3in1: Multi-tone Joint Powering, Clocking, and Communication for Passive IoT

At a Glance

Harvested power vs. distance. 16-tone 3in1 extends the operational range to 22 m — three times that of the single-tone Gen2 baseline — while delivering proportionally more energy at every distance.

The Problem

Battery-free passive IoT devices — RFID tags, backscatter sensors, implants — harvest every milliwatt from ambient RF signals. Three fundamental services compete for those milliwatts:

Today's standard — single-tone AM/ASK signaling (e.g., Gen2 UHF RFID) — can only do one thing at a time. Sending data means dropping power. Improving range means sacrificing data rate. There is no free lunch with a single carrier.

Power advantage. At the same average transmit power, multi-tone envelopes have higher PAPR, boosting the rectifier's harvest sensitivity.

Uninterrupted powering. Single-tone ASK dips the envelope to zero when sending data. 3in1 encodes clock and data into beat frequencies, keeping the envelope — and power delivery — continuous.

3in1: One Waveform, Three Services

3in1 transmits 16 RF tones with 1 MHz spacing, all starting in phase. When the multi-tone signal hits the tag's rectifier, three things happen simultaneously:

System overview. The SDR helper transmits a structured multi-tone waveform. The tag's single rectifier produces DC power, extracts the 2 MHz beat-frequency clock, and decodes downlink data — all from the same received signal.

Function 1

Wireless Power

In-phase multi-tone envelopes have high PAPR, making the rectifier's diodes conduct more efficiently. As tone count grows from 1 to 16, harvested DC power increases monotonically — no extra transmit power required.

Diode conduction improves as tone count increases from 1 → 4 → 16.

Function 2

Wireless Clock

After rectification, adjacent tones mix to produce stable beat frequencies. The 2 MHz beat component — shared across multiple tone pairs — is inherently resilient to multipath fading and is filtered out directly at the tag to serve as a precise reference clock.

Multi-tone provides many beat-frequency paths to the clock frequency, surviving fading that would silence a 2-tone design.

Function 3

Downlink Data

Even-indexed tones are BPSK-modulated (0 or π phase) to encode bits. Odd-indexed tones remain untouched to preserve the clock. Because the envelope amplitude never drops, power delivery is never interrupted — unlike ASK.

Phase flips on half the tones encode data while the other half keeps the clock stable.

FCC-Compliant by Design

Broadcasting 16 narrowband tones risks exceeding FCC power spectral density limits. 3in1 solves this with coordinated frequency hopping: all tones shift together across the ISM band, spreading spectral energy without changing the baseband envelope seen by the tag. The tag's envelope detector responds identically regardless of carrier frequency, so hopping is completely transparent to it.

Coordinated hopping distributes spectral energy across the ISM band to meet FCC limits.

Carrier independence. The baseband envelope after rectification is the same regardless of the carrier frequency, making hopping invisible to the tag.

Hardware Prototype

We built a complete end-to-end prototype to validate 3in1 in real-world conditions. The helper transmitter runs on a USRP X310 SDR generating calibrated multi-tone waveforms at 900 MHz. The passive tag is a fully custom PCB with a single RF-to-DC rectifier, a joint filter bank for separating power, clock, and data, and an MCU for decoding.

SDR Helper Transmitter (USRP X310)

Custom Passive Tag

Experiment setup. We tested 3in1 in an indoor corridor up to 22 m, including through-wall NLOS scenarios.

Evaluation Highlights

3in1 delivers 6× higher power conversion efficiency and 3× longer range than single-tone Gen2. At −10 dBm input power, multi-tone signals enable cold-start in 24 seconds — compared to 7.6 minutes for Gen2 — a 19× improvement that dramatically reduces dead time in intermittent-duty deployments.

PMU cold-start time. 3in1 enables cold-start down to −19 dBm and slashes startup time by 19× compared to Gen2.

The wirelessly delivered 2 MHz clock achieves zero frequency offset and <1 ns jitter at close range, degrading gracefully with distance and remaining usable well beyond the power sensitivity threshold. Clock performance holds under NLOS and frequency hopping.

Clock jitter vs. distance. Sub-nanosecond jitter at close range, remaining stable across tested distances.

The 200 kbps downlink achieves a bit error rate below 10⁻³ across all tested distances, in both LOS and NLOS environments. BPSK modulation preserves clock quality during data transmission — jitter changes by less than 0.1 ns when data is active.

BER vs. distance. Downlink remains reliable (BER < 10⁻³) throughout the operating range.

Citation