Dual Connectivity — Practical Aspects
Introduction
You may wonder why I’m writing about Dual Connectivity. After all, it comes from LTE in Rel-12, when we are currently within the Rel-16/17 timeframe. The reason for this is that the DC from LTE is treated as a baseline. The enhanced version is incorporated within 5G as one of the main features to allow the so-called Multi-RAT DC (MR-DC). If we look from a broader perspective, we have a multitude of use cases for DC, including LTE-DC, NR-DC, LTE-NR-DC, NR-LTE-DC, LWA. Therefore, this post aims at shedding light on the different aspects of DC, being the legacy feature, which can be extrapolated towards the current situation.
Dual Connectivity (DC)
Dual Connectivity (DC) is an LTE Rel-12 feature that enables aggregation of two radio links with non-ideal backhaul without low-latency requirements. To allow this, the links are aggregated at the PDCP level, combining PDCP PDUs. This is different compared to CA (Carrier Aggregation) that combines MAC-layer blocks. The figure below shows the differences between DC and CA.
Dual Connectivity Operation
Dual Connectivity procedures expand the mobility framework from a handover between two cells, incorporating the following procedures [1]:
- SeNB addition
- SeNB modification (intra MeNB handover involving SCG change)
- SeNB release
- Change of SeNB
- MeNB to eNB change (DC-to-Single Connectivity)
- eNB to MeNB change (Single Connectivity-to-DC)
- Inter-MeNB handover without SeNB change
- In the Split bearer case, DC requires flow control i.e., additional RRM algorithm, scheduling PDCP PDUs to different links;
- S cheduling of individual packets at each link is performed separately and independently of each other and thus can be optimized at each link according to individual channel characteristics;
- The advantage of this design is that the signa l ing overhead can be decreased by means of reduction of the number of handovers, as the CP/UP split concept can be realized by keeping the user context at MeNB , while flexibly allocating radio resources among MeNB and different SeNBs ;
- There is a single link for signal ing , always at MeNB . T here is a risk of dropping the connection when the MeNB link deteriorates, even if SeNB has a good channel quality;
- The Traffic Steering/Mobility Load Balancing framework is expanded from the intra/inter-frequency/inter-RAT handover towards a more holistic framework with multiple options .
Example DC Configuration
Pre-requisites needed for SCG activation :
- To perform addition/deletion
- To share load balancing info
- PCI addition of SCG (SeNB) in the DC feature configuration data
- A4 Events Parameters (used for addition process)
- Count for Users who can use DC features
- UE Capability to support DC feature (EUTRAN Bands for DC)
Addition/Deletion of SCG/ SeNBs may happen based on the following conditions:
References
[1] 3GPP TS 36.300
[2] M. Rahnema, M. Dryjanski, “From LTE to LTE-Advanced Pro and 5G”, Artech House, 2017.
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Originally published at https://www.grandmetric.com on May 26, 2020.
