diff --git a/srsue/hdr/stack/mac_nr/mac_nr.h b/srsue/hdr/stack/mac_nr/mac_nr.h
index 40a9b7462..10ba5910c 100644
--- a/srsue/hdr/stack/mac_nr/mac_nr.h
+++ b/srsue/hdr/stack/mac_nr/mac_nr.h
@@ -21,7 +21,7 @@
 #include "srslte/interfaces/ue_nr_interfaces.h"
 #include "srslte/mac/mac_sch_pdu_nr.h"
 #include "srslte/srslog/srslog.h"
-#include "srsue/hdr/stack/mac/mux.h"
+#include "srsue/hdr/stack/mac_nr/mux_nr.h"
 #include "srsue/hdr/stack/ue_stack_base.h"
 
 namespace srsue {
@@ -77,10 +77,11 @@ public:
   uint16_t get_c_rnti();
   void set_c_rnti(uint64_t c_rnti_);
 
-  bool msg3_is_transmitted() { return true; }
-  void msg3_flush() {}
-  void msg3_prepare() {}
-  bool msg3_is_empty() { return true; }
+  void msg3_flush() { mux.msg3_flush(); }
+  bool msg3_is_transmitted() { return mux.msg3_is_transmitted(); }
+  void msg3_prepare() { mux.msg3_prepare(); }
+  bool msg3_is_pending() { return mux.msg3_is_pending(); }
+  bool msg3_is_empty() { return mux.msg3_is_empty(); }
 
   /// stack interface
   void process_pdus();
@@ -140,6 +141,7 @@ private:
 
   // MAC Uplink-related procedures
   proc_ra_nr proc_ra;
+  mux_nr     mux;
 };
 
 } // namespace srsue
diff --git a/srsue/hdr/stack/mac_nr/mux_nr.h b/srsue/hdr/stack/mac_nr/mux_nr.h
new file mode 100644
index 000000000..c07c95869
--- /dev/null
+++ b/srsue/hdr/stack/mac_nr/mux_nr.h
@@ -0,0 +1,46 @@
+/**
+ *
+ * \section COPYRIGHT
+ *
+ * Copyright 2013-2020 Software Radio Systems Limited
+ *
+ * By using this file, you agree to the terms and conditions set
+ * forth in the LICENSE file which can be found at the top level of
+ * the distribution.
+ *
+ */
+
+#ifndef SRSUE_MUX_NR_H
+#define SRSUE_MUX_NR_H
+
+#include "srslte/common/byte_buffer.h"
+#include "srslte/common/common.h"
+#include "srslte/srslog/srslog.h"
+#include "srslte/srslte.h"
+
+namespace srsue {
+class mux_nr
+{
+public:
+  explicit mux_nr(srslog::basic_logger& logger);
+  ~mux_nr(){};
+  void reset();
+  void init();
+
+  void step();
+
+  void msg3_flush();
+  void msg3_prepare();
+  bool msg3_is_transmitted();
+  bool msg3_is_pending();
+  bool msg3_is_empty();
+
+private:
+  srslog::basic_logger&        logger;
+  srslte::unique_byte_buffer_t msg3_buff = nullptr;
+  typedef enum { none, pending, transmitted } msg3_state_t;
+  msg3_state_t msg3_state = none;
+};
+} // namespace srsue
+
+#endif // SRSUE_MUX_NR_H
diff --git a/srsue/hdr/stack/mac_nr/proc_ra_nr.h b/srsue/hdr/stack/mac_nr/proc_ra_nr.h
index 1afe8a1bd..7d63beffb 100644
--- a/srsue/hdr/stack/mac_nr/proc_ra_nr.h
+++ b/srsue/hdr/stack/mac_nr/proc_ra_nr.h
@@ -37,7 +37,9 @@ public:
 
   bool is_rar_opportunity(uint32_t tti);
   uint16_t get_rar_rnti();
-
+  bool has_temp_rnti();
+  uint16_t get_temp_rnti();
+  
   // PHY interfaces
   void prach_sent(uint32_t tti, uint32_t s_id, uint32_t t_id, uint32_t f_id, uint32_t ul_carrier_id);
   void handle_rar_pdu(mac_interface_phy_nr::mac_nr_grant_dl_t& grant);
diff --git a/srsue/src/stack/mac_nr/CMakeLists.txt b/srsue/src/stack/mac_nr/CMakeLists.txt
index c4ced3176..2003bb81b 100644
--- a/srsue/src/stack/mac_nr/CMakeLists.txt
+++ b/srsue/src/stack/mac_nr/CMakeLists.txt
@@ -6,6 +6,6 @@
 # the distribution.
 #
 
