Re: [Discuss-gnuradio] OFDM in n4hy's tree

[Tom Rondeau]

Robert McGwier wrote:
> Like all things in branches, until they are not in branches,  this is 
> absolutely development code and here is no guarantee of completeness 
> or even correctness.  It is the reason for doing this kind of work in 
> branches rather than imposing it on everyone in the trunk.
>
> In the gnuradio-examples/python/ofdm directory, the only functional 
> up-to-date file as of this moment is ofdm_benchmark.py.   It does a 
> loopback through noise test.
>
> ofdm_benchmark_tx.py calls upon a tx_pick_bitrate  which is missing 
> from the single computer where Tom, Matt, and I did the work.  I will 
> make sure that this is not an accidental omission that was actually on 
> Tom's laptop when he arrives tomorrow.
>
> We are attempting a very difficult thing which requires some delicacy 
> and art.  We want to be able to come up with a compact description of 
> any kind of constellation,  cyclic prefix size, or complete absence of 
> cyclic prefix and then have support for the requirement for sync 
> channels of several types.  At the same time,  we have the design goal 
> for this to be reasonably efficient and even dynamic.  We have chosen 
> not to take the fast road to a single exemplar but to abstract as much 
> of this as possible believing this will aid experimentation and serve 
> the greatest possible good.
>
> I have been so busy for the last few weeks that all of this slipped 
> out of the sieve that is left of my mind ;-).
>
> When Tom and I can get some work done tomorrow, one of the things we 
> will be planning is the next big steps in the ofdm tree.  You may 
> expect nothing to work until we say it does work.
>
> Bob

A quick look showed that pick_bitrate.py was not checked into the 
repository, this has been corrected. However, I don't think we've ever 
tested running the OFDM modulator with the USRP yet, as the 
benchmark_ofdm_tx.py is meant to do.

The pick_bitrate.py file was copied over the digital examples; it will 
probably be modified in the future for OFDM-specific operation.

To test the OFDM system, we were using a loopback mode using 
benchmark_ofdm.py, which calls upon receive_path_simple and 
transmit_path_simple, which bypass the calls to the USRP and connect 
directly to each other. A checkout and install had this working with no 
issues. You should just be able to run:
   ./benchmark_ofdm.py
With no argument and you'll see a stream of packet info being spit out. 
You can change a lot of the parameters now, including the SNR.

As Bob said, this is development code. We have no synchronization in the 
receiver yet, so this will definitely not work over the air yet.

Tom