Should interference be avoided? Charging WSNs with efficient placement of wireless chargers

Using multiple fixed chargers to remotely charge wireless sensor networks (WSNs) is a feasible way in harsh terrain. An interesting property is that, due to the radio interference, the charging efficiency with multiple chargers may not simply be the sum of that with single charger if the chargers have the same radio frequency. To avoid the interference, an alternative way is to employ chargers with diverse frequencies, which, however, occupies too much channel resources of WSNs. In this paper, we try to answer such an interesting question: To charge a given WSN, can unique-frequency chargers be comparable to diverse-frequency chargers? To answer this question, we formulate two problems targeting at minimizing the charger number and maximizing the minimum charging power at sensor nodes, respectively. Then, we propose corresponding greedy algorithms with proof of the approximation ratio. Extensive simulation results show that: 1) given threshold of radio power at sensor nodes, the number of unique-frequency chargers required is no more than 1.1 times of that of diverse-frequency chargers and 2) given the charger number, the minimum radio power provided by unique-frequency chargers can be up to 80% of that with diverse-frequency chargers. This shows the quite competitive performance of unique-frequency chargers since they require only one channel instead of a group of channels required by diverse-frequency chargers. In addition, after appropriate placement, we find it needless to further schedule the unique-frequency chargers to improve the minimum charging power that they provide.