diff --git a/src/wifi/model/wifi-phy-state.h b/src/wifi/model/wifi-phy-state.h
index ae43af539..d18fa798d 100644
--- a/src/wifi/model/wifi-phy-state.h
+++ b/src/wifi/model/wifi-phy-state.h
@@ -22,6 +22,8 @@
 #ifndef WIFI_PHY_STATE_H
 #define WIFI_PHY_STATE_H
 
+#include "ns3/fatal-error.h"
+
 /**
   * The state of the PHY layer.
   */
@@ -58,4 +60,35 @@ enum WifiPhyState
   OFF
 };
 
+/**
+* \brief Stream insertion operator.
+*
+* \param os the stream
+* \param state the state
+* \returns a reference to the stream
+*/
+inline std::ostream& operator<< (std::ostream& os, WifiPhyState state)
+{
+  switch (state)
+    {
+    case IDLE:
+      return (os << "IDLE");
+    case CCA_BUSY:
+      return (os << "CCA_BUSY");
+    case TX:
+      return (os << "TX");
+    case RX:
+      return (os << "RX");
+    case SWITCHING:
+      return (os << "SWITCHING");
+    case SLEEP:
+      return (os << "SLEEP");
+    case OFF:
+      return (os << "OFF");
+    default:
+      NS_FATAL_ERROR ("Invalid state");
+      return (os << "INVALID");
+    }
+}
+
 #endif /* WIFI_PHY_STATE_H */
diff --git a/src/wifi/model/wifi-phy.cc b/src/wifi/model/wifi-phy.cc
index 8ecb05827..e9d4f43d4 100644
--- a/src/wifi/model/wifi-phy.cc
+++ b/src/wifi/model/wifi-phy.cc
@@ -4135,30 +4135,6 @@ WifiPhy::AssignStreams (int64_t stream)
   return 1;
 }
 
-std::ostream& operator<< (std::ostream& os, WifiPhyState state)
-{
-  switch (state)
-    {
-    case WifiPhyState::IDLE:
-      return (os << "IDLE");
-    case WifiPhyState::CCA_BUSY:
-      return (os << "CCA_BUSY");
-    case WifiPhyState::TX:
-      return (os << "TX");
-    case WifiPhyState::RX:
-      return (os << "RX");
-    case WifiPhyState::SWITCHING:
-      return (os << "SWITCHING");
-    case WifiPhyState::SLEEP:
-      return (os << "SLEEP");
-    case WifiPhyState::OFF:
-      return (os << "OFF");
-    default:
-      NS_FATAL_ERROR ("Invalid WifiPhy state");
-      return (os << "INVALID");
-    }
-}
-
 } //namespace ns3
 
 namespace {
diff --git a/src/wifi/model/wifi-phy.h b/src/wifi/model/wifi-phy.h
index 05d7e0b4c..040b5163c 100644
--- a/src/wifi/model/wifi-phy.h
+++ b/src/wifi/model/wifi-phy.h
@@ -1998,13 +1998,6 @@ private:
   Callback<void> m_capabilitiesChangedCallback; //!< Callback when PHY capabilities changed
 };
 
-/**
- * \param os          output stream
- * \param state       wifi state to stringify
- * \return output stream
- */
-std::ostream& operator<< (std::ostream& os, WifiPhyState state);
-
 } //namespace ns3
 
 #endif /* WIFI_PHY_H */