What is Direct Sequence Spread Spectrum?

What is DSSS or Direct-Sequence Spread Spectrum?

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- everything RF

Oct 26, 2021

Direct-Sequence Spread Spectrum (DSSS) is a modulation technique used in telecommunications to reduce interference in signals during transmission. It is a spread spectrum technique which means that the frequency of the signal generated with a particular bandwidth is deliberately increased (spreading) resulting in a signal with a wider bandwidth. Transmitting signals with a spread bandwidth have less unintentional/intentional interference and loss/corruption of data. The bandwidth of the original signal before spreading of frequency (DSSS) occurs is called the information bandwidth. At the receiver end, the signal undergoes despreading or removal of DSSS modulation and the information bandwidth is restored.

For DSSS transmissions, the data signal is multiplied with a pseudorandom spreading sequence that has a much higher bit rate than the original data rate. The resulting transmitted signal resembles bandlimited white noise, like an audio recording of "static". The spreading sequence is known at the receiver end and the original data is reconstructed by multiplying the same spreading sequence with the transmitted noise-like signal. This process is known as despreading.

While a transmitted DSSS signal occupies a much wider bandwidth than a simple modulation of the original signal would require, its frequency spectrum can be somewhat restricted for spectrum economy by a conventional analog bandpass filter to give a roughly bell-shaped envelope centered on the carrier frequency. If an undesired transmitter transmits on the same channel but with a different spreading sequence (or no sequence at all), the despreading process reduces the power of that signal. This effect is the basis for the code-division multiple access (CDMA) property of DSSS, which allows multiple transmitters to share the same channel. DS-CDMA (Direct-Sequence Code Division Multiple Access) is a multiple access scheme based on DSSS which works by spreading the signals from/to different users with different codes. It is the most widely used type of CDMA.

Benefits of using DSSS:

  • Resistance to unintended or intended jamming
  • Sharing of a single channel among multiple users
  • Reduced signal/background-noise level hampers interception
  • Determination of relative timing between transmitter and receiver

DSSS is used in several GNSS systems like GPS, Galileo and GLONASS. Cordless phones use DSSS operating in the 900 MHz, 2.4 GHz and 5.8 GHz bands. 2.4 GHz Wi-Fi (IEEE 802.11) and Zigbee, WirelessHART (IEEE 802.15.4) protocols use DSSS. It is also used in automatic meter reading and radio-controlled model automotive, aeronautical & marine vehicle applications.