# Travel Route Planning with Optimal Coverage in Difficult Wireless Sensor Network Environment
## 2 Related Work
### Rendezvous Based Schema  
In this schema, the moving trajectory, as well as the SPs of the mobile collector, are elaborately designed. Data transmission conducts only when the mobile collector stops at the SPs. Sensors far away from the SPs will transmit the data to a relay node close to the sojourn point in advance. Following are some representative protocols using this schema.  
In reference [31], the authors presented a data collection schema that adopts sink mobility technology called MWR. In MWR, the compatible sensor pairs are elected to function as multiple antennas to simulate virtual multiple input and output (vMIMO). In order to address the problem of delay minimization for data gathering using multi-hop communication which has been validated to be an NP-hard problem, authors adopt integer linear program to formulate this problem. Some sensors are defined as polling points (PPs) for the mobile sink to access information collection. Authors used a heuristic approach to find the optimal positions of PPs, according to three crucial metrics, such as the compatibility of non-associated sensors, the capability of covering the non-associated neighbors, and the performance of moving length reducing.  
In reference [32], a routing schema combined with clustering as well as dual data collection is presented, and it is called LBC-DUU. In LBC-DUU, the network is constructed with three-layer, they are sensor layer, CH layer, and mobile sink layer respectively. In sensor and CH layers, authors present a load balanced clustering (LBC) method to automatically partition the network into clusters by means of iteration. Then a sensor car is adopted as a mobile data collector to visit the polling points (PPs) which are handpicked. As the sensor car is equipped with two antennas, multiple CHs can upload their data to it together to reduce the time cost.  
In reference [33], the authors presented a tree-based algorithm called MSMA. In MSMA, different structures of the topologies for different root nodes are scheduled in the initial phase and then the topology information is restored in the corresponding nodes. When the sink moves, the close nodes are elected as root nodes, and the topology is switched using the restored information.  
In reference [34], the authors planned the trajectory of the tour for multiple mobile sinks and presented SHDGP. In SHDGP, authors define polling points as the position where the data collectors can stop for data gathering. Each sensor can be chosen as a polling point or extra polling points can be explored by mobile sinks. Due to the unstable wireless communication, the neighbor set of each polling point will be confirmed by tentative communication. Then a spanning tree is generated by calculating the average cost of each polling point. Finally, the generated tree is broken up into subtrees according to the maximal tour length of each mobile sink.  
Comparisons of different aspects of the mentioned protocols are listed in Table 1.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Protocol** & **Year** & **Targets** & **Routing Schema** & **Sink Type** & **Clustering** & **Topology** & **Controll** & **Contributions** \\ \hline LEACH [19] & 2000 & Energy efficient & Clustering-based & Single static sink & True & Distributed & Hierarchical routing \\ \hline PEGASIS [20] & 2003 & Energy efficient & Clustering-based & Single static sink & False & Distributed & Chain structure routing \\ \hline HEED [21] & 2004 & Energy efficient, & Clustering-based & Single static sink & True & Distributed & Competitional CHs selection \\ \hline EEUC [22] & 2005 & Energy balancing & Clustering-based & Single static sink & True & Distributed & Competitional CHs selection \\ \hline TTDD [25] & 2005 & Efficient data delivery & Data mule based & Multiple mobile sinks & False & Query driven & Virtual grid division, dissemination nodes selection \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison of some mentioned routing protocols.