MAC: Add RACH preamble collision modeling

UEs now pick a random preamble index (0..numPreambles-1) when sending
RAC requests. The eNB buffers all RAC packets arriving during a TTI
and resolves them at the start of the next TTI: if multiple UEs chose
the same preamble, all of them fail (collision). Unique preambles
succeed as before. Failed UEs exercise the existing backoff/retry path.

New NED parameter `numPreambles` (default 64) on both LteMacUe and
LteMacEnb controls the preamble pool size.

While this is a simplified model of the multi-step exchange that actually takes
place in real LTE and NR systems, it is still represents an improvement
over the previous code that never reported collision.

Author: avarga

src/simu5g/stack/mac/LteMacEnb.cc

@@ -64,6 +64,10 @@ LteMacEnb::~LteMacEnb()
 
     for (auto &[key, value] : bsrbuf_)
         delete value;
+
+    for (auto &[preamble, pkts] : pendingRacRequests_)
+        for (auto *pkt : pkts)
+            delete pkt;
 }
 
 /***********************
@@ -130,6 +134,8 @@ void LteMacEnb::initialize(int stage)
 
         cellId_ = nodeId_;
 
+        numPreambles_ = par("numPreambles");
+
         cellInfo_.reference(this, "cellInfoModule", true);
 
         eNodeBCount = par("eNodeBCount");
@@ -454,24 +460,62 @@ void LteMacEnb::sendGrants(std::map<GHz, LteMacScheduleList> *scheduleList)
 
 void LteMacEnb::macHandleRac(cPacket *pktAux)
 {
-    EV << NOW << "LteMacEnb::macHandleRac" << endl;
-
     auto pkt = check_and_cast<Packet *>(pktAux);
+    auto racPkt = pkt->peekAtFront<LteRac>();
+    int preamble = racPkt->getPreambleIndex();
+
+    EV << NOW << "LteMacEnb::macHandleRac - buffering RAC from UE "
+       << pkt->getTag<UserControlInfo>()->getSourceId()
+       << " preamble=" << preamble << endl;
+
+    pendingRacRequests_[preamble].push_back(pkt);
+}
 
-    auto racPkt = pkt->removeAtFront<LteRac>();
-    auto uinfo = pkt->getTagForUpdate<UserControlInfo>();
+void LteMacEnb::resolveRacCollisions()
+{
+    if (pendingRacRequests_.empty())
+        return;
 
-    enbSchedulerUl_->signalRac(uinfo->getSourceId(), uinfo->getCarrierFrequency());
+    EV << NOW << "LteMacEnb::resolveRacCollisions - resolving "
+       << pendingRacRequests_.size() << " preamble groups" << endl;
 
-    // TODO: all RACs are marked as successful
-    racPkt->setSuccess(true);
-    pkt->insertAtFront(racPkt);
+    for (auto& [preamble, pkts] : pendingRacRequests_) {
+        bool collision = (pkts.size() > 1);
 
-    uinfo->setDestId(uinfo->getSourceId());
-    uinfo->setSourceId(nodeId_);
-    uinfo->setDirection(DL);
+        if (collision) {
+            EV << NOW << "LteMacEnb::resolveRacCollisions - COLLISION on preamble "
+               << preamble << " (" << pkts.size() << " UEs)" << endl;
+        }
 
-    sendLowerPackets(pkt);
+        for (auto *pkt : pkts) {
+            auto racPkt = pkt->removeAtFront<LteRac>();
+            auto uinfo = pkt->getTagForUpdate<UserControlInfo>();
+            MacNodeId ueId = uinfo->getSourceId();
+
+            if (collision) {
+                // preamble collision: RAC fails
+                racPkt->setSuccess(false);
+                EV << NOW << "LteMacEnb::resolveRacCollisions - UE " << ueId
+                   << " RAC FAILED (preamble collision)" << endl;
+            }
+            else {
+                // unique preamble: RAC succeeds
+                racPkt->setSuccess(true);
+                enbSchedulerUl_->signalRac(ueId, uinfo->getCarrierFrequency());
+                EV << NOW << "LteMacEnb::resolveRacCollisions - UE " << ueId
+                   << " RAC SUCCESS" << endl;
+            }
+
+            pkt->insertAtFront(racPkt);
+
+            uinfo->setDestId(ueId);
+            uinfo->setSourceId(nodeId_);
+            uinfo->setDirection(DL);
+
+            sendLowerPackets(pkt);
+        }
+    }
+    pendingRacRequests_.clear();
 }
 
 void LteMacEnb::macPduMake(MacCid cid)
@@ -753,6 +797,9 @@ void LteMacEnb::handleSelfMessage()
 
     EV << "-----" << "ENB MAIN LOOP -----" << endl;
 
+    // Resolve any RAC requests buffered since the last TTI (preamble collision detection)
+    resolveRacCollisions();
+
     // Reception
 
     // extract PDUs from all HARQ RX buffers and pass them to unmaker

src/simu5g/stack/mac/LteMacEnb.h

@@ -51,6 +51,13 @@ class LteMacEnb : public LteMacBase
 
     /*******************************************************************************************/
 
