er PDU layer Relay Relay 5G user-plane GTP-U GTP-U 5G user-plane encapsulation encapsulation 5G-AN 5G-AN UDP/IP UDP/IP UDP/IP UDP/IP protocol protocol layers layers 5G-AN (PDU session anchor) Figure 1.72 User-plane protocol stack between UE and UPF [3]. 5G Network Architecture Carrier 1 (F1) ((HH)) User-plane data Carrier 2 (F2) User-plane data Macro- Small cell gNB cell gNB Non-ideal backhaul (Xn) Figure 1.73 Illustration of inter-node radio resource aggregation (dual connectivity concept). 1.3 Dual Connectivity and Multi-connectivity Schemes Dual connectivity (or multi-connectivity) is a term that is used to refer to an operation where a given UE is allocated radio resources provided by at least two different network nodes connected with non-ideal backhaul (see Fig. 1.73). Each access node involved in dual connectivity for a UE may assume different roles. Those roles do not necessarily depend on the access node's power class and can vary for different UEs. To support tight interworking between LTE and NR, where both LTE eNB and NR gNB can act as a master node. It is assumed that the dual connectivity between LTE and NR supports the deployment scenario where LTE eNB is not required to be synchronized with NR gNB. 3GPP NR supports dual connectivity operation in which a UE in the connected mode is con- figured to utilize radio resources provided by two distinct schedulers, located in two gNBs connected via a non-ideal backhaul as shown in Fig. 1.73. The gNBs involved in the dual connectivity for a certain UE may assume two different roles, that is, a gNB may either act as a master gNB (MgNB) or as a secondary gNB (SgNB). Under this condition, a UE is con- nected to one MgNB and one SgNB. Under dual connectivity framework, the radio protocol stack that a radio bearer uses depends on how the radio bearer is setup. There are four bearer types in dual connectivity framework, namely MCG70 bearer, MCG split bearer, SCG7¹ bearer, and SCG split bearer as depicted in Fig. 1.74. Under dual connectivity framework, the UE is 