csi: walk around CSI-RS collision with PDSCH DM-RS

Author: xavierarteaga

apps/units/flexible_du/du_high/du_high_config_translators.cpp

@@ -163,6 +163,14 @@ static void fill_csi_resources(serving_cell_config& out_cell, const du_high_unit
   csi_params.nof_ports     = cell_cfg.nof_antennas_dl;
   csi_params.csi_rs_period = static_cast<csi_resource_periodicity>(csi_cfg.csi_rs_period_msec *
                                                                    get_nof_slots_per_subframe(cell_cfg.common_scs));
+
+  // [Implementation-defined] The default CSI symbols are in symbols 4 and 8, the DM-RS for PDSCH might collide in
+  // symbol index 8 when the number of DM-RS additional positions is 3.
+  if (uint_to_dmrs_additional_positions(cell_cfg.pdsch_cfg.dmrs_add_pos) == dmrs_additional_positions::pos3) {
+    csi_params.csi_ofdm_symbol_index            = 9;
+    csi_params.tracking_csi_ofdm_symbol_indexes = {4, 9, 4, 9};
+  }
+
   if (cell_cfg.tdd_ul_dl_cfg.has_value()) {
     if (not csi_helper::derive_valid_csi_rs_slot_offsets(
             csi_params,

include/srsran/scheduler/config/csi_helper.h

@@ -13,6 +13,7 @@
 #include "srsran/ran/csi_rs/csi_meas_config.h"
 #include "srsran/ran/pci.h"
 #include "srsran/ran/tdd/tdd_ul_dl_config.h"
+#include <array>
 
 namespace srsran {
 namespace csi_helper {
@@ -26,6 +27,10 @@ struct csi_builder_params {
   unsigned nof_rbs;
   /// Number of ports set for the CSI-RS.
   unsigned nof_ports = 1;
+  /// Symbol index for the CSI measurement.
+  unsigned csi_ofdm_symbol_index = 8;
+  /// Symbol indexes for tracking signals.
+  std::array<unsigned, 4> tracking_csi_ofdm_symbol_indexes = {4, 8, 4, 8};
   /// Period of the CSI-RS resources.
   csi_resource_periodicity csi_rs_period = csi_resource_periodicity::slots80;
   /// Slot offset for measurement CSI-RS resources. Note: Should avoid collisions with SSB and SIB1.

lib/scheduler/config/csi_helper.cpp

@@ -165,7 +165,7 @@ static zp_csi_rs_resource make_default_zp_csi_rs_resource(const csi_builder_para
   res.res_mapping.fd_alloc.resize(3);
   res.res_mapping.fd_alloc.set(2, true);
   res.res_mapping.cdm                     = csi_rs_cdm_type::fd_CDM2;
-  res.res_mapping.first_ofdm_symbol_in_td = 8;
+  res.res_mapping.first_ofdm_symbol_in_td = params.csi_ofdm_symbol_index;
   res.res_mapping.freq_density            = csi_rs_freq_density_type::one;
   res.res_mapping.freq_band_rbs           = get_csi_freq_occupation_rbs(params.nof_rbs, params.nof_rbs);
   res.period                              = params.csi_rs_period;
@@ -317,11 +317,10 @@ fill_tracking_nzp_csi_rs_resource(span<nzp_csi_rs_resource> tracking_csi_rs,
   res.res_mapping.cdm          = csi_rs_cdm_type::no_CDM;
   res.res_mapping.freq_density = csi_rs_freq_density_type::three;
 
-  static constexpr unsigned symbol_offsets[]  = {4, 8, 4, 8};
   static constexpr unsigned rel_slot_offset[] = {0, 0, 1, 1};
   for (unsigned i = 0; i != NOF_TRACKING_RESOURCES; ++i) {
     res.res_id                              = static_cast<nzp_csi_rs_res_id_t>(first_csi_res_id + i);
-    res.res_mapping.first_ofdm_symbol_in_td = symbol_offsets[i];
+    res.res_mapping.first_ofdm_symbol_in_td = params.tracking_csi_ofdm_symbol_indexes[i];
     res.csi_res_offset                      = params.tracking_csi_slot_offset + rel_slot_offset[i];
     tracking_csi_rs[i]                      = res;
   }