# Dual Connectivity

## Summary


Dual Connectivity (DC) is a feature that was first introduced in LTE (Release 12) to allow a UE to be connected to two eNBs and send/receive packets via both eNBs. The eNBs are connected via a non-ideal backhaul. 

In 5G (Release 15), this concept was expanded so that a UE can be connected to both LTE E-UTRA and 5G NR nodes. The Core Network (CN) is either LTE EPC or 5G Core. This was later expanded so that both cells can belong to 5G NR, in which case the CN is exclusively 5G Core. These various options come under the general term **Multi-Radio Dual Connectivity (MR-DC)**. MR-DC is a generalization of Intra-E-UTRA Dual Connectivity. 

MR-DC can offer a UE more resources for higher throughput. More commonly, it helps operators improve mobility robustness and handovers in macro/micro-cell deployments. It aids in migrating their networks from 4G to 5G.

## Discussion

### Why do we need MR-DC?

UL/DL separation, user/control plane separation, and user plane traffic aggregation via multiple nodes are some benefits of DC. DC can help operators integrate WLAN with their cellular networks. DC can also be used for redundancy for URLLC services with traffic duplication. Duplicated traffic may go via the same node or a second node. 

An E-UTRA node can provide mobility and initial access to the UE while an NR node joins in later to offer higher throughput. Essentially, E-UTRA master node handles the control plane and both E-UTRA and NR nodes handle the user plane. Thus a more stable LTE macro layer is overlaid on top of a high-throughput NR small cell layer. We could say that LTE provides the anchor while 5G provides more bandwidth. 


### How can MR-DC help operators in their 4G-to-5G migration?

With **Non-Standalone (NSA)**, operators can extend the life of their 4G equipment while deploying 5G. Regardless of the type of Core Network, NSA allows both 4G and 5G nodes to interwork. These options are named Option 3 (and variants 3a and 3x), Option 4 and Option 7.  

The path of least investment is probably Option 3. Operators deploy only 5G gNB nodes to roll out 5G NR. These connect to EPC core via existing 4G eNB nodes. Thus, neither 5G core nor new backhauls need to be installed. A gNB connects to an eNB via a cheaper non-ideal backhaul, called the X2 interface. 

From Option 3, an operator could migrate the core from EPC to 5G Core (Options 4 or 7). Eventually, all eNBs can be replaced with gNBs, leading to **Standalone (SA)** deployment Option 2. In any case, eNBs and gNBs could coexist for a long time since Dynamic Spectrum Sharing (DSS) allows spectrum to be used flexibly between 4G and 5G. 


### What's the terminology used in MR-DC?

In MR-DC, a UE is connected to two nodes. One is called the **Master Node (MN)** and the other is called the **Secondary Node (SN)**. MN is the node to which the UE first connects. Subsequently, using RRC signalling messages via the MN, UE connects to the SN.

Historically, only MN provided the control plane connection between the UE and the Core Network (CN). SN provided only additional resources to carry user plane traffic. However, with the later introduction of split SRB and SRB3, SN can also carry signalling messages.

A MN can be an eNB (in EN-DC), a ng-eNB (in NGEN-DC) or a gNB (in NR-DC and NE-DC). Similarly, a SN can be en-gNB (in EN-DC), a ng-eNB (in NE-DC) or a gNB (in NR-DC and NGEN-DC). 

MR-DC can be used alongside Carrier Aggregation (CA). Thus, both MN and SN may be associated with multiple cells or carriers. These aggregated carriers are collectively called **Master Cell Group (MCG)** and **Secondary Cell Group (SCG)**. Each group has one primary cell (SpCell) and potentially many secondary cells. The primary cell of MCG is called PCell and of SCG is called PSCell. It carries the PUCCH. 


### What are the different configuration options in MR-DC?

MR-DC has four configurations: 

    + **EN-DC (E-UTRA-NR Dual Connectivity)**: UE connects to a LTE eNB as MN and a 5G en-gNB as SN. CN is LTE EPC. en-gNB may connect to EPC via S1-U interface.
    + **NGEN-DC (NG-RAN E-UTRA-NR Dual Connectivity)**: UE connects to ng-eNB as MN and gNB as SN. CN is 5GC.
    + **NE-DC (NR-E-UTRA Dual Connectivity)**: UE connects to gNB as MN and ng-eNB as SN. CN is 5GC.
    + **NR-DC (NR-NR Dual Connectivity)**: UE connects to one gNB as MN and another gNB as SN. Alternatively, a single gNB can function as both MN and SN. CN is 5GC.EN-DC corresponds to Option 3, NGEN-DC to Option 4, NE-DC to Option 7, and NR-DC to Option 2. 

