Frequency Hopping
Understanding Frequency Hopping
Frequency hopping was originally developed for military anti-jamming applications but has become a fundamental multiple access technique in commercial wireless systems, notably Bluetooth and some military tactical radios.
Frequency Hopping Parameters
- Hop rate: Bluetooth: 1600 hops/sec. Military: 100-10,000+ hops/sec. Higher = better jam resistance.
- Hop bandwidth: Total spectrum used by all hop frequencies. Wider = more spreading gain.
- Processing gain: BW_total / BW_channel (in dB). Represents the advantage over narrowband systems.
FH vs DSSS
| Parameter | FH | DSSS |
|---|---|---|
| Spreading method | Frequency jumping | Code multiplication |
| Bandwidth at any instant | Narrow (one channel) | Wide (full spread BW) |
| Near-far problem | Less critical | Severe |
| Synchronization | Time-frequency alignment | Code phase alignment |
Frequently Asked Questions
What is frequency hopping?
FH rapidly changes carrier frequency in a pseudo-random pattern. Benefits: interference resistance (only occasional hops disrupted), LPI (appears as noise bursts), multiple access. Used in Bluetooth (1600 hops/sec) and military radios.
How does FH resist jamming?
A narrowband jammer only disrupts hops landing on the jammed frequency. With 100 hop frequencies, the jammer disrupts only 1% of transmissions. Error correction codes recover the lost hops. Wideband jammers must spread power thin across all frequencies.
Bluetooth uses frequency hopping?
Yes. Bluetooth hops 1600 times/sec across 79 channels in the 2.4 GHz ISM band. This provides coexistence with Wi-Fi and other 2.4 GHz devices by avoiding occupied frequencies adaptively (Adaptive Frequency Hopping).