In telecommunication, the term direct-sequence spread spectrum has the following meanings:
- A system (a) for generating spread-spectrum transmissions by phase-modulating a sine wave pseudorandomly with a continuous string of pseudonoise code symbols, each of duration much smaller than a bit and (b) that may be time-gated, where the transmitter is keyed periodically or randomly within a specified time interval.
- A signal structuring technique utilizing a digital code sequence (PN Sequences) having a chip rate much higher than the information signal bit rate. Each information bit of a digital signal is transmitted as a pseudorandom sequence of chips.
Put simply, 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", except that this noise can be filtered out at the receiving end to recover the original data, by again multiplying the same pseudorandom sequence (PN Sequences) to the received signal (because 1 × 1 = 1, and −1 × −1 = 1).
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 retunes the carrier, instead of adding pseudo-random noise to the data, which results in a uniform frequency distribution whose width is determined by the output range of the pseudo-random number generator.
Comparison of DS SS and FH SS
- DS SS
- Flexible support of variable data rates
- High capacity is possible with enhancements (interference cancellation, adaptive antenna, etc.)
- Suffers from near-far effect ⇒ power control required
- FH SS
- Suitable for ad hoc networks (no near-far problem)
- Robust to interference
- Limited data rate
- Bottleneck of DS SS
- Processing gain can be made larger by employing a longer PN sequence and more chips per bit
- Physical devices used to generate PN sequence impose practical limit on attainable processing gain
- Processing gain may be not enough to overcome effects of some jammers