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.
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.
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.
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.
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
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
UHF RFID Tag Emulator
- Development of new protocols and modulation schemes
- Complex emulation tasks