WO2021142702A1

WO2021142702A1 - Methods and apparatus of uplink feedback and retransmission for nr multicast services

Info

Publication number: WO2021142702A1
Application number: PCT/CN2020/072426
Authority: WO
Inventors: Xuelong Wang
Original assignee: Mediatek Singapore Pte. Ltd.
Priority date: 2020-01-16
Filing date: 2020-01-16
Publication date: 2021-07-22
Also published as: WO2021143869A1

Abstract

Apparatus and methods are provided to support the Uplink feedback and retransmission for NR multicast service. In one novel aspect, an enhanced RLC UM mode is proposed in the invention to mimic the RLC AM mode operation for NR multicast service. The UE is configured to establish an associated unicast RLC channel corresponding to the multicast radio bearer (s) in order to provide Uplink feedback and provide needed retransmission for non-acknowledged RLC packets in unicast manner. The reliability of the transmission of NR multicast service is improved.

Description

METHODS AND APPARATUS OF UPLINK FEEDBACK AND RETRANSMISSION FOR NR MULTICAST SERVICES

TECHNICAL FIELD

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

BACKGROUND

3GPPspecified the support for MBMS transmission but no Uplink feedback is supported so far.

At Rel-13, there is a discussion at 3GPP to support physical layer feedback in the context of SC-PTM transmission. However, eventually it is not specified.

Traditionally, the MBMS service is supported via RLC UM mode for RRC-connected UE at high layer of Uu interface, high layer based feedback is not possible for MBMS. High layer based retransmission is not possible for MBMS.

For RLC UM, mode, when indicated from upper layer (i.e. PDCP) to discard a particular RLC SDU, the transmitting UM RLC entity shall discard the indicated RLC SDU, if neither the RLC SDU nor a segment thereof has been submitted to the lower layers.

For RLC UM, mode, the RLC UM entity submits UMD PDU to lower layer. The UMD PDU may contain a segment or RLC SDU or a whole RLC SDU. It is assumed that RLC UM entity remove the data from its buffer when he delivers RLC SDU, or RLC SDU segment to the lower layer, since no acknowledgement from the peer RLC entity is expected.

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 of the NR Multicast services via Uplink feedback and retransmission at high layer of Uu interface.

SUMMARY

A method is provided to support the Uplink feedback and retransmission for NR multicast service.

In one novel aspect, an enhanced RLC UM mode is proposed in the invention to mimic the RLC AM mode operation.

The enhanced RLC UM mode supports to buffer the transmitted RLC packets for retransmission in unicast manner. The enhanced RLC UM mode also supports the packet discarding to slide the transmission window. The enhanced RLC UM mode also supports to move 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.

The UE is configured to establish an associated unicast RLC channel corresponding to the multicast radio bearer (s) in order to provide Uplink feedback in terms of RLC Status Report PDU. Polling Request sent over the associated unicast RLC channel can pull the RLC Status Report PDU.

The associated unicast RLC channel is used to provide needed retransmission for non-acknowledged RLC packets in unicast manner.

BRIEF DESCRIPTION OF THE 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 NR multicast radio bearer model in accordance with embodiments of the current invention.

Figure 4 illustrates an exemplary NR multicast radio bearer model with one-to-many mapping between associated unicast RLC channel and MRBs in accordance with embodiments of the current invention.

Figure 5 illustrates an exemplary signal flow for Uplink feedback and unicast retransmission for NR multicast service 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.

In order to improve the reliability of the transmission of NR multicast service, the RLC UM mode configured for multicast radio bearer needs to be enhanced. Figure 3 illustrates an exemplary NR multicast radio bearer model in accordance with embodiments of the current invention. The general principle of the NR multicast radio bearer model is that an associated unicast RLC channel in RLC AM mode is configured to assist the NR multicast radio bearer in RLC UM mode. The associated unicast RLC channel can be named by other term but with the same functionality as described in this invention. The associated unicast RLC channel operates based on the establishment of a specific associated unicast RLC AM entity at both BS and UE.

The associated unicast RLC channel is configured for each UE during NR multicast radio bearer establishment via RRC message to RRC connected mode UE. 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 Uplink feedback channel to the NR multicast radio bearer (i.e. MRB) . The intention of establishing this association between unicast RLC channel and MRB is to mimic the RLC AM mode operation for NR MRB.

The associated unicast RLC channel is also used as the backup Downlink transmission channel for the configured NR multicast radio bearer for a particular UE. When one or more RLC SDU, and/or one or more RLC SDU segments, can not be successfully received by a particular UE, and the BS decides to perform retransmission of these packets via unicast. The packets can be sent via this associated unicast RLC channel in AM mode.

The RLC UM mode configured for multicast radio bearer is not a traditional RLC UM mode, and it is an enhanced RLC UM mode (e.g. RLC eUM mode) . Correspondingly, the RLC UM entity for MRB at both BS and UE is an enhanced RLC UM entity (e.g. RLC eUM entity) .

