fix for computing ssb subcarrier offset

common/utils/nr/nr_common.c

@@ -755,11 +755,25 @@ void SLIV2SL(int SLIV,int *S,int *L) {
   }
 }
 
-int get_ssb_subcarrier_offset(uint32_t absoluteFrequencySSB, uint32_t absoluteFrequencyPointA)
+int get_ssb_subcarrier_offset(uint32_t absoluteFrequencySSB, uint32_t absoluteFrequencyPointA, int scs)
 {
-  uint32_t absolute_diff = (absoluteFrequencySSB - absoluteFrequencyPointA);
-  const int scaling_5khz = absoluteFrequencyPointA < 600000 ? 3 : 1;
-  return ((absolute_diff / scaling_5khz) % 24);
+  // for FR1 k_SSB expressed in terms of 15kHz SCS
+  // for FR2 k_SSB expressed in terms of the subcarrier spacing provided by the higher-layer parameter subCarrierSpacingCommon
+  // absoluteFrequencySSB and absoluteFrequencyPointA are ARFCN
+  // NR-ARFCN delta frequency is 5kHz if f < 3 GHz, 15kHz for other FR1 freq and 60kHz for FR2
+  const uint32_t absolute_diff = absoluteFrequencySSB - absoluteFrequencyPointA;
+  int scaling = 1;
+  if (absoluteFrequencyPointA < 600000) // correspond to 3GHz
+    scaling = 3;
+  if (scs > 2) // FR2
+    scaling <<= (scs - 2);
+  int sco_limit = scs == 1 ? 24 : 12;
+  int subcarrier_offset = (absolute_diff / scaling) % sco_limit;
+  // 30kHz is the only case where k_SSB is expressed in terms of a different SCS (15kHz)
+  // the assertion is to avoid having an offset of half a subcarrier
+  if (scs == 1)
+    AssertFatal(subcarrier_offset % 2 == 0, "ssb offset %d invalid for scs %d\n", subcarrier_offset, scs);
+  return subcarrier_offset;
 }
 
 uint32_t get_ssb_offset_to_pointA(uint32_t absoluteFrequencySSB,
@@ -767,16 +781,16 @@ uint32_t get_ssb_offset_to_pointA(uint32_t absoluteFrequencySSB,
                                   int ssbSubcarrierSpacing,
                                   int frequency_range)
 {
+  // offset to pointA is expressed in terms of 15kHz SCS for FR1 and 60kHz for FR2
+  // only difference wrt NR-ARFCN is delta frequency 5kHz if f < 3 GHz for ARFCN
   uint32_t absolute_diff = (absoluteFrequencySSB - absoluteFrequencyPointA);
   const int scaling_5khz = absoluteFrequencyPointA < 600000 ? 3 : 1;
-  int sco = get_ssb_subcarrier_offset(absoluteFrequencySSB, absoluteFrequencyPointA);
-  const int scs_scaling = frequency_range == FR2 ? 1 << (ssbSubcarrierSpacing - 2) : 1 << ssbSubcarrierSpacing;
-  const int scaled_abs_diff = absolute_diff / scaling_5khz;
-  const int ssb_offset_point_a =
-      (scaled_abs_diff - sco) / 12
-      - 10 * scs_scaling; // absoluteFrequencySSB is the central frequency of SSB which is made by 20RBs in total
-  AssertFatal(ssb_offset_point_a % scs_scaling == 0, "PRB offset %d can create frequency offset\n", ssb_offset_point_a);
-  AssertFatal(sco % scs_scaling == 0, "ssb offset %d can create frequency offset\n", sco);
+  const int scaling = frequency_range == FR2 ? 1 << (ssbSubcarrierSpacing - 2) : 1 << ssbSubcarrierSpacing;
+  const int scaled_abs_diff = absolute_diff / (scaling_5khz * scaling);
+  // absoluteFrequencySSB is the central frequency of SSB which is made by 20RBs in total
+  const int ssb_offset_point_a = ((scaled_abs_diff / 12) - 10) * scaling;
+  // Offset to point A needs to be divisible by scaling
+  AssertFatal(ssb_offset_point_a % scaling == 0, "PRB offset %d not valid for scs %d\n", ssb_offset_point_a, ssbSubcarrierSpacing);
   return ssb_offset_point_a;
 }
 

