Method and apparatus for handling conditional handover (CHO) in a wireless communication network

Abstract

The present disclosure relates to communication methods and systems for integrating fifth generation (5G) communication systems with Internet of Things (IoT) technologies to support higher data rates than fourth generation (4G) systems. The present disclosure can be applied to smart services based on 5G communication technologies and IoT-related technologies, such as smart homes, smart buildings, smart cities, smart cars, connected cars, healthcare, digital education, smart retail, security and safety services. The present disclosure provides an apparatus and method for processing conditional handover (CHO) in a wireless communication network.

Description

Technical Field

[0001] The present disclosure relates to the field of wireless communication networks, and more particularly, to controlling enhanced mobility in wireless communication networks by configuring conditional handover (CHO) to a user equipment (UE) and optimizing the performance of CHO. Background Art

[0002] In order to meet the growing demand for wireless data services since the deployment of the 4th generation (4G) communication system, efforts have been made to develop improved 5th generation (5G) or pre-5G (pre-5G) communication systems. Therefore, 5G or pre-5G communication systems are also referred to as "beyond 4G networks" or "post-long term evolution (LTE) systems". 5G communication systems are considered to be implemented in higher frequency (millimeter wave (mmWave)) bands (for example, the 60GHz band), thereby achieving higher data rates. In order to reduce the propagation loss of radio waves and increase the transmission distance, beamforming, large-capacity multiple-input multiple-output (MIMO), full-dimensional MIMO (FD-MIMO), array antennas, analog beamforming and massive antenna technology are discussed in 5G wireless communication systems. In addition, in the 5G communication system, development of system network improvements based on advanced small cells, cloud radio access network (RAN), ultra-dense network, device-to-device (D2D) communication, wireless backhaul, mobile network, cooperative communication, coordinated multi-points (CoMP), receiving end interference elimination, etc. is underway. In the 5G system, hybrid frequency shift keying (FSK) and quadrature amplitude modulation (QAM) modulation (FQAM) and sliding window superposition coding (SWSC) as advanced coding modulation (ACM) have been developed, and filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA) and sparse code multiple access (SCMA) as advanced access technologies have been developed.

[0003] The Internet is a human-centered connected network in which humans generate and consume information. Now the Internet is evolving towards the Internet of Things (IoT), in which distributed entities (such as things) can exchange and process information without human intervention. The Internet of Everything (IoE) has emerged, which is a combination of IoT technology and big data processing technology through connection with cloud servers. As IoT realizes the demand for technical elements such as "sensing technology", "wired / wireless communication and network infrastructure", "service interface technology", and "security technology", sensor networks, machine-to-machine (M2M) communication, machine type communication (MTC), etc. have been studied recently. Such an IoT environment can provide intelligent Internet technology services that create new value for human life by collecting and analyzing data generated between connected things. In this case, IoT can be applied to various fields, including smart homes, smart buildings, smart cities, smart cars or connected cars, smart grids, health care, smart appliances, and advanced medical services, through the convergence and combination between existing information technology (IT) and various industrial applications.

[0004] In view of this, various attempts have been made to apply 5G communication systems to IoT networks. For example, technologies such as sensor networks, machine type communications (MTC), and machine-to-machine (M2M) communications can be implemented through beamforming, MIMO, and array antennas. The application of cloud radio access networks (RANs) as the above-mentioned big data processing technology can also be considered as an example of convergence between 5G technology and IoT technology.

[0005] Meanwhile, various studies on technologies in wireless communication systems, such as Conditional Handover (CHO), are underway to protect and enhance mobility in various communication environments. Summary of the invention

[0006] Technical issues

[0007] A primary object of embodiments herein is to disclose a method and system for controlling enhanced mobility in a wireless communication network by configuring at least one user equipment with a conditional handover (CHO) configuration of a CHO candidate target base station (BS).

[0008] Another object of the embodiments of the present invention is to disclose a method and system for reconfiguring at least one UE with an updated CHO configuration when at least one event occurs after the at least one UE is configured with the CHO configuration, wherein the at least one event includes at least one of a change in the source cell configuration and a change in the target cell configuration.

[0009] Another object of embodiments herein is to disclose a method and system for reconfiguring at least one UE by performing one of a one-step reconfiguration method and a two-step reconfiguration method.

[0010] Another object of embodiments herein is to disclose a method and system for enabling a CHO candidate target BS to release a CHO configuration previously configured on at least one UE when the CHO candidate target cell does not serve the at least one UE.

[0011] Another object of the embodiments herein is to disclose a method and system for enabling a CHO candidate target BS to serve at least one UE by retrieving UE context and sequence number (SN) status transmission from an old source cell when at least one UE encounters at least one failure on the old source cell.

[0012] Another object of embodiments herein is to disclose a method and system for enabling at least one UE to release a CHO configuration of a CHO candidate target cell when transitioning from a Radio Resource Control (RRC) connected state to an RRC inactive state.

[0013] Technical Solution

[0014] Therefore, embodiments of the present invention provide methods and systems for handling conditional handover (CHO) in a wireless communication network. The method disclosed herein includes configuring a user equipment (UE) by a source base station (BS) with at least one CHO candidate target cell configuration of at least one CHO candidate target BS and at least one CHO trigger condition. The method also includes reconfiguring the UE with at least one updated CHO candidate target cell configuration of at least one CHO candidate target BS upon detecting the occurrence of at least one reconfiguration event. The method also includes performing CHO on one of the CHO candidate target cells of at least one CHO candidate target BS by the UE based on the updated CHO candidate target cell configuration of at least one CHO candidate target BS and at least one CHO trigger condition.

[0015] Therefore, an embodiment of the present invention provides a wireless communication network, which includes multiple user equipments (UEs) and multiple base stations (BSs). A source BS among the multiple BSs is configured to configure a UE among the multiple UEs with at least one CHO candidate target cell configuration and at least one CHO trigger condition of at least one CHO candidate target BS among the multiple BSs. The source BS is also configured to reconfigure the UE using at least one updated CHO candidate target cell configuration of at least one CHO candidate target BS when the occurrence of at least one reconfiguration event is detected. The UE is configured to perform CHO on one of the CHO candidate target cells of at least one CHO candidate target BS based on the updated CHO candidate target cell configuration and at least one CHO trigger condition of at least one CHO candidate target BS.

[0016] These and other aspects of the example embodiments herein will be better appreciated and understood when considered in conjunction with the following description and accompanying drawings. However, it should be understood that the following description, while indicating the example embodiments and many of their specific details, is given by way of illustration and not limitation. Many changes and modifications may be made within the scope of the example embodiments of the present disclosure without departing from the spirit of the present disclosure, and the example embodiments of the present disclosure include all such modifications.

[0017] Beneficial Effects

[0018] According to the embodiments, an apparatus and method capable of effectively controlling enhanced mobility in a wireless communication system may be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The embodiments of the present invention are shown in the accompanying drawings, in which similar reference numerals represent corresponding parts in each of the accompanying drawings. The embodiments of the present invention will be better understood through the following description with reference to the accompanying drawings, in which:

[0020] Figure 1 Depicting a wireless communication network according to embodiments disclosed herein;

[0021] Figure 2 is a block diagram depicting hardware components of a base station (BS) / cell according to embodiments disclosed herein;

[0022] Figure 3 is a block diagram depicting hardware components of a user equipment (UE) according to embodiments disclosed herein;

[0023] Figure 4 is a sequence diagram depicting a one-step reconfiguration of a UE according to embodiments disclosed herein;

[0024] Figure 5 is an example sequence diagram depicting a two-step reconfiguration of a UE according to embodiments disclosed herein;

[0025] Fig. 6A , 6B and 6C are example sequence diagrams depicting release of a conditional handover (CHO) target cell configuration initiated by a CHO candidate target cell when the target cell no longer serves the UE according to embodiments disclosed herein;

[0026] Figure 7 is an example sequence diagram depicting rejection of a handover cancellation request of a CHO candidate target cell according to embodiments disclosed herein;

[0027] Fig. 8A , 8B and 8C is an example sequence diagram depicting an example procedure according to embodiments disclosed herein, wherein a radio resource control (RRC) reconfiguration message is sent to a UE to instruct the UE to remove a CHO candidate target cell;

[0028] Fig.9A and 9B is an example diagram depicting a fallback to a connection establishment process after a UE encounters at least one failure on a source cell according to an embodiment disclosed herein;

[0029] Fig. 10A is an example sequence diagram depicting a fallback procedure to connection reestablishment after a UE encounters at least one failure on a source cell according to embodiments disclosed herein;

[0030] Fig. 10B depicts an example RRC reconfiguration complete message including information about an old source cell according to embodiments disclosed herein;

[0031] Fig. 10C Depicted are process example signaling provided to a UE for implementing a fallback to an RRC establishment according to embodiments disclosed herein;

[0032] Fig.11 is an example sequence diagram illustrating retrieval of a sequence number (SN) status transmission by a CHO candidate target cell to serve a UE after the UE encounters at least one failure on a source cell according to embodiments disclosed herein;

[0033] Fig. 12A is an example sequence diagram illustrating association of CHO trigger conditions with CHO target cell configurations of CHO candidate target cells according to embodiments disclosed herein; and

[0034] Fig. 12B Depicted are CHO configurations of CHO candidate target cells according to embodiments disclosed herein. DETAILED DESCRIPTION

[0035] The example embodiments herein and their various features and advantageous details are explained more fully with reference to the non-limiting embodiments shown in the accompanying drawings and described in detail in the following description. Descriptions of known components and processing techniques are omitted so as not to unnecessarily obscure the embodiments herein. The description herein is intended only to help understand the manner in which the example embodiments herein can be practiced, and further to enable those skilled in the art to practice the example embodiments herein. Therefore, this disclosure should not be interpreted as limiting the scope of the example embodiments herein.

[0036] Typically, a wireless communication network monitors the behavior of a user equipment (UE) and provides the necessary resources to the UE to perform one or more operations. Examples of wireless communication networks may be, but are not limited to, long term evolution (LTE) / 4G networks, new radio (NR) / 5G networks, and the like. Examples of one or more operations may be, but are not limited to, uplink data transmission, downlink data transmission, and the like. The signal strength and quality experienced by the UE when performing one or more operations may vary based on the proximity of the UE to the base station / radio access network (RAN) node (gNodeB (gNB) / eNodeB (eNB)) (i.e., the distance from the BS) and the interference experienced by the UE from neighboring BSs. A UE close to a BS / cell may have better signal strength than a UE far from the BS (i.e., the cell edge situation). In addition, when the serving BS / cell and the neighboring BS / cell have low loads, the UE experiences less interference.

[0037] In a wireless communication network, a BS / RAN node may always maintain the context of a UE in an active radio resource control (RRC) connection state. At any point in time, due to various reasons (such as, but not limited to, weak signal conditions, heavy load on the serving BS, etc.), the serving BS / source cell may perform a handover of the UE from its control to another BS / cell (hereinafter referred to as the target BS / cell). Performing a handover involves transferring the entire context of the UE from the source cell to the target cell and changing the primary cell (PCell) of the UE. The serving cell performs a handover of the UE to the target cell upon receiving auxiliary information from the UE. The auxiliary information includes measurement reports of neighboring cells. The serving cell configures the UE to measure the signal strength of the serving cell and neighboring cells on the same frequency or different frequencies that may belong to different BS / cells. The serving cell further configures the UE with specific measurement and reporting criteria. When the configured measurement and reporting criteria are met, the UE sends auxiliary information to the serving cell for handover.

