implementation of basic search for optimal rbgmask for subband CQI

srsenb/hdr/stack/mac/sched_ue.h

@@ -83,6 +83,7 @@ public:
   /// Get total pending bytes to be transmitted in DL.
   /// The amount of CEs to transmit depends on whether enb_cc_idx is UE's PCell
   uint32_t                   get_pending_dl_bytes(uint32_t enb_cc_idx);
+  srsran::interval<uint32_t> get_requested_dl_bytes(uint32_t enb_cc_idx);
   rbg_interval               get_required_dl_rbgs(uint32_t enb_cc_idx);
   uint32_t                   get_pending_dl_rlc_data() const;
   uint32_t                   get_expected_dl_bitrate(uint32_t enb_cc_idx, int nof_rbgs = -1) const;
@@ -139,8 +140,6 @@ public:
   bool pusch_enabled(tti_point tti_rx, uint32_t enb_cc_idx, bool needs_pdcch) const;
 
 private:
-  srsran::interval<uint32_t> get_requested_dl_bytes(uint32_t enb_cc_idx);
-
   bool is_sr_triggered();
 
   tbs_info allocate_new_dl_mac_pdu(sched_interface::dl_sched_data_t* data,

srsenb/hdr/stack/mac/sched_ue_ctrl/sched_dl_cqi.h

@@ -36,15 +36,17 @@ public:
     K(K_),
     wb_cqi_avg(init_dl_cqi),
     bp_list(nof_bandwidth_parts(cell_nof_prb_), bandwidth_part_context(init_dl_cqi)),
-    subband_cqi(srsran_cqi_hl_get_no_subbands(cell_nof_prb), 0)
+    subband_cqi(std::max(1, srsran_cqi_hl_get_no_subbands(cell_nof_prb)), 0)
   {
     srsran_assert(K <= 4, "K=%d outside of {0, 4}", K);
+    srsran_assert(K == 0 or cell_nof_prb_ > 6, "K > 0 not allowed for nof_prbs=6");
   }
 
   /// Set K value from upper layers. See TS 36.331, CQI-ReportPeriodic
   void set_K(uint32_t K_)
   {
     srsran_assert(K <= 4, "K=%d outside of {0, 4}", K);
+    srsran_assert(K == 0 or cell_nof_prb > 6, "K > 0 not allowed for nof_prbs=6");
     K = K_;
   }
 
@@ -158,6 +160,19 @@ public:
 
   tti_point last_cqi_info_tti() const { return last_pos_cqi_tti; }
 
+  int get_wb_cqi_info() const { return wb_cqi_avg; }
+
+  uint32_t rbg_to_sb_index(uint32_t rbg_index) const { return rbg_index * N() / cell_nof_rbg; }
+
+  /// Get CQI of given subband index
+  int get_subband_cqi(uint32_t subband_index) const
+  {
+    if (subband_cqi_enabled()) {
+      return get_wb_cqi_info();
+    }
+    return bp_list[get_bp_index(subband_index)].last_feedback_tti.is_valid() ? subband_cqi[subband_index] : wb_cqi_avg;
+  }
+
 private:
   static const uint32_t max_subband_size    = 8;
   static const uint32_t max_nof_subbands    = 13;
@@ -177,8 +192,6 @@ private:
 
   uint32_t prb_to_sb_index(uint32_t prb_index) const { return prb_index * N() / cell_nof_prb; }
 
-  uint32_t rbg_to_sb_index(uint32_t rbg_index) const { return rbg_index * N() / cell_nof_rbg; }
-
   srsran::interval<uint32_t> get_bp_sb_indexes(uint32_t bp_idx) const
   {
     return srsran::interval<uint32_t>{bp_idx * N() / J(), (bp_idx + 1) * N() / J()};
@@ -212,6 +225,15 @@ private:
   srsran::bounded_vector<float, max_nof_subbands>                     subband_cqi;
 };
 
