What does a frequency hopping (FHSS) system gain over a fixed frequency system?
In FHSS the carrier continuously changes frequency within the band in a defined pattern. A narrowband interference source corrupts only a few hops, and the corrupted packet is caught by CRC and retried. A fixed frequency system is blinded entirely when the same interference source lands on its channel. The XRF protocol AXI uses in the HCT400/HCT402 is built on a frequency hopping architecture.
Most interference on an industrial site is narrowband and moves over time: a telemetry transmitter, a garage remote, a neighbouring wireless device. A fixed frequency receiver can do nothing when such a source lands right on top of it; even if the signal level is sufficient, the SNR is not.
What FHSS gains you:
- Interference resistance: the interference affects only a few channels in the hopping sequence, the remaining channels stay healthy.
- Multiple systems working together: sets using different hopping sequences can work side by side on the same site. Multi-kit operation on the same site is supported for the HCT series.
- Average energy spreading: power is not concentrated on a single channel, and the disturbance given to neighbouring devices is reduced.
FHSS is not a magic solution: wideband noise (such as a welding arc) affects all channels at once and hopping cannot fully compensate for it. Hopping also requires the transmitter and receiver to stay in sync; re-synchronising after a lost link takes a short time. In terms of band use and power limits, FHSS equipment must also comply with the relevant harmonised standard; at 2.4 GHz the upper limit under EN 300 328 is 20 dBm EIRP.
Full topic: 433 MHz or 2.4 GHz?