Thanks Hal;
Transceivers A and B (C etc) would extract clock and 1 PPS from their
own GPS. The idea is to use 1 PPS (or derivative) to reset periodically.
Yes the propagation delay A-B would limit the hopping/spreading rate
unless some mechanism to correct the offset at the receiver end. (I
don't see this as big problem for the experiment).
On 2/16/2011 5:12 PM, Hal Murray wrote:
I'm not familiar with FHSS or DSSS, but pseudo random sequences from LFSRs
are used as scramblers on serial links. The idea is to avoid long strings of
0s that will provoke clock recovery troubles. The transmitter XORs a pseudo
random sequence with the data stream and the receiver does the same to get
the data back.
The problem is to keep both ends in sync.
There is a good section on scramblers in Lee and Messerchmitt: Digital
Communication, page 440.
I don't know how good that book is. It's one I picked up years ago. I've
been happy with it. Mine says copyright 1988, 4th printing 1992.
There are two types of scramblers: Frame Synchronous and Self Synchronizing.
With a Frame Synchronous setup, you get a reset pulse from someplace else.
(magic) It's usually a start-of-packet signal or a specific bit pattern
used as a frame marker, anything both ends can agree on. The idea is to get
back in sync occasionally in case you happened to get out of sync.
So the questions are:
Can both ends get the same reset pulse?
Are the clocks at both ends running at the same speed?
I'd expect it to work as long as both GPS systems are happy.
You will probably have to correct for propagation delays, that is delay the
receive clocking by X ns.
The self synchronizing scramblers probably aren't appropriate for SS work.
(But they might work fine if you know how to use them. I don't see it.)
The idea is to feed the data bit on the wire into the LFSR rather than the
output of the LFSR. If the data is received correctly, a garbage scrambler
on the receive side will get in sync after N bits. If you get a single bit
error on the wire, it will make a multi-bit error in the unscrambled data
stream. That error pattern will be the polynomial used for the LFSR. (It's
easy after you see it.)
--
Joe Leikhim
Leikhim and Associates
Communications Consultants
Oviedo, Florida
www.Leikhim.com
[email protected]
407-982-0446
WWW.LEIKHIM.COM
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