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Inter-Technology Backscatter: Towards Internet Connectivity for Implanted Devices

Vikram Iyer, Vamsi Talla, Bryce Kellogg, Shyamnath Gollakota, Joshua R. Smith

arXiv:1607.04663v1cs.NI

TL;DR

Severely power-constrained implanted devices cannot directly use conventional radios or fully plug-and-play with commodity mobile devices. The paper introduces inter-technology backscatter, which converts Bluetooth transmissions into Wi-Fi and ZigBee-compatible signals using commodity hardware. Prototypes generate 2–11 Mbps standards-compliant Wi-Fi signals, demonstrate ZigBee generation, and support contact-lens and implantable neural-interface proof of concepts.

  • Problem

    Severely power-constrained implanted devices cannot use conventional radios to communicate directly with commodity devices, while passive Wi-Fi requires specialized continuous-wave infrastructure.

  • Method

    Inter-technology backscatter transforms Bluetooth transmissions into Wi-Fi and ZigBee-compatible signals on the air using commodity radios as RF sources and receivers.

  • Results

    2–11 Mbps Wi-Fi signals are generated from Bluetooth transmissions, with ZigBee generation and application proof-of-concepts also demonstrated.

  • Takeaways & Limitations

    The approach supports direct communication between implanted or compact devices and commodity smartphones, watches, and other devices.

  • Takeaways & Limitations

    Conventional sideband backscatter creates a mirror copy that can fall outside the unlicensed ISM band, motivating the paper’s single-sideband design.

Abstract

from arXiv · show

We introduce inter-technology backscatter, a novel approach that transforms wireless transmissions from one technology to another, on the air. Specifically, we show for the first time that Bluetooth transmissions can be used to create Wi-Fi and ZigBee-compatible signals using backscatter communication. Since Bluetooth, Wi-Fi and ZigBee radios are widely available, this approach enables a backscatter design that works using only commodity devices. We build prototype backscatter hardware using an FPGA and experiment with various Wi-Fi, Bluetooth and ZigBee devices. Our experiments show we can create 2-11 Mbps Wi-Fi standards-compliant signals by backscattering Bluetooth transmissions. To show the generality of our approach, we also demonstrate generation of standards-complaint ZigBee signals by backscattering Bluetooth transmissions. Finally, we build proof-of-concepts for previously infeasible applications including the first contact lens form-factor antenna prototype and an implantable neural recording interface that communicate directly with commodity devices such as smartphones and watches, thus enabling the vision of Internet connected implanted devices.

1. INTRODUCTION

Inter-technology backscatter addresses the challenge of enabling severely power-constrained implanted devices to communicate directly with commodity mobile devices. The system transforms Bluetooth transmissions into standards-compatible Wi-Fi or ZigBee signals using commodity radios, with prototype demonstrations spanning medical and non-medical applications.

  • Implanted devices cannot use conventional radios for Wi-Fi, Bluetooth, or ZigBee because of severe power constraints.
  • Inter-technology backscatter transforms wireless transmissions between technologies on the air using commodity devices as RF sources and receivers.The design avoids dedicated backscatter readers and leverages widely available Bluetooth, Wi-Fi, and ZigBee radios.
  • The system creates Wi-Fi signals by backscattering Bluetooth advertising packets, despite differences in physical-layer specifications, carrier frequencies, and receiver requirements.The design transforms Bluetooth into a single tone, shifts it using single-sideband backscatter, and addresses bidirectional communication through Wi-Fi-based amplitude modulation.
  • The single-sideband design creates frequency shifts on only one side of Bluetooth transmissions using complex impedances, avoiding power-consuming 2.4 GHz oscillators.This design supports Wi-Fi signals shifted by tens of megahertz.
  • 2–11 Mbps Wi-Fi signals are generated by backscattering Bluetooth transmissions, while ZigBee signal generation is also demonstrated.The authors implement prototype hardware on an FPGA and estimate 28 µW consumption for backscattering 2 Mbps Wi-Fi signals.
  • Proofs of concept include a contact-lens-form-factor antenna, an implantable neural recording interface, and credit-card-form-factor communicating devices.The contact lens and neural interface are evaluated in vitro, while the credit-card devices use Bluetooth transmissions as the backscatter RF source.

