How do I install UHF RFID readers in close proximity?

Question

Greetings people of the internet, I need help... I work for a company that installs and maintains security and access control equipment. Our latest project involved installing UHF tag readers in the entrance lanes to a residential estate and RFID tags (technical specs below) on the vehicles of the residents who live there so that the gates would open when they approach and they don't need to touch the biometrics scanners and such...

The system is operational and works around 90% of the time, the other 10% presents the issue where the vehicle tag is read by the wrong reader, causing the wrong entrance lane gate to open. The tag readers we are using are the UHF5 series from ZKteco, the tags are Alien Technologies ALN-9654 mounted in the top left corner of the windshield of the vehicles. The readers are mounted on the same wall 4.5 meters above the ground, approximately 4 meters apart, facing in the same direction in their respective entrance lanes, and angled to read tags approximately 5 meters from the gates (which are directly under the readers).

In normal operation, a car will approach one of the gates, that lane's reader will read the tag on the car and open the gate, no issue. The problem comes when a car approaches a gate and the reader in the other lane, 4 meters to the side, picks up the tag and opens the wrong gate, preventing the vehicle from entering the estate. Is there any way to stop this from happening without changing the existing hardware on site?

Notes: From observation and research, my best guess is that that one of the readers is energizing the tags, and the tag is broadcasting it's EPC which is being picked up by the wrong reader, I reckon the RF transmitter is directional but the receiver is not which is why the wrong reader picks up the EPC sometimes, I have no idea if this is even correct though or how to stop it, this is not my area of expertise.

I am aware of RSSI filtering but as far as I can tell, it is not something I can configure on the readers we are using.

Answer 1

Its a common problem with UHF rfid. Antenna placement and power adjustment is the first port of call. One technique I’ve used is sequencing the polling - but in my case i had low level control of the radios. Other things to try are different antennas - pay attention ti the specs on the radiation pattern and sometimes having the antenna inset into the wall can manipulate the radiation pattern.

Answer 2

I think your assumption that "the RF transmitter is directional but the receiver is not which is why the wrong reader picks up the EPC sometimes" is not quite correct, as the transmitter and receiver (in the RFID reader) use the same antenna. The tags themselves are not very directional.

Given your description of the distances, the tag on the car in the "wrong" lane is about 45 degrees off-axis. The flat-panel antennas in those readers would not have a very narrow beam, so there's probably only a 3-6 dB dropoff at the wrong-lane position.

One solution is an RFID reader antenna with a narrower beamwidth. Alternatively you could add a second sensing mechanism that detects "car in lane" (e.g. proximity sensor, photoelectric beam, etc.) and only trigger a gate opening when there is both an RFID detection and a car in that lane.