WO2024197899A1

WO2024197899A1 - Methods, devices and medium for communication

Info

Publication number: WO2024197899A1
Application number: PCT/CN2023/085705
Authority: WO
Inventors: Lin Liang; Gang Wang
Original assignee: NEC Corp
Current assignee: NEC Corp
Priority date: 2023-03-31
Filing date: 2023-03-31
Publication date: 2024-10-03
Anticipated expiration: 2025-09-30
Also published as: EP4690979A1; JP2026514431A; CN121002946A; EP4690979A4

Abstract

Example embodiments of the present disclosure relate to a solution for random access (RA) procedure initiation. In this solution, a terminal device receives, from a first network device, a first physical downlink control channel (PDCCH) order to trigger a random access (RA) procedure for L1/L2 triggered mobility (LTM). The first PDCCH order indicates a first RA resource. The terminal device determines whether the first PDCCH order indicates a first cell of the first network device or a second cell of a second network device for handover. The terminal device is served in the first cell. If the first PDCCH order indicates the second cell, the terminal device initiates the RA procedure towards the second cell using a second RA resource, with the first RA resource in the first PDCCH order ignored. The second RA resource is configured by the second network device for the terminal device.

Description

METHODS, DEVICES AND MEDIUM FOR COMMUNICATION

FIELDS

Example embodiments of the present disclosure generally relate to the field of communication techniques and in particular, to methods, devices, and medium for random access (RA) procedure initiation.

BACKGROUND

Communication systems are widely deployed to provide various telecommunication services. For example, mobility of devices within communication networks have been supported. The functionality of mobility of the communication network can provide continuous coverage for a device such as a terminal device as the device moving from one cell to another cell (also referred to as inter-cell) . For example, as the terminal device will be moving in the communication network, a handover or a cell switch may happen during the mobility. It has been proposed to support a layer one (L1) or layer two (L2) based inter-cell mobility or L1/L2 triggered mobility (LTM) for the handover or cell switch. Works are ongoing regarding RA procedure initiation for LTM.

SUMMARY

In general, embodiments of the present disclosure provide methods, devices and computer storage medium for RA procedure initiation.

In a first aspect, there is provided a communication method performed by a terminal device. The method comprises: receiving, from a first network device, a first physical downlink control channel (PDCCH) order to trigger a random access (RA) procedure, the first PDCCH order indicating a first RA resource; determining whether the first PDCCH order indicates a first cell of the first network device or a second cell of a second network device for handover, the terminal device being served in the first cell; and in accordance with a determination that the first PDCCH order indicates the second cell, initiating the RA procedure towards the second cell using a second RA resource, with the first RA resource in the first PDCCH order ignored, wherein the second RA resource is configured by the second network device for the terminal device.

In a second aspect, there is provided a communication method performed by a second network device. The method comprises: configuring a second random access (RA) resource for a terminal device, the terminal device being served in a first cell of a first network device, the second RA resource comprising a contention-free RA (CFRA) resource for initiating a RA procedure by the terminal device before the terminal device switches to a second cell of the second network device; after the terminal device is switched from the first cell to the second cell, performing a further RA procedure with the terminal device using the second RA resource; and transmitting a further RA response (RAR) for the further RA procedure to the terminal device, regardless of whether the terminal device is configured to monitor a RAR or not, the further RAR at least indicating the TA information for the second cell.

In a third aspect, there is provided a terminal device comprising: a processor configured to cause the terminal device to: receive, from a first network device, a first physical downlink control channel (PDCCH) order to trigger a random access (RA) procedure, the first PDCCH order indicating a first RA resource; determine whether the first PDCCH order indicates a first cell of the first network device or a second cell of a second network device for handover, the terminal device being served in the first cell; and in accordance with a determination that the first PDCCH order indicates the second cell, initiate the RA procedure towards the second cell using a second RA resource, with the first RA resource in the first PDCCH order ignored, wherein the second RA resource is configured by the second network device for the terminal device.

In a fourth aspect, there is provided a second network device comprising: a processor configured to cause the second network device to: configure a second random access (RA) resource for a terminal device, the terminal device being served in a first cell of a first network device, the second RA resource comprising a contention-free RA (CFRA) resource for initiating a RA procedure by the terminal device before the terminal device switches to the second cell; after the terminal device is switched from the first cell to the second cell, perform a further RA procedure with the terminal device using the second RA resource; and transmit a further RA response (RAR) for the further RA procedure to the terminal device, regardless of whether the terminal device is configured to monitor a RAR or not, the further RAR at least indicating the TA information for the second cell.

In a fifth aspect, there is provided a computer readable medium having instructions stored thereon, the instructions, when executed on at least one processor, causing the at least one processor to carry out the method according to the first or second aspect.

Other features of the present disclosure will become easily comprehensible through the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

Through the more detailed description of some example embodiments of the present disclosure in the accompanying drawings, the above and other objects, features and advantages of the present disclosure will become more apparent, wherein:

FIG. 1 illustrates an example communication environment in which example embodiments of the present disclosure can be implemented;

FIG. 2 illustrates a signaling flow for RA procedure initiation according to some example embodiments of the present disclosure;

FIG. 3 illustrates a process for RA procedure initiation according to some example embodiments of the present disclosure;

FIG. 4 illustrates a signaling flow for random access channel (RACH) initiation and RAR monitoring according to some example embodiments of the present disclosure;

FIG. 5 illustrates a signaling flow for RA procedure without RAR monitoring according to some example embodiments of the present disclosure;

FIG. 6 illustrates a process for RA procedure according to some example embodiments of the present disclosure;

FIG. 7A and FIG. 7B illustrate example diagrams showing timing of retransmission according to some example embodiments of the present disclosure, respectively;

FIG. 8A illustrates a process for RA resource selection procedure according to some example embodiments of the present disclosure;

FIG. 8B illustrates another process for RA procedure without RAR monitoring according to some example embodiments of the present disclosure;

FIG. 9A illustrates a process for monitoring RAR with RNTI according to some example embodiments of the present disclosure;

FIG. 9B illustrates another process for monitoring RAR with RNTI according to some example embodiments of the present disclosure;

FIG. 10 illustrates a signaling flow for RA preamble retransmission according to some example embodiments of the present disclosure;

FIG. 11 illustrates a flowchart of a method implemented at a terminal device according to some example embodiments of the present disclosure;

FIG. 12 illustrates another flowchart of a method implemented at a network device according to some example embodiments of the present disclosure; and

FIG. 13 illustrates a simplified block diagram of an apparatus that is suitable for implementing example embodiments of the present disclosure.

Throughout the drawings, the same or similar reference numerals represent the same or similar element.

DETAILED DESCRIPTION

Principle of the present disclosure will now be described with reference to some example embodiments. It is to be understood that these embodiments are described only for the purpose of illustration and help those skilled in the art to understand and implement the present disclosure, without suggesting any limitation as to the scope of the disclosure. Embodiments described herein can be implemented in various manners other than the ones described below.

In the following description and claims, unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skills in the art to which this disclosure belongs.

As used herein, the term ‘terminal device’ refers to any device having wireless or wired communication capabilities. Examples of the terminal device include, but not limited to, user equipment (UE) , personal computers, desktops, mobile phones, cellular phones, smart phones, personal digital assistants (PDAs) , portable computers, tablets, wearable devices, internet of things (IoT) devices, Ultra-reliable and Low Latency Communications (URLLC) devices, Internet of Everything (IoE) devices, machine type communication (MTC) devices, devices on vehicle for V2X communication where X means pedestrian, vehicle, or infrastructure/network, devices for Integrated Access and Backhaul (IAB) , Space borne vehicles or Air borne vehicles in Non-terrestrial networks (NTN) including Satellites and High Altitude Platforms (HAPs) encompassing Unmanned Aircraft Systems (UAS) , eXtended Reality (XR) devices including different types of realities such as Augmented Reality (AR) , Mixed Reality (MR) and Virtual Reality (VR) , the unmanned aerial vehicle (UAV) commonly known as a drone which is an aircraft without any human pilot, devices on high speed train (HST) , or image capture devices such as digital cameras, sensors, gaming devices, music storage and playback appliances, or Internet appliances enabling wireless or wired Internet access and browsing and the like. The ‘terminal device’ can further has ‘multicast/broadcast’ feature, to support public safety and mission critical, V2X applications, transparent IPv4/IPv6 multicast delivery, IPTV, smart TV, radio services, software delivery over wireless, group communications and IoT applications. It may also incorporate one or multiple Subscriber Identity Module (SIM) as known as Multi-SIM. The term “terminal device” can be used interchangeably with a UE, a mobile station, a subscriber station, a mobile terminal, a user terminal or a wireless device.

The term “network device” refers to a device which is capable of providing or hosting a cell or coverage where terminal devices can communicate. Examples of a network device include, but not limited to, a Node B (NodeB or NB) , an evolved NodeB (eNodeB or eNB) , a next generation NodeB (gNB) , a transmission reception point (TRP) , a remote radio unit (RRU) , a radio head (RH) , a remote radio head (RRH) , an IAB node, a low power node such as a femto node, a pico node, a reconfigurable intelligent surface (RIS) , and the like.

The terminal device or the network device may have Artificial intelligence (AI) or Machine learning capability. It generally includes a model which has been trained from numerous collected data for a specific function, and can be used to predict some information.

The terminal or the network device may work on several frequency ranges, e.g., FR1 (e.g., 450 MHz to 6000 MHz) , FR2 (e.g., 24.25GHz to 52.6GHz) , frequency band larger than 100 GHz as well as Tera Hertz (THz) . It can further work on licensed/unlicensed/shared spectrum. The terminal device may have more than one connection with the network devices under Multi-Radio Dual Connectivity (MR-DC) application scenario. The terminal device or the network device can work on full duplex, flexible duplex and cross division duplex modes.

The embodiments of the present disclosure may be performed in test equipment, e.g., signal generator, signal analyzer, spectrum analyzer, network analyzer, test terminal device, test network device, channel emulator. In some embodiments, the terminal device may be connected with a first network device and a second network device. One of the first network device and the second network device may be a master node and the other one may be a secondary node. The first network device and the second network device may use different radio access technologies (RATs) . In some embodiments, the first network device may be a first RAT device and the second network device may be a second RAT device. In some embodiments, the first RAT device is eNB and the second RAT device is gNB. Information related with different RATs may be transmitted to the terminal device from at least one of the first network device or the second network device. In some embodiments, first information may be transmitted to the terminal device from the first network device and second information may be transmitted to the terminal device from the second network device directly or via the first network device. In some embodiments, information related with configuration for the terminal device configured by the second network device may be transmitted from the second network device via the first network device. Information related with reconfiguration for the terminal device configured by the second network device may be transmitted to the terminal device from the second network device directly or via the first network device.

