wifi: Set BER threshold for Ideal remote manager to 1e-6

Otherwise the selected MCS (especially high order ones) would result in high PER

CHANGES.html

@@ -77,6 +77,7 @@ in order to support multi-users (MU) transmissions.
 <h2>Changed behavior:</h2>
 <ul>
 <li>The <b>default Wi-Fi ErrorRateModel</b> for the 802.11n/ac/ax standards has been changed from the NistErrorRateModel to a new TableBasedErrorRateModel.  Users may experience a shift in Wi-Fi link range due to the new default error model, as <b>the new model is more optimistic</b> (the PER for a given MCS will degrade at a lower SNR value). The Wi-Fi module documentation provides plots that compare the performance of the NIST and new table-based model.</li>
+<li>The default value of the <b>BerThreshold</b> attribute in <b>IdealWifiManager</b> was changed from 1e-5 to 1e-6, so as to the correct high order MCS.</li>
 </ul>
 
 <hr>

src/wifi/doc/source/wifi-design.rst

@@ -836,9 +836,17 @@ mode-specific SNR/BER curves.
 
 Available attribute:
 
-* BerThreshold (default 10e-6): The maximum Bit Error Rate
+* BerThreshold (default 1e-6): The maximum Bit Error Rate
   that is used to calculate the SNR threshold for each mode.
 
+Note that the BerThreshold has to be low enough to select a robust enough MCS
+(or mode) for a given SNR value, without being too restrictive on the target BER.
+Indeed we had noticed that the previous default value (i.e. 1e-5) led to the
+selection of HE MCS-11 which resulted in high PER.
+With this new default value (i.e. 1e-6), a HE STA moving away from a HE AP has
+smooth throughput decrease (whereas with 1e-5, better performance was seen further
+away, which is not "ideal").
+
 MinstrelWifiManager
 ###################
 

src/wifi/model/ideal-wifi-manager.cc

@@ -58,7 +58,7 @@ IdealWifiManager::GetTypeId (void)
     .AddConstructor<IdealWifiManager> ()
     .AddAttribute ("BerThreshold",
                    "The maximum Bit Error Rate acceptable at any transmission mode",
-                   DoubleValue (1e-5),
+                   DoubleValue (1e-6),
                    MakeDoubleAccessor (&IdealWifiManager::m_ber),
                    MakeDoubleChecker<double> ())
     .AddTraceSource ("Rate",