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From: J.A. Legris on 2 Jul 2010 11:13
On Jul 1, 12:19 pm, andy baxter <n...(a)earthsong.null.free-
online.co.uk> wrote:
> Hello,
>
> I am working for a small company who are working on a system which needs
> to send digital signals over a noisy medium. We have something working
> which uses a pair of frequencies (in the 100kHz-1Mhz range) to represent
> 0s and 1s, but would like something that gives better noise rejection.
>
> We are looking at spread spectrum signalling for this, but don't have the
> specific skills in house to develop this technique, so want to research
> it a bit further to see if it is something that might be useful.
>
> Can anyone recommend any good books on this subject which would cover a
> range of techniques and give us some idea of which might be useful for
> what we want to do?
>
> Thanks for any help with this,
>
> andy baxter.
>
> --
> (remove 'null' to email me)

Popular introductory textbook on communications theory and technology:
http://www.amazon.com/Electronic-Communication-Techniques-Paul-Young/dp/0130482854

Highly recommended advanced spread-spectrum textbook:
http://www.amazon.com/Spread-Spectrum-Systems-Commercial-Applications/dp/0471593427

--
Joe
From: andy baxter on 2 Jul 2010 11:51
On Fri, 02 Jul 2010 08:13:28 -0700, J.A. Legris wrote:

>
> Popular introductory textbook on communications theory and technology:
> http://www.amazon.com/Electronic-Communication-Techniques-Paul-Young/
dp/0130482854
>
> Highly recommended advanced spread-spectrum textbook:
> http://www.amazon.com/Spread-Spectrum-Systems-Commercial-Applications/
dp/0471593427


Thanks - this is the kind of thing I was looking for.

andy


--
(remove 'null' to email me)
From: Tim Wescott on 2 Jul 2010 12:41
On 07/02/2010 04:14 AM, andy baxter wrote:
> On Thu, 01 Jul 2010 14:44:25 -0700, Tim Wescott wrote:
>
>>
>> Depending on how many transmitters you have, and how organic the network
>> is, there are a number of things that might work.
>>
>> Vladimir's suggestion of just having each transmitter on a different
>> frequency would work if you can assign enough channels and if
>> single-tone signaling works in your environment (which it will if the
>> noise is wideband, or has specific, predictable frequencies that must be
>> avoided).
>
> The problem with this is that the number of transmitters could
> potentially be quite large - several thousand at a guess - so giving each
> one its own frequency might not be so easy.

Argh. Indeed.

>>
>> Spread-spectrum (code division multiplexing, really) makes sense if you
>> have single-tone interference that you can't trust to stay on frequency,
>> but trying to find a scheme that keeps the receiver's cheap may be a
>> challenge. Spread spectrum won't do you much good if the noise is
>> completely random and is already swamping your receivers.
>
> This is something I'm trying to clear up in my own mind - do spread
> spectrum techniques have inherently better noise rejection than
> traditional methods, or does it depend on the situation?

It depends on the situation. We're really used to thinking of the world
in terms of frequency spectrum because it's so easy to make resonant
filters that not only is that one of the first things we learned how to
do with electronics, but the world is full of them. But if you can put
that aside, the basic architecture of a spread spectrum receiver in
cloud-cuckoo land is exactly the same as a regular old superheterodyne
receiver like you find in all but the cheapest radios: the signal from
the antenna is multiplied by a reference signal (the LO in a superhet,
the despreading sequence in a spread spectrum RX) and low-pass filtered.

Spread spectrum radios do better overall than narrow-band radios in
situations where there is lots of narrowband interference. It is also
much easier to multiplex an unknown number of transmitters with CDMA if
you can synchronize them all: with frequency-division multiplexing when
you fill up the spectrum you're done, and if you have 1000 empty
channels and two transmitters that happen to be fixed at one frequency
you're screwed. With CDMA you can make a practically infinite number of
"channels" that are mutually orthogonal, and each active transmitter
just raises the noise floor for all of the receivers. So not only is
planning much easier, but the failure is more graceful.

> Also, how do noise rejection and data rate work together? I'm guessing
> that you can get better noise rejection by lowering the data rate, but
> this is only an intuitive guess, and I was thinking that there are
> probably equations which have been worked out covering this sort of
> question, which we could get from a decent book on the subject.

This is a basic communications question, not really spread spectrum.
Yes, you get better noise rejection by lowering the data rate -- with
totally random noise, when you average a constant signal + noise the
output noise level goes down by the square root of the averaging
interval while the signal strength remains the same.

There are fancier ways to achieve better results when the noise isn't
white, or when it's bursty -- but it all boils down to the fact that
making longer observations generally get you more reliable results at
the expense of data rate.

> Another question - what about chirp spread spectrum? The thing that
> kicked all this off was my boss was talking about how he thought we ought
> to be using spread spectrum techniques, and I offered to do some research
> on the subject, and set up a demo for them of chirp spread spectrum in
> the audio domain using supercollider (an audio synthesis programming
> language). I chose to do the demo using CSS because it looked like it was
> a simpler way of getting the benefits of spread spectrum than CDMA.

I'm not familiar with it specifically, but _any_ spread spectrum
technique is going to have similar advantages and disadvantages. Chirp
spread spectrum may allow you to simplify the receiver, which would be a
good thing.

>> What about time-division multiplexing? Is there any reason you can't
>> have the transmitters cooperate to send at different times, perhaps in
>> round-robin order? Or are you already doing this?
>
> I don't think this would work - the transmitters have to be kept as
> simple and low power as possible, so we want to avoid putting any
> receiving capabilities on them. Something like ethernet where they each
> broadcast a short id code at random intervals might work though, if we
> make the receivers capable of detecting collisions.

