increase tbs until allocation is big enough to fit MAC subheader and RLC header

srsenb/src/stack/mac/scheduler_grid.cc

@@ -826,7 +826,7 @@ void sf_sched::set_dl_data_sched_result(const pdcch_grid_t::alloc_result_t& dci_
     }
 
     if (tbs <= 0) {
-      log_h->warning("SCHED: sched_cell_params[CARRIER_IDX].cfg %s rnti=0x%x, pid=%d, mask=%s, tbs=%d, buffer=%d\n",
+      log_h->warning("SCHED: DL %s failed rnti=0x%x, pid=%d, mask=%s, tbs=%d, buffer=%d\n",
                      is_newtx ? "tx" : "retx",
                      user->get_rnti(),
                      h->get_id(),

srsenb/src/stack/mac/scheduler_ue.cc

@@ -345,6 +345,8 @@ void sched_ue::tpc_dec()
  *
  *******************************************************/
 
+constexpr uint32_t min_mac_sdu_size = 5; // accounts for MAC SDU subheader and RLC header
+
 /**
  * Allocate space for multiple MAC SDUs (i.e. RLC PDUs) and corresponding MAC SDU subheaders
  * @param data struct where the rlc pdu allocations are stored
@@ -356,7 +358,6 @@ uint32_t sched_ue::allocate_mac_sdus(sched_interface::dl_sched_data_t* data, uin
 {
   // TS 36.321 sec 7.1.2 - MAC PDU subheader is 2 bytes if L<=128 and 3 otherwise
   auto     compute_subheader_size = [](uint32_t sdu_size) { return sdu_size > 128 ? 3 : 2; };
-  constexpr uint32_t min_mac_sdu_size = 5; // accounts for MAC SDU subheader and RLC header
   uint32_t rem_tbs                = total_tbs;
 
   // if we do not have enough bytes to fit MAC subheader and RLC header, skip MAC SDU allocation
@@ -542,6 +543,10 @@ int sched_ue::generate_format2a_unlocked(dl_harq_proc*                     h,
     } else if (tb_en[tb] && req_bytes && no_retx) {
       if (carriers[cc_idx].fixed_mcs_dl < 0) {
         tbs = carriers[cc_idx].alloc_tbs_dl(nof_prb, nof_re, req_bytes, &mcs);
+        while (tbs > 0 and (uint32_t) tbs < min_mac_sdu_size and mcs < 28) {
+          mcs++;
+          tbs = srslte_ra_tbs_from_idx((uint32_t)srslte_ra_tbs_idx_from_mcs(mcs, false), nof_prb) / 8;
+        }
       } else {
         tbs = srslte_ra_tbs_from_idx(
                   (uint32_t)srslte_ra_tbs_idx_from_mcs((uint32_t)carriers[cc_idx].fixed_mcs_dl, false), nof_prb) /
@@ -549,12 +554,12 @@ int sched_ue::generate_format2a_unlocked(dl_harq_proc*                     h,
         mcs = carriers[cc_idx].fixed_mcs_dl;
       }
 
+      tbs = allocate_mac_sdus(data, tbs, tb);
+      if (tbs > 0) {
         h->new_tx(user_mask, tb, tti, mcs, tbs, data->dci.location.ncce);
-
-      allocate_mac_sdus(data, tbs, tb);
-
         Debug("SCHED: Alloc format2/2a new mcs=%d, tbs=%d, nof_prb=%d, req_bytes=%d\n", mcs, tbs, nof_prb, req_bytes);
       }
+    }
 
     /* Fill DCI TB dedicated fields */
     if (tbs > 0 && tb_en[tb]) {