Direct-sequence spread spectrum: Difference between revisions

Revision as of 13:57, 10 April 2006

Revision as of 19:53, 11 April 2006

Line 8:

Put simply, direct-sequence spread-spectrum transmissions multiply the data being transmitted by a "noise" signal. This noise signal is a pseudorandom sequence of <code>1</code> and <code>−1</code> 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 Sequencessequences]]) to the received signal (because 1 × 1 = 1, and −1 × −1 = 1). The resulting effect of enhancing [[signal to noise ratio]] on the channel is termed ''processing 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.

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 [[Amplitude modulation|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|pseudo-random]] number generator.

'''Comparison of DSSS and Frequency Hopped SS'''

*DSSS

**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

*FHSS

**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

==References==

Line 36 ⟶ 21:

* [[Frequency-hopping spread spectrum]]

* [[Linear feedback shift register]]

* [[PN Sequences]]

[[Category:Radio modulation modes]]