In a rechargeable wireless sensor network, the data packets are generated by sensor nodes at a specific data rate, and transmitted to a base station. Moreover, the base station transfers power to the nodes by using Wireless Power Transfer (WPT) to extend their battery life. However, inadequately scheduling WPT and data collection causes some of the nodes to drain their battery and have their data buffer overflow, while the other nodes waste their harvested energy, which is more than they need to transmit their packets. In this paper, we investigate a novel optimal scheduling strategy, called EHMDP, aiming to minimize data packet loss from a network of sensor nodes in terms of the nodes' energy consumption and data queue state information. The scheduling problem is first formulated by a centralized MDP model, assuming that the complete states of each node are well known by the base station. This presents the upper bound of the data that can be collected in a rechargeable wireless sensor network. Next, we relax the assumption of the availability of full state information so that the data transmission and WPT can be semi-decentralized. The simulation results show that, in terms of network throughput and packet loss rate, the proposed algorithm significantly improves the network performance.
MAC protocol design for scheduling data transmission and power transfer is given in the literature @cite_36 @cite_30 @cite_22 . It was noted that the power transfer varies depending on surrounding environments. A MAC protocol based on fair polling scheme is studied to achieve a data collection fairness @cite_36 . Moreover, each node contends channel adaptively to their energy harvesting rate. @cite_30 , a RF-MAC protocol is studied to jointly schedule power transmitters and energy harvesting rates in RWSN. RF-MAC focuses on the amount of energy delivery to the nodes, while eliminating disruption to data communication. @cite_22 , multiple WPT transmitters are grouped into different sets based on an estimate of their separation distance from the node to reduce the impact of destructive interference. Moreover, a MAC protocol is presented to transfer power to the nodes on request. Unfortunately, the MAC protocols in literature schedule WPT and data communication with the objective of interference elimination, which is different from the problem in this work.
@cite_36 @cite_22 @cite_30
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Energy harvesting wireless sensor network (EHWSNs) are being actively studied in order to solve the problems faced by battery-operated WSNs, namely the cost for battery replacement and impacts on the environment. In EH-WSNs, each node harvests ambient energy, such as light, heat, vibration, and uses it for sensing, computations, and wireless communications, where the amount of harvested energy of each node varies depending on their surrounding environments. MAC protocols for EH-WSNs need to be designed to achieve high throughput and fairness, however, the conventional MAC protocols proposed for EH-WSNs do not adapt to the harvesting rate of each node, resulting in poor fairness. In this paper, we propose a fair MAC protocol based on polling scheme for EH-WSNs. The proposed scheme adjusts contention probability of each node according to the individual harvesting rate, thereby increasing throughput achieved by nodes with low harvesting rate. We evaluate throughput and fairness of the proposed fair polling scheme by computer simulations, and show that the proposed scheme can improve fairness with little degradation of the overall network throughput. Wireless transfer of energy will help realize perennially operating sensors, where dedicated transmitters replenish the residual node battery level through directed radio frequency (RF) waves. However, as this radiative transfer is in-band, it directly impacts data communication in the network, requiring a fresh perspective on medium access control (MAC) protocol design for appropriately sharing the channel for these two critical functions. Through an experimental study, we first demonstrate how the placement, the chosen frequency, and number of the RF energy transmitters affect the sensor charging time. These studies are then used to design a MAC protocol called RFMAC that optimizes energy delivery to desirous sensor nodes on request. To the best of our knowledge, this is the first distributed MAC protocol for RF energy harvesting sensors, and through a combination of experimentation and simulation studies, we demonstrate 112 average network throughput improvement over the modified unslotted CSMA MAC protocol. Wireless charging through directed radio frequency (RF) waves is an emerging technology that can be used to replenish the battery of a sensor node, albeit at the cost of data communi- cation in the network. This tradeoff between energy transfer and communication functions requires a fresh perspective on medium access control (MAC) protocol design for appropriately sharing the channel. Through an experimental study, we demonstrate how the placement, the chosen frequency, and number of the RF energy transmitters impact the sensor charging time. These studies are then used to design a MAC protocol called RF-MAC that optimizes energy delivery to sensor nodes, while minimizing disruption to data communication. In the course of the protocol design, we describe mechanisms for (i) setting the maximum energy charging threshold, (ii) selecting specific transmitters based on the collective impact on charging time, (iii) requesting and granting energy transfer requests, and (iv) evaluating the respective priorities of data communication and energy transfer. To the best of our knowl- edge, this is the first distributed MAC protocol for RF energy harvesting sensors, and through a combination of experimenta- tion and simulation studies, we observe 300 maximum network throughput improvement over the classical modified unslotted CSMA MAC protocol.