Frequency Hopping Spread Spectrum

Frequency Hopping Spread Spectrum (FHSS) is an invented by George Antheil and Hedy Lamarr spread spectrum for wireless data transmission. It is divided into fast and slow hopping.

Generally, the carrier frequency changes here frequentiv and discreet. The sequence of the frequency change is pseudo-random numbers (see picture ).

The user data are modulated only a narrow band, and then spread in a second modulator by a frequency synthesizer. On the other hand, a frequency synthesizer is connected to the modulator receiving again, which makes the reverse spreading, and then demodulated by conventional methods.

This technique is civil 802.11 and optionally used for example in Bluetooth and great- Fi standard in special operating modes in the context of GSM. In the military, they are called SINCGARS.

Bandwidth and susceptibility

The difference of the spectrum compared to Direct Sequence Spread Spectrum (DSSS) manifests itself in the fact that the spectrum of the modulated signal is as wide as the original signal itself, the spectral spreading is practically only on a time basis, since the carrier frequency is always small for a torque remains constant.

Similar to DSSS also FH -SS news before " eavesdroppers " can hide. Unlike the DSSS signal is not hidden in the noise, but it can therefor not listen because an outsider would never know on which carrier frequency is the signal for the next hop. Furthermore, even the Hopdauer be varied (slow FH ( = SFH ) or almost FH ( = FFH ) ).

FHSS also is characterized by low susceptibility. This is because it is always the same carrier frequency is used only for a short duration. For example, if the frequency disturbed by another station, so only a small part of the data sequence of this disorder will be subject to what can be corrected or recognize mostly by error correction method.

Slow hopping

Here, a minimum of 1 bit per frequency hopping is transmitted, eg 3 bits before the frequency changes.

Fast hopping

Here a maximum of 1 bit is transmitted per frequency hopping, but it may well also 3 frequency jumps within one bit to take place.

FHSS is easy to implement, but uses only a small portion of spectrum at a time, and is not as safe as tap- DSSS.