Using field strength scaling to save energy in mobile HF-band RFID-systems

Radio frequency identification (RFID) is a technology enabling a contactless exchange of data. This technology features the possibility to wirelessly transfer power to the transponder (opponent). HF-RFID is used in mobile devices like smart phones and shows potential for applications like payment, identification, etc. Unfortunately, the needed functionality increases the battery drain of the device. As a countermeasure, power-management techniques are implemented. However, these techniques commonly do not consider the whole system, which also consists of the communication to the transponder, to prevent wasting energy. One cross-system technique of reducing the wasted energy is magnetic field strength scaling, which regulates the power transfer to the transponder. This article shows three investigations made, regarding field strength scaling to prevent this wastage of energy. The results of one investigation, how to use field strength scaling at card detection phase in form of the PTF-Determinator method, is described in detail. This method determines the Power Transfer Function (PTF) during run-time and scales the provided power accordingly to save energy. As a case study the PTF-Determinator is integrated in an application to read digital business cards. The resulting power consumption and timing has been evaluated by simulation and measurement of a development platform for mobile phones. Furthermore, the impact of field strength scaling to the energy consumption of a state of the art NFC-enhanced smart phone has been analyzed. The results of the case study shows that up to 26% less transmission energy (energy drain of NFC) is needed, if field strength scaling is applied (proofen by measurement). According to this result a smart phone’s battery drain (energy drain of the whole system) can be decreased by up to 13% by using field strength scaling for this case study.