common/utils/nr/nr_common.h

@@ -202,7 +202,7 @@ uint32_t get_ssb_offset_to_pointA(uint32_t absoluteFrequencySSB,
                                   uint32_t absoluteFrequencyPointA,
                                   int ssbSubcarrierSpacing,
                                   int frequency_range);
-int get_ssb_subcarrier_offset(uint32_t absoluteFrequencySSB, uint32_t absoluteFrequencyPointA);
+int get_ssb_subcarrier_offset(uint32_t absoluteFrequencySSB, uint32_t absoluteFrequencyPointA, int scs);
 int get_delay_idx(int delay, int max_delay_comp);
 
 void freq2time(uint16_t ofdm_symbol_size,

openair1/PHY/INIT/nr_parms.c

@@ -347,10 +347,12 @@ int nr_init_frame_parms_ue(NR_DL_FRAME_PARMS *fp,
   fp->freq_range = (fp->dl_CarrierFreq < 6e9)? nr_FR1 : nr_FR2;
 
   uint8_t sco = 0;
-  if (((fp->freq_range == nr_FR1) && (config->ssb_table.ssb_subcarrier_offset<24)) ||
-      ((fp->freq_range == nr_FR2) && (config->ssb_table.ssb_subcarrier_offset<12)) ) {
+  if (((fp->freq_range == nr_FR1) && (config->ssb_table.ssb_subcarrier_offset < 24)) ||
+      ((fp->freq_range == nr_FR2) && (config->ssb_table.ssb_subcarrier_offset < 12))) {
     if (fp->freq_range == nr_FR1)
       sco = config->ssb_table.ssb_subcarrier_offset>>config->ssb_config.scs_common;
+    else
+      sco = config->ssb_table.ssb_subcarrier_offset;
   }
 
   fp->ssb_start_subcarrier = (12 * config->ssb_table.ssb_offset_point_a + sco);

openair2/LAYER2/NR_MAC_gNB/config.c

@@ -639,7 +639,8 @@ static void config_common(gNB_MAC_INST *nrmac, nr_pdsch_AntennaPorts_t pdsch_Ant
   cfg->num_tlv++;
   cfg->ssb_table.ssb_subcarrier_offset.value =
        get_ssb_subcarrier_offset(*scc->downlinkConfigCommon->frequencyInfoDL->absoluteFrequencySSB,
-                                 scc->downlinkConfigCommon->frequencyInfoDL->absoluteFrequencyPointA);
+                                 scc->downlinkConfigCommon->frequencyInfoDL->absoluteFrequencyPointA,
+                                 *scc->ssbSubcarrierSpacing);
 
   AssertFatal(cfg->ssb_table.ssb_subcarrier_offset.value < 16,
               "cannot handle ssb_subcarrier_offset %d resulting from Point A %ld SSB %ld: please increase dl_absoluteFrequencyPointA "

openair2/RRC/NR/nr_rrc_config.c

@@ -1768,7 +1768,8 @@ NR_BCCH_BCH_Message_t *get_new_MIB_NR(const NR_ServingCellConfigCommon_t *scc)
   int ssb_subcarrier_offset = 31; // default value for NSA
   if (get_softmodem_params()->sa) {
     ssb_subcarrier_offset = get_ssb_subcarrier_offset(*scc->downlinkConfigCommon->frequencyInfoDL->absoluteFrequencySSB,
-                                                      scc->downlinkConfigCommon->frequencyInfoDL->absoluteFrequencyPointA);
+                                                      scc->downlinkConfigCommon->frequencyInfoDL->absoluteFrequencyPointA,
+                                                      *scc->ssbSubcarrierSpacing);
   }
   mib->message.choice.mib->ssb_SubcarrierOffset = ssb_subcarrier_offset & 15;