+/// Get {RBG index, CQI} tuple which correspond to the set RBG with the lowest CQI
+std::tuple<uint32_t, int> find_min_cqi_rbg(const rbgmask_t& mask, const sched_dl_cqi& dl_cqi);
+
+/// Returns the same RBG mask, but with the RBGs of the subband with the lowest CQI reset
+rbgmask_t remove_min_cqi_subband(const rbgmask_t& rbgmask, const sched_dl_cqi& dl_cqi);
+
+/// Returns the same RBG mask, but with the RBG with the lowest CQI reset
+rbgmask_t remove_min_cqi_rbg(const rbgmask_t& rbgmask, const sched_dl_cqi& dl_cqi);
+
 } // namespace srsenb
 
 #endif // SRSRAN_SCHED_DL_CQI_H

srsenb/hdr/stack/mac/sched_ue_ctrl/sched_ue_cell.h

@@ -93,7 +93,7 @@ tbs_info cqi_to_tbs_dl(const sched_ue_cell& cell,
                        const rbgmask_t&     rbgs,
                        uint32_t             nof_re,
                        srsran_dci_format_t  dci_format,
-                       int                  req_bytes = -1);
+                       uint32_t             req_bytes = std::numeric_limits<uint32_t>::max());
 
 /// Compute UL grant optimal TBS and MCS given UE cell context and UL grant parameters
 tbs_info
@@ -105,6 +105,19 @@ int      get_required_prb_dl(const sched_ue_cell& cell,
                              uint32_t             req_bytes);
 uint32_t get_required_prb_ul(const sched_ue_cell& cell, uint32_t req_bytes);
 
+tbs_info compute_mcs_and_tbs_lower_bound(const sched_ue_cell& ue_cell,
+                                         tti_point            tti_tx_dl,
+                                         const rbgmask_t&     rbg_mask,
+                                         srsran_dci_format_t  dci_format);
+
+bool find_optimal_rbgmask(const sched_ue_cell&       ue_cell,
+                          tti_point                  tti_tx_dl,
+                          const rbgmask_t&           dl_mask,
+                          srsran_dci_format_t        dci_format,
+                          srsran::interval<uint32_t> req_bytes,
+                          tbs_info&                  tb,
+                          rbgmask_t&                 newtxmask);
+
 } // namespace srsenb
 
 #endif // SRSRAN_SCHED_UE_CELL_H

srsenb/src/stack/mac/CMakeLists.txt

@@ -9,7 +9,8 @@
 add_subdirectory(schedulers)
 
 set(SOURCES mac.cc ue.cc sched.cc sched_carrier.cc sched_grid.cc sched_ue_ctrl/sched_harq.cc sched_ue.cc
-        sched_ue_ctrl/sched_lch.cc sched_ue_ctrl/sched_ue_cell.cc sched_ue_ctrl/sched_dl_cqi.cc sched_phy_ch/sf_cch_allocator.cc sched_phy_ch/sched_dci.cc sched_helpers.cc)
+            sched_ue_ctrl/sched_lch.cc sched_ue_ctrl/sched_ue_cell.cc sched_ue_ctrl/sched_dl_cqi.cc
+            sched_phy_ch/sf_cch_allocator.cc sched_phy_ch/sched_dci.cc sched_helpers.cc)
 add_library(srsenb_mac STATIC ${SOURCES} $<TARGET_OBJECTS:mac_schedulers>)
 
 set(SOURCES mac_nr.cc)

srsenb/src/stack/mac/sched_grid.cc

@@ -495,23 +495,25 @@ alloc_result sf_sched::alloc_dl_user(sched_ue* user, const rbgmask_t& user_mask,
     return alloc_result::no_rnti_opportunity;
   }
 
