We present the design of a communication system that enables two devices to communicate using ambient RF as the only source of power. Our approach leverages existing TV and cellular transmissions to eliminate the need for wires and batteries, thus enabling ubiquitous communication where devices can communicate among themselves at unprecedented scales and in locations that were previously inaccessible.To achieve this, we introduce ambient backscatter, a new communication primitive where devices communicate by backscattering ambient RF signals. Our design avoids the expensive process of generating radio waves; backscatter communication is orders of magnitude more power-efficient than traditional radio communication. Further, since it leverages the ambient RF signals that are already around us, it does not require a dedicated power infrastructure as in traditional backscatter communication. To show the feasibility of our design, we prototype ambient backscatter devices in hardware and achieve information rates of 1 kbps over distances of 2.5 feet and 1.5 feet, while operating outdoors and indoors respectively. We use our hardware prototype to implement proof-of-concepts for two previously infeasible ubiquitous communication applications.

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Ambient backscatter: wireless communication out of thin air

In many applications, we have to identify an object and then locate the object to within high precision (centimeter- or millimeter-level). Legacy systems that can provide such accuracy are either expensive or suffering from performance degradation resulting from various impacts, e.g., occlusion for computer vision based approaches. In this work, we present an RFID-based system, Tagoram, for object localization and tracking using COTS RFID tags and readers. Tracking mobile RFID tags in real time has been a daunting task, especially challenging for achieving high precision. Our system achieves these three goals by leveraging the phase value of the backscattered signal, provided by the COTS RFID readers, to estimate the location of the object. In Tagoram, we exploit the tag's mobility to build a virtual antenna array by using readings from a few physical antennas over a time window. To illustrate the basic idea of our system, we firstly focus on a simple scenario where the tag is moving along a fixed track known to the system. We propose Differential Augmented Hologram (DAH) which will facilitate the instant tracking of the mobile RFID tag to a high precision. We then devise a comprehensive solution to accurately recover the tag's moving trajectories and its locations, relaxing the assumption of knowing tag's track function in advance. We have implemented the Tagoram system using COTS RFID tags and readers. The system has been tested extensively in the lab environment and used for more than a year in real airline applications. For lab environment, we can track the mobile tags in real time with a millimeter accuracy to a median of 5mm and 7.29mm using linear and circular track respectively. In our year- long large scale baggage sortation systems deployed in two airports, our results from real deployments show that Tagoram can achieve a centimeter-level accuracy to a median of 6.35cm in these real deployments.

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Tagoram: real-time tracking of mobile RFID tags to high precision using COTS devices

Recently, ambient backscatter communication has been introduced as a cutting-edge technology which enables smart devices to communicate by utilizing ambient radio frequency (RF) signals without requiring active RF transmission. This technology is especially effective in addressing communication and energy efficiency problems for low-power communications systems such as sensor networks, and thus it is expected to realize numerous Internet-of-Things applications. Therefore, this paper aims to provide a contemporary and comprehensive literature review on fundamentals, applications, challenges, and research efforts/progress of ambient backscatter communications. In particular, we first present fundamentals of backscatter communications and briefly review bistatic backscatter communications systems. Then, the general architecture, advantages, and solutions to address existing issues and limitations of ambient backscatter communications systems are discussed. Additionally, emerging applications of ambient backscatter communications are highlighted. Finally, we outline some open issues and future research directions.

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Ambient Backscatter Communications: A Contemporary Survey

RFIDs are emerging as a vital component of the Internet of Things. In 2012, billions of RFIDs have been deployed to locate equipment, track drugs, tag retail goods, etc. Current RFID systems, however, can only identify whether a tagged object is within radio range (which could be up to tens of meters), but cannot pinpoint its exact location. Past proposals for addressing this limitation rely on a line-of-sight model and hence perform poorly when faced with multipath effects or non-line-of-sight, which are typical in real-world deployments. This paper introduces the first fine-grained RFID positioning system that is robust to multipath and non-line-of-sight scenarios. Unlike past work, which considers multipath as detrimental, our design exploits multipath to accurately locate RFIDs. The intuition underlying our design is that nearby RFIDs experience a similar multipath environment (e.g., reflectors in the environment) and thus exhibit similar multipath profiles. We capture and extract these multipath profiles by using a synthetic aperture radar (SAR) created via antenna motion. We then adapt dynamic time warping (DTW) techniques to pinpoint a tag's location. We built a prototype of our design using USRP software radios. Results from a deployment of 200 commercial RFIDs in our university library demonstrate that the new design can locate misplaced books with a median accuracy of 11~cm.