2. SYSTEM DESIGN

The system design transforms Bluetooth transmissions into Wi-Fi signals and supports bidirectional communication by using Wi-Fi OFDM devices as amplitude modulators.

  • The design transforms Bluetooth transmissions into Wi-Fi signals through backscatter communication.
  • The system creates an 802.11b signal from a single-tone Bluetooth transmission.
  • Bidirectional communication uses OFDM Wi-Fi as an amplitude modulator.

2.1 Bluetooth Versus Wi-Fi

Bluetooth and Wi-Fi differ substantially in bandwidth, coding, modulation, and channel organization, creating the physical-layer mismatch that inter-technology backscatter must bridge.

  • Bluetooth LE broadcasts on three advertising channels and otherwise communicates across 36 hopped data channels in the 2.4 GHz ISM band.
  • Wi-Fi uses three non-overlapping 22 MHz channels and supports 1, 2, 5.5, and 11 Mbps 802.11b transmissions with different coding and modulation schemes.

2.2 Bluetooth as an RF source

The design turns Bluetooth into a controllable single-tone RF source by exploiting GFSK and reversing data whitening while accommodating fixed advertising-packet fields.

  • Bluetooth can produce a single-frequency tone when its GFSK input contains a constant stream of ones or zeros.GFSK encodes zero and one using two frequencies, and the Gaussian filter preserves a single tone’s spectral properties.
  • Data whitening: The design reverses Bluetooth data whitening to create the desired constant sequences of ones or zeros.Bluetooth whitening uses a 7-bit linear feedback shift register with polynomial x^7 + x^4 + 1.
  • Packet structure: Bluetooth advertising packets contain fixed preamble and access-address fields, while only the payload can be set arbitrarily.The advertising access address is 0x8E89BED6, and Android exposes only 24 of 32 payload bytes for arbitrary setting.
  • Packet structure: The Bluetooth CRC does not affect the generated Wi-Fi packet because it is transmitted on a different channel.
  • An envelope detector estimates when Bluetooth packets begin and can be configured to trigger only for transmitters up to 8–10 feet away.Energy detection limits false positives but does not accurately identify the packet beginning.

2.3 Generating Wi-Fi using backscatter

The design uses complex-impedance backscatter to shift a single-tone Bluetooth signal onto one sideband, then synthesizes standards-compliant 802.11b signals. It supports multiple Wi-Fi rates while avoiding mirror copies, though advertising-packet limits constrain packet sizes.

  • Single Sideband Backscatter Design: Single-sideband backscatter shifts a single-tone Bluetooth transmission on only one side, avoiding the mirror copy produced by prior approaches.The architecture uses complex impedances to emulate radio operations without power-consuming 2.4 GHz oscillators.
  • Single Sideband Backscatter Design: Square-wave approximations generate the required sine and cosine terms, while complex impedance states implement the resulting values at the backscatter switch.The four impedance states correspond to the complex values needed for frequency-shift generation.
  • Synthesizing 802.11b Signals: At 2 Mbps with a 22 MHz frequency shift, single-sideband backscatter eliminates the strong mirror copy and improves spectral efficiency relative to prior sideband backscatter.The comparison is shown in Figure 6 using the single-sideband and prior double-sideband spectra.
  • Practical Design Considerations: Bluetooth advertising packets support Wi-Fi payloads of 38, 104, and 209 bytes at 2, 5.5, and 11 Mbps, but cannot fit a 1 Mbps Wi-Fi packet.The design focuses on advertising packets because they are easier to control on commodity devices.

2.4 Communication to Backscatter Device

Because conventional receivers for Wi-Fi and Bluetooth require costly carrier synthesis, the system communicates with the backscatter device through amplitude patterns encoded in Wi-Fi OFDM symbols. Constant and random OFDM symbols create distinguishable peaks that a passive receiver can detect, while protocol-aware encoding handles coding and symbol-boundary artifacts.

