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From WPT to Encrypted Telemetry: A Battery-Free Backscattering-based Polarimetric Wireless Sensor

Taki Eddine Djidjekh, Loïc Thomas, Quentin Bernyer, Gaël Loubet, Daniela Dragomirescu, Alexandru Takacs

arXiv:2609.29214v1cs.NI

TL;DR

Many battery-free sensing approaches provide limited on-node processing or cryptographic protection. This work combines multi-sensor environmental sensing, local processing, AES-128 protection, and polarization-separated backscatter in a WPT-powered node; indoor tests demonstrate operation at meter-scale distances with a measured 1.83-mJ complete cycle.

  • Problem

    Many active BFWS demonstrations remain focused on a single sensing modality, with limited edge processing and minimal or absent encryption.

    Source: §I · p. 1

  • Method

    The node combines environmental sensors and a low-power MCU for local calibration, feature extraction, payload construction, and AES-128 protection, with a 1-bit rectenna for harvesting and cross-polarized backscatter.

    Source: §V · pp. 5–6

  • Results

    1.83 mJ was measured for a complete sensing, calibration, and secure-transmission cycle; charging took 3 min 11 s at 5 m and 5 min 35 s at 7 m.

    Source: §IV · p. 5

  • Takeaways & Limitations

    The results demonstrate indoor WPT-powered operation combining multi-parameter environmental sensing, embedded security, and backscatter communication within a stringent energy budget.

    Source: §IV · p. 5

Abstract

from arXiv · show

This work introduces an indoor Battery-Free Wireless Sensing Node powered through radiative Wireless Power Transfer (WPT). The proposed platform targets secure, energyefficient active sensing and overcomes key limitations of many prior battery-free approaches, which commonly provide neither on-node computation nor cryptographic protection. The node combines temperature, humidity, pressure and Volatile Organic Compound (VOC) measurements with a low-power microcontroller that executes sensor calibration, derives a VOC index, formats the payload, and applies AES-128 encryption before wireless transmission. Energy harvesting and communication are enabled by a 1-bit controlled Backscatter Rectenna (BR), which both scavenges incident RF power and produces an orthogonally polarized backscattered signal for robust polarimetric operation. Experimental results validate reliable multi-sensor readout and encrypted data transfer, while maintaining a very low energy budget for the complete sense-compute-encrypt-transmit cycle.

I. INTRODUCTION

Battery-free sensing can support long-term monitoring where wiring and battery replacement are impractical, but existing approaches often lack meaningful on-node computation and cryptographic protection. This work proposes an active, WPT-powered node combining multi-parameter sensing, local processing, AES-128 protection, and polarization-domain backscatter.

I. INTRODUCTION

  • Passive battery-free systems commonly measure low-complexity observables, while little embedded computation leaves interpretation to the reader and cryptographic protection is generally absent,.The described threats include impersonation or spoofing, eavesdropping, and replay or relay attacks.Source: §I · p. 1
  • Many active BFWS demonstrations focus on one sensing modality and provide limited edge processing or encryption, leaving confidentiality and authenticity largely unaddressed -.Dedicated WPT provides a controllable RF energy supply for more deterministic operation and duty-cycled sensing.Source: §I · p. 1
  • The proposed active BFWS senses temperature, humidity, pressure, and air-quality metrics, and processes measurements locally to compute a VOC indicator.Its physical-layer identification and authentication target spoofing, replay or relay, eavesdropping, and man-in-the-middle attempts.Source: §I · p. 1
  • A 1-bit controlled rectenna harvests RF power while encoding encrypted information on a cross-polarized backscatter waveform for polarimetric readout.The environmental payload and identification data are protected using AES-128.Source: §I · pp. 1–2

A. System Hardware Architecture

The hardware combines a multi-parameter sensing front-end, an MCU-based processing and security engine, and a dual-function RF interface for harvesting and polarimetric backscatter. A power-management unit conditions and buffers harvested energy for the sensing and digital subsystems.

A. System Hardware Architecture

  • The node integrates environmental sensors, an MCU, and a dual-polarized 1-bit Backscattering Rectenna, with a PMU conditioning and buffering harvested power.The prototype uses separate evaluation boards for the RF rectifier and RF switch rather than the proposed single-PCB arrangement.Source: A. System Hardware Architecture · pp. 2–3
  • In the default unpowered switch state, incident vertically polarized WPT is routed for energy harvesting; after sufficient stored voltage, an MCU command enables cross-polarized backscatter.Vertical illumination and horizontal-polarized telemetry provide polarization-domain separation that reduces sensitivity to clutter and limits self-interference or cross-jamming.Source: A. System Hardware Architecture · p. 2
  • The sensing front-end combines temperature and relative-humidity sensing, a metal-oxide VOC sensor providing a raw gas-related signal, and barometric pressure sensing; the STM32G431KB MCU handles readout, calibration/compensation, index computation, payload formatting, and AES-128 processing.Source: A. System Hardware Architecture · p. 2
  • The 868-MHz rectifier uses impedance matching, a Schottky diode, and output smoothing, with its measured conversion efficiency characterized across 864–880 MHz for a 10-kΩ load.The rectifier was tuned with the PMU as a load whose input impedance changes under its maximum-power-point tracking; measurements confirm robust operation around 868 MHz.Source: A. System Hardware Architecture · p. 3

B. Energy-Efficient System Software Implementation

The firmware schedules sensing and computation around the VOC sensor’s heater warm-up and uses low-power MCU operation. Its complete sensing, processing, encryption, and transmission cycle consumes 1.83 mJ.

