EN-DC Overview
E-UTRA New Radio-Dual Connectivity (EN-DC) is a 3GPP release 15 feature introduced to support New Radio 5G data with existing LTE core and radio networks and without introducing 5G core network. EN -DC can be a useful feature for Heterogeneous networks (HetNets) where LTE provides reliable coverage and NR can be used for improving data rates. EN-DC ensures better system reliability by reducing service interruptions due to higher propagation loss in mmwave or non-line of sight situations in Massive MIMO (beam sweeping)*
EN-DC Elements
Master Node (LTE):
Master Node or eNB(LTE) in EN-DC is used to access EPC on control plane and user plane by the user equipment(UE). It is used for:
Maintaining connection state transitions
Handling connection setup/ release
Initiating first time secondary node addition (EN-DC setup)
User data over LTE RAN
Secondary Node (NR):
Secondary Node or gNB(NR) in EN-DC is used fo r user Data only. It is connected to LTE EPC only on user plane
UE Device:
A UE device supporting EN-DC should be at least 3GPP release 15 compliant. The UE will have 2 RRC termination points (LTE and NR). Having 2 RRC termination points enable secondary n ode to trigger intra-NR mobility i.e. change/release/modification of secondary Node.
Signalling Radio Bearers(SRB)
Master Cell Group (LTE):
Cell(s) part of LTE network will be part of Master cell group (MCG). SRB1 and SRB2 will be used for reconfiguration including Secondary cell group (SCG) addition.
Split Signalling Radio Bearers (LTE+NR):
EN-DC gives an option to use split bearers. The concept of split bearers enables operators to transmit SRBs via LTE radio network and/or NR 5G radio network. See Figure[2].
Below are the routes a signalling message can take in split bearer configuration:
Master Node RRC messages via RLC, MAC and PHY of either the master node or second ary node.
Master Node RRC messages via RLC, MAC and PHY of both the master node and secondary node.
Master Node RRC message can also embed Secondary Node RRC message.
Split SRB to improve mobility robustness especially for NR high frequency bands.
R RC mes sage can be exchanged via both LTE and NR lower layers for
Duplicating the message on both paths
Selecting the best radio path
Secondary Cell Group (NR):
Signalling message can be transmitted to UE devices using SRB3 via NR5G radio network . It can be use d for SCG reconfiguration in DL and measurement reporting in UL by the UE
Data Radio Bearers(DRB)
Master Cell Group DRBs use only master node lower layers
In Master Cell Group Split DRBs , bearers terminate at master node. The data path can use wither LTE and/or NR lower layers.
Secondary Cell Group DRBs , will use only secondary node lower layers
In Secondary Cell Group Split DRBs , bearers will terminate at NR node and can use lower layers of both LTE and/or NR
Below are some basic differences between LTE DC and EN-DC
ASN Configuration
As mentioned previously, SRB1 on LTE network will be used to add NR SCG for EN-DC. The RrcReconfiguration message will include IEs for NR setup. The RRCReconfiguration message is 36.331 compliant. NR specific configuration is sent in HEX string .
For e.g.
3GPP 38.313 is required to decode nr-RadioBearerConfig2-r15 and nr-SecondaryCellGroupConfig-r15. Configuration related to DRB parameters is included in nr-RadioBearerConfig2-r15
For e.g.
Below are the Need codes and conditions in NR
nr-SecondaryCellGroupConfig-r15 IE will include all common and dedicated parameters for the NR leg of EN-DC. A representative example is shown below
For detailed NR IEs, refer to 3GPP 38.331.
EN-DC Call Setup
For EN-DC to setup, the UE must attach to LTE n/w first. Refer to call flow diagram below –