As used herein, the singular forms ‘a’ , ‘an’ and ‘the’ are intended to include the plural forms as well, unless the context clearly indicates otherwise. The term ‘includes’ and its variants are to be read as open terms that mean ‘includes, but is not limited to. ’ The term ‘based on’ is to be read as ‘at least in part based on. ’ The term ‘one embodiment’ and ‘an embodiment’ are to be read as ‘at least one embodiment. ’ The term ‘another embodiment’ is to be read as ‘at least one other embodiment. ’ The terms ‘first, ’ ‘second, ’ and the like may refer to different or same objects. Other definitions, explicit and implicit, may be included below.

In some examples, values, procedures, or apparatus are referred to as ‘best, ’ ‘lowest, ’ ‘highest, ’ ‘minimum, ’ ‘maximum, ’ or the like. It will be appreciated that such descriptions are intended to indicate that a selection among many used functional alternatives can be made, and such selections need not be better, smaller, higher, or otherwise preferable to other selections.

As used herein, the term “resource, ” “transmission resource, ” “uplink resource, ” or “downlink resource” may refer to any resource for performing a communication, such as a resource in time domain, a resource in frequency domain, a resource in space domain, a resource in code domain, or any other resource enabling a communication, and the like. In the following, unless explicitly stated, a resource in both frequency domain and time domain will be used as an example of a transmission resource for describing some example embodiments of the present disclosure. It is noted that example embodiments of the present disclosure are equally applicable to other resources in other domains.

As mentioned above, the mobility of the communication network can provide continuous coverage for a device such as a terminal device as the device moving from one cell to another cell (also referred to as inter-cell) . For example, as the terminal device will be moving in the communication network, a handover or a cell switch may happen during the mobility. In some mechanism, layer three (L3) mobility has been proposed for inter-cell mobility management such as handover management. However, such L3 based mobility may result in delays and interruptions as the terminal device moves from one cell to another cell.

In some mechanisms, L1/L2 based inter-cell mobility which is also referred to as LTM has been proposed to support the inter-cell mobility for the handover or cell switch. LTM may be applied to reduce mobility latency. For example, centralized unit (CU) /distributed unit (DU) interface signaling is used to support L1/L2 mobility. Details regarding the L1/L2 based inter-cell mobility or LTM need to be discussed and improved.

EXAMPLE OF COMMUNICATION ENVIRONMENT

FIG. 1 illustrates a schematic diagram of an example communication environment 100 in which example embodiments of the present disclosure can be implemented. In the communication environment 100, a plurality of communication devices, including a terminal device 110 and a network device 120-1, a network device 130-2, ..., a network device 120-N, can communicate with each other. The network device 120-1, network device 130-2, ..., and network device 120-N can be collectively referred to as “network device (s) 120” or individually referred to as a “network device 120” . The number N can be any suitable integer number.

In the example of FIG. 1, the terminal device 110 may be a UE and the network device 120 may be a base station serving the UE. The serving area of the network device 120 may be called a cell 122. For example, the serving area of the network device 120-1 is a cell 122-1, the serving area of the network device 120-2 is a cell 122-2, and the serving area of the network device 120-N is a cell 122-N. The cell 122-1, cell 122-2, …and cell 122-N can be collectively referred to as “cell (s) 122” or individually referred to as a “cell 122” .

As shown, the terminal device 120 is served by the cell 122-1 of the network device 120-1 at present. In such cases, the cell 122-1 may be referred to as a serving cell or a first cell, and the network device 120-1 may be referred to as a serving network device or a first network device. In the scenario of handover, the cell 122-2 or cell 122-N may be referred to as a candidate cell or a second cell for handover, and the network device 120-2 or network device 120-N may be referred to as a candidate network device or a second network device.

In some example embodiments, the terminal device 110 may move to another cell. For example, if the terminal device 110 moves to the cell 122-2 of the network device 120-2, a handover or cell switch may happen. After the handover, the cell 122-2 becomes the serving cell, and the cell 122-1 may become a candidate cell. It is to be understood that any cell may become the serving cell or candidate cell under different situations.

In the communication environment 100, the network device 120 and the terminal devices 110 may communicate data and control information to each other. The terminal devices 110 may also communicate with each other.

It is to be understood that the number of devices and their connections shown in FIG. 1 are only for the purpose of illustration without suggesting any limitation. The communication environment 100 may include any suitable number of devices configured to implementing example embodiments of the present disclosure. Although not shown, it would be appreciated that one or more additional devices may be located in the cell, and one or more additional cells may be deployed in the communication environment 100. It is noted that although illustrated as a network device, the network device 120 may be another device than a network device. Although illustrated as a terminal device, the terminal device 110 may be other device than a terminal device.

In the following, for the purpose of illustration, some example embodiments are described with the terminal device 110 operating as a UE and the network device 120 operating as a base station. However, in some example embodiments, operations described in connection with a terminal device may be implemented at a network device or other device, and operations described in connection with a network device may be implemented at a terminal device or other device.

In some example embodiments, if the terminal device 110 is a terminal device and the network device 120 is a network device, a link from the network device 120 to the terminal device 110 is referred to as a downlink (DL) , while a link from the terminal device 110 to the network device 120 is referred to as an uplink (UL) . In DL, the network device 120 is a transmitting (TX) device (or a transmitter) and the terminal device 110 is a receiving (RX) device (or a receiver) . In UL, the terminal device 110 is a TX device (or a transmitter) and the network device 120 is a RX device (or a receiver) .

The communications in the communication environment 100 may conform to any suitable standards including, but not limited to, Global System for Mobile Communications (GSM) , Long Term Evolution (LTE) , LTE-Evolution, LTE-Advanced (LTE-A) , New Radio (NR) , Wideband Code Division Multiple Access (WCDMA) , Code Division Multiple Access (CDMA) , GSM EDGE Radio Access Network (GERAN) , Machine Type Communication (MTC) and the like. The embodiments of the present disclosure may be performed according to any generation communication protocols either currently known or to be developed in the future. Examples of the communication protocols include, but not limited to, the first generation (1G) , the second generation (2G) , 2.5G, 2.75G, the third generation (3G) , the fourth generation (4G) , 4.5G, the fifth generation (5G) communication protocols, 5.5G, 5G-Advanced networks, or the sixth generation (6G) networks.

Principles and implementations of the present disclosure will be described in detail below with reference to the figures.

EXAMPLE OF RA PROCEDURE INITIATION

In order to solve at least part of the above problems or other potential problems, a solution on RA procedure initiation is proposed. According to embodiments of the present disclosure, a terminal device receives, from a first network device, a first physical downlink control channel (PDCCH) order to trigger a random access (RA) procedure for L1/L2 triggered mobility (LTM) . The first PDCCH order indicates a first RA resource. The terminal device determines whether the first PDCCH order indicates a first cell of the first network device or a second cell of a second network device for handover. The terminal device is served in the first cell. If the first PDCCH order indicates the second cell, the terminal device initiates the RA procedure towards the second cell using a second RA resource, with the first RA resource in the first PDCCH order ignored. The second RA resource is configured by the second network device for the terminal device.

In this way, the terminal device can determine to initiate the RA procedure towards the serving cell in which the terminal device is served, or determine to initiate the RA procedure towards a candidate cell based on the PDCCH order. Such simple determination process is benefit for mobility latency reduction.

In some cases, while the terminal device 110 is served by a cell (referred to as a “first cell” ) , it can receive a PDCCH order from the serving network device, to indicate a cell for initiating a RA procedure. FIG. 2 illustrates a signaling flow 200 for RA procedure initiation according to some example embodiments of the present disclosure. As shown in FIG. 2, the signaling flow 200 involves the terminal device 110, the network device 120-1 and the network device 120-2 in FIG. 1. In the shown signaling flow 200, it is assumed that the terminal device 110 is currently served in the cell 122-1 (also referred to as a “first cell” or “serving cell” ) of the network device 120-1. The cell 122-2 (also referred to as a “second cell” or “candidate cell” or “target cell” ) of the network device 120-2 is the cell for handover. As used herein, the network device 120-1 may be referred to as a “first network device” , and the network device 120-2 may be referred to as a “second network device” . It is to be understood that the signaling flow 200 may involves more devices or less devices, and the number of devices illustrated in FIG. 2 is only for the purpose of illustration without suggesting any limitations.

In operation, the network device 120-1 transmits (210) a first PDCCH order to the terminal device 110, to trigger a RA procedure. The first PDCCH order initiates a first RA resource. The PDCCH order may be a medium access control (MAC) entity. As an example, the first RA resource comprises downlink control information (DCI) format 1_0 for RA procedure indicated by the PDCCH order.

In some embodiments, the RA procedure may be initiated by the first PDCCH order, by the MAC entity itself, or by RRC for the events. There is one RA procedure ongoing at any point in time in a MAC entity. The RA procedure on an SCell shall be initiated by a PDCCH order with ra-PreambleIndex different from 0b000000.

If a new RA procedure is triggered while another is already ongoing in the MAC entity, it is up to the implementation at the side of the terminal device 110 whether to continue with the ongoing procedure or start with the new procedure (e.g., for system information (SI) request) .

If there was an ongoing RA procedure that is triggered by a PDCCH order while the terminal device 110 receives another PDCCH order indicating the same RA preamble, physical random access channel (PRACH) mask index and uplink carrier, the RA procedure is considered as the same RA procedure as the ongoing one and not initialized again.

In some embodiments, the cyclic redundancy check (CPC) of the DCI format 1_0 is scrambled by cell-radio network temporary identifier (C-RNTI) and the “Frequency domain resource assignment” fields are of all ones, the DCI format 1_0 is for RA procedure initiated by the first PDCCH order, with all remaining fields set as shown in Table 1.

Table 1

It is to be understood that the above example structure or fields of the RA resource is only for the purpose of illustration, without suggesting any limitation. Any suitable RA resource may be applied. Scope of the present disclosure is not limited in this regard.