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. For the first option, the transmitted packets is cleared from the buffer in case the packets are acknowledged by all of the UEs (by RLC Status Report via associated RLC channel) receiving the multicast service. It means that the transmission window is sliding forward when the packets are acknowledged by all UEs. This option has the drawback of too long latency for UE perception. For the second option, the transmission window is sliding forward under the control of a configured timer. A discard timer can be configured per transmitted packet in RLC layer. For the third option, the transmission window is sliding forward under the counting of the number of packets transmitted, for example, when ten packets (e.g. N, N+1, N+2, …, N+9) was transmitted, the previous packets (e.g. N-10, N-11, etc. ) can be discarded. The value of the number of packets transmitted is configurable. As stated in the options described above, the Packet Discard functionality is supported by this enhanced RLC UM mode for MRB.

The additional characteristic of this enhanced RLC UM mode is that this RLC UM entity 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. The associated RLC AM entity is corresponding to the associated unicasts RLC channel, for a particular UE that indicate his non-acknowledgement of the reception of the RLC packets.

In general, the Uplink feedback at RLC layer for MRB enables the Base Station to retransmit non-acknowledged RLC packets to one or more UEs when multicast transmission from BS to these UEs can not ensure successful reception for the UE (s) . This means, at the same time, these UEs are expected to continue their reception of the new RLC packets from MRB RLC UM entity, and/or other multicast radio bear (s) . From UE perspective, the associated unicast RLC AM entity needs to deliver the received RLC packets to MRB RLC UM entity for packet assembly and/or in order delivery to PDCP layer. Packet assembly occurs at MRB RLC UM entity when some of the RLC SDU segments is already received previously by MRB RLC UM entity and then additional RLC SDU segments received from the associated unicast RLC AM entity is delivered to MRB RLC UM entity for assembly. Non-segmented RLC SDU can be delivered to PDCP layer directly by associated unicast RLC AM entity if no requirement for in order delivery to PDCP layer at the UE.

The associated unicast RLC AM entity at BS can also be used to transmit new RLC packet to the UE, when there is a decision made by BS based on e.g. packet reception status. In this case, the PDCP entity of the MRB at BS delivers the PDCP PDU directly to the associated unicast RLC AM entity. From UE perspective, as shown by the dash line in Figure 3, the RLC SDU can be delivered to PDCP layer directly by associated unicast RLC AM entity if no requirement for in order delivery to PDCP layer at the UE. Otherwise, it is reordered before delivering to PDCP layer.

The associated unicast RLC channel transmits the data in AM mode, so then there is RLC Polling Request PDU of unicast data and RLC Status Report PDU of unicast data transmitted over this channel. In summary, the transmitted data and control PDU (s) over this associated RLC channel include RLC Polling Request of MRB data, RLC Status Report PDU of MRB data, RLC Polling Request of unicast data, RLC Status Report PDU of unicast data, and Unicast data (for both retransmission and new retransmission) .

When the UE is configured with multiple multicast radio bearer (i.e. MRB) to receive multiple multicast service, a single associated unicast RLC channel is established to provide the Uplink feedback. This means one-to-many mapping between associated unicast RLC channel and MRBs. Figure 4 illustrates an exemplary NR multicast radio bearer model with one-to-many mapping between associated unicast RLC channel and MRBs in accordance with embodiments of the current invention.

Correspondingly, as shown at Figure 4, there is one associated unicast RLC AM entity configured at both BS and UE. When multiple MRB shares one associated unicast RLC channel, the ID of MRB, and/or the LCID of MRB needs to be indicated within the RLC Polling Request of MRB data, RLC Status Report PDU of MRB data, RLC Polling Request of unicast data, RLC Status Report PDU of unicast data, and/or Unicast data. The said ID (s) is used by the associated unicast RLC AM entity at receiving side to determine where to deliver the received RLC Polling Request, RLC Status Report PDU, and/or unicast retransmitted RLC packets. For example, as shown in Figure 4, the associated unicast RLC AM at UE side needs to determine if a polling request is forwarded to RLC UM entity of MRB-1 or RLC UM entity of MRB-2.

Figure 5 illustrates an exemplary signal flow for Uplink feedback and unicast retransmission for NR multicast service in accordance with embodiments of the current invention. In Step 1 of Figure 5, the BS configures two multicast radio bearers (MRB-1 and MRB-2) to the UE. Meanwhile, the associated unicast RLC channel is also configured to support the reliable transmission of MRB-1 and MRB-2. Different bearer ID (s) and logic channel ID (s) are assigned for MRB-1 and MRB-2. A specific unicast logic channel is configured for the associated unicast RLC channel. The RLC configuration of associated unicast RLC channel follows the corresponding configuration of the MRB(s) . When multiple MRBs are associated with a single unicast RLC channel, the same RLC configurations (e.g. same RLC SN length) applies to these MRBs in order to enable consistent RLC transmission in terms of RLC status report and RLC retransmission.

