Читать книгу NG-RAN and 5G-NR - Frédéric Launay - Страница 12

Option 3 is the non-standalone EN-DC configuration.

Option 3 uses the MN (Master Node) terminated MCG (Master Cell Group) bearer for signaling: the eNB is the master node, and the gNB (gNodeB) acts as the secondary node. The radio access network is connected to EPC.

The 4G base station (eNB) controls the 5G base station (en-gNB) through the X2 interface.

The eNB supports signaling with the MME (Mobile Management Entity) through the S1-MME interface and supports the user plane traffic (MCG bearer) with the SGW (Serving Gateway) entity through the S1-U interface.

The en-gNB base station supports signaling with the eNB. The 5G-NR interface is activated by the eNB over the X2 interface. Once activated, en-gNB controls its own radio resource allocation. The user traffic is either transmitted from the eNB to en-gNB or transmitted from the 4G core network (SGW) over the S1-U interface to en-gNB.

The master node eNB exchanges data in both directions, uplink and downlink, with the mobile.

The secondary node en-gNB allows us to increase both uplink and downlink data rates.

With a DC mechanism, data is transmitted to the mobile according to one of the following variations (Figure 1.3):

1 – option 3: in plain option 3, all uplink and downlink data flows to and from the LTE part (MCG split bearer) of the LTE/NR base station, i.e. to and from the eNB. The eNB then decides which part of the data it wants to forward to the 5G gNB part of the base station over the Xx interface;

2 – option 3a: both LTE eNG (MCG bearer) and 5G-NR en-gNB (SCG bearer) exchange traffic to the 4G core network directly. This means that a data bearer allocated to a node cannot share its load over the second node. This option does not suit the case of mobile use;

3 – option 3x: user data traffic will directly flow to the 5G gNB part of the base station (SCG split bearer). The traffic is delivered over the 5G-NR interface to the device, and part of the data can be forwarded over the X2 interface to the 4G eNB.

Figure 1.3. Secondary node addition – option 3

Option 3 uses the MN terminated MCG bearer for user traffic. The eNB entity splits the S1 bearer into:

1 – LTE radio support;

2 – NR support.

Option 3x uses the SN terminated SCG bearer for user traffic. The gNB entity splits the S1 bearer into:

1 – LTE radio support;

2 – NR support.