We note that EN-DC uses LTE EPC whereas other MR-DC configurations use 5G Core. MN and SN are connected via the X2 interface (in EN-DC) and Xn interface (NGEN-DC, NE-DC, NR-DC). MN must have a connection to the CN. MN and SN must be connected even if it's a non-ideal backhaul. 


### What's the difference between synchronous and asynchronous DC?

LTE specifies both synchronous and asynchronous DC. Synchronous DC implies that the two eNBs involved in DC need to be time synchronized. UE can cope with 33μs of maximum reception time difference and 35.21μs maximum transmission time difference. With asynchronous DC, no such time synchronization is required. 

Likewise, synchronous and asynchronous DC are applicable for MR-DC configurations. Asynchronous NR-DC was specified in Release 16. 


### What bearers are used in MR-DC?

An MCG bearer is a radio bearer with an RLC bearer only in MCG. Likewise, a SCG bearer has an RLC bearer only in SCG. A Split Bearer is a radio bearer with RLC bearers in both MCG and SCG. Where the bearer is used for signalling, we use the terms MCG SRB, SCG SRB and Split SRB. 

In the control plane, RRC PDUs from SN go via MN unless SRB3 is configured. SRB3 is possible only when SN is a gNB or en-gNB. With SRB3, RRC PDUs can be sent directly from SN to UE. With Split SRB, RRC PDUs from MN can be duplicated and sent via both MCG and SCG. Split SRB is possible for all MR-DC configurations. RRC PDUs from SN can't be duplicated this way. 

In the user plane, MCG/SCG/Split bearer may be terminated either in MN or SN. For example, an SCG bearer can be terminated in MN by passing PDCP PDUs between SN RLC and MN PDCP over the X2 or Xn interface. With split bearers, PDCP does routing and duplication. 


### What's the higher layer signalling for MR-DC?

The UE has a single RRC state based on the MN RRC state. UE connects to the CN via single control plane connection. Initial access (SRB0) and RRC configuration (SRB1) are via the MN but later reconfigurations can be from either MN or SN. EN-DC starts with E-UTRA PDCP but this can be reconfigured to use NR PDCP. 

If there's a link failure in SCG but MCG link is fine, no re-establishment procedure is triggered. Release 16, extended this concept to include fast MCG link recovery; that is, even when MCG link fails but SCG link is fine, no re-establishment procedure will be triggered. Instead, SCG link is used to recover MCG link. 

Full details of signalling for Intra-E-UTRA DC and EN-DC are specified in TS 36.300. RRC signalling for other MR-DC configurations are specified in TS 38.331. 


### How is uplink power control achieved in MR-DC?

Uplink power control procedures defined for E-UTRA and NR continue to be applicable with some additional constraints. The UE is required to conform to the configured maximum transmission power in each cell group. UE is also configured with total maximum transmission power for the applicable MR-DC configuration. Power sharing can be dynamic or semi-static. 

In Intra-E-UTRA DC, power control modes 1 and 2 are available. UE will reduce its power in SCG if subframes in MCG and SCG overlap and total power exceeds a threshold. 

In EN-DC and NE-DC, if the UE supports dynamic power sharing, and \(P\_{MCG}(i\_1) + P\_{SCG}(i\_2) > P\_{Total}\) for slots \(i\_1\) and \(i\_2\), then UE reduces in the power in the NR node (MCG or SCG) so that total transmit power doesn't exceed the total configured value. 

In NR-DC, power control is done independently for MCG and SCG when they use different ranges (FR1 and FR2). Where inter-CG power sharing is applicable, transmission powers for MCG and SCG are determined per frequency range. 


### How is DC different from CA?

DC and CA are not mutually exclusive and can be combined. In both DC and CA, a UE is connected to more than one cell. With CA, all cells belong to the same node (eNB or gNB). Scheduling decisions are made jointly for all cells. With DC, the co-ordination between MCG and SCG is a lot looser and scheduling decisions are made in different nodes. 

Cells from different radio access technologies (RATs) can be used for DC but not for CA. DC is essential for 5G Non-Standalone operation but CA is not. 

