Direct-sequence spread spectrum

Passband modulation v · d · e
Analog modulation
AM · SSB · QAM · FM · PM · SM
Digital modulation
FSK · MFSK · ASK · OOK · PSK · QAM MSK · CPM · PPM · TCM · SC-FDE
Spread spectrum
CSS · DSSS · FHSS · THSS
See also: Demodulation, modem, line coding, PAM, PWM, PCM

In telecommunications, direct-sequence spread spectrum (DSSS) is a modulation technique. As with other spread spectrum technologies, the transmitted signal takes up more bandwidth than the information signal that is being modulated. The name 'spread spectrum' comes from the fact that the carrier signals occur over the full bandwidth (spectrum) of a device's transmitting frequency. Certain IEEE 802.11 standards use DSSS signaling.

Features

1. DSSS phase-modulates a sine wave pseudorandomly with a continuous string of pseudonoise (PN) code symbols called "chips", each of which has a much shorter duration than an information bit. That is, each information bit is modulated by a sequence of much faster chips. Therefore, the chip rate is much higher than the information signal bit rate.
2. DSSS uses a signal structure in which the sequence of chips produced by the transmitter is known a priori by the receiver. The receiver can then use the same PN sequence to counteract the effect of the PN sequence on the received signal in order to reconstruct the information signal.

Transmission method

Direct-sequence spread-spectrum transmissions multiply the data being transmitted by a "noise" signal. This noise signal is a pseudorandom sequence of 1 and −1 values, at a frequency much higher than that of the original signal, thereby spreading the energy of the original signal into a much wider band.

The resulting signal resembles white noise, like an audio recording of "static". However, this noise-like signal can be used to exactly reconstruct the original data at the receiving end, by multiplying it by the same pseudorandom sequence (because 1 × 1 = 1, and −1 × −1 = 1). This process, known as "de-spreading", mathematically constitutes a correlation of the transmitted PN sequence with the PN sequence that the receiver believes the transmitter is using.

For de-spreading to work correctly, the transmit and receive sequences must be synchronized. This requires the receiver to synchronize its sequence with the transmitter's sequence via some sort of timing search process. However, this apparent drawback can be a significant benefit: if the sequences of multiple transmitters are synchronized with each other, the relative synchronizations the receiver must make between them can be used to determine relative timing, which, in turn, can be used to calculate the receiver's position if the transmitters' positions are known. This is the basis for many satellite navigation systems.

The resulting effect of enhancing signal to noise ratio on the channel is called process gain. This effect can be made larger by employing a longer PN sequence and more chips per bit, but physical devices used to generate the PN sequence impose practical limits on attainable processing gain.

If an undesired transmitter transmits on the same channel but with a different PN sequence (or no sequence at all), the de-spreading process results in no processing gain for 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 PN sequences.

As this description suggests, a plot of the transmitted waveform has a roughly bell-shaped envelope centered on the carrier frequency, just like a normal AM transmission, except that the added noise causes the distribution to be much wider than that of an AM transmission.

In contrast, frequency-hopping spread spectrum pseudo-randomly re-tunes the carrier, instead of adding pseudo-random noise to the data, the latter process resulting in a uniform frequency distribution whose width is determined by the output range of the pseudo-random number generator.

Benefits

• Resistance to intended or unintended 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
• 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 OFDM instead)
• 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 vehicles

References

•  This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C".
• NTIA Manual of Regulations and Procedures for Federal Radio Frequency Management

See also

• Frequency-hopping spread spectrum
• Linear feedback shift register

External links

• Civil Spread Spectrum History