[0038] In a typical handover method, the serving cell triggers the handover process by sending a handover (HO) request to the target cell. When receiving an acknowledgement (ACK) from the target cell in response to the HO request, the serving cell initiates the handover of the UE by sending an RRC reconfiguration to the UE. The RRC reconfiguration includes the configuration of the HO command and the target cell. When the RRC reconfiguration is received from the serving cell, the UE initiates a random access process for handover by sending a Physical Random Access Channel (PRACH) preamble to the target cell. However, when the UE moves or rotates when initiating the handover, the UE may experience very rapid degradation of signal strength because the UE is able to perform one or more operations at frequency range 2 (FR2) or higher frequency ranges. At FR2 or higher frequency ranges, changes in signal strength and degradation of signal strength may be more unfavorable than changes at frequency range 1 (FR1) or lower frequency ranges. Such rapid degradation of signal strength may result in a higher probability of handover failure. In addition, due to the rapid degradation of signal strength, the UE may not be able to receive a HO command from the serving cell. Thereby resulting in handover failure.

[0039] To reduce handover failures due to rapid degradation of signal strength, 3GPP defines Conditional Handover (CHO).

[0040] In the conventional CHO method, the serving cell initiates CHO preparation with one or more neighboring cells of the UE upon receiving measurements of one or more neighboring cells from the UE. The serving cell receives confirmation from one or more neighboring cells that have been successfully prepared (i.e., admission control is successful) to serve the UE. The one or more neighboring cells that send confirmation to the serving cell may be candidate target cells / CHO candidate target cells for CHO. The confirmation received from the target / neighboring cell includes the configuration / CHO configuration of the target cell that the UE must apply when performing a CHO switch to the target cell. The serving cell sends an RRC reconfiguration message to the UE, which includes the CHO configuration of one or more candidate target cells. The serving cell additionally provides the UE with one or more trigger conditions associated with the CHO configuration. The UE can use one or more trigger conditions to evaluate whether a switch to any configured CHO candidate target cell can be triggered. Unlike the typical switching method, the UE does not initiate a switch execution when it receives an RRC reconfiguration message including a CHO configuration. The UE stores the received CHO configuration. Once one or more trigger conditions associated with the CHO configuration have been met, the UE evaluates one or more trigger conditions associated with the CHO configuration and automatically initiates a switch execution. Therefore, CHO ensures that the RRC reconfiguration message (including the CHO configuration of the CHO candidate target cell) is provided to the UE in advance to cope with poor radio signaling conditions. Providing the RRC reconfiguration message to the UE in advance reduces the probability of failure due to not receiving the HO command from the serving BS / cell, which further improves mobility robustness.

[0041] In the conventional CHO method, when the UE performs a transition from the RRC connected state to the RRC inactive state, the UE stores the application server (AS) context. The AS context includes the RRC reconfiguration message (including the CHO configuration of the CHO candidate target cell) and the configuration of the special cell (SpCell). Once the UE resumes the RRC connected state from the RRC inactive state, the UE applies the CHO configuration to initiate a HO execution with one of the CHO candidate target cells. However, when the UE resumes the RRC connected state, the CHO configuration may be invalid. Therefore, the amount of signaling increases.

[0042] In addition, when the UE resumes the RRC connected state from the RRC inactive state, the configuration of the serving cell may change. The configuration change of the serving cell also affects the configuration of the CHO candidate target cell. Therefore, in such a case, the UE may have an invalid CHO configuration.

[0043] Furthermore, once a UE is configured with CHO, the load conditions on the CHO candidate target cell may change and the CHO candidate target cell may no longer serve the UE. Conventional CHO methods do not involve any signaling that allows the CHO candidate target cell to modify or cancel an already confirmed and prepared HO for the UE.

[0044] Embodiments herein disclose methods and systems for controlling enhanced mobility in a wireless communication network by configuring a user equipment (UE) with conditional handover (CHO) and optimizing the performance of CHO.

[0045] Referring now to the drawings and more particularly to Figures 1 to 12B , where like reference numerals indicate corresponding features consistently throughout the drawings, example embodiments are shown.

[0046] Figure 1 A wireless communication network 100 according to an embodiment disclosed herein is depicted. Examples of the wireless communication network 100 may be, but are not limited to, a Long Term Evolution (LTE) / 4G network, an Advanced LTE network, a New Radio (NR) / 5G network, a Universal Mobile Telecommunications Service (UMTS), a Global System for Mobile Communications (GSM), and an Enhanced Data Rates for GSM Evolution (EDGE) Radio Access Network (GERAN) system or any other next generation network.

[0047] The wireless communication network 100 includes a plurality of user equipments (UEs) 102 , at least one core network (CN) 104 , and a plurality of base stations / radio access network (RAN) nodes 106 .

[0048] The UE(s) 102 referred to herein may be user equipment capable of supporting the wireless communication network 100. Examples of the UE 102 may be, but are not limited to, a mobile phone, a smart phone, a tablet computer, a handheld phone, a personal digital assistant (PDA), a laptop computer, a computer, a wearable computing device, a vehicle infotainment device, an Internet of Things (IoT) device, a Virtual Reality (VR) device, a Wireless Fidelity (Wi-Fi) router, an adapter (USB dongle), a robot, an automated guided vehicle, or any other device that supports the wireless communication network 100.

[0049] UE 102 may be connected to at least one BS 106 via an interface supported by wireless communication network 100 to perform one or more operations. Examples of interfaces may be, but are not limited to, wired interfaces, wireless interfaces (e.g., air interfaces, Nu interfaces, etc.), wireless fronthaul interfaces, wired or wireless backhaul interfaces, or any structure supporting communication via wired or wireless connections. Examples of operations may be, but are not limited to, uplink data transmission, downlink data transmission, etc. In the examples herein, data may be, but are not limited to, at least one of voice packets, video packets, data packets, etc.

[0050] The UE 102 may operate in various states, such as, but not limited to, a radio resource control (RRC) connected state, an RRC inactive / idle state, etc. The RRC connected state may be a state in which the UE 102 has established an RRC connection with at least one BS 106 to perform one or more operations. The RRC inactive state may be a state in which the UE 102 has not established an RRC connection with any BS 106.

[0051] The CN 104 referred to herein may be one of an Evolved Packet Core (EPC), a 5G Core (5GC) network, etc., but is not limited thereto. The CN 104 may be connected to one or more BSs 106 and an external data network (not shown). Examples of external data networks may be, but are not limited to, the Internet, a Packet Data Network (PDN), an Internet Protocol (IP) multimedia core network subsystem, etc. The CN 104 may be configured to connect at least one UE 102 (connected to an associated BS 106) to an external data network to perform one or more operations.

[0052] The (multiple) BS / RAN nodes 106 referred to herein may be nodes such as, but not limited to, evolved nodes (eNBs), new radio nodes (gNBs), etc. The (multiple) BSs 106 may be associated with one or more cells. In an embodiment, terms such as BS 106 and cell 106 may be used interchangeably throughout the document. The BS 106 may connect one or more UEs 106 to the CN 104. The BS 106 may be configured to perform radio resource management functions such as, but not limited to, radio bearer control, radio admission control, connection mobility control, dynamic allocation of resources to UEs in uplink / downlink (scheduling), etc.

[0053] The (multiple) BSs 106 referred to herein may act as a serving BS / cell or a neighboring BS / cell for the UE 102. The BS 106 connected to the UE 102 is referred to herein as a source BS / cell. The embodiments herein use terms such as, but not limited to, "serving BS / cell", "source BS / cell", "primary cell (PCell)", "primary BS", etc., which are interchangeably used for the BS / cell currently connected to the UE 102. The BS / cell 106 that may exist near the UE 102 is referred to herein as a neighboring BS / cell (i.e., not connected to the UE).

[0054] The source BS / cell 106 may be configured to maintain a context (hereinafter referred to as UE context) of the UE 102. The UE context may be necessary information required to maintain and provide communication services (e.g., voice packets, video packets, data packets, etc.) to the UE 102. The UE context may include information such as, but not limited to, UE state information (e.g., RRC connection state, RRC inactive state, etc.), security information, UE capability information, an identifier of a logical S1 connection associated with the UE, and the like.

[0055] The source BS / cell 106 may also be configured to monitor the behavior / mobility of the UE 102 and perform a handover of the UE 102 to a target BS / cell 106. The target BS / cell 106 may be one of the neighboring BS / cells of the UE 102. The handover of the UE 102 involves transferring the UE context from the source BS / cell 106 to the target BS / cell 106.

[0056] In an embodiment, the source cell 106 may be configured to control enhanced mobility of the UE 102 by configuring a conditional handover (CHO) to the UE 102 and optimizing the performance of the CHO.

[0057] The source cell 106 initiates CHO candidate cell preparation when it receives measurements of one or more neighboring cells from the UE 102. The source cell 106 may receive measurements of one or more neighboring cells from the UE 102 when the UE 102 experiences degradation in the signal strength associated with the source cell and degradation in the quality of the communication service that the UE 102 is accessing. During the CHO candidate cell preparation, the source cell 106 sends its configuration to one or more cells, which is the current configuration available on the UE (hereinafter referred to as the service / source cell configuration). In an embodiment, the one or more cells may be cells of the source BS 106 or neighboring BSs 106. The source cell 106 may send the source cell configuration to the one or more cells 106 as a handover preparation in a handover request. Upon receiving the source cell configuration, one or more cells on the BS (which have received the handover request) may perform admission control and reserve resources for the UE 102. One or more cells that traverse the document, perform admission control, and reserve resources for the UE 102 for handover are referred to as CHO candidate target cells. With respect to reserving resources for the UE 102, each CHO candidate target cell 106 prepares a target cell configuration (i.e., a configuration that the UE 102 may apply if the UE 102 chooses to perform a CHO handover to the corresponding candidate target cell). In an embodiment, each CHO candidate target cell 106 may prepare the target cell configuration as an increment of the source cell configuration. Preparing the target cell configuration as an increment of the source cell configuration means that the target cell configuration has been prepared without releasing the source cell configuration (i.e., the target cell configuration has been prepared relative to the received source cell configuration, or the target cell configuration has been updated on the existing source configuration).

[0058] The one or more CHO candidate target cells 106 may also include a target cell configuration that is prepared as an increment of the source cell configuration in a container. The container referred to herein may be an RRC message that enables the source cell 106 to skip a decoding process that must be performed on the container. The embodiments herein may interchangeably use terms such as "container", "RRC container", "RRC reconfiguration message", etc. to refer to an RRC message that may skip a decoding process. Therefore, the target cell configuration of the one or more CHO candidate target cells 106 may be represented as:

[0059] Target cell configuration = source cell configuration + container (target cell configuration is prepared as an increment of source cell configuration)

[0060] Embodiments herein may interchangeably use terms such as “CHO target cell configuration,” “CHO candidate target cell configuration,” “target cell configuration,” etc. to refer to the configuration of the CHO candidate target cell 106 .

[0061] In an embodiment, one or more CHO candidate target cells 106 may prepare a CHO target cell configuration based on UE capability coordination and source cell configuration. Thus, the source cell or target cell configuration may be reconfigured by ensuring that the reconfigured source cell or target cell configuration may be within the UE capability coordination range and may be complied with by the UE 102. The one or more CHO candidate target cells 106 include the CHO target cell configuration in an RRC reconfiguration message. The one or more CHO candidate target cells 106 send an RRC reconfiguration message to the source cell 106 in a handover request confirmation message.

[0062] The source cell 106 encapsulates the container received from one or more CHO candidate target cells 106 into an RRC reconfiguration message and identifies the CHO target cell configuration included in the RRC reconfiguration message. The source cell 106 prepares a CHO configuration for one or more CHO candidate target cells by associating a trigger condition / CHO trigger condition with the CHO target cell configuration of each CHO candidate target cell. Therefore, the CHO configuration includes the CHO target cell configuration of each CHO candidate target cell 106 and the CHO trigger condition associated with each CHO target cell configuration. The CHO trigger condition may be based on a measurement event such as, but not limited to, event A3, event A5, etc.

