Re: [Discuss-gnuradio] How to align Tx/Rx timestamps

[Josh Blum]

On 03/11/2012 09:35 AM, yend B. wrote:
> Hi,
> 
> I want to implement a TDD system on Gnuradio using stream tags. The
> receiver first detects bursts from another transmitter and sets timestamps
> of its bursts accordingly. I have some experiece in writing custom blocks,
> but I have never touched the UHD api nor the gr_uhd blocks. I want to know
> how to align timestamps of UHD source and sink blocks. Does it matter if I
> use the same or separate boxes? I guidline for general MIMO case would be
> appreciated.
> 


1) The simple option:

Multiple network devices like N210 can be grouped into one block with
multiple channels. Example:
http://old.nabble.com/attachment/32790029/0/4Channel.png

In this situation, all channels are aligned in time. Similarly, with the
usrp sink, all outputs channels are tagged with the same transmit time.
For this option, there is an assumption that the channels are
homogeneous (same sample rate).

2) A more complicated option:

More complication, but more control. You can 1 source and 1 sink block
per USRP device. In this situation, to have transmit alignment, your
producer blocks must tag each channel with a timestamp. Similarly, on
receive, the consumer blocks must be aware of the timestamp on each channel.

A description of tags here:
http://gnuradio.org/cgit/gnuradio.git/tree/gr-uhd/include/gr_uhd_usrp_sink.h#n59
http://gnuradio.org/cgit/gnuradio.git/tree/gr-uhd/include/gr_uhd_usrp_source.h#n59

A tags demonstration:
http://gnuradio.org/cgit/gnuradio.git/tree/gr-uhd/examples/tags_demo.cc

A guide to using stream tags:
http://gnuradio.org/redmine/projects/gnuradio/wiki/BlocksCodingGuide#Stream-Tags

-josh