Yunfei Ma

Accurate Indoor Ranging by Broadband Harmonic Generation in Passive NLTL Backscatter Tags

Millimeter-precision meter-distance real-time indoor ranging capability is challenging due to multipath reflections in a rich scattering environment. Traditional continuous wave (CW) phase-based ranging methods, although simple and flexible, are vulnerable to phase offsets and interferences. We improve the previous CW approach by passive broadband harmonic nonlinear-transmission-line (NLTL) tags. Since phase information is now contained within the second harmonic rather than the fundamental frequency, interferences and phase errors caused by direct reflections of the interrogating signal are greatly reduced. By the broadband property of NLTL, a heuristic multi-frequency CW method is formulated to resolve the phase integer ambiguity and to further improve ranging accuracy and robustness even under large phase errors. We present theoretical and simulation analyses, followed by experimental verification.

Minding the Billions: Ultra-wideband Localization for Deployed RFID Tags

State-of-the-art RFID localization systems fall under two categories. The first category operates with off-the-shelf narrowband RFID tags but makes restrictive assumptions on the environment or the tags movement patterns. The second category does not make such restrictive assumptions; however, it requires designing new ultra-wideband hardware for RFIDs and uses the large bandwidth to directly compute a tags 3D location. Hence, while the first category is restrictive, the second one requires replacing the billions of RFIDs already produced and deployed annually. This paper presents RFind, a new technology that brings the benefits of ultra-wideband localization to the billions of RFIDs in todays world. RFind does not require changing todays passive narrowband RFID tags. Instead, it leverages their underlying physical properties to emulate a very large bandwidth and uses it for localization. Our empirical results demonstrate that RFind can emulate over 220MHz of bandwidth on tags designed with a communication bandwidth of only tens to hundreds of kHz, while remaining compliant with FCC regulations. This, combined with a new super-resolution algorithm over this bandwidth, enables RFind to perform 3D localization with sub-centimeter accuracy in each of the x/y/z dimensions, without making any restrictive assumptions on the tags motion or the environment.

Harmonic-WISP: A passive broadband harmonic RFID platform

In conventional passive radio frequency identification (RFID) systems, downlink (reader to tag) and uplink (tag to reader) overlap on the same carrier frequency, which leads to severe self-jamming and reader collision problems. To resolve these issues, nonlinearity in passive RFID tags can be exploited to generate second or higher order harmonics for uplink data communication. The design of harmonic tag that allows efficient energy harvesting and harmonic generation at the same time is critical. We present Harmonic-WISP, the first harmonic RFID system integrated with the open-source wireless identification and sensing platform (WISP). Harmonic-WISP adopts a new routing strategy to ensure full power utilization in both energy harvesting and harmonic generation modes. The new platform can fundamentally eliminate self-jamming issues and can greatly reduce reader-to-reader interference. By integrating with WISP, the proposed platform can further allow flexible implementation and evaluation of efficient multiplexing and security protocols.

Reflective nonlinear transmission lines for single-antenna non-self-jamming RFID

We present a harmonic radio frequency identification (RFID) transponder based on reflective nonlinear transmission lines (NLTLs), where the tag size is significantly reduced with single antenna configuration. To show the feasibility and design procedure, two open-ended NLTLs are fabricated in IBM 0.13µm CMOS process. Both of the 0.35 mm by 0.95 mm open-ended NLTLs show around −11 dBm reflected second harmonic by 0 dBm small signal input. Distance of wireless transmission is characterized by single dualband/broadband antenna connection.

Passive ranging by low-directivity antennas with quality estimate

Compared to the high-directivity patch and horn antennas, miniaturized omni-directional antennas allow more flexible integration with portable devices because of their smaller size, lower cost, broader angle coverage and less phase center variation. However, locating passive tags with low-directivity antennas in indoor environment becomes even more challenging due to more multi-paths and weaker line-of-sight (LoS) path. We propose a simple solution by digital beamforming with two arrays of 2-element antennas in a broadband harmonic multi-static backscatter system. Millimeter-precision ranging in weak LoS environment is achieved in a broad and sparse frequency scheme with omni-directional antennas which offer peak and average gain of 0.5dBi and -1.9dBi respectively. Our system employs sensing frequencies that are separated wider than typical indoor coherence bandwidth and therefore, we show that ranging robustness can be further enhanced by estimating angles of arrival (AoA) gap, i.e., unreliable measurements from dominant multi-paths can be distinguished and rejected. We present the fundamental theory, simulation and experiments with a homodyne harmonic RFID reader in a rich scattering environment.

