Source-linked AI summary
FM Backscatter: Enabling Connected Cities and Smart Fabrics
Anran Wang, Vikram Iyer, Vamsi Talla, Joshua R. Smith, Shyamnath Gollakota
TL;DR
Outdoor everyday objects need low-power connectivity to cars and smartphones, but existing approaches do not meet the relevant deployment requirements. The paper uses ambient FM signals and a modulation technique that makes backscatter compatible with FM audio receivers. The resulting system reaches up to 3.2 kbps over 5–60 feet while consuming as little as 11.07 µW, with demonstrations on smart fabrics and posters.
Problem
Existing backscatter approaches are unsuitable for outdoor communication to cars and smartphones, motivating a low-power solution using ubiquitous signals and available receivers.
Method
The system uses ambient FM radio as the RF source and transforms RF backscatter multiplication into addition on FM-demodulated audio signals.
Results
Up to 3.2 kbps over 5–60 feet and as little as 11.07 µW were achieved, with audio and data demonstrated on car and smartphone FM receivers.
Takeaways & Limitations
FM backscatter enables everyday objects, including smart fabrics and posters, to transmit information to smartphones and cars outdoors.
Takeaways & Limitations
The current communication range is limited to 4–60 feet, so nearby devices may require separated FM bands or MAC protocols to share spectrum.
Abstract
from arXiv · showhide
This paper enables connectivity on everyday objects by transforming them into FM radio stations. To do this, we show for the first time that ambient FM radio signals can be used as a signal source for backscatter communication. Our design creates backscatter transmissions that can be decoded on any FM receiver including those in cars and smartphones. This enables us to achieve a previously infeasible capability: backscattering information to cars and smartphones in outdoor environments. Our key innovation is a modulation technique that transforms backscatter, which is a multiplication operation on RF signals, into an addition operation on the audio signals output by FM receivers. This enables us to embed both digital data as well as arbitrary audio into ambient analog FM radio signals. We build prototype hardware of our design and successfully embed audio transmissions over ambient FM signals. Further, we achieve data rates of up to 3.2 kbps and ranges of 5-60 feet, while consuming as little as 11.07μW of power. To demonstrate the potential of our design, we also fabricate our prototype on a cotton t-shirt by machine sewing patterns of a conductive thread to create a smart fabric that can transmit data to a smartphone. We also embed FM antennas into posters and billboards and show that they can communicate with FM receivers in cars and smartphones.
1 Introduction
The paper introduces FM backscatter, enabling everyday outdoor objects to transmit audio and data to cars and smartphones. Its modulation design makes RF backscatter compatible with ordinary FM receivers while supporting overlay, stereo, and cooperative techniques.
- Core design: The modulation technique transforms RF multiplication into addition on FM-demodulated audio signals.This makes backscatter compatible with the audio output of FM receivers.
- Overlay backscatter: Overlay backscatter embeds arbitrary audio and digital data in ambient FM signals for reception by ordinary FM receivers or smartphone software.Audio can be heard without additional processing, while digital data requires processing-capable receivers.
- Stereo backscatter: Stereo backscatter uses under-utilized stereo streams, or inserts a pilot signal, to transmit with low or reduced interference.The technique can make mono FM signals operate in stereo mode and use the interference-free stereo stream.
- Cooperative backscatter: Cooperative backscatter uses two nearby smartphones to imitate MIMO and cancel underlying FM audio during decoding.This enables decoding backscatter transmissions without interference from the ambient audio.
- Evaluation and applications: 3.2 kbps, 5–60 ft, and 11.07 µW are the reported maximum data rate, communication range, and integrated-circuit power consumption.The prototype was evaluated with a smartphone and a car FM receiver, and fabricated smart-fabric and poster applications were demonstrated.
- Motivation and contribution: FM backscatter uses ambient FM radio as its signal source to create transmissions decodable on FM receivers in cars and smartphones outdoors.The system transforms everyday objects into FM radio stations.
2 Application Requirements
Connected-city and smart-fabric devices need inexpensive, low-power outdoor communication because many locations lack dedicated power and fabrics require flexible form factors. The paper argues that FM backscatter addresses these requirements using pervasive FM infrastructure and low-cost hardware.
- Connected cities: Connected-city devices must broadcast continuously while maximizing battery life because outdoor signposts and bus stops often lack dedicated power.Reducing maintenance costs is a practical requirement for deployment at scale.