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Dude, where's my card?: RFID positioning that works with multipath and non-line of sight

Communication primitives such as coding and multiple antenna processing have provided significant benefits for traditional wireless systems. Existing designs, however, consume significant power and computational resources, and hence cannot be run on low complexity, power constrained backscatter devices. This paper makes two main contributions: (1) we introduce the first multi-antenna cancellation design that operates on backscatter devices while retaining a small form factor and power footprint, (2) we introduce a novel coding mechanism that enables long range communication as well as concurrent transmissions and can be decoded on backscatter devices. We build hardware prototypes of the above designs that can be powered solely using harvested energy from TV and solar sources. The results show that our designs provide benefits for both RFID and ambient backscatter systems: they enable RFID tags to communicate directly with each other at distances of tens of meters and through multiple walls. They also increase the communication rate and range achieved by ambient backscatter systems by 100X and 40X respectively. We believe that this paper represents a substantial leap in the capabilities of backscatter communication.

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Turbocharging ambient backscatter communication

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.

Phase based spatial identification of UHF RFID tags

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.

Passive tag-to-tag communication

A method and system of determining spatial identification of an object, such as orientation, size, location, range, and/or movement, using an RFID system is disclosed An RFID system can comprise one or more RFID reader receiving antennas that query one or more RFID tags coupled to the object The measurement of the phase of the tag responses at the reader antennas and phase differentials as a function of distance, frequency, and time are the basis of spatial identification The system can work with conventional Gen 2 tags and readers without modification of the tags or protocol

Method and system to determine the position, orientation, size, and movement of RFID tagged objects

An RFID reader directly controls computer network applications on the basis of information collected from an RFID tag. The RFID tag includes certain designated fields that identify a destination computer system and/or application program for data recovered from the RFID tag. The RFID reader can then distribute the collected information in a format and to a destination that is determined by the RFID tag, thereby eliminating the need for intermediary software programs or human operators to make such decisions about the distribution of information. This capability permits RFID tag information to be automatically collected and distributed to network applications for ultimate data processing and collection.

Networking applications for automated data collection

An identification protocol that includes multiple identification methodologies is used to isolate and identify a radio frequency (RF) transponder. In accordance with a preferred embodiment of the present invention, the RF transponder includes a memory device, an RF front end, a random number generator (RNG), a counter, and a signal processing circuit connected thereto and programmed to operate in accordance with an identification protocol (or portions thereof). In embodiments of the present invention, the identification protocol includes a Fail (or Fail_N) command, a Next_Cycle command, a Read_New command, a Cycle_Setup command, a Full_Acknowledgement (FAK) command, a Partial_Acknowledgement (PAK) command, and/or an Ambiguous_Acknowledgement (MK) command. If multiple RF transponders are transmitting their ID numbers simultaneously, the radio frequency identification (RFID) base station may chose to resolve these conflicts later by transmitting the Next_Cycle command, which moves the transponders back to a ready state. Alternatively, the Fail command can be used to isolate and identify individual RF transponders. This command may include an integer (N) that defines the number of “Fail routines” (or isolation routines) that are to be performed in response to receiving the command. Once the transponder is isolated, it can be acknowledged and/or move to a data_exchange state by transmitting a Read_New, FAK, PAK or AAK command, each one having unique characteristics. For example, the Read_New command includes an ID number and identifies data that is to transmitted back. The FAK command includes an ID number and requests that data identified in the Cycle_Setup command be transmitted back. The PAK command includes an ID number but does not request data to be transmitted back. Finally, the AAK command does not include an ID number or a request for data.

Shashi Ramamurthy

Papers

Phase based spatial identification of UHF RFID tags

Passive tag-to-tag communication

Method and system to determine the position, orientation, size, and movement of RFID tagged objects

Networking applications for automated data collection

Enhanced identification protocol for RFID systems