diff --git a/srsenb/hdr/stack/mac/scheduler_ue.h b/srsenb/hdr/stack/mac/scheduler_ue.h
index 1ed9fd814..882ccad21 100644
--- a/srsenb/hdr/stack/mac/scheduler_ue.h
+++ b/srsenb/hdr/stack/mac/scheduler_ue.h
@@ -54,7 +54,7 @@ struct sched_ue_carrier {
   // Harq access
   void          reset_old_pending_pids(uint32_t tti_rx);
   dl_harq_proc* get_pending_dl_harq(uint32_t tti_tx_dl);
-  dl_harq_proc* get_empty_dl_harq();
+  dl_harq_proc* get_empty_dl_harq(uint32_t tti_tx_dl);
   int           set_ack_info(uint32_t tti_rx, uint32_t tb_idx, bool ack);
   ul_harq_proc* get_ul_harq(uint32_t tti);
   uint32_t      get_pending_ul_old_data();
@@ -66,7 +66,7 @@ struct sched_ue_carrier {
   uint32_t                   get_required_prb_ul(uint32_t req_bytes);
   const sched_cell_params_t* get_cell_cfg() const { return cell_params; }
   bool                       is_active() const { return active; }
-  uint32_t                   get_ue_cc_idx() const { return ue_cc_idx; }
+  void                       update_cell_activity();
 
   std::array<dl_harq_proc, SCHED_MAX_HARQ_PROC> dl_harq = {};
   std::array<ul_harq_proc, SCHED_MAX_HARQ_PROC> ul_harq = {};
@@ -94,9 +94,7 @@ private:
   const sched_cell_params_t*       cell_params = nullptr;
   uint16_t                         rnti;
   uint32_t                         ue_cc_idx = 0;
-
-  // state
-  bool active = false;
+  bool                             active    = false;
 };
 
 /** This class is designed to be thread-safe because it is called from workers through scheduler thread and from
@@ -160,8 +158,8 @@ public:
   uint32_t get_pending_dl_new_data_total();
 
   void          reset_pending_pids(uint32_t tti_rx, uint32_t cc_idx);
-  dl_harq_proc* get_pending_dl_harq(uint32_t tti, uint32_t cc_idx);
-  dl_harq_proc* get_empty_dl_harq(uint32_t cc_idx);
+  dl_harq_proc* get_pending_dl_harq(uint32_t tti_tx_dl, uint32_t cc_idx);
+  dl_harq_proc* get_empty_dl_harq(uint32_t tti_tx_dl, uint32_t cc_idx);
   ul_harq_proc* get_ul_harq(uint32_t tti, uint32_t cc_idx);
 
   /*******************************************************
diff --git a/srsenb/src/stack/mac/scheduler_metric.cc b/srsenb/src/stack/mac/scheduler_metric.cc
index 78a023516..094f6ca0c 100644
--- a/srsenb/src/stack/mac/scheduler_metric.cc
+++ b/srsenb/src/stack/mac/scheduler_metric.cc
@@ -81,9 +81,11 @@ bool dl_metric_rr::find_allocation(uint32_t nof_rbg, rbgmask_t* rbgmask)
 
 dl_harq_proc* dl_metric_rr::allocate_user(sched_ue* user)
 {
+  // Do not allocate a user multiple times in the same tti
   if (tti_alloc->is_dl_alloc(user)) {
     return nullptr;
   }
+  // Do not allocate a user to an inactive carrier
   auto p = user->get_cell_index(cc_cfg->enb_cc_idx);
   if (not p.first) {
     return nullptr;
@@ -97,11 +99,7 @@ dl_harq_proc* dl_metric_rr::allocate_user(sched_ue* user)
   uint32_t        req_bytes = user->get_pending_dl_new_data_total();
 
   // Schedule retx if we have space
-#if ASYNC_DL_SCHED
   if (h != nullptr) {
-#else
-  if (h && !h->is_empty()) {
-#endif
     // Try to reuse the same mask
     rbgmask_t retx_mask = h->get_rbgmask();
     code                = tti_alloc->alloc_dl_user(user, retx_mask, h->get_id());
@@ -127,12 +125,8 @@ dl_harq_proc* dl_metric_rr::allocate_user(sched_ue* user)
   }
 
