A pinboard by
Colleen Josephson

PhD Candidate in EE, Stanford University


FreeRider lets us communicate efficiently using OFDM WiFi, ZigBee or Bluetooth.

We introduce the design and implementation of FreeRider, the first backscatter system that enables a tag to embed information on WiFi, ZigBee, or Bluetooth signals while these signals are simultaneously used for productive data communication. Furthermore, we are, to our knowledge, the first to implement and evaluate a multi-tag system. Backscatter is a communication technique that allows devices without active radios to communicate passively by reflecting and modulating information on top of received RF. One very well known example of a backscatter system is RFID. Backscatter systems consist of three parts: a transmitter that sends an RF excitation signal, a tag that receives and reflects the excitation signal, and a receiver that receives and decodes the reflected signal.

Backscatter tags are attractive because they use orders of magnitude less power than a standard WiFi transmitter. Low-power connectivity has become very important with the recent explosion of the Internet of Things (IoT), but RFID does not fill that role well due to a lack of inherent internet connectivity and the need for expensive custom transmitters and receivers.

A number of recent projects have explored alternatives to RFID. While very interesting and important, most of those projects require custom transmitters or receivers, and the transmitters often occupy an entire channel continuously sending a useless signal like a sine wave. This prevents the channel from being used by normal clients. For example, a typical WiFi backscatter transmitter occupies the whole channel and prevents normal clients from using it. Furthermore, none of these projects implement and evaluate a multi-tag system.

Freerider is a very low power connectivity solution for IoT devices that leverages existing infrastructure and allows the transmitter to continue communicating productively with normal clients in addition to backscatter tags. The excitation signals are used for productive communication between other devices (e.g a WiFi transmission between an AP and a laptop or phone, Bluetooth communication between a phone and a headset). Hence, FreeRider enables low-power connectivity for multiple tags without needing any new infrastructure hardware besides the tag itself.

We built a hardware prototype of Freerider , and our empirical evaluations show a data rate of ∼60kbps in single tag mode, 15kbps in multi-tag mode, and a backscatter communication distance up to 42m when operating on 802.11g/n WiFi.


A Review of Passive RFID Tag Antenna-Based Sensors and Systems for Structural Health Monitoring Applications.

Abstract: In recent few years, the antenna and sensor communities have witnessed a considerable integration of radio frequency identification (RFID) tag antennas and sensors because of the impetus provided by internet of things (IoT) and cyber-physical systems (CPS). Such types of sensor can find potential applications in structural health monitoring (SHM) because of their passive, wireless, simple, compact size, and multimodal nature, particular in large scale infrastructures during their lifecycle. The big data from these ubiquitous sensors are expected to generate a big impact for intelligent monitoring. A remarkable number of scientific papers demonstrate the possibility that objects can be remotely tracked and intelligently monitored for their physical/chemical/mechanical properties and environment conditions. Most of the work focuses on antenna design, and significant information has been generated to demonstrate feasibilities. Further information is needed to gain deep understanding of the passive RFID antenna sensor systems in order to make them reliable and practical. Nevertheless, this information is scattered over much literature. This paper is to comprehensively summarize and clearly highlight the challenges and state-of-the-art methods of passive RFID antenna sensors and systems in terms of sensing and communication from system point of view. Future trends are also discussed. The future research and development in UK are suggested as well.

Pub.: 02 Feb '17, Pinned: 28 Jun '17

Modulation in the Air: Backscatter Communication over Ambient OFDM Carrier

Abstract: Ambient backscatter communication (AmBC) enables radio-frequency (RF) powered backscatter devices (BDs) (e.g., sensors, tags) to modulate their information bits over ambient RF carriers in an over-the-air manner. This technology also called "modulation in the air", thus has emerged as a promising solution to achieve green communications for future Internet-of-Things. This paper studies an AmBC system by leveraging the ambient orthogonal frequency division multiplexing (OFDM) modulated signals in the air. We first model such AmBC system from a spread-spectrum communication perspective, upon which a novel joint design for BD waveform and receiver detector is proposed. The BD symbol period is designed to be in general an integer multiplication of the OFDM symbol period, and the waveform for BD bit `0' maintains the same state within a BD symbol period, while the waveform for BD bit `1' has a state transition in the middle of each OFDM symbol period within a BD symbol period. In the receiver detector design, we construct the test statistic that cancels out the direct-link interference by exploiting the repeating structure of the ambient OFDM signals due to the use of cyclic prefix. For the system with a single-antenna receiver, the maximum-likelihood detector is proposed to recover the BD bits, for which the optimal threshold is obtained in closed-form expression. For the system with a multi-antenna receiver, we propose a new test statistic, and derive the optimal detector. Moreover, practical timing synchronization algorithms are proposed, and we also analyze the effect of various system parameters on the system performance. Finally, extensive numerical results are provided to verify that the proposed transceiver design can improve the system bit-error-rate (BER) performance and the operating range significantly, and achieve much higher data rate, as compared to the conventional design.

Pub.: 07 Apr '17, Pinned: 28 Jun '17