-  // Check if allocation would cause segmentation
+  srsran_dci_format_t dci_format = user->get_dci_format();
+  if (dci_format == SRSRAN_DCI_FORMAT1A and not is_contiguous(user_mask)) {
+    logger.warning("SCHED: Can't use distributed RBGs for DCI format 1A");
+    return alloc_result::invalid_grant_params;
+  }
+
+  // Check if allocation is too small to fit headers, BSR or would cause SRB0 segmentation
   const dl_harq_proc& h = user->get_dl_harq(pid, cc_cfg->enb_cc_idx);
   if (h.is_empty()) {
     // It is newTx
-    rbg_interval r = user->get_required_dl_rbgs(cc_cfg->enb_cc_idx);
+    srsran::interval<uint32_t> req_bytes = user->get_requested_dl_bytes(get_enb_cc_idx());
-    if (r.start() > user_mask.count()) {
+    tbs_info                   tb        = compute_mcs_and_tbs_lower_bound(*cc, get_tti_tx_dl(), user_mask, dci_format);
-      logger.debug("SCHED: The number of RBGs allocated to rnti=0x%x will force segmentation", user->get_rnti());
+    if ((int)req_bytes.start() > tb.tbs_bytes) {
+      logger.debug("SCHED: The number of RBGs allocated to rnti=0x%x is too small to fit essential control information",
+                   user->get_rnti());
       return alloc_result::invalid_grant_params;
     }
   }
 
-  srsran_dci_format_t dci_format = user->get_dci_format();
-  if (dci_format == SRSRAN_DCI_FORMAT1A and not is_contiguous(user_mask)) {
-    logger.warning("SCHED: Can't use distributed RBGs for DCI format 1A");
-    return alloc_result::invalid_grant_params;
-  }
-
   bool has_pusch_grant = is_ul_alloc(user->get_rnti()) or cc_results->is_ul_alloc(user->get_rnti());
 
   // Check if there is space in the PUCCH for HARQ ACKs

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

@@ -141,7 +141,7 @@ 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 or req_bytes <= 0) {
+  if (tb_max.tbs_bytes + 8 <= (int)req_bytes or tb_max.mcs == 0) {
     // if mcs cannot be lowered or a decrease in TBS index won't meet req_bytes requirement
     return tb_max;
   }

srsenb/src/stack/mac/sched_ue_ctrl/sched_dl_cqi.cc

@@ -13,7 +13,7 @@
 #include "srsenb/hdr/stack/mac/sched_ue_ctrl/sched_dl_cqi.h"
 #include "srsenb/hdr/stack/mac/schedulers/sched_base.h"
 
-using namespace srsenb;
+namespace srsenb {
 
 rbgmask_t sched_dl_cqi::get_optim_rbgmask(const rbgmask_t& dl_mask, uint32_t req_rbgs, bool max_flag) const
 {
@@ -50,3 +50,56 @@ rbgmask_t sched_dl_cqi::get_optim_rbgmask(const rbgmask_t& dl_mask, uint32_t req
 