   // If could not schedule the reTx, or there wasn't any pending retx, find an empty PID
-#if ASYNC_DL_SCHED
-  h = user->get_empty_dl_harq(cell_idx);
+  h = user->get_empty_dl_harq(tti_dl, cell_idx);
   if (h != nullptr) {
-#else
-  if (h && h->is_empty()) {
-#endif
     // Allocate resources based on pending data
     if (req_bytes > 0) {
       uint32_t  pending_prbs = user->get_required_prb_dl(cell_idx, req_bytes, tti_alloc->get_nof_ctrl_symbols());
diff --git a/srsenb/src/stack/mac/scheduler_ue.cc b/srsenb/src/stack/mac/scheduler_ue.cc
index d9ecda6f9..59d763ea2 100644
--- a/srsenb/src/stack/mac/scheduler_ue.cc
+++ b/srsenb/src/stack/mac/scheduler_ue.cc
@@ -117,19 +117,15 @@ void sched_ue::set_cfg(const sched_interface::ue_cfg_t& cfg_)
       if (ue_idx >= prev_supported_cc_list.size()) {
         // New carrier needs to be added
         carriers.emplace_back(cfg, (*cell_params_list)[cc_cfg.enb_cc_idx], rnti, ue_idx);
-        scell_activation_state_changed |= ue_idx > 0 and carriers.back().is_active();
       } else if (cc_cfg.enb_cc_idx != prev_supported_cc_list[ue_idx].enb_cc_idx) {
         // TODO: Check if this will ever happen.
         // One carrier was added in the place of another
         carriers[ue_idx] = sched_ue_carrier{cfg, (*cell_params_list)[cc_cfg.enb_cc_idx], rnti, ue_idx};
-        scell_activation_state_changed |= ue_idx > 0 and carriers[ue_idx].is_active();
       } else {
-        // The enb_cc_idx, ue_cc_idx match previous configuration.
-        // The SCell state may have changed. In such case we will schedule a SCell Activation CE
-        scell_activation_state_changed = carriers[ue_idx].is_active() != cc_cfg.active and ue_idx > 0;
-        // reconfiguration of carrier might be needed.
+        // The SCell internal configuration may have changed
         carriers[ue_idx].set_cfg(cfg);
       }
+      scell_activation_state_changed |= carriers[ue_idx].is_active() != cc_cfg.active and ue_idx > 0;
     }
     if (scell_activation_state_changed) {
       pending_ces.emplace_back(srslte::sch_subh::SCELL_ACTIVATION);
@@ -254,7 +250,13 @@ bool sched_ue::pucch_sr_collision(uint32_t current_tti, uint32_t n_cce)
 int sched_ue::set_ack_info(uint32_t tti, uint32_t cc_idx, uint32_t tb_idx, bool ack)
 {
   std::lock_guard<std::mutex> lock(mutex);
-  return carriers[cc_idx].set_ack_info(tti, tb_idx, ack);
+  int                         ret = -1;
+  if (cc_idx < carriers.size()) {
+    ret = carriers[cc_idx].set_ack_info(tti, tb_idx, ack);
+  } else {
+    log_h->warning("Received DL ACK for invalid cell index %d\n", cc_idx);
+  }
+  return ret;
 }
 
 void sched_ue::ul_recv_len(uint32_t lcid, uint32_t len)
@@ -293,6 +295,10 @@ void sched_ue::set_dl_ri(uint32_t tti, uint32_t cc_idx, uint32_t ri)
 void sched_ue::set_dl_pmi(uint32_t tti, uint32_t cc_idx, uint32_t pmi)
 {
   std::lock_guard<std::mutex> lock(mutex);
+  if (cc_idx >= carriers.size()) {
+    log_h->warning("Received DL PMI for invalid cell index %d\n", cc_idx);
+    return;
+  }
   carriers[cc_idx].dl_pmi     = pmi;
   carriers[cc_idx].dl_pmi_tti = tti;
 }
@@ -300,8 +306,13 @@ void sched_ue::set_dl_pmi(uint32_t tti, uint32_t cc_idx, uint32_t pmi)
 void sched_ue::set_dl_cqi(uint32_t tti, uint32_t cc_idx, uint32_t cqi)
 {
   std::lock_guard<std::mutex> lock(mutex);
+  if (cc_idx >= carriers.size()) {
+    log_h->warning("Received DL CQI for invalid cell index %d\n", cc_idx);
+    return;
+  }
   carriers[cc_idx].dl_cqi     = cqi;
   carriers[cc_idx].dl_cqi_tti = tti;
+  carriers[cc_idx].update_cell_activity();
 }
 
