5G Stuff: SeNB Addition, modification and release in Dual Connectivity

Signalling flow for dual connectivity operation in SeNB addition/removal/release

SeNB Addition or modification

SeNB release

5G Stuff: Dual Connectivity basics and various User plane (UP) and Control plane (C or CP) options (part 2)

User Plane (UP) Bearer split options for DC

From the previous post part 1, the below are the 3 options

Option 1: S1-U also terminates in SeNB;

Option 2: S1-U terminates in MeNB, no bearer split in RAN;

Option 3: S1-U terminates in MeNB, bearer split in RAN.

Bearer Split options

For S1-U termination at the MeNB, the protocol stack in the SeNB is at least required to support (re-)segmentation due to the non-ideal back haul used and the (re-)segmentation must take place in the same node as the one transmitting the RLC PDUs. Based on this assumption, four families of U-plane alternatives emerge: 

A. Independent PDCPs

B. Master-Slave PDCPs

C. Independent RLCs


D. Master-Slave RLCs

Based on the options for bearer split and U-plane protocol stack above, we obtain the following alternatives:

-     1A: S1-U terminates in SeNB + independent PDCPs (no bearer split);

-     2A: S1-U terminates in MeNB + no bearer split in MeNB + independent PDCP at SeNB;

-     2B: S1-U terminates in MeNB + no bearer split in MeNB + master-slave PDCPs;

-     2C: S1-U terminates in MeNB + no bearer split in MeNB + independent RLC at SeNB;

-     2D: S1-U terminates in MeNB + no bearer split in MeNB + master-slave RLCs;

-     3A: S1-U terminates in MeNB + bearer split in MeNB + independent PDCPs for split bearers;

-     3B: S1-U terminates in MeNB + bearer split in MeNB + master-slave PDCPs for split bearers;

-     3C: S1-U terminates in MeNB + bearer split in MeNB + independent RLCs for split bearers;

-     3D: S1-U terminates in MeNB + bearer split in MeNB + master-slave RLCs for split bearers.

Out of all the above alternative 1A and 3C are considered due to the flexibility and advantages they have in the implementation (based on the Table 8.1.1.11-1 from 3GPP TR 36842).

UP 1A:

The expected benefits of this alternative are:

-     no need for MeNB to buffer or process packets for an EPS bearer transmitted by the SeNB;

-     little or no impact to PDCP/RLC and GTP-U/UDP/IP;

-     no need to route all traffic to MeNB, low requirements on the backhaul link between MeNB and SeNB and no flow control needed between the two;

-     support of local break-out and content caching at SeNB straightforward for dual connectivity UEs.

The expected drawbacks of this alternative are:

-     SeNB mobility visible to CN;

-     offloading needs to be performed by MME and cannot be very dynamic;

-     security impacts due to ciphering being required in both MeNB and SeNB;

-     utilisation of radio resources across MeNB and SeNB for the same bearer not possible;

-     for the bearers handled by SeNB, handover-like interruption at SeNB change with forwarding between SeNBs;

-     in the uplink, logical channel prioritisation impacts for the transmission of uplink data (radio resource allocation is restricted to the eNB where the Radio Bearer terminates).

UP 3C:

The expected benefits of this alternative are:

-     SeNB mobility hidden to CN;

-     no security impacts with ciphering being required in MeNB only;

-     no data forwarding between SeNBs required at SeNB change;

-     offloads RLC processing of SeNB traffic from MeNB to SeNB;

-     little or no impacts to RLC;

-     utilisation of radio resources across MeNB and SeNB for the same bearer possible;

-     relaxed requirements for SeNB mobility (MeNB can be used in the meantime).

The expected drawbacks of this alternative are:

-     need to route, process and buffer all dual connectivity traffic in MeNB;

-     PDCP to become responsible for routing PDCP PDUs towards eNBs for transmission and reordering them for reception;

-     flow control required between MeNB and SeNB;

-     in the uplink, logical channel prioritisation impacts for handling RLC retransmissions and RLC Status PDUs (restricted to the eNB where the corresponding RLC entity resides);

-     no support of local break-out and content caching at SeNB for dual connectivity UEs.

As per 8.1.2, there is no requirement to have co-ordination between eNBs for PRACH resource and any eNB can respond for PRACH when there is no overlap with Random access preamble transmission. It is required in SeNB addition/modification procedure.

From the UE power consumption point of view, DRX co-ordination is beneficial and expected to have separate DRX timers and active time for MeNB and SeNB.

UE side MAC entity is configured per Cell Group, i.e., one MAC for MCG and another one for SCG.

