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Ambient Backscatter Communications: A Contemporary Survey

Nguyen Van Huynh, Dinh Thai Hoang, Xiao Lu, Dusit Niyato, Ping Wang, Dong In Kim

arXiv:1712.04804v1cs.NI

TL;DR

Low-energy wireless systems need communication approaches that avoid active RF transmission, and ambient backscatter offers one option by using surrounding RF signals. This survey synthesizes backscatter fundamentals, architectures, designs, protocols, applications, challenges, and research directions, concluding that ABCSs reduce power and deployment costs while remaining constrained by uncontrollable ambient sources and limited harvested energy.

  • Problem

    Low-power systems such as sensor networks need communication and energy efficiency, while ambient backscatter systems still face source dependence, scheduling, interference, security, interoperability, and limited bitrate and range.

  • Method

    The paper provides a comprehensive survey of backscatter fundamentals, architectures, hardware and protocol techniques, applications, existing solutions, challenges, and future research directions.

  • Results

    Ambient backscatter can lower transceiver power and eliminate dedicated RF-source deployment, while surveyed designs address communication improvement, multiple access, energy management, and related system issues.

  • Takeaways & Limitations

    Ambient backscatter is a promising technology for large-scale self-sustainable wireless networks, including wireless sensor networks and the Internet of Things.

  • Takeaways & Limitations

    Ambient backscatter sources are uncontrollable and unpredictable, while harvested RF energy is usually small, limiting stability, bitrate, and communication range.

Abstract

from arXiv · show

Recently, ambient backscatter communications 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. It is expected to realize numerous Internet-of-Things (IoT) 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.

I. INTRODUCTION

Ambient backscatter communications enables low-energy devices to communicate by reflecting surrounding RF signals, while introducing source-dependence, scheduling, interference, and deployment challenges. The paper surveys fundamentals, architectures, design techniques, applications, limitations, and future directions.

  • Ambient backscatter devices communicate by modulating and reflecting surrounding RF signals from sources such as TV, cellular, and Wi-Fi transmitters.
  • ABCS transmission efficiency depends on ambient-source type, source location, and environment, requiring designs tailored to particular operating conditions.
  • Dynamic ambient signals make transmission scheduling important, while licensed-source operation requires protocols that avoid interfering with licensed users.
  • The paper surveys ABCS fundamentals, architectures, protocols, applications, existing solutions, open issues, and future research directions.
  • Backscatter systems comprise monostatic, bistatic, and ambient architectures, with bistatic systems separating the RF source and receiver to reduce round-trip path loss.
  • Ambient RF sources reduce deployment costs and spectrum-allocation needs, but their unpredictability and lack of controllability complicate performance optimization.

B. Fundamentals of Modulated Backscatter Communications

Modulated backscatter encodes data by switching antenna loads between absorbing and reflecting states rather than generating an RF carrier. Receivers can decode these signals with ADC-based processing or lower-power averaging mechanisms.

  • Backscatter transmitters map bits to RF waveforms by tuning antenna impedance and changing the reflection coefficient.
  • Two-state load modulation typically represents bit 0 with an absorbing, impedance-matched state and bit 1 with a reflecting state.
  • ADC decoding samples received signals and uses differences between average power levels associated with reflecting and non-reflecting states.
  • An averaging mechanism replaces energy-intensive ADCs and oscillators with envelope averaging and threshold calculation to detect transmitted bits.
  • The transmitter and receiver use relatively simple components, supporting low power consumption, low implementation cost, and easier deployment.

C. Antenna Design

Antenna design determines received power, communication distance, and practical implementation in backscatter systems. The survey discusses frequency, gain, polarization, impedance matching, and trade-offs among size, cost, and performance.

  • The Friis-based distance model relates communication range to wavelength, transmit power, antenna gains, polarization efficiency, impedance matching, and receiver power threshold.
  • Higher operating frequencies can reduce antenna dimensions and may increase antenna gain and object immunity without the active-circuit power penalty found in conventional RF systems.
  • SHF backscatter can reuse signals from existing Bluetooth and Wi-Fi devices and exploit wider bandwidth for spread-spectrum or ultra-wideband operation.
  • Impedance matching maximizes absorbed or reflected RF power, and adjusting antenna impedance is more practical than changing chip impedance.

3) Antenna Gain:

Antenna design balances communication range, cost, size, orientation, and material-dependent losses. The section also reviews coding methods that improve robustness and throughput in backscatter systems.

