Talk:Data Coding Theory/Spectrum Spreading

I don't understand how 1 XOR'D with 111 = 011 (as shown in the "Direct CDM" example).

001 XOR 111 should be 110 - so the example is probably wrong, although the layout isn't very clear and it could mean 100 XOR 111 rather instead, which would be 011. Unusual? Quite TalkQu 16:35, 26 December 2008 (UTC)


 * I'm a bit embarrassed that I didn't notice this error before now. When transmitting a data "1" bit, every code chip is inverted - the first chip, and the last chip, and all the chips in-between, until the next data bit arrives. Does it look good now?
 * I'm a bit worried that this simplified example is a bit misleading -- real DSS systems typically have dozens or hundreds of code chips per data bit. --DavidCary (talk) 21:12, 29 December 2008 (UTC)


 * What you are saying in the example is, I presume this:

1    0     1     1   X 101 X 100 X 111 X 000 - - - -    010   100   000   111


 * XOR 1 three times (i.e., with 1, 0 and 1) - that is equivalent to XOR 7 and 5 which gives 2. Then XOR 0 three times (with 1, 0 and 0) which is equivalent to XOR 0 and 4 which gives 4 and so on. If this is right, then your example is correct. And yes, looks better! Unusual? Quite TalkQu 16:56, 30 December 2008 (UTC)


 * Yes, thank you. Another way of explaining this is that there are 2 spreading sequences that continually repeat, over and over:

010011000111... -- sequence transmitted on "1" data 101100111000... -- sequence transmitted on "0" data (practical spreading sequences are much, much longer than this)
 * and the data bit selects either one or the other code to actually be transmitted at any particular instant.
 * Please edit this book to make it easier to understand. --DavidCary (talk) 19:58, 12 January 2009 (UTC)