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:

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))

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!