WO2021007721A1

WO2021007721A1 - Apparatus and method to control mobility procedure to reduce handover interruption

Info

Publication number: WO2021007721A1
Application number: PCT/CN2019/095854
Authority: WO
Inventors: Yuanyuan Zhang; Chun-Fan Tsai; Li-Chuan Tseng; Chia-Chun Hsu
Original assignee: Mediatek Singapore Pte. Ltd.
Priority date: 2019-07-12
Filing date: 2019-07-12
Publication date: 2021-01-21

Abstract

Apparatus and methods are provided to support dual protocol stacks to achieve 0ms mobility interruption time in LTE and NR system. In one novel aspect, a UE support both normal type of PDCP entity and a new type of PDCP entity. The 'extended type' of PDCP entity includes two security-handling modules and is able to perform integrity verification and deciphering separately for both the source cell and the target cell. In one embodiment, upon reception of the HO command, UE keeps the UP protocol for the source cell. UE creates a MAC entity for the target cell. Then UE modifies/extends the PDCP entities to include more PDCP functions. UE establishes RLC entities for all the DRBs established. In one embodiment, UE only establishes RLC entities for the DRBs for which extendPDCPis set. Upon reception of the reconfiguration message to release the source cell connection, UE releases the RLC entities and the associated logical channels of the source cell and reconfigure the PDCP entity in accordance with the pdcp-Config, i.e. change the extended PDCP entity to normal PDCP entity.

Description

APPARATUS AND METHOD TO CONTROL MOBILITY PROCEDURE TO REDUCE HANDOVER INTERRUPTION

TECHNICAL FIELD

The disclosed embodiments relate generally to wireless communication, and, more particularly, to reduce mobility interruption time through dual protocol stacks in the new radio access system.

BACKGROUND

5G radio access technology will be a key component of the modern access network. It will address high traffic growth and increasing demand for high-bandwidth connectivity. It will also support massive numbers of connected devices and meet the real-time, high-reliability communication needs of mission-critical applications. Both the standalone NR deployment and non-standalone NR with LTE/eLTE deployment will be considered. In order to improve the UE experience quality, it’s desirable to reduce the mobility interruption time during handover. Mobility interruption time means the shortest time duration supported by the system during which a user terminal cannot exchange user plane packets with any base station during transitions. The target for mobility interruption time should be 0ms, which is intended for both intra-frequency and inter-frequency mobility for intra-NR mobility.

As shown in table 1, in LTE (Rel-8/9) , the latency during handover execution is nearly 50 ms from step 7 (RRCConnectionReconfiguration) to step 11 (RRCConectionReconfigurationComplete) , which cannot satisfy the mobility interruption requirement in NR. In order to minimize the service interruption in mobility events, two solutions are considered, i.e. RACH-less handover and “make-before-break” handover. In RACH-less HO, RACH procedure can be skipped during handover. Although the interruption time can be reducedby4.5/8.5ms with RACH-less HO without performing from step 9.3 to step 10, addition interruption is expected before the preconfigured periodical UL resource is available. The make-before-break solution means that UE continues downlink and uplink with the source cell until the UE performs the first transmission through PUSCH or PRACH to the target eNB. In “make-before-break” HO, UE can continue the data transmission with the source cell after receiving the handover command until RACH is initiated. The interruption time can be reduced by 35ms, since data transmission continues with source cell during the HO execution from step 7 to step 9.2. Although the interruption is reduced by 35ms, the interruption due to random access procedure and delivering RRCConnectionReconfigurationComplete message can’t be avoided.

Table 1: Minimum/Typical radio access latency components (Rel. 8/Rel. 9) during handover

In this invention, apparatus and mechanisms are sought to reduce mobility interruption time during HO through dual active protocol stacks to 0ms or close to 0ms.

SUMMARY

Apparatus and methods are provided to support dual active protocol stacks to reduce mobility interruption time in both LTE and NR system. In one novel aspect, a UE support both normal type of PDCP entity and a new type of PDCP entity. It is called as ‘extended’ PDCP entity. AUE reconfigure/modify/extend the normal PDCP entity to the extended PDCP entity upon reception of a particular HO command from the network. The particular HO command includes a flag, indicating to perform HO with dual active protocol stack. The ‘extended type’ of PDCP entity includes two security handling modules and is able to perform integrity verification and deciphering separately for both the source cell and the target cell. In another embodiment, the ‘extended type’ of PDCP entity further includes two header decompression modules and is able to perform header decompression separately for both the source cell and the target cell.

