[lug] Linux Journal article about software radio

[J. Wayde Allen]

On Fri, 21 May 2004 robmohr at earthnet.net wrote:

> Is software radio just a parlor trick?  

In classical radio systems you'd build a tuner circuit followed by a
circuit that could demodulate and amplify the signal.  The development of
high speed digital samplers (A/D converters) has now made it possible to
perform many of the radio functions mathematically in the computer.  

The idea is that if you have a high speed broadband digitizer you could
convert the signal picked up by an antenna into a digital data stream.  
The digital data can then be operated on using digital filters, fast
Fourier transforms (FFT's), etc. to realize a radio as an almost
completely mathematical construct.  For a system like this most of the
hardware exists at the radio's front-end leading up to the digitizer.

> Or does it have immediate applications?

There are immediate applications.  Think of it this way.  If you build a
radio using hardware its capabilities are limited by the design of the
hardware.  You'll get an AM radio or and FM radio for instance, but not
both simultaneously.

With a software defined radio how the radio works can be changed as fast
as you can change the signal processing algorithm.  This can be very
valuable if for example you have a radio system that needs to interact
with a variety of other radios.  This can be ideal for a public safety
or military radio system where you may need to talk to a cellular network,
two-way police, fire department, or even spy on the enemy.  Let's see,
there are number of examples but the one I turned up first is
<http://www.spectrumsignal.com/Wireless_systems/Signals_Intelligence.asp>.

There is even a GNU Radio project in case you'd like to build your own
Linux based system, check it out at
<http://www.gnu.org/software/gnuradio/>.

> I am guessing here, are cell phones more software than hardware or
> equal parts of each?

Cell phones are a little of both.

> Pointless question?

Nope, it is a pretty darn hot topic these days.  The next level above
software defined radio is the so-called "Cognitive
Radio" <http://www.eetimes.com/showArticle.jhtml?articleID=18700443>.  These
radios actively look at the radio environment and work out the parameters
needed to communicate.

- Wayde
  (wallen at village.org)