tbf: constify the IMSI argument for the downlink assignment
diff --git a/src/pcu_l1_if.h b/src/pcu_l1_if.h
index 715f89f..f01b95a 100644
--- a/src/pcu_l1_if.h
+++ b/src/pcu_l1_if.h
@@ -40,7 +40,7 @@
         uint32_t fn, uint8_t block_nr);
 void pcu_l1if_tx_agch(bitvec * block, int len);
 
-void pcu_l1if_tx_pch(bitvec * block, int plen, char *imsi);
+void pcu_l1if_tx_pch(bitvec * block, int plen, const char *imsi);
 
 int pcu_l1if_open(void);
 void pcu_l1if_close(void);