How to pass time tag into usrp sink as a parameter ?[hurry.]

GNU Radio
1

Hello,
I would like to implement the process as follows at the Rx side: Record
the time when the first sample arrived, and then wait for a certain
interval and then send ack back to Tx. Meanwhile, finish the cfo
calculating and compensating.
Here is my whole flow graph design. (Each means a block)

                        pkt_dtc-> cfo_calc -> file_sink
                                         cfo_calc -> cfo_cpst
                                    file_source -> cfo_cpst ->

usrp_sink

where pkt_dtc, grab_time_tag and cfo_calc was done myself with C++.
However, My question are:

  1. Is it correct of my whole desgin (I want to implement them in one top
    block) or are there any easier solutions ?
  2. After the block grab_time_tag, how should I pass the time information
    (directly got from the method get_tags_in_range) into the usrp_sink as a
    parameter so as to send ack at the intended time? Should I write in
    Python(but they are not the same level) or C++(but I can’t rewrite a
    usrp_sink) ?

Any doubts or suggestions are highly appreciated !

2

Hello Rui,

why do you take the detour through a file? That looks unnecessary.

the USRP sink understands transmission times you specify using tags [1],
so this will be the easiest way to deal with this. You just need to
write a really short block (in C++ or python, whatever you prefer) which
takes the rx_time tag that the USRP source added to the sample stream
and generates a tag with the desired point in time [2].

There are some examples of the rx_time and tx_time tags usage in gr-uhd,
so just search for “tx_time” or “rx_time” in gnuradio/gr-uhd/examples/ .

Greetings,
Marcus

[1] http://gnuradio.org/doc/doxygen/page_stream_tags.html
[2] http://gnuradio.org/doc/doxygen/page_stream_tags.html

3

Hi, Marcus,
I read the doc and examples you recommended, but I still don’t
understand fully how to implement my wok.
Sorry for my previous omission that the first line of my flow graph
design is:
usrp_source -> ptk_dtc -> grab_time_tag -> usrp_sink

  1. Here is the work function of my grab_time_tag block(not finished), I
    don’t know how to code next:
    const float *sigin = (const float *) input_items[0];
    const unsigned char *detect = (const unsigned char *)
    input_items[1];
    pmt::pmt_t *out = (pmt::pmt_t *) output_items[0];

    std::vectorgr::tag_t rx_time_tags;
    const uint64_t samp0_count = this->nitems_read(0);
    for(i = 0; i < ninput_items; i++){
    if (detect[0]){
    get_tags_in_range(rx_time_tags, 0, samp0_count,
    samp0_count+1,
    pmt::string_to_symbol(“rx_time”));
    break;
    }
    }

  2. I want to send acks at an intended time at usrp_sink, how to pass
    this time information into usrp_sink?
    I know a C++ method gr::uhd::usrp_sink::set_command_time, but don’t know
    how to use it.
    What also confuses me is:
    if I construct a usrp_sink object in python, how to pass the time
    information posted from grab_time_tag into it? (They are not at the same
    level after all). IMHO, Python is not something valuable. It’s just to
    build block objects and connect them.
    but if I do the job in C++, does it mean I have to rewrite a new
    usrp_sink block(sounds impratical) to receive the desired time value
    posted from grab_time_tag ? or else?

BTW, the file_sink/source was just for testing.
I 'm purely a green hand. If something is wrongly understood, please
correct me with no hesitation.
Hope your kind reply in specific details on the two query above. Many
thanks!

??? 2014???12???17??? 00:34, Marcus M??ller ???:

4

On Wed, Dec 17, 2014 at 3:44 PM, Rui Z. [email protected] wrote:

const unsigned char *detect = (const unsigned char *) input_items[1];
}
2. I want to send acks at an intended time at usrp_sink, how to pass this
time information into usrp_sink?

You could tag your ACK burst with “tx_time” to send your ACK after a
prefered delay.
Please look in to Bastian’s packet-pad implementation or pre-cog for
reference

[1] gr-foo/lib/packet_pad_impl.cc at maint-3.9 · bastibl/gr-foo · GitHub
[2]
https://github.com/jmalsbury/pre-cog/blob/master/python/packet_framer.py