WO2021142770A1

WO2021142770A1 - Methods and apparatus of lossless handover for nr multicast services

Info

Publication number: WO2021142770A1
Application number: PCT/CN2020/072728
Authority: WO
Inventors: Xuelong Wang
Original assignee: Mediatek Singapore Pte. Ltd.
Priority date: 2020-01-17
Filing date: 2020-01-17
Publication date: 2021-07-22
Also published as: WO2021143868A1

Abstract

Apparatus and methods are provided to support lossless handover during cell change for a particular UE to enable reliable multicast transmission for NR multicast service. In one novel aspect, an associated unicast RLC channel is established between the UE and the target cell during handover. One associated unicast RLC channel may correspond to multiple multicast radio bearers. The associated unicast RLC channel is used to retransmit the buffered or non-acknowledged data from the target cell to the UE. The RLC packets including both RLC SDU (s) and RLC segment (s) is forwarded from the source cell to the target cell for retransmission.

Description

METHODS AND APPARATUS OF LOSSLESS HANDOVER FOR NR MULTICAST SERVICES

TECHNICAL FIELD

The disclosed embodiments relate generally to wireless communication, and, more particularly, to enable lossless handover for NR multicast based transmission for purpose of improving the reliability of NR multicast services.

BACKGROUND

3GPPspecified the support of MBMS transmission for LTE but no lossless handover is supported so far.

Mobility procedures for LTE MBMS reception allow the UE to start or continue receiving MBMS service (s) via MBSFN or SC-PTM when changing cell (s) . For each MBMS service provided using SC-PTM, E-UTRAN indicates in the SC-MCCH the list of neighbour cells providing this MBMS service so that the UE can request unicast reception of the service before changing to a cell not providing the MBMS service using SC-PTM.

For MBSFN transmission, LTE procedures provide support for service continuity with respect to mobility within the same MBSFN area. UEs that are receiving MBMS service (s) in RRC_IDLE state performing cell reselection or are in RRC_CONNECTED state obtain target cell (SC-) MTCH information from the target cell (SC-) MCCH. To avoid the need to read MBMS related system information and potentially (SC-) MCCH on neighbour frequencies, the UE is made aware of which frequency is providing which MBMS services via MBSFN or SC-PTM through the combination of the following MBMS assistance information: user service description (USD) and system information.

3GPP has specified the support of lossless handover for unicast. The basic principle is that the non-acknowledged PDCP PDU is forwarded from the source Base Station to the target Base Station in order to enable reliable transmission for data bearers in RLC AM mode.

In Dec 2019, 3GPP approved a work item (WI) on the support of NR Broadcast and Multicast Services. Within the scope of the WI, the reliable transmission of the NR Multicast services is the key objective.

In this invention, it is sought to achieve reliable transmission between the network and the UE via loss handover during connected mode mobilityfor NR Multicast services.

SUMMARY

A method is provided to support lossless handover during cell change for a particular UE to enable reliable multicast transmission for NR multicast service.

In one novel aspect, an associated unicast RLC channel is established between the UE and the target cell during handover. One associated unicast RLC channel may correspond to multiple multicast radio bearers. The associated unicast RLC channel is used to retransmit the buffered or non-acknowledged data forwarded by source cell from the target cell to the UE. The configuration of the associated unicast RLC channel is sent to the UE via RRC message e.g. handover command during handover preparation stage.

The RLC packets including both RLC SDU (s) and RLC segment (s) is forwarded from the source cell to the target cell for retransmission. The RLC packets can be forwarded in either transparent mode or non-transparent mode. The RLC packets can be forwarded within a gNB-DU or among different gNBDUs belonging to same gNB CU or different gNB CU.

BRIEF DESCRIPTION OF DRAWINGS

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

Figure 1 (a) is a schematic system diagram illustrating an exemplary Base Station (i.e. BS) in accordance with embodiments of the current invention.

Figure 1 (b) is a schematic system diagram illustrating an exemplary UE in accordance with embodiments of the current invention.

Figure 2 illustrates an exemplary NR wireless system in accordance with embodiments of the current invention.

Figure 3 illustrates an exemplary multicast-to-multicast radio bearer change during intra-gNB and intra-DU cell change in accordance with embodiments of the current invention.

Figure 4 illustrates an exemplary multicast-to-unicast radio bearer change during intra-gNB and intra-DU cell change in accordance with embodiments of the current invention.

Figure 4a illustrates an exemplary multicast-to-unicast radio bearer change during intra-gNB and intra-DU cell change with the establishment of a new PDCP entity in accordance with embodiments of the current invention.

Figure 5 illustrates an exemplary multicast-to-multicast radio bearer change during intra-gNB and inter-DU cell change in accordance with embodiments of the current invention.