- FM signal availability: 0.7 dBm standard deviation over 24 hours indicates that received FM power is roughly constant over time at a fixed outdoor location.The measurements support FM signals as a reliable backscatter source.
- Connected cities: Almost 3 years of continuous transmission is achievable with the backscatter system, while an FM radio chip costs over $4 and backscatter can cost a few cents.The comparison motivates backscatter as a lower-power, lower-cost alternative for connected-city objects.
- Alternative technologies: BLE broadcast is limited to short packets every 100 ms and is therefore unsuitable for streaming audio; car antenna placement also limits outdoor interaction.The passage notes that car Bluetooth antennas are positioned inside the vehicle and may be shielded from smart objects.
- Smart fabrics: Smart fabrics require thin, flexible form factors and low current draw, but flexible batteries are limited to a 10 mA peak current.Conductive thread can produce flexible FM antennas on textile substrates, while available batteries constrain radio choices.
3 System Design
The system uses ambient FM broadcasts as a reliable outdoor backscatter source and designs modulation compatible with ordinary FM receivers. It exploits FM signal structure to transmit audio and data, including through underused stereo streams.
- 3.1 Survey of FM Radio Signals: FM radio satisfies the system’s requirements through ubiquitous high-power broadcasts and FM receivers integrated into smartphones.FM stations can transmit at powers up to 100 kW, and FM receivers are included in almost every smartphone’s LTE and Wi-Fi chipsets.
- 3.1 Survey of FM Radio Signals: 0.7 dBm is the standard deviation of strongest-station power over 24 hours, indicating a roughly constant and reliable ambient source.The measurements were collected once per minute at a fixed outdoor location.
- 3.2 Structure of FM Radio Transmissions: FM broadcasts combine a 30 Hz–15 kHz mono stream with an optional 23–53 kHz stereo difference stream and a 19 kHz stereo pilot.Mono carries L+R, while stereo carries L-R; RDS occupies 56–58 kHz.
- 3.3 Backscattering FM Radio: The modulation converts RF-domain backscatter multiplication into addition of ambient and backscatter audio at a receiver tuned to fc + fback.This allows digital data or audio to be embedded while remaining compatible with FM demodulation.
- 3.3 Backscattering FM Radio: A large fraction of the 100 FM channels are unoccupied, while the median nearest-channel frequency shift is 200 kHz and the worst case is below 800 kHz.The backscatter frequency should be selected where ambient FM power is lowest.
- 3.3.1 FM Backscatter capabilities: Underused stereo streams provide a lower-interference channel for backscattered data or audio, and a pilot can enable stereo decoding for mono broadcasts.The prototype avoids backscattering the existing 19 kHz pilot tone.
- 3.4 Data Encoding with Backscatter: 3.2 kbps is the maximum achieved data rate using 4-FSK with frequency-division multiplexing across four frequency sets.The system transmits eight bits per symbol at 400 symbols per second; higher symbol rates substantially degrade BER.
4 Implementation
The implementation combines an off-the-shelf baseband and FM-modulation setup with an integrated-circuit design for compact, scalable deployments. The IC uses digitally controlled frequency modulation and an antenna impedance-switching backscatter stage.
- Off-the-shelf design: The prototype uses an NI myDAQ for analog baseband generation and a Tektronix 3252 arbitrary waveform generator for FM modulation.This setup supports evaluation of both audio and data modulation.
- IC Design: The integrated-circuit design targets reduced size and cost and scalable fabrication for smart fabrics and posters.It is implemented in the TSMC 65 nm LP CMOS process.
- IC Design: A digitally controlled LC-tank oscillator uses an 8-capacitor binary-weighted bank to modulate the oscillator frequency.The capacitor bank is connected in parallel to an off-chip 1.8 mH inductor.
- Backscatter switch: 11.07 µW is the simulated total power consumption of the FM backscatter system, including a 0.13 µW switch operating at 600 kHz.The NMOS switch toggles the antenna between open and short impedance states.
5 Evaluation
The evaluation characterizes FM backscatter across frequency, range, data rate, interference, and audio quality. Results show usable audio-band operation, low BER at practical distances, and trade-offs between throughput, combining, and ambient-signal strength.
- Range: 20 ft is reachable at -30 dBm received power at the backscatter device, while -50 dBm remains reasonably strong at close distances.The authors attribute this close-range performance to FM receiver sensitivity relative to prior TV-based approaches.
