Frequency hopping spread spectrum

Frequency hopping diagram

Frequency Hopping Spread Spectrum ( FHSS ), German frequency hopping , is one of George Antheil and Hedy Lamarr invented frequency spreading for wireless data transfer . It is divided into fast and slow hopping.

In general, the carrier frequency changes here frequently and discretely. The frequency change sequence is determined synchronously by a sequence of pseudo-random numbers for the transmitter and receiver .

The useful data is first modulated in a narrow band and then spread in a second modulator by a frequency synthesizer. On the opposite side, a frequency synthesizer is connected to the receiving modulator, which reverses the spreading and then demodulates conventionally. This technology is used civilly, for example with Bluetooth and in the original WLAN standard 802.11 . In the military they are called SINCGARS .

Bandwidth and susceptibility to failure

The difference between the spectrum and Direct Sequence Spread Spectrum (DSSS) is that the spectrum of the modulated signal is just as wide as the original signal itself. The spectral spreading is practically only based on time, as the carrier frequency is always only for a small one Moment remains constant.

Similar to DSSS, FHSS messages can also be hidden from "eavesdroppers". In contrast to the DSSS, the signal is not hidden in the noise, rather it cannot be intercepted because an outsider never knows which carrier frequency the signal will be on after the next hop . The hop duration can also be varied (slow FH (= SFH) or almost FH (= FFH)). ${\displaystyle T_{h}}$

FHSS is also particularly characterized by its low susceptibility to failure. This is because the same carrier frequency is only used for a short period of time. If, for example, the frequency is disturbed by another transmitter, only a small part of the data sequence will be subject to this disturbance, which can usually be corrected or recognized by appropriate error correction methods. ${\displaystyle f_{t1}}$

Slow hopping

A minimum of one bit is transmitted per frequency hop, so e.g. B. three bits before the frequency changes.

Fast hopping

A maximum of one bit is transmitted per frequency hop, but three frequency hops can also take place within one bit.

FHSS is easy to implement, but only uses a narrow range of the spectrum at a time and is not as bug-proof as DSSS .

literature

• Rodger E. Ziemer: Fundamentals of Spread Spectrum Modulation . tape 3 from the series: Synthesis lectures on communications. Morgan & Claypool Publishers, 2007, ISBN 978-1-59829-264-0 .