Figure 6 illustrates an exemplary multicast-to-unicast radio bearer change during intra-gNB and inter-DU cell change in accordance with embodiments of the current invention.

Figure 6a illustrates an exemplary multicast-to-unicast radio bearer change during intra-gNB and inter-DU cell change with the establishment of a new PDCP entity in accordance with embodiments of the current invention.

Figure 7 illustrates an exemplary multicast-to-multicast radio bearer change during inter-gNB cell change in accordance with embodiments of the current invention.

Figure 8 illustrates an exemplary multicast-to-unicast radio bearer change during inter-gNB cell change in accordance with embodiments of the current invention.

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 (a) is a schematic system diagram illustrating an exemplary Base Station (i.e. BS) in accordance with embodiments of the current invention. The BS 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. The Base Station 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 functions. Memory stores program instructions and data to control the operations of Base Station.

Figure 1 (b) is a schematic system diagram illustrating an exemplary UE in accordance with embodiments of the current invention. The UE may also be referred to as a mobile station, a mobile terminal, a mobile phone, smart phone, wearable, an IoT device, a table let, a laptop, or other terminology used in the art. 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. 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.

Figure 2 illustrates an exemplary NR wireless system in accordance with embodiments of the current invention. Different protocol split options between Central Unit and Distributed Unit of gNB nodes may be possible. 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.

The handover procedures described in the invention aims to achieve lossless handover or to reduce the loss of packets during handover. It assumes that an enhanced RLC UM entity (i.e. RLC UM entity in enhanced RLC UM mode) configured for multicast radio bearer performs the transmission of NR multicast service. An associated unicast RLC channel in RLC AM mode is configured to assist the NR multicast radio bearer in the enhanced RLC UM mode. The associated unicast RLC channel is configured for the UE during NR multicast radio bearer establishment. For Downlink, the Base Station sends the Polling request to the UE through the associated unicast RLC channel. For Uplink, the UE sends the RLC Status Report PDU response to the Base Station through the associated unicast RLC channel. The associated unicast RLC channel serves as the channel for Uplink feedback and data retransmission for NR multicast radio bearer (i.e. MRB) . The characteristic of enhanced RLC UM mode is that this enhanced RLC UM mode requires the transmitter side (i.e. BS) to buffer the transmitted packets (e.g. in terms of RLC SDU and RLC SDU segment) in the buffer for a period after transmission in order to wait for the Uplink feedback from the UE (s) . Packet Discard functionality is supported by this enhanced RLC UM mode for MRB. This RLC UM entity in enhanced RLC UM mode moves one or more transmitted packets (i.e. RLC SDU, and/or RLC SDU segments) from its buffer during packet discarding to the associated unicast RLC AM entity for retransmission, if there is a need to retransmit the transmitted packets to the UE via unicast mode.

Figure 3 illustrates an exemplary multicast-to-multicast radio bearer change during intra-gNB and intra-DU cell change in accordance with embodiments of the current invention. During intra-gNB and intra-DU cell change (i.e. handover) , the multicast service continuity is ensured if the target cell also supports multicast transmission of the NR multicast service.

As depicted by Figure 3, the multicast radio bearer is already available. Then a new associated RLC AM entity is established at both target cell and UE during cell change (i.e. handover) in order to establish an associated RLC unicast channel to support Uplink feedback and needed unicast based retransmission after handover. The configuration of the new associated RLC AM entity and corresponding associated RLC unicast channel including the RLC configuration and MAC configuration (e.g. logical channel configuration) is sent to UE during handover e.g. via Handover Command message. The UE applies these configurations when it is connected to the target cell.

For the case described at Figure 3, lossless data delivery is achieved via retransmission of the buffered or non-acknowledged data packets at target cell. As shown in the dash line of Figure 3 from source associated RLC AM entity to target associated RLC AM entity, the buffered or non-acknowledged RLC SDU and RLC SDU segments are forwarded to the target associated RLC AM entity from the source associated RLC AM entity. The target associated RLC AM entity initiates retransmission of these RLC packets (i.e. RLC SDU and RLC SDU segments) when the UE successfully hands over to the target cell. The UE assemblies the RLC SDU and RLC SDU segments received from target cell with the packets he received from source cell before. As the target cell and source cell shares the PDCP entity, the PDCP SN is consistent after handover.

Figure 4 illustrates an exemplary multicast-to-unicast radio bearer change during intra-gNB and intra-DU cell change in accordance with embodiments of the current invention. During intra-gNB and intra-DU cell change, the multicast transmission is not supported by the target cell. In this case, during cell change for UE, a new unicast RLC AM entity is established at target cell during cell change (i.e. handover) .

As first option, within the UE, the RLC UM entity for receiving MRB and the associated unicast RLC AM entity are replaced by a new unicast RLC AM entity. As second option, the RLC UM entity for receiving MRB is released and the associated unicast RLC AM entity is reconfigured to be a new unicast RLC AM entity to support the succeeding unicast transmission between target cell and the UE.

