Patent Publication Number: US-2023156505-A1

Title: Method and apparatus for reporting failure information in a communication system

Description:
TECHNICAL FIELD 
     The present disclosure generally relates to wireless communication technology, and more particularly to failure information report in a wireless communication system. 
     BACKGROUND 
     In the 3rd Generation Partnership Project (3GPP), deployment of Relay Nodes (RNs) in a wireless communication system is promoted. One objective of deploying RNs is to enhance the coverage area of a Base Station (BS, also called gNB in 5G networks) by improving the throughput of a mobile device (also known as a User Equipment (UE)) that is located in a coverage hole or far from the BS, which can result in relatively low signal quality. 
     In a wireless communication system which employs RNs, a BS that can provide a connection to at least one RN is called a donor BS (or a donor node or a donor). An RN is connected to a donor BS by a backhaul link. The RN may hop through one or more RNs before reaching the donor BS, or may be directly connected to the donor BS. For New Radio (NR) communication networks, 3GPP is envisioning an Integrated Access and Backhaul (IAB) architecture for supporting multi-hop relays, wherein a donor node with multi-connectivity is also supported by an IAB node. That is, the IAB node may have a plurality of active routes to the donor BS via multiple parent IAB nodes. A multi-hop network may provide more range extension than a single-hop network. This is relatively more beneficial with respect to wireless communications at frequencies above 6 GHz, which have limited ranges when using single-hop backhauling. Multi-hop backhauling further enables backhauling around obstacles, e.g., buildings in an urban environment for in-cluster deployments. 
     There is a need for handling a failure in the backhaul link when multi-connectivity is supported in a wireless communication system. 
     SUMMARY OF THE DISCLOSURE 
     An embodiment of the present disclosure provides a method. The method may include: transmitting, at an access node, a failure information message to a base station, wherein the access node is connected to the base station via a master node and a secondary node, and wherein the failure information message includes a failure type of a backhaul link, and the failure type is based on a failure type set including at least the following: an expiry of a physical layer problem timer, a random access problem, and reaching a maximum number of retransmission in response to radio link failure (RLF). 
     Another embodiment of the present disclosure provides a method. The method may include: receiving, at a base station from an access node, a failure information message, wherein the access node is connected to the base station via a master node and a secondary node, the failure information message includes a failure type of a backhaul link, and the failure type is based on a failure type set including at least the following: an expiry of a physical layer problem timer, a random access problem, and reaching a maximum number of retransmission in response to radio link failure (RLF). 
     Yet another embodiment of the present disclosure provides an apparatus. According to some embodiments of the present disclosure, the apparatus includes: at least one non-transitory computer-readable medium having computer executable instructions stored therein; at least one receiving circuitry; at least one transmitting circuitry; and at least one processor coupled to the at least one non-transitory computer-readable medium, the at least one receiving circuitry and the at least one transmitting circuitry, wherein the at least one non-transitory computer-readable medium and the computer executable instructions are configured to, with the at least one processor, to cause the apparatus to perform a method according to some embodiments of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to describe the manner in which the advantages and features of the disclosure can be obtained, a description of the disclosure is rendered by reference to specific embodiments thereof, which are illustrated in the appended drawings. These drawings depict only exemplary embodiments of the disclosure and are not therefore to be considered limiting of its scope. 
         FIG.  1    illustrates a schematic wireless communication system according to some embodiments of the present disclosure; 
         FIG.  2 A  illustrates an exemplary procedure of reporting a SCG failure according to some embodiments of the present disclosure; 
         FIG.  2 B  illustrates an exemplary fast MCG link recovery procedure according to some embodiments of the present disclosure; 
         FIG.  3 A  illustrates a wireless communication system according to some embodiments of the present disclosure; 
         FIG.  3 B  illustrates a wireless communication system according to some embodiments of the present disclosure; 
         FIG.  4    illustrates an exemplary fast MCG link recovery procedure according to some embodiments of the present disclosure; 
         FIG.  5    illustrates an exemplary procedure of reporting a SCG failure according to some embodiments of the present disclosure; 
         FIG.  6    illustrates an exemplary procedure of failure information transmission according to some embodiments of the present disclosure; 
         FIG.  7    illustrates an exemplary procedure of handling failure information according to some embodiments of the present disclosure; and 
         FIG.  8    illustrates an example block diagram of an apparatus according to some embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The detailed description of the appended drawings is intended as a description of preferred embodiments of the present disclosure and is not intended to represent the only form in which the present disclosure may be practiced. It should be understood that the same or equivalent functions may be accomplished by different embodiments that are intended to be encompassed within the spirit and scope of the present disclosure. 