  • 3) Antenna Gain:: Higher antenna gain generally extends transmission range, but high-gain antennas cost more and are larger.Low-gain antennas are preferred when devices are close or incoming-signal direction is unavailable.
  • 3) Antenna Gain:: On-object gain penalty depends on material properties, object geometry, frequency, and antenna type, so simulations and measurements are commonly used.
  • 3) Antenna Gain:: Polarization mismatch can prevent communication when transmitter and receiver antennas are oriented orthogonally.Parallel antennas maximize received power, whereas a 90° displacement creates complete polarization mismatch.
  • 3) Antenna Gain:: Circular polarization in monostatic systems limits both uplink and downlink polarization mismatch to 3dB.
  • D. Channel Coding and Decoding: Balanced block coding increases throughput by 50% compared with conventional techniques such as FM0.Short block-length cyclic coding uses polynomial-based encoding and can be implemented with a simple shift register.

E. Modulation and Demodulation

Backscatter systems use ASK, FSK, and PSK according to rate, noise, and multiple-access needs, while receiver detection trades simplicity against bitrate. The section also introduces basic and multi-antenna channel models and their link budgets.

  • E. Modulation and Demodulation: ASK, FSK, and PSK are the three basic modulation schemes, corresponding to changes in carrier amplitude, frequency, and phase.
  • E. Modulation and Demodulation: FSK is favored in bistatic systems because it fits FDMA, supports simultaneous transmitters, and resists noise and signal-strength variations.
  • E. Modulation and Demodulation: PSK is mainly adopted in ambient backscatter systems because it can support high-rate transmission using few RF cycles.
  • E. Modulation and Demodulation: Noncoherent detection is common because it avoids carrier-phase estimation and reduces receiver complexity, but coherent detection can increase bitrate.Coherent detection requires carrier-phase knowledge and is more complex; PSK usually uses coherent detection.
  • 1) Backscatter Communications Channels:: A basic backscatter channel contains an RF source, backscatter transmitter, and receiver connected through forward and backscatter links.The received signal combines the carrier, information signal, two channel responses, and noise.
  • 1) Backscatter Communications Channels:: A dyadic channel models M, L, and N antennas at the RF source, transmitter, and receiver as an M × L × N channel.Its matrix formulation represents the two-fold nature of the backscatter path.

2) Theoretical Analyses and Experimental Measurements:

Experimental and theoretical studies evaluate backscatter channels through link budgets, fading analysis, and BER measurements. Multiple antennas can improve reliability and communication range, while performance depends on several system factors.

  • 2) Theoretical Analyses and Experimental Measurements:: Link-budget measurements across cardboard, aluminum, and pine-plywood antenna materials show that antenna materials affect backscatter-system performance.
  • 2) Theoretical Analyses and Experimental Measurements:: Link-envelope correlation can couple fading across forward and backscatter links even when fading within each link is uncorrelated.
  • 2) Theoretical Analyses and Experimental Measurements:: BER performance varies with antenna configuration, detector, channel coding, and modulation scheme.Using multiple backscatter-transmitter antennas can significantly improve BER performance.
  • 2) Theoretical Analyses and Experimental Measurements:: The survey synthesizes modulated-backscatter fundamentals, channel models, and designs for bistatic and ambient backscatter systems.

III. BISTATIC BACKSCATTER COMMUNICATIONS SYSTEMS

Bistatic backscatter communications systems separate the carrier emitter from the backscatter receiver and support low-power, low-cost, scalable wireless networks. Their main advantages include reduced transmitter power and implementation cost, while performance remains constrained by hardware limitations and unresolved system issues.

  • BBCSs target low-cost, low-power, large-scale networks and have applications in wireless sensor networks, IoT, and smart agriculture.
  • BBCSs comprise backscatter transmitters, a backscatter receiver, and a physically separate carrier emitter that supplies the RF signals.The carrier emitter transmits RF signals to the backscatter transmitter, which reflects modulated data toward the receiver.
  • 0.3 µW and 7.2 mW are the reported power consumptions of an RF switch and controller in one low-power backscatter transmitter.Another cited backscatter transmitter consumes 10.6 µW for its operations.
  • $10 is the reported implementation cost for 100 backscatter sensor transmitters, enabled partly by battery-less designs and reduced electronic complexity.The section contrasts this with higher-cost active sensing and RFID alternatives.
  • Bistatic deployment can extend coverage because carrier emitters placed near transmitters reduce emitter-to-transmitter path loss.The section also states that bistatic systems usually offer greater communication ranges and transmission rates than monostatic systems.
  • BBCS performance remains limited by battery-less, hardware-constrained transmitters, while multiple access and energy management remain important issues.

