Also known as: FHSS, frequency hopping
Frequency-hopping spread spectrum (FHSS) repeatedly retunes the carrier across many narrow channels following a pseudo-random hop pattern known to both ends, so the signal’s energy is spread across a wide band over time rather than all at once.1 Any given transmission lingers on one channel for a short dwell time before jumping to the next, so a narrowband interferer or fade only corrupts the hops that happen to land on it — the rest get through, and forward error correction stitches the message back together.
How it works
A pseudo-random number generator — often driven by a maximal-length sequence or a keyed cipher — produces the sequence of channel indices, the hop set. Transmitter and receiver run the same generator from the same seed and stay time-synchronized, so the receiver’s local oscillator retunes in lockstep and always lands where the signal is. Symbols are usually carried by a simple modulation (FSK or GFSK) within each hop channel.
Two regimes are distinguished by how the hop rate compares to the symbol rate:
- Slow hopping sends many symbols per hop (e.g. Bluetooth: 1600 hops/s, hundreds of symbols per dwell). Simpler, but a hit on one channel loses a whole block of symbols.
- Fast hopping changes frequency several times per symbol, giving strong anti-jam and frequency diversity at the cost of a fast, agile synthesizer.
Processing gain comes from the ratio of the total hopped bandwidth to the instantaneous channel bandwidth. Unlike DSSS, the instantaneous signal is narrowband — it just never stays in one place — which makes FHSS tolerant of the near-far problem and easy to build with conventional narrowband radios.
In practice
Multiple FHSS users can share a band with different hop patterns; when two happen to collide on the same channel in the same slot, both lose that hop and rely on FEC and retransmission — a form of CDMA by orthogonal hopping rather than orthogonal codes. Adaptive schemes (Bluetooth’s Adaptive Frequency Hopping) drop channels that are persistently busy from the hop set.
Relevance to SDR
Bluetooth and Bluetooth Low Energy are the most familiar FHSS systems; military radios (SINCGARS, HAVE QUICK) use it for anti-jam and LPI, and some proprietary telemetry and cordless systems hop as well. The land-mobile trunking protocols GopherTrunk targets do not hop their traffic channels — P25, DMR, and TETRA assign fixed frequencies via a control channel, and a scanner follows those grants rather than a hop pattern.
That distinction is the practical takeaway for a scanner operator: to receive a genuinely frequency-hopping transmission you need the hop set and hop timing, not just a center frequency — without the pattern and synchronization, each intercepted dwell is an isolated, unintelligible fragment. GopherTrunk therefore does not attempt to follow hopped links; it decodes the fixed-channel FDMA/TDMA systems where every voice and control channel sits at a known frequency.
Sources
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Frequency-hopping spread spectrum — Wikipedia, for the hop-pattern mechanism, dwell time, and slow/fast hopping. ↩