The terminal device 110 receives 220 the first PDCCH order. The terminal device 110 determines 230 whether the first PDCCH order indicates a first cell (such as the cell 122-1) of the network device 120-1 or a second cell (such as the cell 122-2) of the network device 120-2 for handover.

If the first PDCCH order indicates the first cell such as the cell 122-1, the terminal device 110 initiates 245 the RA procedure towards the network device 120-1 using the first RA resource in the first PDCCH order. For example, if a candidate cell indication field in the DCI format 1_0 indicated by the first PDCCH order indicates an indication of the cell 122-1, the terminal device 110 initiates the RA procedure towards the network device 120-1 using the first RA resource in the first PDCCH order.

In one example, if the value of “Random Access Preamble index” is not all zeros and candidate cell indication field, if configured, indicates the cell 122-1, the SS/PBCH index field (for example, 6 bits) indicates the SS/PBCH that shall be used to determine the RACH occasion for the PRACH transmission. Otherwise, this field is reserved.

In another example, if the ra-PreambleIndex is not 0b000000 and the candidate cell indication field, if configured, indicates the cell 122-1, the PREAMBLE_INDEX is set to the signalled ra-PreambleIndex; and the SSB signalled by PDCCH is selected. This process may be specified in Table 2 below.

Table 2

If the first PDCCH order indicates the cell 122-2, the terminal device initiates 240 the RA procedure towards the cell 122-2 using a second RA resource, with the first RA resource in the first PDCCH order ignored. The second RA resource is configured by the network device 120-2 for the terminal device 110. In some embodiments, the RA procedure on a secondary cell (SCell) such as the cell 122-2 may be initiated by a PDCCH order with ra-PreambleIndex different from 0b000000.

For example, if a candidate cell indication field in the DCI format 1_0 indicated by the first PDCCH order may comprise an indication of the cell 122-2, the terminal device initiates 240 the RA procedure towards the cell 122-2 using the second RA resource. By way of example, the second RA resource comprises a contention-free RA (CFRA) resource.

In some example embodiments, the second RA resource is the RACH recourse configured by radio resource control (RRC) signaling. For example, the terminal device 110 may be configured by RRC RACH resources for each candidate cell including the cell 122-2. The configured RRC RACH resource may include at least RA preamble indices and indications of RACH occasions with the associated synchronization signal block (SSB) indices for each candidate cell.

In some example embodiments, if the candidate cell indication field in the first RA resource does not indicates the cell 122-1, or the candidate cell indication field indicates the cell 122-2, at least one of field (s) of RA Preamble index, UL/SUL indicator, SS/PBCH index (e.g., 6 bits) , or PRACH Mask index in the PDCH order may be ignored or reserved.

By initiating the RA procedure based on the cell indicated by the PDCCH order, the source cell such as the cell 122-1 may not need to know the RACH resources reserved for the terminal device 110 for CFRA in the target cell such as the cell 122-2 configured by the cell 122-2. The source cell can configure any value in the field and the terminal device 110 can initiate CFRA based on the RRC configuration by the target cell.

Example embodiments regarding the RA procedure initiation have been described with respect to FIG. 2. By using the present RA procedure initiation, the terminal device may choose to use the PDCCH ordered resource or the RRC configured resource to perform the RA procedure based on the PDCCH order. Such RA procedure initiation can be benefit for mobility latency reduction. Further example embodiments of RA procedure initiation will be described with respect to FIG. 3.

FIG. 3 illustrates a process 300 for RA procedure initiation according to some example embodiments of the present disclosure. For the purpose of discussion, the process 300 will be described from the perspective of the terminal device 110 in FIG. 1. In the shown process 300, it is assumed that the terminal device 110 is currently served in the cell 122-1 (also referred to as a “first cell” or “serving cell” ) of the network device 120-1. The cell 122-2 (also referred to as a “second cell” or “candidate cell” or “target cell” ) of the network device 120-2 is the cell for handover. As used herein, the network device 120-1 may be referred to as a “first network device” , and the network device 120-2 may be referred to as a “second network device” .

At block 310, the terminal device 110 determines whether the first PDCCH order indicates the first cell (such as the cell 122-1) or the second cell (such as the cell 122-2) . If the first PDCCH order indicates the first cell, at block 350, the terminal device 110 initiates the RA procedure towards the network device 120-1 using the first RA resource.

If the first PDCCH order indicates the second cell, at block 320, the terminal device 110 may determine whether at least one synchronization signal block (SSB) of a plurality of candidate SSBs associated with a RA occasion has a signal quality above a first quality threshold. As one example, the first quality threshold may be predefined or configured. For example, the first quality threshold may be configured as rsrp-ThresholdSSB.

In some embodiments, L1 enhancements for inter-cell beam management such as L1 measurement and reporting and beam indication is applied. The signal quality may be a RSRP from L1 measurement.

If the at least one SSB has a signal quality above the first quality threshold, at block 330, the terminal device 110 initiates the RA procedure using the CFRA resource based on the at least one SSB. Alternatively, if none of the plurality of candidate SSBs has a signal quality above the first quality threshold, at block 340, the terminal device 110 initiates the RA procedure using the CFRA resource based on at least one of the plurality of candidate SSBs. For example, the terminal device 110 may select any SSB amongst the associated SSBs. The terminal device 110 may set the PREAMBLE_INDEX to a ra-PreambleIndex corresponding to the selected SSB. The above process may be described in Table 3 below.

Table 3

By comparing the signal quality and initiating the RA procedure according to the above process, contention free RACH is performed for the first PDCCH order RACH transmission on candidate cells even when reference signal received power (RSRP) is not above the first quality threshold. Comparing with some mechanisms where the terminal device will not initiate the RA procedure using the CFRA resource based on at least one of the plurality of candidate SSBs but instead perform a contention-based RACH, the present RA initiating method will not involve the contention based RACH. In this way, contention-based RACH will not perform, and RACH latency can be reduced.

Example embodiments regarding the RA procedure initiation have been described. By using the present RA procedure initiation, the terminal device may choose to use the PDCCH ordered resource or the RRC configured resource to perform the RA procedure based on the PDCCH order. Such RA procedure initiation can be benefit for mobility latency reduction.

EXAMPLE OF RACH INITIATION AND RAR MONITORING

In some scenarios, the terminal device is currently served by a serving cell (referred to as a first cell) of a first network device. The terminal device is configured by RRC with RACH resources such as the resources specified in Table 1 by a second network device. The terminal device may be indicated by a PDCCH order to initiate RACH on a candidate cell (referred to as a second cell) of the second network device. According to embodiments of the present disclosure, a solution is proposed to improve the RACH initiation and RAR monitoring.

In the solution, the second network device configures a second random access (RA) resource for a terminal device. The second RA resource comprises a contention-free RA (CFRA) resource for initiating a RA procedure by the terminal device before the terminal device switches to the second cell of the second network device. After the terminal device is switched from the first cell to the second cell, the second network device performs a further RA procedure with the terminal device using the second RA resource. The network device transmits a further RA response (RAR) for the further RA procedure to the terminal device, regardless of whether the terminal device is configured to monitor a RAR or not. The further RAR at least indicates the timing advance (TA) information for the second cell.

In this way, the second network device can configure or indicate the TA information the terminal device after the terminal device switched to the second cell of the second network device. By indicating the TA information, the handover latency for obtaining TA information of the second cell can be reduced, and TA management can be improved.

FIG. 4 illustrates a signaling flow 400 for RACH initiation and RAR monitoring according to some example embodiments of the present disclosure. As shown in FIG. 4, the signaling flow 400 involves the terminal device 110 and the network device 120-2 (referred to as a “second network device” ) in FIG. 1. The terminal device 110 is served in the cell 122-1 (also referred to as a “first cell” or “serving cell” ) of the network device 120-1 (referred to as a “first network device” ) . The cell 122-2 (also referred to as a “second cell” or “candidate cell” or “target cell” ) of the network device 120-2 is the cell for handover. It is to be understood that the signaling flow 400 may involves more devices or less devices, and the number of devices illustrated in FIG. 4 is only for the purpose of illustration without suggesting any limitations.

In operation, the network device 120-2 configures a second random access (RA) resource for the terminal device 110. The second RA resource comprises a CFRA resource for initiating a RA procedure by the terminal device 110 before the terminal device 110 switches to a second cell (such as the cell 122-2) of the network device 120-2. That is, the terminal device 110 is configured by RRC RACH resources, including at least RA preamble indices and indication of RACH occasions with the associated SSB indices for each candidate cell.

The terminal device 110 may be indicated by the PDCCH order to initiate RACH on the candidate cell such as the cell 122-2. For example, the terminal device 110 may receive the PDCCH order from the network device 120-1 to initiate RACH on the cell 122-2.

In some embodiments, the terminal device 110 may receive a handover command such as from the network device 120-1. In some example embodiments, RRC configured measurements on the cell 122-1 and cell 122-2. The terminal device 110 may measure and report the results to the network. RRC may indicate a handover command (RRC reconfiguration message including reconfigurationWithSync IE) by indicating target cell common configuration of the cell 122-2, contention free RACH resources (preamble index, RACH occasion, etc. ) for the cell 122-2, C-RNTI used for the cell 122-2.

In response to the handover command, the terminal device 110 performs 420 a handover procedure from the cell 122-1 to the cell 122-2. That is, the terminal device 110 may synchronize with the cell 122-2. For example, the terminal device 110 may perform the following actions shown in Table 4 to execute a reconfiguration with sync.

Table 4

After the terminal device is switched from the cell 122-1 to the cell 122-2, a further RA procedure may be performed 440 by the terminal device 110 and the network device 120-2 using the second RA resource. For example, the terminal device 110 may transmit the RACH preamble to the network device 120-2. It is to be understood that after the cell switching, the terminal device is served by the cell 122-2 of the network device 120-2. The cell 122-1 of the network device 120-1 becomes a candidate cell, accordingly.

In some example embodiments, the terminal device 110 may determine 430 whether timing advance (TA) information for the cell 122-2 is unavailable or invalid. In some embodiments, for PDCCH ordered-RACH for candidate cell (s) , RAR reception may be configured/indicated. If reception of RAR is not configured/indicated (without RAR) , the TA value of candidate cell may be indicated in cell switch command or handover command.

