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Ask Eric Blossom about Software-Defined Radio
Eric Blossom is an electrical engineer with a history of working with radio and communications security. He gave a presentation at the recent H2K2 conference about his work with GNU Radio, which is, bar none, the single most exciting software project in existence today. (Imagine computing devices that communicate seamlessly across the entire electromagnetic spectrum.) As usual, we'll forward some of the best questions to Eric and post his responses when we receive them.
Isn't the Slashdot blurb.. how shall I put this.. completely and totally incorrect?
The Slashdot story implies this project will bring: computing devices that communicate seamlessly across the entire electromagnetic spectrum.
Surely this is an exaggeration.
It is possible to imagine real-time processing of kHz, and possibly MHz signals in software.
There are issues with getting and antenna with decent response over a very wide range, but we can sidestep that. The point is, the ADC and actual processing of the waveform with the CPU is not unreasonable with x86 hardware.
But if you're looking at microwave communications... that just isn't plausible to me at all.
The ADC time resolution would surely require dedicated equipment. You could put this on a PCI card, but the PCI bus obviously does not have bandwidth for a GHz signal.
I would think the point of this is not to cover a wide range of the spectrum at all, but instead to process a range such as 1 kHz - 10 kHz in new and unusual ways.
And of course the applications are not limited to wireless devices! This could be just as useful with transmission down coax.
You can just use a soundcard if that's all you have. It will get you 44 kHz of bandwidth. Then you'd need a rf tuner in front of that.
On the high end, we use a $1k+ 20Ms/s card with a cable tuner.
It is widely believed, but not yet proven, that you can coax raw samples from a BT8x8 video capture card, which would be a great boon, as those are cheap.
It allows you to encode/decode different signals on the airwaves...
For example, they have demonstrated it decoding two FM radio broadcasts at the same time using their ADC and a cable modem tuner.
It replaces the hardware components involved in decoding signals... you could built a cell phone that only needs a tuner and and ADC, the rest in software. Depending on the tuning range of the tuner you use, your computer could become MANY different kinds of radios.
That is the cool thing about SDR... when a new fancy modulation technique comes along, you only have to upgrade the software... cell companies love this. I would assume those base stations can get pricey...
It has always been possible to generate waves in software. What's novel here is that it's being done in real time in the IF range of frequencies. Computers are still not quite fast enough to do this in the frequency range of the carrier signal.
When you tune your FM radio, your receiver accepts a signal centered at around 90-100 MHz and downshifts it to 10.7 MHz. The 90-100 MHz is called "radio frequency", the 10.7 MHz is called "intermediate frequency". No matter what station you listen to, it gets shifted down to the same intermediate frequency, so that once it's there, it can go thru exactly the same frequency.
The conversion from RF to IF is a pretty simple process that ignores the actual audio content of the signal. Once it's in IF, an FM demodulator picks off the audio that was modulated onto the carrier at the transmitter. AM radio also uses an intermediate frequency, but it's 455 kHz instead of 10.7 MHz.
GNU Radio depends upon commodity computers to sample and process the signal in real time. The Nyquist sampling theorem says you must sample at twice the frequency of interest as a theoretical minimum; in practice you'd like to sample more like threee or four times the frequency. So when you run GNU Radio, samples are whipping through your computer at 30 or 40 megasamples per second, at least for the IF processing. The audio processing can be done at a much more leisurely pace.
Since the computer can't hope to keep up with the 90-100 MHz raw carrier (yet, anyway) there must still be some external circuitry to perform the RF->IF downshift. But that's the relatively simple circuitry - by specifying everything else in software you get a hugely flexible radio. I've drooled with envy watching people use all-mode radios to talk to satellites in orbit, but those suckers are expensive! You need to be able to do AM modulation on the 2 meter band (144-148 MHz). Now maybe I can try it one of these days.
WWJD for a Klondike Bar?
Perhaps one of the best current applications of software radio is to act as a bridge between multiple radio systems.
Take for example an event, such as a terrorist attack, that warrants the use of many agencies that span a wide spectrum of roles and budgets. In the US, this means from local fire, EMS, law enforcement, response teams, HazMat, right on up to FBI, US Marshalls, Secret Service, FEMA and across to public works and everyone else involved in the response effort.
Communications in these situation has always been a big problem. It was highlighted nearly a year ago in New York and Arlington.
What a software radio unit would allow you to do is set up on site and when in operation the unit allows all the different radio systems of the agencies - who of course have different budgets, and hence equipment - to communicate relatively easily with each other as the software radio does the transformations from one radio signal and channel to another. And this can be done without reprogramming the frequencies on the radios, as you can use agencies predesiginated channels.
Perhaps the coolest thing is that it can be set up to work with current equipment, so apart from the bridge, and associated hardware, agencies preexisting radios will work. This is all good especially when you consider that people at the Pentagon were carrying around combos like 2 radios, 3 cell phones, and 3 pagers just to keep on top of everything.
By moving it to software, you can target everything to nearly one device - to the point where you plug phone lines (could be cable/sattelite/rf) into the bridge, and people on radios can make phone calls.
Another benefit is that it could be used in radio dense environments to bridge radio communications as radio devices increase in popularity by allowing them to use frequencies they weren't originially designed for. I think thats pretty cool.
Cheers