From 38b4db155b13db4c8d2a44944da332ee4557f042 Mon Sep 17 00:00:00 2001
From: Stefano Avallone <stavallo@unina.it>
Date: Tue, 12 Sep 2023 20:25:33 +0200
Subject: [PATCH] wifi: Take channel switch delay into account when main PHY
 switches to take over UL TXOP

---
 .../model/eht/eht-frame-exchange-manager.cc   | 51 +++++++++++++++--
 src/wifi/model/eht/emlsr-manager.cc           | 55 ++++++++-----------
 src/wifi/model/eht/emlsr-manager.h            |  4 +-
 src/wifi/model/wifi-phy-common.h              |  3 +
 src/wifi/test/wifi-emlsr-test.cc              |  3 +
 5 files changed, 79 insertions(+), 37 deletions(-)

diff --git a/src/wifi/model/eht/eht-frame-exchange-manager.cc b/src/wifi/model/eht/eht-frame-exchange-manager.cc
index 1fda0309e..040e9b4e1 100644
--- a/src/wifi/model/eht/eht-frame-exchange-manager.cc
+++ b/src/wifi/model/eht/eht-frame-exchange-manager.cc
@@ -39,6 +39,7 @@ namespace ns3
 /// aRxPHYStartDelay value to use when waiting for a new frame in the context of EMLSR operations
 /// (Sec. 35.3.17 of 802.11be D3.1)
 static constexpr uint8_t RX_PHY_START_DELAY_USEC = 20;
+
 /**
  * Additional time (exceeding 20 us) to wait for a PHY-RXSTART.indication when the PHY is
  * decoding a PHY header.
@@ -176,6 +177,8 @@ EhtFrameExchangeManager::StartTransmission(Ptr<Txop> edca, uint16_t allowedWidth
 {
     NS_LOG_FUNCTION(this << edca << allowedWidth);
 
+    std::optional<Time> timeToCtsEnd;
+
     if (m_staMac && m_staMac->IsEmlsrLink(m_linkId))
     {
         // Cannot start a transmission on a link blocked because another EMLSR link is being used
@@ -204,6 +207,13 @@ EhtFrameExchangeManager::StartTransmission(Ptr<Txop> edca, uint16_t allowedWidth
             return false;
         }
 
+        if (!m_phy)
+        {
+            NS_LOG_DEBUG("No PHY is currently operating on EMLSR link " << +m_linkId);
+            NotifyChannelReleased(edca);
+            return false;
+        }
+
         if (auto mainPhy = m_staMac->GetDevice()->GetPhy(emlsrManager->GetMainPhyId());
             mainPhy != m_phy)
         {
@@ -229,6 +239,39 @@ EhtFrameExchangeManager::StartTransmission(Ptr<Txop> edca, uint16_t allowedWidth
                 NotifyChannelReleased(edca);
                 return false;
             }
+
+            const auto rtsTxVector =
+                GetWifiRemoteStationManager()->GetRtsTxVector(m_bssid, allowedWidth);
+            const auto rtsTxTime =
+                m_phy->CalculateTxDuration(GetRtsSize(), rtsTxVector, m_phy->GetPhyBand());
+            const auto ctsTxVector =
+                GetWifiRemoteStationManager()->GetCtsTxVector(m_bssid, rtsTxVector.GetMode());
+            const auto ctsTxTime =
+                m_phy->CalculateTxDuration(GetCtsSize(), ctsTxVector, m_phy->GetPhyBand());
+
+            // the main PHY shall terminate the channel switch at the end of CTS reception;
+            // the time remaining to the end of CTS reception includes two propagation delays
+            timeToCtsEnd = rtsTxTime + m_phy->GetSifs() + ctsTxTime +
+                           MicroSeconds(2 * MAX_PROPAGATION_DELAY_USEC);
+
+            auto switchingTime = mainPhy->GetChannelSwitchDelay();
+
+            if (mainPhy->IsStateSwitching())
+            {
+                // the main PHY is switching (to another link), hence the remaining time to the
+                // end of the current channel switch needs to be added up
+                switchingTime += mainPhy->GetDelayUntilIdle();
+            }
+
+            if (switchingTime > timeToCtsEnd)
+            {
+                // switching takes longer than RTS/CTS exchange, do not transmit anything to
+                // avoid that the main PHY is requested to switch while already switching
+                NS_LOG_DEBUG("Main PHY will still be switching channel when RTS/CTS ends, thus it "
+                             "will not be able to take over this TXOP");
+                NotifyChannelReleased(edca);
+                return false;
+            }
         }
     }
 