B. Energy-Efficient System Software Implementation

  • The MCU uses 16-MHz low-power operation with the PLL disabled, I²C sensor access, and RTC-supported sleep to reduce global energy consumption.After the raw VOC measurement, the heater is disabled and the VOC Index is computed; device identification, freshness protection, and environmental readings are packaged in a 20-byte backscatter frame.Source: B. Energy-Efficient System Software… · pp. 3–4, Table II · p. 4
  • The firmware overlaps the VOC sensor’s approximately 170-ms heater warm-up with flash operations, temperature/humidity acquisition, AES initialization, and pressure measurement.These tasks take 38.36 ms of active time; the remaining approximately 132 ms are spent in STOP1 with RTC wake-up before the VOC sample is read.Source: B. Energy-Efficient System Software… · pp. 3–4
  • 1.83 mJ is required for a complete sensing, processing, encryption, and transmission cycle at 3.3 V.The PMU provides enough energy for one whole cycle, and the measured current trace identifies VOC warm-up overhead as the dominant contribution.Source: B. Energy-Efficient System Software… · p. 4, Figure 5 · p. 4

A. Measurement Setup

The indoor setup uses a radiative 868-MHz WPT source and a separate spectrum-analyzer receive chain to capture cross-polarized backscatter. The prototype was positioned between 1.50 and 7 meters from the source.

A. Measurement Setup

  • The test bench illuminates the prototype with an 868-MHz, vertically polarized WPT signal and receives telemetry with a horizontally polarized antenna connected to a spectrum analyzer.The source was set to 30 dBm EIRP in the described setup.Source: A. Measurement Setup · p. 4, Figure 6 · p. 4
  • The prototype was tested at separations from 1.50 to 7 meters, with the figure showing placements at 1.5–3 m and 5–7 m.Source: Figure 6 · p. 4, A. Measurement Setup · p. 4

B. Functional Demonstration and Data Recovery

Under continuous RF powering, the node periodically completed a full sensing-to-transmission cycle, and offline decryption recovered the expected sensor values from a captured frame. Charging measurements characterized operation across indoor distances and RF power levels, with corridor and room results affected by propagation conditions.

B. Functional Demonstration and Data Recovery

IV. DISCUSSION

The discussion reports successful indoor end-to-end operation of the WPT-powered sensing and secure telemetry platform. It also quantifies cycle energy and charging behavior, and notes that higher data rates would require adapting other parts of the communication chain.

IV. DISCUSSION

  • Indoor experiments confirmed end-to-end sensing, local processing, payload construction, AES-128 protection, and polarimetric backscatter telemetry within a stringent energy budget.The platform covered temperature, relative humidity, barometric pressure, and VOC-related air-quality monitoring.Source: §IV · p. 5
  • 1.83 mJ was measured for a complete sensing, calibration, and secure-transmission cycle on the 3.3 V rail; corridor charging took 3 min 11 s at 5 m and 5 min 35 s at 7 m.Office measurements also quantified how recharge time depends on RF source power.Source: §IV · p. 5
  • The 400 kHz switch modulation frequency with Manchester encoding yields a 200 kbps effective frame rate and a 0.8 ms duration for the 20-byte frame.Each information bit is represented by two modulation states.Source: §IV · p. 5
  • The current data rate is sufficient for the compact sensing payload, while higher rates would require adapting MCU timing, receiver bandwidth, signal-to-noise ratio, and demodulation.The RF switch can operate at up to 6 MHz.Source: §IV · p. 5
  • The architecture advances beyond common single-sensor, reader-centric paradigms by combining multi-sensor acquisition, embedded processing, lightweight security, and polarization-separated backscatter in a WPT-powered node.Source: §IV · p. 5

V. CONCLUSION

The conclusion presents a WPT-powered sensing node that integrates environmental measurements, local processing, AES-128 protection, and polarimetric backscatter. Indoor validation supports its practical feasibility, while further integration, miniaturization, and performance optimization remain future work.

V. CONCLUSION

  • The platform combines active temperature, humidity, pressure, and VOC-related sensing with MCU-based calibration, feature extraction, payload construction, and AES-128 ECB protection.Source: §V · p. 5
  • An MCU-controlled 1-bit polarization-shift rectenna supports energy harvesting and backscattering, embedding protected data in a cross-polarized waveform that isolates powering from telemetry.The separation improves robustness against indoor clutter and cross-interference.Source: §V · pp. 5–6
  • Indoor validation confirmed practical feasibility; future work targets a compact single-PCB design, a dual circularly polarized antenna, and further energy-efficiency and communication-performance optimization.Source: §V · p. 6
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