# Autonomous Active Tag Using Energy Harvesting Strategies
## 1 Introduction  
The world of tags has evolved over the last few years. Passive tags appeared first, and are now very widespread in our society, for example, in contactless cards or as anti-theft systems. They store limited data and the reader has to be close to power and communicate with these passive tags. These characteristics mean they have to be used in a reduced physical space with a restricted amount of data. Then came active tags, which allowed communication over longer distances and greater information storage capacities. This brought a paradigm change as all the data required for an application can now be stored in the tag, instead of, for example, a remote server. Anyone with permission can access the history of a tag without the need for a server. However, they need to continuously power the elements required to perform these functions. Batteries are usually the way this is achieved, but they have to be recharged regularly. Supercapacitors are currently being used to replace them; although they have to be recharged like batteries, they have a longer life. The use of energy harvesting techniques is now widespread to provide energy in this kind of scenarios in order to convert the tag into an autonomous device. Autonomous active tags are electronic devices that obtain, store, and transmit data from the environment in which they are placed without any interaction of any user. The difference between them and passive tags is basically the internal source of power that active tags include to feed the device, because passive tags do not need their own energy source to operate. The main energy harvesting techniques to consider for the autonomous requirement are: radio-frequency, thermal, mechanical, and solar [1; 2; 3; 4]. There are other energy harvesting techniques that are being developed at the present time, such as fuel cells [5] and bio cells [6]. All these technologies generate energy to be stored in batteries and supercapacitors. Integrating energy harvesting techniques with storage elements in active tags turns them into autonomous active tags. Due to the evolution to an autonomous and connected world, here we are making an effort to develop a device adaptable to a great many applications that can establish a connection to other devices, is wireless and uses the energy that nature provides as much as possible. We shall now study the devices with different features and different harvesting techniques to obtain power. Some systems use the radio-frequency technique, for example, as the energy source to monitor when food has gone bad [7]. The system that manages the power and the data is at a distance of 6 m from the source antenna, or, as a monitor of temperature, at a distance of 50 cm, highlighting the large antenna that is needed to receive the power [8]. This last has a feature to store energy in a supercapacitor. Other systems use the thermal technique to obtain energy; for example, implementing a bracelet surrounded by heat sinks with a thermoelectric module (TEM) as a wearable for different applications powered by a battery [9]. The mechanical technique is used in [10], where a large sized system is developed using a piezoelectric combined with a triboelectric to obtain energy to power 50 LEDs (Light-Emitting Diodes) or some electronic devices such as a calculator or a temperature sensor. A great majority of the systems use solar energy as the technique to obtain power. One of these solar systems power wireless sensor nodes to extend their life, enabling them to wake up for less time, and increase the default transmission range [11]. The solar technique is used even in space, as satellites recollect the energy, save it in a battery and use it for transmissions [12]. There is an animal monitoring device that uses solar energy to know their behavior and follow their route using GPS (Global Positioning System). The device stores the energy in a supercapacitor [13].