WO2024207446A1

WO2024207446A1 - Methods and apparatus of rlc enhancement during intra-du ltm

Info

Publication number: WO2024207446A1
Application number: PCT/CN2023/086955
Authority: WO
Inventors: Yao PENG; Xiaonan Zhang; Yuanyuan Zhang
Original assignee: Mediatek Singapore Pte. Ltd.
Priority date: 2023-04-07
Filing date: 2023-04-07
Publication date: 2024-10-10

Abstract

This disclosure describes methods and apparatus to perform the RLC retransmission when performing the cell switch by intra-DU LTM, further comprising the steps of receiving the LTM cell switch command from the network, switching to the target cell and performing the RLC retransmission for the UE when performing the cell switch by LTM. The RLC retransmission can be performed with/without the RLC status report by different implementations. In one embodiment, the RLC retransmission is performed when HARQ continuation is not supported for LTM. In one embodiment, the RLC bearer is the RLC UM bearer.

Description

METHODS AND APPARATUS OF RLC ENHANCEMENT DURING INTRA-DU LTM

FIELD

The present disclosure relates generally to communication systems, and more particularly, the method of RLC enhancement during intra-DU LTM.

BACKGROUND

In the conventional network of the 3rd generation partnership project (3GPP) 5G new radio (NR) , when the UE moves from the coverage area of one cell to another cell with better signal quality, at some point a serving cell change needs to be performed. Currently serving cell change is triggered by L3 measurements and is done by RRC signaling triggered by reconfiguration with synchronization for change of PCell and PSCell, as well as release/add for SCells when applicable. All cases involve complete L2 (and L1) resets, leading to longer latency, larger overhead, and longer interruption time than beam switch mobility. A further enhancement in 5G NR is the improvement of inter-cell mobility. In order to reduce the latency, overhead, and interruption time during UE mobility, the mobility mechanism can be enhanced to enable a serving cell to change via beam management with L1/L2 signaling. The L1/L2 based inter-cell mobility (LTM, L1L2-triggered Mobility) should support the different scenarios, including intra-DU/inter-DU inter-cell cell change, FR1/FR2, intra-frequency/inter-frequency, and source and target cells may be synchronized or non-synchronized.

In legacy HO design controlled by a series of L3 procedures including RRM measurement and RRC Reconfiguration, ping-pong effects should be avoided with relatively long ToS (time of stay) in order to reduce the occurrences of HOs, accompanied with which is the reduction of signaling overhead and interruption during the overall lifetime of RRC connection. However, the drawback is that UE can’t achieve the optimized instantaneous throughput if the best beam is not belonging to the serving cell. L1/L2 based inter-cell mobility is more proper for the scenarios of intra-DU and inter-DU cell change.

During the cell switch, UE received the handover command, then trigger an RLC reset, and a MAC reset to perform HO between the source cell and the target cell. For intra-DU LTM, RLC layer doesn’t need to be re-established. Therefore, all RLC SDUs, RLC SDU segments, and RLC PDUs are kept and not discarded and the RLC state machine is kept at LTM cell switch. The basic HO that L2 is always reset, data loss due to L2 reset at LTM cell switch will be an issue (especially for RLC UM bearers) considering the high number of LTM cell switch execution. A method for UE to perform RLC enhancement should be considered during intra-DU LTM.

In this invention, apparatus and mechanisms are sought for a method for RLC enhancement in the intra-DU LTM scenario.

SUMMARY

The following presents a simplified summary of one or more aspects in order to provide a basic understanding of such aspects. This summary is not an extensive overview of all contemplated aspects, and is intended to neither identify key or critical elements of all aspects nor delineate the scope of any or all aspects. Its sole purpose is to present some concepts of one or more aspects in a simplified form as a prelude to the more detailed description that is presented later.

In an aspect of the disclosure, a method, a computer-readable medium, and an apparatus are provided. The apparatus may be a UE.

