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The Journey of Radios From Hardware to Software
An anonymous reader writes "The New York Times is carrying a story all about the process of replacing radios with software. The article tells the tale of Vanu Bose, son of the man who started the Bose company, and his quest to bring software to what was previously a hardware-only enterprise. He met a lot of resistance in the 90s to his ideas, because processor technology was not up to the task. Now that technology has caught up with Vanu, his software (and other products like it) are increasingly replacing now-outdated hardware components. 'Well-established companies like Motorola and Ericsson now use elements of software-defined radio for their base stations. But Mr. Bose was the first to come to market with software that could handle multiple networks with the same equipment. Software radio appears to offer an elegant solution to what has been a vexing problem: how to have a single handset, like a cellphone, communicate across multiple networks. For instance, the G.S.M. standard, for global system for mobile communications, is used broadly in Europe, and most notably in the United States by AT&T.'"
It's not that a single software-defined radio is all that important. It's that you can do the transforms on the incoming waveform and then extract N different channels with one signal processing system. That's what's been making cellular base stations go for almost two decades. (All the hard work is on the receive side; transmission is easy.)
First generation cellular base stations (i.e. AMPS) had one big analog card per channel, each heavily shielded from its neighbors. The amount of hardware required was huge, and cell sites tended not to be fully populated with channel cards, so they were easy to overload.
Then things started to go digital, with combinations of analog and DSP components processing the signal. Both GSM and CDMA inherently assume digital processing, and in early systems, hard-wired special purpose components were used. As CPUs get faster, there's a steady trend toward using general purpose CPUs.
It's still rare to actually process RF directly in software. Usually, there's a local oscillator and mixer to down-convert the desired band to a working IF frequency, which is then digitized and processed. So it's only necessary to digitize at maybe 10-100MHz, not up in the gigahertz range.
For lower bands, though, a true software RF receivers are available. These just suck up everything from 0 to 30MHz and digitize it. An attached PC does all the hard work.
GNU Radio.
Well, yeah. Square wave outputs do tend to generate alllll sorts of harmonics. But it does technically work. If I were serious about it, I'd at least add a capacitor across the output, to make some attempt at filtration.
The "schematic" involved an 8-pin microprocessor, with two outputs each connected to a 1k resistor. The other ends of both resistors were connected to the antenna. Not very efficient, but as a proof of concept, it was a cool toy. Tuning was completely via software (tweak the timing loop to provide the correct waveform.)
You want pictures? Happy to oblige. (The idea was to see just how simple a transmitter I could make...)
http://www.intellectualism.org/electronics/schematic.jpg
http://www.intellectualism.org/electronics/Closeup.jpg
Paleotechnologist and connoisseur of pretty shiny things.
My company has IP for a FPGA based FM radio that meets all FCC harmonics requirements - and it has the same kind of external requirements. - I believe in our case it's an RC on the TTL compatible output. This isn't software defined in the traditional meaning of the term - but the FPGA is implementing the same algorithms the software is implementing. So it's soft in the way an FPGA is soft.- but the idea is identical to the MPC version mentioned above as far as the outputs are concerned.
Bottom line - the harmonics can be taken care of by wave-shaping, i.e. you take the output load configuration into equation as you design the radio.
Have you compiled your kernel today??