However, this noise-like signal is used to exactly reconstruct the original data at the receiving end, by multiplying it by the same spreading sequence (because 1 × 1 = 1, and −1 × −1 = 1). The resulting transmitted signal resembles bandlimited white noise, like an audio recording of "static". The receiver can then use the same spreading sequence to counteract its effect on the received signal in order to reconstruct the information signal.ĭirect-sequence spread-spectrum transmissions multiply the data being transmitted by a pseudorandom spreading sequence that has a much higher bit rate than the original data rate. DSSS uses a signal structure in which the spreading sequence produced by the transmitter is already known by the receiver.Therefore, the chip rate is much higher than the information bit rate. That is, each information bit is modulated by a sequence of much faster chips. DSSS phase-shifts a sine wave pseudorandomly with a continuous string of chips, each of which has a much shorter duration than an information bit.Some practical and effective uses of DSSS include the code-division multiple access (CDMA) method, the IEEE 802.11b specification used in Wi-Fi networks, and the Global Positioning System. The smaller the chip duration, the larger the bandwidth of the resulting DSSS signal more bandwidth multiplexed to the message signal results in better resistance against interference. The modulation of the message bits scrambles and spreads the pieces of data, and thereby results in a bandwidth size nearly identical to that of the spreading sequence. Each spreading-sequence bit, which is known as a chip, has a much shorter duration (larger bandwidth) than the original message bits. With DSSS, the message bits are modulated by a pseudorandom bit sequence known as a spreading sequence. The first known scheme for this technique was introduced by a Swiss inventor, Gustav Guanella. The direct-sequence modulation makes the transmitted signal wider in bandwidth than the information bandwidth.Īfter the despreading or removal of the direct-sequence modulation in the receiver, the information bandwidth is restored, while the unintentional and intentional interference is substantially reduced. In telecommunications, direct-sequence spread spectrum ( DSSS) is a spread-spectrum modulation technique primarily used to reduce overall signal interference.
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