       FIG.  1    illustrates a schematic dual-connectivity wireless communication system  100  according to some embodiments of the present disclosure. 
     Referring to  FIG.  1   , the dual-connectivity wireless communication system  100  may include a UE (e.g., UE  30 ), which may be connected to two base stations (e.g., BS  10 A and BS  10 B). Although a specific number of UE and BSs are depicted in  FIG.  1   , it is contemplated that wireless communication system  100  may also include more or fewer BSs and UEs. 
     The UE and the base stations may support communication based on, for example, 3G, long-term evolution (LTE), LTE-advanced (LTE-A), new radio (NR), or other suitable protocol(s). For example, BS  10 A or BS  10 B may include an eNB or a gNB. UE  30  may include, for example, but is not limited to, a computing device, a wearable device, a mobile device, an IoT device, a vehicle, etc. Persons skilled in the art should understand that as technology develops and advances, the terminologies described in the present disclosure may change, but should not affect or limit the principles and spirit of the present disclosure. 
     BS  10 A may act as a Master Node (MN), and BS  10 B may act as a Secondary Node (SN). UE  30  may communicate with BS  10 A or BS  10 B via, for example, a Uu interface. BS  10 A and BS  10 B may communicate with each other via a network interface such as an Xn interface. At least the MN (e.g., BS  10 A) is connected to a Core Network (not shown in  FIG.  1   ). In some embodiments of the present disclosure, one of BS  10 A and BS  10 B may provide NR access and the other one of BS  10 A and BS  10 B may provide either E-UTRA (Evolved Universal Terrestrial Radio Access) or NR access. 
     UE  30  may connect to a Master Cell Group (MCG) and a Secondary Cell Group (SCG) through BS  10 A and BS  10 B, respectively. The MCGs and SCGs may be groups of cells associated with BS  10 A and BS  10 B, respectively, including a primary cell (PCell)/primary SCell (PSCell), and optionally one or more secondary cells (SCells). BS  10 A may be configured to add or release SCGs associated with BS  10 B. The link between a UE (e.g., UE  30 ) and an MN (e.g., BS  10 A) may also be referred to as a MCG link, and the link between a UE (e.g., UE  30 ) and an SN (e.g., BS  10 B) may also be referred to as a SCG link. 
     UE  30  may be configured to monitor the SCG of BS  10 B and may detect SCG failures. In some embodiments of the present disclosure, UE  30  may initiate a procedure to report SCG failures when, for example, one of the following conditions is met:
     upon detecting radio link failure for the SCG;   upon reconfiguration with sync failure of the SCG;   upon SCG configuration failure; and   upon integrity check failure indication from SCG lower layers concerning SRB3 (Signaling Radio Bearer type 3).   

     When an SCG failure is detected, UE  30  may report a SCG failure information message to an MN (e.g., BS  10 A). BS  10 A may handle the SCG failure information message, and may decide to keep, change, or release the SN/SCG. 
     UE  30  may also detect MCG failures. In some embodiments of the present disclosure, UE  30  may initiate a MCG recovery procedure to inform the MN (e.g., BS  10 A) about an MCG link failure via the SCG link. UE  30  may be configured with split SRB1 (Signaling Radio Bearer type 1) or SRB3 to report MCG failures when a Radio Link Failure (RLF) on a MCG link happens. 
     In some examples, when SRB1 is configured as split SRB, UE  30  may start a timer (e.g., T316) and submit a MCG failure information message to lower layers for transmission to the MN (e.g., BS  10 A) via SRB1. In some examples, when SRB3 is configured, UE  30  may encapsulate the MCG failure information message in another Radio Resource Control (RRC) message (e.g., uplink information transfer message for Multi-Radio Dual Connectivity (MRDC) or ULInformationTransferMRDC message), which is used for communication between a UE and a MN, and transmit the another RRC message via SRB3. After BS  10 A receives the MCG failure information message, BS  10 A may transmit a reconfiguration with sync message or a release message to UE  30 . 
       FIG.  2 A  illustrates an exemplary procedure  200 A of reporting a SCG failure according to some embodiments of the present disclosure. Details described in all of the foregoing embodiments of the present disclosure are applicable for the embodiments shown in  FIG.  2 A . 
     The exemplary procedure  200 A shows a procedure of a UE (e.g., UE  32 ) communicating with a MN (e.g., MN  12 A). In some examples, UE  32  may function as UE  30  in  FIG.  1   , and MN  12 A may function as BS  10 A or BS  10 B in  FIG.  1   . 