@@ -238,7 +281,7 @@ EhtFrameExchangeManager::StartTransmission(Ptr<Txop> edca, uint16_t allowedWidth
     {
         // notify the EMLSR Manager of the UL TXOP start on an EMLSR link
         NS_ASSERT(m_staMac->GetEmlsrManager());
-        m_staMac->GetEmlsrManager()->NotifyUlTxopStart(m_linkId);
+        m_staMac->GetEmlsrManager()->NotifyUlTxopStart(m_linkId, timeToCtsEnd);
     }
 
     return started;
@@ -924,8 +967,8 @@ EhtFrameExchangeManager::ReceiveMpdu(Ptr<const WifiMpdu> mpdu,
         }
     }
 
-    // We impose that an aux PHY is only able to receive an ICF or a management frame (we are
-    // interested in receiving mainly Beacon frames). Note that other frames are still
+    // We impose that an aux PHY is only able to receive an ICF, a CTS or a management frame
+    // (we are interested in receiving mainly Beacon frames). Note that other frames are still
     // post-processed, e.g., used to set the NAV and the TXOP holder.
     // The motivation is that, e.g., an AP MLD may send an ICF to EMLSR clients A and B;
     // A responds while B does not; the AP MLD sends a DL MU PPDU to both clients followed
@@ -934,7 +977,7 @@ EhtFrameExchangeManager::ReceiveMpdu(Ptr<const WifiMpdu> mpdu,
     // through the aux PHY.
     if (m_staMac && m_staMac->IsEmlsrLink(m_linkId) &&
         m_mac->GetLinkForPhy(m_staMac->GetEmlsrManager()->GetMainPhyId()) != m_linkId &&
-        !icfReceived && !mpdu->GetHeader().IsMgt())
+        !icfReceived && !mpdu->GetHeader().IsCts() && !mpdu->GetHeader().IsMgt())
     {
         NS_LOG_DEBUG("Dropping " << *mpdu << " received by an aux PHY on link " << +m_linkId);
         return;
diff --git a/src/wifi/model/eht/emlsr-manager.cc b/src/wifi/model/eht/emlsr-manager.cc
index 695189523..0067f44aa 100644
--- a/src/wifi/model/eht/emlsr-manager.cc
+++ b/src/wifi/model/eht/emlsr-manager.cc
@@ -383,7 +383,7 @@ EmlsrManager::NotifyIcfReceived(uint8_t linkId)
 }
 
 void
-EmlsrManager::NotifyUlTxopStart(uint8_t linkId)
+EmlsrManager::NotifyUlTxopStart(uint8_t linkId, std::optional<Time> timeToCtsEnd)
 {
     NS_LOG_FUNCTION(this << linkId);
 
@@ -403,21 +403,27 @@ EmlsrManager::NotifyUlTxopStart(uint8_t linkId)
         }
     }
 
-    // if this TXOP is being started by an aux PHY, wait until the end of RTS transmission and
-    // then have the main PHY (instantaneously) take over the TXOP on this link. We may start the
-    // channel switch now and use the channel switch delay configured for the main PHY, but then
-    // we would have no guarantees that the channel switch is completed in RTS TX time plus SIFS.
+    // if this TXOP is being started by an aux PHY, schedule a channel switch for the main PHY
+    // such that the channel switch is completed by the time the CTS response is received. The
+    // delay has been passed by the FEM.
     if (m_staMac->GetLinkForPhy(m_mainPhyId) != linkId)
     {
         auto stateHelper = m_staMac->GetWifiPhy(linkId)->GetState();
         NS_ASSERT(stateHelper);
         NS_ASSERT_MSG(stateHelper->GetState() == TX,
                       "Expecting the aux PHY to be transmitting (an RTS frame)");
-        Simulator::Schedule(stateHelper->GetDelayUntilIdle(),
-                            &EmlsrManager::SwitchMainPhy,
-                            this,
-                            linkId,
-                            true); // channel switch should occur instantaneously
+        NS_ASSERT_MSG(timeToCtsEnd.has_value(),
+                      "Aux PHY is sending RTS, expected to get the time to CTS end");
+
+        auto mainPhy = m_staMac->GetDevice()->GetPhy(m_mainPhyId);
+
+        // the main PHY shall terminate the channel switch at the end of CTS reception;
+        // the time remaining to the end of CTS reception includes two propagation delays
+        const auto delay = *timeToCtsEnd - mainPhy->GetChannelSwitchDelay();
+
+        NS_ASSERT(delay.IsPositive());
+        NS_LOG_DEBUG("Schedule main Phy switch in " << delay.As(Time::US));
+        Simulator::Schedule(delay, &EmlsrManager::SwitchMainPhy, this, linkId, false);
     }
 
     DoNotifyUlTxopStart(linkId);
@@ -436,31 +442,16 @@ EmlsrManager::NotifyTxopEnd(uint8_t linkId)
 