Correspondingly, a new unicast RLC channel specific to the UEis established to support unicast transmission between target cell and the UE. The unicast based retransmission after handover occurs at the unicast RLC channel at target side. For the case described at Figure 4, lossless data delivery is achieved via retransmission at target cell of the buffered or non-acknowledged data packets forwarded by source cell.

As shown in the dash line of Figure 4 from source associated RLC AM entity to target unicast RLC AM entity, non-acknowledged RLC SDU and RLC SDU segments are forwarded to the target unicast RLC AM entity from the source associated RLC AM entity. The target unicast RLC AM entity initiates retransmission of these RLC packets (i.e. RLC SDU and RLC SDU segments) when the UE successfully hands over to the target cell. The UE assemblies the RLC SDU and RLC SDU segments received from target cell with the packets he received from source cell. As the target cell and source cell shares the PDCP entity, the PDCP SN is consistent after handover.

As shown in Figure 4, the unicast RLC AM entity established at the target cell also delivers the packets he receives from PDCP entity (for multicast) . This presents the case where one multicast PDCP entity corresponds to different RLC entities including AM mode RLC entity (for unicast) and UM mode RLC entity (for multicast) .

Figure 4a illustrates an exemplary multicast-to-unicast radio bearer change during intra-gNB and intra-DU cell change with the establishment of a new PDCP entity in accordance with embodiments of the current invention. As depicted in Figure 4a, a new PDCP entity is established corresponding to the multicast SDAP entity at gNB CU. As first option, this PDCP entity shares the same PDCP configuration (including security key configuration) as multicast PDCP entity. As second option, this PDCP entity uses different PDCP configuration (including security key configuration) from the configuration of multicast PDCP entity. In this case, the PDCP configuration needs to configured to the UE via RRC message e.g. handover command.

For the scenario depicted in Figure 4a, the buffered or non-acknowledged PDCP PDU is forwarded from source multicast PDCP entity to target unicast PDCP entity. The SN of buffered or non-acknowledged PDCP PDU is decided based on the delivery indication from MRB RLC UM entity and the feedback from MRB RLC UM entity and/or the associated RLC AM entity at source cell side. The delivery indication from MRB RLC UM entity follows the legacy behavior, which indicates the delivery of a particular of PDCP PDU to PDCP entity. The MRB RLC UM entity and/or the associated RLC AM entity in source cell feedbacks to PDCP entity the unsegmented RLC SDU (i.e. PDCP PDU) he received from PDCP entity, which is transmitted but is not successfully received by the UE based on the UL feedback. The source PDCP entity forwards the received PDCP PDU to the established PDCP entity for the target cell. In addition, if one RLC SDU segment of a particular RLC SDU is not transmitted but is not successfully received by the UE based on the UL feedback, the while RLC SDU needs to be feedbacked to PDCP entity. This requires the RLC entity at the source cell to buffer the while RLC SDU in case one of the segments is not successfully received by the UE. This operation will lead to the fact that some of the RLC SDU segment of a particular RLC SDU is successfully received by the UE, but the UE needs to discard the RLC SDU segment since the additional RLC SDU segments is not successfully received. As stated above, the whole RLC SDU will be retransmitted by the target cell. The RLC SDU segments received from source cell is out of date.

For the scenario depicted in Figure 4a, during the handover stage, the new established unicast PDCP entity needs to ensure synchronous PDCP SN numbering with the source cell multicast PDCP entity. To be specific, the multicast PDCP entity forwards the latest SN he allocated e.g. X to the unicast PDCP entity. Then the first SN allocated by unicast PDCP entity for data flow from SDAP entity should be X+1. The expected effect should be that the same SN allocated by source PDCP entity and unicast PDCP entity for the data coming from SDAP entity in order to maximize the service continuity for the UE.

For the scenario depicted in Figure 4a, The new PDCP entity delivers the retransmission PDU (s) as received from the source cell before delivering any new PDCP PDUs to the RLC entity.

Figure 5 illustrates an exemplary multicast-to-multicast radio bearer change during intra-gNB and inter-DU cell change in accordance with embodiments of the current invention. During intra-gNB and inter-DU cell change, the multicast service continuity is ensured if the target cell also supports multicast transmission of the NR multicast service. As depicted by Figure 5, a new associated RLC AM entity is established at both target cell and UE during cell change (i.e. handover) in order to establish an associated RLC unicast channel to support Uplink feedback and needed unicast based retransmission after handover. The configuration of the new associated RLC AM entity and corresponding associated RLC unicast channel including the RLC configuration and MAC configuration (e.g. logical channel configuration) is sent to UE during handover e.g. via Handover Command message. The UE applies these configurations when it is connected to the target cell. In this scenario, the buffered RLC SDU, and RLC SDU segments can be put into transparent container and then be retransmitted from the associated RLC AM entity in source cell to the associated RLC AM entity in target cell via F1 interface between gNB-CU and gNB-DU. The forwarding operation can be performed by the PDCP entity within gNB-CU. The UE assemblies the RLC SDU and RLC SDU segments received from target cell with the packets he received from source cell in order to support lossless handover.