  • Motivation: Conventional receivers for phase- and frequency-modulated Wi-Fi and Bluetooth signals require high-frequency carrier synthesis, unlike amplitude-modulation backscatter receivers.Wi-Fi uses DBPSK/DQPSK and Bluetooth uses frequency modulation, giving both signals relatively constant amplitudes.
  • AM Encoding: 802.11g OFDM symbols can encode amplitude patterns because random symbols spread energy across time samples, whereas constant symbols concentrate energy in the first sample.The design constructs constant symbols by using constant modulated bits across OFDM frequency bins.
  • Encoding Pipeline: The system preserves constant encoded sequences through convolutional encoding, interleaving, and modulation by using all-zero or all-one scrambled inputs.All-zero or all-one encoded bits remain uniform after interleaving and map to the same constellation point across OFDM bins.
  • Encoding Pipeline: Pilot bits and convolutional-encoder memory perturb ideal constant symbols, so the design accounts for uncontrollable pilots and preceding-symbol state.The encoder delay length is 7, meaning the previous six data bits affect the current symbol's first encoded bits.
  • AM Encoding: A passive peak detector distinguishes random and constant OFDM symbols, but consecutive constant symbols can create false peaks at each symbol beginning.The receiver output is shown as a red peak-detection trace in Figure 7.
  • Robust Detection: Each bit is encoded with two OFDM symbols to reduce false detections from consecutive constant symbols, and the preceding random symbol is selected to avoid cyclic-prefix glitches.The preceding symbol's final sample is chosen to have high amplitude so the peak detector sees a valid boundary peak.

2.5 Putting it all together

The system uses a query-reply protocol to coordinate communication between a Wi-Fi device and backscatter devices. This supports multiple backscatter devices by querying them sequentially.

  • Protocol: The Wi-Fi device queries a backscatter device over a reverse channel, and the device replies over the backscatter channel.The protocol separates the query and response paths described in Sections 2.4 and 2.3.
  • Protocol: Multiple backscatter devices are supported because the Wi-Fi device can query them one after another.

3. FPGA AND IC DESIGN

The prototype combines an FPGA-based RF front end and baseband circuit with a later low-power CMOS implementation. The hardware uses switched impedances for backscatter modulation and passive energy detection for reception.

  • Development Platform: The initial hardware was developed on an FPGA platform to characterize the system and build proof-of-concept applications.The design was subsequently translated into an IC to quantify power consumption.
  • RF Front End: The RF front end contains a backscatter modulator and passive receiver isolated by an SPDT switch between transmit and receive modes.The receiver is an energy detector built from passive analog components and a comparator.
  • RF Front End: The backscatter modulator switches among four impedance states using a cascaded two-stage SPDT network.The four states use a 3 pF capacitor, open impedance, a 1 pF capacitor, and a 2 nH inductor.
  • Signal Generation: Figure 9 contrasts random-data Bluetooth transmissions with the single-tone transmission created by interscatter.Random application data is shown in red and the created single-tone transmission in green.
  • IC Design: The IC implementation uses TSMC 65 nm low-power CMOS and comprises a frequency synthesizer, baseband processor, and backscatter modulator.

4. EVALUATION

The evaluation measures generated Wi-Fi performance, single-sideband interference, reverse-link reception, and ZigBee generation. Results show useful Wi-Fi range, similar packet loss at 2 and 11 Mbps, reduced interference from single-sideband design, and feasible reverse and ZigBee links.

