# Analytical Model for the Duty Cycle in Solar-Based EH-WSN for Environmental Monitoring
## 2 Related Work
The core contribution is an algorithm that dynamically adjusts the sleeping period of nodes in order to allow them for harvesting sufficient RF energy from a surrounding LTE eNodeB. Finally, another sink-initiated MAC protocol is AH-MAC (Adaptive Hierarchical MAC) [45], which is suitable for low-rate large-scale wireless sensor networks supporting active monitoring applications or event-driven alarm systems. AH-MAC combines the benefits of LEACH [46] and IEEE 802.15.4 to implement a scalable, self-configurable and self-healing wireless sensor network that incorporates energy-harvesting at a reasonable cost: only predetermined cluster heads are equipped with energy-harvesting circuits in AH-MAC. Accordingly, the activity of regular battery-powered nodes is limited to uploading their data to the cluster heads, which support most network tasks. In receiver-initiated asynchronous MAC protocols, it is every receiver that requests data transmission from its senders. For instance, in the EH-MAC protocol proposed in [47], a receiver uses a probabilistic polling mechanism to request data packets. Compared to deterministic polling, this mechanism is more consistent with the unpredictable state of potential senders, given the temporal and spatial variability of the energy that can be harvested from the environment. Moreover, the contention probability value contained in the polling packet is dynamically adjusted by the receiver in order to reduce data packet collisions and maximize network throughput. Note that EH-MAC is very similar to the protocol proposed in [43], with the main difference that the polling activity in EH-MAC is transferred to the receivers. Another receiver-initiated MAC protocol is ODMAC (On-Demand MAC) [48], which exploits the fact that typically sensor networks are low traffic networks. In ODMAC the communication is on demand, meaning that a sensor node transmits a packet only upon being requested by a receiver. To do this, receivers periodically broadcast a beacon frame to indicate senders that they are ready to receive. Senders only wake up when they have queued packets to be transmitted to the sink; after waking up, they listen to the channel waiting for a beacon and may enter a contention period if other senders react to the same beacon. So, the burden of communication (idle listening) is transferred from receivers to transmitters, fact that benefits the overall energy balance because most of time transmission queues are empty (low traffic condition). Additionally, each receiver dynamically adjusts the beacon period according to the energy profiles of its sending nodes. As a result, the per-node duty cycle is kept as large as possible to maximize performance while, at the same time, energy neutral operation is preserved. LEB-MAC (Load and Energy Balancing MAC) [49] follows a similar approach, but in this case a receiving node stamps the next time it will wake up with a certain probability in the beacon message. This probability is locally computed based on recent energy-harvesting history, and thus it takes into account the variability of environmental factors. Upon receiving a beacon message, sending nodes can synchronize their duty cycles with the receiver duty cycle as long as they have sufficient energy. So, the resulting duty cycles and node throughputs are consistent with current energy states. ODMAC and LEB-MAC do not avoid contention between multiple senders trying to transmit data to the same receiver. In contrast, the authors of QAEE-MAC (QoS-Aware Energy Efficient) [50] propose an exchange of beacon messages between senders and receivers in order to reduce contention. Specifically, if a sender node wants to transmit a data packet, it first sends a Tx-beacon and waits for a beacon from the intended receiver (Rx-beacon). For its part, the receiver periodically wakes up and listens to the channel to receive all sender beacon frames.