## 6 Conclusions and Further Work  
In this paper, we have presented an analysis of the energy consumption during the localization process of a wireless sensor network. We have optimized the transmission strategy (in terms of transmission power and packet transmission rate) at each node with the aim of obtaining certain accuracy in the RSS or TOA measurements (expressed by the number of packets that the receiver node should average) while minimizing the energy consumption in the network. We have shown that the optimum transmission strategy consists in sending packets with such a transmission power \(P_{TX}^{*}\) that the \(snr\) at the receiver is the optimum and a packet rate of \(1/T^{*}\), where \(T^{*}\) depends on the desired accuracy and the optimum value of the \(snr\). However, we have also seen that the value of \(P_{TX}^{*}\) depends on the distance between nodes, which is not known in advance, as it is the parameter that is estimated during localization. In order to apply the results above, the transmission power has to be estimated iteratively.  
We have also proposed some modifications to the theoretical optimal strategy in order to correct the effects of the propagation channel when applying this strategy in a real scenario. In particular, we have proposed how to correct these effects in the case of RSS-based localization, by adding a correction margin to the estimated transmission power and correcting the number of transmitted packets by a factor that depends on the packet length and the value of \(W/v_{b}\). The proposed strategy with its corrections was proved to guarantee a desired number of received packets at the receiver (necessary to obtain a given accuracy in the distance estimations) while having at the same time lower energy consumption than other strategies.  
In further work we are planning to evaluate the performance of the proposed strategy using real-field deployments. In particular, we are implementing the embedded versions of the strategy in MICAz motes (under TinyOS 2.1), to experimentally measure and evaluate the energy consumption during the localization when the proposed strategy is executed in these resource-constrained devices.  
Future work on studying the effects that the accuracy of the radio measurement has on the distance estimation for TOA-based and RSS-based localization techniques will allow extracting some guidelines for the calculation of the required average number of received packets, depending on the localization accuracy requirements and on the geometric configuration of the network. Further work would also include the analysis of other channel and receiver models and the evaluation of variant channels. Furthermore, the value of the localization period \(T_{0}\), which we have considered as constant, may change in order to adapt to the mobility characteristics of the mobile node. The combination of the proposed optimization with the optimization of the localization period may lead to further energy savings and could be a promising line of future work.  
This work has been supported by the Government of Madrid under grant S2009 TIC-1485 (CONTEXTS) and by the Spanish Ministry of Science and Innovation under grant TIN2011-28620-C02-02.