## 2 Literature Review
In addition, the authors have introduced a layered architecture for topology that supports elasticity in multi-hop communication and adopted unequal size in partitioning the network to form clusters to consume less energy and improve the reliability and lifetime. [PERSON] et al. [24] have studied the problem of enhancing the lifetime of a WSN by exploiting mobility wherein, a few number of nodes in the network move around in search of energy to recharge and aid the energy depleted stationary nodes to get charged from them. It has been shown through a simple analytical framework that their proposed mobile energy management scheme can improve the network's lifetime if the total energy utilization rate is less than the rate at which the energy is harvested together with sufficient number of nodes that can move around to support a self-sustaining eco system. The authors in [25] considered the problem of area charging by designing an efficient charging scheme based on Medial Axis based Charging (MAC) for a Wireless Mobile Charger (WMC). The WMC employed by the authors is directional, which can charge an area that has the shape of a sector. In addition, the WMC is capable of recharging sensors at the boundary of an Area of Interest (AoI). Furthermore, WMC has been designed to recharge any of the sensors inside the AoI with sufficient energy with minimum charging time. In order to deal with area charging, the AoIs are partitioned into rectangular and sector shapes subareas based on the medial axis of the AoIs. The authors proposed a rectangle-based moving approach and rotating approach for the first and second type of partitions, respectively, and calculated the charging time. They evaluated their proposal by conduction extensive simulation trails, and proved that their algorithm achieved a stable approximation ratio. In [26], the authors developed an Energy Stimulated Time Sync (ESTS) method based on a new time synchronization approach to provide ultra-low-power energy harvesting for wireless networks. The ESTS scheme proposed by the authors does not depend on time stamp exchange. However, it employs short radio frequency tones to synchronize energy harvesting nodes with that of the central node. They implemented their proposal on a real Microchip ATmega 256 RFR2 System-on-Chip (SoC) and Powercast P110B radio platform and analyzed its performance with the aid of extensive experiments to demonstrate that their scheme considerably reduces the energy consumption of the energy harvesting nodes than that of the timestamp-based synchronization approaches. Also, they showed that ESTS exhibited an accuracy of 5 ms on an average. The research in [27] demonstrated with the aid of in-field experimental study on RF wireless charging conducted based on the commercially available transmitters to measure the amount of power obtained from Powercast. The investigation reported by the authors reveal that the power transferred to a few meters of distance employing millimeter-sized antennas with small form factor was from mW to \(\mu\)W, i.e., at 40 cm distance, the setup is capable of harvesting mill watts of power continuously and for 140 cm up to 592 \(\mu\)W was harvestable. This range of power was found to be sufficient to charge small IoT devices. Specifically, the authors' focus was on the Qi- and NFC-aided charging. They performed several experimental evaluations and showed that the RF power transfer is more promising for many of the application scenarios and can charge devices with an obstacle between transmitter and receiver. In addition, their study showed that at 1.6 m distance, an output power 135 uW and at 1 m distance more than 1 mW was harvested despite the small size of the chip antennas. Furthermore, they advocate that if the impact of obstacles is restricted, RF wireless charging can turn out to be an appealing technology that can be utilized in a wide range of realistic use cases.