 void sched_ue::set_ul_cqi(uint32_t tti, uint32_t cc_idx, uint32_t cqi, uint32_t ul_ch_code)
@@ -898,18 +909,27 @@ void sched_ue::reset_pending_pids(uint32_t tti_rx, uint32_t cc_idx)
 dl_harq_proc* sched_ue::get_pending_dl_harq(uint32_t tti_tx_dl, uint32_t cc_idx)
 {
   std::lock_guard<std::mutex> lock(mutex);
-  return carriers[cc_idx].get_pending_dl_harq(tti_tx_dl);
+  if (cc_idx < carriers.size() and carriers[cc_idx].is_active()) {
+    return carriers[cc_idx].get_pending_dl_harq(tti_tx_dl);
+  }
+  return nullptr;
 }
 
-dl_harq_proc* sched_ue::get_empty_dl_harq(uint32_t cc_idx)
+dl_harq_proc* sched_ue::get_empty_dl_harq(uint32_t tti_tx_dl, uint32_t ue_cc_idx)
 {
   std::lock_guard<std::mutex> lock(mutex);
-  return carriers[cc_idx].get_empty_dl_harq();
+  if (ue_cc_idx < carriers.size() and carriers[ue_cc_idx].is_active()) {
+    return carriers[ue_cc_idx].get_empty_dl_harq(tti_tx_dl);
+  }
+  return nullptr;
 }
 
-ul_harq_proc* sched_ue::get_ul_harq(uint32_t tti_tx_ul, uint32_t cc_idx)
+ul_harq_proc* sched_ue::get_ul_harq(uint32_t tti_tx_ul, uint32_t ue_cc_idx)
 {
-  return carriers[cc_idx].get_ul_harq(tti_tx_ul);
+  if (ue_cc_idx < carriers.size() and carriers[ue_cc_idx].is_active()) {
+    return carriers[ue_cc_idx].get_ul_harq(tti_tx_ul);
+  }
+  return nullptr;
 }
 
 dl_harq_proc* sched_ue::find_dl_harq(uint32_t tti_rx, uint32_t cc_idx)
@@ -929,14 +949,15 @@ dl_harq_proc* sched_ue::get_dl_harq(uint32_t idx, uint32_t cc_idx)
 
 std::pair<bool, uint32_t> sched_ue::get_cell_index(uint32_t enb_cc_idx) const
 {
-  auto it =
-      std::find_if(cfg.supported_cc_list.begin(),
-                   cfg.supported_cc_list.end(),
-                   [enb_cc_idx](const sched_interface::ue_cfg_t::cc_cfg_t& u) { return u.enb_cc_idx == enb_cc_idx; });
-  if (it == cfg.supported_cc_list.end()) {
-    return std::make_pair(false, 0);
+  auto it = std::find_if(
+      cfg.supported_cc_list.begin(),
+      cfg.supported_cc_list.end(),
+      [enb_cc_idx](const sched_interface::ue_cfg_t::cc_cfg_t& u) { return u.enb_cc_idx == enb_cc_idx and u.active; });
+  if (it != cfg.supported_cc_list.end()) {
+    uint32_t ue_cc_idx = std::distance(cfg.supported_cc_list.begin(), it);
+    return {carriers[ue_cc_idx].is_active(), enb_cc_idx};
   }
-  return std::make_pair(true, it->enb_cc_idx);
+  return {false, 0};
 }
 
 void sched_ue::finish_tti(const tti_params_t& tti_params, uint32_t enb_cc_idx)
@@ -1080,6 +1101,9 @@ sched_ue_carrier::sched_ue_carrier(const sched_interface::ue_cfg_t& cfg_,
   log_h(srslte::logmap::get("MAC ")),
   ue_cc_idx(ue_cc_idx_)
 {
+  // only PCell starts active. Remaining ones wait for valid CQI
+  active = ue_cc_idx == 0;
+
   // Init HARQ processes
   for (uint32_t i = 0; i < dl_harq.size(); ++i) {
     dl_harq[i].init(i);
@@ -1125,7 +1149,7 @@ void sched_ue_carrier::reset()
 
