Minimizing service delay in directional sensor networks

In directional sensor networks, sensors can steer around to serve multiple target points. Most previous works assume there are always enough deployed sensors so that all target points can be served simultaneously. However, this assumption may not hold when the mission requirement changes or when more target points need to be served. Since it is not always practical to deploy new sensors, we propose to reconfigure the network by letting existing sensors steer and serve the targets periodically. As a result, targets may not be served continuously, and the service delay affects the quality of service. One important problem is how to choose the optimal set of targets to serve by each sensor such that the maximum service delay is minimized. We first show that this problem is NP-complete, and then we propose a centralized protocol whose performance is bounded by a logarithm factor of the optimal solution. We also propose a distributed protocol which achieves the same performance as the centralized protocol. Finally, we extend the optimization model and the protocols by considering the rotation delay, which is critical for some applications but ignored by previous work.