Reuse old TDA when allocating DL retransmissions

- reusing old TDA whenever possible helps allocate DL retransmission when TDA chnges
- e.g. nrOfSymbols of new TDA > nrOfSymbols old TDA (leading to TBS size mismatching and allocation failure)
- This bugifx improves DL throughput stability in a multi-UE operation

openair2/LAYER2/NR_MAC_gNB/gNB_scheduler_dlsch.c

@@ -408,16 +408,27 @@ bool allocate_dl_retransmission(module_id_t module_id,
   const int tda = get_dl_tda(nr_mac, scc, slot);
   AssertFatal(tda>=0,"Unable to find PDSCH time domain allocation in list\n");
 
-  if (tda == retInfo->time_domain_allocation &&
+  /* Check first whether the old TDA can be reused
+  * this helps allocate retransmission when TDA changes (e.g. new nrOfSymbols > old nrOfSymbols) */
+  NR_tda_info_t temp_tda = nr_get_pdsch_tda_info(dl_bwp, tda);
+  const uint16_t temp_bitmap = SL_to_bitmap(temp_tda.startSymbolIndex, temp_tda.nrOfSymbols);
+  NR_tda_info_t *old_tda = &retInfo->tda_info;
+  const uint16_t old_bitmap = SL_to_bitmap(old_tda->startSymbolIndex, old_tda->nrOfSymbols);
+  bool reuse_old_tda = (old_bitmap & temp_bitmap) == old_bitmap;
+  LOG_D(NR_MAC, "[UE %x] %s old TDA, %s number of layers\n",
+    UE->rnti,
+    reuse_old_tda ? "reuse" : "do not reuse",
+    layers == retInfo->nrOfLayers ? "same" : "different");
+
+  if ((reuse_old_tda || tda == retInfo->time_domain_allocation) &&
       layers == retInfo->nrOfLayers) {
 
-    NR_tda_info_t *tda_info = &retInfo->tda_info;
     /* Check that there are enough resources for retransmission */
     while (rbSize < retInfo->rbSize) {
       rbStart += rbSize; /* last iteration rbSize was not enough, skip it */
       rbSize = 0;
 
-      const int slbitmap = SL_to_bitmap(tda_info->startSymbolIndex, tda_info->nrOfSymbols);
+      const uint16_t slbitmap = reuse_old_tda ? old_bitmap : temp_bitmap;
       while (rbStart < bwpSize && (rballoc_mask[rbStart] & slbitmap) != slbitmap)
         rbStart++;
 
@@ -434,7 +445,6 @@ bool allocate_dl_retransmission(module_id_t module_id,
   } else {
     /* the retransmission will use a different time domain allocation, check
      * that we have enough resources */
-    NR_tda_info_t temp_tda = nr_get_pdsch_tda_info(dl_bwp, tda);
     NR_pdsch_dmrs_t temp_dmrs = get_dl_dmrs_params(scc,
                                                    dl_bwp,
                                                    &temp_tda,