 void sched_ue_carrier::set_cfg(const sched_interface::ue_cfg_t& cfg_)
 {
-  if (cfg != nullptr and cfg->maxharq_tx == cfg_.maxharq_tx and active == cfg->supported_cc_list[ue_cc_idx].active) {
+  if (cfg != nullptr and cfg->maxharq_tx == cfg_.maxharq_tx) {
     // nothing changed
     return;
   }
@@ -1135,7 +1159,6 @@ void sched_ue_carrier::set_cfg(const sched_interface::ue_cfg_t& cfg_)
     dl_harq[i].set_cfg(cfg->maxharq_tx);
     ul_harq[i].set_cfg(cfg->maxharq_tx);
   }
-  active = cfg->supported_cc_list[ue_cc_idx].active;
 }
 
 void sched_ue_carrier::reset_old_pending_pids(uint32_t tti_rx)
@@ -1162,7 +1185,10 @@ void sched_ue_carrier::reset_old_pending_pids(uint32_t tti_rx)
 
 dl_harq_proc* sched_ue_carrier::get_pending_dl_harq(uint32_t tti_tx_dl)
 {
-#if ASYNC_DL_SCHED
+  if (not ASYNC_DL_SCHED) {
+    dl_harq_proc* h = &dl_harq[tti_tx_dl % SCHED_MAX_HARQ_PROC];
+    return h->is_empty() ? nullptr : h;
+  }
 
   int      oldest_idx = -1;
   uint32_t oldest_tti = 0;
@@ -1180,16 +1206,16 @@ dl_harq_proc* sched_ue_carrier::get_pending_dl_harq(uint32_t tti_tx_dl)
     h = &dl_harq[oldest_idx];
   }
   return h;
-
-#else
-  return &dl_harq[tti % SCHED_MAX_HARQ_PROC];
-#endif
 }
 
-dl_harq_proc* sched_ue_carrier::get_empty_dl_harq()
+dl_harq_proc* sched_ue_carrier::get_empty_dl_harq(uint32_t tti_tx_dl)
 {
-  auto it =
-      std::find_if(dl_harq.begin(), dl_harq.end(), [](dl_harq_proc& h) { return h.is_empty(0) and h.is_empty(1); });
+  if (not ASYNC_DL_SCHED) {
+    dl_harq_proc* h = &dl_harq[tti_tx_dl % SCHED_MAX_HARQ_PROC];
+    return h->is_empty() ? nullptr : h;
+  }
+
+  auto it = std::find_if(dl_harq.begin(), dl_harq.end(), [](dl_harq_proc& h) { return h.is_empty(); });
   return it != dl_harq.end() ? &(*it) : nullptr;
 }
 
@@ -1202,7 +1228,6 @@ int sched_ue_carrier::set_ack_info(uint32_t tti_rx, uint32_t tb_idx, bool ack)
       return h.get_tbs(tb_idx);
     }
   }
-
   Warning("SCHED: Received ACK info for unknown TTI=%d\n", tti_rx);
   return -1;
 }
@@ -1326,6 +1351,14 @@ uint32_t sched_ue_carrier::get_required_prb_ul(uint32_t req_bytes)
   return n;
 }
 
+void sched_ue_carrier::update_cell_activity()
+{
+  if (ue_cc_idx > 0 and active != cfg->supported_cc_list[ue_cc_idx].active) {
+    active = cfg->supported_cc_list[ue_cc_idx].active;
+    log_h->info("SCell index=%d is now %s\n", ue_cc_idx, active ? "active" : "inactive");
+  }
+}
+
 /*******************************************************
  *                   MAC CE Command
  ******************************************************/
diff --git a/srsenb/test/mac/scheduler_ca_test.cc b/srsenb/test/mac/scheduler_ca_test.cc
index 0b8ab6c8e..13a82fc18 100644
--- a/srsenb/test/mac/scheduler_ca_test.cc
+++ b/srsenb/test/mac/scheduler_ca_test.cc
@@ -110,13 +110,6 @@ int run_sim1()
   uint32_t prach_tti = 1, msg4_tot_delay = 10; // TODO: check correct value
   uint32_t msg4_size = 20;                     // TODO: Check
   uint32_t duration  = 1000;
-  //  auto     process_ttis = [&generator, &tti_start, &tester]() {
-  //    for (; tester.tti_counter <= generator.tti_counter;) {
-  //      uint32_t tti = (tti_start + tester.tti_count) % 10240;
-  //      log_global->step(tti);
-  //      tester.run_tti(generator.tti_events[tester.tti_count]);
-  //    }
-  //  };
 