In one embodiment, upon reception of the HO command, UE keeps the UP protocol for the source cell. UE creates a MAC entity for the target cell. Then UE modifies/extends the PDCP entities to include more PDCP functions. The normal type of PDCP entity is changed to an ‘extended type’ of PDCP entity. In one embodiment, UE modifies the PDCP entities for all the DRBs established. In one embodiment, UE modifies the PDCP entities for the DRBs if the extendPDCP is set. UE establishes additional RLC entities for the DRBs and associates the RLC entities to the corresponding extended PDCP entity. In one embodiment, UE establishes RLC entities for all the DRBs established. In one embodiment, UE only establishes RLC entities for the DRBs for which extendPDCPis set. UE derives the security keys of the target cell. UE configures the ‘extended PDCP’ entity to apply the new keys of the target cell for the data transmission/reception to/from the target cell and continues to use the old keys of the source cell for data transmission/reception to/from the source cell.

In one embodiment, UE extends the PDCP entities for all the DRBsconfigured with pdcp-config that are established upon reception of the HO command with dualActiveProtocol is configured. UE establishes a new MCG RLC entity and an associated DTCH logical channel. UE associates theseRLC entities with the extended PDCP entities with the same value of drb-Identity within the current UE configuration.

In one embodiment, the RRC reconfiguration message includes a flag e.g. releaseSourceCell to release the connection with the source cell. In one embodiment for DRB addition/modification (e.g. in LTE) , UE releases the RLC entities and the associated logical channels of the source cell and reconfigure the PDCP entity in accordance with the pdcp-Config, i.e. change the extended PDCP entity to normal PDCP entity.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, where like numerals indicate like components, illustrate embodiments of the invention.

Figure 1 is a schematic system diagram illustrating an exemplary wireless network in accordance with embodiments of the current invention.

Figure 2 illustrates an exemplary NR wireless system with centralization of the upper layers of the NR radio stacks in accordance with embodiments of the current invention.

Figure 3 illustrates an exemplary NR wireless system supporting inter gNB mobility scenario in accordance with embodiments of the current invention.

Figure 4 illustrates an exemplary user-plane data transmission in PDCP layer with inter-gNB mobility in accordance with embodiments of the current invention.

Figure 5 illustrates an exemplary user-plane data reception in PDCP layer with inter-gNB mobility in accordance with embodiments of the current invention.

Figure 6 shows an exemplary RRC procedure when performing HO with dual active protocol stacks in accordance with embodiments of the current invention.

Figure 7 shows an exemplary RRC procedure to control PDCP extension for all DRBs when performing HO with dual active protocol stacks in accordance with embodiments of the current invention.

Figure 8 shows an exemplary RRC procedure to control PDCP extension for DRBs with extendPDCP flag when performing HO with dual active protocol stacks in accordance with embodiments of the current invention.

Figure 9 shows an exemplary RRC procedure to maintain two sets of security keys for both source cell and the target cell during HO.

Figure 10 shows an exemplary RRC procedure to release the connection with the source cell after HO is successfully completed.

DETAILED DESCRIPTION

Reference will now be made in detail to some embodiments of the invention, examples of which are illustrated in the accompanying drawings.

Figure 1 is a schematic system diagram illustrating an exemplary wireless network in accordance with embodiments of the current invention. Wireless system includes one or more fixed base infrastructure units forming a network distributed over a geographical region. The base unit may also be referred to as an access point, an access terminal, a base station, a Node-B, an eNode-B, a gNB, or by other terminology used in the art. As an example, base stations serve a number of mobile stations within a serving area, for example, a cell, or within a cell sector. In some systems, one or more base stations are coupled to a controller forming an access network that is coupled to one or more core networks. gNB1andgNB2 are base stations in NR, the serving area of which may or may not overlap with each other. As an example, UE1 or mobile station is only in the service area of gNB1 and connected with gNB1. UE1 is connected with gNB1only, gNB1is connected with gNB 102 via Xn interface. UE2 is in the overlapping service area of gNB1 and gNB2. In one embodiment, UE2 is configured with dual protocol stacks and can be connected with gNB1and gNB2simultaneously.

Figure 1 further illustrates simplified block diagrams for UE2 and gNB2, respectively. UE has an antenna, which transmits and receives radio signals. A RF transceiver, coupled with the antenna, receives RF signals from antenna, converts them to baseband signal, and sends them to processor. In one embodiment, the RF transceiver may comprise two RF modules (not shown) . A first RF module is used for transmitting and receiving on one frequency band, and the other RF module is used for different frequency bands transmitting and receiving which is different from the first transmitting and receiving. RF transceiver also converts received baseband signals from processor, converts them to RF signals, and sends out to antenna. Processor processes the received baseband signals and invokes different functional modules to perform features in UE. Memory stores program instructions and data to control the operations of mobile station. UE also includes multiple function modules that carry out different tasks in accordance with embodiments of the current invention.