For LTM, consider a method for UE to perform RLC SDUs/segments retransmission. In one embodiment, the RLC bearer is RLC UM bearer, and two UM RLC entities for transmitting side and reception side are maintained during cell switch of LTM. In one embodiment, the RLC SDUs/segments retransmission is performed with RLC STATUS report. In one embodiment, UE is receiving side and initiates RLC STATUS report. In one embodiment, UE is transmitting side and receives the RLC STATUS report from network. In one embodiment, the RLC STATUS report is triggered by the received cell switch command. In one embodiment, the RLC STATUS report is triggered by the network indication or network implementation. In one embodiment, the RLC STATUS report is triggered when channel condition or reception quality meets certain threshold.

In one embodiment, the RLC SDUs/segments retransmission is performed without status report. In one embodiment, the transmitting side checks the HARQ status for each HARQ process and decodes the TBs which are not successfully transmitted to check the sequence number of RLC SDUs/segments. In one embodiment, the method by which the transmitting side checks the HARQ status and the behavior of send the check results to the RLC entity is up to the UE/network implementation. In one embodiment, the transmitting side initiates the RLC retransmission after the cell switch for LTM.

To the accomplishment of the foregoing and related ends, the one or more aspects comprise the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative features of the one or more aspects. These features are indicative, however, of but a few of the various ways in which the principles of various aspects may be employed, and this description is intended to include all such aspects and their equivalents.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates an exemplary NR wireless system with centralization of the upper layers of the NR radio stacks in accordance with the embodiments of the current invention.

Figure 2 illustrates an exemplary deployment scenario for intra-DU inter-cell LTM in accordance with the embodiments of the current invention.

Figure 3 illustrates an exemplary flow chart for the receiving side of RLC retransmission with status report for LTM intra-DU cell switch in accordance with embodiments of the current invention.

Figure 4 illustrates an exemplary flow chart for the transmitting side of RLC retransmission with status report for LTM intra-DU cell switch in accordance with embodiments of the current invention.

Figure 5 illustrates an exemplary flow chart for the receiving side of RLC retransmission without status report for LTM intra-DU cell switch in accordance with embodiments of the current invention.

Figure 6 illustrates an exemplary flow chart for the transmitting side of RLC retransmission without status report for LTM intra-DU cell switch in accordance with embodiments of the current invention.

Figure 7 illustrates an exemplary RLC retransmission when HARQ buffer is flushed and HARQ continuation is not supported during intra-DU LTM cell switch in accordance with embodiments of the current invention.

Figure 8 illustrates an exemplary block diagram of a UE 800 according to an embodiment of the disclosure.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appended drawings is intended as a description of various configurations and is not intended to represent the only configurations in which the concepts described herein may be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of various concepts. However, it will be apparent to those skilled in the art that these concepts may be practiced without these specific details. In some instances, well known structures and components are shown in block diagram form in order to avoid obscuring such concepts.

Several aspects of telecommunication systems will now be presented with reference to various apparatus and methods. These apparatus and methods will be described in the following detailed description and illustrated in the accompanying drawings by various blocks, components, circuits, processes, algorithms, etc. (collectively referred to as “elements” ) . These elements may be implemented using electronic hardware, computer software, or any combination thereof. Whether such elements are implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system.

Aspects of the present disclosure provide methods, apparatus, processing systems, and computer-readable mediums for NR (new radio access technology, or 5G technology) or other radio access technology. NR may support various wireless communication services. These services may have different quality of service (QoS) requirements e.g. latency and reliability requirements.

Figure 1 illustrates an exemplary NR wireless system with centralization of the upper layers of the NR radio stacks in accordance with the embodiments of the current invention. Different protocol split options between the 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. Each Central Unit may control multiple Distributed Units. In one embodiment, SDAP and PDCP layers are located in the central unit, while RLC, MAC, and PHY layers are located in the distributed unit.

