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frequency hopping spread spectrum (modulation) (FHSS)

Frequency Hopping Spread Spectrum (FHSS) is a frequency hopping technique used in mobile communications and WLANs. In this technique, the wanted signal is modulated onto an abruptly changing carrier frequency.

The frequency changes take place in a pseudo-random or predetermined bit sequence whose code is known to both the transmitter and the receiver. In WLANs according to IEEE 802.11, 79 non-overlapping frequency hops per second are available in Europe and the USA in the available ISM band between 2.4000 GHz and 2.4835 GHz.

The hopping sequence is determined by a pseudorandom generator, and the minimum hopping 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) hopping frequencies are available in a smaller frequency band.

Frequency hopping technique

Frequency hopping technique

Depending on the speed of the frequency change, a distinction is made between normal frequency hopping( FH), fast frequency hopping (FFH) and slow frequency hopping (SFH). In Fast Frequency Hopping (FFH), which uses frequency shift ke ying or multi-frequency shift keying( MFSK), one data bit is split over several frequency hops, making the detection of coherent signals problematic. The situation is different with slow frequency hopping (SFH), in which the transmission frequencies are changed regularly. The transmission time on one radio frequency is longer than the duration of several bits to be transmitted.

Comparison of radio technologies

Comparison of radio technologies

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 reliable, 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 conceptualized in mobile networks. Due to the constant frequency changes, frequency hopping avoids interference and prevents eavesdropping on the changing transmission frequencies.

Informations:
Englisch: frequency hopping spread spectrum (modulation) - FHSS
Updated at: 05.12.2020
#Words: 324
Links: frequency hopping (FH), mobile communications, indium (In), signal, carrier
Translations: DE
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