   return emptymask;
 }
+
+std::tuple<uint32_t, int> find_min_cqi_rbg(const rbgmask_t& mask, const sched_dl_cqi& dl_cqi)
+{
+  if (mask.none()) {
+    return std::make_tuple(mask.size(), -1);
+  }
+
+  int rbg = mask.find_lowest(0, mask.size());
+  if (not dl_cqi.subband_cqi_enabled()) {
+    return std::make_tuple(rbg, dl_cqi.get_wb_cqi_info());
+  }
+
+  int      min_cqi = std::numeric_limits<int>::max();
+  uint32_t min_rbg = mask.size();
+  for (; rbg != -1; rbg = mask.find_lowest(rbg, mask.size())) {
+    uint32_t sb  = dl_cqi.rbg_to_sb_index(rbg);
+    int      cqi = dl_cqi.get_subband_cqi(sb);
+    if (cqi < min_cqi) {
+      min_cqi = cqi;
+      min_rbg = rbg;
+    }
+    rbg = (int)srsran::ceil_div((sb + 1U) * mask.size(), dl_cqi.nof_subbands()); // skip to next subband index
+  }
+  return min_cqi != std::numeric_limits<int>::max() ? std::make_tuple(min_rbg, min_cqi) : std::make_tuple(0u, -1);
+}
+
+rbgmask_t remove_min_cqi_subband(const rbgmask_t& rbgmask, const sched_dl_cqi& dl_cqi)
+{
+  std::tuple<uint32_t, int> tup = find_min_cqi_rbg(rbgmask, dl_cqi);
+  if (std::get<1>(tup) < 0) {
+    return rbgmask_t(rbgmask.size());
+  }
+  uint32_t sb        = dl_cqi.rbg_to_sb_index(std::get<0>(tup));
+  uint32_t rbg_begin = sb * rbgmask.size() / dl_cqi.nof_subbands();
+  uint32_t rbg_end   = srsran::ceil_div((sb + 1) * rbgmask.size(), dl_cqi.nof_subbands());
+
+  rbgmask_t ret(rbgmask);
+  ret.fill(rbg_begin, rbg_end, false);
+  return ret;
+}
+
+rbgmask_t remove_min_cqi_rbg(const rbgmask_t& rbgmask, const sched_dl_cqi& dl_cqi)
+{
+  std::tuple<uint32_t, int> tup = find_min_cqi_rbg(rbgmask, dl_cqi);
+  if (std::get<1>(tup) < 0) {
+    return rbgmask_t(rbgmask.size());
+  }
+  rbgmask_t ret(rbgmask);
+  ret.set(std::get<0>(tup), false);
+  return ret;
+}
+
+} // namespace srsenb

srsenb/src/stack/mac/sched_ue_ctrl/sched_ue_cell.cc

@@ -13,6 +13,7 @@
 #include "srsenb/hdr/stack/mac/sched_ue_ctrl/sched_ue_cell.h"
 #include "srsenb/hdr/stack/mac/sched_helpers.h"
 #include "srsenb/hdr/stack/mac/sched_phy_ch/sched_dci.h"
+#include "srsenb/hdr/stack/mac/schedulers/sched_base.h"
 #include <numeric>
 
 namespace srsenb {
@@ -234,7 +235,7 @@ tbs_info cqi_to_tbs_dl(const sched_ue_cell& cell,
                        const rbgmask_t&     rbgs,
                        uint32_t             nof_re,
                        srsran_dci_format_t  dci_format,
-                       int                  req_bytes)
+                       uint32_t             req_bytes)
 {
   bool     use_tbs_index_alt = cell.get_ue_cfg()->use_tbs_index_alt and dci_format != SRSRAN_DCI_FORMAT1A;
   uint32_t nof_prbs          = count_prb_per_tb(rbgs);
@@ -242,10 +243,10 @@ tbs_info cqi_to_tbs_dl(const sched_ue_cell& cell,
   tbs_info ret;
   if (cell.fixed_mcs_dl < 0 or not cell.dl_cqi().is_cqi_info_received()) {
     // Dynamic MCS configured or first Tx
-    uint32_t dl_cqi_avg = cell.dl_cqi().get_grant_avg_cqi(rbgs);
+    uint32_t dl_cqi = std::get<1>(find_min_cqi_rbg(rbgs, cell.dl_cqi()));
 
     ret = compute_min_mcs_and_tbs_from_required_bytes(
-        nof_prbs, nof_re, dl_cqi_avg, cell.max_mcs_dl, req_bytes, false, false, use_tbs_index_alt);
+        nof_prbs, nof_re, dl_cqi, cell.max_mcs_dl, req_bytes, false, false, use_tbs_index_alt);
 
     // If coderate > SRSRAN_MIN(max_coderate, 0.932 * Qm) we should set TBS=0. We don't because it's not correctly
     // handled by the scheduler, but we might be scheduling undecodable codewords at very low SNR
@@ -296,7 +297,7 @@ int get_required_prb_dl(const sched_ue_cell& cell,
   auto compute_tbs_approx = [tti_tx_dl, &cell, dci_format](uint32_t nof_prb) {
     uint32_t  nof_re       = cell.cell_cfg->get_dl_lb_nof_re(tti_tx_dl, nof_prb);
     rbgmask_t min_cqi_rbgs = cell.dl_cqi().get_optim_rbgmask(nof_prb, false);
-    tbs_info  tb           = cqi_to_tbs_dl(cell, min_cqi_rbgs, nof_re, dci_format, -1);
+    tbs_info  tb           = cqi_to_tbs_dl(cell, min_cqi_rbgs, nof_re, dci_format);
     return tb.tbs_bytes;
   };
 