Figure 2 illustrates an exemplary deployment scenario for intra-DU inter-cell LTM in accordance with the embodiments of the current invention. A CU (Central Unit) is connected to two DUs (Distributed Unit) through the F1 interface, and two DUs are connected to multiple RUs respectively. A cell may consist of a range covered by one or more RUs under the same DU. In this scenario, a UE is moving from the edge of one cell to another cell, in which two belong to the same DU and share a common protocol stack. Intra-DU LTM can be used in this scenario to replace the legacy handover process to reduce the interruption and improve the throughput of UE. In one embodiment, a single protocol stack at the UE side (common RLC/MAC) is used to handle LTM mobility.

Figure 3 illustrates an exemplary flow chart for the receiving side of RLC retransmission with status report for LTM intra-DU cell switch in accordance with embodiments of the current invention. For DL(downlink) transmission, the apparatus may be a UE. In one embodiment, the RLC bearer is RLC UM bearer. When UE receives the LTM cell switch command from the network, In one embodiment, UE initiates RLC STATUS report. In one embodiment, the RLC STATUS report is immediately triggered by UE upon receive cell switch command. In one embodiment, the RLC STATUS report is triggered by the network indication. In one embodiment, the UE triggers the RLC STATUS report when channel condition or reception quality meets certain threshold. In one embodiment, UE generates the RLC STATUS report based on its own RLC reception buffer. When LTM cell switch is performed, in one embodiment, the two UM RLC entities for transmitting and receiving side are maintained. In one embodiment, UE receives the RLC retransmitted SDUs from the network after cell switch. In one embodiment, the received RLC SDUS/segments are retransmitted based on the RLC STATUS report generated by the receiving side. Likewise, for the UL (uplink) transmission, the network may perform the similar procedure as UE as the figure 3 shown, e.g., initiates RLC STATUS report and receives the retransmitted RLC RLC SDUS/segments from the UE.

Figure 4 illustrates an exemplary flow chart for the transmitting side of RLC retransmission with status report for LTM intra-DU cell switch in accordance with embodiments of the current invention. For UL(uplink) transmission, the apparatus may be a UE. In one embodiment, the RLC bearer is RLC UM bearer. When UE receives the LTM cell switch command from the network, in one embodiment, UE suspend the new RLC transmission and stall the RLC reception window, waiting for the RLC STATUS report from the network. In one embodiment, the network send RLC STATUS report together with cell switch command to UE. In one embodiment, the network triggers RLC STATUS report when the channel conditions of UE meet certain threshold. In one embodiment, the network triggers RLC STATUS report through its implementation. When LTM cell switch is performed, in one embodiment, the two UM RLC entities for transmitting and receiving side are maintained. In one embodiment, UE initiates the RLC RLC SDUS/segments retransmission after the cell switch. In one embodiment, the retransmitted RLC SDUS/segments are generated based on the RLC STATUS report received from the receiving side. Likewise, for the DL transmission, the network may perform the similar procedure as UE as the figure 4 shown, e.g., receives the RLC STATUS report from UE and initiates the RLC retransmission for LTM cell switch.

Figure 5 illustrates an exemplary flow chart for the receiving side of RLC retransmission without status report for LTM intra-DU cell switch in accordance with embodiments of the current invention. For DL (downlink) transmission, the apparatus may be a UE. In one embodiment, the RLC bearer is RLC UM bearer. When LTM cell switch is performed, in one embodiment, the two UM RLC entities for transmitting and receiving side are maintained. In one embodiment, UE receives the retransmitted RLC SDUS/segments from the network after cell switch. Likewise, for the UL (uplink) transmission, the network may perform the similar procedure as UE as the figure 5 shown.

