diff --git a/src/pdch_ul_controller.c b/src/pdch_ul_controller.c
index 9cbe845..2ae553f 100644
--- a/src/pdch_ul_controller.c
+++ b/src/pdch_ul_controller.c
@@ -157,7 +157,7 @@
 }
 
 /* Get next free (unreserved) FN which is not located in time before "start_fn" */
-uint32_t pdch_ulc_get_next_free_fn(struct pdch_ulc *ulc, uint32_t start_fn)
+uint32_t pdch_ulc_get_next_free_fn(const struct pdch_ulc *ulc, uint32_t start_fn)
 {
 	struct rb_node *node;
 	struct pdch_ulc_node *it;
diff --git a/src/pdch_ul_controller.h b/src/pdch_ul_controller.h
index 9857c8f..a488e9b 100644
--- a/src/pdch_ul_controller.h
+++ b/src/pdch_ul_controller.h
@@ -84,7 +84,7 @@
 bool pdch_ulc_fn_is_free(struct pdch_ulc *ulc, uint32_t fn);
 
 int pdch_ulc_get_next_free_rrbp_fn(struct pdch_ulc *ulc, uint32_t fn, uint32_t *poll_fn, unsigned int *rrbp);
-uint32_t pdch_ulc_get_next_free_fn(struct pdch_ulc *ulc, uint32_t start_fn);
+uint32_t pdch_ulc_get_next_free_fn(const struct pdch_ulc *ulc, uint32_t start_fn);
 
 struct pdch_ulc_node *pdch_ulc_get_node(struct pdch_ulc *ulc, uint32_t fn);
 struct pdch_ulc_node *pdch_ulc_pop_node(struct pdch_ulc *ulc, uint32_t fn);