Spread Spectrum: FHSS and DSSS
Instead of shouting on one frequency and hoping nobody talks over you, spread spectrum whispers across many frequencies at once — making your signal nearly invisible to interference.
In plain English
Imagine talking across a crowded party. If you speak loudly on one note, someone playing that same note drowns you out. But if you rapidly whisper across 79 different pitches while your partner knows which pitch comes next, nobody else can follow the conversation — even someone trying to jam you has to shout across all 79 pitches at once. That's spread spectrum: security and interference immunity through unpredictable frequency use.
Shannon's channel capacity theorem tells us that capacity grows with bandwidth as well as SNR: C = B · log₂(1 + S/N). Spread spectrum exploits this by accepting a large bandwidth penalty in exchange for the ability to operate at very low signal-to-noise ratios — even below the noise floor. The processing gain (ratio of spread bandwidth to data bandwidth) quantifies the SNR improvement at the receiver: 10 dB processing gain means the receiver can tolerate 10 dB more interference or noise.
There are two fundamental spread spectrum techniques: Frequency Hopping Spread Spectrum (FHSS) continuously moves the carrier between pseudo-randomly selected frequency channels, and Direct Sequence Spread Spectrum (DSSS) multiplies the data with a high-rate pseudo-noise (PN) code that widens the signal's bandwidth. Both require a shared secret key (the hopping sequence or PN code) between transmitter and receiver — a party without the key receives only what appears to be noise.
Invented originally for military secure communications (including by actress Hedy Lamarr, who co-patented FHSS torpedo guidance in 1942), spread spectrum was declassified in the 1980s. The FCC's 1985 Part 15 ruling allowed unlicensed spread spectrum devices, directly enabling the WiFi, Bluetooth, and cordless phone industries. Today, DSSS in various forms underlies CDMA (3G), GPS, and UWB ranging systems.
The transmitter hops between 8 frequency channels following a pseudo-random sequence. The blue block shows the current channel; fading blocks show the transmission history. An observer without the hopping sequence sees intermittent narrowband bursts — no single channel carries enough information to be easily jammed or intercepted.
GPS L1 C/A
Operates 20 dB below noise floor. Usable with 25 cm position accuracy.
CDMA IS-95
64 simultaneous users per 1.25 MHz channel. Soft handoff between base stations.
Bluetooth FHSS
1,600 hops/s across 79 channels in 2.4 GHz ISM band. Coexists with WiFi.
FHSS
- Mechanism
- Rapid carrier frequency changes
- Interference immunity
- Avoids bad channels (AFH)
- Key metric
- Number of channels × hop rate
- Best for
- Coexistence, LPI/LPD comms
- Examples
- Bluetooth, military VHF/UHF
- Weakness
- Slow hopping can be partially jammed
DSSS
- Mechanism
- PN code multiplication widens BW
- Interference immunity
- Processing gain vs all interference
- Key metric
- Processing gain (chip rate / bit rate)
- Best for
- Multiple access (CDMA), ranging (GPS)
- Examples
- GPS, 3G CDMA, 802.11b, UWB
- Weakness
- Near-far problem without power control