1. Field of the Invention
The present invention generally relates to a radio frequency transmitter device and a method for adapting its transmit power.
2. Description of the Related Technology
Next generation sensor networks are powered by energy harvesting techniques, thus getting rid of batteries. Energy harvesting (also known as power harvesting or energy scavenging) is the process by which energy is derived from external sources (e.g., solar power, thermal energy, wind energy, salinity gradients, and kinetic energy), captured and stored.
Among the various harvesting options photovoltaic and RF are the most promising ones. With RF harvesting actually a wireless energy transfer is meant, where the source of energy is a transmitting device (1), designed especially for this purpose and placed in close proximity of devices with RF power harvesting (2) (see FIG. 1). The maximum distance between the detector or sensor and the RF source depends on the transmitted power.
The International Commission on Non-Ionizing Radiation Protection (ICNIRP) has issued guidelines for limiting exposure to time-varying electric, magnetic and electromagnetic fields up to 300 GHz in “Guidelines for Limiting Exposure to Time-Varying Electric, Magnetic and Electromagnetic Fields (up to 300 GHz)” (Health Physics, Vol. 74, No. 4, April 1998, pp. 494-522). Following these guidelines provides protection against known adverse health effects. An adverse health effect causes detectable impairment of the health of the exposed individual or of his/her offspring. A biological effect, on the other hand, may or may not result in an adverse health effect.
It is important that the recommended levels not be exceeded, especially in public spaces, like office buildings, where sensor/detector networks are commonly used. The maximum power that can be transmitted poses an upper limit to the distance between the transmitter and the detector and/or to the maximum power that can be used by the detector and/or to the minimum time needed to transfer a required amount of energy. These restrictions also limit the possible time instances that the RF transmitters may send wireless energy for powering up the sensor/detector network.
Hence, a need arises for devices and method that allow a safe energy transfer towards a sensor or a network of sensors, whereby the energy levels prescribed for safe transmission in the presence of living beings are respected.