[0063] The CHO triggering condition of each CHO candidate target cell 106 includes a measurement object and a reporting configuration. The measurement object may be, but is not limited to, a reference signal received power (RSRP), a reference signal received quality (RSRQ), etc. The measurement object may indicate the frequency at which the CHO candidate target cell 106 can operate. The reporting configuration may indicate that at least one condition of the associated CHO candidate target cell must be met so that the UE 102 can initiate a CHO execution for switching to one of the CHO candidate target cells 106. The source cell 106 is designed for a reporting configuration for CHO, regardless of whether the switching is configured to the same frequency. In addition to normal switching, the source cell 106 can also use CHO. Normal switching can be used for improved network control, and CHO can be used to improve the robustness of switching. The source cell 106 can design a reporting configuration for CHO based on one or more options. Examples of these options may be, but are not limited to, using a separate reporting configuration (i.e., blind configuration) for CHO, using a reporting configuration that has been used for normal switching for CHO, using a reporting configuration for CHO by adding a CHO offset in a reporting configuration for normal switching, etc. Similarly, the source cell 106 may design the measurement object based on one or more options. Examples of options may be, but are not limited to, reusing measurement objects configured for other radio resource management (RRM) purposes (the same parameter values ​​may be used), configuring measurement objects separately for CHO (i.e., blind configuration), etc.

[0064] The source cell 106 uses a measurement identifier (e.g., ID or IE) to link the measurement object and the reporting configuration. The measurement identifier holds a unique integer value that cannot be applied to any other CHO candidate target cell 106. Therefore, using the measurement identifier, the UE 102 can uniquely identify the CHO candidate target cell associated with the CHO trigger condition. Alternatively, the source cell 106 can configure a new identifier or measurement identifier within the CHO configuration to link the measurement object and the reporting configuration. The new identifier may incur more overhead, but the new identifier can be used if the same CHO trigger condition is applied to multiple candidates. In addition, the use of the new identifier automatically avoids RRM (i.e., measurement reporting is not triggered when the CHO trigger condition is met).

[0065] The source cell 106 may provide a measurement identifier associated with the CHO triggering condition together with the source cell configuration in the handover request to one or more target cells 106. One or more CHO candidate target cells 106 may add the measurement identifier in a container carrying the CHO target cell configuration. Adding the measurement identifier in the container enables the UE 102 to fully understand the association between the CHO triggering condition and the CHO target cell configuration. Therefore, the CHO configuration may include a container with the CHO target cell configuration and the measurement identifier corresponding to the CHO triggering condition. Alternatively, the source cell 106 may include the reporting configuration and the measurement object (configured for CHO) within the existing measurement configuration rather than within the CHO target cell configuration (i.e., not within the RRC container). Alternatively, the source cell 106 may enable the target cell 106 to add the CHO triggering condition outside the RRC container. In such a scenario, if the UE 102 performs an autonomous release of the CHO configuration, the UE 102 must remove the CHO triggering condition separately.

[0066] In an embodiment, the source cell 106 may associate the same CHO trigger condition with the CHO target cell configuration of one or more CHO candidate target cells. In such a case, the source cell 106 uses a new unique identifier to uniquely identify the CHO candidate cell associated with the CHO trigger condition. The source cell 106 signals the new unique identifier to the one or more CHO candidate target cells 106. The one or more CHO candidate target cells 106 may add a value indicating the new unique identifier in the CHO target cell configuration. The one or more CHO candidate target cells 106 may also add a value indicating the new unique identifier as part of the RRC reconfiguration message within the container.

[0067] In an embodiment, the source cell 106 may associate multiple CHO trigger conditions with the CHO target cell configuration of one or more CHO candidate target cells. The source cell 106 may associate multiple CHO trigger conditions for each CHO candidate target cell 106 by combining two events (such as but not limited to event A3, event A5, etc.). In order to avoid the measurement burden on the UE 102, the source cell 106 may use events on different trigger quantities. For example, event A3 may be an RSRP-based event, while event A5 may be an RSRQ-based event, or vice versa. The source cell 106 may combine the two events using a single logical "AND" operation. Support for two trigger conditions combined by a single logical "AND" operation can only be an enhancement to the CHO operation. CHO trigger conditions (including a combination of two events) cannot be applied to measurement reports. The measurement ID or a new equivalent ID used for linking and reporting configurations within the CHO configuration may have the option of linking two reporting configurations (including two events).

[0068] The source cell 106 sends the CHO configuration of one or more CHO candidate target cells 106 to the UE 102, wherein each CHO configuration corresponding to each CHO candidate target cell may include a CHO target cell configuration and an associated CHO trigger condition. Upon receiving the CHO configuration of one or more CHO candidate target cells, the UE 102 stores the CHO configuration in a CHO candidate target list. Since the received CHO target cell configuration includes a combination of a source cell configuration and a container, and the container has an incremental target cell configuration prepared as a source cell configuration, the UE 102 may apply the source cell configuration before performing the CHO execution to continue in the source cell 106. The UE 102 may perform a CHO execution for handover when a CHO trigger condition associated with one of the CHO candidate target cells is met. The CHO execution involves applying an incremental target cell configuration prepared as a source cell configuration to handover to one of the CHO candidate target cells that meets the CHO trigger condition. Therefore, handover failures may be reduced by providing the CHO configuration to the UE 102 in advance under poor cell edge radio conditions.

[0069] In an embodiment, when the CHO trigger condition associated with two or more CHO candidate target cells has been met, the UE 102 may select one of the CHO candidate target cells for handover based on its implementation. In an embodiment, when the CHO trigger condition associated with two or more CHO candidate target cells has been met, the UE 102 may select the CHO candidate target cell for handover (from the two or more CHO candidate target cells) that has the highest CHO trigger condition in terms of radio quality (i.e., reference signal received power). In an embodiment, when the CHO trigger condition associated with two or more CHO candidate target cells has been met, the UE 102 may select the CHO candidate target cell from the two or more CHO candidate target cells based on random access RA resource configuration (e.g., if contention-free resources are configured, etc.). In an embodiment, when the CHO trigger condition associated with two or more CHO candidate target cells has been met, the UE 102 may select the CHO candidate target cell from the two or more CHO candidate target cells based on a combination of two events (combined events) associated with each CHO candidate target cell. In an example, the combined event may include an RSRP-based event A3 and an RSRP-based event A4. In such a scenario, the UE 102 may specify the combined event by applying a weight factor w to event A3 and (1-w) to event A4, where the value of the weight factor w is less than 1. In another example, the combined event may include an RSRP-based event A3 and an RSRQ-based event A4, or vice versa. In such a case, the UE 102 may specify the combined event by applying a weight factor w to event A3 and (1-w) to event A4, where the value of the weight factor w is less than 1. Alternatively, the UE 102 may apply equal weights to the two events. When selecting a CHO candidate cell 106 for switching, the UE 102 applies the CHO target cell configuration of the selected CHO candidate target to switch to the selected CHO candidate cell 106.

[0070] Embodiments herein enable UE 102 to release a stored CHO configuration when performing a transition from an RRC connected state to an RRC inactive state. When a CHO configuration of one or more candidate target cells 106 is received from a source cell 106, UE 102 may transition to an RRC inactive state. UE 102 may receive an RRC release message from a source cell 106 utilizing a suspension configuration, which enables UE 102 to transition from an RRC connected state to an RRC inactive state. However, the suspension configuration does not explicitly instruct UE 102 to release the CHO configuration. In such a scenario, UE 102 releases the stored CHO configuration when transitioning to an RRC inactive state. Therefore, due to changes in the CHO configuration, handover failures may be reduced.

[0071] Embodiments herein enable the source cell 106 to reconfigure the UE 102 with an updated CHO configuration upon detecting the occurrence of at least one event after providing the CHO configuration of one or more CHO candidate target cells 106 to the UE 102. Examples of the at least one event may be, but are not limited to, a change in the source cell configuration, a change in the target cell configuration, etc.

[0072] In an embodiment, upon detecting the occurrence of at least one event after providing the CHO configuration of one or more CHO candidate target cells 106 to the UE 102, the source cell 106 may reconfigure the UE 102 independently with the updated source cell configuration and the target cell configuration. Consider an example event, wherein the source cell configuration has changed after the CHO configuration is provided to the UE 102, and the changed source cell configuration does not affect the target cell configuration. In such an event, the source cell 106 may provide only the changed / updated source cell configuration to the UE 102 in the RRC reconfiguration message. The source cell configuration may be reconfigured as an increment of the existing source cell configuration. Throughout this document, the reconfigured / updated source cell configuration is referred to as an incremental source cell configuration. Therefore, the UE 102 may receive an updated source cell configuration without changing the already stored CHO target cell configuration. Consider another example event, wherein the CHO target cell configuration of the CHO candidate target cell 106 has been changed / reconfigured after the CHO configuration is provided to the UE 102, and the changed target cell configuration does not affect the source cell configuration. In such an event, the source cell 106 receives a reconfigured / updated target cell configuration from the CHO candidate target cell 106. The target cell configuration may be reconfigured as a delta of the source cell configuration available to the CHO candidate target cell 106. Throughout this document, the reconfigured / updated CHO target cell configuration is hereinafter referred to as the delta CHO target cell configuration. The source cell 106 provides the delta CHO target cell configuration to the UE 102. The UE 102 forms a CHO configuration of the CHO candidate target cell for handover as a combination of the source cell configuration currently available at the UE 102 and the received delta CHO target cell configuration.

[0073] In an embodiment, the source cell 106 does not independently provide an updated source cell configuration and an updated CHO target cell configuration to the UE 102 in one or more scenarios, wherein the source cell configuration affects the CHO target cell configuration and vice versa. Consider an example scenario, wherein the source cell configuration has changed after the CHO configuration is provided to the UE 102, and the changed source cell configuration may affect the CHO target cell configuration. In such a scenario, if the source cell 106 sends an RRC reconfiguration message to the UE 102 that includes only the source cell configuration, it may result in a potential CHO target cell configuration, which may be generated as a combination of the changed / updated source cell configuration and the already available CHO target cell configuration. Consider another example scenario, wherein the CHO target cell configuration of the CHO candidate target cell 106 changes after the CHO configuration is provided to the CHE configuration, and the target cell configuration may affect the source cell configuration. In such a scenario, updating the UE 102 with a changed / updated CHO target cell configuration (before updating the source cell configuration) may result in an invalid CHO target cell configuration, which has been generated as a combination of the source cell configuration available at the UE 102 and the updated incremental target cell configuration (sent by the CHO candidate target cell to the UE 102). Therefore, in the above example scenario, the source cell 106 does not update the source cell configuration and the target cell configuration to the UE 102 independently.

[0074] In an embodiment, upon detecting at least one event after providing the CHO configuration to the UE 102, the source cell 106 may reconfigure the UE 102 by performing a one-step reconfiguration. The one-step reconfiguration of the UE 102 includes sending both updated / incremental source and CHO target cell configurations to the UE 102 at once in a single RRC reconfiguration message. When the source cell configuration has changed after providing the CHO configuration to the UE 102, the source cell 106 sends an incremental source cell configuration (i.e., a change in the source cell configuration relative to an earlier / previous source cell configuration based on which each candidate target cell generates the CHO configuration) to the CHO candidate target cell in a handover request. Upon receiving the incremental source cell configuration, the CHO candidate target cell 106 prepares the updated / incremental CHO target cell configuration in a container. Preparing the updated CHO configuration involves updating the CHO target cell configuration to an increment of the incremental source cell configuration. Therefore, the updated / incremental CHO target configuration includes an incremental source cell configuration and a container including a target cell configuration prepared as an increment of the incremental source cell configuration. The CHO candidate target cell 106 sends the incremental CHO target cell configuration in the handover request confirm message to the source cell 106. Upon receiving the handover request confirm message, the source cell 106 sends an RRC reconfiguration message to the UE 102. The RRC reconfiguration message includes the incremental CHO target cell configuration of the CHO candidate target cell 106. The incremental CHO target cell configuration of each CHO candidate target cell 106 includes an incremental source cell configuration and a container including a corresponding target cell configuration prepared as an increment of the incremental source cell configuration.

