Backscatter

Backscatter Communication in RFID

Backscatter is the fundamental communication mechanism that enables passive tags to transmit data without an onboard power source. Instead of generating its own RF signal, a passive RFID tag modulates the reader's continuous-wave carrier and reflects a portion of that energy back. The reader then demodulates the reflected signal to extract the tag's data payload, typically an EPC or other identifier.

This technique is what makes passive RFID economically viable. Because the tag does not need a local oscillator, mixer, or power amplifier, the tag IC can be manufactured with a minimal transistor count and an extremely small die size, keeping unit costs below ten cents at volume.

How Backscatter Works

When the reader transmits a continuous-wave signal, the tag antenna absorbs energy and charges the IC's rectifier capacitor. Once the IC reaches its activation threshold (tag sensitivity), it begins toggling its impedance between two states — matched and mismatched — at a rate defined by the data encoding scheme. In the matched state the antenna absorbs most incident power; in the mismatched state it reflects it. This impedance modulation creates amplitude or phase variations in the reflected wave that the reader's receiver can decode.

The backscattered signal is inherently weak — typically 60 to 80 dB below the reader's transmitted power. This asymmetry is the reason the reverse link (tag-to-reader) almost always limits the achievable read range rather than the forward link.

Encoding and Modulation

EPC Gen2 defines two baseband encoding options for backscatter: FM0 and Miller encoding (Miller-2, Miller-4, Miller-8). FM0 is faster but more susceptible to noise; Miller sub-carrier encoding trades throughput for improved signal-to-noise ratio, making it the preferred choice in dense reader mode environments.

The reader selects the encoding via the Query command, allowing dynamic adaptation to environmental conditions. Higher Miller modulation indices provide narrower spectral occupation, which reduces adjacent-channel interference when many readers share the ISM band.

Practical Considerations

Backscatter performance degrades near metals and liquids because these materials alter the antenna impedance match and absorb or reflect the carrier wave before it reaches the tag. On-metal tags address this by incorporating a spacer layer or patch antenna that compensates for the metallic ground plane. Engineers evaluating a deployment should measure the actual backscattered power using a spectrum analyser or the reader's RSSI telemetry and compare it against the link budget to verify adequate margin.