# Multi-Source Cooperative Data Collection with a Mobile Sink for the Wireless Sensor Network
## 2 Multi-Source Sparse Cooperation Models with a Mobile Sink
### One-Round Cooperation Model  
In this model, each source node \(s_{i},1\leq i\leq N\) broadcasts test data to the sink node \(d\) and its neighbors sequentially when the mobile sink \(d\) approaches and awakes it. Specifically, when the source node \(s_{i}\) transmits signals, its neighbors and the sink node \(d\) listen to \(s_{i}\). Let \(R(s_{i})\) denote the node set, in which the member nodes can correctly receive the data from the source \(s_{i}\). It is assumed that at least one neighbor can obtain the correct data from the node \(s_{i}\) and this assumption is not difficult to be guaranteed in densely-deployed wireless sensor networks. If multiple neighbor nodes successfully receive the data from the node \(s_{i}\), we choose only one node \(s_{j}\in R(s_{i})\) as the partner of  
Figure 1: Architecture of a wireless sensor network with a mobile sink.  
the node \(s_{i}\) among all the candidates. The selected node \(s_{j}\) volunteers to not only assist the node \(s_{i}\) but also transmit its own data to the approaching sink node \(d\) during the data collection. In this way, a cooperative link is formed one by one as shown in Figure 2. Note that the cooperation model should be formed based on the moving trajectory of the sink node \(d\) and the reception results \(R(s_{i})\), \(1\leq i\leq N\), for some nodes may fail to decode the data of some other sources due to the fading and noise of inter-source channels.  
In Figure 2, it is a line-like cooperation model, where each source node in the cooperative link except the first node \(s_{1}\) and the last node \(s_{N}\) has a single partner to help itself and it also assists only another partner. For the two special nodes, the first node \(s_{1}\) assisted by the node \(s_{2}\) does not help any node, while the last node \(s_{N}\) participating the data transmission of the node \(s_{N-1}\) has no node to assist itself. Without loss of generality, the description about the cooperation relation refers to the general source nodes in this paper. We assume that such a cooperative link can be formed in the densely-deployed wireless sensor networks and the specific partner selection strategies are beyond the scope of the paper. Indeed, the cooperative network can be formed based on the geographical information of the source nodes, the inter-node channels, and the moving trajectory of the sink node. In Figure 2, the filled nodes denote the active nodes in this line-like cooperative model and the unfilled nodes denote the sleeping nodes in this snapshot. In this paper, we assume that the source nodes with data to upload are all included in the cooperative link and the sleeping nodes do not have data to transmit at this moment. Indeed, if some source nodes not included in the active link have data to report, they can transmit data to the nearest active nodes to assist them uploading the data through the active cooperative link. This is out of the investigation of this paper and we only aim at the joint data collection from the active nodes in the cooperative link.  
For the cooperative link, we will design effective coded cooperation scheme with LDPC codes to achieve significant performance with quite low complexity in the next section. Note that the cooperative link in Figure 2 is quite different from the traditional multi-hop transmission. On the one hand, each node \(s_{i}\) in the link jointly encodes and transmits its own data and the correctly received data from its selected partner during the cooperation, while the relay usually simply forwards the partner data in the traditional multi-hop transmission. On the other hand, the data rate does not decrease as \(N\) increases and the performance can be improved when more source nodes participate in the cooperative link in Figure 2. However, the transmission rate in the multi-hop transmission reduces as the number of hops increases.