# Energy-Aware Control of Error Correction Rate for Solar-Powered Wireless Sensor Networks
## 1 Introduction  
Wireless sensor nodes consume limited hardware resources and have a short lifetime, especially owing to small battery capacities. For this reason, many studies are actively being conducted in order to overcome the problems of restricted energy in wireless sensor networks (WSNs). In particular, studies focusing on utilizing ambient energy to power WSNs are being carried out [1]. Among various ambient energies, solar energy is most frequently employed for WSNs because it not only has a high energy density, but also satisfies periodicity. However, because the amount of solar energy that can be harvested varies dynamically depending on the environment (e.g., time of day, weather, season and location), the energy consumption of a node must be carefully controlled with the consideration of the following objectives:  
* Energy-neutral operation (ENO): The energy input during a harvesting period should not be less than the amount consumed during the same period. Because the energy harvested in one day can vary owing to environmental conditions, a node should adapt its power consumption rate to match the harvested energy for eternal operations. * Minimizing the waste of harvested energy: In order to satisfy ENO, it is important to control the energy consumption such that it is below the energy input. This means that the less energy a nodeconsumes, the better for satisfying the ENO conditions. However, in this case, a different problem can occur. Assuming that there is continuous sunny weather over several days, there may be a surplus of energy in excess of that required for a node's general operation; thus, overcharged energy would inevitably be discarded owing to the capacity constraints of a rechargeable battery. Therefore, it is important to not only satisfy the ENO conditions, but also keep the remaining energy from exceeding the battery capacity. * Reserving energy for the non-harvesting time: There is no energy to be harvested when the Sun has set. However, in many applications, the user wants to collect data at all times. Therefore, a node should reserve a certain amount of energy for the non-harvesting time. This requirement allows a node to operate regardless of the time of day. Meanwhile, error control is typically performed for data transmission between nodes through an automatic repeat request (ARQ) or forward error correction (FEC) on a data-link layer. Using the ARQ method, data are re-transmitted if acknowledgment (ACK) is not delivered, as ACK indicates that data have successfully been received before timeout. On the contrary, in the FEC method, parity is generated by means of data encoding and then transmitted along with the data. If an error occurs when the target node receives these data, then the broken data can be restored by decoding the parity bits. In general, it is well known that errors are frequently generated in the channels of most WSNs [2; 3]. Therefore, when the ARQ method is applied in a WSN environment, the number of retransmission requests increases, and thus, the amount of consumed energy increases proportionally. Therefore, the FEC method is more appropriate than the ARQ method for WSNs [4]. The FEC method restores errors based on the units of symbols, and the number of restorable symbols depends on the parity length. When a relatively long parity is applied, the number of restorable symbols becomes large. However, the amount of energy consumed also increases, and vice versa. Because of this, during FEC, the data-loss rate and energy-consumption rate have a trade-off relationship. In this study, we attempt to improve the amount of data collected from a WSN by applying an adaptive FEC scheme to a solar-powered node. Note that we employ the Reed-Solomon (RS) method [5] among the many FEC schemes, since it has been shown to be one of the most efficient methods.