[0075] In an embodiment, if the source cell 106 initiates a configuration change that affects the target cell configuration, the source cell 106 sends a handover modification request to one or more CHO candidate target cells 106. The source cell 106 waits to receive a confirmation and an incremental CHO target cell configuration from the one or more candidate target cells 106. The source cell 106 forms an RRC reconfiguration message including the incremental source cell configuration and the incremental CHO target cell configuration. The source cell 106 sends the RRC reconfiguration message to the UE 102.

[0076] Upon receiving the RRC reconfiguration message, UE 102 first updates the source cell configuration in the stored CHO configuration with the received incremental source cell configuration. UE 102 then updates the CHO target cell configuration in the stored CHO configuration with the received incremental CHO target cell configuration. Thus, the stored CHO configuration is updated.

[0077] One-step reconfiguration of UE 102 may be applicable to scenarios where an updated source cell or target cell configuration may be delayed to be sent to UE 102. For example, transmission of an updated / incremental source cell configuration to UE 102 may be delayed until source cell 106 receives an updated / incremental CHO target cell configuration from CHO candidate target cell 106. For example, transmission of an updated / incremental CHO target cell configuration to UE 102 may be delayed until source cell 106 generates an updated / incremental source cell configuration. However, in scenarios where transmission of an updated cell configuration to UE 102 cannot be delayed, the updated source cell configuration must be sent immediately to UE 102 to ensure good quality of service (QoS) and uninterrupted service (i.e., changes in the source cell configuration must be updated immediately at UE 102 without waiting for confirmation from CHO candidate target cell 106). In such a scenario, if only the updated source cell configuration is provided to the UE 102, the CHO target cell configuration may become invalid because the CHO target cell configuration stored at the UE 102 may be a combination of the updated source cell configuration and the existing CHO target cell configuration. Therefore, in such a scenario, embodiments herein enable the source cell 106 to reconfigure the UE 102 by performing a two-step reconfiguration.

[0078] In a two-step reconfiguration of the UE 102, when the source cell 106 reconfigures its configuration, the source cell 106 sends a first RRC reconfiguration message to the UE 102. The first RRC reconfiguration message includes an updated / incremental source cell configuration and a release of the CHO configuration corresponding to all CHO candidate target cells 106. Upon receiving the first RRC reconfiguration message from the source cell 106, the UE 102 releases the stored CHO configuration and stores the updated / incremental source cell configuration received from the source cell 106. Upon sending the first RRC message to the UE 102, the source cell 106 sends a handover modification request to the CHO candidate target cell 106. In response to the handover modification request, the source cell 106 receives an updated / incremental CHO target cell configuration from the CHO candidate target cell 106 in a handover request acknowledgment. Once the updated / incremental CHO target cell configuration is received, the source cell 106 sends a second RRC reconfiguration message to the UE 102, wherein the second RRC reconfiguration message includes a container with the incremental target cell configuration. In addition, the source cell 106 may include a source cell configuration that does not affect the CHO target cell configuration in the second RRC reconfiguration message. Upon receiving the second RRC reconfiguration message, the UE 102 stores the CHO target cell configuration of the CHO candidate target cell 106 by including an incremental CHO target cell configuration relative to the received incremental source cell configuration in the first RRC reconfiguration message.

[0079] Embodiments herein enable the CHO candidate target cell 106 to send a handover cancel request to the source cell 106 if the CHO candidate target cell 106 can no longer serve the UE 102 that has completed CHO preparation. Based on the handover request received from the source cell 106, the CHO candidate target cell 106 performs admission control and reserves resources for the UE 102. While reserving resources for the UE 102, in response to the handover request, the CHO candidate target cell 106 sends a handover request confirmation to the source cell 106. However, when sending the handover request confirmation to the source cell 106, the CHO candidate target cell 106 may reallocate resources to the UE 102 in one or more scenarios. Examples of scenarios may be, but are not limited to, a load change of the CHO candidate target cell 106, a reallocation of resources of the UE 102 configured with the CHO configuration of other UEs 102, and the like. For example, the UE 102 may perform CHO execution to handover to only one CHO candidate target cell 106 when the associated CHO trigger condition is met. Since there is uncertainty about when the UE 102 may initiate the actual CHO execution, the CHO candidate target cell 106 may reallocate the reserved resources of the UE 102 configured with the CHO configuration to other UEs that are attempting to access the CHO candidate target cell. Therefore, the CHO candidate target cell 106 does not want to serve the UE 102 configured with its CHO configuration. For example, the load on the CHO candidate target cell 106 may increase when sending the handover request confirmation to the source cell 106. In such a scenario, the CHO candidate target cell 106 does not want to serve the UE 102 configured with its CHO configuration.

[0080] When the CHO candidate target cell 106 does not wish to serve the UE 102 configured with its CHO configuration, the CHO candidate target cell 106 initiates the release of the CHO configuration provided for the UE 102. The CHO candidate target cell 106 initiates the release of the CHO configuration by sending a handover release / cancellation request or a CHO configuration release request to the source cell 106. Upon receiving the handover cancel request, the source cell 106 accepts the handover cancel request and sends a handover cancel request confirmation to the CHO candidate target cell 106. In addition, upon receiving the handover cancel request, the source cell 106 requests the UE 102 to release the CHO configuration of the CHO candidate target cell 106.

[0081] In an embodiment, the source cell 106 may accept the handover cancellation request by ensuring that the UE 102 has released the CHO configuration of the CHO candidate target cell 106 and sending a handover cancellation request confirmation to the CHO candidate target cell 106. Upon receiving the handover cancellation request or the CHO configuration release request from the CHO candidate target cell 106, the source cell 106 sends an RRC reconfiguration message to the UE 102 to release the CHO configuration of the corresponding CHO candidate target cell 106. In an embodiment, upon receiving the RRC reconfiguration message for releasing the CHO configuration of the candidate target cell 106, the UE 102 releases the CHO configuration of the candidate target cell 106 and sends an RRC reconfiguration complete message to the source cell 106. Releasing the CHO configuration on the UE 102 involves removing the stored CHO target cell configuration and the associated CHO trigger condition of the CHO candidate target cell 106 from the CHO candidate target cell list. In an embodiment, upon receiving the RRC reconfiguration message for releasing the CHO configuration of the candidate target cell 106, the UE 102 releases the CHO configuration of the candidate target cell 106 and sends a radio link control (RLC) confirmation as an RRC reconfiguration complete message to the source cell 106. Upon receiving the RRC reconfiguration complete message from the UE 102, the source cell 106 accepts the handover cancellation of the CHO candidate target cell and sends a handover cancellation request confirmation to the CHO candidate target cell 106.

[0082] Embodiments herein enable the source cell 106 to reject a handover cancel received from a CHO candidate target cell 106 when the UE 102 is unable to receive an RRC reconfiguration message from the source cell 106 to release the CHO configuration of the CHO candidate target cell 106. The UE 102 configured with the CHO configuration of the CHO candidate target cell 106 continuously evaluates the associated CHO triggering conditions to initiate a CHO execution for handover. Once the CHO triggering condition associated with one of the CHO candidate target cells is satisfied, the UE 102 initiates a CHO execution to handover to the corresponding CHO candidate target cell 106. During the CHO execution, sometimes, due to its hardware capabilities, the UE 102 may not be able to connect to the source cell 106 to receive any information from the source cell 106. In such a scenario, the UE 102 instead tunes its radio frequency (RF) to the frequency of the CHO candidate target cell 106 (for which the CHO triggering condition is satisfied) for cell synchronization and subsequent handover execution procedures, such as initiating a random access on the CHO candidate target cell 106. However, when the CHO candidate target cell 106 does not wish to serve the UE 102 configured with its CHO configuration, the CHO candidate target cell 106 may send a handover cancel request to the source cell 106. Therefore, a scenario is caused in which the UE 102 attempts to initiate a CHO execution to handover to a CHO candidate cell that has sent a request to the source cell 106 to cancel / release the CHO configuration, and the handover of the UE 102 may fail.

[0083] In an embodiment, in order to avoid handover failure, when the UE 102 cannot receive an RRC reconfiguration for releasing the CHO configuration of the CHO candidate target cell 106 from the source cell 106, the source cell 106 rejects the handover cancel request received from the CHO candidate target cell 106. Upon receiving the handover cancel request or the CHO configuration release request from the CHO candidate target cell 106, the source cell 106 sends an RRC reconfiguration message to the UE 102 to release the CHO configuration of the CHO candidate target cell 106. If the UE 102 cannot receive the RRC configuration message from the source cell 106, the source cell 106 rejects the handover cancel request received from the CHO candidate target cell 106 and sends a handover cancel reject message to the CHO candidate target cell 106. Upon receiving the handover cancel reject message from the source cell 106, the CHO candidate target cell 106 retains the CHO configuration provided to the UE 102. Therefore, handover failure can be avoided.

[0084] Embodiments herein enable the source cell 106 to send an RRC reconfiguration message in various procedures / methods to instruct the UE 102 to release the CHO configuration of the CHO candidate target cell.

[0085] In the example process, the source cell 102 sends an RRC reconfiguration message including a CHO release message to the UE 102 to release the CHO configuration of the CHO candidate target cell 106. The CHO release message indicates the CHO configuration of the CHO candidate target cell that must be released. In such a scenario, when releasing the CHO configuration of the indicated candidate target cell 106, the UE 102 sends an RRC reconfiguration complete message to the source cell 106.

[0086] In another example process, the source cell 106 sends an RRC reconfiguration message including only the source cell configuration to the UE 102 for releasing the CHO configuration of the CHO candidate target cell 106. In such a scenario, when releasing the CHO configuration of the candidate target cell 106, the UE 102 sends an RRC reconfiguration complete message to the source cell 106.

[0087] In another example process, the source cell 106 sends an RRC reconfiguration message including only the CHO target cell configuration to the UE 102 for releasing the CHO configuration of the CHO candidate target cell 106. In such a scenario, the UE 102 does not send an RRC reconfiguration complete message to the source cell 106. The UE 102 may send an RRC reconfiguration complete message to the CHO candidate target cell 106 when the CHO execution is successfully completed.

[0088] In another example process, the source cell 106 sends an RRC reconfiguration message including an updated / incremental source cell configuration and a CHO target cell configuration of the CHO candidate target cell 106 to the UE 102 for releasing the CHO configuration of the CHO candidate target cell 106. In such a scenario, the UE 102 sends an RRC reconfiguration complete message to the source cell 106, indicating that the UE 102 has complied with the received updated / incremental source cell configuration. The UE 102 does not send an RRC reconfiguration complete message to the source cell 106 to confirm the update of the CHO target cell configuration. However, when the CHO execution for the CHO candidate target cell 106 is successfully completed, the UE 102 sends a reconfiguration complete message to the CHO candidate target cell 106.

[0089] In another example process, the source cell 106 sends an RRC reconfiguration message including an updated / incremental CHO target cell configuration to the UE 102 to release the CHO configuration. In such a scenario, the UE 102 sends an RRC reconfiguration complete message to the source cell 106, indicating that the UE 102 has released the CHO configuration of the indicated CHO candidate target cell.