-set(SOURCES mac_nr.cc proc_ra_nr.cc)
+set(SOURCES mac_nr.cc proc_ra_nr.cc mux_nr.cc)
 add_library(srsue_mac_nr STATIC ${SOURCES})
 target_link_libraries(srsue_mac_nr srslte_mac)
\ No newline at end of file
diff --git a/srsue/src/stack/mac_nr/mac_nr.cc b/srsue/src/stack/mac_nr/mac_nr.cc
index 1ca19c888..5c46c071e 100644
--- a/srsue/src/stack/mac_nr/mac_nr.cc
+++ b/srsue/src/stack/mac_nr/mac_nr.cc
@@ -17,7 +17,7 @@
 namespace srsue {
 
 mac_nr::mac_nr(srslte::ext_task_sched_handle task_sched_) :
-  task_sched(task_sched_), logger(srslog::fetch_basic_logger("MAC")), proc_ra(logger)
+  task_sched(task_sched_), logger(srslog::fetch_basic_logger("MAC")), proc_ra(logger), mux(logger)
 {
   tx_buffer  = srslte::make_byte_buffer();
   rlc_buffer = srslte::make_byte_buffer();
@@ -45,6 +45,8 @@ int mac_nr::init(const mac_nr_args_t& args_, phy_interface_mac_nr* phy_, rlc_int
   
   proc_ra.init(phy, this, &task_sched);
 
+  mux.init();
+
   started = true;
 
   return SRSLTE_SUCCESS;
@@ -104,11 +106,9 @@ mac_interface_phy_nr::sched_rnti_t mac_nr::get_dl_sched_rnti_nr(const uint32_t t
     return {proc_ra.get_rar_rnti(), srslte_rnti_type_ra};
   }
 
-#if 0
   if (proc_ra.has_temp_rnti() && has_crnti() == false) {
-    return proc_ra->get_temp_rnti();
+    return {proc_ra.get_temp_rnti(), srslte_rnti_type_c};
   }
-#endif
 
   if (has_crnti()) {
     return {get_crnti(), srslte_rnti_type_c};
@@ -159,7 +159,7 @@ void mac_nr::write_pcap(const uint32_t cc_idx, mac_nr_grant_dl_t& grant)
   if (pcap) {
     for (uint32_t i = 0; i < SRSLTE_MAX_CODEWORDS; ++i) {
       if (grant.tb[i] != nullptr) {
-        if (SRSLTE_RNTI_ISRAR(grant.rnti)) { // TODO: replace with proc_ra->get_rar_rnti()
+        if (grant.rnti == proc_ra.get_rar_rnti()) {
           pcap->write_dl_ra_rnti_nr(grant.tb[i]->msg, grant.tb[i]->N_bytes, grant.rnti, true, grant.tti);
         } else if (grant.rnti == SRSLTE_PRNTI) {
           pcap->write_dl_pch_nr(grant.tb[i]->msg, grant.tb[i]->N_bytes, grant.rnti, true, grant.tti);
@@ -219,23 +219,35 @@ void mac_nr::get_ul_data(const mac_nr_grant_ul_t& grant, phy_interface_stack_nr:
   tx_buffer->clear();
   tx_pdu.init_tx(tx_buffer.get(), grant.tbs, true);
 
-  while (tx_pdu.get_remaing_len() >= MIN_RLC_PDU_LEN) {
-    // read RLC PDU
-    rlc_buffer->clear();
-    uint8_t* rd      = rlc_buffer->msg;
-    int      pdu_len = rlc->read_pdu(args.drb_lcid, rd, tx_pdu.get_remaing_len() - 2);
+  if (mux.msg3_is_pending()) {
+    // If message 3 is pending pack message 3 for uplink transmission
+    tx_pdu.add_crnti_ce(proc_ra.get_temp_rnti());
 
-    // Add SDU if RLC has something to tx
-    if (pdu_len > 0) {
-      rlc_buffer->N_bytes = pdu_len;
-      logger.info(rlc_buffer->msg, rlc_buffer->N_bytes, "Read %d B from RLC", rlc_buffer->N_bytes);
+    srslte::mac_sch_subpdu_nr::lcg_bsr_t sbsr = {};
+    sbsr.lcg_id                               = 0;
+    sbsr.buffer_size                          = 1;
+    tx_pdu.add_sbsr_ce(sbsr);
 
