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MüllerMarcus (talk | contribs) →Transmission method: fun fact: WORDS MATTER. DSSS is on symbols, not bits, what is perceived as audio static is not white at all (so, bad, but analogy, but irrelevant here->strike), the spectrum is only white if the sequence is chosen accordingly. Not all DSSS is with a bipodal sequence. Kill all the useless and mostly incorrect filler words, while we're at it. Not all filters are "bell shaped" (the fewest are) |
MüllerMarcus (talk | contribs) →Transmission method: The positive comparison with FHSS w.r.t. pulse shaping doesn't work at all; the same power loss applies in both cases, and most channels that you use FHSS on are frequency-selective anyways, so that the system needs to be robust against that to begin with (not rely on amplitude or keep normalization per hop frequency, i.e. single-tap FD equalization) |
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Direct-sequence spread-spectrum transmissions multiply the symbol sequence being transmitted with a spreading sequence that has a higher rate than the original message rate. Usually, sequences are chosen such that the resulting spectrum is spectrally [[white noise|white]]. Knowledge of the same sequence is used to reconstruct the original data at the receiving end. This is commonly implemented by the element-wise multiplication with the spreading sequence, followed by summation over a message symbol period. This process, ''despreading'', is mathematically a [[correlation]] of the transmitted spreading sequence with the spreading sequence. In an AWGN channel, the despreaded signal's [[signal-to-noise ratio]] is increased by the spreading factor, which is the ratio of the spreading-sequence rate to the data rate.
While a transmitted DSSS signal occupies a wider bandwidth than the direct modulation of the original signal would require, its spectrum can be restricted by conventional [[Pulse shaping|pulse-shape filtering]]
If an undesired transmitter transmits on the same channel but with a different spreading sequence (or no sequence at all), the despreading process reduces the power of that signal. This effect is the basis for the [[code-division multiple access]] (CDMA) property of DSSS, which allows multiple transmitters to share the same channel within the limits of the [[cross-correlation]] properties of their spreading sequences.
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