sched: fix ul max coderate derivation

srsenb/src/stack/mac/sched_phy_ch/sched_dci.cc

@@ -94,7 +94,9 @@ tbs_info compute_mcs_and_tbs(uint32_t nof_prb,
   assert((is_ul or not ulqam64_enabled) && "DL cannot use UL-QAM64 enable flag");
 
   uint32_t max_Qm = (is_ul) ? (ulqam64_enabled ? 6 : 4) : (use_tbs_index_alt ? 8 : 6);
+  if (!is_ul) {
     max_coderate = std::min(max_coderate, 0.930F * max_Qm);
+  }
 
   int mcs = 0;
   do {
@@ -123,7 +125,9 @@ tbs_info compute_mcs_and_tbs(uint32_t nof_prb,
     // update max coderate based on mcs
     srsran_mod_t mod = (is_ul) ? srsran_ra_ul_mod_from_mcs(mcs) : srsran_ra_dl_mod_from_mcs(mcs, use_tbs_index_alt);
     uint32_t     Qm  = srsran_mod_bits_x_symbol(mod);
+    if (!is_ul) {
       max_coderate = std::min(0.930F * Qm, max_coderate);
+    }
 
     if (coderate <= max_coderate) {
       // solution was found
@@ -151,7 +155,7 @@ tbs_info compute_min_mcs_and_tbs_from_required_bytes(uint32_t nof_prb,
 {
   // get max MCS/TBS that meets max coderate requirements
   tbs_info tb_max = compute_mcs_and_tbs(nof_prb, nof_re, cqi, max_mcs, is_ul, ulqam64_enabled, use_tbs_index_alt);
-  if (tb_max.tbs_bytes + 8 <= (int)req_bytes or tb_max.mcs == 0) {
+  if (tb_max.tbs_bytes + 8 <= (int)req_bytes or tb_max.mcs == 0 or req_bytes <= 0) {
     // if mcs cannot be lowered or a decrease in TBS index won't meet req_bytes requirement
     return tb_max;
   }

srsenb/test/mac/sched_dci_test.cc

@@ -271,6 +271,43 @@ int test_min_mcs_tbs_specific()
   return SRSRAN_SUCCESS;
 }
 
+void test_ul_mcs_tbs_derivation()
+{
+  uint32_t cqi     = 15;
+  uint32_t max_mcs = 28;
+
+  sched_cell_params_t cell_params;
+  prbmask_t           prbs;
+
+  auto compute_tbs_mcs = [&prbs, &cell_params, &max_mcs, &cqi](uint32_t Nprb, uint32_t prb_grant_size) {
+    sched_interface::cell_cfg_t   cell_cfg   = generate_default_cell_cfg(Nprb);
+    sched_interface::sched_args_t sched_args = {};
+    cell_params.set_cfg(0, cell_cfg, sched_args);
+    prbs.resize(Nprb);
+    prbs.fill(2, prb_grant_size);
+    uint32_t req_bytes = 1000000;
+    uint32_t N_srs     = 0;
+    uint32_t nof_symb  = 2 * (SRSRAN_CP_NSYMB(cell_params.cfg.cell.cp) - 1) - N_srs;
+    uint32_t nof_re    = nof_symb * prbs.count() * SRSRAN_NRE;
+    return compute_min_mcs_and_tbs_from_required_bytes(
+        prbs.count(), nof_re, cqi, max_mcs, req_bytes, true, false, false);
+  };
+
+  cqi = 0;
+  TESTASSERT(compute_tbs_mcs(25, 25 - 4).mcs == 0);
+  TESTASSERT(compute_tbs_mcs(50, 50 - 5).mcs == 0);
+
+  cqi = 5;
+  TESTASSERT(compute_tbs_mcs(25, 25 - 4).mcs == 9);
+  TESTASSERT(compute_tbs_mcs(50, 50 - 5).mcs == 9);
+
+  cqi = 15;
+  TESTASSERT(compute_tbs_mcs(25, 25 - 4).mcs == 28);
+  TESTASSERT(compute_tbs_mcs(50, 50 - 5).mcs == 28);
+  TESTASSERT(compute_tbs_mcs(75, 75 - 5).mcs == 28);
+  TESTASSERT(compute_tbs_mcs(100, 100 - 5).mcs == 28);
+}
+
 } // namespace srsenb
 
 int main()
@@ -286,6 +323,7 @@ int main()
   TESTASSERT(srsenb::test_mcs_lookup_specific() == SRSRAN_SUCCESS);
   TESTASSERT(srsenb::test_mcs_tbs_consistency_all() == SRSRAN_SUCCESS);
   TESTASSERT(srsenb::test_min_mcs_tbs_specific() == SRSRAN_SUCCESS);
+  srsenb::test_ul_mcs_tbs_derivation();
 
   printf("Success\n");
   return 0;