Direct Sequence Spread Spectrum
Direct Sequence Spread Spectrum ( DSSS ) is a frequency spreading method for data transmission via radio . The idea is to spread an output signal (useful signal) using a specified bit sequence. Spreading in this context refers to the frequency spectrum which is occupied by the signal to be transmitted after the application of the DSSS method. The method is used to make the useful signal more robust against a certain form of interference in radio transmission (narrowband interference). This is done by translating bits of the original bit stream into several sub-bits, so-called chips . The chip sequence is also referred to as a spreading code or chipping sequence . The same link also takes place in the recipient. While the useful signal is being reconstructed, the narrow-band interference signal added on the transmission path is now spread in the receiver (analogous to the spreading of the useful signal in the transmitter). This spreading of the interfering signal distributes its energy density accordingly, which reduces its interfering effect.
history
During the Second World War, the patent system and the exchange of technical knowledge between the USA and Europe were hindered, so that research into frequency spreading methods was largely independent in the USA and Europe . Only after the war was it possible to gain an overview of the individual contributions. The US standard work Spread Spectrum Communications Handbook by Marvin Kenneth Simon gives an overview of the development and the current state of the art. It honors the remarkably early contribution of the Swiss inventor Gustav Guanella , who led to an important Swiss patent application as early as 1942. The patent is entitled Method for the transmission of messages which are disguised with the help of control signals and was applied for by Guanella's employer company Brown, Boveri & Cie (BBC), based in Baden (Switzerland). In addition to other patents granted abroad, the corresponding German patent was granted in 1952.
General
With DSSS the symbol energy is distributed over a large bandwidth . For this purpose, the useful data stream is multiplied by the spreading code , the data rate of which is higher than that of the useful data stream. This code sequence is called chips or pseudo-statistical codes ( PN codes). The spreading requires a larger bandwidth for transmission. The longer the spreading code, the more bandwidth is required.
At the same time, the energy density in the spectrum is also reduced, so that other signals are less disturbed. The user data stream can only be reconstructed at the receiver by using the correct chip sequence. DSSS is used with GPS , WLAN , UMTS , ultra broadband , IEEE 802.15.4 , wireless USB and in the model building sector with certain remote control systems in the 2.4 GHz band.
The signal disappears in the background noise - this is used in military applications because for eavesdropping or in order to be able to detect a transmission at all, knowledge of the pseudo-random sequence used for band spreading is necessary.
Another property is made use of in the so-called CDMA ( Code Division Multiple Access ) method : Each transmitter is assigned its own unique spreading code. All transmitters can then transmit at the same time and the receiver can reconstruct the individual signals and thereby differentiate the transmitters.
Spreading means that a channel uses more bandwidth. On the other hand, the signal-to-noise ratio improves during despreading. Since the noise signal does not correlate with the spreading code, its signal strength does not increase in contrast to the useful signal.
DSSS is insensitive to narrow-band interference, as an interference signal is multiplied by the spread signal at the receiver. As a result, the interference signal, like the data signal in the transmitter, is spread. The power density of the interference signal is reduced by the spreading factor and can therefore no longer interfere with the despread data signal. As provided, the data signal is multiplied a second time with the spreading code and thus despread again. The interference signal is drowned out in the noise and has no influence on the useful signal.
See also
Individual evidence
- ^ Marvin Simon: Spread Spectrum Communications Handbook, Electronic Edition. McGraw Hill Professional, 2001, ISBN 9780071395700 , p. 44. Limited preview in Google Book Search
- ↑ Espacenet -Originaldokument DE846562 (C) - 1952-08-14. In: espacenet.com. Retrieved December 30, 2014 .
Web links
- Spread spectrum modulations ( PDF ; 337 kB)