Direct-sequence spread spectrum

1. 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.

2. 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 add pseudorandom "noise" to the data being transmitted, which is then removed by the receiver. The resulting signal resembles the result of a sender transmitting an audio recording of static, except that the "static" can be filtered out at the receiving end to recover the original data. 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.

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 generator.

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