Re: [Discuss-gnuradio] How to best use new GR features for TDMA systems ?

[bob wole]

I think GNU Radio apps should look a bit like this:
At the top (think "MAC") blocks pass data as PDUs. The further down you go
("PHY"), you'll eventually need regular blocks. Here, tagged streams will
do what you want.
I guess gr-easymac was an early demo of this principle, although its
functionality can now be reproduced with pure GNU Radio. At CEL, we've
developed a FHSS network using pure GNU Radio, which works quite well
(someone still needs to release some code, and I hope they're reading this
:). There will be a presentation on this at FOSDEM, btw.

Also, I've been working on a way to do TDMA using the OFDM blocks (on one
of my lower-priority branches, though, so don't get too excited right now).
In principle, the idea is:
* At MAC level, create a package with a tx time stamp as a PDU
* Pass this to the OFDM mod. The time stamp will propagate along with the
length tag, and stay at the front of the streamed packet (this feature was
merged into GNU Radio a while back and is available in the latest release).
* The hardware driver should take the streamed packet and the metadata in
the tags (the tx time, though it could also be the centre frequency for FH)
and tx accordingly.

Hi MB,

I am using tags (tx_time, tx_eob, tx_sob) for  burst transmission. You mentioned above that tags can also be used to tell the centre frequency for FH? Which tag does so ? Is it a custom tag you implemented ?

Thanks,

Bob

 

* At the receive side, all we need to do is remember the rx time of samples
when we do a packet modulation.

There's a couple of things that are hard here: On the TX side, we just hope
that the packet will reach the hardware in time. What if it doesn't? Well,
bad luck.

Bottom line: GNU Radio has only recently become capable of doing these
things. This means we are still in the process of figuring out of how
exactly to write TDMA PHYs with corresponding MACs, which is a feature that
we've always wanted and now seems achievable. I guess the more we try out,
the more we will identify what's missing in order to do this.

MB