Control Plane (CP) for DC

In dual connectivity operation, a UE always stays in a single RRC state, i.e., either RRC_CONNECTED or RRC_IDLE. With this principle, the main two architecture alternatives for RRC are the following:

Option C1: Only the MeNB generates the final RRC messages to be sent towards the UE after the coordination of RRM functions between MeNB and SeNB. The UE RRC entity sees all messages coming only from one entity (in the MeNB) and the UE only replies back to that entity.

L2 transport of these messages depends on the chosen UP architecture and the intended solution.

Option C2: MeNB and SeNB can generate final RRC messages to be sent towards the UE after the coordination of RRM functions between MeNB and SeNB and may send those directly to the UE (depending on L2 architecture) and the UE replies accordingly. How and whether to distinguish source and destination RRC entity are FFS. How to route UL messages is FFS. L2 transport of these messages depends on the chosen UP architecture and the intended solution.

Based on the performance comparison C1 is selected as baseline to support the following qualities.

Configuration delay

Synchronization of RRC parameter change

Signalling and processing overload

Complexity in the UE side

Complexity in the NW side

5G Stuff: Dual Connectivity basics and various User plane (UP) and Control plane (C or CP) options (part 1)

Why do we need DC?

In Het Nets, when Macro and small cells are deployed on the same frequency (Scenario 1) /different frequency (Scenario 2) and connected via non-ideal backhaul, some of the below challenges are seen

1. Increased Handover Failures (HOF) or Radio Link Failures (RLF) in mobility between macro and small cells

2. UL/DL imbalance between macro and small cells

3. Increased CN signaling load due to frequent handovers

4. Difficulty in improving per-user throughput even after utilizing resources from more than one eNB

5. Network planning and configuration effort 

When only small cells on one of more carrier frequencies are connected via non-ideal backhaul (Scenario 3), above 1, 3 and 5 challenges are observed.

To address some of the above challenges, one of the solutions proposed is Dual Connectivity (DC).

Below are the various terms used in the context of DC (based on 3GPP TR 36842).

Bearer Split: in dual connectivity, refers to the ability to split a bearer over multiple eNBs.

Dual Connectivity: Operation where a given UE consumes radio resources provided by at least two different network points (Master and Secondary eNBs) connected with non-ideal backhaul while in RRC_CONNECTED.

Master Cell Group: the group of the serving cells associated with the MeNB.

Master eNB: in dual connectivity, the eNB which terminates at least S1-MME and therefore act as mobility anchor towards the CN.

Secondary Cell Group: the group of the serving cells associated with the SeNB.

Secondary eNB: in dual connectivity, an eNB providing additional radio resources for the UE, which is not the Master eNB.

Xn: interface between MeNB and SeNB. Since the current E-UTRAN architecture was selected as baseline in this study, Xn in this TR means X2.

To address the above 4), improve per-user throughput for the scenario 2, Inter-node radio resource aggregation is considered as potential solution. Here, higher user throughput can be achieved by aggregating radio resources in more than one eNB.

Based on the experiments mentioned in 3GPP TR 36842, to achieve higher throughput for UEs in Pico cells, Network should deploy both Pico and macro on different frequencies, as in Scenario B. The gain is achieved due to lack of strong interference in Pico cells from Macro cells.

To achieve higher throughput in macro UEs, Network should deploy both eNBs on the same frequency as in Scenario A.

Simulation scenarios for inter-node radio resource aggregation

RLF challenge or mobility robustness, above 1), could be addressed with RRC diversity. In this case, RRC signalling related to HO could additionally be sent from the potential target cell when the UE is in "handover region or RRC diversity region", this way UE maintains connection with at least one cell and RLF could be prevented as shown below. From the simulation results, it can be seen that this proposal gives significant gains for both Scenario 1 and 2.

RRC Diversity

Another challenge UL/DL imbalance, due to large difference in the transmit power of the macro and pico, is address via load balancing from Macro and Pico cells. Using DC this could be achieved by allowing the UE to connected in DL to the cell which offers highest DL throughput while being connected in UL to the cell which offers highest UL throughput. In this way, network has the flexibility to shift more DL traffic to macro eNB and UL traffic to Pico eNB (when macro eNB is loaded in UL) for both intra and inter frequency cases.

There are other solutions proposed to improve user throughput using CA+eICIC for scenario 2, but down prioritized due to low cost Pico cell deployment.

Another solution being the mobility anchor with the intention to reduce/hide the signalling load towards the CN by hiding the subsequent mobility involving SeNBs which seems independent of DC solution.