A measurement module, which controls the RRM measurement according to network’s configuration. A mobility controller, which receives RRC message for mobility, e.g. HO command and transmits the response message for HO command. A protocol stack controller, which manage to add or remove the protocol stack associated to source gNB and target gNB. A security hander, which associate the different security keys to different gNBs. A ROHC handler, which determines to use one or more ROHC profiles according to network configuration for different base station and apply the ROHC profile in the corresponding Protocol stack. Protocol Stack 1 and Protocol Stack 2 include RLC, MAC and PHY layers. In one embodiment, the SDAP layer is optionally configured.

In one embodiment, the PDCP layer supports the functions of transfer of data, maintenance of PDCP SN, header compression and decompression using the ROHC protocol, ciphering and deciphering, integrity protection and integrity verification, timer based SDU discard, routing for split bearer, duplication, re-ordering and in-order delivery; out of order delivery and duplication discarding. In one embodiment, one PDCP entity per DRB supporting simultaneous data transmission/reception with both the source cell and the target cell is modelled at the UE side.

Similarly, gNB2 has an antenna, which transmits and receives radio signals. A RF transceiver, coupled with the antenna, receives RF signals from antenna, converts them to baseband signals, and sends them to processor. RF transceiver also converts received baseband signals from processor, converts them to RF signals, and sends out to antenna. Processor processes the received baseband signals and invokes different functional modules to perform features in gNB2. Memory stores program instructions and data to control the operations of gNB2. gNB2 also includes multiple function modules that carry out different tasks in accordance with embodiments of the current invention.

A measurement module, which controls the RRM measurement through RRC configuration and receives measurement report from the UE side. A mobility controller, which determines the target gNB for mobility. It coordinates with other candidate gNB through Xn interface, make the HO decision and sends HO command to UE. A protocol stack controller, which manage to add or remove the protocol stack associated to source gNB and target gNB. A security handler, which generateone security key corresponding to the gNB. A ROHC handler determines one or more ROHC profiles are configured and associates the ROHC profile to the corresponding base station for each DRB. Protocol Stack includesSDAP, PDCP, RLC, MAC and PHY layers. In one embodiment, the SDAP layer is optionally configured.

Figure 2 illustrates an exemplary NR wireless system with centralization of the upper layers of the NR radio stacks in accordance with embodiments of the current invention. Different protocol split options between Central Unit and lower layers of gNB nodes may be possible. The functional split between the Central Unit and lower layers of gNB nodes may depend on the transport layer. Low performance transport between the Central Unit and lower layers of gNB nodes can enable the higher protocol layers of the NR radio stacks to be supported in the Central Unit, since the higher protocol layers have lower performance requirements on the transport layer in terms of bandwidth, delay, synchronization and jitter. In one embodiment, SDAP and PDCP layer are located in the central unit, while RLC, MAC and PHY layers are located in the distributed unit.

Figure 3 illustrates an exemplary NR wireless system supporting inter gNB mobility scenario in accordance with embodiments of the current invention. The intra 5G intra-RAT handover is normally based on Xn-based handover. HO is performed between gNBs through Xn interface, which are connected to the NR corn network. Each gNB has the protocol stacks including SDAP, PDCP, RLC, MAC and PHY layers.

Figure 4 illustrates an exemplary DL user-plane data processing in PDCP layer with inter-gNB mobility in accordance with embodiments of the current invention. In one embodiment, DL transmission operation at the network side includes:

·The source eNB assigns PDCP SN and forwards PDCPSDUs and the SN assigned to each SDU to target eNB;

·The source eNB and the target eNB perform header compression separately with their own ROHC profile;

·The source eNB and the target eNB perform ciphering separately with their own security keys;

In one embodiment, a new type of PDCP entity called ‘extended’ PDCP entity is used at the UE side. With the extended PDCP entity:

·UE performs deciphering for the DL PDCPSDUs received from the source eNB and target eNB separately.

·UE performs header decompression for the DL PDCPSDUs received from the source eNB and target eNB separately with the corresponding ROHC profile;

·UE stores the PDCPSDUs received from the source eNB and target eNB in the common PDCP reception buffer and performs PDCP reordering;

·UE delivers the PDCPSDUs to upper layers in ascending order of the associated COUNT value.

