# Analytical Model for the Duty Cycle in Solar-Based EH-WSN for Environmental Monitoring
## 1 Introduction  
A wireless sensor network is a wireless network of low-cost, low-power and small-in-size multifunctional nodes, which have limited sensing, processing and communicating capabilities and cooperate with each other to relay sensed data from a region of interest to one or multiple sinks. From its emergence in the early nineties, the main design goal faced by researchers in this area has been network lifetime. This is due to the fact that, for many years, it has been assumed that sensor nodes are battery-powered, and so it has become essential to develop solutions aimed at optimizing energy consumption. These solutions span all components of a sensor node and, in the case of the communication component, all layers of the protocol stack. However, the main research efforts have been devoted to the design of energy-aware MAC (Medium Access Control) protocols, since major sources of energy consumption take place at this layer; channel-sensing, packet transmission and reception, packet overhearing, collisions and idle listening. However, in contrast to channel-sensing, packet transmission and reception, packet overhearing and collisions, which represent energy expenses on the scale of the relatively short packet duration, idle listening is especially harmful, because a node might spend a great amount of time listening to the channel without receiving any packet. This is particularly true in the case of event-driven sensor networks designed for rare event detection,and also in the case of time-driven sensor networks supporting environmental monitoring applications, where reporting times are typically much larger than the duration of packets. Hence, in order to reduce the impact of idle listening, the majority of MAC protocols for battery-powered sensor networks are based on duty-cycling the communication activity (these protocols are described in more detail in the next section). Nevertheless, even the best of such protocols cannot avoid node batteries to become exhausted after a finite (and relatively short) period of time. On the other hand, the new vision of Internet of Things (IoT), from which WSN (Wireless Sensor Networks) will become an integral part, claims enhanced energy management mechanisms that lead to much longer network lifetimes while providing QoS (Quality Of Service) guarantees. To achieve this goal, the scientific and industrial communities have recently intensified their efforts towards the introduction of energy-harvesting technologies in wireless sensor networks, with the ultimate goal of taking self-sustained WSN from vision to reality. The size compatibility between harvesting devices and the small sensor nodes has been and continues being one of the main obstacles, since the efficiency of the former is directly proportional to their surface or volume. Fortunately, research efforts have made it possible nowadays to develop various cost-effective energy scavenging mechanisms tailored to the intrinsic characteristics of sensor nodes. Among them, photovoltaic cells constitute the most widely deployed alternative in spite of the unavailability of solar power during night times or under bad weather conditions. This is due to their higher energy conversion efficiency compared to other mechanisms, such as vibrational, electromagnetic, thermoelectric, wind, etc. References [1; 2; 3; 4; 5] provide good surveys on energy scavenging mechanisms for EH-WSN. However, size compatibility between harvesting devices and sensor nodes is not the only challenge faced by researchers in EH-WSN. In fact, this new paradigm poses additional and even more difficult challenges due to the time-varying nature and unpredictability of most energy sources.