EW Academy · Contents · Chapter 16 · §16.5 · Nederlandse versie

§16.5 FHSS and DSSS

Open §16.5 in the academy

In plain terms

Transmitter and receiver jump together between many frequencies according to a secret pattern. A jammer on a single frequency only hits a jump now and then.

In practice

The SINCGARS combat net radio hops some hundred times per second across the VHF band, and the Link 16 data link of NATO aircraft jumps between 51 frequencies. GPS works on the other principle, DSSS: one wide, noise-like code that disappears below the noise.

There are two main forms of spread spectrum:

FHSS — Frequency-Hopping Spread Spectrum

The transmitter constantly hops between many different frequencies according to a pseudo-random code; transmitter and receiver hop in sync. A narrowband jammer therefore only hits occasionally — it misses most of the “hops”. Well-known examples: the military radio SINCGARS and, in the civilian world, Bluetooth.

DSSS — Direct-Sequence Spread Spectrum

Each data bit is multiplied by a much faster pseudo-random chip sequence. The result is a wide, noise-like signal with a low spectral power density — it disappears, as it were, below the noise floor (exactly the picture above). An example you already know: the GPS C/A code (GNSS and navigation warfare) is in fact DSSS, and that is why an extremely weak satellite signal still works.

FHSS: hopping between channelsjammertimefrequencythe jammer hits 1 of the 12 hopsDSSS: spreading over a wide banddata: narrow, highnoise floorfrequencyafter spreading: wide and below the noisea narrow jammer only hits a sliver
Two forms of spread spectrum. Left, FHSS: the signal hops from channel to channel following a pseudo-random code, so a jammer on one frequency only hits now and then. Right, DSSS: the signal is spread over a wide band and sinks below the noise floor, where it is hard to find and to jam.

Because a spread signal can disappear below the noise, it is by nature LPI/LPD (low probability of intercept/detection): the opponent has trouble even finding it, let alone jamming it. This is the same principle as LPI radar (Radar), but applied to communications.