[0090] In another example process, the source cell 106 sends an RRC reconfiguration message including an updated / incremental source cell configuration and a CHO release message to the UE 102 to release the CHO configuration of the CHO candidate target cell 106. In such a scenario, the UE 102 sends a joint / combined RRC reconfiguration complete message to the source cell 106. The joint RRC reconfiguration complete message includes a reconfiguration complete message for the updated / incremental source cell configuration and an embedded reconfiguration complete message indicating the release of the CHO configuration of the CHO candidate target cell 106. Alternatively, the RRC reconfiguration complete message includes a reconfiguration complete message for the updated / incremental source cell configuration and a new indicator (IE) indicating the release of the CHO configuration of the indicated CHO candidate target cell 106.

[0091] When the UE 102 encounters at least one failure on the source cell 106, the embodiments of the present invention enable the target cell 106 to send an RRC release message for RRC reestablishment. When the CHO-configured UE 102 configured with the CHO candidate target cell 106 encounters at least one failure, the UE 102 initiates a recovery process by performing a cell selection process. Examples of failures may be, but are not limited to, Radio Link Failure (RLF), handover failure when accessing the CHO candidate target cell, integrity verification failure, reconfiguration failure of the RRC message, and the like. The cell selection process involves selecting at least one BS / cell present in the vicinity of the UE 102 to camp on. When selecting a cell by performing cell selection, the UE 102 uses the CHO configuration stored in the CHO candidate target cell list to determine whether the selected cell is a CHO candidate target cell. If the selected cell is not a CHO candidate target cell, the UE initiates an RRC reestablishment process.

[0092] If the selected cell is a CHO candidate target cell configured to the UE 102 before encountering at least one failure on the source cell 106, the UE 102 performs CHO execution and sends an RRC reconfiguration complete message to the selected cell / CHO candidate target cell 106. The RRC reconfiguration complete message sent to the selected cell / CHO candidate cell may be an unexpected message for the selected cell, since the selected cell / CHO candidate target cell 106 has already released the UE context and the resources reserved for the UE 102. Upon receiving an indication from the source cell 106 after identifying at least one failure encountered by the UE 102, the CHO candidate target cell 106 may release the UE context and the resources reserved for the UE 102. Therefore, the CHO candidate target cell 106 may only expect an RRC reestablishment request message from the UE 102 when the UE 102 encounters at least one failure. In such a scenario, in response to the unexpected RRC reconfiguration message, the CHO candidate target cell 106 may send an RRC release message to the UE 102. Upon receiving the RRC release message, UE 102 may initiate an RRC connection establishment procedure.

[0093] Embodiments herein enable a target cell 106 to perform a fallback to connection establishment procedure when a UE 102 configured with a CHO configuration encounters at least one failure on a source cell 102. The target cell 106 may be a cell selected by the UE 102 when encountering at least one failure, and the selected cell is a CHO candidate target cell. In the fallback to connection establishment procedure, upon receiving an unexpected RRC reconfiguration complete message, the CHO candidate target cell 106 may initiate a connection establishment procedure with the UE 102. The connection establishment procedure involves sending an RRC establishment message to the UE 102 in response to the unexpected RRC reconfiguration complete message. The RRC establishment message includes the configuration of the CHO candidate target cell 106. Initiating the connection establishment procedure by the CHO candidate target cell 106 reduces over-the-air signaling and enables data bearers (Signaling Radio Bearer (SRB) and Dedicated Radio Bearer (DRB)) to resume data transmission that was in progress before the at least one failure encountered by the UE 102. Upon receiving the RRC setup message from the CHO candidate target cell 106 , the UE 102 releases all existing CHO configurations of the CHO candidate target cell and applies the configuration of the target cell 106 received in the RRC setup message for connecting with the CHO candidate target cell 106 .

[0094] When a UE 102 configured with a CHO configuration encounters at least one failure on a source cell 106, embodiments herein enable a target cell 106 to serve the UE 102 by retrieving a UE context from the source cell 106. The target cell 106 may be a cell selected by the UE 102 when encountering at least one failure, and the selected cell is a CHO candidate target cell.

[0095] Upon encountering at least one failure on the source cell, the UE 102 includes information about the source cell 106 in an unexpected RRC reconfiguration complete message. The source cell 106 may be an old source cell that has served the UE 102 before encountering at least one failure. The UE 102 sends an unexpected RRC reconfiguration complete message with the information of the old source cell 106 to the CHO candidate target cell 106. The information about the source cell 106 includes an indicator indicating an identity of the old source cell 106 and an authentication token of the old source cell 106 (which is required to authenticate the UE 102).

[0096] Upon receiving an unexpected RRC reconfiguration complete message with information of the old source cell 106 from the UE 102, the CHO candidate target cell 106 may determine the old source cell 106 of the UE 102. The CHO candidate target cell 106 sends a RETREIVE UE CONTEXT message to the old source cell 106 and requests a UE context from the old source cell 106. The UE context may provide information about bearers that have been established at the UE 102 before encountering at least one failure, so that the corresponding bearers may be reestablished on the CHO candidate target cell 106. The old source cell 106 provides the UE context to the CHO candidate target cell 106.

[0097] Upon receiving the UE context from the old source cell 106, the CHO candidate target cell 106 sends an RRC reconfiguration message to the UE 102 based on the received UE context. The RRC reconfiguration message may include a new configuration of the CHO candidate target cell 106. Upon receiving the RRC reconfiguration message from the CHO candidate target cell 106, the UE 102 applies the new configuration and sends an RRC reconfiguration complete message to the CHO candidate target cell 106.

[0098] Embodiments herein enable a target cell 106 to serve the UE 102 when a UE 102 configured with a CHO configuration encounters at least one failure on a source cell 106 by retrieving a UE context and a sequence number (SN) status transmission from the source cell 106. The target cell 106 may be a cell selected by the UE 102 when encountering at least one failure, and the selected cell is a CHO candidate target cell.

[0099] Upon receiving the unexpected RRC reconfiguration complete message including information about the old source cell 106 from the UE 102, the CHO candidate target cell 106 requests the old source cell 106 and retrieves the UE context. Upon receiving the UE context from the old source cell 106, the CHO candidate target cell 106 sends an RRC reconfiguration message to the UE 102 based on the received UE context. The RRC reconfiguration message may include the configuration of the CHO candidate target cell 106. Upon receiving the RRC reconfiguration message from the CHO candidate target cell 106, the UE 102 applies the received configuration and sends an RRC reconfiguration complete message to the CHO candidate target cell 106.

[0100] Upon receiving the RRC reconfiguration complete message from the UE 102, the CHO candidate target cell 106 requests the old source cell 106 to perform an SN state transmission. The SN state transmission includes a Packet Data Convergence Protocol (PDCP) sequence number. If at least one failure is encountered on the UE 102 during uplink data transmission, the PDCP sequence number may be the first lost data unit. If at least one failure is encountered on the UE 102 during downlink data transmission, the PDCP sequence number may be the next sequence number that must be assigned to the UE 102. In an embodiment, for the SN state transmission message, the CHO candidate target cell 106 sends an XN-U address indication. The XN-U address indication includes a forwarding address to the source cell 106, thereby notifying the source cell 106 of the address of the CHO candidate target cell to which state transmission and data forwarding must be performed. The CHO candidate target cell 106 receives the SN state transmission from the source cell 106 in response to the sent XN-U address indication. The CHO candidate target cell 106 can use the SN state transmission message to ensure less failure recovery loss.

[0101] In an embodiment, if the SN status transmission was available on the CHO candidate target cell 106 before the RRC reconfiguration complete message was received from the UE 102, the CHO candidate target cell 106 resumes data transmission to the UE 102. Data transmission may be resumed in increasing order of SNs indicated in the SN status transmission received from the old source cell 106 on which the UE 106 experienced at least one failure.

[0102] In an embodiment, if the cell selected by the UE 102 by performing cell selection (after encountering at least one failure) is not a CHO candidate target cell, the UE 102 sends a reestablishment request including information about the old source cell 106 to the selected cell / new target cell 106. In such a scenario, the new target cell 106 requests the old source cell and retrieves the UE context from the old source cell 106. Upon successful reestablishment of the connection between the UE 102 and the new target cell 106, the new target cell 106 may further request the old source cell 106 to perform an SN status transmission. Upon receiving the SN status transmission from the old source cell 106, the new target cell 106 resumes data transmission to the UE 102 in the increasing order of the SNs indicated in the received SN status transmission.

[0103] Figure 1 Exemplary units of the wireless communication network 100 are shown, but it should be understood that other embodiments are not limited thereto. In other embodiments, the wireless communication network 100 may include fewer or more units. In addition, the labels or names of the units are only for illustrative purposes and do not limit the scope of the embodiments herein. One or more units may be combined together to perform the same or substantially similar functions in the wireless communication network 100.

[0104] Figure 2 2 is a block diagram depicting hardware components of BS 106 according to embodiments disclosed herein. BS 106 (source BS / cell and target BS / cell) includes memory 202, communication interface 204, and controller 206. BS 106 also includes at least one antenna, at least one RF transceiver, processing circuitry, etc. (not shown).

[0105] The memory 202 may store at least one of a UE context, a source cell configuration, a CHO target cell configuration, an incremental source cell configuration, an incremental source cell configuration, a CHO configuration about the UE 102 configuration, an SN state transmission, etc. Examples of the memory 202 may be, but are not limited to, a NAND, an embedded Multimedia Card (embedded Multimedia Card, eMMC), a Secure Digital (Secure Digital, SD) card, a Universal Serial Bus (Universal Serial Bus, USB), a Serial Advanced Technology Attachment (Serial Advanced Technology Attachment, SATA), a solid-state drive (solid-state drive, SSD), etc. The memory 202 may also include one or more computer-readable storage media. The memory 202 may also include a non-volatile storage element. Examples of such non-volatile storage elements may include a magnetic hard disk, an optical disk, a floppy disk, a flash memory, or an electrically programmable memory (electrically programmable memories, EPROM) or an electrically erasable and programmable memory (electrically erasable and programmable, EEPROM). In addition, in some examples, the memory 202 may be considered to be a non-temporary storage medium. The term "non-transitory" may indicate that the storage medium is not embodied in a carrier wave or propagating signal. However, the term "non-transitory" should not be interpreted to mean that the memory 202 is non-removable. In some examples, non-transitory storage media may store data that may change over time (e.g., in random access memory (RAM) or cache).

[0106] The communication interface 204 may be configured to enable the BS 106 to communicate with at least one external entity (e.g., UE 102, other BSs 106, CN 104, etc.), examples of which may be at least one of a wired or wireless fronthaul interface, a wired or wireless backhaul interface, or any structure supporting communication over a wired or wireless connection.

[0107] The controller 206 may be at least one of a single processor, multiple processors, multiple homogeneous or heterogeneous cores, multiple different types of central processing units (CPUs), microcontrollers, special media, and other accelerators. The controller 206 may be a general-purpose processor such as a central processing unit (CPU), an application processor (AP), a digital signal processor (DSP), etc.

[0108] When the BS 106 is the source cell 106, the controller 206 is configured to prepare a source cell configuration or an updated / incremental source configuration. When the BS 106 is the source cell 106, the controller 206 is further configured to enable the source cell 106 to:

[0109] sending a handover request to a CHO candidate target cell 106 for CHO target cell configuration, wherein the CHO candidate target cell 106 may be a cell of the source BS 106 or at least one neighboring BS 106;

[0110] associating a CHO trigger condition with a CHO target cell configuration of each CHO candidate target cell 106;

[0111] configuring the UE 102 with the CHO configurations of the CHO candidate target cells 106, wherein the CHO configuration of each CHO candidate cell includes a CHO target cell configuration and an associated CHO trigger condition;

[0112] independently reconfiguring the UE 102 with the updated source cell configuration and the CHO target cell configuration;

[0113] performing a one-step reconfiguration of UE 102;

[0114] performing a two-step reconfiguration of UE 102; and

[0115] When the CHO candidate target cell can no longer serve the UE 102, the UE 102 is requested to release the CHO configuration of the CHO candidate target cell.

