| Passband modulation |
|---|
| 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 |
| Multiplex techniques |
|---|
| Circuit mode (constant bandwidth) |
| TDM · FDM · SDM Polarization multiplexing Spatial multiplexing (MIMO) |
| Statistical multiplexing (variable bandwidth) |
| Packet mode · Dynamic TDM FHSS · DSSS OFDMA · SC-FDM · MC-SS |
| Related topics |
| Channel access methods Media Access Control (MAC) |
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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.
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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.
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.
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.
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