In Step 1 of Figure 5, the MRB (s) is configured with enhanced RLC UM mode for multicast service. The associated unicast RLC entity is configured in AM mode.

In Step 2 of Figure 5, the UE in connected mode receives the two multicast radio bearers (MRB-1 and MRB-2) . Meanwhile, there may be ongoing unicast DRB (s) for the UE.

In Step 3 of Figure 5, the BS sends the Polling request to the UE. As the first option, the Polling request can be carried by the RLC data PDU over MRB-1. As the second option, it can be carried in a RLC control PDU over associated unicast RLC channel. In this case, a new type of RLC control PDU is introduced. The Bear ID of MRB-1 is included in the Polling Request. As the third option, a specific MAC CE can be used to indicate the Polling Request over associated unicast RLC channel. In this case, the Bear ID of MRB-1 or the LCID of MRB-1is included in the subheader of the MAC CE on top of the legacy structure of the subheader of the MAC CE .

In Step 4 of Figure 5, after receiving the multicast related Polling request (i.e. for MRB-1) from the BS, the UE sends the RLC Status Report to BS for MRB-1. One top of legacy RLC Status Report, an additional field is added to indicate the ID of radio bearer (i.e. MRB-1) in terms of Bear ID of MRB-1 or the LCID of MRB-1.

In Step 5 of Figure 5, according to the reception status of the MRB-1 at UE, the BS’s RLC UM entity of MRB-1 may initiate retransmission over MRB-1 for one or more RLC packets. The BS decides to use unicast to retransmit the RLC packets when the BS slides the transmission window forward without receive the acknowledgement from the UE for one or more transmitted RLC packets (i.e. RLC SDU or RLC SDU segment) for MRB-1. At this time, the BS’s RLC UM entity of MRB-1 delivers the corresponding RLC SDU (s) or RLC SDU segment (s) to the associated unicast RLC AM entity for the UE before discarding the packets.

In Step 6 of Figure 5, the associated unicast RLC AM entity at BS sends the RLC packets to the UE via unicast mode. From MAC layer perspective, the data is identified via the logic channel ID of the associated unicast RLC channel. In order to distinguish the data for different MRB (s) , the LCID or Bearer ID should be included within the data PDU. As the first option, the Bearer ID of MRB is included within the RLC header of the RLC data PDU, or within the subheader of the MAC data sub PDU. As the second option, the LCID of MRB is included within the RLC header of the RLC data PDU, or within the subheader of the MAC data sub PDU. The receiving side of the associated unicast RLC channel delivers the RLC packets to the corresponding RLC UM entity of MRB following the bearer ID and/or LCID indication.

During Step 6 of Figure 5, as shown in Step 6a and 6b, the associated unicast RLC AM entity at BS may send the Polling request to UE to pull the RLC Status Report from UE. The Polling request and RLC Status Report for uncast retransmission data is differed from the Polling request and RLC Status Report for multicast retransmission data via bearer ID and/or LCID indication.

In Step 7 of Figure 5, the BS decides to resume multicast transmission for MRB-1 for the UE.

In Step 8 of Figure 5, the BS performs multicast transmission for MRB-1 and MRB-2 for the UE.

The description in this invention can be also applicable to the Uplink feedback 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 Base Station and UE,

Performing Uplink feedback for multicast radio bearer from UE to Base Station over an associated unicast RLC channel, and

Retransmitting the non-acknowledged RLC packets from Base Station to UE over an associated unicast RLC channel for NR multicast.
The method of claim 1, wherein the multicast radio bearer is established together with an associated unicast RLC channel based on the RRC configuration message during establishment of the MRB.
The method of claim 1, wherein the associated unicast RLC channel is associated with multiple MRBs.
The method of claim 1, wherein the associated unicast RLC channel requires the establishment of associated unicast RLC AM entity in both BS and UE.
The method of claim 1, wherein the multicast radio bearer requires the establishment of an RLC UM entity in enhanced RLC UM mode.
The method of claim 5, wherein the RLC UM entity supports packet discarding based on the number of the transmitted RLC packets.
The method of claim 5, wherein the RLC UM entity supports to buffer the transmitted packets before discarding.
The method of claim 5, wherein the RLC UM entity supports to move 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.
The method of claim 1, wherein Uplink feedback for multicast radio bearer from UE to Base Stationis sent over an associated unicast RLC channel in terms of RLC Status Report PDU.
The method of claim 9, wherein the RLC Status Report PDU is identified by a Bearer ID or a logic channel ID for MRB.
The method of claim 9, wherein the RLC Status Report PDU is triggered by Polling Request sent over an associated unicast RLC channel.
The method of claim 11, wherein the Polling Request is identified by a Bearer ID or a logic channel ID.
The method of claim 1, wherein the non-acknowledged RLC packets retransmitted from Base Station to UE is identified by a Bearer ID or a logic channel ID.