Real-time 3D robotic arm tracking in indoor environment by RF nonlinear backscattering

We demonstrate accurate real-time 3D robotic arm tracking in indoor environment via broadband RF nonlinear backscattering from passive tags, which is immune to mild visual obstruction from clothes and interior walls as well as static magnetic field distortion from the metallic structures and motors. The marker tag on the robotic arm has nonlinear backscattering built on an open-source RFID platform in order to resolve the self-jamming and multi-path problems in conventional RFID system. The sparsely selected frequencies in a broad bandwidth and the antenna spatial diversity enable efficient and reliable phase-based 3D localization against the ghost images from multi-path interference. We utilized one Tx and four Rx antennas with five frequencies from 829MHz to 1080MHz. The measured 20 Hz sampling had centimeter-level accuracy against an estimated 3D ground truth and millimeter-level deviation in many repeated measurements.

Accurate indoor ranging by broadband passive NLTL tags

Millimeter-precision, decameter-distance real-time indoor ranging capability is challenging due to multipath reflections in rich scattering environment. We present a solution by passive broadband nonlinear transmission line (NLTL) tags. Via harmonic backscattering, reader self-jamming and errors caused by direct reflections from ambient scatterers can be greatly reduced. By the broadband property of NLTL, a simple, robust method by multi-frequency continuous wave (MFCW) is formulated to resolve the phase integer ambiguity and to further improve ranging accuracy in the occasion of large phase errors. We present theoretical and simulation analyses, followed by experimental verification.

A passive wireless sensor with reflective nonlinear transmission lines for capacitive signal transduction

A passive RFID transponder with reflective nonlinear transmission lines (NLTLs) has been developed for capacitive sensor detection. By detecting the phase change in the harmonic signal generated by NLTLs, the capacitive sensor information can be monitored wirelessly. To show the feasibility and design procedure, two open-ended NLTLs with terminating capacitive sensors are implemented in IBM 0.13μm CMOS process. The NLTLs with Bragg cutoff frequency fB = 10 GHz and 15 GHz achieve an average of 0.25°/fF and 0.61°/fF in the phase of the reflected harmonic signal, when the modeled sensor capacitance changes from 290 fF to 900 fF and from 117 fF to 362 fF, respectively.

Real-time code-division multi-tag localization with centimeter accuracy

In conventional indoor passive radio frequency identification (RFID) systems, time-division multiple access (TDMA) protocols are designed to counter multi-path interference and inter-tag collision for digital ID retrieval. However, high-accuracy (centimeter-level and below) localization to track multiple tags simultaneously still awaits a feasible solution. Overlap on the same carrier frequency of the downlink (from reader to tags) and uplink (from tags to reader) leads to severe self-jamming and reader collision, which makes tag localization especially difficult. The present narrow bandwidth also limits the localization accuracy and reliability. Starting from the polling process and synchronization by TDMA, we propose the code division multiple access (CDMA) within the broadband harmonic backscatter RFID system to enable the simultaneous real-time (15 Hz) tag localization with centimeter accuracy and millimeter deviation. We investigate the impacts of the number of tags with respect to the sampling rate, tag power consumption and locating errors originated from the inter-tag collision, and then present an experimental prototype for verification.

Ubiquitous tagless object locating with ambient harmonic tags

Extremely low-cost passive RFID tags have become key components for the Internet of Things (IoT), where ubiquitous object locating is one of the most important functions. Although the number of tags in indoor environment continues growing fast, objects cannot be always assumed tagged, whether intentionally or unintentionally. Compared to tagged objects, a tagless target which neither emits nor modulates signal is much more difficult to locate, especially when the size is small. In this paper, we achieve accurate tagless object locating through a dense and wide non-uniform sampling in the Fourier domain of target reflectivity with the help of ambient passive RFID tags as landmarks. Unlike conventional RFID systems, we leverage nonlinearity in passive tags to backscatter second harmonic signals. The frequency separation of uplink and downlink in harmonic RFID allows ready interference cancellation. We embrace spatial diversity enabled by ambient tags and frequency diversity by broadband harmonic backscattering to improve accuracy and robustness. We present the fundamental theory and a prototype system to verify the proposed approach.