Direct-sequence spread spectrum

Direct-sequence spread spectrum (DSSS) is a modulation method that spreads a transmitted signal across a wider bandwidth than the original information would normally occupy, mainly to cut down on signal interference. At the receiver, the process of despreading restores the original bandwidth while sharply reducing both accidental and deliberate interference.

The technique works by multiplying each message symbol with a spreading sequence—a series of complex values. Each element of this sequence, called a chip, is much shorter than the original symbol. This multiplication scrambles and spreads the signal across the spectrum, so the final bandwidth matches that of the spreading sequence. A shorter chip duration leads to a wider DSSS signal, and the extra bandwidth helps the signal resist narrowband interference more effectively.

Swiss inventor Gustav Guanella first proposed a method for secret signals using this approach. In practice, the spreading sequence is chosen so the resulting spectrum appears white. The receiver must know the same sequence to reconstruct the original data, typically by multiplying the incoming signal element-wise with the sequence and summing over a symbol period—a mathematical correlation. In an additive white Gaussian noise channel, this despreading boosts the signal-to-noise ratio by the spreading factor, which is the ratio of the chip rate to the data rate.

Although a DSSS signal occupies more bandwidth than a directly modulated signal would, conventional pulse-shape filtering can restrict its spectrum. If another transmitter uses the same channel but a different spreading sequence, the despreading process reduces that unwanted signal's power. This property is the basis for code-division multiple access (CDMA), which lets multiple users share the same channel as long as their spreading sequences have low cross-correlation.

Key benefits include resistance to jamming, the ability for many users to share a single channel, reduced detectability due to a lower signal-to-background-noise level, and the ability to determine relative timing between transmitter and receiver.

DSSS is used in several major satellite navigation systems: the United States' GPS, Europe's Galileo, and Russia's GLONASS (early GLONASS combined DSSS with a single spreading sequence and FDMA, while later versions use DSSS for CDMA with multiple sequences).