[0116] When the BS 106 is the target cell 106, the controller 206 is configured to prepare the target cell configuration as an incremental or updated / incremental target cell configuration of the source cell configuration. When the BS 106 is the target cell 106, the controller 206 is further configured to enable the target cell 106 to:

[0117] Sending the CHO target cell configuration to the source cell 106;

[0118] When the target cell 106 can no longer serve the UE 102, a release of the CHO configuration of the UE 102 is initiated;

[0119] performing a fallback connection procedure when an unexpected RRC reconfiguration message is received from the UE 102 after encountering at least one failure on the source cell 106;

[0120] Upon receiving an unexpected RRC reconfiguration message from UE 102 including information about old source cell 106, retrieving a UE context from old source cell 106 to serve UE 102; and

[0121] Upon receiving an unexpected RRC reconfiguration message from UE 102 including information about old source cell 106 , an XN-U address indication is sent to old source cell 106 and an SN status transmission is retrieved from old source cell 106 to serve UE 102 .

[0122] Figure 2 Exemplary units of BS 106 are shown, but it should be understood that other embodiments are not limited thereto. In other embodiments, BS 106 may include fewer or more units. In addition, the labels or names of the units are only for illustrative purposes and do not limit the scope of the embodiments herein. One or more units may be combined together to perform the same or substantially similar functions in BS 106.

[0123] Figure 3 is a block diagram depicting hardware components of UE 102 according to embodiments disclosed herein. UE 102 includes memory 302, communication interface 304, and controller 306. UE 102 also includes at least one antenna, at least one RF transceiver, processing circuitry, a display, input / output (I / O) ports, etc. (not shown).

[0124] The memory 302 may store, but is not limited to, at least one of a list of CHO candidate target cells, a CHO configuration of the CHO candidate target cell 106, an updated source / CHO target cell configuration, information about the source cell 106, and the like. Examples of the memory 302 may be, but are not limited to, NAND, an embedded multimedia card (eMMC), a secure digital (SD) card, a universal serial bus (USB), a serial advanced technology attachment (SATA), a solid-state drive (SSD), and the like. The memory 302 may also include one or more computer-readable storage media. The memory 302 may also include a non-volatile storage element. Examples of such non-volatile storage elements may include a magnetic hard disk, an optical disk, a floppy disk, a flash memory, or an electrically programmable memory (EPROM) or an electrically erasable programmable memory (EEPROM). In addition, in some examples, the memory 302 may be considered to be a non-transitory storage medium. The term "non-transitory" may indicate that the storage medium is not embodied in a carrier or propagation signal. However, the term "non-transitory" should not be interpreted as meaning that the memory 302 is non-removable. In certain examples, non-transitory storage media may store data that may change over time (eg, in random access memory (RAM) or cache).

[0125] The communication interface 304 may be configured to enable the UE 102 to communicate with at least one external entity (BS 106, other UEs 102, etc.) via an interface supported by the wireless communication network 100, a wired or wireless backhaul interface, or any structure supporting communication via a wired or wireless connection.

[0126] The controller 306 may be at least one of a single processor, multiple processors, multiple homogeneous or heterogeneous cores, multiple different types of central processing units (CPUs), microcontrollers, special media, and other accelerators. The controller 306 may be a general-purpose processor such as a central processing unit (CPU), an application processor (AP), a digital signal processor (DSP), etc.

[0127] The controller 306 may be configured to enable the UE 102 to:

[0128] storing the CHO configuration of the CHO candidate target cell in the CHO candidate target cell list in the memory 302;

[0129] enabling CHO execution for handover when a CHO trigger condition associated with one of the CHO candidate target cells is satisfied;

[0130] selecting a CHO candidate target cell for handover 106 when CHO trigger conditions associated with two or more CHO candidate target cells are satisfied;

[0131] Upon receiving updated source cell and CHO target cell configurations from the source cell 106, updating the CHO configuration;

[0132] Upon receiving an RRC reconfiguration message for releasing the CHO configuration from the source cell 106, releasing the stored CHO configuration of the CHO candidate target cell 106; and

[0133] When performing a transition from the RRC connected state to the RRC inactive state, the stored CHO configuration of the CHO candidate target cell 106 is released.

[0134] Figure 3 Exemplary units of UE 102 are shown, but it should be understood that other embodiments are not limited thereto. In other embodiments, UE 102 may include fewer or more units. In addition, the labels or names of the units are only for illustrative purposes and do not limit the scope of the embodiments herein. One or more units may be combined together to perform the same or substantially similar functions in UE 102.

[0135] Figure 4 1 is a sequence diagram depicting a one-step reconfiguration of a UE 102 according to an embodiment disclosed herein. The embodiments herein further explain that the source and target cells 106 are considered as gNBs, but it is obvious to those skilled in the art that the source and target cells 106 can be any other RAN nodes. The source cell 106 may be referred to as SgNB 106 hereinafter, and the target cell 106 may be referred to as TgNB 106 hereinafter.

[0136] In step 401, UE 102 sends a measurement report to SgNB 106, wherein the measurement report includes measurements of TgNB 106. In step 402, upon receiving the measurement report from UE 102, SgNB 106 sends a handover request (CHO) to TgNB 106. The handover request includes a source cell configuration. In step 403, upon receiving the handover request from SgNB 106, TgNB 106 performs admission control and reserves resources for UE 102. While reserving resources for UE 102, TgNB 106 prepares a CHO target cell configuration. The CHO target cell configuration includes a source cell configuration and a container (including a target cell configuration that is prepared as an increment of the received source cell configuration). In step 404, TgNB 106 sends a handover request confirmation to SgNB 106, which includes the CHO target cell configuration.

[0137] In step 405, upon receiving the CHO target cell configuration from TgNB 106, SgNB 106 associates the CHO trigger condition with the CHO target cell configuration and sends the CHO configuration of TgNB 106 to UE 102 in an RRC reconfiguration message. The CHO configuration of TgNB 106 includes the CHO target cell configuration and the CHO trigger condition. In step 406, UE 102 applies the source cell configuration received in the CHO configuration to continue on the source cell. If the CHO trigger condition of TgNB 106 has been met, UE 102 performs CHO execution. CHO execution involves applying the CHO target cell configuration of TgNB 106 for handover.

[0138] In step 407, the SgNB 106 reconfigures the source cell configuration. The SgNB 106 may reconfigure the source cell configuration as an increment over the existing source cell configuration (incremental source cell configuration). In step 408, the SgNB 106 sends a handover modification request to the TgNB 106, wherein the handover modification request includes the incremental source cell configuration. In step 409, upon receiving the incremental source cell configuration, the TgNB 106 updates the CHO target cell configuration by updating the CHO target cell configuration as an increment over the incremental source cell configuration (incremental CHO target cell configuration). The TgNB 106 sends the incremental CHO target cell configuration to the SgNB 106 in a handover request acknowledgment.

[0139] In step 410, upon receiving the incremental CHO target cell configuration, the SgNB 106 sends the incremental source cell configuration and the incremental CHO target cell configuration to the UE 102 in an RRC reconfiguration message. In step 411, the UE 102 updates the CHO configuration of the TgNB 106. The UE 102 first updates the source cell configuration in the stored CHO configuration (of the CHO configuration) with the incremental source configuration. Subsequently, the UE 102 updates the CHO target configuration stored in the CHO configuration of the TgNB 106 with the incremental target cell configuration. In step 412, upon updating the CHO configuration of the TgNB 106, the UE 102 sends an RRC reconfiguration complete message to the SgNB 106, indicating the successful update of the CHO configuration of the TgNB 106.

[0140] Figure 5 is an example sequence diagram depicting a two-step reconfiguration of the UE 102 according to embodiments disclosed herein.

[0141] In step 501, UE 102 sends a measurement report to SgNB 106, wherein the measurement report includes measurements of TgNB 106. In step 502, upon receiving the measurement report from UE 102, SgNB 106 sends a handover request (CHO) to TgNB 106. The handover request includes a source cell configuration. In step 503, upon receiving the handover request from SgNB 106, TgNB 106 performs admission control and reserves resources for UE 102. While reserving resources for UE 102, TgNB 106 prepares a CHO target cell configuration. The CHO target cell configuration includes a source cell configuration and a container (including a target cell configuration that is prepared as an increment of the received source cell configuration). In step 504, TgNB 106 sends a handover request confirmation to SgNB 106, which includes the CHO target cell configuration.

[0142] At step 505, upon receiving the CHO target cell configuration from TgNB 106, SgNB 106 associates the CHO trigger condition with the CHO target cell configuration and sends the CHO configuration of TgNB 106 to UE 102 in an RRC reconfiguration message. The CHO configuration of TgNB 106 includes the CHO target cell configuration and the CHO trigger condition. At step 506, UE 102 applies the source cell configuration received in the CHO configuration to continue on the source cell. If the CHO trigger condition of TgNB 106 has been met, UE 102 performs CHO execution. CHO execution involves applying the CHO target cell configuration of TgNB 106 for handover.

[0143] In step 507, SgNB 106 reconfigures the source cell configuration. SgNB 106 may reconfigure the source cell configuration as an increment of the existing source cell configuration (incremental source cell configuration). In step 508, when reconfiguring the source cell configuration, SgNB 106 sends a first RRC reconfiguration message to UE 102. The first RRC reconfiguration message includes the incremental source cell configuration and a request to release the CHO target cell configuration of TgNB 106 in the CHO configuration. When sending the first RRC reconfiguration message to UE 102, in step 509, SgNB 106 sends a handover modification request to TgNB 106.

[0144] At step 510, upon receiving the first RRC reconfiguration message from SgNB 106, UE 102 updates the source cell configuration in the CHO configuration with the incremental source cell configuration and releases the CHO target cell configuration of TgNB 106. At step 511, upon releasing the CHO target cell configuration of TgNB 106, UE 102 sends an RRC reconfiguration complete message to SgNB 106 in response to the first RRC reconfiguration message.

[0145] At step 512, upon receiving the Handover Modify Request from the SgNB, the TgNB 106 updates the CHO target cell configuration and sends the updated / incremental CHO target cell configuration in the Handover Request Ack to the SgNB 106. The incremental CHO target cell configuration may be prepared by updating the CHO target cell configuration to the increment of the received incremental source cell configuration.

[0146] At step 513, upon receiving the incremental CHO target cell configuration, SgNB 106 sends a second RRC reconfiguration message to UE 102. The second RRC reconfiguration message includes the incremental CHO target cell configuration. At step 514, upon receiving the second RRC reconfiguration message, UE 102 updates the CHO target cell configuration in the CHO configuration of TgNB 106 with the received incremental CHO target cell configuration.

[0147] Fig. 6A , Figure 6B and Figure 6C is an example sequence diagram depicting release of a CHO target cell configuration initiated by a CHO candidate target cell 106 when the target cell 106 no longer serves the UE 102 according to embodiments disclosed herein.

[0148] In an embodiment, Fig. 6AAs shown, TgNB 106 initiates the release of the CHO target cell configuration of UE 102 by sending a handover cancel request to SgNB 106, and SgNB 106 immediately accepts the handover cancel request. When TgNB 106 no longer serves UE 102 due to at least one of allocating resources reserved for UE 102 to another UE 102, increasing load on TgNB, etc., TgNB 106 may initiate the release of the CHO target cell configuration for UE 102. Fig. 6A As shown, in step 611, TgNB 106 sends a handover cancel request or a CHO configuration release request to SgNB 106 to allow TgNB to release the CHO target cell configuration previously configured on UE 102. In step 612, SgNB 106 accepts the received handover cancel request and sends a handover cancel request confirmation to TgNB 106.

