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'[EE]: Spread Spectrum systems'
2002\10\30@081645 by Jonathan Johnson

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Has anyone had any experience or know anything about hybrid Direct sequence/
Frequency Hopping spread spectrum systems? I'm looking at  using it in a
system I'm working on and was wondering how effective it is in real life
situations, any tips, pointers or comments would be extremely welcomed. I'm
curious as to how big the spread width would be for a given number of
frequency hops, is it better to have more hops or a wider spread, like I
said any comments would be appreciated

Thanks

Jonathan

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2002\10\30@085626 by Gordon Varney

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Every thing you ever wanted to know about spread spectrum, but was
afraid to ask........:-)

http://www.sss-mag.com


Gordon Varney
http://www.voiceactiveremote.com
http://www.talk2it.com



{Quote hidden}

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2002\10\30@104430 by Jim

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Firstly, SS does not equal 'frequency hopping'.

The key to this is: look at the differences in the
bandwidths of the IF filters used in each ...

How effective is SS?

Well, GPS uses it (all birds signals' occupy
the same BW), several PCS carriers use it ...

Also visit the http://www.qualcomm.com site as at one
time they had some some intro material on SS
(qualcomm is a pioneer and patent holder on a
LOT of spread spectrum technology).

RF Jim


{Original Message removed}

2002\10\30@115716 by Dave Tweed

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Jim <spam_OUTjvpollTakeThisOuTspamDALLAS.NET> wrote:
> "Jonathan Johnson" <.....jonathanKILLspamspam@spam@OUTEREDGE.NET> wrote:
> > Has anyone had any experience or know anything about hybrid Direct
> > sequence/Frequency Hopping spread spectrum systems?
>
> Firstly, SS does not equal 'frequency hopping'.

As the OP already indicated, spread spectrum can be either direct sequence
(as used in GPS) or frequency hopping (as used by hams in the VHF bands).
There is also something called "chirp spreading", but this is rare.

> The key to this is: look at the differences in the bandwidths of the IF
> filters used in each ...

Before or after de-spreading?

> > I'm curious as to how big the spread width would be for a given number
> > of frequency hops, is it better to have more hops or a wider spread,

With frequency-hopping, the "spread width" is simply a matter of the
difference between the lowest frequency you use and the highest. Choosing
the hop rate and the number of carrier frequencies you use is a combination
of what frequencies you're licensed to use, how good your frequency-agile
oscillator is, and how well your receiver synchronizer works.

The ARRL Spread Spectrum Sourcebook is a good reference to get you started.

-- Dave Tweed

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2002\10\30@124707 by Jim

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   "Before or after de-spreading?"

In a SS system the LO is not hopped, but
in a hopping system the LO must be 'hopped'
in order to go after and get the signal
centered back into the IF ... effectively
recapturing the 'hopped' signal (IOW you have
to go after the hopping data 'burst'). You
can hear freq hopped system on HF, for instance,
as a series as seeming randon clicks ...

In a SS system you can safely let everything
come down the WIDE IF - where the "de-spreading"
code (demodulation process) will spread out coherent
carriers (or other *different* spreading codes)
*but* will properly render the desired cohernet SS
code ... typically your speading (or chipping code)
is 100x your desired data rate and your modulator
is simply an XOR gate (FAR cry form hopping your
LO).

Please, do not confuse 'freq hopped' systems with
true DSSS (Direct Sequence Spread Spectrum).

RF Jim

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2002\10\30@185100 by Jonathan Johnson

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> -----Original Message-----
> From: pic microcontroller discussion list
> [PICLISTspamKILLspamMITVMA.MIT.EDU]On Behalf Of Jim
> Sent: Thursday, 31 October 2002 1:43 AM
> To: .....PICLISTKILLspamspam.....MITVMA.MIT.EDU
> Subject: Re: [EE]: Spread Spectrum systems
>
>
> Firstly, SS does not equal 'frequency hopping'.
>


I understand this, but it IS one form of SS...



{Quote hidden}

Thanks Jim I'm on my way

Regards

Jonathan

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2002\10\30@185112 by Jonathan Johnson

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Thanks Gordon, I've had a look at that site but alas....no joy to be found
on the front of Hybrid systems

Cheers

Jonathan

> {Original Message removed}

2002\10\30@191144 by Jonathan Johnson

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{Quote hidden}

What is Chirp spreading? Is it something like sweeping the carrier frequency
instead of hopping or gaussian spreading it?





{Quote hidden}

What I was really proposing was something along the lines of the little
diagram below:



hop1   _hop2   _hop3   _hop4   _hop5   _hop6
*******_*******_*******_*******_*******_*******

in this diagram the number of hops or carriers is 6 and the DS spread width
is 7*, a '*' is just a unit of bandwidth for example so to occupy the same
bandwidth, is it better to have:


hop1_hop2_hop3_hop4_hop5_hop6_hop7_hop8_hop9_hop10
****-****-****-****-****-****-****-****-****-****

where hops or carriers = 10

DS spread width = 4

total bandwidth = 10 x 4 = 40 units of bandwidth

or


 hop1  -  hop2  -  hop3  -  hop4  -  hop5
********-********-********-********-********


where hops or carriers = 5

DS spread width = 8

total bandwidth =5 x 8 =40 units of bandwidth


> The ARRL Spread Spectrum Sourcebook is a good reference to get
> you started.
>

thanks Dave I'll check that one out.

Best Regards

Jonathan

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2002\10\31@221445 by Dave Tweed

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Jonathan Johnson <TakeThisOuTjonathanEraseMEspamspam_OUTOUTEREDGE.NET> wrote:
> What is Chirp spreading? Is it something like sweeping the carrier frequency
> instead of hopping or gaussian spreading it?

Exactly.

{Quote hidden}

I see where you're headed, but I can't imagine why you'd go to this much
trouble. It seems like you'd end up with all the disadvantages of both
systems, plus a few from trying to combine them, for no overall benefit.

If you have the ability to do DS, just spread over the whole band and
forget about FH.

-- Dave Tweed

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