A UE has a single MAC entity for CA. For DC, a UE has two MAC entities. A DC UE also requires 2 TX. In CA, MAC splits the traffic among Component Carriers (CCs). In DC, traffic is split at PDCP. 

All traffic due to CA are transported via ideal backhauls. This is not the case with DC. Non-ideal backhauls (with higher latency) may be used for the X2 or Xn interface. In fact, it's been said that, 

> Dual connectivity can be seen as carrier aggregation extended to the case of non-ideal backhaul.


### What are the engineering challenges with implementing MR-DC?

Self-interference is one of the challenges. In DC, a UE will be transmitting on multiple carriers. Their intermodulation products may fall within the receiver band. To avoid this interference, RF circuitry will need to be designed with less non-linearity, which may increase cost and power consumption. One way to avoid this problem is to constrain the UE to **single-TX DC** for some band combinations. UE is not allowed to do simultaneous transmissions towards eNB and gNB. Schedulers in these nodes coordinate to satisfy this constraint. 

The figure shows an example of B41/n41 EN-DC spectral mask measured at one of the two antennas. We can see that the transmission fails spectral mask requirements due to low frequency intermodulation product. 


### Which are the main specifications relevant to Dual Connectivity?

Overall descriptions of Dual Connectivity can be found in **TS 36.300** (Intra-E-UTRA DC) and **TS 37.340** (MR-DC involving E-UTRA and NR, or NR only).  

For Intra-E-UTRA-DC, DC valid band combinations are specified in **TS 36.101**, section 5.5C. For example, *DC\_1-20* combines DL bands 2110-2170 MHz with 791-821 MHz (and their paired UL FDD bands). 

Valid band combinations for EN-DC, NGEN-DC and NE-DC are specified in a series of documents: **TS 37.716**, **TS 37.717** (Release 17), and **TS 37.718** (Release 18). Likewise, for NR-DC we have **TS 38.716**, **TS 38.717** and **TS 38.718**. These documents also include bands for inter-band Carrier Aggregation (CA). Some band combinations may result in self-interference and these are noted in these documents. 

## Milestones

Dec  
2013

As part of Release 12 work, 3GPP publishes the report **TR 36.842** that describes small cell enhancements. Dual connectivity is introduced in this report as one of the possible solutions. 

Mar  
2015

Release 12 of LTE-Advanced is largely completed and functionally frozen. This release introduces **Intra-E-UTRA Dual Connectivity**.  

Aug  
2015

Ericsson in partnership with KT demonstrates Dual Connectivity. The demo uses two LTE cells, a macro cell and a small cell. 

Mar  
2016

Release 13 3GPP specifications is published. This introduces **LTE-WLAN Aggregation (LWA)**. Although called aggregation, it's more like DC than CA. This is the first time that a UE can connect simultaneously to two different RATs. 

Dec  
2017

3GPP publishes Release 15 "early drop". **Dual connectivity between E-UTRA and NR** is supported in this release. This enables DC in Non-Standalone deployment scenarios but not in Standalone scenarios. 

Jun  
2018

3GPP publishes Release 15 "main drop". **NR-DC** is supported in this release, thus enabling DC even in 5G Standalone deployment scenarios. 

May  
2020

Nokia claims to achieve a world record speed of 4.7 Gbps on a commercially deployed network in the U.S. This is achieved using EN-DC with 40 MHz of LTE spectrum and 8 x 100 MHz of millimeter wave spectrum in 28 GHz and 39 GHz bands. A 5G cloud RAN is used for this test. 

Jun  
2020

Release 16 is completed and functionally frozen. This includes many enhancements or additions to DC including idle/inactive mode measurements, SCG/SCell configuration during transition from idle/inactive state, dormant SCell in NR, fast recovery from MCG link failure, and support for asynchronous NR-DC operation. UL Tx Switching (EN-DC) is another new feature in this release. 

Apr  
2021

Ericsson and MediaTek demonstrate NR-DC and achieve 5.1 Gbps using 8 x 100 MHz (@ 28 GHz, n261) and 60 MHz (@ 3.7 GHz, n77). In December 2021, Ericsson and Singtel achieve 5.4 Gbps using NR-DC at 3.5 GHz (mid-band) and 28 GHz (mmWave). This sort of speed enables high-performance low-latency applications including immersive gaming, AR/VR, autonomous vehicles and robotic control.