+    /// Number of RACH preambles for contention-based random access
+    int numPreambles_ = 64;
+
+    /// Pending RAC requests received during this TTI, grouped by preamble index.
+    /// Resolved at the start of handleSelfMessage() to detect collisions.
+    std::map<int, std::vector<inet::Packet *>> pendingRacRequests_;
+
     /// Buffer for the BSRs
     /// In the key (MacCid), lcid is a BsrType: one of SHORT_BSR, D2D_SHORT_BSR, D2D_MULTI_SHORT_BSR.
     std::map<MacCid, LteMacBuffer*> bsrbuf_;
@@ -154,10 +161,16 @@ class LteMacEnb : public LteMacBase
     void macHandleFeedbackPkt(cPacket *pkt) override;
 
     /*
-     * Receives and handles RAC requests.
+     * Buffers incoming RAC requests for preamble collision detection.
      */
     void macHandleRac(cPacket *pkt) override;
 
+    /*
+     * Resolves buffered RAC requests: detects preamble collisions and sends
+     * success/failure responses. Called at the start of each TTI.
+     */
+    void resolveRacCollisions();
+
     /*
      * Update UserTxParam stored in every lteMacPdu when an RTX changes this information.
      */

src/simu5g/stack/mac/LteMacEnb.ned

@@ -77,6 +77,8 @@ simple LteMacEnb extends LteMacBase
 
         string pilotMode @enum(IN_CQI,MAX_CQI,AVG_CQI,MEDIAN_CQI,ROBUST_CQI) = default("ROBUST_CQI");
 
+        int numPreambles = default(64);        // number of RACH preambles available for contention-based random access
+
         string cellInfoModule;
         string rlcUmModule = default("^.rlc.um");
         string pdcpModule = default("^.pdcp");

src/simu5g/stack/mac/LteMacUe.cc

@@ -60,6 +60,7 @@ void LteMacUe::initialize(int stage)
         bool isNr = strcmp(getFullName(), "nrMac") == 0;
         nodeId_ = MacNodeId(networkNode_->par(isNr ? "nrMacNodeId" : "macNodeId").intValue());
 
+        numPreambles_ = par("numPreambles");
         maxRacTryouts_ = par("maxRacAttempts");
         minRacBackoff_ = par("racBackoffMin");
         maxRacBackoff_ = par("racBackoffMax");
@@ -876,6 +877,7 @@ void LteMacUe::checkRAC()
         auto pkt = new Packet("RacRequest");
 
         auto racReq = makeShared<LteRac>();
+        racReq->setPreambleIndex(intuniform(0, numPreambles_ - 1));
         pkt->insertAtFront(racReq);
 
         GHz carrierFrequency = phy_->getPrimaryChannelModel()->getCarrierFrequency();
@@ -887,7 +889,8 @@ void LteMacUe::checkRAC()
 
         sendLowerPackets(pkt);
 
-        EV << NOW << " UE  " << nodeId_ << " cell " << cellId_ << " ,RAC request sent to PHY " << endl;
+        EV << NOW << " UE  " << nodeId_ << " cell " << cellId_ << " ,RAC request sent to PHY (preamble="
+           << racReq->getPreambleIndex() << ")" << endl;
 
         // wait at least "raRespWinStart_" TTIs before another RAC request
         raRespTimer_ = raRespWinStart_;

src/simu5g/stack/mac/LteMacUe.h

@@ -56,6 +56,7 @@ class LteMacUe : public LteMacBase
 
     // RAC and BSR configuration
     // TODO adjust C++ names to match NED parameter names
+    int numPreambles_ = 64;
     unsigned int maxRacTryouts_ = 0;
     unsigned int minRacBackoff_ = 0;
     unsigned int maxRacBackoff_ = 0;

src/simu5g/stack/mac/LteMacUe.ned

@@ -27,6 +27,7 @@ simple LteMacUe extends LteMacBase
         @class("LteMacUe");
         string collectorModule = default("");
 
+        int numPreambles = default(64);        // number of RACH preambles available for contention-based random access
         int maxRacAttempts = default(10);      // preambleTransMax; max RAC attempts (0 = give up after first failure)
         int racBackoffMin = default(0);        // min random backoff (TTIs) after failed RAC
         int racBackoffMax = default(20);       // max random backoff (TTIs) after failed RAC (0 = no backoff, instant retry)

src/simu5g/stack/mac/LteMacUeD2D.cc

@@ -524,11 +524,13 @@ void LteMacUeD2D::checkRAC()
         pkt->addTagIfAbsent<UserControlInfo>()->setFrameType(RACPKT);
 
         auto racReq = makeShared<LteRac>();
+        racReq->setPreambleIndex(intuniform(0, numPreambles_ - 1));
 
         pkt->insertAtFront(racReq);
         sendLowerPackets(pkt);
 
-        EV << NOW << " Ue  " << nodeId_ << " cell " << cellId_ << ", RAC request sent to PHY " << endl;
+        EV << NOW << " Ue  " << nodeId_ << " cell " << cellId_ << ", RAC request sent to PHY (preamble="
+           << racReq->getPreambleIndex() << ")" << endl;
 
         // wait at least  "raRespWinStart_" TTIs before another RAC request
         raRespTimer_ = raRespWinStart_;

src/simu5g/stack/mac/packet/LteRac.msg

@@ -18,5 +18,7 @@ class LteRac extends inet::FieldsChunk
 {
     // meaningful only for DL (response) RAC packets : if true RAC request has been admitted.
     bool success;
+    // preamble index chosen by UE (0..numPreambles-1); used for collision detection at eNB
+    int preambleIndex = -1;
     chunkLength = inet::B(1); // TODO: size 0
 }