Spread spectrum
With spread spectrum (english spread spectrum ) is called in communications technology, a process in which a narrow-band signal is converted into a signal needed with a larger bandwidth than the information transfer. The transmission energy that was previously concentrated in a smaller frequency range is thereby spread over a wider frequency range.
Another recent importance has received in digital technology, the term frequency spread. The aim here is to reduce the spectral density of the clock signals. Although this meeting standards of electromagnetic compatibility is achieved for noise at discrete frequencies, on the other hand, the energy can be eliminated " smeared " over a whole frequency range.
Advantages and Disadvantages
Advantages:
- Greater robustness against narrow-band interference
- Confidentiality: an eavesdropper can overhear news content without authorization; it is hard to realize that even a transfer to take place
Cons:
- Higher complexity when receiving
- Greater bandwidth not applicable at low transmission frequencies may from frequency- technical reasons
Use
Frequency spread on the one hand used for message transmission. Examples are the military intelligence technology, but also in the civilian sector in data transmission over wireless LAN or Wireless USB. The second generation (GSM ) and third generation mobile UMTS also uses the spread spectrum contained in CDMA.
Another application is the Störminimierung. So the so-called Spread Spectrum Clocking (SSC ) is used to reduce the electromagnetic interference in clock signals in digital circuits. A group of class D amplifiers, the spread-spectrum class D amplifiers, frequency spreading allows to dispense with a complicated filtering of the switching frequency in accordance with the amplifier stage.
Method
In Direct Sequence Spread Spectrum (DSSS) procedure, the user data are in direct sequence ( direct sequence ) by exclusive-or ( XOR) associated with a spreading code and then modulated onto a carrier. Without knowledge of the spreading code, a recovery of the user information on the receiver side is not possible. The spreading code has characterized the skilful choice of the function of a cryptographic key.
The direct sequence spread spectrum, is commonly used in combination with the CDMA technique. It is inter alia in wireless LANs according to IEEE 802.11 standard and UMTS are used, as well as in the RC model in remote control systems in the 2.4 GHz band (spectrum DSM, DSM2 ).
Frequency Hopping Spread Spectrum (FHSS)
The frequency hopping ( frequency hopping spread spectrum, short FHSS ), the information to be transmitted is sequentially distributed to multiple channels; at a time only one frequency channel is always used. This result, although each channel has a smaller bandwidth for the total signal a wider bandwidth. The receiver must be synchronized with the transmitter start the same channels. By skillfully selecting the hopping sequence it appears to be a pseudo-random and has a function of a cryptographic key. The patent for this process was issued in 1942, Hedy Lamarr and George Antheil; it should be used for the control of torpedoes.
The difference from the conventional FHSS frequency division multiplexing is that the FHSS channel assignment is performed sequentially, and the conventional frequency-division multiplexing the signal components are present simultaneously in the separate channels.
This method has been used for instance in Bluetooth, as well as in the RC model in remote control systems in the 2.4 GHz band ( Sanwa FHSS Futaba FASST )
Chirp Spread Spectrum (CSS )
Chirp Spread Spectrum or " chirp spread spectrum " uses the frequency spread so-called chirp pulses ( = chirping ). This special modulation method is used, among other things, to wireless data transmissions over short distances within the wireless personal area network according to IEEE 802.15.4a.
Adaptive Frequency Hopping Spread Spectrum
The adaptive frequency hopping method, which has Bluetooth 1.2 installed there, for example, provides better protection against interference by already used or disturbed frequency channels in the hopping sequence (temporarily) be avoided.
Time Hopping Spread Spectrum
, The time hopping method requires the presence of time slots. The position in time of the transmitted and usually modulated DSSS carrier signal jumps pseudo randomly within the time slots. The disadvantage of this technique is the difficulty of synchronization.
Spread Spectrum in the EMC
To avoid disturbing, high peaks of a pure clock signal in digital technology, the clock over a small frequency range is varied slightly ( Spread Spectrum Clocking ). Thus, a single, very high interference is avoided. Rather, now a variety of relatively weak noise is generated. Adherence to standards of electromagnetic compatibility ( EMC) can be ensured.