[0149] In an embodiment, Figure 6B As shown, TgNB 106 initiates the release of the CHO target cell configuration of UE 102 by sending a handover cancellation request to SgNB 106, and when the CHO target cell configuration of TgNB 106 is successfully released at UE 102, SgNB 106 accepts the handover cancellation request. Figure 6B As shown, in step 621, TgNB 106 sends a handover cancel request to SgNB 106 to allow TgNB to release the CHO target cell configuration previously configured on UE 102. In step 622, SgNB 106 sends an RRC reconfiguration message to UE 102 to release the CHO target cell configuration of TgNB 106. In step 623, UE 102 releases the CHO target cell configuration of TgNB 106 and sends an RRC reconfiguration complete message to SgNB 106, indicating the release of the indicated CHO target cell configuration of TgNB 106. In step 624, upon receiving the RRC reconfiguration complete message from UE 102, SgNB 106 accepts the handover cancel request received from TgNB 106 and sends a handover cancel request confirmation to TgNB 106.

[0150] In an embodiment, Figure 6C As shown, upon receiving the RLC confirmation from UE 102, SgNB 106 allows TgNB 106 to release the CHO target cell configuration previously provided to UE 102. Figure 6CAs shown, in step 631, TgNB 106 sends a handover cancel request to SgNB 106 to allow TgNB to release the CHO target cell configuration previously configured on UE 102. In step 632, SgNB 106 sends an RRC reconfiguration message to UE 102 to release the CHO target cell configuration of TgNB 106. In step 633, UE 102 releases the CHO target cell configuration of TgNB 106 and sends an RLC confirmation to SgNB 106, indicating the release of the indicated CHO target cell configuration of TgNB 106. In step 634, upon receiving the RRC reconfiguration complete message from UE 102, SgNB 106 accepts the handover cancel request received from TgNB 106 and sends a handover cancel request confirmation to TgNB 106.

[0151] Figure 7 is an example sequence diagram depicting the rejection of a handover cancel request of a CHO candidate target cell 106 according to embodiments disclosed herein.

[0152] In step 701, TgNB 106 sends a handover cancel request to SgNB 106 to allow TgNB to release the CHO target cell configuration previously configured on UE 102. In step 702, SgNB 106 sends an RRC reconfiguration message to UE 102 to release the CHO target cell configuration of TgNB 106, and determines that UE 102 cannot receive the RRC reconfiguration message. In step 703, upon determining that UE 102 cannot receive the RRC reconfiguration message, SgNB 106 cancels the received handover cancel request of TgNB 106 and sends a handover cancel rejection to TgNB 106.

[0153] Fig. 8A , 8B 8C are example sequence diagrams depicting example procedures according to embodiments disclosed herein, in which a radio resource control (RRC) reconfiguration message is sent to the UE 102 to instruct the UE 102 to remove the CHO candidate target cell 106.

[0154] In such Fig. 8A In the example process shown, at step 811, SgNB 106 sends an RRC reconfiguration message to UE 102 to release the CHO target cell configuration of TgNB 106, wherein the RRC reconfiguration message includes a CHO release message. The CHO release message indicates the CHO configuration / CHO target cell configuration of the TgNB that must be released. At step 812, UE 102 releases the CHO target cell configuration of TgNB 106 and sends an RRC reconfiguration complete message to SgNB 106.

[0155] In another example process, Figure 8B As shown, at step 821, SgNB 106 sends an RRC reconfiguration message to UE 102 to release the CHO target cell configuration of TgNB 106, wherein the RRC reconfiguration message includes the source cell configuration and the CHO release message. At step 822, UE 102 releases the CHO target cell configuration of TgNB 106 and sends a joint / combined RRC reconfiguration complete message to SgNB 106. The combined RRC reconfiguration complete message includes a reconfiguration complete message for the updated / incremental source cell configuration and an embedded reconfiguration complete message for indicating the release of the CHO target cell configuration of TgNB 106.

[0156] In another example process, Figure 8C As shown, in step 831, SgNB 106 sends an RRC reconfiguration message to UE 102 to release the CHO target cell configuration of TgNB 106, wherein the RRC reconfiguration message includes the source cell configuration and the CHO release message. In step 832, UE 102 releases the CHO target cell configuration of TgNB 106 and sends an RRC reconfiguration complete message to SgNB 106. The RRC reconfiguration complete message includes a CHO release indicator (IE) indicating the successful release of the CHO target cell configuration of the indicated TgNB 106.

[0157] Fig.9A and 9B is an example diagram depicting a fallback to connection establishment process after the UE 102 encounters at least one failure on the source cell 106 according to embodiments disclosed herein.

[0158] like Fig.9A and 9B As shown, in step 901, UE 102 sends a measurement report to SgNB 106, wherein the measurement report includes measurements of TgNB 106. In step 902, upon receiving the measurement report from UE 102, SgNB 106 sends a handover request (CHO) to TgNB 106. The handover request includes a source cell configuration. In step 903, upon receiving the handover request from SgNB 106, TgNB 106 performs admission control and reserves resources for UE 102. While reserving resources for UE 102, TgNB 106 prepares a CHO target cell configuration. The CHO target cell configuration includes a source cell configuration and a container (including a target cell prepared as an increment of the received source cell configuration). In step 904, TgNB 106 sends a handover request confirmation to SgNB 106, which includes the CHO target cell configuration.

[0159] At step 905, upon receiving the CHO target cell configuration from TgNB 106, SgNB 106 associates the CHO trigger condition with the CHO target cell configuration and sends the CHO configuration of TgNB 106 to UE 102 in an RRC reconfiguration message. The CHO configuration of TgNB 106 includes the CHO target cell configuration and the CHO trigger condition. At step 906, UE 102 applies the source cell configuration received in the CHO configuration to continue on the source cell. If the CHO trigger condition of TgNB 106 has been met, UE 102 performs CHO execution. CHO execution involves applying the CHO target cell configuration of TgNB 106 for handover.

[0160] At step 907, UE 102 encounters at least one failure (e.g., RLF) on SgNB 106. At step 908, upon encountering at least one failure, UE 102 performs a cell selection process and selects a gNB present in the vicinity of UE 102, wherein the selected gNB may be a CHO candidate target cell / TgNB 106. At step 909, UE 102 sends an RRC reconfiguration complete message to TgNB 106. At step 910, because TgNB 106 does not have a UE context, TgNB 106 treats the received RRC reconfiguration complete message as an unexpected RRC reconfiguration complete message.

[0161] In such Fig.9A In the illustrated embodiment, in step 911a, upon receiving an unexpected RRC reconfiguration message, TgNB 106 sends an RRC release message to UE 102. Upon receiving the RRC release message, UE 102 may initiate an RRC connection establishment procedure.

[0162] In such Fig. 9B In the illustrated embodiment, at step 911b, upon receiving the unexpected RRC reconfiguration message, TgNB 106 sends an RRC setup message to UE 102, wherein the RRC setup message includes the configuration of TgNB 106. At step 911c, upon receiving the RRC setup message, UE 102 applies the configuration of TgNB 106 in the received RRC setup message and sends an RRC setup complete message to TgNB 106.

[0163] Fig. 10A is an example sequence diagram depicting a fallback procedure to connection reestablishment after the UE 102 encounters at least one failure on the source cell 106 according to embodiments disclosed herein.

[0164] In step 1001, UE 102 sends a measurement report to SgNB 106, wherein the measurement report includes measurements of TgNB 106. In step 1002, upon receiving the measurement report from UE 102, SgNB 106 sends a handover request (CHO) to TgNB 106. The handover request includes a source cell configuration. In step 1003, upon receiving the handover request from SgNB 106, TgNB 106 performs admission control and reserves resources for UE 102. While reserving resources for UE 102, TgNB 106 prepares a CHO target cell configuration. The CHO target cell configuration includes a source cell configuration and a container (including a target cell configuration that is prepared as an increment of the received source cell configuration). In step 1004, TgNB 106 sends a handover request confirmation to SgNB 106, which includes the CHO target cell configuration.

[0165] At step 1005, upon receiving the CHO target cell configuration from TgNB 106, SgNB 106 associates the CHO trigger condition with the CHO target cell configuration and sends the CHO configuration of TgNB 106 to UE 102 in an RRC reconfiguration message. The CHO configuration of TgNB 106 includes the CHO target cell configuration and the CHO trigger condition. At step 1006, UE 102 applies the source cell configuration received in the CHO configuration to continue on the source cell. If the CHO trigger condition of TgNB 106 has been met, UE 102 performs CHO execution. CHO execution involves applying the CHO target cell configuration of TgNB 106 for handover.

[0166] At step 1007, the UE 102 encounters at least one failure (e.g., RLF) on the SgNB 106. At step 1008, upon encountering at least one failure, the UE 102 performs a cell selection process and selects a gNB present in the vicinity of the UE 102, wherein the selected gNB may be a CHO candidate target cell / TgNB 106. At step 1009, the UE 102 sends an RRC reconfiguration complete message to the TgNB 106, wherein the RRC reconfiguration complete message includes information about the old SgNB 106 (the gNB used as the source cell 106 of the UE 102 before encountering at least one failure). The information about the old SgNB 106 includes information about an identity of the old SgNB 106 and an authentication token of the old SgNB 106. Fig. 10B An example RRC reconfiguration complete message including information about the old source cell is depicted in FIG.

[0167] At step 1010, upon receiving the RRC reconfiguration message including information about the old SgNB 106, the TgNB 106 requests the old SgNB 106 and retrieves the UE context. At step 1011, upon retrieving the UE context, the TgNB 106 sends signaling to the UE 102 to perform RRC reestablishment. Fig. 10C 1012, UE 102 performs RRC reestablishment and sends RRC reestablishment completion to TgNB 106.

[0168] Fig.11 is an example sequence diagram illustrating retrieval of SN status transmissions by a CHO candidate target cell to serve the UE 102 after the UE 102 encounters at least one failure on the source cell 106 according to embodiments disclosed herein.

[0169] In step 1101, UE 102 sends a measurement report to SgNB 106, wherein the measurement report includes measurements of TgNB 106. In step 1102, upon receiving the measurement report from UE 102, SgNB 106 sends a handover request (CHO) to TgNB 106. The handover request includes a source cell configuration. In step 1103, upon receiving the handover request from SgNB 106, TgNB 106 performs admission control and reserves resources for UE 102. While reserving resources for UE 102, TgNB 106 prepares a CHO target cell configuration. The CHO target cell configuration includes a source cell configuration and a container (including a target cell configuration that is prepared as an increment of the received source cell configuration). In step 1104, TgNB 106 sends a handover request confirmation to SgNB 106, which includes the CHO target cell configuration.

[0170] At step 1105, upon receiving the CHO target cell configuration from TgNB 106, SgNB 106 associates the CHO trigger condition with the CHO target cell configuration and sends the CHO configuration of TgNB 106 to UE 102 in an RRC reconfiguration message. The CHO configuration of TgNB 106 includes the CHO target cell configuration and the CHO trigger condition. At step 1106, UE 102 applies the source cell configuration received in the CHO configuration to continue on the source cell. If the CHO trigger condition of TgNB 106 has been met, UE 102 performs CHO execution. CHO execution involves applying the CHO target cell configuration of TgNB 106 for handover.

[0171] At step 1107, the UE 102 encounters at least one failure (e.g., RLF) on the SgNB 106. At step 1108, upon encountering at least one failure, the UE 102 performs a cell selection process and selects a gNB present in the vicinity of the UE 102, wherein the selected gNB may be a CHO candidate target cell / TgNB 106. At step 1109, the UE 102 sends an RRC reconfiguration complete message to the TgNB 106, wherein the RRC reconfiguration complete message includes information about the old SgNB 106 (the gNB used as the source cell 106 of the UE 102 before encountering at least one failure). The information about the old SgNB 106 includes information about an identity of the old SgNB 106 and an authentication token of the old SgNB 106. At step 1110, upon receiving the RRC reconfiguration message including the information about the old SgNB 106, the TgNB 106 determines that the TgNB 106 does not have a UE context. At step 1111, TgNB 106 identifies old SgNB 106 from the RRC reconfiguration message received from UE 102, and sends a retrieve UE context to old SgNB 106 for the UE context. At step 1112, old SgNB 106 sends a retrieve UE context response including the UE context to TgNB 106.