@@ -340,4 +341,96 @@ uint32_t get_required_prb_ul(const sched_ue_cell& cell, uint32_t req_bytes)
   return req_prbs;
 }
 
+/// Computes the minimum TBS/MCS achievable for provided UE cell configuration, RBG mask, TTI, DCI format
+tbs_info compute_mcs_and_tbs_lower_bound(const sched_ue_cell& ue_cell,
+                                         tti_point            tti_tx_dl,
+                                         const rbgmask_t&     rbg_mask,
+                                         srsran_dci_format_t  dci_format)
+{
+  uint32_t nof_prbs = count_prb_per_tb(rbg_mask);
+  if (nof_prbs == 0) {
+    return tbs_info{};
+  }
+  uint32_t nof_re_lb = ue_cell.cell_cfg->get_dl_lb_nof_re(tti_tx_dl, nof_prbs);
+  return cqi_to_tbs_dl(ue_cell, rbg_mask, nof_re_lb, dci_format);
+}
+
+bool find_optimal_rbgmask(const sched_ue_cell&       ue_cell,
+                          tti_point                  tti_tx_dl,
+                          const rbgmask_t&           dl_mask,
+                          srsran_dci_format_t        dci_format,
+                          srsran::interval<uint32_t> req_bytes,
+                          tbs_info&                  tb,
+                          rbgmask_t&                 newtxmask)
+{
+  // Find the largest set of available RBGs possible
+  newtxmask = find_available_rbgmask(dl_mask.size(), dci_format == SRSRAN_DCI_FORMAT1A, dl_mask);
+
+  // Compute MCS/TBS if all available RBGs were allocated
+  tb = compute_mcs_and_tbs_lower_bound(ue_cell, tti_tx_dl, newtxmask, dci_format);
+
+  if (not ue_cell.dl_cqi().subband_cqi_enabled()) {
+    // Wideband CQI case
+    // NOTE: for wideband CQI, the TBS is directly proportional to the nof_prbs, so we can use an iterative method
+    //       to compute the best mask given "req_bytes"
+
+    if (tb.tbs_bytes < (int)req_bytes.start()) {
+      // the grant is too small. it may lead to srb0 segmentation or not space for headers
+      return false;
+    }
+    if (tb.tbs_bytes <= (int)req_bytes.stop()) {
+      // the grant is not sufficiently large to fit max required bytes. Stop search at this point
+      return true;
+    }
+    // Reduce DL grant size to the minimum that can fit the pending DL bytes
+    srsran::bounded_vector<tbs_info, MAX_NOF_RBGS> tb_table(newtxmask.count());
+    auto compute_tbs_approx = [tti_tx_dl, &ue_cell, dci_format, &tb_table](uint32_t nof_rbgs) {
+      rbgmask_t search_mask(ue_cell.cell_cfg->nof_rbgs);
+      search_mask.fill(0, nof_rbgs);
+      tb_table[nof_rbgs - 1] = compute_mcs_and_tbs_lower_bound(ue_cell, tti_tx_dl, search_mask, dci_format);
+      return tb_table[nof_rbgs - 1].tbs_bytes;
+    };
+    std::tuple<uint32_t, int, uint32_t, int> ret = false_position_method(
+        1U, tb_table.size(), (int)req_bytes.stop(), compute_tbs_approx, [](int y) { return y == SRSRAN_ERROR; });
+    uint32_t upper_nprb = std::get<2>(ret);
+    int      upper_tbs  = std::get<3>(ret);
+    if (upper_tbs >= (int)req_bytes.stop()) {
+      tb = tb_table[upper_nprb - 1];
+    }
+    return true;
+  }
+
+  // Subband CQI case
+  // NOTE: There is no monotonically increasing guarantee between TBS and nof allocated prbs.
+  //       One single subband CQI could be dropping the CQI of the whole TB.
+  //       We start with largest RBG allocation and continue removing RBGs. However, there is no guarantee this is
+  //       going to be the optimal solution
+
+  // Subtract whole CQI subbands until objective is not met
+  // TODO: can be optimized
+  rbgmask_t smaller_mask;
+  tbs_info  tb2;
+  do {
+    smaller_mask = remove_min_cqi_subband(newtxmask, ue_cell.dl_cqi());
+    tb2          = compute_mcs_and_tbs_lower_bound(ue_cell, tti_tx_dl, smaller_mask, dci_format);
+    if (tb2.tbs_bytes >= (int)req_bytes.stop() or tb.tbs_bytes <= tb2.tbs_bytes) {
+      tb        = tb2;
+      newtxmask = smaller_mask;
+    }
+  } while (tb2.tbs_bytes > (int)req_bytes.stop());
+  if (tb.tbs_bytes <= (int)req_bytes.stop()) {
+    return true;
+  }
+  do {
+    smaller_mask = remove_min_cqi_rbg(newtxmask, ue_cell.dl_cqi());
+    tb2          = compute_mcs_and_tbs_lower_bound(ue_cell, tti_tx_dl, smaller_mask, dci_format);
+    if (tb2.tbs_bytes >= (int)req_bytes.stop() or tb.tbs_bytes <= tb2.tbs_bytes) {
+      tb        = tb2;
+      newtxmask = smaller_mask;
+    }
+  } while (tb2.tbs_bytes > (int)req_bytes.stop());
+
+  return true;
+}
+
 } // namespace srsenb