And if you make the duty cycles of the transmitters short enough, and
the hold-off intervals random enough -- if you're really going to have
thousands of transmitters then you could easily reach a situation where
you'd never have a clear space long enough for a transmitter to send in
the clear.

Something that spread spectrum could do for you in theory is to let you
decode transmissions that overlap, as long as they start at sufficiently
different times. But trying to do so will seriously complicate the
receiver. CSS looks like it would complicate such a receiver the least,
which is nice. My brain boils with possible ways to do this that would
(a) work, and (b) keep the receiver at least moderately practical.

>> Like Vladimir, I could help you with this. I think you should seriously
>> consider getting an expert to at least help you kick things off -- there
>> are a lot of ins and outs in this sort of thing, and I very much doubt
>> that they're all going to be collected in one book for you. In
>> particular, you don't want to get dazzled by some complicated technique
>> when there's a simple expedient that would work well.
>
> We have considered this, but the point for us is that even if we do get
> someone else in to work on this part of the project, it would be useful
> to know a bit more about the subject ourselves first just to have a
> better idea of whether this is justified, and to be able to assess the
> work they are doing if we do take someone else on. Which is why I
> suggested that we should look for a good book that would at least teach
> us the language of the subject and give us an idea of what is what in
> that field.

Understood. A good expert would start by being your consultant -- i.e.
they'd start by evaluating what you're trying, and let you know if what
you want to do is possible, whether it's feasible, and whether it's
practical given your budget. Only after they're done with that would
they take your money to start actually designing the system.

They'd also leave you as educated as they could, so that you'd at least
understand how to use what they did.

But yes, studying up on it a bit first would be good. The texts that I
have sitting on my shelf that pertain to this, at the level that you're
probably interested in, are:

"Information Transmission, Modulation and Noise", Mischa Schwartz, 1980,
McGraw-Hill

"Principals of Communications Engineering", Wozencraft and Jacobs, 1965,
Wiley

"Principals of Data Transmission", A.P.Clark, 1983, Wiley

Unfortunately, these will all give you lots of foundation for
understanding your problem without actually connecting directly. To
bracket your problem while still not hitting it on center, get a copy of
the ARRL Handbook -- that has a ton of applied theory for wireless
communications, but it (a) doesn't touch heavily on spread spectrum and
(b) misses what you want in the other direction -- the Handbook tends to
give solutions without tying things back to theory, while the three
books I cited tend to give theory without tying it to solutions.

If you're going to talk to a consultant, the theory books are probably
the ones to go with -- but the Handbook is more fun, and a better door
stop in a high wind. People with more up to date libraries than mine
often recommend that people "Get the Proakis", and John Proakis has a
number of titles out there. The "Digital Communications" book by
Proakis that's on Amazon looks like a likely find -- it's table of
contents looks like it covers what the Clark book I cited covers, and
it's probably more up to date.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html
From: Tim Wescott on 2 Jul 2010 12:43
On 07/02/2010 08:13 AM, J.A. Legris wrote:
> On Jul 1, 12:19 pm, andy baxter<n...(a)earthsong.null.free-
> online.co.uk> wrote:
>> Hello,
>>
>> I am working for a small company who are working on a system which needs
>> to send digital signals over a noisy medium. We have something working
>> which uses a pair of frequencies (in the 100kHz-1Mhz range) to represent
>> 0s and 1s, but would like something that gives better noise rejection.
>>
>> We are looking at spread spectrum signalling for this, but don't have the
>> specific skills in house to develop this technique, so want to research
>> it a bit further to see if it is something that might be useful.
>>
>> Can anyone recommend any good books on this subject which would cover a
>> range of techniques and give us some idea of which might be useful for
>> what we want to do?
>>
>> Thanks for any help with this,
>>
>> andy baxter.
>>
>> --
>> (remove 'null' to email me)
>
> Popular introductory textbook on communications theory and technology:
> http://www.amazon.com/Electronic-Communication-Techniques-Paul-Young/dp/0130482854
>
> Highly recommended advanced spread-spectrum textbook:
> http://www.amazon.com/Spread-Spectrum-Systems-Commercial-Applications/dp/0471593427

From other things you've said I think you want to start reading the
introductory text first. It'll answer many of the questions you've
asked on other sub-threads.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html
From: andy baxter on 2 Jul 2010 13:43
On Fri, 02 Jul 2010 09:41:40 -0700, Tim Wescott wrote:

>
> Understood. A good expert would start by being your consultant -- i.e.
> they'd start by evaluating what you're trying, and let you know if what
> you want to do is possible, whether it's feasible, and whether it's
> practical given your budget. Only after they're done with that would
> they take your money to start actually designing the system.
>
> They'd also leave you as educated as they could, so that you'd at least
> understand how to use what they did.
>
> But yes, studying up on it a bit first would be good. The texts that I
> have sitting on my shelf that pertain to this, at the level that you're
> probably interested in, are:
>
> "Information Transmission, Modulation and Noise", Mischa Schwartz, 1980,
> McGraw-Hill
>
> "Principals of Communications Engineering", Wozencraft and Jacobs, 1965,
> Wiley
>
> "Principals of Data Transmission", A.P.Clark, 1983, Wiley
>

Thanks - this (and the rest of your reply which I have snipped) is really
helpful. :)

Cheers,

andy

--
(remove 'null' to email me)
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