Direct-sequence spread spectrum: Difference between revisions

Revision as of 04:10, 7 October 2004

Revision as of 22:58, 1 January 2005

Line 1:

In [[telecommunication]], the term '''direct-sequence spread spectrum''' has the following meanings:

<b>1.</b> # A [[system]] (a) for generating spread-spectrum transmissions by [[phase]]-modulating a [[sine wave]] [[pseudorandom]]ly 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.

<b>2.</b> # A [[signal]] structuring technique utilizing a [[digital]] code [[sequence]] 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.

<b>2.</b> A [[signal]] structuring technique utilizing a [[digital]] code [[sequence]] 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 a "noise" signal to the data being transmitted. 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 to the received signal (because 1 x&times; 1 = 1, and -1 x&times; -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 [[Amplitude modulation|AM]] transmission, except that the added noise causes the distribution to be much wider than that of an AM transmission.

Line 13 ⟶ 12:

By 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.

==References==

Source: from [[Federal Standard 1037C]] and from the [[NTIA Manual of Regulations and Procedures for Federal Radio Frequency Management]]

* [[Federal Standard 1037C]]

Source: from [[Federal Standard 1037C]] and from the* [[NTIA Manual of Regulations and Procedures for Federal Radio Frequency Management]]

==See also:==

* [[Frequency-hopping spread spectrum]]

* [[Linear feedback shift register]]