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Re: [Discuss-gnuradio] Using USRP to transmit and receive samples

From: JPL
Subject: Re: [Discuss-gnuradio] Using USRP to transmit and receive samples
Date: Fri, 18 Oct 2013 18:00:48 -0500


I found out two *.grc files, tx_ofdm and rx_ofdm, under gr_digital/examples/ofdm.

Don't know if this can be realized, 
first test running the steps without changing any parameters in *.grc.
(A) use *.dat generated from Matlab (I saved a set of wav audio), 
(B) place it as input in the tx_ofdm.grc, 
(C) send through antenna, 
(D) the other USRP execute rx_ofdm.grc
(E) put the output *.dat into Matlab analyzing.

(1) should I just replace the "Vector Source block" into "File Source" in tx_ofdm.grc?
(2) the rx_ofdm.grc, again, am I right just replace "tag debug" with "file sink"?

Is this possible?


On Fri, Oct 18, 2013 at 5:43 AM, Marcus Müller <address@hidden> wrote:

.mat is really just a complicated container format for all kind of matlab data -- GNU Radio can't directly deal with that, although with SciPy you could create something that will be able to parse .mat files; but that is quite useless, as you could as well use Matlab to write something that can be directly used with other software.

So to answer question (1.), I'd agree with Nathan: Best way to do it is using the existing GNU Radio OFDM tools, not writing code that has already been written several times, and start with something that already works. Thereby dropping Matlab as your signal processing framework, and only using it for data analysis and visualisation.

To comment on (II): GNU Radio has blocks like "file_sink". They will just save the samples to a file, in this case, in the format of raw float32s (1 for real, 1 for imag part) one after another.

To answer (III): If you really want to do that, see the GNU Radio source tree, gnuradio-core/src/utils/read_{float,complex,...}_binary.m. There is the same with "write" instead of "read".


On 10/18/2013 06:53 AM, West, Nathan wrote:
On Thu, Oct 17, 2013 at 11:14 PM, JPL <address@hidden> wrote:

We have used Matlab to generate *.mat file, a file around 1966240 complex number OFDM samples.
Thinking to (I) transmit between two USRPs, (II) let the receiver side saving data into file, and later (III) putting the file on Matlab for demodulation.

1. What is the best way to do it? like create GRC blocks? or UHD example?
2. How can I make sure that the TX part will stop when those 1966240 samples are sent. and RX part will stop when completely receiving those 1966240 samples?
3. Should the file type be the *.dat?



I'll attempt to respond to both questions since they are really the same thing.

First, this is kind of a weird way to use GNU Radio. GNU Radio provides you an environment to do all of the signal processing you want to do in c++ or (if you want) python. The flowgraph of file -> transmit antenna --> over the air --> receive antenna -> file followed by signal processing in Matlab is sort of not the point of GNU Radio. There's nothing saying you cannot do this, but you might look in to  implementing whatever signal processing you are doing in GNU Radio. This will reduce the round trip time of testing and make the whole experience a little better. ( you're basically doing an even slower version of what we call flying blind: http://gnuradio.org/redmine/projects/gnuradio/wiki/FAQ#Flying-Blind )

Re: your other email and question about GR file sink/source:
About the file sinks... http://gnuradio.org/redmine/projects/gnuradio/wiki/FAQ#What-is-the-file-format-of-a-gr_file_sink
Basically it's a binary file just like you would expect if you open a file with 'b' flag in most languages I'm familiar with. If you're dumping complex floats in there then you'll need to read two float values.

2. There is a block called head. It takes a number of samples to pass through and then quits. But again, you might consider your approach here.

3. I've never actually looked in to the format of a .mat file, but connecting that (regardless of data inside the file) to a UHD source would be spewing garbage out. The .dat extension you might see in GR literature is just a convention we use to denote it's data; it's not really a special format.


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