In this paper we investigate the benefits of placing optimal number of sinks for a wireless sensor network (WSN) to prolong the network lifetime, provided that the number of hops from each sensor to its nearest sink is no more than h>=1 and the sink location space is given in advance. We first formulate this problem as a joint optimization problem, which consists of finding the optimal number of sinks for placement and devising an energy-efficient routing protocol for data collection. Due to the NP-hardness of the problem, we then propose a novel heuristic by decomposing the problem into two sub-problems and solving them separately. As a result, the proposed optimization framework improves network performance from several aspects, including the network lifetime prolongation, network scalability improvement, and the average data delivery delay reduction. Furthermore, it also enhances the network robustness substantially, since the sensing data generated by all sensors will be collected by multiple deployed sinks regardless of the network connectivity. We finally conduct extensive experiments by simulations to evaluate the performance of the proposed algorithm. The experimental results demonstrate that the proposed algorithm outperforms another popular heuristic significantly in terms of network lifetime prolongation.
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