Using NI PXI Modular Instruments and LabVIEW to Build UHF RFID Development Platforms
Figure 1: Investigating Enhancements on UHF RFID Performance Using the NI PXI Software-Defined Radio Platform
"By using NI PXI modular instruments, we performed proof-of-concept enhancements for RFID systems to be validated not only through simulations, but also through actual hardware experimentation. This led to more meaningful, valuable results that are closer to real-world implementation."
- Joel Joseph S. Marciano Jr.,
University of the Philippines, Diliman
Developing and testing a variety of hardware and software techniques to improve the reliability and functionality of ultra-high frequency (UHF) radio frequency identification (RFID) systems in real-world environments.
Using NI PXI modular instruments and NI LabVIEW software to build reconfigurable UHF RFID reader and tag emulation platforms to rapidly experiment and prototype new ideas.
Joel Joseph S. Marciano Jr. - University of the Philippines, Diliman
N. Fernando Bautista - University of the Philippines, Diliman
Leonard Bryan B. Paet - University of the Philippines, Diliman
RFID is rapidly becoming the technological solution of choice for many applications such as inventory management, vehicular traffic management, real-time location services, and ubiquitous sensing networks. The emergence of these new applications demands several improvements to current RFID systems.
At the University of the Philippines Diliman, we are developing and testing possible improvements on passive RFID readers and tags operating in the 860 MHz to 960 MHz UHF band conformant to the Electronic Product Code Class 1 Generation 2 (EPC C1G2) protocol. These improvements include enhancing the signal-to-noise ratio (SNR) in the communication link and extending the read range or coverage of the system (see Figure 1).
To do this, we built a flexible test bed that emulates actual RFID readers and tags using NI PXI hardware and LabVIEW software. This RFID test bed serves as a platform for prototyping enhancements, which include a novel decoding technique for FM0 signals in tag-to-reader communication and merging “smart antenna” concepts with RFID readers and tags.
Reader Emulation Platform
The reader emulation platform simulates the operation of the transmitter and receiver sections of an EPC C1G2-compliant reader. The transmit section is composed of an NI PXIe-8108 embedded controller, an NI PXI-5652 RF signal generator, an NI PXIe-5450 I/Q waveform generator, and an NI PXIe-5611 I/Q vector modulator. The receive section, on the other hand, is an “RFID tag sniffer” that uses the NI PXIe-5641R IF transceiver. This transceiver digitizes the received signal and performs quadrature demodulation and downconversion to baseband. The system uses the onboard Xilinx field-programmable gate array (FPGA) to realize a bit slicer and an enhanced FM0 decoder. External components of the platform include an Alien ALR-9800 RFID reader and a UPM Raflatac passive RFID tag, whose communication is captured over the air and subsequently processed by the reader emulator.
To control the operation of the RFID reader emulator, we wrote a LabVIEW VI. The VI gathers relevant RFID reader-to-tag input parameters such as power-up/power-down duration, preamble values, and reader command sequences. These parameters generate encoded sequences that constitute a complex-valued baseband message waveform, which is then upconverted to the UHF band for transmission. This implementation transmits the reader waveforms that conform to the EPC C1G2 UHF RFID standard and the FCC Part 15 regulation used in automated tests.
Tag Emulation Platform
The tag emulation platform simulates the operation of the receiver section of an EPC C1G2-compliant tag. An innovation introduced in this study is the incorporation of multiple antennas for implementing receiver diversity and optimum combining to achieve performance improvements such as enhancing the SNR and extending the read range or coverage in RFID tags. The multiple-antenna tag test bed uses the NI PXIe-8108 embedded controller, the NI PXI-5652 RF signal generator, the 16-bit NI PXIe-5622 IF digitizer, the NI PXIe-5601 RF downconverter, and custom-made UHF printed dipole antennas. We used these NI modules to realize the following blocks of a UHF passive RFID tag: (1) the power harvester, (2) the amplitude shift keying (ASK) demodulator, and (3) the baseband processor.
The tag power harvester is comprised of a voltage multiplier and a voltage regulator that provides power to the tag. The ASK demodulator is a simple envelope detector that demodulates the RF signal, which is then sent to the baseband processor. The baseband processor decodes the envelope-detected signal, then extracts and interprets the RFID commands. Figure 2 shows the block diagram of the RFID reader and tag implementation on the NI PXI system.
Improvements in RFID Reader and Tag Performance
We developed and tested a novel FM0 decoder for UHF RFID readers and integrating smart antenna techniques, such as spatial diversity combining schemes, in RFID tags. Current FM0 decoding schemes, particularly the prevalent correlation-based schemes, are susceptible to the large +/- 22 percent FM0 data rate variations expected from the tag's backscattered signal.
We usually compensate for data rate variations by employing complex frequency synchronization schemes, but at the cost of high FPGA resource use and long processing delays. To address this issue, we developed a novel FM0 decoding scheme on the RFID reader emulation platform. The decoder uses duty cycle measurements that are independent of data rate variations as the main decoding parameter. Figure 3a shows the LabVIEW VI block diagram of the enhanced FM0 decoder.
FPGA synthesis reports that the enhanced FM0 decoder uses only 5.9 percent of the total FPGA slices and incurs only eight clock cycles of processing delay. The functionality of the decoder is verified with actual backscattered tag signals from commercial tags. In this setup, we used the reader platform configured as an RFID sniffer to capture the backscattered signal from RFID reader and tag communication. Figure 3b shows a sample received backscattered tag reply and the corresponding decoding waveforms. The enhanced FM0 decoder can decode an actual backscattered signal from the tag. Bit error rate (BER) measurements show that the enhanced FM0 decoder achieves higher performance gains as the number of samples per symbol used for duty cycle calculation increases.
We also conducted an investigation on improving RFID tag performance by integrating diversity combining schemes. We created pre and postenvelope detection combining techniques for passive UHF RFID tags using two antennas on the tag emulation platform. The combining techniques include selection diversity combining, predetection direct additive combining, predetection equal gain combining, predetection maximal ratio combining, postdetection direct additive combining, and postdetection ratio squared combining. Figure 4a shows the LabVIEW VI of the RFID tag system incorporated with the diversity combining schemes.
The system conducts a test to measure the improvements in the tag’s read coverage. The reader emulation platform sends 20,000 predetermined reader commands that are received and read by the tag emulation platform. The number of successful reads by the tag is recorded for each of the different diversity combining schemes at reader and tag separation of 0.6 m to 6 m in steps of 0.3 m. Figure 4b shows the tag emulation platform front panel with a successful read. Results of the experiments conducted using these platforms show an improvement of up to 26.67 percent in the read coverage of passive UHF RFID systems obtained by incorporating receive diversity schemes into RFID tags.
Benefits of Using National Instruments Hardware and Software
By using NI PXI modular instruments, we performed proof-of-concept enhancements to validate RFID systems not only through simulations, but also through actual hardware experimentation. This led to more meaningful, valuable results that are closer to real-world implementation. The reconfigurability feature of the NI PXI platform provided the needed versatility in exploring various hardware configurations—in this case, for smart antennas in UHF RFID systems—such as using multiple receiver chains and different spatial diversity schemes. Furthermore, the intuitive programming interface and the comprehensive library of built-in LabVIEW VIs significantly shortens the development period of our systems, such as the proposed decoding scheme for RFID tag signals.
We plan to expand the current RFID emulation platform to include real-time and automated testing of existing RFID systems, as well as testing of other RFID protocols. We will enhance the platform to support further research such as merging RFID with smart antenna technology, which we hope will lead to the development of the next generation of RFID systems.
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