wifi: Fix rate used to send BlockAckReq frames

Fixes a regression introduced with 1a8b3718

RELEASE_NOTES.md

@@ -41,6 +41,7 @@ Release 3-dev
 - (wifi) - Stop A-MSDU aggregation when an A-MSDU is found in the queue
 - (lr-wpan) !1769 - `DoDispose` SIGSEGV and beacon fixes
 - (wifi) - ReportAmpduTxStatus called twice when sending explicit BAR upon missed BlockAck
+- (wifi) - Fix regression causing BlockAckReq frames to be sent with data rates instead of control rates
 
 Release 3.40
 ------------

examples/wireless/wifi-simple-ht-hidden-stations.cc

@@ -25,6 +25,7 @@
 #include "ns3/ipv4-address-helper.h"
 #include "ns3/log.h"
 #include "ns3/mobility-helper.h"
+#include "ns3/rng-seed-manager.h"
 #include "ns3/ssid.h"
 #include "ns3/string.h"
 #include "ns3/udp-client-server-helper.h"
@@ -64,6 +65,9 @@ main(int argc, char* argv[])
     double minExpectedThroughput = 0;
     double maxExpectedThroughput = 0;
 
+    RngSeedManager::SetSeed(1);
+    RngSeedManager::SetRun(5);
+
     CommandLine cmd(__FILE__);
     cmd.AddValue("nMpdus", "Number of aggregated MPDUs", nMpdus);
     cmd.AddValue("payloadSize", "Payload size in bytes", payloadSize);

src/wifi/model/ht/ht-frame-exchange-manager.cc

@@ -570,6 +570,12 @@ HtFrameExchangeManager::SendMpduFromBaManager(Ptr<WifiMpdu> mpdu,
         return false;
     }
 
+    NS_ABORT_IF(txParams.m_acknowledgment->method != WifiAcknowledgment::BLOCK_ACK);
+
+    // the BlockAckReq frame is sent using the same TXVECTOR as the BlockAck frame
+    auto blockAcknowledgment = static_cast<WifiBlockAck*>(txParams.m_acknowledgment.get());
+    txParams.m_txVector = blockAcknowledgment->blockAckTxVector;
+
     // we can transmit the BlockAckReq frame
     SendPsduWithProtection(GetWifiPsdu(mpdu, txParams.m_txVector), txParams);
     return true;