The disclosed embodiments relate generally to wireless communication, and, more particularly, to a method for L1/L2-based mobility enhancement in 5G New Radio (NR) cellular communication networks.
The wireless communications network has grown exponentially over the years. A long-term evolution (LTE) system offers high peak data rates, low latency, improved system capacity, and low operating cost resulting from simplified network architecture. LTE systems, also known as the 4G system, also provide seamless integration to older wireless network, such as GSM, CDMA and universal mobile telecommunication system (UMTS). In LTE systems, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNodeBs or eNBs) communicating with a plurality of mobile stations, referred to as user equipments (UEs). The 3rd generation partner project (3GPP) network normally includes a hybrid of 2G/3G/4G systems. The next generation mobile network (NGMN) board has decided to focus the future NGMN activities on defining the end-to-end requirements for 5G new radio (NR) systems. In 5G NR, the base stations are also referred to as gNodeBs or gNBs.
Frequency bands for 5G NR are being separated into two different frequency ranges. Frequency Range 1 (FR1) includes sub-6 GHz frequency bands, some of which are bands traditionally used by previous standards, but has been extended to cover potential new spectrum offerings from 410 MHz to 7125 MHz. Frequency Range 2 (FP2) includes frequency bands from 24.25 GHz to 52.6 GHz. Bands in FR2 in this millimeter wave range have shorter range but higher available bandwidth than bands in FR1. For UEs in RRC Idle mode mobility, cell selection is the procedure through which a UE picks up a specific cell for initial registration after power on, and cell reselection is the mechanism to change cell after UE is camped on a cell and stays in idle mode. For UEs in RRC Connected mode mobility, handover is the procedure through which a UE hands over an ongoing session from the source gNB to a neighboring target gNB.
The concept of “candidate cell” has been introduced. for conditional handover (CHO) and conditional PSCell addition/change (CPA/CPC). The UE applies candidate cell configurations received earlier upon cell switch (CHO/CPAC execution). Upon CHO/CPC/CPA execution, other candidates are released. Note that CHO configuration can be delta configuration, meaning that once applied, UE cannot switch back to source cell without another RRC configuration from target cell (e.g., to add source cell as a candidate cell). In R18 L1/L2-based inter cell mobility, pre-configured candidate cells are also considered. L1/L2 signaling (e.g., MAC CE) is used to trigger UE switching to a candidate cell.
With the existing CHO/CPAC procedures, a candidate cell must be released and configured again. With L1/L2 mobility, candidate cells should be kept for enhanced performance. This is because in FR2, due to smaller coverage and denser deployment, UE is likely to switch back-and-forth among a set of candidate/active cells. It is thus desired to allow UE to switch to another candidate cell, or back to original serving cell, without receiving additional RRC reconfiguration message.
A method of L1/L2-based mobility with signaling optimization using base (reference) configuration and replaceable (delta) configuration for candidate cells is proposed. All candidates can be configured at different levels, provided by Radio Resource Control (RRC) signaling, e.g., RRCReconfiguration. To support L1/L2-triggered mobility (LTM), there is a common base configuration for candidates, and the candidates are modelled as replaceable configurations which are delta configurations on top of the common base configuration (instead of UE's current RRC configuration). The common and replaceable configurations are also referred to as reference configuration and candidate delta configuration. The candidate delta configuration is applied on top of the reference configuration to form a complete candidate configuration for handover.
In one embodiment, a UE maintains a configuration in a serving cell of a mobile communication network, wherein the configuration comprises a base configuration and one or more replaceable configurations for corresponding candidate cells. The UE receives a cell-switching command from the network to handover from the serving cell to a first target cell belonging to the candidate cells. The UE applies a first replaceable configuration of the first target cell on top of the base configuration upon receiving the cell switching command, wherein the first replaceable configuration and the base configuration form a complete configuration for the first target cell.
Other embodiments and advantages are described in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims.
The accompanying drawings, where like numerals indicate like components, illustrate embodiments of the invention.
