Re: [Discuss-gnuradio] Half-Duplex Relay

[Martin Braun]

On 01/10/2014 03:06 PM, David Halls wrote:
> Hi all,
> 
> Hopefully a very easy question! How do I implement a relay such that it
> will not begin transmitting until it has received a whole ‘burst’ of
> data. As there will be a direct path from source to destination, I don’t
> want the relay to start transmitting until the source has finished
> transmitting. Thus I want to implement a TDMA restriction where source
> transmits in time slot 1, then transmits nothing during time slot 2
> (easy), then I want the relay to listen only in time slot 1 and then not
> begin transmitting until time slot 2.
> 
> I was wondering if I should use some kind of tagging?

Most likely, yes. Although I know one student at CEL wrote a relay
before tags were around (I doubt the code is still available...).

Not transmitting is not as trivial as it sounds :)
I'm assuming you're using UHD devices (USRPs). In this case, have a look
at the tx_sob and tx_eob tags and what they do in the UHD sink (they
shut down the transmitter and fire it up again, so your USRP doesn't
expect samples when you're in an idle slot).

There's a couple of things to consider. If you're doing some
relay-specific experiment, you probably have dedicated code for source,
relay and destination.

The source will only send out a burst (use the mentioned tags to mark
that) and wait. Alternatively, you can also send out zeros between tags.

The destination node is even simpler -- you rx all the time and pass
packets to an upper layer for combining. Nothing special here.
Same with the relay, although you'll need the tags again for
retransmission. Also, you should keep timing in mind, which can
sometimes be a bit random in GNU Radio. If you're expecting decoding
within a certain time after decoding (or just reception if you do AaF).

Have a look at the manual page for tagged streams and PDUs as well as
the examples for packet-based transceivers (e.g. the OFDM code).
If you need anything more specific, just ask here!

MB