fix calculation of ssb frequency.

lib/include/srsran/common/band_helper.h

@@ -105,9 +105,19 @@ public:
    */
   double get_center_freq_from_abs_freq_point_a(uint32_t nof_prb, uint32_t freq_point_a_arfcn);
 
+  /**
+   * @brief Compute the absolute pointA for a NR carrier from its bandwidth and the center frequency
+   *
+   * @param nof_prb Carrier bandwidth in number of RB
+   * @param center_freq double Frequency in Hz
+   * @return Absolute Point A frequency in Hz
+   */
+  double get_abs_freq_point_a_from_center_freq(uint32_t nof_prb, double center_freq);
+
   /**
    * @brief Compute the absolute frequency point A for a arfcn
    *
+   * @param band nr frequency band.
    * @param nof_prb Number of PRBs.
    * @param arfcn Given ARFCN.
    * @return frequency point A in arfcn notation.
@@ -116,10 +126,9 @@ public:
 
   /**
    * @brief Compute the absolute frequency of the SSB for a DL ARFCN and a band. This selects an SSB center frequency
-   * following the band SS/PBCH frequency raster provided by 38.104 table 5.4.3.1-1 as close as possible to PointA
+   * following the band SS/PBCH frequency raster provided by TS38.104 table 5.4.3.1-1 as close as possible to PointA
    * without letting any SS/PBCH subcarrier below PointA
    *
-   * @param band nr frequency band.
    * @param scs ssb subcarrier spacing.
    * @param freq_point_a_arfcn frequency point a in arfcn notation.
    * @return absolute frequency of the SSB in arfcn notation.
@@ -136,36 +145,77 @@ public:
   class sync_raster_t
   {
   protected:
-    sync_raster_t(uint32_t f, uint32_t s, uint32_t l) : first(f), step(s), last(l), gscn(f) {}
+    sync_raster_t(uint32_t gscn_f, uint32_t gscn_s, uint32_t gscn_l) :
+      gscn_first(gscn_f), gscn_step(gscn_s), gscn_last(gscn_l), gscn(gscn_f)
+    {
+      // see TS38.104 Table 5.4.3.1-1
+      if (gscn_last <= 7498) {
+        N_first = 1;
+        N_last  = 2499;
+      } else if (7499 <= gscn_last and gscn_last <= 22255) {
+        N_last = 14756;
+      } else if (22256 <= gscn_last and gscn_last <= 26639) {
+        N_last = 4383;
+      }
+
+      N = N_first;
+    }
     uint32_t gscn;
+    uint32_t N;
+    uint32_t M[3]  = {1, 3, 5};
+    uint32_t M_idx = 0;
 
   private:
-    uint32_t first;
-    uint32_t step;
-    uint32_t last;
+    uint32_t gscn_first;
+    uint32_t gscn_step;
+    uint32_t gscn_last;
+    uint32_t N_first = 0;
+    uint32_t N_last  = 0;
 
   public:
-    bool valid() const { return step != 0; }
+    bool valid() const { return gscn_step != 0; }
 
     void next()
     {
-      if (gscn <= last) {
-        gscn += step;
+      if (gscn_last <= 7498 and M_idx < 3) {
+        M_idx += 1;
+        if (M_idx == 3 and N <= N_last) {
+          M_idx = 0;
+          N += 1;
+        }
+      } else if (N <= N_last) {
+        N += 1;
       }
     }
 
-    bool end() const { return (gscn > last or step == 0); }
+    bool end() const { return (N > N_last or gscn_step == 0); }
 
-    void reset() { gscn = first; }
+    void reset()
+    {
+      N     = N_first;
+      M_idx = 0;
+    }
+
+    void gscn_next()
+    {
+      if (gscn <= gscn_last) {
+        gscn += gscn_step;
+      }
+    }
+
+    bool gscn_end() const { return (gscn > gscn_last or gscn_step == 0); }
+
+    void gscn_reset() { gscn = gscn_first; }
 
     double get_frequency() const;
+
+    uint32_t get_gscn() const;
   };
 
   sync_raster_t get_sync_raster(uint16_t band, srsran_subcarrier_spacing_t scs) const;
 
 private:
   // internal helper
-  double get_abs_freq_point_a_from_center_freq(uint32_t nof_prb, double center_freq);
 
   // Elements of TS 38.101-1 Table 5.2-1: NR operating bands in FR1
   struct nr_operating_band {

lib/src/common/band_helper.cc

@@ -136,24 +136,26 @@ double srsran_band_helper::get_abs_freq_point_a_from_center_freq(uint32_t nof_pr
 uint32_t
 srsran_band_helper::get_abs_freq_ssb_arfcn(uint16_t band, srsran_subcarrier_spacing_t scs, uint32_t freq_point_a_arfcn)
 {
-  double ssb_bw_hz = SRSRAN_SSB_BW_SUBC * SRSRAN_SUBC_SPACING_NR(scs);
-
   sync_raster_t sync_raster = get_sync_raster(band, scs);
   if (!sync_raster.valid()) {
     return 0;
   }
 
