looking for books on digital signalling

I am Vladimir Vassilevsky, and I am at your service.

andy baxter wrote:

What's your data rate? What's the data like? How are uou applying the modulation? How are you detecting it?

Spread-spectrum won't necessarily give better performance, unless you have some special-case interferance problems.

John

The data rate is pretty low - the situation is that we have a large number of transmitters (which have to use as little power as possible), and a smaller number of receivers. The problem is for each receiver to continuously detect which transmitters are connected on the same physical channel as it is operating on. We can send analog electronic signals over this channel, but it is very lossy and subject to noise. (Sorry not to be more specific but my boss has asked me not to talk about the actual application in too much detail).

The data is just a unique id of each transmitter, sent around 10 times a second.

The modulation technique we are using at the moment is to generate pulses of one of two given frequencies using a programmable oscillator connected to a microcontroller, and then decode them by putting them through a pair of tuned filters.

andy baxter schrieb:

Hello,

you should look for the modulation used for the GPS system to transmit the almanac data with low data rate. Each satellite uses the same frequency and the signal at receivers is below the noise level, but due to the clever digital modulation it is possible to detect and to separate the signals from different satellites. A lot of computing power is needed using a signal processor.

Bye

So, frequency-shift keying...

Filters have a Q, so there'll be a delay... could you use synchronous rectification as your keying filter, instead? You'd still want a bandpass filter to include your signals and exclude the larger noise bandwidth.

POTS modems use multiple frequencies and get good noise performance through rejection of 'illegal' combinations. That might help you out, and the benefit of pushing data faster means your low-power devices can shut off more often.

That's simple. Assign unique FFT bin frequency for each of the transmitters. Do an FFT at the receiver side. Provided your channel is frequency flat and impaired with white noise, this scheme would be as energy efficient as it could ever be.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

What is a noisy medium? What you are looking for is probably a good old modem.

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).

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.

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?

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.

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.

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?

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.

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 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.

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.

andy

Popular introductory textbook on communications theory and technology:

Highly recommended advanced spread-spectrum textbook:

-- Joe

dp/0130482854

dp/0471593427

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

andy

Argh. Indeed.

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.

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.

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.

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.

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.

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.

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

Cheers,

andy

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No, not at all, it is quite as easy to fill the code space as it is to fill up the frequency space. Especially if you want short transmissions. Gold codes work well for GPS because the transmission is continuous. The Viterbi patents are closer to what OP seems to need.

See Shannon's law. It relates BW, signaling rate, and S/N ratio in a very useful way.

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Another issue is keeping the transmitters cost effective.

Come to think of it, OP has not told us whether or not freespace propagation or bounded media is involved and how much support of existing infrastructure will be required.

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