If the handover command does not include the TA information or the handover command indicates an invalid TA value, the TA information is considered to be unavailable or invalid. If RAR is considered to be successful, and if the TA information or TA command is valid, the terminal device 110 may apply the TA command in RAR and consider the RACH is successful completed. If RACH is considered to be successful completed, the terminal device 110 may consider handover is completed.

If the TA information is unavailable or invalid, the terminal device 110 may initiate a further RA procedure towards the cell 122-2 using the CFRA resource.

In some example embodiments, if the terminal device 110 receives the handover command and the TA information is unavailable or invalid if the terminal device 110 is configured to monitor RAR for the cell 122-2, the terminal device 110 may switch to the cell 122-2 and may not perform any UL transmission on the cell 122-2 except the RA preamble due to timeAlignmentTimer which the cell 122-2 belongs is not running.

Alternatively, or in addition, if the terminal device 110 receives the handover command and the terminal device 110 does not receive RAR in the cell 122-2 to complete RACH if the terminal device 110 is configured to monitor RAR for the cell 122-2, the terminal device 110 may switch to the cell 122-2 and may not perform any UL transmission on the cell 122-2 except the RA preamble due to timeAlignmentTimer which the cell 122-2 belongs is not running.

In some example embodiments, the network device 120-2 transmits 450 a further RAR for the further RA procedure to the terminal device 110, regardless of whether the terminal device 110 is configured to monitor a RAR or not. The further RAR at least indicates the TA information for the cell 122-2. The terminal device 110 may monitor or receive 460 the further RAR for the further RA procedure from the network device 120-2, regardless of whether the terminal device 110 is configured to monitor a RAR or not. That is, the terminal device 110 may initiate RACH on the cell 122-2 using RACH resources configured for the cell 122-2 and monitor RAR. The network device 120-2 may transmit 450 RAR on the cell 122-2 regardless of whether the terminal device 110 is configured to monitor RAR or not. By doing so, it can give chance for the terminal device 110 to get TA information when the terminal device 110 has switched to the cell 122-2 but is configured to not monitor RAR.

In this way, if RACH is not successfully completed on the candidate cell such as the cell 122-2 while channel condition require handover immediately, the terminal device 110 can firstly switch to the candidate cell and then using CFRA RACH to the candidate cell to reduce latency.

In some embodiments, the TA field in the command to trigger switching the cell may have special value to indicate invalid TA information. It can apply to situation where handover should be triggered but candidate cell does not detect RACH or TA information has not been received from candidate cell yet.

Example embodiments regarding RACH initiation and RAR monitoring have been described with respect to FIG. 4. With the RACH initiation and RAR monitoring according to the present disclosure, the mobility latency or handover latency can be reduced.

EXAMPLE OF RA PROCEDURE WITHOUT RAR MONITORING

In some scenarios, after a terminal device transmits a RA preamble, the terminal device needs to monitor a RAR. According to some legacy RA mechanisms, a terminal device may perform actions illustrated by Table 5 below.

Table 5

According to legacy some RA mechanisms, a terminal device may perform actions such as RACH power ramping according to Table 6 below.

Table 6

As discussed above, according to some legacy mechanisms, the terminal device performs RAR monitoring during the RA procedure. However, in some scenarios where a RA procedure towards a candidate cell or non-serving cell is initiated based on a PDCCH order, the RAR monitoring may result in latency. In order to solve at least part of the above problems or other potential problems, several solutions on RA procedure without RAR monitoring are proposed. These solutions will be described with respect to FIG. 5 to FIG. 7B.

In the following embodiments with respect to FIG. 5 to FIG. 7B, it is assumed that the terminal device 110 is currently served by the cell 122-1 (referred to as a “first cell” or “serving cell” ) of the network device 120-1 (referred to as a “first network device” or “serving network device” ) . The terminal device 110 receives a PDCCH order from the network device 120-1, to indicate a cell for initiating a RA procedure. The cell 122-2 (also referred to as a “second cell” or “candidate cell” or “target cell” ) of the network device 120-2 (referred to as a “second network device” ) is the cell for handover.

FIG. 5 illustrates a signaling flow 500 for performing the RA procedure without RAR monitoring according to some example embodiments of the present disclosure. As shown in FIG. 5, the signaling flow 500 involves the terminal device 110 and the network device 120-2 in FIG. 1. It is to be understood that the signaling flow 500 may involves more devices or less devices, and the number of devices illustrated in FIG. 5 is only for the purpose of illustration without suggesting any limitations.

In operation, the terminal device 110 determines 510 whether the terminal device 110 is configured to monitor a RAR for the RA procedure. For example, the terminal device 110 may determine 510 whether it is configured to monitor RAR based on RRC RACH resources or other suitable signaling.

In some example embodiments, if the terminal device 110 determines 510 that the terminal device 110 is configured not to monitor the RAR for the RA procedure, the terminal device 110 determines 540 that the RA procedure is successfully completed after the RA procedure is initiated. That is, after performing RACH preamble transmission towards the target candidate cell such as the cell 122-2, the terminal device 110 may consider the RA procedure successfully completed. In this way, the terminal device 110 can quickly complete the RACH procedure and return back to source cell which can reduce interruption time.

Alternatively, or in addition, in some example embodiments, if the terminal device 110 determines 510 that the terminal device 110 is configured not to monitor the RAR for the RA procedure, determine that the RA procedure is not successfully completed after the RA procedure is initiated. The terminal device 110 may transmit 520 a RA preamble for a predetermined number of times to the network device 120-2 during the RA procedure. The network device 120-2 may receive 530 the RA preamble (s) .

In some embodiments, after the RA preamble transmission, the terminal device 110 may consider the RACH procedure is not completed because RAR is not received. Alternatively, in some embodiments, the terminal device 110 may determine 540 that the RA procedure is successfully completed after the RA preamble is transmitted for the predetermined number of times. The predetermined number may be configured. For example, the predetermined number may be less than preambleTransMax. For another example, the predetermined number may be be defined as a minimum value of preambleTransMax. In such cases, the terminal device 110 may further consider power ramping step is zero or may not increase the power ramping counter such as PREAMBLE_POWER_RAMPING_COUNTER.

In this way, autonomous repetition can be achieved. The terminal device 110 may determine the next occasion for preamble transmission after the previous preamble transmission. FIG. 7A illustrates an example diagram 700 showing timing of retransmission in such cases. As shown, the terminal device 110 performs an initial transmission 710 at the RACH occasion (RO) 702. If the initial transmission 710 is not completed after the RO 702, the terminal device 110 may perform an autonomous retransmission 720 at RO 704. In such embodiments, the terminal device 110 does not monitor RAR, thus the terminal device 110 may perform the autonomous retransmission 720 at RO 704 without waiting for search spaces (SSs) . It is to be understood the numbers of ROs and SSs are only for the purpose of illustration, without suggesting any limitation.

Still refers to FIG. 5, in some example embodiments, if the terminal device 110 determines 510 that the terminal device 110 is configured to monitor the RAR, the terminal device 110 may perform further actions. For example, the terminal device 110 may perform actions according to Table 5 above. If the terminal device 110 performs actions according to Table 5, the terminal device 110 may monitor the RAR with a RAR timer such as ra-ResponseWindow configured in RACH-ConfigCommon.

Alternatively, in some embodiments, the terminal device 110 may perform a process 600 for RA procedure as shown in FIG. 6. For the purpose of discussion, the process 600 will be described from the perspective of the terminal device 110 in FIG. 1.

At block 610, the terminal device 110 monitors a RAR for the RA procedure by setting a RAR timer as zero. That is, the terminal device 110 monitors the RAR but considers that the RAR timer is zero.

At block 620, the terminal device 110 determines whether the RAR timer expires. In cases where the RAR timer is set as zero at block 610, the terminal device 110 determines that the RAR timer expires at block 620.

If the RAR timer expires, at block 630, the terminal device 110 determines that a RA preamble in the RA procedure is transmitted on a secondary cell (SCell) such as the cell 122-2. The terminal device 110 considers RACH transmission on the target candidate cell as transmission on the SCell.

If the RA preamble is transmitted on the SCell such as the cell 122-2, at block 640, the terminal device 110 may determine that the RA procedure is unsuccessfully completed. The terminal device 110 may further consider power ramping step is zero or may not increase the power ramping counter such as PREAMBLE_POWER_RAMPING_COUNTER.

In this way, autonomous repetition may be achieved. The terminal device 110 may determine the next occasion for preamble transmission after the next occasion after RAR expires. FIG. 7B illustrates an example diagram 750 showing timing of retransmission in such cases. As shown, similar to FIG. 7A, the terminal device 110 performs an initial transmission 710 at the RACH occasion (RO) 702. What is different from FIG. 7A is that if the initial transmission 710 is not completed after the RO 702, the terminal device 110 may perform an autonomous retransmission 760 at RO 762, instead of the RO 704. This is because the terminal device 110 monitors RAR, thus the terminal device 110 may perform the autonomous retransmission 720 after the SSs. It is to be understood the numbers of ROs and SSs are only for the purpose of illustration, without suggesting any limitation.

It is to be understood that although in the process 600, RAR monitoring is performed, the time duration of the RAR monitoring is quite small because the RAR timer is set to be zero. RA process with such short RAR monitoring may be considered as a RA procedure without RAR monitoring.

Still refers to FIG. 5, in some example embodiments, if the terminal device 110 determines 510 that the terminal device 110 is configured not to monitor a RAR for the RA procedure, the terminal device 110 may determine 540 to perform a process 800 or process 860 which will be described with respect to FIG. 8A and FIG. 8B, respectively.

FIG. 8A illustrates the process 800 for performing a RA resource selection procedure according to some example embodiments of the present disclosure. For the purpose of discussion, the process 800 will be described from the perspective of the terminal device 110 in FIG. 1.

At block 810, the terminal device 110 determines that the RA procedure is not completed. For example, the terminal device 110 may determine that the RA procedure is on hold. At block 820, the terminal device 110 may increment a retransmission counter of a RA preamble in the RA procedure. For example, the terminal device 110 may increase the retransmission counter by one.

At block 830, the terminal device 110 determines whether the RA procedure is unfinished (or not completed) and the retransmission counter is within a retransmission upper threshold. The retransmission upper threshold may be predefined or configured. For example, the retransmission upper threshold may be preambleTransMax.

