RFID tag tracking using phase likelihood

A radio frequency (RF) system to estimate a tag state of an RFID tag wherein the tag state includes a position and a velocity in a coordinate system, the system comprising:

• a local oscillator;
• a modulator connected to the local oscillator;
• an RF power amplifier connected to the modulator to send a transmit signal to cause a tag signal to be returned from the RFID tag;
• a receive path to receive a tag signal from the RFID tag, the receive path including a quadrature mixer;
• and
• a processor connected to the quadrature mixer, the processor operable to recursively determine a measured phase of the tag signal by making a plurality of phase measurements, determine a plurality of possible actual phases of the tag signal using a prior estimate of the tag state, produce a channel phase model including a sum of periodically spaced distribution functions based on a modulo function of a fraction of an RF carrier wavelength, and recursively update a current tag state estimate to produce a plurality of updated tag states, map the plurality of updated tag states, each to one of the plurality of possible actual phases, use the measured phase as an assumed central location in a phase distribution in the channel phase model to produce a plurality of phase likelihoods, and produce the current tag state estimate using the plurality of phase likelihoods.

Apparatus for estimating a tag state of an RFID tag wherein the tag state includes a position and a velocity in a coordinate system, the apparatus comprising:

• means for recursively determining a measured phase of a tag signal at an RFID receiver;
• means for determining a plurality of possible actual phases of the tag signal from a prior estimate of the tag state;
• means for producing a plurality of updated tag states by recursively updating a current tag state estimate;
• means for mapping each of the plurality of updated tag states to one of the plurality of possible actual phases;
• means for producing a plurality of phase likelihoods using the measured phase as an assumed central location in a phase distribution;
• and
• means for producing the current tag state estimate using the plurality of phase likelihoods associated with the updated tag states.