[Discuss-gnuradio] Re: few queries on OFDM implementation

[Tom Rondeau]

On Thu, Jun 18, 2009 at 7:45 AM, Sheshanandan
KN<address@hidden> wrote:
> Hi all,
> I am yet to figure out the answers to the following basic questions
> which was posted sometime back and somehow got missed out. It will
> help in proper understanding of 'ofdm' implemented on gnuradio and
> inturn will aid me to take it to the next level of my experimentation.
> Please help me get answers to these questions.
>
> Tom,
> if you can give some inputs, it will be great.
>
> Here we specify the center frequency at which the ofdm signal is going
> to be transmitted.
>
> 1) Is this the center frequency of the sub-carrier of a particular
> channel? If so, what is the bandwidth used?

This is the center frequency of the symbol transmitted over the air.
The bandwidth is based on the interpolation rate you use. The number
of subcarriers is specified by the user and this number and the
bandwidth selected determines the bandwidth of the subcarrers.

> 2) If not, how the sub-carrier is differentiated from the channel? How
> many sub-carriers are supported?

use the -h to see all of the options available. The number of
subcarriers is one of them.

> 3) The modulation parameter is the modulation technique defined for a
> particular sub-carrier, right? If so, can we also define a different
> modulation technique to be used at runtime once we switch to a
> different frequency?

No, the modulation used is for all subcarriers. Different modulations
for each subcarrier is currently not supported in this code. Allowing
per-subcarrier modulation would be a real benefit to the code and
community, though!

> 4) Why the interpolation/decimation values need to be specified for
> this experiment?

These map directly to the bandwidth of the symbol. The transmitted
bandwidth is (dac_rate)/(interp_rate).


> 5) My present setup consists of: 2 USRPs connected to 2 different PCs.
> The PCs are connected to a LAN. I do OFDM transmission at a frequency
> which is sensed to be free. I want to send this information to the
> OFDM receiver so that it also shifts its frequency of reception. I am
> not getting how this can be achieved in gnuradio?  Can we do this by
> extracting this information through the ethernet connection between
> the 2 PCs?
>
> Shesh

Yes, you could do it over Ethernet for experimental purposes.

To do this all over-the-air, you'll need to work on the upper layers
withe some protocol that specifies that this is a control packet.
Then, the framer should treat this differently than data packets. This
is going to require work in both the C++ and Python worlds and is not
trivial. It would be very helpful to see how you solve this problem.

Tom