wifi: Set wifi-spatial-reuse.cc to use 20 MHz BW

This commit adapts the example to use the new way of specifying channel,
bandwidth and band via the ChannelSettings attribute, solving a
regression that caused it to use an 80 MHz bandwidth instead of the 20
MHz it was originally calibrated with.

In addition, it also expands the script description to reflect the
discussions in !873 and #566.

examples/wireless/wifi-spatial-reuse.cc

@@ -44,22 +44,46 @@
 //  The following command will display all of the available run-time help options:
 //    ./ns3 run "wifi-spatial-reuse --help"
 //
-//  By default, the script shows the benefit of the OBSS_PD spatial reuse script:
-//    ./ns3 run wifi-spatial-reuse
-//    Throughput for BSS 1: 6.6468 Mbit/s
-//    Throughput for BSS 2: 6.6672 Mbit/s
+//  According to the Wi-Fi models of ns-3.36 release, throughput with
+//  OBSS PD enabled (--enableObssPd=True) was roughly 6.5 Mbps, and with
+//  it disabled (--enableObssPd=False) was roughly 3.5 Mbps.
 //
-// If one disables the OBSS_PD feature, a lower throughput is obtained per BSS:
-//    ./ns3 run "wifi-spatial-reuse --enableObssPd=0"
-//    Throughput for BSS 1: 5.8692 Mbit/s
-//    Throughput for BSS 2: 5.9364 Mbit/
+//  This difference between those results is because OBSS_PD spatial
+//  reuse enables to ignore transmissions from another BSS when the
+//  received power is below the configured threshold, and therefore
+//  either defer during ongoing transmission or transmit at the same
+//  time.
 //
-// This difference between those results is because OBSS_PD spatial
-// enables to ignore transmissions from another BSS when the received power
-// is below the configured threshold, and therefore either defer during ongoing
-// transmission or transmit at the same time.
+//  Note that, by default, this script configures a network using a
+//  channel bandwidth of 20 MHz. Changing this value alone (through
+//  the --channelWidth argument) without properly adjusting other
+//  parameters will void the effect of spatial reuse: since energy is
+//  measured over the 20 MHz primary channel regardless of the channel
+//  width, doubling the transmission bandwidth creates a 3 dB drop in
+//  the measured energy level (i.e., a halving of the measured
+//  energy). Because of this, when using the channelWidth argument
+//  users should also adjust the CCA-ED Thresholds (via --ccaEdTrSta1,
+//  --ccaEdTrSta2, --ccaEdTrAp1, and --ccaEdTrAp2), the Minimum RSSI
+//  for preamble detection (via --minimumRssi), and the OBSS PD
+//  Threshold (via --obssPdThreshold) appropriately. For instance,
+//  this can be accomplished for a channel width of 40 MHz by lowering
+//  all these values by 3 dB compared to their defaults.
+//
+//  In addition, consider that adapting the adjustments shown above
+//  for an 80 MHz bandwidth (using a 6 dB threshold adjustment instead
+//  of 3 dB) will not produce any changes when enableObssPd is enabled
+//  or disabled. The cause for this is the insufficient amount of
+//  traffic that is generated by default in the example: increasing
+//  the bandwidth shortens the frame durations, and lowers the
+//  collision probability. Collisions between BSSs are a necessary
+//  condition to see the improvements brought by spatial reuse, and
+//  thus increasing the amount of generated traffic by setting the
+//  interval argument to a lower value is necessary to see the
+//  benefits of spatial reuse in this scenario. This can, for
+//  instance, be accomplished by setting --interval=0.0001.
 //
 
+#include "ns3/abort.h"
 #include "ns3/command-line.h"
 #include "ns3/config.h"
 #include "ns3/string.h"
@@ -110,6 +134,8 @@ main (int argc, char *argv[])
   double ccaEdTrSta2 = -62; // dBm
   double ccaEdTrAp1 = -62; // dBm
   double ccaEdTrAp2 = -62; // dBm
+  double minimumRssi = -82; // dBm
+  int channelBandwidth = 20; // MHz
   uint32_t payloadSize = 1500; // bytes
   uint32_t mcs = 0; // MCS value
   double interval = 0.001; // seconds
@@ -131,6 +157,9 @@ main (int argc, char *argv[])
   cmd.AddValue ("ccaEdTrSta2", "CCA-ED Threshold of STA2 (dBm)", ccaEdTrSta2);
   cmd.AddValue ("ccaEdTrAp1", "CCA-ED Threshold of AP1 (dBm)", ccaEdTrAp1);
   cmd.AddValue ("ccaEdTrAp2", "CCA-ED Threshold of AP2 (dBm)", ccaEdTrAp2);
+  cmd.AddValue ("minimumRssi", "Minimum RSSI for the ThresholdPreambleDetectionModel", minimumRssi);
+  cmd.AddValue ("channelBandwidth", "Bandwidth of the channel in MHz [20, 40, or 80]", channelBandwidth);
+  cmd.AddValue ("obssPdThreshold", "Threshold for the OBSS PD Algorithm", obssPdThreshold);
   cmd.AddValue ("mcs", "The constant MCS value to transmit HE PPDUs", mcs);
   cmd.Parse (argc, argv);
 
@@ -149,9 +178,23 @@ main (int argc, char *argv[])
 
   spectrumPhy.SetChannel (spectrumChannel);
   spectrumPhy.SetErrorRateModel ("ns3::YansErrorRateModel");
-  spectrumPhy.Set ("Frequency", UintegerValue (5180)); // channel 36 at 20 MHz
-  spectrumPhy.SetPreambleDetectionModel ("ns3::ThresholdPreambleDetectionModel");
-  //TODO: add parameter to configure CCA-PD
+  switch (channelBandwidth)
+    {
+    case 20:
+      spectrumPhy.Set ("ChannelSettings", StringValue ("{36, 20, BAND_5GHZ, 0}"));
+      break;
+    case 40:
+      spectrumPhy.Set ("ChannelSettings", StringValue ("{62, 40, BAND_5GHZ, 0}"));
+      break;
+    case 80:
+      spectrumPhy.Set ("ChannelSettings", StringValue ("{171, 80, BAND_5GHZ, 0}"));
+      break;
+    default:
+      NS_ABORT_MSG ("Unrecognized channel bandwidth: " << channelBandwidth);
+      break;
+    }
+  spectrumPhy.SetPreambleDetectionModel ("ns3::ThresholdPreambleDetectionModel",
+                                         "MinimumRssi", DoubleValue (minimumRssi));
 
   WifiHelper wifi;
   wifi.SetStandard (WIFI_STANDARD_80211ax);