[0172] At step 1113, upon receiving the UE context from the old SgNB 106, the TgNB 106 sends an RRC Reestablishment message to the UE 102. At step 1114, upon receiving the RRC Reestablishment message, the UE 102 sends an RRC Reestablishment Complete message to the TgNB 106. At step 1115, the TgNB 106 sends an Xn-U address indication to the old SgNB 106 for SN status transmission. At step 1116, the old SgNB 106 sends the SN status transmission to the TgNB 106 so that the TgNB 106 can resume the data transmission to the UE 102 that was in progress before the at least one failure encountered by the UE 102.

[0173] Fig. 12A is an example sequence diagram depicting the association of CHO triggering conditions with CHO target cell configurations of the CHO candidate target cells 106 according to embodiments disclosed herein.

[0174] Embodiments herein enable the SgNB 106 to associate a CHO trigger condition with a CHO target cell configuration of the TgNB 106 during CHO preparation of the UE 102. The CHO trigger condition may include a measurement object and a reporting configuration. The SgNB 106 uses a measurement identifier or any other unique ID to link the measurement object and the reporting configuration. The SgNB 106 further enables the TgNB 106 to include the measurement identifier in a container of its CHO target cell configuration.

[0175] like Fig. 12A As shown, at step 1201, SgNB 106 sends a handover request to TgNB 106, the handover request including a source cell configuration and a measurement identifier. At step 1202, upon receiving the handover request, TgNB 106 prepares a CHO target cell configuration and sends the CHO target cell configuration to SgNB 106 in a handover request confirmation. The CHO target cell configuration includes a source cell configuration and an RRC container, wherein the RRC container includes a target cell configuration prepared for the source cell configuration and an increment on the measurement identifier.

[0176] In step 1203, upon receiving the CHO target cell configuration, SgNB 106 prepares the CHO configuration of TgNB 106 by associating the CHO trigger condition with the CHO target cell configuration, and sends the CHO configuration of TgNB 106 in the RRC reconfiguration message to UE 102. UE 102 stores the CHO configuration of TgNB 106 in the CHO candidate target cell list (already stored in memory 202).

[0177] like Fig. 12B As shown, the CHO configuration of TgNB 106 in the RRC reconfiguration message includes CHO target cell configuration, measurement objects, report configuration, identified measurements, etc. The CHO target cell configuration includes a source cell configuration and a container, wherein the RRC container includes a target cell configuration prepared for the source cell configuration and an increment on the measurement identifier.

[0178] The embodiments disclosed herein may be implemented by at least one software program running on at least one hardware device and performing network management functions to control these elements. Figure 1-12A The elements shown in the figure may be at least one of a hardware device, or a combination of a hardware device and a software module.

[0179] The embodiments disclosed herein describe methods and systems for managing conditional handover (CHO) in wireless communication networks. Therefore, it should be understood that the scope of protection extends to such a program, and in addition to a computer-readable device having a message therein, such a computer-readable storage device also includes a program code device for implementing one or more steps of the method, when the program is run on a server, a mobile device, or any suitable programmable device. The method is implemented in a preferred embodiment by or with a software program written in another programming language such as a very high speed integrated circuit hardware description language (VHDL), or by one or more VHDL or multiple software modules executed on at least one hardware device. The hardware device can be any programmable portable device. The device can also include a device, which can be, for example, a hardware device (such as an ASIC), or a combination of hardware and software devices (such as an ASIC and an FPGA), or at least one microprocessor and at least one memory having a software module located therein. The method embodiments described herein can be implemented partially in hardware and partially in software. Alternatively, the present disclosure can be implemented on different hardware devices, for example, using multiple CPUs.

[0180] The above description of the specific embodiments will fully reveal the general nature of the embodiments herein, so that others can easily modify and / or adapt the various applications of these specific embodiments by applying current knowledge without departing from the general concept, and therefore, such adaptations and modifications should and are intended to be understood within the equivalent meaning and scope of the disclosed embodiments. It should be understood that the phrases or terms used herein are for description rather than for limitation. Therefore, although the embodiments of this article have been described according to the embodiments, those skilled in the art will recognize that the embodiments of this article can be practiced by modification within the spirit and scope of the embodiments described herein.

Claims

1. A method performed by a source base station BS in a wireless communication system, the method comprising: Sending (502) a handover request message for the terminal to the candidate target BS, the handover request message including source cell configuration information; receiving (504) a handover request confirmation message including first conditional handover (CHO) configuration information corresponding to the source cell configuration information from the candidate target BS; Based on the handover request confirmation message, sending (505) a first radio resource control RRC message including first CHO configuration information to the terminal; as well as sending (508) a second RRC message for configuring updated source cell configuration information of the source BS for the terminal to the terminal, Wherein, based on the second RRC message, the first CHO configuration information is released.

2. The method according to claim 1, further comprising: sending (509) a handover request message including updated source cell configuration information to the candidate target BS; receiving (512) a handover request confirmation message including second CHO configuration information corresponding to the updated source cell configuration information from the candidate target BS; and sending (513) a third RRC message including second CHO configuration information to the terminal, Wherein, when the triggering condition of the CHO based on the second CHO configuration information is satisfied, the terminal performs the CHO to the candidate target BS.

3. The method according to claim 1, further comprising: sending a fourth RRC message to the terminal, instructing the terminal to enter an RRC inactive state, Wherein, based on the fourth RRC message, the first CHO configuration information is released.

4. The method according to claim 2, further comprising: Receive (611, 621, 631) a CHO cancel message from the candidate target BS; and Based on the CHO cancel message, sending (622, 632) a fifth RRC message to the terminal for releasing the stored CHO configuration information, Wherein, based on the fifth RRC message, the second CHO configuration information is released.

5. A method performed by a candidate target base station BS in a wireless communication system, the method comprising: receiving (502) a handover request message for the terminal from the source BS, the handover request message including source cell configuration information; Sending (504) a handover request confirmation message including first conditional handover CHO configuration information corresponding to the source cell configuration information to the source BS; receiving (509) a handover request message including updated source cell configuration information of the source BS from the source BS; sending (512) a handover request confirmation message including second CHO configuration information corresponding to the updated source cell configuration information to the source BS, and receiving, from the terminal, a message for performing CHO to the candidate target BS in a case where a triggering condition of CHO based on the second CHO configuration information is satisfied, Wherein, based on the updated source cell configuration information of the source BS, the first CHO configuration information is released.

6. The method according to claim 5, further comprising: Send (611, 621, 631) CHO cancellation message to the source BS; in, Based on the CHO cancel message, a radio resource control RRC message is sent to the terminal to release the stored CHO configuration information, and Wherein, based on the RRC message from the source BS, the second CHO configuration information is released.

7. A method performed by a terminal in a wireless communication system, the method comprising: receiving (505) a first radio resource control RRC message from a source base station BS, the first RRC message including first conditional handover CHO configuration information for a terminal to a candidate target BS, the first CHO configuration information corresponding to the source cell configuration information; receiving (508) from the source BS a second RRC message for configuring updated source cell configuration information of the source BS; Based on the second RRC message, releasing (510) the first CHO configuration information; receiving (513) a third RRC message including second CHO configuration information from the source BS; Based on the second CHO configuration information, determining (514) whether a triggering condition for CHO of the candidate target BS is satisfied; and In case the trigger condition is satisfied, CHO to the candidate target BS is performed, wherein the second CHO configuration information corresponds to updated source cell configuration information of the source BS.

8. The method according to claim 7, further comprising: receiving (611, 621, 631) a fourth RRC message for releasing stored CHO configuration information or for instructing the terminal to enter an inactive state from the source BS; and Based on the fourth RRC message, the second CHO configuration information is released.

9. A source base station BS in a wireless communication system, the source BS comprising: a transceiver configured to transmit and receive signals; and A controller is coupled to the transceiver and is configured to: Sending a handover request message for the terminal to the candidate target BS, the handover request message including source cell configuration information, receiving a handover request confirmation message including first conditional handover (CHO) configuration information corresponding to the source cell configuration information from the candidate target BS, Based on the handover request confirmation message, a first radio resource control RRC message including first CHO configuration information is sent to the terminal, and sending a second RRC message to the terminal for configuring updated source cell configuration information of the source BS for the terminal, Wherein, based on the second RRC message, the first CHO configuration information is released.

10. The source BS according to claim 9, in, The controller is also configured to: Sending a handover request message including updated source cell configuration information to the candidate target BS, receiving a handover request confirm message including second CHO configuration information corresponding to the updated source cell configuration information from the candidate target BS, and sending a third RRC message including second CHO configuration information to the terminal, and Wherein, when the triggering condition of the CHO based on the second CHO configuration information is satisfied, the terminal performs the CHO to the candidate target BS.

11. The source BS according to claim 10, in, The controller is further configured to send a fourth RRC message to the terminal, indicating that the terminal enters the RRC inactive state, and Wherein, based on the fourth RRC message, the second CHO configuration information is released, or The controller is further configured to receive a CHO cancellation message from the candidate target BS; and based on the CHO cancellation message, send a fifth RRC message to the terminal for releasing the stored CHO configuration information, and based on the fifth RRC message, the second CHO configuration information is released.

12. A candidate target base station BS in a wireless communication system, the candidate target BS comprising: a transceiver configured to transmit and receive signals; and A controller is coupled to the transceiver and is configured to: receiving a handover request message for the terminal from the source BS, the handover request message including source cell configuration information, Sending a handover request confirmation message including first conditional handover CHO configuration information corresponding to the source cell configuration information to the source BS, receiving a handover request message including updated source cell configuration information of the source BS from the source BS, sending a handover request confirmation message including second CHO configuration information corresponding to the updated source cell configuration information to the source BS, and receiving, from the terminal, a message for performing CHO to the candidate target BS in a case where a triggering condition of CHO based on the second CHO configuration information is satisfied, Wherein, based on the updated source cell configuration information of the source BS, the first CHO configuration information is released.

13. The candidate target BS according to claim 12, in, The controller is also configured to: Send a CHO cancellation message to the source BS; wherein, based on the CHO cancel message, a radio resource control RRC message is sent to the terminal to release the stored CHO configuration information, and Wherein, based on the RRC message from the source BS, the second CHO configuration information is released.

14. A terminal in a wireless communication system, the terminal comprising: a transceiver configured to transmit and receive signals; and A controller is coupled to the transceiver and is configured to: receiving a first radio resource control RRC message from a source base station BS, the first RRC message including first conditional handover CHO configuration information for the terminal to the candidate target BS, the first CHO configuration information corresponding to the source cell configuration information, receiving, from the source BS, a second RRC message for configuring updated source cell configuration information of the source BS; Based on the second RRC message, releasing the first CHO configuration information, receiving, from the source BS, a third RRC message including second CHO configuration information; determining, based on the second CHO configuration information, whether a triggering condition for a CHO of the candidate target BS is satisfied; and When the trigger condition is met, a CHO to the candidate target BS is performed. The second CHO configuration information corresponds to updated source cell configuration information of the source BS.

15. The terminal according to claim 14, wherein: The controller is also configured to: receiving from the source BS a fourth RRC message for releasing the stored CHO configuration information or for instructing the terminal to enter an RRC inactive state, and Based on the fourth RRC message, the second CHO configuration information is released.

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