Figure 6 illustrates an exemplary multicast-to-unicast radio bearer change during intra-gNB and inter-DU cell change in accordance with embodiments of the current invention. During intra-gNB and inter-DU cell change, the multicast transmission is not supported by the target cell. In this case, during cell change for UE, a new unicast RLC AM entity continuity is established at both target cell and UE during cell change (i.e. handover) . Correspondingly, a new unicast RLC channel is established to support unicast transmission for the UE. The unicast based retransmission after handover occurs at the unicast RLC channel at target side. The lossless data delivery is achieved via retransmission of the buffered or non-acknowledged data packets at target cell. In this scenario, the buffered RLC SDU, and RLC SDU segments can be put into transparent container and then be retransmitted from the associated RLC AM entity in source cell to the associated RLC AM entity in target cell via F1 interface between gNB-CU and gNB-DU. The forwarding operation can be performed by the PDCP entity or other entity within the gNB-CU. The UE assemblies the RLC SDU and RLC SDU segments received from target cell with the packets he received from source cell in order to support lossless handover.

Figure 6a illustrates an exemplary multicast-to-unicast radio bearer change during intra-gNB and inter-DU cell change with the establishment of a new PDCP entity in accordance with embodiments of the current invention. As depicted in Figure 6a, a new unicast PDCP entity is established corresponding to the multicast SDAP entity at gNB CU. The buffered or non-acknowledged PDCP PDU is forwarded from source multicast PDCP entity to target unicast PDCP entity. The decision on the set of PDCP PDU subject to forwarding uses the same mechanism as described for Figure 4a. The SN allocation mechanism described for Figure 4a applies also to the scenario described at Figure 6a. The same data retransmission mechanism as described for Figure 4a also applies to the scenario described at Figure 6a.

Figure 7 illustrates an exemplary multicast-to-multicast radio bearer change during inter-gNB cell change in accordance with embodiments of the current invention. As depicted in Figure 7, the buffered or non-acknowledged RLC SDU and RLC segments are forwarded to the target from source associated RLC AM entity to the target associated RLC AM entity for purpose of retransmission. The aligned PDCP SN is assumed for source PDCP entity and target PDCP entity for NR multicast service transmission. The similar mechanisms described for other scenarios within this invention apply to scenario depicted in Figure 7.

Figure 8 illustrates an exemplary multicast-to-unicast radio bearer change during inter-gNB cell change in accordance with embodiments of the current invention. As depicted in Figure 8, the buffered or non-acknowledged RLC SDU and RLC segments are forwarded to the target from source associated RLC AM entity to the target RLC AM unicast entity for purpose of retransmission. The similar mechanisms described for other scenarios within this invention apply to scenario depicted in Figure 8.

For all of the scenarios described in this invention, when the UE receives multiple multicast services at source cell of a source Base Station, during handover, a single associated unicast RLC AM channel is established between the UE and the target cell of target Base Station. This configuration is indicated to the UE via RRC message e.g. handover command during handover preparation.

The description in this invention can be also applicable to the lossless handover procedure for NR broadcast services.

While aspects of the present disclosure have been described in conjunction with the specific embodiments thereof that are proposed as examples, alternatives, modifications, and variations to the examples may be made. Accordingly, embodiments as set forth herein are intended to be illustrative and not limiting. There are changes that may be made without departing from the scope of the claims set forth below.

Claims

A method comprising:

establishing a multicast radio bearer and an associated unicast RLC channel at target cell between a target Base Station and the UE,

forwarding buffered or non-acknowledged packets from the source cell of a source Base Station to a target cell of a target Base Station, and

retransmitting buffered or non-acknowledged packets by a target cell of a target Base Station to UE.
The method of claim 1, wherein the associated unicast RLC channel requires the establishment of the associated unicast RLC AM entity at both target cell and UE.
The method of claim 1, wherein the associated unicast RLC channel can correspond to multiple multicast radio bearer established at target cell and UE.
The method of claim 1, wherein the buffered or non-acknowledged packets is RLC packets (including RLC SDU and RLC SDU segments) or PDCP packets.
The method of claim 1, wherein buffered or non-acknowledged packets can be retransmitted to the UE via the associated unicast RLC channel when multicast radio bearer is supported at target cell.
The method of claim 1, wherein buffered or non-acknowledged packets can be retransmitted to the UE via a unicast RLC channel when multicast radio bearer is not supported at target cell.