Reference will now be made in detail to some embodiments of the invention, examples of which are illustrated in the accompanying drawings.
Frequency bands for 5G NR are being separated into two different frequency ranges. Frequency Range 1 (FR1) includes sub-6 GHz frequency bands, some of which are bands traditionally used by previous standards, but has been extended to cover potential new spectrum offerings from 410 MHz to 7125 MHz. Frequency Range 2 (FR2) includes frequency bands from 24.25 GHz to 52.6 GHz. Bands in FR2 in this millimeter wave range have shorter range but higher available bandwidth than bands in FR1. The 5G core function receives all connection and session related information and is responsible for connection and mobility management tasks. For UEs in radio resource control (RRC) Idle mode mobility, cell selection is the procedure through which a UE picks up a specific cell for initial registration after power on, and cell reselection is the mechanism to change cell after UE is camped on a cell and stays in idle mode. For UEs in RRC Connected mode mobility, handover is the procedure through which a UE hands over an ongoing session from the source gNB to a neighboring target gNB.
In the example of
The concept of “candidate cell” has been introduced for conditional handover (CHO) and conditional PSCell addition/change (CPA/CPC). The UE applies candidate cell configurations received earlier upon cell switch (CHO/CPAC execution). Upon CHO/CPC/CPA execution, other candidates are released, e.g., a candidate cell must be released and configured again. With L1/L2 mobility, candidate cells should be kept for enhanced performance. This is because in FR2, due to smaller coverage and denser deployment, UE is likely to switch back-and-forth among a set of candidate/active cells.
In accordance with one novel aspect, a method of for L1/L2-based mobility with candidate cell configuration is proposed. All candidates can be configured at different levels, provided by Radio Resource Control (RRC) signaling, e.g., RRCReconfiguration. To support L1/L2-triggered mobility (LTM), there is a common base configuration for candidates, and the candidates are modelled as replaceable configurations which are delta configurations on top of the common base configuration (instead of UE's current RRC configuration). The common and replaceable configurations are also referred to as reference configuration and candidate delta configuration. The candidate delta configuration is applied on top of the reference configuration to form a complete candidate configuration for handover. Under the proposed method, UE is allowed to switch to different candidate cells or back to the original serving cell without receiving additional RRC reconfiguration message.
In the example of
Similarly, the UE 201 has a memory 202, a processor 203, and an RF transceiver module 204. The RF transceiver 204 is coupled with the antenna 405, receives RF signals from the antenna 205, converts them to baseband signals, and sends them to the processor 203. The RF transceiver 204 also converts received baseband signals from the processor 203, converts them to RF signals, and sends out to the antenna 205. The processor 203 processes the received baseband signals (e.g., comprising an SCell/PSCell addition/activation command) and invokes different functional modules and circuits to perform features in the UE 201. The memory 202 stores data and program instructions 210 to be executed by the processor 203 to control the operations of the UE 201. Suitable processors include, by way of example, a special purpose processor, a Digital Signal Processor (DSP), a plurality of micro-processors, one or more micro-processor associated with a DSP core, a controller, a microcontroller, Application Specific Integrated Circuits (ASICs), File Programmable Gate Array (FPGA) circuits, and other type of Integrated Circuits (ICs), and/or state machines. A processor associated with software may be used to implement and configure features of the UE 201.
The UE 201 also includes a protocol stack 260 and a set of control function modules and circuits 270. The protocol stack 260 may include a NAS layer to communicate with an AMF/SMF/MME entity connecting to the core network, an RRC layer for high layer configuration and control, a PDCP/RLC layer, a MAC layer, and a PHY layer. The Control function modules and circuits 270 may be implemented and configured by software, firmware, hardware, and/or combination thereof. The control function modules and circuits 270, when executed by the processor 203 via program instructions contained in the memory 202, interwork with each other to allow the UE 201 to perform embodiments and functional tasks and features in the network. In one example, the control function modules and circuits 270 include a configuration circuit 271 for obtaining measurement and configuration information of candidate cells, a measurement circuit 272 for performing and reporting measurements, and a handover handling circuit 273 for performing synchronization and handover procedure based on the common base configuration and replaceable configuration for the candidate cells received from the network. The base configuration and the replaceable configuration form a complete configuration of the candidate target cells.