+  // double abs_freq_ssb_hz = sync_raster.get_frequency();
+  double freq_point_a_hz = nr_arfcn_to_freq(freq_point_a_arfcn);
+  double ssb_bw_hz       = SRSRAN_SSB_BW_SUBC * SRSRAN_SUBC_SPACING_NR(scs);
+
+  while (!sync_raster.end()) {
     double abs_freq_ssb_hz = sync_raster.get_frequency();
 
-  while (abs_freq_ssb_hz < (nr_arfcn_to_freq(freq_point_a_arfcn) + ssb_bw_hz / 2)) {
-    sync_raster.next();
-    if (sync_raster.end()) {
-      return 0;
-    }
-    abs_freq_ssb_hz = sync_raster.get_frequency();
+    if ((abs_freq_ssb_hz > (freq_point_a_hz + ssb_bw_hz / 2)) and
+        ((uint32_t)std::round(abs_freq_ssb_hz - freq_point_a_hz) % SRSRAN_SUBC_SPACING_NR(scs) == 0)) {
+      return freq_to_nr_arfcn(abs_freq_ssb_hz);
     }
 
-  return freq_to_nr_arfcn(abs_freq_ssb_hz);
+    sync_raster.next();
+  }
+  return 0;
 }
 
 srsran_ssb_patern_t srsran_band_helper::get_ssb_pattern(uint16_t band, srsran_subcarrier_spacing_t scs) const
@@ -213,7 +215,7 @@ srsran_duplex_mode_t srsran_band_helper::get_duplex_mode(uint16_t band) const
 
 struct sync_raster_impl : public srsran_band_helper::sync_raster_t {
 public:
-  sync_raster_impl(uint32_t f, uint32_t s, uint32_t l) : sync_raster_t(f, s, l)
+  sync_raster_impl(uint32_t gscn_f, uint32_t gscn_s, uint32_t gscn_l) : sync_raster_t(gscn_f, gscn_s, gscn_l)
   {
     // Do nothing
   }
@@ -221,22 +223,20 @@ public:
 
 double srsran_band_helper::sync_raster_t::get_frequency() const
 {
+  // see TS38.104 table 5.4.3.1-1
+
   // Row 1
-  if (gscn >= 2 and gscn <= 7498) {
-    double N = std::ceil((gscn - 1) / 3.0);
-    double M = (gscn - 3 * N) / 2.0 + 3.0;
-    return N * 1200e3 + M * 50e3;
+  if (gscn_last <= 7498) {
+    return N * 1200e3 + M[M_idx] * 50e3;
   }
 
   // Row 2
-  if (gscn >= 7499 and gscn <= 22255) {
-    double N = gscn - 7499;
+  if (7499 <= gscn_last and gscn_last <= 22255) {
     return 3000e6 + N * 1.44e6;
   }
 
   // Row 3
-  if (gscn >= 22256 and gscn <= 26639) {
-    double N = gscn - 22256;
+  if (22256 <= gscn_last and gscn_last <= 26639) {
     return 2425.08e6 + N * 17.28e6;
   }
 
@@ -244,6 +244,19 @@ double srsran_band_helper::sync_raster_t::get_frequency() const
   return NAN;
 }
 
+uint32_t srsran_band_helper::sync_raster_t::get_gscn() const
+{
+  if (gscn_last <= 7498) {
+    return 3 * N + (M[M_idx] - 3) / 2;
+  } else if (7499 <= gscn_last and gscn_last <= 22255) {
+    return 7499 + N;
+  } else if (22256 <= gscn_last and gscn_last <= 26639) {
+    return 22256 + N;
+  }
+
+  return 0;
+}
+
 srsran_band_helper::sync_raster_t srsran_band_helper::get_sync_raster(uint16_t                    band,
                                                                       srsran_subcarrier_spacing_t scs) const
 {

srsenb/src/enb_cfg_parser.cc

@@ -1540,6 +1540,12 @@ int set_derived_args_nr(all_args_t* args_, rrc_nr_cfg_t* rrc_cfg_, phy_cfg_t* ph
       cfg.ssb_absolute_freq_point =
           band_helper.get_abs_freq_ssb_arfcn(cfg.band, srsran_subcarrier_spacing_30kHz, cfg.dl_absolute_freq_point_a);
     }
+
+    if (cfg.ssb_absolute_freq_point == 0) {
+      ERROR("Can't derive SSB freq point for dl_arfcn %d and band %d", cfg.dl_arfcn, cfg.band);
+      return SRSRAN_ERROR;
+    }
+
     // Convert to frequency for PHY
     cfg.phy_cell.carrier.ssb_center_freq_hz = band_helper.nr_arfcn_to_freq(cfg.ssb_absolute_freq_point);