The ‘extended type’ of PDCP entity includes two security handling modules and is able to perform integrity verification and deciphering separately for both the source cell and the target cell. In another embodiment, the ‘extended type’ of PDCP entity further includes two header decompression modules and is able to perform header decompression separately for both the source cell and the target cell.

Figure 5 illustrates an exemplary UL user-plane data processing in PDCP layer with inter-gNB mobility in accordance with embodiments of the current invention. UE stops UL new data transmission with the source eNB upon reception of the first UL grant for data transmission from the target eNB after RA procedure towards the target eNB is successfully completed (UE continues UL ACK/NACK and other CSI kind of feedback with source eNB) .

Figure 6 shows an exemplary RRC procedure when performing HO with dual active protocol stacks in accordance with embodiments of the current invention. In one embodiment, the HO command includes a flag, e.g. dualActiveProtocol indicating that UE needs to perform simultaneous transmission reception with both the source cell and the target cell during the HO procedure. Therefore, UE needs to continue data transmission/reception with the source cell when performing random access with the target cell. Upon reception of the HO command, UE keeps the UP protocol for the source cell, i.e. UE doesn’t reset the MCG MAC entity for the source cell, doesn’t re-establish PDCP for the DRBs and doesn’t re-establish RLC for the DRBs. UE creates a MAC entity for the target cell. Then UE modifies/extends the PDCP entities to include more PDCP functions. The normal type of PDCP entity is changed to an ‘extended type’ of PDCP entity. In one embodiment, UE modifies the PDCP entities for all the DRBs established. In one embodiment, UE modifies the PDCP entities for the DRBs if the extendPDCP is set. The PDCP parameters for the ‘extended type’ of PDCP entity is provided by pdcp-Config, which is extended to include the required parameters for the target cell. UE establishes additional RLC entities for the DRBs and associates the RLC entities to the corresponding extended PDCP entity. In one embodiment, UE establishes RLC entities for all the DRBs established. In one embodiment, UE only establishes RLC entities for the DRBs for which extendPDCPis set. UE derives the security keys of the target cell. UE configures the ‘extended PDCP’ entity to apply the new keys of the target cell for the data transmission/reception to/from the target cell and continues to use the old keys of the source cell for data transmission/reception to/from the source cell.

In one embodiment, the HO command is RRCReconfig message with reconfigurationWithSync in NR. In one embodiment, the HO command is RRCConnectionReconfiguration message with mobilityControlInfo) . In one embodiment of LTE, the flagdualActiveProtocolis carried bymobilityControlInfo. In one embodiment of NR, the flag dualActiveProtocolis carried byspCellConfig with reconfigurationWithSync.

In one embodiment, the MAC entity created for the target cell is also a MCG MAC entity. From UE aspect, UE has two MCGs and two MAC entities for the source cell and the target cell respectively.

Figure 7 shows an exemplary RRC procedure to control PDCP extension for all DRBswhen performing HO with dual active protocol stacks in accordance with embodiments of the current invention. In one embodiment, UE extends the PDCP entities for all the DRBs configured with pdcp-config that are established upon reception of the HO command with dualActiveProtocol is configured. UE establishes a new MCG RLC entity and an associated DTCH logical channel. UE associates these RLC entities with the extended PDCP entities with the same value of drb-Identity within the current UE configuration.

Figure 8 shows an exemplary RRC procedure to control PDCP extension for DRBs with extendPDCP flag when performing HO with dual active protocol stacks in accordance with embodiments of the current invention. In one embodiment, UE extends the PDCP entities for the DRBs if the extendPDCP is set for the DRB. The network provides the drb-Identity value in the drb-ToAddModList and sets the extendPDCP flag for each DRB in the drb-ToAddModList. In one embodiment for DRB addition/modification (e.g. in NR) , for each drb-Identity value included in the drb-ToAddModList that is part of the current UE configuration, if the extendPDCP is set, trigger the PDCP entity of this DRB to perform PDCP extension procedure. In one embodiment for RLC bearer addition/modification, for each RLC-BearerConfig received in therlc-BearerToAddModList IE the UE shall: if the UE's current configuration doesn’t contains an RLC bearer with the received logicalChannelIdentity, establish an RLC entity in accordance with the received rlc-Config, configure this MAC entity with a logical channel in accordance to the received mac-LogicalChannelConfig, and associate this logical channel with the extended PDCP entity identified by servedRadioBearer. drb-Identityof servedRadioBearer is identical to the drb-Identity value in the drb-ToAddModListwithextendPDCPconfigured. In one embodiment, UE shall associated this logical channel with the security keys/algorithm provided by the HO command and the new header compression protocol in the extend PDCP entity identified by servedRadioBearer.

