[time-nuts] PN sequence generation using GPS

[Bob Camp]

Hi

System design is always about compromise. 

If you hop slowly, you "stomp" on each channel pretty hard. You are likely
to get noticed when you do. The idea is to stomp so rarely and for so short
a time that you aren't noticed. 

If you hop fast, you need to adjust the sequence to match. If you have a max
distance of 50 miles / 270 us and a 270 us chip rate things are still pretty
easy. You only need to be able to adjust over a +/- 1 chip range. More or
less like a fine tuning knob. 

If you are trying this with a conventional radio, there are some other
issues. The PLL lock times are way longer than the 270 us you are looking at
for distance. Based on using radios as scanners, anything dimensioned in
hops/second is likely sound bad. 

If it's a software radio, then it's got all the channels all the time.
Adjusting the sequence is just another little software task hidden away
among many. 

Lots of compromises.

Bob 

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Joe Leikhim
Sent: Thursday, February 17, 2011 10:22 AM
To: time-nuts at febo.com
Subject: Re: [time-nuts] PN sequence generation using GPS

I am trying to stay within the FCC Part 97 rules. The spreading or hopping
will be of a narrowband (25 KHz BW) FM signal. I haven't decided on either
the FHSS or DSSS approach. I had thought of a FH approach that exploited
time of day to address a frequency look up table, but I think that is
outside Part 97, although arguably it is a "net scheduler."

1) If I go with a FH approach running at around 10-20 hops/second will the 1
pps be sufficient?

2) Part 97 says the shift register cannot be reset by anything other than by
itself during a transmission. Clearly many of the synchronization methods
discussed in the ARRL SS Sourcebook utilize a synch method transmitted
across the link or derived from a TV broadcast signal etc. I don't see the
reason this rule exists?

3) If I block the 1 PPS during PTT, and the receiving end asserts 1 PPS
reset, will synch be lost? If not how effective will the freewheeling be
during a 30 second exchange with 10 MHz GPS derived clock reduced to 20
hops/second?

4) 20 hops per second is a 50 ms chip. Two radios 50 miles apart would be
270 us delayed. I don't think that should impair the analog FM. So don't
envision needing any correlation adjustment. Comments?


    From: Bob Camp


    Hi

    The key item here is that the system is going to work via amateur
    radio here in the US. The FCC only lets you use three very specific
    PN sequences. The three are called out explicitly in the rules. The
    requirement that they not be reset while transmitting except by
    feedback is also called out explicitly in the rules. So no fancy
    stuff with multiple spreaders and the like.

    The fact that they work out to primes is not important other than
    there's not much use trying to factor them. Without any common
    factors, and with 1 pps as your reset, you get a pretty specific
    trigger point for reset. It's long enough to not be of much use.

    There's a much easier way to do it and keep everybody in sync. GPS
    gives you time of day along with the 1 pps signal. You agree that at
    this or that time, everybody starts in sync. It's just time math to
    figure out where you should be at any 1 pps point. If you aren't
    where you are supposed to be, you take action.

    I suspect you could do all the checking on a 50 cent processor.
    Latch the data at 1 pps and read the time string. Do the math to
    check it. Fire the alarm flag it's not right. Not a lot to get done
    in a second.

    Bob

-- 
Joe Leikhim

Leikhim and Associates
Communications Consultants
Oviedo, Florida

www.Leikhim.com

JLeikhim at Leikhim.com

407-982-0446
WWW.LEIKHIM.COM

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