     DoNotifyTxopEnd(linkId);
 
-    // if the main PHY starts switching link at the end of the TXOP (which happens, e.g., when the
-    // aux PHY does not switch link), we have to keep blocking transmissions until the channel
-    // switch is complete. Otherwise, an aux PHY may get channel access, transmit an RTS and
-    // request the main PHY to switch to its link, but the main PHY may still be completing the
-    // previous channel switch
     Simulator::ScheduleNow([=, this]() {
-        auto mainPhy = m_staMac->GetDevice()->GetPhy(m_mainPhyId);
-        auto delay = mainPhy->IsStateSwitching() ? mainPhy->GetDelayUntilIdle() : Seconds(0);
-        NS_LOG_DEBUG("Main PHY is switching, postpone unblocking links by " << delay.As(Time::US));
-
-        // block transmissions on the link on which the TXOP was carried out
-        m_staMac->BlockTxOnLink(linkId, WifiQueueBlockedReason::USING_OTHER_EMLSR_LINK);
-        m_staMac->GetChannelAccessManager(linkId)->NotifyStartUsingOtherEmlsrLink();
-
-        Simulator::Schedule(delay, [=, this]() {
-            // unblock transmissions and resume medium access on other EMLSR links
-            for (auto id : m_staMac->GetLinkIds())
+        // unblock transmissions and resume medium access on other EMLSR links
+        for (auto id : m_staMac->GetLinkIds())
+        {
+            if (m_staMac->IsEmlsrLink(id))
             {
-                if (m_staMac->IsEmlsrLink(id))
-                {
-                    m_staMac->UnblockTxOnLink(id, WifiQueueBlockedReason::USING_OTHER_EMLSR_LINK);
-                    m_staMac->GetChannelAccessManager(id)->NotifyStopUsingOtherEmlsrLink();
-                }
+                m_staMac->UnblockTxOnLink(id, WifiQueueBlockedReason::USING_OTHER_EMLSR_LINK);
+                m_staMac->GetChannelAccessManager(id)->NotifyStopUsingOtherEmlsrLink();
             }
-        });
+        }
 
         StartMediumSyncDelayTimer(linkId);
     });
diff --git a/src/wifi/model/eht/emlsr-manager.h b/src/wifi/model/eht/emlsr-manager.h
index 2c614562b..7d25b8c4a 100644
--- a/src/wifi/model/eht/emlsr-manager.h
+++ b/src/wifi/model/eht/emlsr-manager.h
@@ -178,8 +178,10 @@ class EmlsrManager : public Object
      * Notify the start of an UL TXOP on the given link
      *
      * \param linkId the ID of the given link
+     * \param timeToCtsEnd time remaining to the end of CTS reception, in case the UL TXOP is
+     *                     started by an aux PHY
      */
-    void NotifyUlTxopStart(uint8_t linkId);
+    void NotifyUlTxopStart(uint8_t linkId, std::optional<Time> timeToCtsEnd);
 
     /**
      * Notify the end of a TXOP on the given link.
diff --git a/src/wifi/model/wifi-phy-common.h b/src/wifi/model/wifi-phy-common.h
index 87f5bbd54..976baf5be 100644
--- a/src/wifi/model/wifi-phy-common.h
+++ b/src/wifi/model/wifi-phy-common.h
@@ -48,6 +48,9 @@ class WifiNetDevice;
 class WifiMode;
 class Time;
 
+/// maximum propagation delay
+static constexpr uint8_t MAX_PROPAGATION_DELAY_USEC = 1;
+
 /**
  * typedef for a pair of start and stop frequencies in Hz to represent a band
  */
diff --git a/src/wifi/test/wifi-emlsr-test.cc b/src/wifi/test/wifi-emlsr-test.cc
index d190d17c0..fe85bdb97 100644
--- a/src/wifi/test/wifi-emlsr-test.cc
+++ b/src/wifi/test/wifi-emlsr-test.cc
@@ -2304,6 +2304,9 @@ EmlsrUlTxopTest::DoSetup()
     Config::SetDefault("ns3::EhtConfiguration::MsdMaxNTxops", UintegerValue(m_msdMaxNTxops));
     Config::SetDefault("ns3::ChannelAccessManager::GenerateBackoffIfTxopWithoutTx",
                        BooleanValue(m_genBackoffIfTxopWithoutTx));
+    // Channel switch delay should be less than RTS TX time + SIFS + CTS TX time, otherwise
+    // UL TXOPs cannot be initiated by aux PHYs
+    Config::SetDefault("ns3::WifiPhy::ChannelSwitchDelay", TimeValue(MicroSeconds(75)));
 
     EmlsrOperationsTestBase::DoSetup();