Brad Huntting <huntting@glarp.com> asks:
[...]this would mean the sender and reciever would need to be synced to well within 500ns of each other. Isn't this a bit difficult? How do they do it?
You are right. This is perhaps *the* central design issue of this kind of spread spectrum system. One standard solution is to use "gold codes". Gold codes are special in that they are very self dissilimar. That is they look very unlike any shifted version of themselves. So you can build a very simple corelator which tries all the possible shiftings of a code to the signal, until one pops up with "low frequency" data rather than "high frequency" noise. Another is to begin a transmission with a special sync header (and concievably intersperse additional ones bassed on the expected frequency of loss of lock). Currently available PLL's working at 900MHz have very low phase noise, and I can imagine the construction of fixed frequency PLL's with even lower phase noise. A third is to transmit BOTH the spreading code, and the data. You can think of this technique as sending two channels of data, one which is all 1's (or 0's), the other which is a little more interesting. The two channels are then combined at the reciever to yield the data. A fourth is to use an externaly generated sync signal -- for example a radio transmission that both sender and reciever can hear. (For this aplication, I don't see how this would be used...) Aditional solutions are possible. (What is this S.A.W. thing I read about??) j'