| Direct-sequence Spread Spectrum |
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transmitted signal takes up more Bandwidth than the information signal that is being modulated, which is the reason that it is called Spread Spectrum . DSSS has the following features: # 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 . # 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). 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 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. REFERENCES
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