If the RA procedure is not completed and the retransmission counter is within the retransmission upper threshold, at block 840, the terminal device 110 determines whether a second PDCCH order indicating the second cell such as the cell 122-2 is received. If the second PDCCH order indicates that the second cell is received, at block 850, the terminal device 110 may perform a RA resource selection procedure.

That is, the terminal device 110 may not perform the RA resource selection procedure until a PDCCH order indicating the same candidate cell is received. In some embodiments, if the terminal device 110 receives the PDCCH order indicating the same candidate cell such as the cell 122-2, the terminal device 110 may not initiate the retransmission counter such as PREAMBLE_TRANSMISSION_COUNTER and the power ramping counter such as PREAMBLE_POWER_RAMPING_COUNTER.

In some embodiments, the terminal device 110 may reset the power ramping counter such as PREAMBLE_POWER_RAMPING_COUNTER after a configured timer expires or a new RRC configuration or MAC CE command or indication in PDCCH order.

FIG. 8B illustrates a process 860 performed during the RA procedure without RAR monitoring. In some embodiments, if the terminal device 110 is configured not to monitor the RAR, the terminal device 110 may perform the process 860. For the purpose of discussion, the process 860 will be described from the perspective of the terminal device 110 in FIG. 1.

At block 860, the terminal device 110 determines whether a signal quality of a SSB for initiating the RA procedure is above a second quality threshold. The second quality threshold may be predefined or configured. The signal quality may be synchronization signal based reference signal received power (SS-RSRP) or any other signal quality parameter.

If the signal quality of the SSB is above the second quality threshold, at block 890, the terminal device 110 may determine that the RA procedure is successfully completed. For example, the terminal device 110 may perform no autonomous retransmission.

Alternatively, if the signal quality of the SSB is not above the second quality threshold, at block 880, the terminal device 110 may determine that the RA procedure is not successfully completed. That is, the terminal device 110 may perform an autonomous retransmission if the signal quality such as the SS-RSRP is not above the second quality threshold.

Embodiments regarding the RA procedure without RAR monitoring or with a short RAR monitoring have been described above. It is to be understood that the above signaling flow 500 or process 600, 800 or 860 may be performed separately, or in any suitable combination. By using these signaling flow or processes, of the time duration of the RA procedure can be reduced. The mobility latency can thus be reduced.

EXAMPLE OF RAR MONITORING

In some scenarios, during the RA procedure of a non-serving cell (also referred to as a second cell) initiated by a serving cell (also referred to as a first cell) , monitoring RAR enables the terminal device to obtain TA earlier. Therefore, in some situations, the terminal device may perform RAR monitoring to achieve benefits.

According to some legacy mechanism, a set of PDCCH candidates for a terminal device to monitor is defined in terms of PDCCH SS sets. A SS set can be a common search space (CSS) set or a UE-specific search space (USS) set. The terminal device monitors PDCCH candidates in one or more of the following search spaces sets: a Type1-PDCCH CSS set configured by ra-SearchSpace in PDCCH-ConfigCommon for a DCI format with CRC scrambled by a RA-RNTI or a temporary C-RNTI (TC-RNTI) on the primary cell.

If the terminal device is provided with one or more search space sets by corresponding one or more of searchSpaceZero, searchSpaceSIB1, searchSpaceOtherSystemInformation, pagingSearchSpace, ra-SearchSpace, and a C-RNTI, an modulation and coding scheme (MCS) -C-RNTI, or a configured scheduling (CS) -RNTI, the terminal device monitors PDCCH candidates for DCI format 0_0 and DCI format 1_0 with CRC scrambled by the C-RNTI, the MCS-C-RNTI, or the CS-RNTI in the one or more search space sets in a slot where the terminal device monitors PDCCH candidates for at least a DCI format 0_0 or a DCI format 1_0 with CRC scrambled by system information (SI) -RNTI, RA-RNTI or padding (P) -RNTI.

According to the legacy mechanisms described above, the terminal device used various RNTI for RAR monitoring under different conditions. Details actions regarding how to use RNTI for RAR monitoring or which RNTI is used for RAR monitoring need to be discussed. According to present disclosure, several detail solutions regarding RAR monitoring with RNTI have been proposed, which will be described with respect to FIG. 9A to FIG. 10.

In the following embodiments with respect to FIG. 9A to FIG. 10, it is assumed that the terminal device 110 is currently served by the cell 122-1 (referred to as a “first cell” or “serving cell” ) of the network device 120-1 (referred to as a “first network device” or “serving network device” ) . The terminal device 110 receives a PDCCH order from the network device 120-1, to indicate a cell for initiating a RA procedure. The cell 122-2 (also referred to as a “second cell” or “candidate cell” or “target cell” ) of the network device 120-2 (referred to as a “second network device” ) is the cell for handover.

FIG. 9A illustrates a process 900 for monitoring RAR with RNTI according to some example embodiments of the present disclosure. For the purpose of discussion, the process 900 will be described from the perspective of the terminal device 110 in FIG. 1.

At block 910, the terminal device 110 determines whether the terminal device 110 is configured to monitor a RAR for the RA procedure. If the terminal device 110 is configured to monitor the RAR, at block 920, the terminal device 110 monitors the RAR by monitoring DCI using at least one of: a first C-RNTI configured for the first cell such as the cell 122-1 or a second C-RNTI configured for the second cell such as the cell 122-2. As one example, the terminal device 110 may monitor the RAR by monitoring DCI using the second C-RNTI configured for the cell 122-1 without using the first C-RNTI.

By using the process 900, the terminal device 110 may start applying common cell configuration including type-1 SS configuration and/or C-RNTI value configured for the candidate cell such as the cell 122-2. The terminal device 110 monitors DCI on search space for the serving cell such as the cell 122-1 using C-RNTI of the serving cell and the terminal device 110 monitors DCI on search space for the non-serving cell (candidate cell) such as the cell 122-2 using C-RNTI of the non-serving cell. Thus, the PDCCH monitoring can be improved.

FIG. 9B illustrates another process 950 for monitoring RAR with RNTI according to some example embodiments of the present disclosure. For the purpose of discussion, the process 950 will be described from the perspective of the terminal device 110 in FIG. 1.

Similar to process 900, at block 910, the terminal device 110 determines whether the terminal device 110 is configured to monitor a RAR for the RA procedure. If the terminal device 110 is configured to monitor the RAR, at block 960, the terminal device 110 may monitor the RAR by skipping monitoring of DCI using a second C-RNTI configured for the second cell such as the cell 122-2.

At block 970, the terminal device 110 determines whether a search space for monitoring DCI is configured for the second cell such as the cell 122-2. If the search space is configured for the second cell, at block 980, the terminal device 110 may skip monitoring of DCI by skipping at least one DCI occasion in the search space.

That is, the terminal device 110 may not apply C-RNTI configured for the candidate cell such as the cell 12202. The terminal device 110 doesn’t monitor PDCCH candidates for DCI format 0_0 and DCI format 1_0 with CRC scrambled by the C-RNTI in the one or more search space sets in a slot where the terminal device 110 monitors PDCCH candidates for at least a DCI format 0_0 or a DCI format 1_0 with CRC scrambled by SI-RNTI, RA-RNTI or P-RNTI if the search space is on the non-serving cell (candidate cell for handover) such as the cell 122-2.

The reason for doing so is that C-RNTI is a per cell configured value. Candidate cell may configure the same C-RNTI to another terminal device. If the terminal device still monitors this C-RNTI on the candidate cell, the scheduling for the other terminal device will be wrongly performed for the terminal device. Therefore, by using the process 950, the RAR monitoring and resource scheduling can be improved.

In some embodiments, the terminal device 110 may ignore the UL grant on the candidate cell such as the cell 122-2. If RACH is completed on the candidate cell such as the cell 122-2, the terminal device 110 may stop applying common cell configuration including type-1 Search Space configuration and/or C-RNTI value configured for the candidate cell.

In embodiments where the terminal device 110 monitoring the RAR by monitoring DCI using at least one of the first C-RNTI configured for the first cell such as the cell 122-1 or the second C-RNTI configured for the second cell such as the cell 122-2, the terminal device 110 may perform RA preamble retransmission under some situations. FIG. 10 illustrates a signaling flow 1000 for RA preamble retransmission according to some example embodiments of the present disclosure. As shown in FIG. 10, the signaling flow 1000 involves the terminal device 110 and the network device 120-2 in FIG. 1. The terminal device 110 is served in the cell 122-1 (also referred to as a “first cell” or “serving cell” ) of the network device 120-1 (referred to as a “first network device” ) . The cell 122-2 (also referred to as a “second cell” or “candidate cell” or “target cell” ) of the network device 120-2 is the cell for handover. It is to be understood that the signaling flow 1000 may involves more devices or less devices, and the number of devices illustrated in FIG. 10 is only for the purpose of illustration without suggesting any limitations.

In operation, the terminal device 110 and the network device 120-1 performs 1010 a RA procedure before the terminal device 110 switches to the cell 122-2. The network device 120-2 may transmit 1020, to the terminal device 110, a third PDCCH order in DCI using a second C-RNTI configured for the cell 122-2. The third PDCCH order indicates the cell 122-2. In some example embodiments, the third PDCCH order further indicates the second RA resource. The terminal device 110 may receive 1030 the third PDCCH order.

In some embodiments, the terminal device 110 may determine 1040 whether a reception of the RAR is not successfully completed and DCI is detected from the network device 120-2 using the second C-RNTI. If the reception of the RAR is not successfully completed and DCI is detected from the network device 120-2 using the second C-RNTI, the terminal device 110 may determine 1050 whether the third PDCCH order comprised in the DCI indicates the second cell such as the cell 122-2. If the third PDCCH order indicates the cell 122-2, the terminal device 110 may stop 1070 monitoring of the RAR. In addition, if the third PDCCH order indicates the cell 122-2, the terminal device 110 may perform a RA preamble retransmission in the RA procedure. For example, the terminal device 110 may transmit 1070 the RA preamble to the network device 120-2. The network device 120-2 may receive 1080 the RA preamble if the transmission succeeds.

Alternatively, or in addition, in some embodiments, if the third PDCCH order indicates the second cell, the terminal device 110 may determine whether the third PDCCH order further indicates the second RA resource. If the third PDCCH order indicates the second RA resource, the terminal device 110 may stop 1060 monitoring of the RAR and perform a RA preamble retransmission in the RA procedure.

