band_helper: add helper to derive UL ARFCN from DL ARFCN

lib/include/srsran/common/band_helper.h

@@ -59,6 +59,17 @@ public:
    */
   uint16_t get_band_from_dl_arfcn(uint32_t arfcn) const;
 
+  /**
+   * @brief Get the respective UL ARFCN of a DL ARFCN
+   *
+   * For paired spectrum (FDD) the function returns the respective ARFCN in the same band.
+   * For unparied spectrum (TDD) the function returns the same ARFCN.
+   *
+   * @param dl_arfcn   The DL ARFCN
+   * @return uint32_t
+   */
+  uint32_t get_ul_arfcn_from_dl_arfcn(uint32_t dl_arfcn) const;
+
   /**
    * @brief Selects the SSB pattern case according to the band number and subcarrier spacing
    * @remark Described by TS 38.101-1 Table 5.4.3.3-1: Applicable SS raster entries per operating band

lib/src/common/band_helper.cc

@@ -32,7 +32,7 @@ double srsran_band_helper::nr_arfcn_to_freq(uint32_t nr_arfcn)
   return (params.F_REF_Offs_MHz * 1e6 + params.delta_F_global_kHz * (nr_arfcn - params.N_REF_Offs) * 1e3);
 }
 
-// Implements 5.4.2.1 in TS 38.401
+// Implements 5.4.2.1 in TS 38.104
 std::vector<uint32_t> srsran_band_helper::get_bands_nr(uint32_t                             nr_arfcn,
                                                        srsran_band_helper::delta_f_raster_t delta_f_raster)
 {
@@ -79,6 +79,24 @@ uint16_t srsran_band_helper::get_band_from_dl_arfcn(uint32_t arfcn) const
   return UINT16_MAX;
 }
 
+uint32_t srsran_band_helper::get_ul_arfcn_from_dl_arfcn(uint32_t dl_arfcn) const
+{
+  // return same ARFCN for TDD bands
+  if (get_duplex_mode(get_band_from_dl_arfcn(dl_arfcn)) == SRSRAN_DUPLEX_MODE_TDD) {
+    return dl_arfcn;
+  }
+
+  // derive UL ARFCN for FDD bands
+  for (const auto& band : nr_band_table_fr1) {
+    if (band.band == get_band_from_dl_arfcn(dl_arfcn)) {
+      uint32_t offset = (dl_arfcn - band.dl_nref_first) / band.dl_nref_step;
+      return (band.ul_nref_first + offset * band.ul_nref_step);
+    }
+  }
+
+  return UINT16_MAX;
+}
+
 srsran_ssb_patern_t srsran_band_helper::get_ssb_pattern(uint16_t band, srsran_subcarrier_spacing_t scs) const
 {
   // Look for the given band and SCS