MR-DC?

What is MR-DC?

MR-DC (Multi-RAT Dual Connectivity) is similar to Intra LTE (E-UTRA) Dual Connectivity in which UEs with multiple TRX configured to utilize resources provided by two different node (eNB/gNB) which are connected via non-ideal backhaul. One node provides E-UTRA access and the other NR access.

Out of two nodes one node acts as a Master Node (MN) and the other as Secondary Node (SN). At least one node (MN) have access to or connected to Core network.

What is an Ideal Backhaul and non Ideal Backhaul?

Ideal Backhaul : Very High Throughput and Very Low Latency backhaul such as dedicated Point-to-Point connection using optical fiber.

An Ideal Backhaul is defined as Latency less than 2.5µs and throughput up to 10Gbps. Usually between eNB and RRH (Remote Radio Head)

Backhaul Technology	Latency (One way)	Throughput	Priority (1 is the highest)
Fiber Access 4 (NOTE 1)	less than 2.5 us (NOTE2)	Up to 10Gbps	1

Non-Ideal Backhaul: This is the typical backhaul widely used in the market for example xDSL, Microwave link, relaying ..etc

In simple, any backhaul which does not satisfy above requirements (Latency and Throughput) are considered as Non-Ideal Backhaul.

Backhaul Technology	Latency (One way)	Throughput	Priority (1 is the highest)
Fiber Access 1	10-30ms	10M-10Gbps	1
Fiber Access 2	5-10ms	100-1000Mbps	2
Fiber Access 3	2-5ms	50M-10Gbps	1
DSL Access	15-60ms	10-100 Mbps	1
Cable	25-35ms	10-100 Mbps	2
Wireless Backhaul	5-35ms	10Mbps – 100Mbps typical, maybe up to Gbps range	1

Based on the core network access, MR-DC can be as below.

(i) MR-DC with EPC and
(ii) MR-DC with 5GC (work still ongoing)

Let's see how option (i) is implemented.

Below picture is self explanatory.

EN-DC, E-UTRA-NR Dual Connectivity where an UE is connected to one eNB which acts as a Master Node (MN) and one en-gNB acting as Secondary Node (SN).

eNB is connected to EPC via S1-U interface where as it is optional for the en-gNB to connect to EPC. eNB and en-gNBs talk to each other over X2 interface.

MR-DC with 5GC :

It is basically to which node the UE is connected to and the Core Network type that the MN is connected to. In this case the Core Network type is 5GC. UE can be connected to either gNB node or ng-eNB.

(a). E-UTRA-NR DC and

(b). NR-E-UTRA DC

Ref :
1. 3GPP 37.304
2. 3GPP 36.932