- BER performance: At 100 bps, BER is nearly zero through 6 ft across -20 to -60 dBm and exceeds 12 ft when power is greater than -60 dBm.This operating point supports smartphone transmission across all surveyed locations described by the authors.
- BER performance: At 1.6 and 3.2 kbps, BER remains low through 16 ft when received power exceeds -40 dBm, while increasing bit rate reduces range.At 1.6 kbps, BER remains low to 3 ft at -60 dBm and 6 ft at -50 dBm.
- BER performance: Combining two consecutive transmissions with MRC is sufficient to significantly reduce BER, but MRC also decreases effective receiver throughput.The evaluation uses 1.6 kbps at -40 dBm and compares combining two through four transmissions.
- Stereo and audio performance: Stereo backscatter improves BER and audio quality by reducing interference, but requires stronger ambient FM signals to detect the 19 kHz pilot.For audio, stereo backscatter outperforms overlay at high powers; transforming mono transmissions into stereo operates at -40 dBm, while cooperative backscatter operates at -50 dBm.
- Stereo and audio performance: At -50 dBm, audio backscatter shows similar performance through 12 ft, while audio requires more power than data because data can use modulation and coding at lower rates.A PESQ score of 2 is described as fair to good for white noise, although the evaluated interference is real ambient FM audio.
6 Proof-of-concept Applications
The paper demonstrates FM backscatter in public posters and wearable fabrics. Poster antennas transmit data and audio to smartphones and cars, while a sewn shirt prototype transmits data during standing, walking, and running.
- Talking Posters: Posters can broadcast audio, notifications, links, and directions to smartphones, extending static advertising with interactive functions.The motivation specifically includes discounted event tickets and directions to an event.
- Talking Posters: A bus-stop poster uses a 40”x60” half-wavelength dipole, while a 24”x36” Super A1 poster uses a bowtie antenna fabricated with copper tape.Both poster antennas are designed as poster form factors for outdoor deployment.
- Talking Posters: At a bus stop with -35 to -40 dBm ambient power, the prototype poster transmits data and audio from a local 94.9 MHz station using overlay backscatter.The backscatter signal is created at 95.3 MHz and evaluated with a smartphone.
- Talking Posters: The poster is also evaluated with a car receiver positioned 10 ft away, without direct line of sight to the FM transmitter.The setup uses a poster mounted 5 ft above the ground and a 2010 Honda CRV.
- Smart Fabric: A cotton shirt prototype integrates a meander dipole antenna sewn from conductive 316L stainless-steel thread.The antenna is designed to fit the front of a standard 15-inch-wide adult small T-shirt and withstand repeated use or washing without thread oxidation.
7 Related Work
Prior backscatter systems rely on dedicated infrastructure, unsuitable outdoor signals, or additional user-carried devices. FM backscatter instead targets ubiquitous receivers and offers lower-power, longer-range connectivity than several alternatives.
- RFID backscatter requires expensive reader infrastructure, limiting its suitability for outdoor deployments beyond inventory tracking.
- Ambient TV backscatter is unsuitable for smartphones because they lack TV receivers, while broadcast TV channels are declining with digital-TV transition.
- Wi-Fi and Bluetooth backscatter approaches depend on scarce outdoor Wi-Fi, plugged-in infrastructure, or two devices carried by users.
- A poster can send notifications about discounted local concert tickets, illustrating an outdoor content-delivery application.
- FM backscatter is orders of magnitude lower power and cheaper than Bluetooth, avoids QR-code camera pointing, and exceeds NFC’s few-centimeter range.
- Personal FM transmitters are unsuitable for the target applications because they consume orders of magnitude more power.
8 Discussion and Conclusion
The paper identifies ambient FM as a new backscatter source and discusses deployment, scaling, range, power, and future capability extensions. Demonstrations include smart fabric communication under mobility scenarios.
- The work opens a new direction by showing that ambient FM radio signals can serve as signal sources for backscatter communication.
- Smart fabric is demonstrated with a cotton shirt carrying a sewn conductive-thread antenna, alongside BER measurements across mobility scenarios.
- 4–60 feet is the current range limit, but nearby devices can use different backscatter frequencies or MAC protocols to share spectrum.
- Coding could improve range, while imperceptible-audio techniques could make embedded data inaudible to users.
- Future power strategies include harvesting ambient FM or TV energy, solar power, and duty cycling transmissions based on proximity.
- Future extensions include dual-antenna indoor Wi-Fi and outdoor FM operation, phone-to-device links, and headphone-based FM antennas.