Re: [Discuss-gnuradio] Good CDMA codes

[Marcus Müller]

Hi Henry,

I recently wrote an app to go through all the binary permutations up to 2^20 and report which ones have an equal number of 0’s and 1’s

... which would simply be "all permutations of (00000000001111111111)", right?(For a rather quick method of calcu

Having the same numbers of zeros and ones is not a measure for orthogonality – it *is* a useful measure for spreading codes, because you'd typically want all data to be coded to have the same energy, but it's a completely separate aspect of these codes.

As mentioned by P, starting off with known good codes is probably a very good idea – finding good codes has been (and still is) a very hard problem, and there's a lot of algebra and comm theory behind doing it optimally.

Also, techniques like DSSS are practical, because they use well-understood and easily available spreader and despreader "components" – in fact, you'd use a linear feedback shift register to generate the pseudorandom binary sequence that is used for coding in DSSS systems, and one of the side-effects of chosing a good generator polynomial for that shift register is that he sequence is "white", and hence doesn't have a DC component (on average), and hence has about as many 0 as 1. Hence, the spreading sequence is not arbitrarily chosen from the set of all potential n-bit-strings, but needs to be generatable by a finite length shift register. Hence, DSSS-CDMA is a bit special, because you need to come up with *different* polynomials, which can get pretty hard (as finding a single one isn't inherently trivial).  Which is yet another reason to stick with P's recommendation!

Best regards,

Marcus

On 15.07.2016 04:54, Henry Barton wrote:

I’m designing a CDMA system with a spreading factor of 20. I recently wrote an app to go through all the binary permutations up to 2^20 and report which ones have an equal number of 0’s and 1’s, or at least differ by only one. It came up with so many “hits” that I have to wonder if they're really orthogonal. Does anyone know offhand how many good spreading codes I can realistically expect from 1048576 possible entries?

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