In CHO, when one CHO candidate (Cell B) is chosen, its configuration is applied (on top of Cell A), and other candidates (including Cell C) are released because their configuration (C|A) is no longer applicable after reconfiguration (Cell B). Therefore, subsequent CHO (without RRC reconfiguration) is not supported. In subsequent LTM, in addition to the current RRC configurations (A, B, or C), the UE maintains a separate reference configuration (R), and the candidate configurations (B|R, C|R) and provided as delta configurations on top of the reference configuration. Upon cell switch, the target configuration based on R is applied, and this allows subsequent LTM. It is also possible that the network performs RRC reconfiguration after candidate configurations (A to A′) or even after cell switch (B to B′). With reference configuration maintained separately, the candidate configurations can still be applied upon LTM cell switch, as long as the reference configuration is not modified.
In the example of
Candidate configuration at cell group (CG) level is depicted by 510. Network provided multiple MCG (Main cell group) and multiple SCG (Secondary cell group) configurations to UE. The common base MCG and SCG configuration may comprise MAC config, RLC bearer config, SpCell config, and serving cell config, etc. Upon L1/L2-based cell-switching command, the UE activates corresponding CG configuration (including RLC, MAC, and PHY parts), and then the UE performs MAC reset. UE may perform RLC re-establishment, depending on network indication. Configuration above CG level remains the same.
Candidates configuration at serving cell level is depicted by 520. Network configures multiple cells in each cell group for a UE. The cells can be SpCell (Special cell) or SCell (Secondary cell). Upon L1/L2-based cell-switching command, UE activates a different subset of configured cells. The subset contains one SpCell and may contain one or more SCells. Note that SpCell and SCells are all serving cells.
Under L1/L2-based mobility, UE 601 performs handover from Cell #0 to Cell #1. UE 601 applies the target cell configuration for Cell #1 by combining the first replaceable configuration (delta configuration #1) on top of the common base configuration. In one example, the first delta configuration #1 comprises a first physical cell ID for Cell #1, SSB configuration, etc. Later on, UE 601 performs handover from Cell #1 to Cell #2. UE 601 applies the target cell configuration for Cell #2 by combining the second replaceable configuration (delta configuration #2) on top of the common base configuration. In one example, the first delta configuration #2 comprises a second physical cell ID for Cell #2, SSB configuration, etc. UE 601 may hand over to the Cell #0 at a later time, provided that a delta configuration for Cell #0 is provided by the network and maintained by UE 601.
In step 731, UE 701 performs synchronization for candidate cells. In the downlink, UE 701 performs fine time-frequency tracking for at least some beams of the candidate cells. In the uplink, UE 701 performs pre-RACH for timing advance (optional). In step 741, UE 701 sends measurement or beam report to the source gNB. In step 742, the source gNB makes cell-switch decision. In step 743, the source gNB sends a cell-switch command to UE 501. Upon receiving the cell switching command, the cell switching command can be L1/L2 signal. In step 751, UE 701 applies target cell configuration, which is the replaceable (delta) configuration on top of the base (reference) configuration. UE 701 detaches from the source cell, but UE 701 may keep the RRC configurations of the source cell and the candidate cells. In step 761, the handover procedure is completed. In step 771, UE 701 starts data transmission and reception in the target cell.
Although the present invention has been described in connection with certain specific embodiments for instructional purposes, the present invention is not limited thereto. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the claims.
This application claims priority under 35 U.S.C. § 119 from U.S. Provisional Application No. 63/370,290, entitled “Candidate Cell Configuration for L1/L2-based Mobility”, filed on Aug. 3, 2022, the subject matter of which is incorporated herein by reference.
Number | Date | Country | |
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63370290 | Aug 2022 | US |