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David H. wrote: Thanks for the info, Caveat. Looks like I may have had run across one of your links in all my searching ![]() There are many question I have regarding SS, but one that's bothering me in particular. Regarding the PN spreading sequence, these sequences obviously have to be aligned perfectly in both transmitter and receiver. Naturally they could be kept in sync if both circuits were initialized at the same time. However, 3 things: 1) The circuits will not be initialized at the same time in 99% of most cases, as in the use of, say, a portable field radio. 2) If they were synchronized at the same time, well, no clock or oscillator is perfect. It would eventually drift. 3) As I understand it, there is no initial "handshake" signal at the beginning of transmission with the receiver to initialize/syncronize the PN sequences on both ends. So in short, how do the PN sequences became and remain synchronized through time? Thanks. The ones I'm familiar with - the Barker codes used in 802.11 DSS - have the interesting characteristic that their auto-correlation is extremely specific. That is: if you take two copies of the Barker sequence, put 'em above one another, multiply them together (treating the two states as 1 and -1 rather than 1 and 0) and then sum up the products, you get a very high positive value (11, in the case of an 11-bit Barker code). If you invert one of the two before multiplying, then you end up with a very large negative value (e.g. -11). However, if you shift one of the sequences one or more bit positions to either side (in a circular-shift fashion), and do the multiply- and-add thing again, the final sum is very close to zero. In other words, the correlation between a Barker code, and a time-shifted version of that code, is very small. So - in order to transmit using DSS, you take the incoming bits, multiply (XOR) them with a Barker sequence of chips running at (e.g.) 11x the bit rate, and transmit. To receive, you take the incoming (11x rate) pattern, and perform the "multiply and add" process against the original Barker chip pattern at the full 11x rate. If nothing's being transmitted, and the input signal is simply noise, the correlation between the noise and the Barker pattern will be very low, and the sums coming out of the adder will be close to zero. During a transmission, the sums from the correleator will also be close to 0 when the bits don't line up. Every 11th chip-time, though, when a complete bit's worth of chips from the receiver has entered the correlator, the sum will jump up to a high absolute value (+11 or -11 in the example case, if none of the chips were corrupted by noise). This sudden jump to a high absolute value tells you what the original data bit was (1 or 0) and can also create a synchronization pulse which you can use to discipline your receive oscillator. The above description is crude, inexact, and may contain errors, but perhaps it gives the flavor of the method. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
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