I am working on a project that detect angle of arrival of RFID Tag by processing the received phases of multiple antennas and I am using the ThingMagic M6 RFID Reader.
The phase of this Reader have an ambiguity of 180 degree, mean that when I read a phase ALPHA, I don't know if that is ALPHA or ALPHA + 180. I have already ask for support of ThingMagic and they answered that because the signal is modulated by the Transition and not the absolue value, so even the signal is inversed, they can demodulate correctly the signal.
But when I read this artical "Anchor-free Backscatter Positioning for RFID Tags with High Accuracy ". I see that with the Reader from Impinj they don't have that phase ambiguity. they can have phase from 0 to 360 degrees. But I found a doc from Impinj "Speedway® Revolution Reader Application Note" that says:
In addition, the Speedway Revolution reader receive signal processing introduces PI radians of ambiguity such that the reported phase can be the true phase (theta) or the true phase plus PI radians (theta+180)
So I am very confused that do we have some reader that can have range 360 degrees of phase ? Is this 180 degree ambiguity a standard characteristic of RFID Reader, every type of Reader have this problems or it is depend on which reader we use ?
I can confirm that the (current) Impinj readers have the same 180 degree phase ambiguity.
The answer from Thingmagic is correct, for correct data demodulation the absolute phase is not needed, therefore many COTS rfid readers don't provide you with this information.
In theory it should be possible to correct for this ambiguity by taking the received data into account. There is even some literature from 2013... http://ieeexplore.ieee.org/document/6694499
I have not found any COTS reader yet that is able to perform this correction.