-      // add to MAC PDU and pack
-      if (tx_pdu.add_sdu(args.drb_lcid, rlc_buffer->msg, rlc_buffer->N_bytes) != SRSLTE_SUCCESS) {
-        logger.error("Error packing MAC PDU");
+  } else {
+    // Pack normal UL data PDU
+    while (tx_pdu.get_remaing_len() >= MIN_RLC_PDU_LEN) {
+      // read RLC PDU
+      rlc_buffer->clear();
+      uint8_t* rd      = rlc_buffer->msg;
+      int      pdu_len = rlc->read_pdu(args.drb_lcid, rd, tx_pdu.get_remaing_len() - 2);
+
+      // Add SDU if RLC has something to tx
+      if (pdu_len > 0) {
+        rlc_buffer->N_bytes = pdu_len;
+        logger.info(rlc_buffer->msg, rlc_buffer->N_bytes, "Read %d B from RLC", rlc_buffer->N_bytes);
+
+        // add to MAC PDU and pack
+        if (tx_pdu.add_sdu(args.drb_lcid, rlc_buffer->msg, rlc_buffer->N_bytes) != SRSLTE_SUCCESS) {
+          logger.error("Error packing MAC PDU");
+        }
+      } else {
+        break;
       }
-    } else {
-      break;
     }
   }
 
diff --git a/srsue/src/stack/mac_nr/mux_nr.cc b/srsue/src/stack/mac_nr/mux_nr.cc
new file mode 100644
index 000000000..1d5b0c8d0
--- /dev/null
+++ b/srsue/src/stack/mac_nr/mux_nr.cc
@@ -0,0 +1,51 @@
+/**
+ *
+ * \section COPYRIGHT
+ *
+ * Copyright 2013-2020 Software Radio Systems Limited
+ *
+ * By using this file, you agree to the terms and conditions set
+ * forth in the LICENSE file which can be found at the top level of
+ * the distribution.
+ *
+ */
+
+#include "srsue/hdr/stack/mac_nr/mux_nr.h"
+#include "srslte/common/buffer_pool.h"
+
+namespace srsue {
+
+mux_nr::mux_nr(srslog::basic_logger& logger_) : logger(logger_)
+{
+  msg3_buff = srslte::make_byte_buffer();
+};
+
+void mux_nr::init() {}
+
+void mux_nr::msg3_flush()
+{
+  msg3_buff->clear();
+  msg3_state = msg3_state_t::none;
+}
+
+void mux_nr::msg3_prepare()
+{
+  msg3_state = msg3_state_t::pending;
+}
+
+bool mux_nr::msg3_is_transmitted()
+{
+  return msg3_state == msg3_state_t::transmitted;
+}
+
+bool mux_nr::msg3_is_pending()
+{
+  return msg3_state == msg3_state_t::pending;
+}
+
+bool mux_nr::msg3_is_empty()
+{
+  return msg3_buff->N_bytes == 0;
+}
+
+} // namespace srsue
diff --git a/srsue/src/stack/mac_nr/proc_ra_nr.cc b/srsue/src/stack/mac_nr/proc_ra_nr.cc
index f855360a5..454052e7e 100644
--- a/srsue/src/stack/mac_nr/proc_ra_nr.cc
+++ b/srsue/src/stack/mac_nr/proc_ra_nr.cc
@@ -93,6 +93,16 @@ uint16_t proc_ra_nr::get_rar_rnti()
   return rar_rnti;
 }
 
+bool proc_ra_nr::has_temp_rnti()
+{
+  return temp_rnti != SRSLTE_INVALID_RNTI;
+}
+
+uint16_t proc_ra_nr::get_temp_rnti()
+{
+  return temp_rnti;
+}
+
 void proc_ra_nr::timer_expired(uint32_t timer_id)
 {
   if (prach_send_timer.id() == timer_id) {
@@ -228,7 +238,7 @@ void proc_ra_nr::ra_completion()
   mac->set_c_rnti(temp_rnti);
   srslte::console("Random Access Complete.     c-rnti=0x%x, ta=%d\n", mac->get_c_rnti(), current_ta);
   logger.info("Random Access Complete.     c-rnti=0x%x, ta=%d", mac->get_c_rnti(), current_ta);
-  temp_rnti = 0;
+  temp_rnti = SRSLTE_INVALID_RNTI;
   reset();
 }