  • 4.2 Measuring the Wi-Fi RSSI: 20 dBm transmit power achieves a Wi-Fi reporting range of around 90 feet.Higher Bluetooth transmit power increases the RSSI of backscatter-generated Wi-Fi packets.
  • 4.2 Measuring the Wi-Fi RSSI: 2 Mbps and 11 Mbps Wi-Fi transmissions have similar packet loss rates.Both use small payloads and preambles and headers encoded at the same bit rate.
  • 4.3 Efficacy of single sideband backscatter: Single-sideband hardware has negligible impact on concurrent iperf throughput at higher backscatter rates.Prior double-sideband hardware reduces throughput by creating a mirror copy on Wi-Fi channel 6; the single-sideband design therefore doubles spectral efficiency.
  • 4.4 Communication in reverse direction: The off-the-shelf peak-detector receiver achieves bit error rates less than 0.01 up to a distance of 18.The receiver has -32 dBm sensitivity for 160 kbps, with higher range possible through custom IC implementation.
  • 4.5 Generating ZigBee Using Backscatter: The system generates ZigBee-compliant packets by backscattering Bluetooth transmissions.The demonstration adapts techniques used to generate 802.11b signals.

5. PROOF-OF-CONCEPT APPLICATIONS

The prototypes apply inter-technology backscatter to contact-lens, implantable neural-recording, and passive-card form factors. These demonstrations evaluate communication feasibility while leaving security and usability outside the study’s scope.

  • 5. PROOF-OF-CONCEPT APPLICATIONS: The application evaluation covers communication only, while security and usability are outside the scope of the work.The paper evaluates prototypes of three applications enabled by inter-technology backscatter.
  • 5.1 Smart Contact Lens: A smart contact lens prototype communicates with commodity Wi-Fi and Bluetooth radios without dedicated reader hardware.The 1 cm diameter loop antenna, immersed in contact lens solution, achieves ranges of more than 24 inches.
  • 5.1 Smart Contact Lens: The contact-lens range is smaller than earlier plots because the antenna is smaller and liquid causes high signal attenuation.The measured range is similar to prior smart contact lenses using a dedicated RFID-like reader.
  • 5.2 Implantable Neural Recording: The paper evaluates an implantable neural-recording antenna using muscle tissue whose electromagnetic properties are similar to grey matter at 2.4 GHz.The in-vitro setup inserts the antenna into a 0.75 inch thick pork chop 0.0625 inch from the surface.
  • 5.3 Passive Cards: Passive cards communicate by backscattering single-tone Bluetooth transmissions, with evaluation using an 18 bit payload at 100 kbps.The prototype uses energy detectors to synchronize with Bluetooth transmissions and varies receiver location.

6. RELATED WORK

Related work includes ambient and Wi-Fi backscatter, full-duplex reception, passive Wi-Fi, and RFID readers. Inter-technology backscatter differs by transforming commodity Bluetooth transmissions into other standards without specialized infrastructure or readers.

  • Ambient and Wi-Fi Backscatter: Ambient and Wi-Fi backscatter encode information by reflecting or not reflecting existing transmissions, limiting control over packet content.Their backscattered bits are encoded at the granularity of transmitter packets, producing lower bit rates and requiring many flooded packets.
  • Full-Duplex Backscatter: Full-duplex backscatter systems achieve high data rates of 5–330 Mbps but require custom full-duplex radios at the receiver.This requirement prevents operation with existing devices.
  • Passive Wi-Fi: Passive Wi-Fi generates 802.11b transmissions by backscattering continuous-wave signals using double-sideband subcarrier modulation.The paper builds on this work while pursuing a different commodity-device architecture.
  • RFID Readers: RFID readers can connect smartphones through a headphone jack, but they cost $185 and lack the economy of scale of Wi-Fi or Bluetooth radios.Existing devices already include Wi-Fi and Bluetooth radios.

7. DISCUSSION AND CONCLUSION

The paper demonstrates Bluetooth-to-Wi-Fi and Bluetooth-to-ZigBee backscatter and prototypes implanted-device applications using commodity devices. It also identifies higher-throughput Bluetooth packets and OFDM-based protocols as future directions.

  • 7. DISCUSSION AND CONCLUSION: Bluetooth transmissions can generate Wi-Fi- and ZigBee-compatible signals through backscatter communication.The approach uses commodity-device radios and supports proof-of-concept implanted-device applications.
  • 7. DISCUSSION AND CONCLUSION: Future work could improve throughput using faster, longer BLE data packets and OFDM-based protocols such as 802.11g/n/ac.The paper states that OFDM-based protocols could improve achieved bit rates by an additional order of magnitude.
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