# Multi-Source Cooperative Data Collection with a Mobile Sink for the Wireless Sensor Network
## 4 Performance Evaluation with Simulations
One the other hand, a little more complex cooperation relation is formed so as to construct more efficient distributed LDPC codes when the sink node approaches each source node twice in wireless sensor networks. Figure 6: Performance of the one-round sparse cooperation with different number of source nodes, \(L=1024\) bits. Figure 7: Performance of the two-round sparse cooperation with different number of source nodes, \(L=1024\) bits. Fourthly, the moving trajectories of the sink node are discussed. We have addressed that the moving trajectory during the second round should be different from the trajectory in the first round so as to construct the distributed LDPC code with good performance. Taking the network with \(N=8\) cooperative nodes as an example, the different moving trajectories I and II for the second round are illustrated in Figures 9 and 10, respectively. Here, the moving trajectories for the second round are denoted by the solid lines and the trajectories for the first round denoted by the dashed lines are the same in the both figures. Figure 11 shows the BER performance of the two-round cooperation schemes with different moving trajectories of the sink node. It is observed that the moving trajectory I achieves a little better performance than the trajectory II. There is one small cycle consisting of \(S_{6}\) and \(S_{7}\) in Figure 9, while the nodes \(S_{1}\), \(S_{2}\), \(S_{6}\) and \(S_{7}\) compose two small cycles of the distributed LDPC code for the moving trajectory II in Figure 10. It is well known that the small cycles decrease the performance of the LDPC codes. The small cycles may decrease the BER performance of the distributed LDPC codes, even though the influence is reduced, attributed to the extension for the parity-check matrix. Therefore, small cycles should be formed as few as possible by optimizing the moving trajectory of the sink node and the cooperative partner selection of each source node. In addition, such small cycles can be greatly reduced as the number of cooperative nodes increases. Figure 8: Performance comparison of the one-round cooperation and the two-round cooperation schemes, \(L=1024\) bits. Figure 9: Moving trajectory I (solid lines) of the mobile sink during the second round, \(N=8\). Finally, the performance of the two-round cooperation schemes with different data sizes is compared in Figure 12. On the one hand, the increasing of the information data length \(K\) results in the longer LDPC codes with the code length \(L=K\times N/r\) at the sink node, which improves the BER performance attributed the coding gain. On the other hand, we assume the data from each source node undergo independent and identically distributed quasi-static Rayleigh fading, which varies from one frame to another. Thus, the block fading channels also vary when the frame length changes. Specifically, the increasing of the frame length also enlarges the fading block, which decreases the BER performance of the distributed LDPC codes. Here, the BER performance is almost the same for the three different data sizes because the improved coding gains may be used to compensate for the worse block fading channels. The frame error rate (FER) performance is also given in Figure 7.0 It is observed that the FER performance decreases as the frame length increases. Indeed, the influence of the data size on the performance is complicated because it is difficult to evaluate the performance of the LDPC codes under block fading channels with the variant block sizes. In addition, the time-delay  
Figure 7.1 Performance of the the two-round cooperation schemes with different moving trajectories of the mobile sink, \(N=8\), \(L=1024\) bits. Figure 7.2 Moving trajectory II (solid lines) of the mobile sink during the second round, \(N=8\). also increases for the bigger frame and thus the data size should be chosen considering these issues in practical wireless sensor networks.