Can an NFC phone act as an RFID Tag, which can be read at distances higher than 4 inches?
To be more specific, I'm trying to implement an Android application, which forces the phone to operate as an RFID tag. This tag is read by scanners placed around doors (thus the > 4 inches constraint) identifying the person and his whereabouts.
The phone will act as a smart tag, thus providing further use cases.
Thank you.
Answer
Most of the NFC enabled Android phones have low level capacity to emulate cards. The NXP chip that is used in the phones as well as the android kernel supports card emulation. However, the high level Android API does not expose these features most likely because some obvious security problems with such feature. To be able to read existing cards and emulate them would make it possible to easily duplicate access cards and other RFID/NFC tags and even some smart cards. This is already possible with the right equipment but to put that capability in the hands of every person with an Android phone might upset whole industries. For example, all physical access control systems rely on cards that could easily be duplicated by using an android phone. I think this feature is not there because enabling it would cause HUGE upset in multiple industries.
You can see that the kernel has support for card emulation (that is what Google Wallet Tap & Pay is using to enable you to use your phone for all your credit cards etc.):
git clone https://android.googlesource.com/platform/external/libnfc-nxp
If you do low level "hacking" you can get access to these features.