Part 2 will explain the architecture and protocol enhancements to realize the solutions described above.

5G Stuff: Control Plane protocol stack architecture for DC between NR and LTE

Below figure describes the control plane protocol stack entities. Here RRC/PDCP/MAC/PHY are terminated at UE and gNB side and NAS is terminated at NextGen CP.

For DC between LTE and NR, any RAT can be Master node and other can be Secondary node. Secondary Node owns  its radio resources and its responsibility to assign them to cells. To support this, some kind of co-ordination between Master and Secondary nodes are expected.

UE has a single RRC state machine and is based on Master node RAT. Depends on the network implementation, UE can be communicating with both Nodes or only to Master node.

When C-Plane established only with Master node
       In this case, RRC PDUs and Inter node PDUs from Secondary Node are sent to Master Node and these are embedded by Master Node along with its RRC PDUs when transported to UE. 

When C-Plane established with both nodes
       In this case, SRB can be configured at UE with SCG  so that RRC PDUs can be sent to the UE directly from the Secondary Node. No co-ordination with Master node is needed.

Below one shows the control plane for DC between NR and LTE, when either LTE being the Master node or NR being the Master node along with UEs' RRC State. This structure is largely based on Option C1 as mentioned in 3GPP TR 36842 V12.0.0

5G Stuff: Bearer types in for DC between NR and LTE

Few words about DC:
         When UE gets connected to 2 or more eNBs at the same time for user plane, one being the Master eNB (MeNB) and the rest are called Secondary eNBs (SeNB). Depends on User plane (UP) architecture, bearer level or packet level splits are implemented in the RAN/CN.

Similarly to support DC between NR and LTE, the following bearer types are possible

         - Split bearer via MCG (Master Cell Group)

         - SCG bearer

         - Split bearer via SCG (Secondary Cell Group)

The following reconfiguration of bearer types are supported

-     reconfiguration between an SCG bearer and an MCG bearer;

-     reconfiguration of an SCG bearer between two secondary nodes;

-     reconfiguration between an MCG bearer and an MCG split bearer.

5G Stuff: Radio Interface Protocol Architecture

Dual Connectivity (DC) seems to play important role in inter-working between LTE and NR. In this case, any RAT can act as master node, as described in earlier posts

To ease the implementation, i assume, LTE eNB is not synchronized with NR gNB in DC between LTE and NR deployment case.

Like in LTE, NR also supports Carrier Aggregation of carriers at lower layers. It is also referred DC as upper layer aggregation.

NR user plane protocol stack (below) containes New AS sublayer along with PDCP/RLC/MAC/PHY at UE side and they are terminated at gNB on the network side. 

5G Stuff: NG-RAN Architecture elements

The below figure shows NG-RAN (NextGen - RAN) architecture. It consists of

          - NG-RAN consists of gNBs, providing both control and user data plane terminations towards UE.
                   - Here gNBs are interconnected with Xn interface

          - NGC (NextGen Core) consists of AMF/UPF
                   - Here gNBs and AMF/UPF are interconnected with NG-CU interface

gNB has similar functionalities, like eNB in LTE. They are below (as per 3GPP TR 38.804 V14.0, sec 5.1)
       - Radio Resource Management
       - IP header compression and encryption of user plane data
       - selection of AMF during UE attachment when no routing to an AMF can be determined from the information provided by the UE
       - Routing of User plane data to UPF
       - Scheduling and transmission of paging messages, System Broadcast Infos 
       - Measurement and Measurement reporting config

AMF Functionality can be as follows
      - NAS signalling termination;

      - NAS signalling security;

      - AS Security control;

      - Inter CN node signalling for mobility between 3GPP access networks;

      - Idle mode UE Reachability (including control and execution of paging retransmission);

      - Tracking Area list management (for UE in idle and active mode);

      - AMF selection for handovers with AMF change;

      - Access Authentication;

      - Access Authorization including check of roaming rights.

UPF:

-     Anchor point for Intra-/Inter-RAT mobility (when applicable);

-     External PDU session point of interconnect to Data Network;

-     Packet routing & forwarding;

-     Packet inspection and User plane part of Policy rule enforcement;

-     Traffic usage reporting;

-     Uplink classifier to support routing traffic flows to a data network;

-     Branching point to support multi-homed PDU session;

-     QoS handling for user plane, e.g. packet filtering, gating, UL/DL rate enforcement;

-     Uplink Traffic verification (SDF to QoS flow mapping);

-     Transport level packet marking in the uplink and downlink;

-     Downlink packet buffering and downlink data notification triggering.

