It is not a new concept to use the reported phase from a UHF GEN 2 RFID tag to calculate direction of travel as well as velocity. Phase data can also be used to filter out stray tag reads, assuming the stray reads are static/non-moving tags while desired tags are in motion (i.e. through a portal), using an approach known as TD-PDOA (Time Domain Phase Difference of Arrival).
Here are a couple of resources to help you with implementing this technique. The first is a white paper which gives an overview and some theory on this approach and why it is the best versus some other options:
Phase Based Spatial Identification of UHF RFID Tags
Next is an application note from Impinj which gives some practical specifics on using the Speedway Revolution reader to get the phase data from the tag and how to use it to calculate velocity (and thus direction of travel):
Use of Phase Data with Impinj Speedway Revolution
The latest version of firmware for the Speedway Revolution reader from Impinj will also allow you to read out doppler frequency directly. However, you actually need to slow down the read rate such that using that particular approach to measure tagged items moving at a walking or even forklift pace is not very practical.
Remember that phase resolution and time window are inversely related; you need a very long time window (packet length) to get down to the few Hertz of doppler resolution required at UHF frequencies and typical velocities. So, for most applications, utilizing phase data is the best approach.