US-20260127396-A1 - ANTENNA ARRAYS AND SIGNAL PROCESSING FOR RFID TAG READERS

Abstract

Radio-frequency identification (RFID) systems use readers to query and locate passive RFID tags in stores, warehouses, and other environments. An interrogation signal emitted by an antenna array from a reader powers up the tag, which replies by modulating and backscattering incident radiation toward the reader. The antenna array in the reader detects the modulated and backscattered radiation, which is usually several of orders of magnitude weaker than the interrogation signal, as the tag's reply. Unfortunately, crosstalk between the antenna elements in the antenna array limits the reader's sensitivity, which in turn limits the range at which the reader can detect and locate tags. Increasing the pitch of the antenna array to greater than half the wavelength of the interrogation signal reduces crosstalk but introduces grating lobes that produce spurious replies. Fortunately, filtering these spurious replies yields sensitive measurements from an antenna array with a pitch large enough to suppress crosstalk.

Inventors

• James Morse
• Joe Mueller
• Prokopios Panagiotou
• Phillip A. Lindsey
• Paul Petrus

Assignees

• Automaton, Inc.

Dates

Publication Date

20260507

Application Date

20260105

Claims (4)

1. 1 . A method of deploying a radio-frequency identification (RFID) tag location system in a room, the method comprising: installing RFID tag readers on a ceiling of the room; measuring locations of the RFID tag readers in the room; aligning the RFID tag readers; and connecting the RFID tag readers to a power supply and/or to a system controller.
2. 2 . The method of claim 1 , wherein installing RFID tag readers on the ceiling comprises placing at least one of the RFID tag readers in an opening in a suspension grid of a drop ceiling.
3. 3 . The method of claim 1 , wherein measuring the locations of the RFID tag readers in the room comprises: measuring a distance from a first RFID tag reader to a point on a floor; and measuring a distance from the point on the floor to a corner of the room.
4. 4 . The method of claim 1 , wherein aligning the RFID tag readers comprises adjusting an orientation of at least one of the RFID tag readers with respect to the ceiling.

Description

CROSS-REFERENCE TO RELATED APPLICATION(S) This application is a continuation of U.S. application Ser. No. 18/743,725, filed on Jun. 14, 2024, which is a bypass continuation of International Application No. PCT/US 2022/081761, filed on Dec. 16, 2022, which claims the priority benefit, under 35 U.S.C. 119(e), of U.S. Application No. 63/290,326, filed on Dec. 16, 2021. Each of these applications is incorporated herein by reference in its entirety for all purposes. BACKGROUND A radio-frequency identification (RFID) tag reader, also called an RFID tag interrogator or simply a reader or sensor, is a device that communicates with RFID tags. A passive RFID tag is powered by the signal transmitted by the reader. The maximum distance or range between the antenna(s) of a reader and a passive RFID tag depends on the maximum power of the RF signal transmitted by the reader toward the RFID tag, the minimum turn-on power or sensitivity of the RFID tag, the maximum power of the tag's reply; loss, noise, and interference, in the communications channel between the reader and the RFID tag; and the sensitivity of the reader. The round-trip channel loss generally increases with range, so increasing the reader's range generally involves some combination of increasing the transmitted signal power, increasing the tag back-scattering efficiency and sensitivity, reducing noise and interference, and improving the reader sensitivity. Unfortunately, the FCC limits the maximum signal power transmitted to the tag (and hence the amount of power available for the tag's reply) and thermal noise fundamentally limits the reader sensitivity. With some conventional systems, FCC regulations on maximum transmitted power and path loss limit the maximum achievable range from a reader to a passive RFID tag to about 15 meters. SUMMARY In an RFID tag reader with an antenna array for transmitting interrogation signals and receiving RFID tag replies, crosstalk between antennas in the antenna array can also limit the maximum round-trip range. Generally, crosstalk is the undesired transfer of signals between communication channels. In an antenna array, crosstalk occurs when one antenna element in the antenna array receives an interrogation signal transmitted by a neighboring antenna element in the antenna array. Crosstalk can also occur via unwanted coupling between the circuit elements that are connected to the different antenna elements in the antenna array. Unlike noise, crosstalk between antenna elements is difficult to suppress because it is correlated with the signals transmitted by the antenna elements. Fortunately, crosstalk between antenna elements can be reduced by increasing the distance between adjacent antenna elements, e.g., so that the antenna elements are arrayed at a pitch, or center-to-center spacing, of more than half a wavelength at the carrier frequency of the interrogation signals. Unfortunately, increasing the pitch causes grating lobes to appear when the antenna array is steered to angles off boresight. Grating lobes introduce gain in unwanted directions and can confuse the direction-finding process applied to the signal from the antenna array. Fortunately, spurious measurements caused by the grating lobes can be discarded by filtering or by correlating angle-of-arrival (AOA) measurements made by several RFID tag readers simultaneously as described in greater detail below. An RFID tag reader that filters spurious signals caused by grating lobes can comprise a signal generator, an antenna array, and a processor. In operation, the signal generator generates an interrogation signal at a carrier frequency. The antenna array, which is operably coupled to the signal generator, transmits the interrogation signal to an RFID tag in a direction of a main lobe of an antenna pattern of the antenna array and receives a reply to the interrogation signal from the RFID tag. The antenna array comprises antenna elements arrayed at a pitch, or center-to-center spacing, of more than half a wavelength of the carrier frequency of the interrogation signal, so it also emits radiations along the direction of a grating lobe that appears in the antenna pattern when the main lobe is steered off boresight (normal to the antenna array). The processor, which is operably coupled to the antenna array, distinguishes the reply to the interrogation signal from the RFID tag from a spurious reply caused by the grating lobe when the main lobe is steered off boresight and estimates a location of the RFID tag based on the reply to the interrogation signal from the RFID tag. The pitch of the antenna elements can be at least 190 millimeters. Setting the pitch of the antenna elements to be more than half the wavelength of the carrier frequency reduces crosstalk between the antenna elements. The antenna array can have an antenna gain on boresight that is at least 27 dB greater than the antenna gain at an elevation of 90° off boresight. The antenna array can also include a gro