B. Performance Improvement for Bistatic Backscatter Communications Systems

Research on bistatic backscatter performance improvement addresses communication range, receiver impairments, modulation and coding, multiple access, and energy-related design constraints. Reported approaches extend range under varied operating conditions and use frequency or time division to manage collisions and interference.

  • 1) Communication Improvement: 60 meters is the reported communication range at 1 kbps and 30 dBm emitter power after carrier-frequency-offset elimination and near-optimal detection.The receiver uses an absolute operator to separate noiseless and noise signals before amplitude-based detection.
  • 1) Communication Improvement: 100 meters is the reported communication range with less than 1 mW emitter power at 868 MHz and 100 kbps after two-phase data smoothing.Histogram filtering is followed by Savitzky-Golay smoothing to reduce transmission errors and increase receiver SNR.
  • 1) Communication Improvement: 250 meters is the reported communication range with a 1 MHz sampling rate and 13 dBm carrier emitter power using a 50% duty-cycle design.
  • 2) Multiple Access: FSK generally outperforms OOK in BER performance, while FDM assigns unique sub-carrier frequencies to reduce collisions among simultaneous transmitters.The section identifies FDM as a commonly used multiple-access scheme in bistatic backscatter models.
  • 2) Multiple Access: Ten humidity sensor transmitters achieved collision-free communication using distinct resistor-capacitor components for FDM and two 20 mW carrier emitters.
  • 2) Multiple Access: Environmental temperature variations can drift reserved sub-carrier frequencies, forcing wider bandwidth allocation and reducing the number of simultaneously operating transmitters.The section identifies a scalability trade-off involving environmental parameters in FDM.
  • 2) Multiple Access: TDM is used when multiple BBCSs operate at the same location, ensuring that only one carrier emitter is active in each time frame.

3) Energy Consumption Reduction:

Bistatic backscatter transmitters rely on small amounts of harvested energy, motivating circuit and system designs that reduce consumption and exploit additional energy sources. The reviewed literature emphasizes energy reduction as less studied than communication-range improvement and identifies security as another open concern.

  • BBCS backscatter transmitters harvest environmental energy for internal operations such as modulation and transmission, but the harvested amount is typically small.
  • 28 µW is the reported power consumption of a backscatter transmitter using 65 nm low-power CMOS technology in its idle state.
  • A plant-assisted sensor transmitter can harvest energy from both a carrier emitter and the plant, whose power-voltage characteristic ranges from 0.52 to 0.67 V.An energy-storage capacitor accumulates the harvested biological energy for internal operations.
  • Existing BBCS studies focus more on improving communication range than on reducing energy consumption or addressing multiple access.
  • Simple FSK and OOK modulation and coding schemes may leave BBCSs vulnerable to security attacks that adversely affect performance and reliability.The survey states that BBCS security has received marginal study and requires effective protection schemes.

2) Ambient Backscatter Design:

Ambient backscatter designs harvest energy from ambient RF signals, reflect them for communication, and use low-power receiver processing to address interference, range, bitrate, and reliability constraints.

  • Architecture: Ambient backscatter transceivers combine a harvester, backscatter transmitter, and backscatter receiver connected to one antenna.The harvester extracts energy from ambient RF signals, while modulation and reflection transmit data.
  • Receiver Design: Averaging separates backscattered signals from ambient RF signals when their bitrates differ substantially.The backscatter transmitter uses a lower frequency than the ambient RF source.
  • Receiver Design: Analog envelope processing and thresholding enable demodulation without an energy-intensive analog-to-digital converter.The receiver smooths signals with an envelope circuit before distinguishing zero and one voltage levels.
  • Advantages and Limitations: 0.25 µW TX and 0.54 µW RX analog-component consumption compare with 2.32 µW TX and 18 µW RX for the WISP system.Ambient RF sources also avoid dedicated carrier emitters or RFID readers and introduce almost negligible licensed-spectrum interference.
  • Advantages and Limitations: Ambient backscatter cannot typically control RF-source quality-of-service, and it may face security issues and insufficient harvested energy.Its transmission efficiency also depends on ambient-source type, location, and environment.
  • Communication Improvement: At BER 10^-2, the first ambient backscatter system achieved 1 kbps up to 2.5 feet outdoors and 1.5 feet indoors.These limited range and bitrate motivate communication-efficiency improvements.
  • Communication Improvement: 1 kbps at 100 meters with a TV tower was theoretically achieved through passive coherent processing using clutter cancellation, correlation, and time-frequency analysis.The method recovers reflected signals from received ambient, reflected, and noise components.
  • Communication Improvement: 90% higher network capacity than conventional BPSK was achieved by jointly optimizing reflection coefficient α and code rate ρ.The optimization uses line-search algorithms and exposes a trade-off between α and ρ.

