3 ms·
DSSS is an exceptionally interesting topic, IMO. It uses linear feedback shift registers which some CS types may have encountered as a very cheap PRNG. But it a
by brian-armstrong 7y ago
DSSS is an exceptionally interesting topic, IMO. It uses linear feedback shift registers which some CS types may have encountered as a very cheap PRNG. But it also makes these interesting spectral properties, and it's orthogonal to time-delayed versions of itself. That's quite a lot of somewhat unexpected properties for a fairly simple shift register construction.
- henrikeh 7y agoTo shed some light on the connection between these properties: they arise from the fact that the coding sequence _is_ pseudo random noise. Specifically, white noise. White noise has this nice property that its autocorrelation is ideally a discrete delta function, aka a 1 at t = 0 and 0 everywhere else. In the time domain, this means that only an exactly aligned code sequence matches itself – ideal for communication symbols. Spectrally, white noise is flat and fills the entire available frequency spectrum. This produces the “spreading” of digital spread spectrum sequences (DSSS). Any kind of white noise could be used for this, but as far as I remember, the Gold codes used in GPS are: easy to generate and maximally different from each other.