wifi: Add WifiTxVector::GetModulationClass for convenience

In case the TxVector is MU, we don't need to specify any STAID
because the modulation class is unique. Getting the modulation
class through the WifiMode requires to specify the STAID instead.

src/wifi/model/wifi-tx-vector.cc

@@ -119,6 +119,23 @@ WifiTxVector::GetMode (uint16_t staId) const
   return m_mode;
 }
 
+WifiModulationClass
+WifiTxVector::GetModulationClass (void) const
+{
+  NS_ABORT_MSG_IF (!m_modeInitialized, "WifiTxVector mode must be set before using");
+
+  // TODO Replace the following with IsMu when available
+  if (m_preamble == WIFI_PREAMBLE_VHT_MU
+      || m_preamble == WIFI_PREAMBLE_HE_MU
+      || m_preamble == WIFI_PREAMBLE_HE_TB)
+    {
+      NS_ASSERT (!m_muUserInfos.empty ());
+      // all the modes belong to the same modulation class
+      return m_muUserInfos.begin ()->second.mcs.GetModulationClass ();
+    }
+  return m_mode.GetModulationClass ();
+}
+
 uint8_t
 WifiTxVector::GetTxPowerLevel (void) const
 {

src/wifi/model/wifi-tx-vector.h

@@ -137,6 +137,13 @@ public:
   */
   void SetMode (WifiMode mode, uint16_t staId);
 
+  /**
+   * Get the modulation class specified by this TXVECTOR.
+   *
+   * \return the Modulation Class specified by this TXVECTOR
+   */
+  WifiModulationClass GetModulationClass (void) const;
+
   /**
    * \returns the transmission power level
    */