2) Power Reduction:

Power-reduction research targets the limited energy harvested from ambient RF signals by using low-power hardware and retransmitting only erroneous packet subsets.

  • Power Reduction: Ambient RF harvesting may provide insufficient power for backscatter-transmitter operation.Proposed remedies largely use the low-power circuit components also applied in bistatic backscatter systems.
  • Power Reduction: Passive Wi-Fi backscatter transmitters reduce energy use with low-power analog devices, RF-switch modulation, and a 65 nm LP CMOS baseband.The transmitter harvests energy from a Wi-Fi access point.
  • Power Reduction: 11.7 µW of power was consumed by a low-power backscatter transmitter using off-the-shelf modulation, RF-switch, and baseband components.A related implementation also saves energy by retransmitting only a subset of bits after errors.

3) Multiple Access:

Multiple-access designs coordinate simultaneous backscatter transmitters by selecting favorable links or exploiting multiplicative multiple-access channels to reduce interference and improve rates.

  • Multiple Access: Ambient backscatter networks require multiple-access schemes because several transmitters may operate simultaneously.The objective is improved network performance.
  • Multiple Access: A three-sub-slot selection protocol lets a receiver choose the transmitter with the best condition before data transmission.The other transmitters remain silent during the selected transmission, avoiding interference.
  • Multiple Access: The selection technique enabled successful reception from 8 backscatter transmitters in simulation.Each slot contains an initial observation phase, sequential transmitter activity, and selected-transmitter data transmission.
  • Multiple Access: The multiplicative multiple-access channel achieved a larger rate region than conventional TDMA and improved performance over 0–30 dB direct-link SNR.The scheme targets direct-link interference in ambient backscatter systems.

C. Potential Applications

Ambient backscatter supports low-maintenance sensing, wearable and implantable monitoring, logistics, and tag-to-tag RFID communication, while broader deployments still face access, cost, complexity, and security challenges.

  • Potential Applications: Ambient backscatter enables devices to operate independently with minimal human intervention in smart-life, logistics, and biomedical applications.Its D2D capability supports these application areas.
  • Potential Applications: Smart-home sensors can operate for long periods without additional power sources or maintenance while detecting gases, monitoring movement, and supporting surveillance.Passive transmitters can be placed inside walls, ceilings, and furniture.
  • Potential Applications: Wearable and implantable monitoring benefits from small, long-lasting devices, including a battery-free shoe platform with sensors and ambient-backscatter modules.The cited prototype places a sensor and module in each shoe.
  • Potential Applications: A grocery-store logistics system broadcasts each item's identification number every 5 seconds and stores neighboring transmitters' identities.Each item is equipped with a backscatter transmitter.
  • Discussion: Most reviewed systems use two communicating transmitters, leaving simultaneous multi-transmitter access and security less studied.Additional antennas could improve reception but may increase cost, complexity, and size.
  • Tag-to-tag RFID Systems: RFID tag-to-tag communication can operate passively when the source signal is strong, or semi-passively with batteries while still avoiding active RF transmission.The system uses backscattered signals from an RF source for direct tag communication.
  • Tag-to-tag RFID Systems: A Dual-ACK virtual carrier-sensing method updates network allocation only after RTS, CTS, and acknowledgment detection, reducing timer and memory demands.Two acknowledgment messages address the hidden-node problem.

B. RF-Powered Cognitive Radio Networks and Backscatter Communication

RF-powered cognitive-radio backscatter systems combine energy harvesting, ambient backscatter, and active transmission to improve operation under variable primary-channel conditions. The surveyed designs optimize time allocation, access, pricing, and hybrid transmitter operation.

