diff --git a/openair2/LAYER2/NR_MAC_gNB/config.c b/openair2/LAYER2/NR_MAC_gNB/config.c
index 0a1d7bce2e..b7882906bc 100644
--- a/openair2/LAYER2/NR_MAC_gNB/config.c
+++ b/openair2/LAYER2/NR_MAC_gNB/config.c
@@ -424,7 +424,7 @@ void config_common(int Mod_idP, int ssb_SubcarrierOffset, int pdsch_AntennaPorts
 
 }
 
-extern uint16_t sl_ahead;
+
 int rrc_mac_config_req_gNB(module_id_t Mod_idP,
                            int ssb_SubcarrierOffset,
                            int pdsch_AntennaPorts,
@@ -447,17 +447,16 @@ int rrc_mac_config_req_gNB(module_id_t Mod_idP,
                 "could not allocate memory for RC.nrmac[]->UL_tti_req_ahead[]\n");
     /* fill in slot/frame numbers: slot is fixed, frame will be updated by
      * scheduler */
+    uint16_t sf_ahead = (uint16_t) ceil((float)6/(0x01<<(*scc->ssbSubcarrierSpacing)));
+    uint16_t sl_ahead = sf_ahead * (0x01<<(*scc->ssbSubcarrierSpacing));
+    /* consider that scheduler runs sl_ahead: the first sl_ahead slots are
+     * already "in the past" and thus we put frame 1 instead of 0!  Note that
+     * extern variable sl_ahead is not initialized yet, so computing it above */
     for (int i = 0; i < n; ++i) {
       nfapi_nr_ul_tti_request_t *req = &RC.nrmac[Mod_idP]->UL_tti_req_ahead[0][i];
-      /* consider that scheduler runs sl_ahead: the first sl_ahead slots are
-       * already "in the past" and thus we put frame 1 instead of 0!  Note that
-       * variable sl_ahead seems to not be correctly initialized, but I leave
-       * it for information purposes here (the fix would always put 0, what
-       * happens now, too) */
-      req->SFN = i < sl_ahead;
+      req->SFN = i < (sl_ahead-1);
       req->Slot = i;
     }
-
     RC.nrmac[Mod_idP]->common_channels[0].vrb_map_UL =
         calloc(n * MAX_BWP_SIZE, sizeof(uint16_t));
     AssertFatal(RC.nrmac[Mod_idP]->common_channels[0].vrb_map_UL,