficient routing strategies are not exclusive, therefore they could be combined properly with OLSRp to increase the energy saved by the network nodes. However, it is not clear what effect those combinations can have on the network traffic, particularly on the data throughput and packet delivery ratio. The Kinetic Multipoint Relaying protocol [27] focuses on predicting the node mobility to improve routing and save energy. This approach selects relay nodes on the basis of the current relay configuration and the future network topology prediction. Similar to the previous category, the proposals that use tuning of the nodes transmission power can also be combined with proposals that reduce energy consumption using a predictor. Concerning the strategies based on the use of sleeping nodes, [PERSON] _et al_. [28] proposed a robust energy-conserving protocol applied to a very large number of small sensors with short battery lifetime. The protocol deals with geometric information to derive redundancy, and thus it allows redundant nodes to be turned off. A similar strategy has been used by [PERSON] and [PERSON] [29], by [PERSON] _et al_. [30], and by [PERSON] _et al_. [31] in wireless _ad hoc_ networks. The proposals in this category do not make sense for use in HWSN, due to the fact that all nodes are important actors in the sensing process. Therefore, reducing the number of available nodes will only jeopardize the interaction among devices, and decrease the trustworthiness of the information obtained from the sensing process. An interesting strategy that reduces the volume of transferred information was presented by [PERSON] _et al_. [32]. They propose a new MAC technique for event-driven wireless sensor network, which creates a hierarchical topology where every node synchronizes the information with its neighbors to avoid resending the same data. In this way, if a node receives the same information that was sent to the same destination, it will remove this message as it assumes that it has already been sent by one of its neighbors, and thus the nodes save energy. [PERSON] _et al_. [33] and also [PERSON] _et al_. [34] have made particular proposals to reduce the information overhead in MANETs, thus, dealing with the problem of energy consumption in mobile devices participating in oppnets. However, none of these proposals use message prediction as the instrument that allows reaching such a goal, which is the particular strategy analyzed in this study.