Frequency Hopping Spread Spectrum (FHSS) is a frequency hopping technique used in mobile communications and WLANs. In this technique, the useful signal is modulated onto a carrier
frequency that changes by leaps and bounds. The frequency changes take place in a pseudo-random or predetermined bit sequence, the code of
are available inEurope and the USA
in the availableISM band between 2.4000 GHz
and 2.4835 GHz. Thehop sequence is determined by a pseudo-random generator, whereby the minimum hop distance is at least 6 channels
In addition, a minimum of 20 frequency hops per second is required for frequency hopping. In some European countries, only 35 (Spain) or 23 (France) frequency hopping frequencies are available in a smaller frequency bandFast Frequency Hopping (FFH), which uses frequency shift keying or multi-frequency shift keying (MFSK), one bit of data is divided among several frequency hops, making the detection of coherent signals problematic. Slow frequency hopping (SFH) is different, where transmission frequencies are changed periodically. The transmission time on a radio frequency is longer than the duration of several bits
to be transmitted.
In order for the signal to be detected by the receiver, the receiver and the receiver must know in advance the sequence of frequency changes. This is done by negotiating the frequency sequence between the transmitter and receiver. This technique is considered very secure, but the transmitter and receiver must be precisely synchronized. This requires a relatively complex electronic circuit and consequently high manufacturing costs. Frequency hopping is used in WLANs and is also conceptually provided for in mobile radio networks. Due to the constant frequency changes, frequency hopping avoids interference and prevents eavesdropping on the changing transmission frequencies.