In one embodiment, the additional parameters for the extended PDCP entity is provided by extending PDCP-Config, which further provides the header (de) compression paramter for the target cell, when HO command configures dualActiveProtocol.

Figure 9 shows an exemplary RRC procedure to maintain two sets of security keys for both source cell and the target cell during HO. In one embodiment for security key updating (e.g. for LTE) , if dualActiveProtocol is configured and the securityConfigHO is included in the RRCConnectionReconfiguration, UE continues to use the old KeNB, K _RRCint, K _UPint, K _RRCenc, K _UPencas well ascurrent integrity algorithm and ciphering algorithm for data transmission/reception with the source cell. UE derives and stores a new set of keys for the target cell. The UE configures the ‘extended PDCP’ entity to apply the new keys of the target cell for the data transmission/reception to/from the target cell.

Figure 10 shows an exemplary RRC procedure to release the connection with the source cell after HO is successfully completed. In one embodiment, the RRC reconfiguration message includes a flag e.g. releaseSourceCell to release the connection with the source cell. In one embodiment for DRB addition/modification (e.g. in LTE) , UE releases the RLC entities and the associated logical channels of the source cell and reconfigure the PDCP entity in accordance with the pdcp-Config, i.e. change the extended PDCP entity to normal PDCP entity. In one embodiment for RLC bearer release, for each logicalChannelIdentity value that is to be released as the result of source cell release, UE releases the RLC entity or entities and releases the corresponding logical channels. UE reconfigure the PDCP entity in accordance with the pdcp-Config, i.e. change the extended PDCP entity to normal PDCP entity.

Claims

A method in RRC layer to perform HO with dual active protocol stacks corresponding to the source cell and the target cell seperately to minimize the HO interruption comprising:

receiving a HO command with a flag e.g. dualActiveProtocol indicating to maintain the UP protocol of the source cell when perform HO to the target cell;

creating a MAC entity for the target cell;

modifying the PDCP entity from the normal PDCP entity to the ‘extended’ PDCP entity for the DRBs;

establishing an additional RLC entity for the DRBs;

deriving the security keys for the target cell; and

configuring the extended PDCP entity to apply the new keys for the target cell and continue to use the old keys of the source cell for the data transmitted/received to/from the target cell and the source cell respectively.
The method of claim 1, wherein the flag e.g. dualActiveProtocol is carried by mobilityControlInfo or carried by spCellConfig with reconfigurationWithSync.
The method of claim 1, wherein UEdoesn’t reset MCG MAC entity for the source cell, doesn't re-establish PDCP and doesn’t re-establish RLC for the DRBs to maintain the UP protocol of the source cell.
The method of claim 1, wherein the created MAC entity for the target cell is also a MCG MAC entity and UE keeps two MCG MAC entities for the source cell and the target cell separately.
The method of claim 1, wherein the normal PDCP entity is modified to include one additional security-handling module to perform integrity protection/verification and ciphering/deciphering for the target cell.
The method of claim 1, wherein the normal PDCP entity is modified to include one additional header compression/decompression module to perform header compression/decompression for the target cell.
The method of claim 6, wherein the header compression parameters are included in pdcp-Config, which is extended to provide the header compression parameters for the target cell.
The method of claim 1, wherein UE extends the PDCP entity for all the configured DRBswith pdcp-Config that are established and establishes an additional RLC entity and an associated logical channel for all those DRBs.
The method of claim 1, wherein UE extends the PDCP entity for the DRBs if the extendPDCP of it is configured and establishes an additional RLC entity and an associated logical channel for those DRBs.
The method of claim 9, wherein the extendPDCPis provided in pdcp-Config.
The method of claim 1, further comprising associating those established RLC entities and the logical channels with the extended PDCP entities with the same value of drb-Identity.
The method of claim 11, further comprising associating the established RLC entity and logical channel with the additional security-handling module, the security keys/algorithm provided by the reconfiguration message and the additional header compression protocol for each DRB.
The method of claim 1, further comprising releasing the connection with the source cell when receiving a reconfiguration message to release the source cell connection after HO is successfully completed.
The method of claim 13, further comprising:

releasing the RLC entities and the associated logical channels of the source cell;

reconfiguring the extended PDCP entities to normal PDCP entities.