spectrum: Make it possible to chain propagation loss models

For example, this change makes it possible to chain fading model with
frequency-dependent pathloss model.

src/spectrum/model/multi-model-spectrum-channel.cc

@@ -438,14 +438,21 @@ void
 MultiModelSpectrumChannel::AddPropagationLossModel (Ptr<PropagationLossModel> loss)
 {
   NS_LOG_FUNCTION (this << loss);
-  NS_ASSERT (m_propagationLoss == 0);
+  if (m_propagationLoss)
+    {
+      loss->SetNext (m_propagationLoss);
+    }
   m_propagationLoss = loss;
 }
 
 void
 MultiModelSpectrumChannel::AddSpectrumPropagationLossModel (Ptr<SpectrumPropagationLossModel> loss)
 {
-  NS_ASSERT (m_spectrumPropagationLoss == 0);
+  NS_LOG_FUNCTION (this << loss);
+  if (m_spectrumPropagationLoss)
+    {
+      loss->SetNext (m_spectrumPropagationLoss);
+    }
   m_spectrumPropagationLoss = loss;
 }
 

src/spectrum/model/single-model-spectrum-channel.cc

@@ -239,7 +239,10 @@ void
 SingleModelSpectrumChannel::AddPropagationLossModel (Ptr<PropagationLossModel> loss)
 {
   NS_LOG_FUNCTION (this << loss);
-  NS_ASSERT (m_propagationLoss == 0);
+  if (m_propagationLoss)
+    {
+      loss->SetNext (m_propagationLoss);
+    }
   m_propagationLoss = loss;
 }
 
@@ -248,7 +251,10 @@ void
 SingleModelSpectrumChannel::AddSpectrumPropagationLossModel (Ptr<SpectrumPropagationLossModel> loss)
 {
   NS_LOG_FUNCTION (this << loss);
-  NS_ASSERT (m_spectrumPropagationLoss == 0);
+  if (m_spectrumPropagationLoss)
+    {
+      loss->SetNext (m_spectrumPropagationLoss);
+    }
   m_spectrumPropagationLoss = loss;
 }
 

src/spectrum/model/spectrum-channel.h

@@ -55,7 +55,7 @@ public:
   static TypeId GetTypeId (void);
 
   /**
-   * Set the single-frequency propagation loss model to be used
+   * Add the single-frequency propagation loss model to be used
    * \warning only models that do not depend on the TX power should be used.
    *
    * \param loss a pointer to the propagation loss model to be used.
@@ -63,13 +63,13 @@ public:
   virtual void AddPropagationLossModel (Ptr<PropagationLossModel> loss) = 0;
 
   /**
-   * Set the frequency-dependent propagation loss model to be used
+   * Add the frequency-dependent propagation loss model to be used
    * \param loss a pointer to the propagation loss model to be used.
    */
   virtual void AddSpectrumPropagationLossModel (Ptr<SpectrumPropagationLossModel> loss) = 0;
 
   /**
-   * Set the  propagation delay model to be used
+   * Set the propagation delay model to be used
    * \param delay Ptr to the propagation delay model to be used.
    */
   virtual void SetPropagationDelayModel (Ptr<PropagationDelayModel> delay) = 0;