srsenb/src/stack/mac/schedulers/sched_base.cc

@@ -153,22 +153,25 @@ alloc_result try_dl_newtx_alloc_greedy(sf_sched& tti_sched, sched_ue& ue, const
   }
 
   // If there is no data to transmit, no need to allocate
-  rbg_interval req_rbgs = ue.get_required_dl_rbgs(tti_sched.get_enb_cc_idx());
+  srsran::interval<uint32_t> req_bytes = ue.get_requested_dl_bytes(tti_sched.get_enb_cc_idx());
-  if (req_rbgs.stop() == 0) {
+  if (req_bytes.stop() == 0) {
     return alloc_result::no_rnti_opportunity;
   }
 
-  // Find RBG mask that accommodates pending data
+  sched_ue_cell* ue_cell = ue.find_ue_carrier(tti_sched.get_enb_cc_idx());
-  bool      is_contiguous_alloc = ue.get_dci_format() == SRSRAN_DCI_FORMAT1A;
+  srsran_assert(ue_cell != nullptr, "dl newtx alloc called for invalid cell");
-  rbgmask_t newtxmask           = find_available_rbgmask(req_rbgs.stop(), is_contiguous_alloc, current_mask);
+  srsran_dci_format_t dci_format = ue.get_dci_format();
-  if (newtxmask.none() or newtxmask.count() < req_rbgs.start()) {
+  tbs_info            tb;
+  rbgmask_t           opt_mask;
+  if (not find_optimal_rbgmask(
+          *ue_cell, tti_sched.get_tti_tx_dl(), current_mask, dci_format, req_bytes, tb, opt_mask)) {
     return alloc_result::no_sch_space;
   }
 
   // empty RBGs were found. Attempt allocation
-  alloc_result ret = tti_sched.alloc_dl_user(&ue, newtxmask, h.get_id());
+  alloc_result ret = tti_sched.alloc_dl_user(&ue, opt_mask, h.get_id());
   if (ret == alloc_result::success and result_mask != nullptr) {
-    *result_mask = newtxmask;
+    *result_mask = opt_mask;
   }
   return ret;
 }