Refactor: add common helper functions to construct 5G-S-TMSI

* 5G-S-TMSI construction is defined in 3GPP TS 23.003 and
  relevant at both gNB and UE side
* Add helper functions to be later used in NG Paging
* Adopt in NAS UE

common/utils/nr/nr_common.c

@@ -1487,3 +1487,35 @@ float get_beta_dmrs(int num_cdm_groups_no_data, bool is_type2)
   }
   return beta_dmrs_pusch;
 }
+
+/** @brief Construct full 5G-S-TMSI from 5G-S-TMSI components
+ * @param amf_set_id AMF Set ID (10 bits)
+ * @param amf_pointer AMF Pointer (6 bits)
+ * @param m_tmsi 5G-TMSI (32 bits)
+ * @return Full 5G-S-TMSI (48 bits)
+ * @note The 5G-S-TMSI is constructed as a 48-bit value:
+ *       - Bits 38-47: AMF Set ID (10 bits)
+ *       - Bits 32-37: AMF Pointer (6 bits)
+ *       - Bits 0-31:  5G-TMSI (32 bits)
+ * @ref 3GPP TS 23.003 */
+uint64_t nr_construct_5g_s_tmsi(uint16_t amf_set_id, uint8_t amf_pointer, uint32_t m_tmsi)
+{
+  // Construct full 5G-S-TMSI: <AMF Set ID (10 bits)><AMF Pointer (6 bits)><5G-TMSI (32 bits)>
+  return ((uint64_t)amf_set_id << 38) | ((uint64_t)amf_pointer << 32) | m_tmsi;
+}
+
+/** @brief Construct 5G-S-TMSI-Part1 from 5G-S-TMSI components
+ * @param amf_set_id AMF Set ID (10 bits)
+ * @param amf_pointer AMF Pointer (6 bits)
+ * @param m_tmsi 5G-TMSI (32 bits)
+ * @return 5G-S-TMSI-Part1 (rightmost 39 bits of the full 5G-S-TMSI)
+ * @note 5G-S-TMSI-Part1 is the rightmost 39 bits of the full 5G-S-TMSI:
+ *       - Bits 32-37: AMF Pointer (6 bits)
+ *       - Bits 0-31:  5G-TMSI (32 bits)
+ * @ref 3GPP TS 23.003 */
+uint64_t nr_construct_5g_s_tmsi_part1(uint16_t amf_set_id, uint8_t amf_pointer, uint32_t m_tmsi)
+{
+  // Construct full 5G-S-TMSI and extract Part1: rightmost 39 bits
+  uint64_t full_s_tmsi = nr_construct_5g_s_tmsi(amf_set_id, amf_pointer, m_tmsi);
+  return full_s_tmsi & ((1ULL << 39) - 1);
+}

common/utils/nr/nr_common.h

@@ -357,6 +357,12 @@ frequency_range_t get_freq_range_from_band(uint16_t band);
  */
 float get_beta_dmrs(int num_cdm_groups_no_data, bool is_type2);
 
+/** @brief Construct full 5G-S-TMSI from 5G-S-TMSI components */
+uint64_t nr_construct_5g_s_tmsi(uint16_t amf_set_id, uint8_t amf_pointer, uint32_t m_tmsi);
+
+/** @brief Construct 5G-S-TMSI-Part1 from 5G-S-TMSI components */
+uint64_t nr_construct_5g_s_tmsi_part1(uint16_t amf_set_id, uint8_t amf_pointer, uint32_t m_tmsi);
+
 #define CEILIDIV(a,b) ((a+b-1)/b)
 #define ROUNDIDIV(a,b) (((a<<1)+b)/(b<<1))
 #define BOUNDED_EVAL(a, b, c) (min(c, max(a, b)))

openair3/NAS/NR_UE/nr_nas_msg.c

@@ -38,6 +38,7 @@
 #include <string.h>
 #include <unistd.h>
 #include "common/utils/ds/byte_array.h"
+#include "common/utils/nr/nr_common.h"
 #include "AuthenticationResponseParameter.h"
 #include "FGCNasMessageContainer.h"
 #include "FGSDeregistrationRequestUEOriginating.h"
@@ -1787,7 +1788,7 @@ static void send_nas_5gmm_ind(instance_t instance, const Guti5GSMobileIdentity_t
   MessageDef *msg = itti_alloc_new_message(TASK_NAS_NRUE, 0, NAS_5GMM_IND);
   nas_5gmm_ind_t *ind = &NAS_5GMM_IND(msg);
   LOG_I(NR_RRC, "5G-GUTI: AMF pointer %u, AMF Set ID %u, 5G-TMSI %u \n", guti->amfpointer, guti->amfsetid, guti->tmsi);
-  ind->fiveG_STMSI = ((uint64_t)guti->amfsetid << 38) | ((uint64_t)guti->amfpointer << 32) | guti->tmsi;
+  ind->fiveG_STMSI = nr_construct_5g_s_tmsi(guti->amfsetid, guti->amfpointer, guti->tmsi);
   itti_send_msg_to_task(TASK_RRC_NRUE, instance, msg);
 }