Frequency Hopping

Frequency Hopping

Frequency hopping is a spread-spectrum technique mandated by the FCC for UHF RFID readers operating in the 902-928 MHz ISM band in the United States and similar regulatory regions. The reader rapidly switches its transmit frequency across 50 or more discrete channels, spending no more than 400 milliseconds on any single channel before hopping. This prevents any one reader from monopolising a frequency and distributes interference across the band.

Regulatory Context

Different regions employ different channel access strategies for UHF RFID:

The FCC approach allocates a wide 26 MHz band and requires hopping to share it fairly. The ETSI approach allocates a narrow 3 MHz band and requires listen-before-talk to avoid collisions. Reader manufacturers must implement region-specific firmware profiles.

How Frequency Hopping Works in Practice

The reader maintains a pseudo-random hopping sequence covering all available 500 kHz channels (the FCC band supports 52 such channels). Before each hop, the reader completes any in-progress tag transaction, switches the synthesiser to the next channel, and resumes operations. Modern readers perform a hop in under 1 ms, making the transition nearly invisible to application software.

From a tag perspective, frequency hopping is transparent. Passive tags are broadband devices — their antennas are designed to operate across the entire UHF band. The tag simply responds to whatever frequency the reader is currently transmitting.

Impact on Performance

Frequency hopping provides natural frequency diversity. If a tag is positioned at a null point for one frequency (due to multipath reflections), it is unlikely to be at a null for all 50+ channels. The reader will eventually hop to a frequency where the tag is readable. This diversity effect significantly improves read reliability in reflective environments such as warehouses with metal shelving.

However, hopping introduces a throughput overhead compared to a fixed-frequency system. Each hop interrupts the singulation process for a brief interval, and the Q-algorithm state may partially reset. For dense populations, this means slightly longer total inventory times compared to a theoretical single-channel system.

Deployment Considerations

System integrators in FCC regions generally do not need to manually configure hopping sequences — the reader firmware handles compliance automatically. However, understanding hopping behaviour is important when analysing read-rate logs. Apparent dips in tag reads every 400 ms are normal and reflect channel transitions, not hardware faults.