   /* Simulation */
 
@@ -161,7 +154,8 @@ int run_sim1()
     user->ue_cfg->supported_cc_list[i].enb_cc_idx = i;
   }
   tester.test_next_ttis(generator.tti_events);
-  // When a new DL tx takes place, it should also encode the CE
+
+  // When a DL newtx takes place, it should also encode the CE
   for (uint32_t i = 0; i < 100; ++i) {
     TESTASSERT(tester.tti_info.dl_sched_result[pcell_idx].nof_data_elems > 0);
     if (tester.tti_info.dl_sched_result[pcell_idx].data[0].nof_pdu_elems[0] > 0) {
@@ -172,11 +166,24 @@ int run_sim1()
     }
     generator.step_tti();
     tester.test_next_ttis(generator.tti_events);
-    // now we have two CCs
   }
-  // now we have two CCs
+  for (uint32_t i = 0; i < TX_DELAY; ++i) {
+    generator.step_tti();
+  }
+  tester.test_next_ttis(generator.tti_events);
+  // The UE has now received the CE
+
+  // Event: Generate a bit more data, it should *not* go through SCells until we send a CQI
+  generate_data(5, P_dl, P_ul_sr);
+  tester.test_next_ttis(generator.tti_events);
+  TESTASSERT(tester.sched_stats->users[rnti1].tot_dl_sched_data[0] > 0);
+  TESTASSERT(tester.sched_stats->users[rnti1].tot_dl_sched_data[1] == 0);
+  TESTASSERT(tester.sched_stats->users[rnti1].tot_ul_sched_data[0] > 0);
+  TESTASSERT(tester.sched_stats->users[rnti1].tot_ul_sched_data[1] == 0);
 
-  // Event: Generate a bit more data, now it should go through both cells
+  // Event: Scheduler receives dl_cqi for SCell. Data should go through SCells
+  const uint32_t cqi = 14;
+  tester.dl_cqi_info(tester.tti_info.tti_params.tti_rx, rnti1, 1, cqi);
   generate_data(10, 1.0, 1.0);
   tester.test_next_ttis(generator.tti_events);
   TESTASSERT(tester.sched_stats->users[rnti1].tot_dl_sched_data[0] > 0);
diff --git a/srsenb/test/mac/scheduler_test_rand.cc b/srsenb/test/mac/scheduler_test_rand.cc
index bb59b2a0d..eaa5c525f 100644
--- a/srsenb/test/mac/scheduler_test_rand.cc
+++ b/srsenb/test/mac/scheduler_test_rand.cc
@@ -164,7 +164,9 @@ void sched_tester::before_sched()
     d.has_ul_retx             = hul->has_pending_retx();
     d.has_ul_tx               = d.has_ul_retx or d.ul_pending_data > 0;
     srsenb::dl_harq_proc* hdl = user->get_pending_dl_harq(tti_info.tti_params.tti_tx_dl, CARRIER_IDX);
-    d.has_dl_tx = (hdl != nullptr) or (it.second.get_empty_dl_harq(CARRIER_IDX) != nullptr and d.dl_pending_data > 0);
+    d.has_dl_tx =
+        (hdl != nullptr) or
+        (it.second.get_empty_dl_harq(tti_info.tti_params.tti_tx_dl, CARRIER_IDX) != nullptr and d.dl_pending_data > 0);
     d.has_ul_newtx = not d.has_ul_retx and d.ul_pending_data > 0;
     tti_data.ue_data.insert(std::make_pair(rnti, d));
     tti_data.total_ues.dl_pending_data += d.dl_pending_data;