
 The set of sensor nodes. 
Ł  The set of links. 
 The communication radius of sensor. 
 The set of RPs. 
 The optimal set of RPs with minimum data transmission cost. 
 The energy consumption parameter. 
 The stationary point where the Sink is located. 
 The total innetwork communication cost. 
 The maximum data collection delay. 
 The network lifetime. 
 The initial energy of each sensor. 
 The maximum length of ME’s trajectory. 
 The adjacent matrix of the treeshaped topology. 
 The length of trajectory composed of . 
 The energy consumption for transmitting one data packet from a node to its neighbor. 
 The multihop transmission link between the node and nearest RP. 
 The optimal data collection trajectory with minimum data transmission cost. 
 Data generation rate (packet/min) at node . 
 The subtree which rooted at node . 
 The average velocity of the Mobile Element. 
 The path between node and node . 
 Wireless link between node and node . 
 The ETX of link Ł. 
 The sets of outgoing links at node . 
 The sets of incoming links at node . 
 The ETX sum of the link between node i and corresponding RP, . 
 The sum of the data rate at all the decedent nodes of ,. 
 We define , where . This equation can be paraphrased as how many transmission count can be saved after we extend the path from node to node . 
