Direct Sequence Spread Spectrum
Direct Sequence Spread Spectrum (DSSS ) is a spread spectrum for wireless data transmission. The idea here is to spread an output signal ( desired signal ) by a predetermined bit sequence. Spread refers in this context to the frequency spectrum, which is covered after application of the direct sequence spread spectrum method of the signal to be transmitted. The method is used to make the desired signal more robust against a certain form of interference with radio transmission ( narrow band interference ). This is done by, translated bits of the original bit stream into a plurality of subbits, so-called chips. The chip sequence is referred to as a spreading code or chipping sequence. The same link will again be held in the receiver. While thus the useful signal is reconstructed, the additional narrowband jamming signal on the transmission path is now spread in the receiver (similar to the spreading of the useful signal in the transmitter ). Through this spreading of the interference signal is its energy density distributed according with which his interference effect decreases.
History
During the Second World War, the patent system and the technical knowledge between the U.S. and Europe were disabled, so that was widely researched in the U.S. and Europe, regardless of frequency spreading. Only after the war they won overview of the individual contributions. The U.S. standard factory Spread Spectrum Communications Handbook by Marvin Kenneth Simon gives an overview of development and the present state of the art. Therein, the remarkable contribution of the early Swiss inventor Gustav Guanella will be appreciated that led to a major Swiss patent application in 1942. The patent entitled Method for transmitting messages that are disguised by means of control signals and was Guanellas employer company Brown, Boveri & Cie ( BBC ), based in Baden ( Switzerland ), filed. Among other members abroad patents the corresponding German patent was granted in 1952.
General
DSSS the symbol energy is spread over a wide bandwidth. To the user data stream is to the spreading code for which the data rate is higher than that of the user data stream, is multiplied. This code sequence is called chips or pseudo-random code ( PN code ). By spreading a wider bandwidth for transmission is necessary. The longer the spreading code, the more bandwidth is required.
At the same time reduces the energy density in the range, so that other signals are less disturbed. The payload data can be reconstructed only by using the right chip sequence at the receiver. DSSS is used in GPS, WLAN, UMTS, Ultra-wideband, ZigBee, Wireless USB and the model construction in certain remote control systems in the 2.4 GHz band.
The signal disappears into the background noise - in military applications one uses this, as for listening or to even be able to determine a transfer, knowing the pseudo-random sequence used to band spreading is necessary.
Another property makes you look at the so-called CDMA (Code Division Multiple Access) advantage: It assigns each station its own unique spreading code to. All stations can transmit simultaneously and then the receiver can reconstruct the individual signals back and distinguish the transmitter thereby.
With expanding claims a channel more bandwidth. In return, the signal -to-noise ratio is improved when despreading. Since the noise signal is not correlated with the spreading code, the signal strength is not increased in contrast to the useful signal.
DSSS is insensitive to narrow-band interference because an interfering signal at the receiver is multiplied by the spread signal. Thereby, the interference signal as the data signal is spread in the transmitter. The power density of the noise signal is reduced by the spreading factor and can therefore no longer interfere with the despread data signal. The data signal is provided as a second time multiplied by the spreading code, thereby despreading again. The interference signal is lost in the noise and does not affect the wanted signal.