Figure 6 illustrates an exemplary flow chart for the transmitting side of RLC retransmission without status report for LTM intra-DU cell switch in accordance with embodiments of the current invention. For UL(uplink) transmission, the apparatus may be a UE. In one embodiment, the RLC bearer is RLC UM bearer. When UE receives the LTM cell switch command from the network, in one embodiment, the transmitting side checks the HARQ status for each HARQ process and decodes the TBs which are not successfully transmitted to check the sequence number of RLC SDUS/segments. In one embodiment, the method by which the transmitting side checks the HARQ status and the behavior of send the check results to the RLC entity is up to the UE/network implementation. When LTM cell switch is performed, in one embodiment, the two UM RLC entities for transmitting and receiving side are maintained. In one embodiment, UE initiates the RLC retransmission after cell switch, UE obtains the sequence number of the RLC SDUS/segments to be retransmitted by checking HARQ buffer and retransmit the unsuccessfully delivered RLC SDUS/segments via RLC bearer. Likewise, for the DL transmission, the network may perform the similar procedure as UE as the figure 6 shown. E. g., check the unsuccessfully transmitted TBs and retransmit the RLC SDUS/segments via RLC bearer.

Figure 7 illustrates an exemplary cell switch RLC UM operations when HARQ buffer is flushed and HARQ continuation is not supported during intra-DU LTM in accordance with embodiments of the current invention. In one embodiment, the RLC bearer is RLC UM bearer. To avoid the data loss and additional delay of data recovery, the RLC is enhanced to retransmit the missing RLC SDUS/segments during cell switch. The RLC SDUs/segments with SN=1, 2, 3 are transmitted from the source cell to UE. RLC SDUs/segments with SN=3 is successfully delivered and RLC SDUs/segments with SN 1, 2 are lost due to HARQ flush at LTM cell switch. Then RLC SDUs/segments with SN=1, 2 will be retransmitted through the target cell after LTM cell switch. In one embodiment, the RLC SDUs/segments retransmission is performed with RLC STATUS report. The network gets the sequence number 1, 2 of the RLC SDUS/segments to be retransmitted from the RLC STATUS report from UE. In one embodiment, the RLC SDUs/segments retransmission is performed without RLC STATUS report, the DU of network check the HARQ status of TBs for each HARQ process and decodes the sequence number of RLC SDUS/segments from TB which are not successfully transmitted. Then the DU can retransmit the RLC SDUs/segments 1, 2 from the target cell after LTM cell switch if those RLC SDUs/segments are identified as not successfully delivered.

Figure 8 illustrates an exemplary block diagram of a UE 800 according to an embodiment of the disclosure. The UE 800 can include a processor 810, a memory 820, and a radio frequency (RF) module 830 that are coupled together as shown in Figure 8. In different examples, the UE 800 can be a mobile phone, a tablet computer, a desktop computer, a vehicle carried device, and the like.

The processor 810 can include signal processing circuitry to process received or to be transmitted data according to communication protocols specified in, for example, LTE and NR standards. Additionally, the processor 810 may execute program instructions, for example, stored in the memory 820, to perform functions related with different communication protocols. The processor 810 can be implemented with suitable hardware, software, or a combination thereof. For example, the processor 810 can be implemented with application specific integrated circuits (ASIC) , field programmable gate arrays (FPGA) , and the like, that includes circuitry. The circuitry can be configured to perform various functions of the processor 810.

In one example, the memory 820 can store program instructions that, when executed by the processor 810, cause the processor 810 to perform various functions as described herein. The memory 820 can include a read only memory (ROM) , a random access memory (RAM) , a flash memory, a solid state memory, a hard disk drive, and the like.

The RF module 830 can be configured to receive a digital signal from the processor 810 and accordingly transmit a signal to a base station in a wireless communication network via an antenna 840. In addition, the RF module 830 can be configured to receive a wireless signal from a base station and accordingly generate a digital signal which is provided to the processor810. The RF module 830 can include digital to analog/analog to digital converters (DAC/ADC) , frequency down/up converters, filters, and amplifiers for reception and transmission operations. For example, the RF module 830 can include converter circuits, filter circuits, amplification circuits, and the like, for processing signals on different carriers or bandwidth parts.

The UE 800 can optionally include other components, such as input and output devices, additional CPU or signal processing circuitry, and the like. Accordingly, the UE 800 may be capable of performing other additional functions, such as executing application programs, and processing alternative communication protocols.

