## 2 Background and Motivation
### Applying Opportunistic Mobile Collectors as Mobile Power Chargers  
Inspired by the recent breakthrough in WPT technology, this study aims to re-examine the network lifetime model for IoT networks concerned with data aggregation and transmissions. We envision the use of opportunistic mobile vehicles for both data collection and wireless power charging operations. The mobile vehicles (referred here as data mules) will carry power charging systems that can be used to wirelessly charge the IoT nodes while opportunistically traversing the network for data collection from the sensor-nodes. This is fundamentally a scenario that tends to exploit vehicular networks whereby vehicles are used as both mobile data collectors (data mules) and wireless energy transmitters (battery chargers), which traverse the network and wirelessly replenish the energy of the sensor nodes. Thus, data and energy exchange are enabled between the mules (data collectors and energy transmitters) and the sensor nodes (data transmitters and energy receivers). The transmitting node receives power from the mule as it transmits data to the Mule. As long as the data mules are within the vicinity of the sensor nodes, they can collect data and be able to wirelessly charge the nodes. We therefore propose a scheme whereby vehicles can be exploited as data mules for both opportunistic data collection and wireless power charging in order to save cost and optimize efficiency. The separate components of this scheme--(i) mobile data collection using vehicles as data mules and (ii) wireless mobile charging using a wireless charging vehicle (WCV)--have been well studied in [9; 10; 11; 21; 22] and [4; 12], respectively, and shown to be energy efficient for WSN applications. For instance, the data mule scheme seen in [9; 10; 11] was used for opportunistic data gathering and distribution for SC solutions. Accordingly, the authors posit that the data mule scheme has the potential to address two major concerns of data collection for WSN, which include monitoring of spatially separated regional areas (unconnected regions) and optimizing energy efficiency of the battery-powered sensor nodes. Essentially, mules can roam over the different regions [9], collect data from the sensors at a good proximity, buffer it and deliver it to the big data collection center or a remote access point (AP) [4; 22]. Moreover, the opportunistic style of data collection has been shown to be cost effective with low node-energy consumption value, since the collection protocol overheads are effectively transferred from the sensors to the mules. At this juncture, it is instructive to note that this paper significantly differs from other surveyed papers discussing only WCV or periodic wireless recharge with a mobile charging vehicle [4]. Our proposed scheme is significantly a type of delay-tolerant network (DTN) that employs different delivery mechanisms including using data mules among others [9]. Although the use of WCV [4; 12] or WPT has some similarity to our proposed scheme (i.e., using mobile data collectors as wireless mobile power chargers or energy transmitters), there are however some fundamental disparities between them. Firstly, from the surveyed papers, a WCV is only deployed for wireless power charging and not for data collection and transmission, whereas our proposed scheme opportunistically exploits the duo advantages of using the same mobile data collectors as wireless mobile power chargers or wireless power transmitters (WPTs). This is predicated on the notion that exploiting the same mobile data collectors as wireless mobile power chargers will further minimize the large delay associated with typical DTNs and further optimize collection efficiency and costs. However, because of the opportunistic style of data collection and wireless energy transmission, large delays can be tolerated even as real-time data exchange and energy transmission between the sensor-nodes and the Mules can assume minimal delay-tolerant.