By performing the signaling flow 1000, if RAR is not successful completed and a PDCCH order addressed to the target C-RNTI for the target cell configured by RRC is received, and/or if PDCCH order indicates the same Random Access Preamble, PRACH mask index and uplink carrier configured by RRC, the terminal device 110 can stop RAR timer and consider the RACH is not completed. For example, the terminal device 110 may perform RACH preamble retransmission immediately.

By doing so, it enables early indication of preamble mis-detection by network and corresponding retransmission indication to the terminal device 110 to further reduce the latency of RACH towards the candidate cell and/or interruption time of serving cell. The terminal device 110 thus can perform earlier preamble retransmission to reduce latency when RAR is configured to monitor.

It would be appreciated that some example specifications and embodiments are provided above, and the detailed description may be varied.

Example embodiments for RA procedure initiation, RACH initiation and RAR monitoring, RA procedure without RAR monitoring, and RAR monitoring are described above with reference to the signaling flows 200, 400, 500 and 1000, and processes 300, 600, 800, 860, 900 and 950. In some embodiments, embodiments described with reference to two or more of the above signaling flows and processed may be combined. By using these signaling flows and/or processes, L1/L2 based inter-cell mobility or LTM can be improved. Particularly, the mobility latency can be reduced.

EXAMPLE METHODS

FIG. 11 illustrates a flowchart of a communication method 1100 implemented at a terminal device in accordance with some embodiments of the present disclosure. For the purpose of discussion, the method 1100 will be described from the perspective of the terminal device 110 in FIG. 1.

At block 1110, the terminal device 110 receives, from a first network device such as the network device 120-1, a first physical downlink control channel (PDCCH) order to trigger a random access (RA) procedure. The first PDCCH order indicates a first RA resource.

At block 1120, the terminal device 110 determines whether the first PDCCH order indicates a first cell of the first network device or a second cell of a second network device such as the network device 120-2 for handover. The terminal device 110 is served in the first cell.

At block 1130, if the first PDCCH order indicates the second cell, the terminal device 110 initiates the RA procedure towards the second cell using a second RA resource, with the first RA resource in the first PDCCH order ignored. The second RA resource is configured by the second network device for the terminal device 110.

In some example embodiments, if the first PDCCH order indicates the first cell, the terminal device 110 may initiate the RA procedure towards the first network device using the first RA resource.

In some example embodiments, the second RA resource comprises a contention-free RA (CFRA) resource.

In some example embodiments, if the first PDCCH order indicates the second cell, the terminal device 110 may determine whether at least one synchronization signal block (SSB) of a plurality of candidate SSBs associated with a RA occasion has a signal quality above a first quality threshold. If the at least one SSB has a signal quality above the first quality threshold, the terminal device 110 may initiate the RA procedure using the CFRA resource based on the at least one SSB. If none of the plurality of candidate SSBs has a signal quality above the first quality threshold, the terminal device 110 may initiate the RA procedure using the CFRA resource based on at least one of the plurality of candidate SSBs.

In some example embodiments, in response to a handover command, the terminal device 110 may perform a handover procedure from the first cell to the second cell. If timing advance (TA) information for the second cell is unavailable or invalid, the terminal device 110 may initiate a further RA procedure towards the second cell using the CFRA resource; and monitor a further RA response (RAR) for the further RA procedure from the second network device, regardless of whether the terminal device 110 is configured to monitor a RAR or not. The further RAR at least indicates the TA information for the second cell.

In some example embodiments, if the terminal device 110 is configured not to monitor a RAR for the RA procedure, the terminal device 110 may determine that the RA procedure is successfully completed after the RA procedure is initiated.

In some example embodiments, if the terminal device 110 is configured not to monitor a RAR for the RA procedure, the terminal device 110 may determine that the RA procedure is not successfully completed after the RA procedure is initiated. The terminal device 110 may transmit, to the second network device, a RA preamble for a predetermined number of times during the RA procedure. The terminal device 110 may determine that the RA procedure is successfully completed after the RA preamble is transmitted for the predetermined number of times.

In some example embodiments, the terminal device 110 may monitor a RAR for the RA procedure by setting a RAR timer as zero. If the RAR timer expires, the terminal device 110 may determine that a RA preamble in the RA procedure is transmitted on a secondary cell (SCell) . If the RA preamble is transmitted on the SCell, the terminal device 110 may determine that the RA procedure is unsuccessfully completed.

In some example embodiments, if the terminal device 110 is configured not to monitor a RAR for the RA procedure, the terminal device 110 may determine that the RA procedure is not completed. The terminal device 110 may increment a retransmission counter of a RA preamble in the RA procedure. If the RA procedure is not completed and the retransmission counter is within a retransmission upper threshold, the terminal device 110 may determine whether a second PDCCH order indicating the second cell is received. If the second PDCCH order indicating the second cell is received, the terminal device 110 may perform a RA resource selection procedure.

In some example embodiments, if the terminal device 110 is configured not to monitor a RAR for the RA procedure, the terminal device 110 may determine whether a signal quality of a SSB for initiating the RA procedure is above a second quality threshold. If the signal quality of the SSB is above the second quality threshold, the terminal device 110 may determine that the RA procedure is successfully completed. If the signal quality of the SSB is not above the second quality threshold, the terminal device 110 may determine that the RA procedure is not successfully completed.

In some example embodiments, if the terminal device 110 is configured to monitor a RAR for the RA procedure, the terminal device 110 may monitor the RAR by monitoring downlink control information (DCI) using at least one of: a first cell-radio network temporary identifier (C-RNTI) configured for the first cell or a second C-RNTI configured for the second cell.

In some example embodiments, if a reception of the RAR is not successfully completed and DCI is detected from the second network device using the second C-RNTI, the terminal device 110 may determine whether a third PDCCH order comprised in the DCI indicates the second cell. If the third PDCCH order indicates the second cell, the terminal device 110 may stop monitoring of the RAR and perform a RA preamble retransmission in the RA procedure.

In some example embodiments, if the third PDCCH order indicates the second cell, the terminal device 110 may determine whether the third PDCCH order further indicates the second RA resource. If the third PDCCH order indicates the second RA resource, the terminal device 110 may stop monitoring of the RAR and perform a RA preamble retransmission in the RA procedure.

In some example embodiments, if the terminal device 110 is configured to monitor a RAR for the RA procedure, the terminal device 110 may monitor the RAR by skipping monitoring of DCI using a second C-RNTI configured for the second cell.

In some example embodiments, the terminal device 110 may determine whether a search space for monitoring DCI is configured for the second cell. If the search space is configured for the second cell, the terminal device 110 may skip monitoring of DCI by skipping at least one DCI occasion in the search space.

FIG. 12 illustrates a flowchart of a communication method 1200 implemented at a second network device in accordance with some embodiments of the present disclosure. For the purpose of discussion, the method 1200 will be described from the perspective of the network device 120-2 in FIG. 1.

At block 1210, the network device 120-2 configures a second random access (RA) resource for a terminal device such as the terminal device 110. The terminal device is served in a first cell of a first network device such as the network device 120-1. The second RA resource comprises a contention-free RA (CFRA) resource for initiating a RA procedure by the terminal device before the terminal device switches to the second cell.

At block 1220, after the terminal device is switched from the first cell to the second cell, the network device 120-2 performs a further RA procedure with the terminal device using the second RA resource.

At block 1230, the network device 120-2 transmits a further RA response (RAR) for the further RA procedure to the terminal device, regardless of whether the terminal device is configured to monitor a RAR or not. The further RAR at least indicates the TA information for the second cell.

In some example embodiments, the network device 120-2 may perform the RA procedure with the terminal device before the terminal device switches to the second cell. The network device 120-2 may transmit, to the terminal device, a third PDCCH order in DCI using a second cell-radio network temporary identifier (C-RNTI) configured for the second cell. The third PDCCH order indicates the second cell. The network device 120-2 may receive, from the terminal device, a RA preamble retransmission in the RA procedure.

In some example embodiments, the third PDCCH order further indicates the second RA resource.

EXAMPLE DEVICE

FIG. 13 is a simplified block diagram of a device 1300 that is suitable for implementing embodiments of the present disclosure. The device 1300 can be considered as a further example implementation of any of the devices as shown in FIG. 1. Accordingly, the device 1300 can be implemented at or as at least a part of the terminal device 110 or the network device 120.

As shown, the device 1300 includes a processor 1310, a memory 1320 coupled to the processor 1310, a suitable transceiver 1340 coupled to the processor 1310, and a communication interface coupled to the transceiver 1340. The memory 1310 stores at least a part of a program 1330. The transceiver 1340 may be for bidirectional communications or a unidirectional communication based on requirements. The transceiver 1340 may include at least one of a transmitter 1342 and a receiver 1344. The transmitter 1342 and the receiver 1344 may be functional modules or physical entities. The transceiver 1340 has at least one antenna to facilitate communication, though in practice an Access Node mentioned in this application may have several ones. The communication interface may represent any interface that is necessary for communication with other network elements, such as X2/Xn interface for bidirectional communications between eNBs/gNBs, S1/NG interface for communication between a Mobility Management Entity (MME) /Access and Mobility Management Function (AMF) /SGW/UPF and the eNB/gNB, Un interface for communication between the eNB/gNB and a relay node (RN) , or Uu interface for communication between the eNB/gNB and a terminal device.

The program 1330 is assumed to include program instructions that, when executed by the associated processor 1310, enable the device 1300 to operate in accordance with the embodiments of the present disclosure, as discussed herein with reference to FIGS. 1 to 12. The embodiments herein may be implemented by computer software executable by the processor 1310 of the device 1300, or by hardware, or by a combination of software and hardware. The processor 1310 may be configured to implement various embodiments of the present disclosure. Furthermore, a combination of the processor 1310 and memory 1320 may form processing means 1350 adapted to implement various embodiments of the present disclosure.

The memory 1320 may be of any type suitable to the local technical network and may be implemented using any suitable data storage technology, such as a non-transitory computer readable storage medium, semiconductor based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory, as non-limiting examples. While only one memory 1320 is shown in the device 1300, there may be several physically distinct memory modules in the device 1300. The processor 1310 may be of any type suitable to the local technical network, and may include one or more of general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs) and processors based on multicore processor architecture, as non-limiting examples. The device 1300 may have multiple processors, such as an application specific integrated circuit chip that is slaved in time to a clock which synchronizes the main processor.

