Christopher R. Valenta

Rectenna performance under power-optimized waveform excitation

Power-optimized waveforms (POWs) have been shown to increase the power-conversion efficiency in energy harvesting circuits. This paper is the first attempt to study the effects of a POW on these circuits and determine how its design can affect circuit performance. As an example, a 5.8 GHz single-shunt rectenna was designed to investigate the variation of POW parameters. It was shown that there is an optimal range of subcarrier spacing to maximize POW gain and minimize voltage ripple. Furthermore, a relationship between the maximum number of equal energy subcarriers, ripple voltage, and circuit parameters has been determined.

Theoretical Energy-Conversion Efficiency for Energy-Harvesting Circuits Under Power-Optimized Waveform Excitation

Closed-form equations have been developed that calculate the maximum output power and energy-conversion efficiency for an energy-harvesting circuit under power-optimized waveform excitation. The theoretical model predicts how signals with high peak-to-average power ratios increase the output power available at low input powers and decrease the maximum energy-conversion efficiency at high input powers. The model shows agreement to within 0.7 dB with ideal simulated components. Additionally, the model provides a theoretical bound for a realized microwave energy-harvesting circuit prototyped at 5.8 GHz.

Multi-antenna techniques for enabling passive RFID tags and sensors at microwave frequencies

Multi-antenna techniques are typically avoided in passive RFID because of the large footprints required. However, the smaller footprints required at microwave frequencies such as the 5.8 GHz industrial, scientific, and medical (ISM) band allow the use of multiple antennas. Two new multi-antenna technologies are featured in this paper to provide power and communications to a passive wireless tag in the 5.8 GHz ISM band. A four-layer FR-4 PCB is presented, which uses a staggered-pattern charge collector (SPCC) and a retrodirective array phase modulator (RAPM). An SPCC is an energy harvester that has two independent antenna arrays. These arrays provide increased gain and beamwidth over a single-antenna source. A RAPM backscatters the reader-transmitted signal directly back to the reader and provides quadrature phase-shift keyed (QPSK) signaling.

Modulation and sensitivity limits for backscatter receivers

This paper discusses how a low-powered RFID tag or sensor motes backscatter modulation scheme - limited in ways that conventional digital wireless systems are not - may be adapted to the peculiar non-white noise properties of a backscatter receivers radio frequency electronic chain to maximize detection. The analysis and results in this paper enable longer-range operation of todays passive, or semi-passive RFID tags as well as future backscatter sensor links that operate in higher frequency bands.

Range improvement of backscatter radio systems at 5.8GHz using tags with multiple antennas

This paper presents an improvement of the read range, radar cross section (RCS) and reliability of radio frequency identification (RFID) tags using multiple antennas. Measurements were performed using custom built, semi-passive RFID tags with co-planar segmented-loop antennas. Results show an increased read range as compared to RFID tag with single segmented-loop antenna.

Backscatter channel measurements at 5.8 GHz across high-voltage corona

This study characterizes 5.8 GHz backscatter radio links in a transient, high-voltage power line environment. The measured results demonstrate how increased RF carrier frequency provides additional resistance to the noise, interference, and corona shielding of communication antennas that operate near high-voltage lines. The results lead to important design rules for low-powered wireless sensor applications deployed in the future smart grid.

Power Conversion Gain as a design metric for RFID systems

This paper presents a novel method for determining the power available to a passive radio-frequency identification (RFID) tag, the Power Conversion Gain (PCG). The power conversion gain of a tag combines the tags antenna pattern with its charge pump efficiency to give the system designer the tags output power as a function of angle and maximum received input power. This metric is more useful for system designers because it helps to determine the tags sensitivity to orientation which is exaggerated by the non-linearity of the charge pump. This paper demonstrates the use of PCG using measured results from a 4-stage Dickson charge pump operating at 5.8 GHz and a directional antenna as a worst case scenario.

DC power pattern analysis of N-by-N staggered pattern charge collector and N 2 rectenna array

This paper compares a 4-by-4 dipole staggered pattern charge collector (SPCC) rectenna array to a 16 dipole rectenna array arranged in a four series-four parallel configuration. While each of these energy harvesting circuits has an identical footprint, the SPCC has a DC power pattern which simultaneously provides high gain and wide beamwidth. This profile provides RF energy harvesting sensors with additional range and makes them less sensitivity to orientation. At 0.1 m range, the N2 rectenna has a peak DC voltage 20% larger than the SPCC, but the SPCCs beamwidth is 400% larger than the N2 rectenna array. At 1 m range, the SPCC beamwidth is 600% larger, and the peak voltage is 300% larger than the N2 rectenna control case.

Omnidirectional loop antenna for a 5.8 GHz microwave backscatter RFID tag

A novel miniaturized printed circular loop antenna with perimeter of nearly one wavelength is designed for operation with a radio-frequency identification (RFID) tag at 5.8 GHz. RFID tags and RFID-enabled sensors which operate at this frequency offer many benefits over traditional UHF and LF tags, such as smaller footprints and increased bandwidth. The loop is broken into conducting segments that alternate on either side of a substrate for capacitive loading needed to maintain a constant current and generate a near omni-directional radiation pattern. The performance of the loop antenna is measured in-situ with the tag and the reader configured as a microwave backscatter radio communication system at 5.8 GHz. An overview of the RFID tag design is presented along with measured backscattered pattern.

R.E.S.T. — A flexible, semi-passive platform for developing RFID technologies

The Radio-Frequency Identification (RFID)-Enabled Sensing Testbed (R.E.S.T.) is a flexible platform for developing RFID technologies (sensors, RF front ends, coding, etc.) across frequency bands from low-frequency through microwave and millimeter. Paired with a USRP N-200 software defined radio, the R.E.S.T. allows researchers to streamline RFID product development or experiment with new sensing techniques/RF architectures without the need for rebuilding an entire RFID tag. Instead, interchangeable daughterboards can be swapped with minimal code modification. This paper will present an overview of the R.E.S.T. platform; its usefulness as a research, teaching, and development tool; and some sample applications which have used the R.E.S.T. platform to date, including the development of a 5.8 GHz RFID backscatter radio system.