On 17.06.2014 04:24, Michael R. wrote:
I have a relatively high level question regarding gnuradio and software
radio in general. Is it a fair generalization to say that gnuradio is
operating at the application layer and is essentially emulating a
physical layer implementation (or the implementation of other lower
layer protocols)? For example, if I have a link between two USRPs (more
specifically, N series USRPs), the digitally sampled received data comes
in on the ethernet NIC and moves up the stack to the software radio
“application.” The signal processing that would typically be done in
lower layers is then handled by the application.
In most cases, GNU Radio handles pretty much what comes out of the A/D
converter (well, not quite, there’s some decimation and filtering in
there. But for the sake of this argument, this is irrelevant).
So, we have this:
ADC -> GNU Radio -> DAC
To say GNU Radio “operates at application layer” would therefore be not
good description of what it does; it doesn’t “emulate a PHY”, it is
the PHY for all intents and purposes.
This is the software radio principle: Using A/D converters, we bring the
software very close to the antenna.
This is not an academic view on things. While most wireless devices (and
I guess that means mobile phones) are what you could call “hardware
radio”, i.e. they have dedicated circuits to do the PHY, many other
applications such as base stations are actually driven by software.
So when you say data “moves up the stack”, it’s not going up the stack
in terms of ISO/OSI layers. The means of getting the sample data (which
is the raw signal, even before the PHY layer) to the software are just
slightly more elaborate than having the PHY chip connected directly to
GNU Radio was specifically designed to implement PHYs, it can also
implement MACs. Above that, you would probably use something else.
The second part of my question is, given a flow graph in gnuradio, what
sort of steps would be necessary to “push it back down the stack” or
implement in a chipset such that it can be used as an interface in a
typical network stack? Is this something that anyone using gnuradio has
considered or should I assume the next step would involve re-implementation?
I’m not sure what you’re trying to do here. Maybe what you’re trying to
do is what Balint and John have done with their gr-mac module, which
allows you to create a TCP/IP connection over your own user-defined PHY?
NOTE: This is by no means for a commercial product, but rather for
demonstration. My research has led me to use gnuradio for some proof of
concept implementations and I’m curious how much additional effort would
be required to port the work to a practical device - for example,
implementation on a smart phone. (you can read this as “will it cause me
to postpone graduation a few weeks? months? years?”)
GNU Radio works on some embedded devices, which might be what you’re
interested in. If you have no experience in embedded development, it can
take you months to years to figure out, but if you’re a smart guy you
might be able to get some results sooner than that. Note that I’m not
talking about smartphones here, but rather commercially available
embedded SDR devices, such as the Ettus E100.