According to embodiments of the present disclosure, a terminal device comprising a circuitry is provided. The circuitry is configured to: receive, from a first network device, a first physical downlink control channel (PDCCH) order to trigger a random access (RA) procedure, the first PDCCH order indicating a first RA resource; determine whether the first PDCCH order indicates a first cell of the first network device or a second cell of a second network device for handover, the terminal device being served in the first cell; and in accordance with a determination that the first PDCCH order indicates the second cell, initiate the RA procedure towards the second cell using a second RA resource, with the first RA resource in the first PDCCH order ignored, wherein the second RA resource is configured by the second network device for the terminal device. According to embodiments of the present disclosure, the circuitry may be configured to perform any method implemented by the terminal device as discussed above.

According to embodiments of the present disclosure, a second network device comprising a circuitry is provided. The circuitry is configured to: configure a second random access (RA) resource for a terminal device, the terminal device being served in a first cell of a first network device, the second RA resource comprising a contention-free RA (CFRA) resource for initiating a RA procedure by the terminal device before the terminal device switches to the second cell; after the terminal device is switched from the first cell to the second cell, perform a further RA procedure with the terminal device using the second RA resource; and transmit a further RA response (RAR) for the further RA procedure to the terminal device, regardless of whether the terminal device is configured to monitor a RAR or not, the further RAR at least indicating the TA information for the second cell. According to embodiments of the present disclosure, the circuitry may be configured to perform any method implemented by the second network device as discussed above.

According to embodiments of the present disclosure, a communication method comprising a circuitry is provided. The circuitry is configured to: determining whether the first PDCCH order indicates a first cell of the first network device or a second cell of a second network device for handover, the terminal device being served in the first cell; and in accordance with a determination that the first PDCCH order indicates the second cell, initiating the RA procedure towards the second cell using a second RA resource, with the first RA resource in the first PDCCH order ignored, wherein the second RA resource is configured by the second network device for the terminal device. According to embodiments of the present disclosure, the circuitry may be configured to perform any method implemented by the communication method as discussed above.

The term “circuitry” used herein may refer to hardware circuits and/or combinations of hardware circuits and software. For example, the circuitry may be a combination of analog and/or digital hardware circuits with software/firmware. As a further example, the circuitry may be any portions of hardware processors with software including digital signal processor (s) , software, and memory (ies) that work together to cause an apparatus, such as a terminal device or a network device, to perform various functions. In a still further example, the circuitry may be hardware circuits and or processors, such as a microprocessor or a portion of a microprocessor, that requires software/firmware for operation, but the software may not be present when it is not needed for operation. As used herein, the term circuitry also covers an implementation of merely a hardware circuit or processor (s) or a portion of a hardware circuit or processor (s) and its (or their) accompanying software and/or firmware.

In summary, embodiments of the present disclosure provide the following aspects.

In an aspect, it is proposed a terminal device comprising: a processor configured to cause the terminal device to: receive, from a first network device, a first physical downlink control channel (PDCCH) order to trigger a random access (RA) procedure, the first PDCCH order indicating a first RA resource; determine whether the first PDCCH order indicates a first cell of the first network device or a second cell of a second network device for handover, the terminal device being served in the first cell; and in accordance with a determination that the first PDCCH order indicates the second cell, initiate the RA procedure towards the second cell using a second RA resource, with the first RA resource in the first PDCCH order ignored, wherein the second RA resource is configured by the second network device for the terminal device.

In some embodiments, the processor is further configured to cause the terminal device to: in accordance with a determination that the first PDCCH order indicates the first cell, initiate the RA procedure towards the first network device using the first RA resource.

In some embodiments, the second RA resource comprises a contention-free RA (CFRA) resource.

In some embodiments, the processor is further configured to cause the terminal device to: in accordance with a determination that the first PDCCH order indicates the second cell, determine whether at least one synchronization signal block (SSB) of a plurality of candidate SSBs associated with a RA occasion has a signal quality above a first quality threshold; in accordance with a determination that the at least one SSB has a signal quality above the first quality threshold, initiate the RA procedure using the CFRA resource based on the at least one SSB; and in accordance with a determination that none of the plurality of candidate SSBs has a signal quality above the first quality threshold, initiate the RA procedure using the CFRA resource based on at least one of the plurality of candidate SSBs.

In some embodiments, the processor is further configured to cause the terminal device to: in response to a handover command, perform a handover procedure from the first cell to the second cell; in accordance with a determination that timing advance (TA) information for the second cell is unavailable or invalid, initiate a further RA procedure towards the second cell using the CFRA resource; and monitor a further RA response (RAR) for the further RA procedure from the second network device, regardless of whether the terminal device is configured to monitor a RAR or not, the further RAR at least indicating the TA information for the second cell.

In some embodiments, the processor is further configured to cause the terminal device to: in accordance with a determination that the terminal device is configured not to monitor a RAR for the RA procedure, determine that the RA procedure is successfully completed after the RA procedure is initiated.

In some embodiments, the processor is further configured to cause the terminal device to: in accordance with a determination that the terminal device is configured not to monitor a RAR for the RA procedure, determine that the RA procedure is not successfully completed after the RA procedure is initiated; transmit, to the second network device, a RA preamble for a predetermined number of times during the RA procedure; and determine that the RA procedure is successfully completed after the RA preamble is transmitted for the predetermined number of times.

In some embodiments, the processor is further configured to cause the terminal device to: monitor a RAR for the RA procedure by setting a RAR timer as zero; in accordance with a determination that the RAR timer expires, determine that a RA preamble in the RA procedure is transmitted on a secondary cell (SCell) ; and in accordance with a determination that the RA preamble is transmitted on the SCell, determine that the RA procedure is unsuccessfully completed.

In some embodiments, the processor is further configured to cause the terminal device to: in accordance with a determination that the terminal device is configured not to monitor a RAR for the RA procedure, determine that the RA procedure is not completed; increment a retransmission counter of a RA preamble in the RA procedure; in accordance with a determination that the RA procedure is not completed and the retransmission counter is within a retransmission upper threshold, determine whether a second PDCCH order indicating the second cell is received; and in accordance with a determination that the second PDCCH order indicating the second cell is received, perform a RA resource selection procedure.

In some embodiments, the processor is further configured to cause the terminal device to: in accordance with a determination that the terminal device is configured not to monitor a RAR for the RA procedure, determine whether a signal quality of a SSB for initiating the RA procedure is above a second quality threshold; in accordance with a determination that the signal quality of the SSB is above the second quality threshold, determine that the RA procedure is successfully completed; and in accordance with a determination that the signal quality of the SSB is not above the second quality threshold, determine that the RA procedure is not successfully completed.

In some embodiments, the processor is further configured to cause the terminal device to: in accordance with a determination that the terminal device is configured to monitor a RAR for the RA procedure, monitor the RAR by monitoring downlink control information (DCI) using at least one of: a first cell-radio network temporary identifier (C-RNTI) configured for the first cell or a second C-RNTI configured for the second cell.

In some embodiments, the processor is further configured to cause the terminal device to: in accordance with a determination that a reception of the RAR is not successfully completed and DCI is detected from the second network device using the second C-RNTI, determine whether a third PDCCH order comprised in the DCI indicates the second cell; in accordance with a determination that the third PDCCH order indicates the second cell, stop monitoring of the RAR; and perform a RA preamble retransmission in the RA procedure.

In some embodiments, the processor is further configured to cause the terminal device to: in accordance with a determination that the third PDCCH order indicates the second cell, determine whether the third PDCCH order further indicates the second RA resource; and in accordance with a determination that the third PDCCH order indicates the second RA resource, stop monitoring of the RAR; and perform a RA preamble retransmission in the RA procedure.

In some embodiments, the processor is further configured to cause the terminal device to: in accordance with a determination that the terminal device is configured to monitor a RAR for the RA procedure, monitor the RAR by skipping monitoring of DCI using a second C-RNTI configured for the second cell.

In some embodiments, the processor is further configured to cause the terminal device to: determine whether a search space for monitoring DCI is configured for the second cell; in accordance with a determination that the search space is configured for the second cell, skip monitoring of DCI by skipping at least one DCI occasion in the search space.

In an aspect, it is proposed a second network device comprising: a processor configured to cause the second network device to: configure a second random access (RA) resource for a terminal device, the terminal device being served in a first cell of a first network device, the second RA resource comprising a contention-free RA (CFRA) resource for initiating a RA procedure by the terminal device before the terminal device switches to the second cell; after the terminal device is switched from the first cell to the second cell, perform a further RA procedure with the terminal device using the second RA resource; and transmit a further RA response (RAR) for the further RA procedure to the terminal device, regardless of whether the terminal device is configured to monitor a RAR or not, the further RAR at least indicating the TA information for the second cell.

In some embodiments, the processor is further configured to cause the second network device to: perform the RA procedure with the terminal device before the terminal device switches to the second cell; transmit, to the terminal device, a third PDCCH order in DCI using a second cell-radio network temporary identifier (C-RNTI) configured for the second cell, the third PDCCH order indicating the second cell; and receive, from the terminal device, a RA preamble retransmission in the RA procedure.

In some embodiments, the third PDCCH order further indicates the second RA resource.

In an aspect, a terminal device comprises: at least one processor; and at least one memory coupled to the at least one processor and storing instructions thereon, the instructions, when executed by the at least one processor, causing the device to perform the method implemented by the terminal device discussed above.

In an aspect, a second network device comprises: at least one processor; and at least one memory coupled to the at least one processor and storing instructions thereon, the instructions, when executed by the at least one processor, causing the device to perform the method implemented by the second network device discussed above.

In an aspect, a computer readable medium having instructions stored thereon, the instructions, when executed on at least one processor, causing the at least one processor to perform the method implemented by the terminal device discussed above.

In an aspect, a computer readable medium having instructions stored thereon, the instructions, when executed on at least one processor, causing the at least one processor to perform the method implemented by the second network device discussed above.

In an aspect, a computer program comprising instructions, the instructions, when executed on at least one processor, causing the at least one processor to perform the method implemented by the terminal device discussed above.

In an aspect, a computer program comprising instructions, the instructions, when executed on at least one processor, causing the at least one processor to perform the method implemented by the second network device discussed above.

