Frequency Hopping Spread Spectrum (FHSS)
Frequency Hopping Spread Spectrum (FHSS) is a radio transmission technique in which the carrier frequency of the signal is rapidly switched among many discrete frequencies in a predetermined sequence. It is used to improve resistance to interference, enable coexistence in shared bands, and fulfill regulatory requirements for unlicensed communication systems.
Scope and Application
FHSS is widely used in license-exempt frequency bands, including:
902–928 MHz (ISM sub-GHz band)
2.4 GHz (2400–2483.5 MHz ISM band)
Common technologies using FHSS:
Bluetooth Classic (BR/EDR)
Zigbee Sub-GHz and proprietary ISM systems
Legacy FHSS WLAN (pre-802.11b)
FHSS is allowed under:
ETSI EN 300 220 (sub-GHz)
SRRC and other national frameworks for unlicensed devices
Technical Principle
The transmitter changes (or "hops") its carrier frequency at fixed intervals (typically milliseconds or less) according to a pseudo-random hop sequence known to both transmitter and receiver.
This makes the signal:
Less susceptible to narrowband interference
More difficult to intercept or jam
Able to coexist with other devices in the same band
Regulatory Requirements
Example: FCC §15.247
Minimum number of hopping channels: ≥ 75 (or reduced limits for fewer)
Minimum 20 dB bandwidth: ≤ 1 MHz per channel
Minimum channel spacing: ≥ 25 kHz or two-thirds of 20 dB bandwidth
Max output power: 1 W conducted (with restrictions on antenna gain)
Example: ISED RSS-247
Same channel count and spacing rules as FCC
FHSS systems must operate with uniform hopping across defined channels
Reduced requirements apply to systems using fewer channels with reduced power
Certification and Testing
FHSS-based devices must undergo compliance testing as part of:
FCC Certification (via TCB)
ISED Certification
Conformity assessment under RED / EN 300 220 for Europe
Measurements include:
Channel occupancy and dwell time
Hopping sequence validation
Output power and emission compliance