System for NFC and audio multiplexing

In this paper we present the design of a system that can seamlessly multiplex NFC signal over earphones which are meant for transmitting audio signals. The system consists of modifying the existing hardware on earphones and adding custom hardware consisting of a mixer and splitter, and making changes to the NFC antenna on mobile phones. The modifications can be made quite easily and no software changes are required. This multiplexed system lets the user to use their ear phones to make NFC transactions. Our solution is cost effective and minute enough to be implemented on a very small earphone. The hardware is designed around the fact that the NFC system filters out 13MHz so any other frequencies that are induced are filtered out, similarly the human audible range spans from 20 Hz till 20 KHz which forms a band pass filter. In our approach, the inherent nature of the system and wide spread of frequencies are leveraged to design a simple passive circuit that aims at selective impedance rather than filtering the frequencies. The hardware design ensures that the system is compatible with existing NFC applications (apps) in mobile devices. We present details of a simple proof of concept implementation of the system. We also show that different kinds of applications can be installed on a mobile phone using our system.