Direct-sequence spread spectrum

In telecommunications, direct-sequence spread spectrum (DSSS) is a spread-spectrum modulation technique primarily used to reduce overall signal interference. The direct-sequence modulation makes the transmitted signal wider in bandwidth than the information bandwidth. After 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.

Swiss inventor, Gustav Guanella proposed a "means for and method of secret signals". With DSSS, the message symbols are modulated by a sequence of complex values known as spreading sequence. Each element of the spreading sequence, a so-called chip, has a shorter duration than the original message symbols. The modulation of the message symbols scrambles and spreads the signal in the spectrum, and thereby results in a bandwidth of the spreading sequence. 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.

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.

Transmission method

Direct-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. The resulting transmitted signal resembles bandlimited white noise, like an audio recording of "static". 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). This process, known as despreading, is mathematically a correlation of the transmitted spreading sequence with the spreading sequence that the receiver already knows the transmitter is using. After the despreading, the signal-to-noise ratio is approximately increased by the spreading factor, which is the ratio of the spreading-sequence rate to the data rate.

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. In contrast, frequency-hopping spread spectrum pseudorandomly retunes the carrier and requires a uniform frequency response since any bandwidth shaping would cause amplitude modulation of the signal by the hopping code.

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 within the limits of the cross-correlation properties of their spreading sequences.

Benefits

• 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

Uses

• The United States GPS, European Galileo and Russian GLONASS satellite navigation systems; earlier GLONASS used DSSS with a single spreading sequence in conjunction with FDMA, while later GLONASS used DSSS to achieve CDMA with multiple spreading sequences.
• DS-CDMA (Direct-Sequence Code Division Multiple Access) is a multiple access scheme based on DSSS, by spreading the signals from/to different users with different codes. It is the most widely used type of CDMA.
• Cordless phones operating in the 900 MHz, 2.4 GHz and 5.8 GHz bands
• IEEE 802.11b 2.4 GHz Wi-Fi, and its predecessor 802.11-1999. (Their successor 802.11g uses both OFDM and DSSS)
• Automatic meter reading
• IEEE 802.15.4 (used, e.g., as PHY and MAC layer for Zigbee, or, as the physical layer for WirelessHART)
• Radio-controlled model Automotive, Aeronautical and Marine vehicles

See also

• Complementary code keying
• Frequency-hopping spread spectrum
• Linear-feedback shift register
• Orthogonal frequency-division multiplexing

References

• The Origins of Spread-Spectrum Communications
•  This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on January 22, 2022.
• NTIA Manual of Regulations and Procedures for Federal Radio Frequency Management

External links

• Civil Spread Spectrum History