It is understood that the specific order or hierarchy of blocks in the processes/flowcharts disclosed is an illustration of exemplary approaches. Based upon design preferences, it is understood that the specific order or hierarchy of blocks in the processes/flowcharts may be rearranged. Further, some blocks may be combined or omitted. The accompanying method claims present elements of the various blocks in a sample order, and are not meant to be limited to the specific order or hierarchy presented.

The previous description is provided to enable any person skilled in the art to practice the various aspects described herein. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects. Thus, the claims are not intended to be limited to the aspects shown herein, but is to be accorded the full scope consistent with the language claims, wherein reference to an element in the singular is not intended to mean “one and only one” unless specifically so stated, but rather “one or more. ” The word “exemplary” is used herein to mean “serving as an example, instance, or illustration. ” Any aspect described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects. Unless specifically stated otherwise, the term “some” refers to one or more. Combinations such as “at least one of A, B, or C, ” “one or more of A, B, or C, ” “at least one of A, B, and C, ” “one or more of A, B, and C, ” and “A, B, C, or any combination thereof” include any combination of A, B, and/or C, and may include multiples of A, multiples of B, or multiples of C. Specifically, combinations such as “at least one of A, B, or C, ” “one or more of A, B, or C, ” “at least one of A, B, and C, ” “one or more of A, B, and C, ” and “A, B, C, or any combination thereof” may be A only, B only, C only, A and B, A and C, B and C, or A and B and C, where any such combinations may contain one or more member or members of A, B, or C. All structural and functional equivalents to the elements of the various aspects described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. The words “module, ” “mechanism, ” “element, ” “device, ” and the like may not be a substitute for the word “means. ” As such, no claim element is to be construed as a means plus function unless the element is expressly recited using the phrase “means for. ”

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 for UE to perform RLC retransmission when performing cell switch by LTM, comprising the step of:

receiving the LTM cell switch command from network;

switching to the target cell;

performing RLC retransmission.
The method of claim 1, wherein the RLC retransmission is performed when HARQ continuation is not supported for LTM.
The method of claim 1, wherein the RLC bearer is RLC UM bearer.
The method of claim 1, further comprising UE to maintain the RLC entity when switching to the target cell.
The method of claim 1, wherein the RLC retransmission is performed with RLC status report.
The method of claim 5, wherein the transmission is downlink transmission and UE is the receiving side.
The method of claim 6, further comprising UE to initiate the RLC status report according to RLC reception buffer.
The method of claim 7, wherein the initiation of RLC status report is triggered when UE receives the cell switch command.
The method of claim 7, wherein the initiation of RLC status report is triggered when UE receives the explicit indication of sending RLC status report from network.
The method of claim 7, wherein the initiation of RLC status report is triggered when certain threshold for UE reception quality is met.
The method of claim 6, further comprising UE to receive the RLC retransmitted SDUs/segments from the network after cell switch.
The method of claim 5, wherein the transmission is uplink transmission and UE is the transmitting side.
The method of claim 12, further comprising UE to receives the RLC STATUS report from network.
The method of claim 13, further comprising UE suspends the RLC SDUS/segments transmission and waits for the RLC status report from network.
The method of claim 12, further comprising UE to initiate the RLC retransmission after cell switch.
The method of claim 15, further comprising UE initiates the RLC retransmission base on the RLC status report from network.
The method of claim 1, wherein the RLC retransmission is performed without RLC status report.
The method of claim 17, wherein the transmission is downlink transmission and UE is the receiving side.
The method of claim 18, further comprising UE to receive the RLC retransmitted SDUs/segments from the network after cell switch.
The method of claim 17, wherein the transmission is uplink transmission and UE is the transmitting side.
The method of claim 20, further comprising UE to prepare to retransmit the unsuccessfully transmitted RLC SDUS/segments.
The method of claim 21, further comprising UE detects the TBs in HARQ buffer which are not successfully transmitted, decodes the sequence number of RLC SDUS/segments from TB and prepare to retransmit RLC SDUS/segments via RLC entity.
The method of claim 20, further comprising UE to initiate the RLC retransmission after cell switch.