Frequency Hopping (FHSS)

Frequency-hopping spread spectrum (FHSS) transmits a signal by rapidly switching the carrier frequency among many channels spread across a wide band, following a pseudorandom hop sequence known to both transmitter and receiver. By dwelling only briefly on each frequency, an FHSS link sidesteps narrowband interference and is harder to jam or eavesdrop on, since an attacker who does not know the sequence sees only fleeting fragments.

Where you encounter it

Bluetooth is the textbook example: it hops up to 1,600 times per second across channels in the 2.4 GHz ISM band, and modern versions add Adaptive Frequency Hopping (AFH) to dynamically skip channels that are already congested. FHSS is distinct from the Chirp Spread Spectrum that ordinary LoRa uses — a chirp continuously sweeps frequency within one channel rather than jumping between discrete channels. However, the LoRa family has added LR-FHSS, a long-range frequency-hopping mode that boosts network capacity for dense deployments.

Why it matters off-grid

FHSS earns its keep in noisy, contested spectrum. The same ISM bands that carry mesh traffic are shared by Wi-Fi, cordless devices, and countless other emitters; hopping spreads a transmission's risk across many channels so a single source of interference only corrupts a fraction of the hops. The trade-off is added complexity and the need for both ends to stay synchronised on the hop pattern, which is why simpler point-to-point sovereign links often favour the steadier chirp modulation instead.

FHSS lives in the same license-free ISM band spectrum as the chirp-based LoRa radios used for off-grid mesh.