Not getting timestamps from UHD received stream tags

dubstep · February 7, 2012, 3:14am

I am trying to read incoming RX timestamps from the UHD sample stream.
I’d
like to calculate the approximate time a preamble is received in the
OFDM
code.

The issue is that, despite getting incoming frames and preambles, I do
not
seem to be getting timestamp values. I use get_tags_in_range() on
“rx_time”:

github.com

gnychis/grforwarder/blob/8f8c4e6cf79a2d635a0eb3bcad9b5ab325f1e373/gr-digital/lib/digital_ofdm_sampler.cc#L78




int
digital_ofdm_sampler::general_work (int noutput_items,
          gr_vector_int &ninput_items,
          gr_vector_const_void_star &input_items,
          gr_vector_void_star &output_items)
{
// Use the stream tags to the timestamp
std::vector<gr_tag_t> rx_time_tags;
const uint64_t nread = this->nitems_read(0); //number of items read on port 0
this->get_tags_in_range(rx_time_tags, 0, nread, nread+ninput_items[0], TIME_KEY);


const gr_complex *iptr = (const gr_complex *) input_items[0];
const char *trigger = (const char *) input_items[1];


gr_complex *optr = (gr_complex *) output_items[0];
char *outsig = (char *) output_items[1];


//FIXME: we only process a single OFDM symbol at a time; after the preamble, we can 
// process a few at a time as long as we always look out for the next preamble.

However, the size of the rx_time tags is consistently zero:

github.com

gnychis/grforwarder/blob/8f8c4e6cf79a2d635a0eb3bcad9b5ab325f1e373/gr-digital/lib/digital_ofdm_sampler.cc#L99


unsigned int index=d_fft_length;  // start one fft length into the input so we can always look back this far


outsig[0] = 0; // set output to no signal by default


// Search for a preamble trigger signal during the next symbol length
while((d_state != STATE_PREAMBLE) && (index <= (d_symbol_length+d_fft_length))) {
  if(trigger[index]) {
    outsig[0] = 1; // tell the next block there is a preamble coming
    d_state = STATE_PREAMBLE;


    std::cout << "got a preamble, size of time_tags: " << rx_time_tags.size() << "\n";


    for(int t=rx_time_tags.size()-1; t>=0; t--) {
      std::cout << "... t: " << t << "\n";
      if(rx_time_tags[t].offset <= index) {  // the current rx_time tag includes our preamble
        const uint64_t sample_offset = index - rx_time_tags[t].offset;  // distance from sample to timestamp in samples
        double rate = relative_rate();
        const pmt::pmt_t &value = rx_time_tags[t].value;
          
        std::cout << boost::format("Full seconds %u, Frac seconds %f, abs sample offset %u")
            % pmt::pmt_to_uint64(pmt_tuple_ref(value, 0))

The output of the OFDM receiver code:
ok: True pktno: 637 n_rcvd: 18 n_right: 17
got a preamble, size of time_tags: 0
ok: True pktno: 638 n_rcvd: 19 n_right: 18
got a preamble, size of time_tags: 0
ok: True pktno: 639 n_rcvd: 20 n_right: 19
got a preamble, size of time_tags: 0
…

Is there something I am missing to enable the RX timestamp on the
incoming
samples, or am I not properly reading the stream tag? My branch came
from
the GR master 4 days ago (309ca769af).

Thanks a bunch.

George

george_n · February 7, 2012, 3:19am

On 02/06/2012 06:13 PM, George N. wrote:

https://github.com/gnychis/grforwarder/blob/8f8c4e6cf79a2d635a0eb3bcad9b5ab325f1e373/gr-digital/lib/digital_ofdm_sampler.cc#L99

Is there something I am missing to enable the RX timestamp on the incoming
samples, or am I not properly reading the stream tag? My branch came from
the GR master 4 days ago (309ca769af).

The timestamps are only sent on the very first packet and after overflow
conditions. Are you seeing at least one tag at the very beginning of
streaming?

-Josh

george_n · February 7, 2012, 3:36am

Bingo, didn’t realize they were only incoming at those points in time.
Problem solved, thanks Josh!