Miller Encoding

Miller Encoding

Miller encoding (also called Miller modulated sub-carrier) is a baseband encoding scheme used in the tag-to-reader backscatter link of EPC Gen2 RFID systems. It provides superior noise immunity compared to FM0 encoding, making it the preferred option in dense reader mode environments where multiple readers operate in close proximity.

Encoding Principle

In Miller encoding, each data bit is represented by a pattern of transitions (or lack thereof) within the bit period. A data-1 produces a mid-bit phase inversion; a data-0 maintains the current phase. The resulting waveform is then multiplied by a square-wave sub-carrier at a frequency that is 2, 4, or 8 times the base link frequency (BLF), producing Miller-2, Miller-4, and Miller-8 variants respectively.

This sub-carrier multiplication concentrates the backscattered signal energy at a defined spectral offset from the reader's carrier frequency. The offset is equal to BLF x M/2, where M is the Miller index. This spectral separation is what enables dense reader mode — neighbouring readers can transmit on adjacent channels without their carriers falling into each other's tag response bands.

Miller Variants and Trade-Offs

Higher Miller indices provide better noise rejection and tighter spectral confinement but reduce the effective data rate proportionally. The choice depends on the deployment's density and throughput requirements.

When to Use Miller Encoding

Multi-reader installations: In logistics distribution centres with dozens of portal readers, Miller-4 is the typical choice. It provides enough spectral separation to avoid inter-reader interference while maintaining acceptable read rates of 200-400 tags per second.

Single-reader deployments: FM0 encoding is faster and should be used when only one reader is active in the area, such as a single handheld device during cycle counting.

High-noise environments: Manufacturing floors with motors, welders, and other RF emitters benefit from Miller-8's superior noise immunity, even if only one reader is present.

Reader Configuration

The reader selects the encoding mode via the Query command's M field. All EPC Gen2 compliant tags must support all Miller variants, so the choice is purely a reader-side configuration decision. Most reader firmware exposes this as a "link profile" or "mode" setting in the LLRP ROSpec or proprietary API.