Generally, various embodiments of the present disclosure may be implemented in hardware or special purpose circuits, software, logic or any combination thereof. Some aspects may be implemented in hardware, while other aspects may be implemented in firmware or software which may be executed by a controller, microprocessor or other computing device. While various aspects of embodiments of the present disclosure are illustrated and described as block diagrams, flowcharts, or using some other pictorial representation, it will be appreciated that the blocks, apparatus, systems, techniques or methods described herein may be implemented in, as non-limiting examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof.

The present disclosure also provides at least one computer program product tangibly stored on a non-transitory computer readable storage medium. The computer program product includes computer-executable instructions, such as those included in program modules, being executed in a device on a target real or virtual processor, to carry out the process or method as described above with reference to FIGS. 1 to 12. Generally, program modules include routines, programs, libraries, objects, classes, components, data structures, or the like that perform particular tasks or implement particular abstract data types. The functionality of the program modules may be combined or split between program modules as desired in various embodiments. Machine-executable instructions for program modules may be executed within a local or distributed device. In a distributed device, program modules may be located in both local and remote storage media.

Program code for carrying out methods of the present disclosure may be written in any combination of one or more programming languages. These program codes may be provided to a processor or controller of a general purpose computer, special purpose computer, or other programmable data processing apparatus, such that the program codes, when executed by the processor or controller, cause the functions/operations specified in the flowcharts and/or block diagrams to be implemented. The program code may execute entirely on a machine, partly on the machine, as a stand-alone software package, partly on the machine and partly on a remote machine or entirely on the remote machine or server.

The above program code may be embodied on a machine readable medium, which may be any tangible medium that may contain, or store a program for use by or in connection with an instruction execution system, apparatus, or device. The machine readable medium may be a machine readable signal medium or a machine readable storage medium. A machine readable medium may include but not limited to an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples of the machine readable storage medium would include an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM) , a read-only memory (ROM) , an erasable programmable read-only memory (EPROM or Flash memory) , an optical fiber, a portable compact disc read-only memory (CD-ROM) , an optical storage device, a magnetic storage device, or any suitable combination of the foregoing.

Further, while operations are depicted in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Likewise, while several specific implementation details are contained in the above discussions, these should not be construed as limitations on the scope of the present disclosure, but rather as descriptions of features that may be specific to particular embodiments. Certain features that are described in the context of separate embodiments may also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment may also be implemented in multiple embodiments separately or in any suitable sub-combination.

Although the present disclosure has been described in language specific to structural features and/or methodological acts, it is to be understood that the present disclosure defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.

Claims

A terminal device comprising:

a processor configured to cause the terminal device to:

receive, from a first network device, a first physical downlink control channel (PDCCH) order to trigger a random access (RA) procedure, the first PDCCH order indicating a first RA resource;

determine whether the first PDCCH order indicates a first cell of the first network device or a second cell of a second network device for handover, the terminal device being served in the first cell; and

in accordance with a determination that the first PDCCH order indicates the second cell, initiate the RA procedure towards the second cell using a second RA resource, with the first RA resource in the first PDCCH order ignored,

wherein the second RA resource is configured by the second network device for the terminal device.
The terminal device of claim 1, wherein the processor is further configured to cause the terminal device to:

in accordance with a determination that the first PDCCH order indicates the first cell, initiate the RA procedure towards the first network device using the first RA resource.
The terminal device of claim 1, wherein the second RA resource comprises a contention-free RA (CFRA) resource.
The terminal device of claim 3, wherein the processor is further configured to cause the terminal device to:

in accordance with a determination that the first PDCCH order indicates the second cell, determine whether at least one synchronization signal block (SSB) of a plurality of candidate SSBs associated with a RA occasion has a signal quality above a first quality threshold;

in accordance with a determination that the at least one SSB has a signal quality above the first quality threshold, initiate the RA procedure using the CFRA resource based on the at least one SSB; and

in accordance with a determination that none of the plurality of candidate SSBs has a signal quality above the first quality threshold, initiate the RA procedure using the CFRA resource based on at least one of the plurality of candidate SSBs.
The terminal device of claim 3, wherein the processor is further configured to cause the terminal device to:

in response to a handover command, perform a handover procedure from the first cell to the second cell;

in accordance with a determination that timing advance (TA) information for the second cell is unavailable or invalid, initiate a further RA procedure towards the second cell using the CFRA resource; and

monitor a further RA response (RAR) for the further RA procedure from the second network device, regardless of whether the terminal device is configured to monitor a RAR or not, the further RAR at least indicating the TA information for the second cell.
The terminal device of claim 1, wherein the processor is further configured to cause the terminal device to:

in accordance with a determination that the terminal device is configured not to monitor a RAR for the RA procedure, determine that the RA procedure is successfully completed after the RA procedure is initiated.
The terminal device of claim 1, wherein the processor is further configured to cause the terminal device to:

in accordance with a determination that the terminal device is configured not to monitor a RAR for the RA procedure, determine that the RA procedure is not successfully completed after the RA procedure is initiated;

transmit, to the second network device, a RA preamble for a predetermined number of times during the RA procedure; and

determine that the RA procedure is successfully completed after the RA preamble is transmitted for the predetermined number of times.
The terminal device of claim 1, wherein the processor is further configured to cause the terminal device to:

monitor a RAR for the RA procedure by setting a RAR timer as zero; and

in accordance with a determination that the RAR timer expires, determine that a RA preamble in the RA procedure is transmitted on a secondary cell (SCell) ;

in accordance with a determination that the RA preamble is transmitted on the SCell, determine that the RA procedure is unsuccessfully completed.
The terminal device of claim 1, wherein the processor is further configured to cause the terminal device to:

in accordance with a determination that the terminal device is configured not to monitor a RAR for the RA procedure, determine that the RA procedure is not completed;

increment a retransmission counter of a RA preamble in the RA procedure; and

in accordance with a determination that the RA procedure is not completed and the retransmission counter is within a retransmission upper threshold, determine whether a second PDCCH order indicating the second cell is received; and

in accordance with a determination that the second PDCCH order indicating the second cell is received, perform a RA resource selection procedure.
The terminal device of claim 1, wherein the processor is further configured to cause the terminal device to:

in accordance with a determination that the terminal device is configured not to monitor a RAR for the RA procedure, determine whether a signal quality of a SSB for initiating the RA procedure is above a second quality threshold; and

in accordance with a determination that the signal quality of the SSB is above the second quality threshold, determine that the RA procedure is successfully completed; and

in accordance with a determination that the signal quality of the SSB is not above the second quality threshold, determine that the RA procedure is not successfully completed.
The terminal device of claim 1, wherein the processor is further configured to cause the terminal device to:

in accordance with a determination that the terminal device is configured to monitor a RAR for the RA procedure, monitor the RAR by monitoring downlink control information (DCI) using at least one of: a first cell-radio network temporary identifier (C-RNTI) configured for the first cell or a second C-RNTI configured for the second cell.
The terminal device of claim 11, wherein the processor is further configured to cause the terminal device to:

in accordance with a determination that a reception of the RAR is not successfully completed and DCI is detected from the second network device using the second C-RNTI, determine whether a third PDCCH order comprised in the DCI indicates the second cell;

in accordance with a determination that the third PDCCH order indicates the second cell, stop monitoring of the RAR; and

perform a RA preamble retransmission in the RA procedure.
The terminal device of claim 12, wherein the processor is further configured to cause the terminal device to:

in accordance with a determination that the third PDCCH order indicates the second cell, determine whether the third PDCCH order further indicates the second RA resource; and

in accordance with a determination that the third PDCCH order indicates the second RA resource,

stop monitoring of the RAR; and

perform a RA preamble retransmission in the RA procedure.
The terminal device of claim 1, wherein the processor is further configured to cause the terminal device to:

in accordance with a determination that the terminal device is configured to monitor a RAR for the RA procedure, monitor the RAR by skipping monitoring of DCI using a second C-RNTI configured for the second cell.
A second network device comprising:

a processor configured to cause the second network device to:

configure a second random access (RA) resource for a terminal device, the terminal device being served in a first cell of a first network device, the second RA resource comprising a contention-free RA (CFRA) resource for initiating a RA procedure by the terminal device before the terminal device switches to a second cell of the second network device;

after the terminal device is switched from the first cell to the second cell, perform a further RA procedure with the terminal device using the second RA resource; and

transmit a further RA response (RAR) for the further RA procedure to the terminal device, regardless of whether the terminal device is configured to monitor a RAR or not, the further RAR at least indicating the timing advance (TA) information for the second cell.
The second network device of claim 15, wherein the processor is further configured to cause the second network device to:

perform the RA procedure with the terminal device before the terminal device switches to the second cell;

transmit, to the terminal device, a third PDCCH order in DCI using a second cell-radio network temporary identifier (C-RNTI) configured for the second cell, the third PDCCH order indicating the second cell; and

receive, from the terminal device, a RA preamble retransmission in the RA procedure.
The second network device of claim 16, wherein the third PDCCH order further indicates the second RA resource.
A communication method comprising:

receiving, by a terminal device and from a first network device, a first physical downlink control channel (PDCCH) order to trigger a random access (RA) procedure, the first PDCCH order further indicating a first RA resource;

determining whether the first PDCCH order indicates a first cell of the first network device or a second cell of a second network device for handover, the terminal device being served in the first cell; and

in accordance with a determination that the first PDCCH order indicates the second cell, initiating the RA procedure towards the second cell using a second RA resource, with the first RA resource in the first PDCCH order ignored,

wherein the second RA resource is configured by the second network device for the terminal device.
A communication method comprising:

configuring, by a second network device, a second random access (RA) resource for a terminal device, the terminal device being served in a first cell of a first network device, the second RA resource comprising a contention-free RA (CFRA) resource for initiating a RA procedure by the terminal device before the terminal device switches to the second cell;

after the terminal device is switched from the first cell to the second cell, performing a further RA procedure with the terminal device using the second RA resource; and

transmitting a further RA response (RAR) for the further RA procedure to the terminal device, regardless of whether the terminal device is configured to monitor a RAR or not, the further RAR at least indicating the TA information for the second cell.
A computer readable medium having instructions stored thereon, the instructions, when executed on at least one processor, causing the at least one processor to perform the method according to claim 18 or the method according to claim 19.