# Multi-Source Cooperative Data Collection with a Mobile Sink for the Wireless Sensor Network
## 2 Multi-Source Sparse Cooperation Models with a Mobile Sink
### Two-Round Cooperation Model  
In this model, the moving sink node traverses twice along the distribution region of the active source nodes \(s_{1},s_{2},\ldots,s_{N}\). The two rounds are along the different trajectories and construct different cooperative links. Each cooperative link can be similarly formed as in the one-round cooperation  
Figure 2: Multi-source sparse cooperation model with one-round moving of the sink node.  
model above. Here, each source node also performs in sequence when the mobile sink node approaches and awakes it. Figure 3 illustrates an example with 14 active sensor nodes. In the figure, the dashed and solid lines depict the two different cooperative links during the two rounds, respectively. It is shown that each node except three special nodes including the first node \(s_{1}\) in the first round, the last nodes \(s_{14}\) in the first round and the last node \(s_{12}\) in the second round, has two parters to help itself and also assists two partners. For the node \(s_{j}\in S\), we assume it assists \(s_{i}\) and \(s_{k}\) in the two rounds, respectively. For example, the node \(s_{6}\) assists \(s_{5}\) and \(s_{8}\) in the first round and the second round, respectively, as illustrated in Figure 3. For the special node \(s_{1}\), it is assisted by \(s_{2}\) and \(s_{7}\) and only helps \(s_{9}\). For the special nodes \(s_{14}\) and \(s_{12}\), the node \(s_{14}\) only has one parter \(s_{8}\) to assist it and helps another node \(s_{13}\), while the node \(s_{12}\) helps two parters \(s_{11}\) and \(s_{13}\) and only has the parter \(s_{13}\) to assist itself.  
In order to obtain the promising performance, the second trajectory of the moving sink node \(d\) should be different from the first one, though the two trajectories both traverse the nodes one by one in the cooperative group \(S=\{s_{1},s_{2},\ldots,s_{N}\}\). Moreover, when multiple neighbors can successfully obtain the data from the broadcasting source nodes, the parter selection algorithm in each step during the two rounds may affect the overall performance. In this paper, we mainly address the strategy of cooperative data collection and the optimal parter selection algorithm will not be investigated.  
Indeed, each moving round plays different roles in the cooperative data collection. In the next section, an efficient distributed encoding scheme is designed using LDPC codes to further illustrate the two-round cooperation model. Generally, for the first moving round, the sink node collects the information data broadcasted by each source node sequentially and each source node keeps the correctly received information data from its partner according to the dashed line in Figure 3. For the second round, each source node receives the encoded parity-check bits from its partner according to the solid line in Figure 3, and then jointly encodes its own data and the data collected from its selected partners during the two rounds. For the sink node, it collects the parity-check bits transmitted by each source node sequentially during the second round. The cooperative processing and encoding procedure will be explained in detail in the following section.