Radio Frequency Identificaton (RFID)

Identification of items by using passive farfield tags is an evolving market. With read ranges up to 10 m, passive ultra high frequency (UHF) radio frequency identification (RFID) can be used for various applications. They are used for example in logistics, apparel industry, and for road pricing. Some applications require not only identification but also localization of the tags, e.g., identifying the tag which is closest to the reader.

Localization of RFID tags based on received signal strength indication (RSSI) or angle of arrival (AoA) techniques is rather inaccurate due to unresolved multipath effects. To get precise ranging information of RFID tags a novel time of flight (ToF) based method was developed which enables wideband time of flight measurements with standard conformal RFID tags. This method is based on a superposition and clever averaging of a wideband-ranging signal. Thereby, it is possible to get a wideband ranging result without additional cooperation from the tag, which would be problematic due to stringent energy constrains on the tag.

Testbed for Localization of RFID tag by using a superimposed wideband ranging sequence

A UHF RFID reader testbed for evaluation and verification of a novel ranging method was implemented on a software defined radio (SDR) platform, namely the USRP 2292 from National Instruments. A custom lightweight EPC decoder, which delivers precise timing information of the sub-bit edges in addition to the EPC decoding, which is one requirement for the ToF based ranging, was implemented in FPGA user logic. The reader design consists of user logic written in VHDL for the signal processing where real-time requirements have to be respected. On top of this custom user logic is a microcontroller, which is used for configuration of the testbed and EPC compliant communication between reader and tag. The setup consists further of a custom Matlab class, which controls the reader via Ethernet.

Hardware modifications to optimize the RF performance of the USRP 2292 for UHF RFID operation with broadband ranging were conducted. External in- and outputs of the local oscillator signals for better phase coherence were attached to the RF daughterboard. Furthermore, the baseband filters were changed to allow broadband ranging.

To evaluate the ranging method a tag positioning system was built with winches mounted in the corners of a room, such that an RFID tag could be placed on multiple locations with high reproducibility and accuracy.

Measurement setup for localizing an RFID tag. A software defined radio platform, communication antennas, PC running Matlab and the RFID tag hung onto four lines.

© Holger Arthaber

Measurement setup used for evaluation of the wideband time of flight ranging

Extension of an SDR UHF RFID Testbed for MIMO and Monostatic Time of Flight Based Ranging

To gain more flexibility and enable multiple-input multiple-output measurements (MIMO) the testbed was extended such that multiple SDRs can be synchronized in baseband such that synchronous processing of the tag response is enabled. To decrease the influence of the phase noise also the local oscillator signals can be derived from only one source and be distributed among the SDRs.

This setup enables MIMO localization with multiple antennas, the separation between identification and localization to optimize each part separately for performance, and the use of dual frequency tags.

Blockdiagram of the software defined MIMO setup. Multiple slave units receiving a local oscillator signal fromm the master. Ethernet connection and synchronization are shown.

© Holger Arthaber

Block diagram showing the essential connections between multiple SDRs for MIMO measurements

Picture: 4 software defined radio units connected to a local oscillator distribution unit. All units are connected to an ethernet switch and a synchronization unit.

© Holger Arthaber

Picture showing a measurement setup with four SDRs

Dual Frequency Tag for Localization in the ISM Band

Ranging accuracy in multipath environments is strongly dependent on the bandwidth that is available for the ranging process. Since RFID tags are designed for a high sensitivity in the narrowband UHF RFID band, their delta radar cross section is strongly frequency dependent and very narrowband. Furthermore, the available power spectral density for the ranging signal is rather low due to stringent regulations in the UHF band. Therefore, a dual frequency tag was developed with an industrial partner. This dual frequency tag has two antenna ports. One is used for the EPC communication and power transfer just like a normal tag. The other port is only used for modulation of a second antenna, which can be designed for any frequency band. The benefit is that this second antenna could be designed for a flat frequency response of the delta radar cross section in the ISM band. Using the ISM band also allows using a higher power level of the ranging sequence and therefore the signal to noise ratio can be improved. Furthermore, the self-interference due to the continuous wave carrier signal for energy supply of the tag is avoided.

Dual frequency RFID tag inside an anechoic chamber with pyramidally shaped radio frequency absorbers. The tag is mounted on a material which is transparent for radio frequencies.

Dual frequency tag in the anechoic chamber

Measurement of the delta radar cross section of RFID tags

The delta radar cross-section of RFID tags and other backscatter modulation based systems is an essential parameter, for instance in the context of the emerging topic of indoor localization. It describes the generally complex-valued difference between the two possible states of reflection, which such a system can show during the tag-to-interrogator communication. For this, an extension of the scalar radar cross-section to a complex value is necessary. A comprehensive measurement system capable of determining the delta radar cross-section in different frequency bands, for different power levels, and for different incident angles of the electromagnetic waves has not been commercially available so far. Such a fully functional system was developed and assembled. This system can be used to examine entirely passive or battery assisted tags in the UHF frequency range around 900 MHz and in the ISM band around 2.45 GHz and 5.8 GHz in an anechoic environment. In addition, a supply carrier in the 900 MHz range can be provided for ISM band measurements. The conducted measurements revealed for example a highly nonlinear power dependency of the delta radar cross-section of passive RFID tags.

Inside an anechoic room with pyramidally shaped wave absorbers, an RFID tag is mounted on top of a rotary stage. The probe antenna is mounted on a swing arm.

© Holger Arthaber

Anechoic measurement setup

Modularly assembled test system for measuring the complex reflection coefficients of RFID tags. Vector signal analyzer, power amplifier, power sensor, circulators and RF modules.

© Holger Arthaber

Advanced measurement setup with carrier cancellation

Further research activities

  • Dynamic Channel Measurements and Emulation for DSRC and RFID applications
    • 868 MHz and 5.8 GHz pathloss and Doppler profile emulation
    • Anechoic chambers, antennas, channel emulation SW&HW, FPGA
Block diagram: The roadside unit as well as the onboard unit is wirelessly connected in separate anechoic chambers with a channel emulator between both chambers.

© Holger Arthaber

Channel emulator for DSRC communication, block diagram

Picture: A box with a front panel featuring a power switch, a rotary switch, a display and multiple status LEDs.

© Holger Arthaber

Channel emulator for DSRC communication, channel emulator unit

Graph: typical attenuation over time behaviors which can be emulated.

© Holger Arthaber

Channel emulator for DSRC communication, graph

Picture: An anechoic chamber with its door opened. On the inside are pyramidally shaped radio frequency absorbers.

© Holger Arthaber

Channel emulator for DSRC communication, anechoic chamber

UHF RFID reader development platform and tolling reader implementation

  • Multiprotocol capabilities
  • Entire RF⁄IF⁄BB system implementation
  • Parallelized correlative receiver in mixed VHDL⁄embedded design
  • CU⁄UL certified
Picture: Several modules with many cables and unused connectors

© Holger Arthaber

RFID reader: module-based setup for development

Picture: Assembled PCB containing the radio frequency components and an FPGA for baseband processing.

© Holger Arthaber

RFID reader: final PCB

UHF RFID Tag Emulator

  • Development of new protocols and modulation schemes
  • Complex emulation tasks
Picture: A stacked PCB setup with 3 optical fibers connected. On the PCB, an antenna structure as well as an FPGA is visible.

© Holger Arthaber

Laser powered RFID tag emulator