## 1 Introduction
### Our Motivations and Contributions  
In this paper, we focus on the source node location privacy protection in WSNs, and our aim is to deal with the aforementioned problems, advance the existing researches and improve the security performance of source node. Different from the existing schemes which determine the PN by the SoN, in this paper, we propose to use powerful sink node to help the source node to determine the phantom node candidate set (PNCS) and propose two new grid-based source location privacy protection schemes in WSNs: The grid-based single phantom node source location privacy protection scheme (SPS) and the grid-based dual phantom node source location privacy protection scheme (DPS). In our proposed schemes, the PNCS is firstly determined by the SN, and then the SoN randomly selects a PN from the PNCS with equal probability. After the source node transmits the data packets to the selected PN via multi-hop routing, the PN sends the data packets to the SN via the single path routing. As the energy of SN is unlimited, the method that the SN helps the SoN to select the PNCS, not only avoids a large amount of energy consumed by the ordinary sensor nodes when determining the PN, but also distributes the PNs randomly throughout the network, thus greatly increases the security of the source node and provides better privacy preservation for them. According to whether the PN is replaced, the above two schemes are further divided into two cases respectively: Not replacing the PN and replacing the PN. In the case of not replacing the PN, once the SoN chooses a PN from the PNCS, it will not be replaced. While in the case of replacing the PN, after a PN is used for a while, the SoN can replace the PN with another. When the PNCS is empty, that is, all the PNs selected by the SN are used up, the SoN sends a new request packet to the SN, requesting the SN to determine a new PNCS for it, which ensures that enough PNs can be used by the SoN during the panda's stay. Moreover, since the SN knows about the basic location information of each node in the entire network, the PNs selected in our schemes are not concentrated near the SoN, but distribute anywhere in the network, and therefore have stronger positional randomness compared with other existing schemes. At the same time, as our schemes select more PNs to ensure the diversification of routing paths, the security performance of privacy protection is further improved. Our proposed new method greatly increases the security of the source node and provides better privacy preservation for them. In this paper, the SPS scheme in the case of replacing the PN is called the RSPS scheme, the SPS scheme in the case of not replacing the PN is called the NRSPS scheme, the DPS scheme in the case of replacing the PN is called the RDPS scheme, and the DPS scheme in the case of not replacing the PN is called the NRDPS scheme. In the performance simulation of Section 5, we explore the impact of replacing PNs on security performance, and the impact of the number of PNs on security performance and total communication overhead. Simulation results verify the effectiveness of our proposed schemes. For example, compared with the shortest path algorithm, RPBMP [16] and EPUSBRF [18], the average safety period of our proposed NRSPS scheme, is increased by 6.08 times, 2.78 times and 3.57 times respectively; compared with RPBMP and EPUSBRF, the total communication overhead of our proposed NRSPS scheme is reduced by 88% and 95%, respectively. We summarize our main contributions as follows:  
(1) We proposed two new grid-based source location privacy protection schemes.