SMF:

-      Session Management;

-     UE IP address allocation and management;

-     Selection and control of UP function;

-     Configures traffic steering at UPF to route traffic to proper destination;

-     Control part of policy enforcement and QoS;

-     Downlink Data Notification.

5G Stuff: Definitions and Abbreviations used in 5G NR

As per various 3GPP specifications, following (list will be growing as I read more and more) are definitions and abbreviations related to 5G Technology

gNB:      NR node

K_eNB:      eNB key

NextGen Core: Core Network for Next Generation System.

NG:        The interface between a gNB and a NextGen Core.

Multi-Connectivity:           Mode of operation whereby a multiple Rx/Tx UE in the connected mode is configured to utilise radio resources amongst E-UTRA and/or NR provided by multiple distinct schedulers connected via non-ideal backhaul

S-K_eNB:   SeNB key

Transmission Reception Point:      Antenna array with one or more antenna elements available to the network located at a specific geographical location for a specific area.

NR-PSS/SSS:        Primary and Secondary synchronisation signal for NR.

AMF                       Access and Mobility Management Function

CA                          Carrier Aggregation

CSI-RS                  Channel State Information Reference Signal

DC                          Dual Connectivity

MCG                      Master Cell Group, has one Pcell and one or more Scells

MN                         Master Node

NG-U                     NG for the user plane

NR                          New Radio

Pcell                      Primary cell, one of the cells belong to MCG
PSCell                    Primary SCell, one of the cells belong to SCG, also configured UL CC in DC

SCG                       Secondary Cell Group, has one PScell and one or more SScells

SeNB                     Secondary eNB
SMF                       Session Management Function

SN                          Secondary Node

TRxP                     Transmission Reception Point
UDM                       Unified Data Management
UDR                        Unified Data Repository
UPF                        User Plane Function

URLLC                 Ultra-Reliable and Low Latency Communications

WT                         WLAN Termination
5GS                      5G System

Other terminologies taken from the existing technologies

Co-located cell: Both 5G NR and LTE ( cells  can be macro or small) eNBs located in the same location

Non-located cell : 5G NR and LTE ( again cells  can be macro or small) eNBs located in different location 

Cells belong to MCG are connected/controlled by MeNB, similarly cells belong to SCG are connected/controlled by SeNB.

5G Stuff: Cell topology and CN-RAN connection in 5G NR

Like in earlier cellular technologies, 5G is also assumed to support the following cell layouts

 - Homogeneous, where all cells provide similar coverage and these can be macro or small cells

 - Heterogeneous, where cells can be of different sizes and may be overlapped, like macro and small cells

Depends on the operator requirements, one of the above can be used while deploying 5G NR along with LTE.

As one option, both LTE and 5G NR cells are over-laid and co-located to provide the same coverage. In this case, all cells are in homogeneous layout and the eNodeBs co-located.

In another case, due to LTE co-existence various combinations are to be considered:
     
        - both LTE and 5G NR cells co-located and overlaid
        - both LTE and 5G NR cells non co-located and overlaid

For ex: when LTE is used to serve as macro, 5G NR can be used as small.

If needed, 5G NR is used to serve as macro and LTE can be used as small.

Depends on the deployment scenarios following CN-RAN connections are possible

1. NR gNB as master node and is connected  to NextGen Core
         - Standalone NR gNB deployment
         - NR gNB (macro cell) is also connected to eLTE eNB (small cell and only User plane is connected to NextGen Core) to support Data flow aggregation
         - NR gNB (macro cell) is also connected to another NR gNB (small cell and only User plane is connected to NextGen Core) to support Data flow aggregation

2. eNB as master node and connected to EPC
         - LTE eNB (macro cell) is also connected to NR gNB (small cell and only User plane is connected to EPC) to support Data flow aggregation

3. LTE eNB as master node and is connected to NextGen Core
         - Standalone eLTE eNB is connected to NextGen Core
         - LTE eNB (macro cell) is also connected to NR gNB (small cell and only User plane is connected to NextGen Core) to support Data flow aggregation

4. Mobility between 5G NR and LTE system
        - eLTE eNB is connected to EPC and 5G NR gNB is interfaced to NextGenCore. Here EPC and NextGen Core may be connected (For Further Study - FFS?)
        - eLTE eNB and NR gNB are connected to NextGen Core

5. WLAN integration
        - WLAN interworking with NR via NextGen Core

        - WLAN aggregation with NR via NextGen Core

        

More details can be found in 3GPP TR38.804 V14.0, 4.1.2/4.1.3