Calculate distance to RFID tag?

Question

Is there a way to calculate/estimate the physical distance to a long-distance passive RFID tag when reading it with a tag reader? E.g. to determine the order of books in a shelf, or telling if one object is close or far away.

If the answer is 'No - not according to the standard', would it be possible to build a reader with this feature? (I guess the only way to achieve this would be to measure the time between call and response very precisely).

Answer 1

It is possible, but to what extent end precision depends on a lot of factors: reader and tag performance, the quality of the software and the resources you are willing to invest in such a software (both time and people in R&D).

There are mainly two ways this can be achieved: The first one relies on getting the RSSI, which is basically the signal strength. The main difficulty using this indicator is that signal strength depends on a lot of factors that can influence it like, reflections if the signal needs to pass a wood cabinet or a wall, the quality of the tag, etc.

The second one is use the time the response is received to an enquiry (Time Differece of Arrival between tags). Given that you know the speed of the beam you can estimate the distance given a very precise timer. The problem here is that this also is influenced by a lot of factors: the mean time the tag needs to complete a cycle (which you should know, and should be the same for every tag used), the timer precision which is not built precisely for these purposes.

Naturally a combination of both should be employed for maximum precission and both are actually used by companies that rely on these algorithms to provide RTLS (Real Time Location Systems) application through Triangulation and Trilateration.

For further information you can check: RTLS, RSSI, TDOA, Trilateration (and Multilateration).

Answer 2

It is possible. As far as I know the company below (I'm not working there, I just happen to know someone who worked there a year before):

http://www.lambda4.com/

is working on such a technology.

Answer 3

It may not be possible if you have a single reader; however if you have multiple receivers and reasonably clear lines of sight , "estimating" the distance becomes possible by looking at signal strengths. It's not trivial though, since the power radiated by a RFID tag is not isotropic (I.e. not uniform in all directions) due to the antenna design; if you have three receivers and a uniform source of RF, you can solve for the distance, but when you add in the antenna pattern and other factors like signal path attenuation and multiparty, it becomes really hard - especially when there are multiple devices in the vicinity.

This is at least in part because the RFID was not designed with an output pattern that helps optimize localization, such as a frequency chirp, short power bursts, or other modulation features that allow estimating the time of flight of the signal from source to receiver and back.

Answer 4

General equation to find distance to RFID tag is Ploss = 20⋅log[ (4 π ⋅ d) /λ]

In case of UHF RFID, the equation to find the gap or distance to passive tag from the reader is Pgap = 22.6(dB)+ Patt, where 22.6dB is the power for near field(λ =c/f ≈ 35cm), where f is frequency operated, Patt is the magnitude of POWER ATTENUATOR

             22.6+Patt = 20⋅log[(4 π ⋅ d)/λ],

In free space, by using the above equation, the approximate distance to RFID tag may be acheived..