  • RF-powered backscatter CRNs: RF-powered backscatter CRNs let secondary transmitters harvest primary signals and backscatter data, addressing low harvested energy or low channel-idle probability.Backscatter and harvesting cannot be performed efficiently at the same time, creating an operating trade-off.
  • Time allocation: Three subperiods allocate time to backscattering, energy harvesting, and direct data transmission according to whether the primary channel is busy or idle.When the channel is busy, the secondary transmitter backscatters or harvests; when idle and sufficiently energized, it transmits directly.
  • Optimization and access: Optimization studies identify trade-offs among backscatter, harvesting, and transmission time for overlay, underlay, and multi-transmitter cognitive-radio networks.The multi-transmitter formulation has a convex throughput objective, yielding a globally optimal time trade-off and time sharing among secondary transmitters.
  • Hybrid transmitter: A hybrid transmitter integrates ambient backscatter, wireless-power harvesting, and active RF transmission through a shared antenna.The design supports longer duty cycles and larger transmission ranges than ambient-backscatter-only or wireless-powered-only transmitters.
  • Hybrid transmitter: A multiple-access scheme for ambient-backscatter-assisted wireless-powered cognitive-radio networks maximizes the sum throughput of secondary transmitters.Numerical results demonstrate superiority of the proposed hybrid transmitter over traditional designs.

C. Wireless-Powered Communication Networks and Backscatter Communication

Wireless-powered networks integrate bistatic, ambient, and hybrid backscatter to reduce energy-acquisition delays, extend coverage, and improve throughput. The surveyed applications include heterogeneous networks, hybrid device-to-device links, relays, and visible-light backscatter.

  • Motivation: Backscatter is integrated with wireless-powered communication because active transmitters may require long energy-acquisition times, making system performance suboptimal.Backscatter can transmit data while using existing RF signals, complementing wireless-powered active transmission.
  • Bistatic backscatter: A bistatic backscatter design uses a carrier emitter and hybrid access point to provide far transmitters with sufficient operating energy.Its two-phase protocol transfers wireless energy downlink, then uses FSK-modulated backscatter uplink.
  • Hybrid backscatter: Hybrid backscatter communication selects bistatic or ambient backscatter according to indoor or outdoor location and energy status.The proposed time-allocation problem is concave and solved using KKT conditions; numerical results show increased system throughput.
  • Relay networks: Relay architectures extend limited single-hop range, including RFly drones that forward reader queries and tag replies while filtering relay self-interference.Full-duplex backscatter relaying also enables source-to-destination transmission through a nearby relay.
  • Relay networks: A relay enables successful source-to-destination delivery with source-to-relay and relay-to-destination bitrates up to 2 kbps and 1 kbps, respectively.The evaluation uses a 539 MHz TV tower transmitting at 10 kW, with both relay links set to 1 meter.
  • Visible-light backscatter: Visible-light backscatter increases bitrate using 8-PAM, achieving 600 bps at 2 meters versus 200 bps with OOK.Trend-based modulation further reaches up to 1 kbps by exploiting partial transparency changes before the display fully switches state.

F. Long-range LoRa Backscatter Communications

LoRa backscatter targets wide-area communication by adapting chirp spread spectrum to low-power backscatter hardware. The survey also identifies source heterogeneity, interference, interoperability, security, and high-frequency operation as continuing research concerns.

  • LoRa backscatter: LoRa backscatter uses chirp spread spectrum to support wide-area transmissions with sensitivity and resilience to fading, Doppler, and interference.Bits are represented by continuous or cyclically shifted chirps.
  • LoRa backscatter: 475 meters is the reported maximum distance between the RF source and backscatter receiver for the LoRa backscatter system.The design combines digital frequency planning, a low-power DAC, and harmonic cancellation to approximate cleaner CSS signals.
  • Open issues: UWB backscatter broadens source compatibility from source-specific designs to ambient signals operating across 80 MHz to 900 MHz.Different source characteristics still require optimized transmission strategies, including joint FM and digital-TV use for throughput.
  • Open issues: Ambient backscatter can interfere with licensed users, and interference behavior may differ for higher-bitrate systems.Experiments found no noticeable TV-receiver glitches below 10 kbps at distances greater than 7.2 inches, but the survey calls for broader interference modeling.
  • Open issues: Proprietary testbeds and protocols using different encoding mechanisms make backscatter devices less interoperable or incompatible.The survey identifies communication-standard development as an urgent need.
  • Open issues: Simple coding and modulation make backscatter communications vulnerable to eavesdropping and jamming, while resource constraints limit conventional security solutions.The survey calls for simple, effective security methods for ambient backscatter without dedicated readers.
  • Open issues: Millimeter-wave backscatter is identified as a high-speed direction, with prior MBCS work demonstrating a 4 Gigabit backscatter transmission rate using binary modulation.Millimeter-wave systems require line-of-sight channels and miniaturized high-gain antennas or arrays.
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