Gregory D. Durgin

New analytical models and probability density functions for fading in wireless communications

This paper presents new envelope probability density functions (PDFs) that describe small-scale, local area fading experienced by narrow-band wireless receivers. The paper also develops novel PDFs that describe the local area fading of two specular multipath components in the presence of other diffusely propagating waves. These PDFs are studied in the context of classical fading PDFs such as the Rayleigh, Rician, and other distributions.

Complete Link Budgets for Backscatter-Radio and RFID Systems

Backscatter radio - wireless communication by modulating signals scattered from a transponder (RF tag) - is fundamentally different from conventional radio because it involves two distinct links: the power-up link for powering passive RF tags, and the backscatter link for describing backscatter communication. Because of severe power constraints on the RF tag, a thorough knowledge of the backscatter channel is necessary to maximize backscatter-radio and radio-frequency identification (RFID) system performance. This article presents four link budgets that account for the major propagation mechanisms of the backscatter channel, along with a detailed discussion of each. Use of the link budgets is demonstrated by a practical UHF RFID portal example. The benefits of future 5.8 GHz multi-antenna backscatter-radio systems are shown. An intuitive analogy for understanding the antenna polarization of RF tag systems is presented.

RF Tag Antenna Performance on Various Materials Using Radio Link Budgets

Passive radio frequency (RF) tags in the UHF and microwave bands have drawn considerable attention because of their great potential for use in many radio frequency identification (RFID) applications. However, more basic research is needed to increase the range and reliability of a passive RF tags radio link, particularly when the RF tag is placed onto any lossy dielectric or metallic surface. This paper presents two new useful forms of the radio link budget that describe the power link of an RF tag system when the tag is attached to an object. These radio link budgets are dependent upon the gain penalty, a term which quantifies the reduction in RF tag antenna gain due to material attachment. A series of measurements, or radio assay, was used to measure the far-field gain pattern and gain penalty of several flexible 915 MHz antennas when attached to cardboard, pine plywood, acrylic, deionized water, ethylene glycol, ground beef, and an aluminum slab. It is shown that the gain penalty due to material attachment can result in more than 20 dB of excess loss in the backscatter communication link.

Gains For RF Tags Using Multiple Antennas

Backscatter radio systems, including high frequency radio frequency identification (RFID), operate in the dyadic backscatter channel - a two-way pinhole channel that has deeper small-scale fades than that of a conventional one-way channel. This paper shows that pinhole diversity is available in a rich scattering environment caused by modulating backscatter with multiple RF tag antennas - no diversity combining at the reader, channel knowledge, or signaling scheme change is required. Pinhole diversity, along with increased RF tag scattering aperture, can cause up to a 10 dB reduction in the power required to maintain a constant bit-error-rate for an RF tag with two antennas. Through examples, it is shown that this gain results in increased backscatter radio system communication reliability and up to a 78% increase in RF tag operating range.

Theory of multipath shape factors for small-scale fading wireless channels

This paper presents a new theory of multipath shape factors that greatly simplifies the description of small-scale fading statistics of a wireless receiver. A method is presented for reducing a multipath channel with arbitrary spatial complexity to three shape factors that have simple intuitive geometrical interpretations. Furthermore, these shape factors are shown to describe the statistics of received signal fluctuations in a fading multipath channel. Analytical expressions for level-crossing rate, average fade duration, envelope autocovariance, and coherence distance are all derived using the new shape factor theory and then applied to several classical examples for comparison.

Effects of multipath angular spread on the spatial cross-correlation of received voltage envelopes

This paper presents a simple formula relating multipath angular spread to small-scale fading statistics. This formula is then applied to find an approximate spatial cross-correlation function for received voltage envelopes. The analytical approach is compared to 67 cross-correlation functions simulated with non-omnidirectional multipath. The equations in this paper provide insight for applying spatial diversity techniques to receivers operating in the presence of non-omnidirectional multipath.

Power-optimized waveforms for improving the range and reliability of RFID systems

A major limitation in passive radio frequency identification (RFID) is the read range from the reader to tag, which is limited by the power available to the radio frequency integrated circuit (RFIC). The power-optimized waveform (POW) is a new type of multiple-tone carrier and modulation scheme that is designed to improve the read range and power efficiency of charge pump-based passive RFICs. This paper presents the POW concept, an estimation of effects on existing class 1 generation 2 RFID systems, several example POWs, simulation results, and measurement results of read range gains using a POW.

Link Envelope Correlation in the Backscatter Channel

High-frequency backscatter radio systems operate in the dyadic backscatter channel, a pinhole channel whose envelope probability density function and bit-error-rate performance are strongly affected by link envelope correlation - the envelope correlation between the forward and backscatter links of the dyadic backscatter channel. This paper shows that link envelope correlation is most detrimental for backscatter radio systems using co-located reader transmitter and receiver antennas and a single RF transponder antenna. It is shown that using separate reader antennas and multiple RF transponder antennas will decrease link envelope correlation effects and a near maximum bit-error-rate can be achieved with link envelope correlation less than 0.6.

Survey of range improvement of commercial RFID tags with Power Optimized Waveforms

The power sensitivity of passive Radio Frequency Identification (RFID) tags heavily affects the read reliability and range. Inventory tracking systems rely heavily on strong read reliability while animal tracking in large fields rely heavily on long read range. Power Optimized Waveforms (POWs) provide a solution to improving both read reliability and read range by increasing RFID tag RF to DC power conversion efficiency. This paper presents a survey of the increases and decreases to read range of common RFID tags from Alien and Impinj with Higgs, Higgs 2, Higgs 3, Monza 3, and Monza 4 RFICs. In addition, POWs are explained in detail with examples and methods of integration into a reader.

Optimal Sliding Correlator Channel Sounder Design

The sliding correlator technique remains one of the most versatile and effective methods for sounding the radio propagation channel in next-generation wireless systems. Despite their utility, there has never been a comprehensive set of metrics and rules for the design of a sliding correlator channel sounder. This paper presents quantitative guidelines for balancing the many system parameters to achieve optimal levels of temporal resolution, dynamic range, processing gain, and Doppler resolution. The design procedure presented at the end of the paper will allow researchers to probe the new radioscapes that result as wireless systems are pushed to higher carrier frequencies, wider bandwidths, multiple antennas, and ubiquitous operation.