[time-nuts] PN sequence generation using GPS

[Joe Leikhim]

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

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