gnss_sync: fix timestamp to TTI conversion

* Improve reliability of timestamp to tti conversion

Difftime is not reliable as it might use 32 bit calculation, depending on the system. This leads to wrong frame numbers and subframe indices.
I encountered this Issue when testing the pssch_ue exampel on my system and the conversion from timestamp to frame number and sf idx was wrong.

* Improve GNSS Sync 

Added loop to check for GNSS alignment while syncing.
If the received GNSS signal is weak, synchronization errors might occur while syncing.

lib/src/phy/ue/ue_sync.c

@@ -928,31 +928,44 @@ int srsran_ue_sync_run_find_gnss_mode(srsran_ue_sync_t* q,
   INFO("Calibration samples received start at %ld + %f", q->last_timestamp.full_secs, q->last_timestamp.frac_secs);
 
   // round to nearest second
-  srsran_timestamp_t ts_next_rx;
+  srsran_timestamp_t ts_next_rx, ts_next_rx_tmp, ts_tmp;
   srsran_timestamp_copy(&ts_next_rx, &q->last_timestamp);
   ts_next_rx.full_secs++;
   ts_next_rx.frac_secs = 0.0;
 
-  INFO("Next desired recv at %ld + %f", ts_next_rx.full_secs, ts_next_rx.frac_secs);
+  srsran_timestamp_copy(&ts_next_rx_tmp, &ts_next_rx);
+  INFO("Next desired recv at %ld + %f\n", ts_next_rx_tmp.full_secs, ts_next_rx_tmp.frac_secs);
 
   // get difference in time between second rx and now
-  srsran_timestamp_sub(&ts_next_rx, q->last_timestamp.full_secs, q->last_timestamp.frac_secs);
-  srsran_timestamp_sub(&ts_next_rx, 0, 0.001); ///< account for samples that have already been rx'ed
+  srsran_timestamp_sub(&ts_next_rx_tmp, q->last_timestamp.full_secs, q->last_timestamp.frac_secs);
+  srsran_timestamp_sub(&ts_next_rx_tmp, 0, 0.001); ///< account for samples that have already been rx'ed
 
-  uint64_t align_len = srsran_timestamp_uint64(&ts_next_rx, q->sf_len * 1000);
+  uint64_t align_len = srsran_timestamp_uint64(&ts_next_rx_tmp, q->sf_len * 1000);
 
-  DEBUG("Difference between first recv is %ld + %f, realigning %" PRIu64 " samples",
-        ts_next_rx.full_secs,
-        ts_next_rx.frac_secs,
+  DEBUG("Difference between first recv is %ld + %f, realigning %" PRIu64 " samples\n",
+        ts_next_rx_tmp.full_secs,
+        ts_next_rx_tmp.frac_secs,
         align_len);
 
   // receive align_len samples into dummy_buffer, make sure to not exceed buffer len
   uint32_t sample_count = 0;
-  while (sample_count < align_len) {
-    uint32_t actual_rx_len = SRSRAN_MIN(align_len, DUMMY_BUFFER_NUM_SAMPLES);
-    actual_rx_len          = SRSRAN_MIN(align_len - sample_count, actual_rx_len);
+  while (align_len > q->sf_len) {
+    uint32_t actual_rx_len = SRSRAN_MIN(align_len, q->sf_len);
     q->recv_callback(q->stream, dummy_offset_buffer, actual_rx_len, &q->last_timestamp);
-    sample_count += actual_rx_len;
+
+    srsran_timestamp_copy(&ts_tmp, &ts_next_rx);
+    srsran_timestamp_sub(&ts_tmp, q->last_timestamp.full_secs, q->last_timestamp.frac_secs);
+    srsran_timestamp_sub(&ts_tmp, 0, 0.001); ///< account for samples that have already been rx'ed
+    align_len = srsran_timestamp_uint64(&ts_tmp, q->sf_len * 1000);
+
+    if (align_len > q->sf_len * 1000) {
+      ts_next_rx.full_secs++;
+      ts_next_rx.frac_secs = 0.0;
+      srsran_timestamp_copy(&ts_tmp, &ts_next_rx);
+      srsran_timestamp_sub(&ts_tmp, q->last_timestamp.full_secs, q->last_timestamp.frac_secs);
+      srsran_timestamp_sub(&ts_tmp, 0, 0.001); ///< account for samples that have already been rx'ed
+      align_len = srsran_timestamp_uint64(&ts_tmp, q->sf_len * 1000);
+    }
   }
 
   DEBUG("Received %d samples during alignment", sample_count);
@@ -1017,39 +1030,22 @@ int srsran_ue_sync_set_tti_from_timestamp(srsran_ue_sync_t* q, srsran_timestamp_
   time_t t_cur = rx_timestamp->full_secs;
   DEBUG("t_cur=%ld", t_cur);
 
-  // time_t of reference UTC time on 1. Jan 1900 at 0:00
-  // If we put this date in https://www.epochconverter.com it returns a negative number
-  time_t t_ref = {0};
-#if 0
-  struct tm t = {0};
-  t.tm_year = 1900; // year-1900
-  t.tm_mday = 1;          // first of January
-  // t.tm_isdst = 0;        // Is DST on? 1 = yes, 0 = no, -1 = unknown
-  t_ref = mktime(&t);
-#endif
-
-  DEBUG("t_ref=%ld", t_ref);
+  // 3GPP Reference UTC time is 1. Jan 1900 at 0:00
+  // If we put this date in https://www.epochconverter.com it returns a negative number (-2208988800)
+  // as epoch time starts at 1. Jan 1970 at 0:00
+  uint64_t epoch_offset_3gpp = 2208988800;
 
   static const uint32_t MSECS_PER_SEC = 1000;
 
-  DEBUG("diff=%f", difftime(t_cur, t_ref));
-
-  double time_diff_secs = difftime(t_cur, t_ref);
-
-  if (time_diff_secs < 0) {
-    fprintf(stderr, "Time diff between Rx timestamp and reference UTC is negative. Is the timestamp correct?\n");
-    return SRSRAN_ERROR;
-  }
-
-  DEBUG("time diff in s %f", time_diff_secs);
+  uint64_t time_3gpp_secs = t_cur + epoch_offset_3gpp;
 
   // convert to ms and add fractional part
-  double time_diff_msecs = time_diff_secs * MSECS_PER_SEC + rx_timestamp->frac_secs;
-  DEBUG("time diff in ms %f", time_diff_msecs);
+  uint64_t time_3gpp_msecs = (time_3gpp_secs + rx_timestamp->frac_secs) * MSECS_PER_SEC;
+  DEBUG("rx time with 3gpp base in ms %lu\n", time_3gpp_msecs);
 
   // calculate SFN and SF index according to TS 36.331 Sec. 5.10.14
-  q->frame_number = ((uint32_t)floor(0.1 * (time_diff_msecs - q->sfn_offset))) % 1024;
-  q->sf_idx       = ((uint32_t)floor(time_diff_msecs - q->sfn_offset)) % 10;
+  q->frame_number = (uint32_t)(((uint64_t)floor(0.1 * (time_3gpp_msecs - q->sfn_offset))) % 1024);
+  q->sf_idx       = (uint32_t)(((uint64_t)floor(time_3gpp_msecs - q->sfn_offset)) % 10);
 
   return SRSRAN_SUCCESS;
 }