Rene Martinez

Power reflection coefficient analysis for complex impedances in RFID tag design

Kurokawas method of calculating the power reflection coefficient from the Smith chart in the situation when one complex impedance is directly connected to another is applied to passive RFID tag design, where power reflection is important, as it determines the tag characteristics. The performance analysis of a specific RFID tag is presented together with experimental data, which is in close agreement with the theory.

Phase based spatial identification of UHF RFID tags

In this paper, we give an overview of spatial identification (determining position and velocity) of modulated backscatter UHF RFID tags using RF phase information. We describe three main techniques based on PDOA (Phase Difference of Arrival): TD (Time Domain), FD (Frequency Domain), and SD (Spatial Domain). The techniques are illustrated with modeling and simulation example in free space and in presence of multipath using a multi-ray channel model for amplitude and phase of the received tag signal in deterministic environment. We also present and discuss the experiments performed in a real RFID warehouse portal environment.

Sensitivity and Impedance Measurements of UHF RFID Chips

In this paper, we describe the sensitivity and impedance measurement method for UHF RF identification (RFID) chips. The measurements are performed using an RFID tester (RFID reader with variable output power and frequency) and a vector network analyzer. No special impedance matching is required: chips can be connected to standard 50-Omega connectors allowing the sensitivity and threshold impedance to be measured directly in a fast and efficient way. We present experimental data for two UHF Gen2 chips (NXP UCODE G2XM and Impinj Monza 2) in thin-shrink small outline packages. The results have been verified using two chip assemblies matched to 50 Omega . This method can also be applied to chips in other packages: flip-chip, strap, etc.

Passive tag-to-tag communication

In this paper, we describe a novel passive RFID system capable of direct tag-to-tag communication in the presence of external radio frequency field. Tags talk by modulating the external field and thus backscattering the commands to each other. We present the system concept and show its hardware implementation based on TI MSP430 microcontroller. We also provide the theoretical model for modulation depth vs. distance which agrees with experimental results (maximum tag-to-tag communication distance). Finally, we discuss possible applications and outline future work.

A technique for simultaneous multiple tag identification

We present for the first time ever, a technique for simultaneously decoding multiple tag responses (i.e., a collision). Additionally the technique provides a considerable degree of robustness and alleviates the problem of missed tags. A theoretical discussion of the underlying principle is done. We provide simulation results when this technique is used specifically for the EPC Gen 2 protocol; the resultant increase in identification rate is 20% when two tag signals are simultaneously decoded and 30% when three tag signals are simultaneously decoded. Experimental results presented show that the extracted individual tag signals (when there is a collision of two or more tag responses) are virtually identical to the single tag signal. We believe that the signal extraction technique discussed here will eventually be a standard feature of all base stations.

A stacked antenna broad-band RFID front-end for UHF and microwave bands

A stacked antenna front-end for RFIDs is presented. Schottky diode based voltage doubler charge pumps are used on the front-end. The use of schottky diodes permits broadband operation for UHF (915MHz) as well as Microwave (2.45GHz). The entire chip has a low power EEPROM, digital state machine, clock recovery and data demodulation circuits; and the chip implements the ISO 18000-6B protocol for passive UHF (915MHz) operation. The chip is built on n-well, dual poly digital process with schottky diodes and EEPROM and comparisons are made between an unoptimized front-end built on 1um and an optimized front-end built on a 0.35 process. We provide a theoretical derivation for the superior orientational performance of a stacked antenna transponder as compared to a single antenna transponder. The tag front-end impedance is measured at UHF and Microwave bands for various powers and the threshold power for the tag is calculated. Based on the threshold power measured, the range of the tag is estimated and the omni-directional superiority of a stacked dipole is shown.

Wireless aircraft fuel quantity indication system

A wireless fuel quantity indication system (FQIS) has been developed using an RFID-enabled sensing platform. The system comprises a fully passive tag, modified reader protocol, capacitive fuel probe, and auxiliary antenna for additional energy harvesting. Results of fluid testing show sensitivity to changes in fluid height of less than 0.25in. An RF-DC harvesting circuit was developed, which delivers up to 5dBm of input power through a remote radio frequency (RF) source. Testing was conducted in a loaded reverberation chamber to emulate the fuel tank environment. Results demonstrate feasibility of the remote source to power the sensor with less than 1W of maximum transmit power and under 100ms dwell time (100mW average power) into the tank. This indicates adequate coverage for large transport aircraft at safe operating levels with a sample rate of up to 1 sample/s.