     Referring to  FIG.  2 A , at the beginning, beside MN  12 A, UE  32  may be in communication with a SN (not shown in  FIG.  2   ), though which UE  32  may connect to a SCG. In operation  211 , UE  32  may detect a SCG failure. In operation  213 , Upon the SCG failure, UE  32  may report a SCG failure information message to MN  12 A. In operation  215 , MN  12 A may determine to keep, change, or release the failed SCG link. In the case that MN  12 A determines to change or release the SCG, MN  12 A may transmit, in operation  217  (denoted by dotted arrow as an option), an RRC reconfiguration message or an RRC release message to UE  32 . In the case that MN  12 A determines to keep the SCG, operation  217  may be eliminated. 
     It should be appreciated by persons skilled in the art that the sequence of the operations in exemplary procedure  200 A may be changed and some of the operations in exemplary procedure  200 A may be eliminated or modified, without departing from the spirit and scope of the disclosure. 
       FIG.  2 B  illustrates an exemplary MCG recovery procedure  200 B according to some embodiments of the present disclosure. Details described in all of the foregoing embodiments of the present disclosure are applicable for the embodiments shown in  FIG.  2 B . 
     The exemplary procedure  200 A shows a procedure of a UE (e.g., UE  33 ) communicating with a MN (e.g., MN  13 A) and a SN (e.g., SN  13 B). In some examples, UE  33  may function as UE  30  in  FIG.  1   , and MN  12 A and SN  13 B may function as BS  10 A or BS  10 B in  FIG.  1   . 
     Referring to  FIG.  2 B , at the beginning, UE  33  may be in communication with MN  13 A, though which UE  33  may connect to a MCG. In operation  221 , UE  33  may detect a MCG failure. UE  33  may then generate a MCG failure information message indicating a failure type, which may indicate a link failure due to, for example, one of the following:
     an expiry of a physical layer problem timer (e.g., T310);   a random access problem; and   reaching a maximum number of retransmission.   

     In some embodiments of the present disclosure, UE  33  may be configured with SRB3 to report MCG failures. In this case, UE  33  may encapsulate the MCG failure information message into an RRC message for communication between a UE and a MN (e.g., an ULInformationTransferMRDC message). In operation  223 , UE  33  may transmit the RRC message to SN  13 B via SRB3. After receiving the RRC message, SN  13 B would know it is for the MN. So SN  13 B will not decode the MCG failure information message in the RRC message, and may transfer the RRC message to MN  13 A in operation  225 . 
     In operation  227 , MN  13 A may decode the MCG failure information message, and may determine to keep, change, or release the failed MCG link. In the case that MN  13 A determines to change or release the MCG, MN  13 A may encapsulate an RRC reconfiguration message or an RRC release message into an RRC message for communication between a UE and a MN (e.g., a DLInformationTransferMRDC message). MN  13 A may transmit, in operation  229  (denoted by dotted arrow as an option), the RRC message to SN  13 B, which may transfer, in operation  231  (denoted by dotted arrow as an option), the message to UE  33  via SRB3. In the case that MN  13 A determines to keep the MCG, operations  229  and  231  may be eliminated. 
     It should be appreciated by persons skilled in the art that the sequence of the operations in exemplary procedure  200 B may be changed and some of the operations in exemplary procedure  200 B may be eliminated or modified, without departing from the spirit and scope of the disclosure. 
     As mentioned above, for NR communication networks, 3GPP is envisioning an IAB architecture for supporting multi-hop relays, wherein a donor node with multi-connectivity is also supported by an IAB node. 
       FIG.  3 A  illustrates an exemplary wireless communication system  300 A according to some embodiments of the present disclosure. Details described in all of the foregoing embodiments of the present disclosure are applicable for the embodiments shown in  FIG.  3 A . 
     Referring to  FIG.  3 A , the wireless communication system  100 A may include a BS (e.g., IAB donor  140 ), some access nodes including IAB nodes (e.g., IAB node  150 A and IAB node  150 B) and UEs (e.g., UE  160 A, UE  160 B, and UE  160 C), and a Next-Generation Core (NGC)  170 . 
     An IAB donor (e.g., IAB donor  140 ) may support gNB-DU functionality and gNB-CU functionality. The gNB-DU functionality and the gNB-CU functionality are defined in 3GPP specification TS 38.401. 
     An IAB node (e.g., IAB node  150 A and IAB node  150 B) may support gNB-DU functionality to terminate the NR access interface to UEs and next-hop IAB nodes and to terminate the F1 protocol to gNB-CU functionality on an IAB donor. In addition to the gNB-DU functionality, an IAB-node may also support a subset of the UE functionality referred to as IAB-MT (Mobile Termination). The above descriptions with respect a UE may also be applied to IAB-MT. An IAB MT may include, for example, physical layer, layer-2, RRC and Non-Access Stratum (NAS) functionality to connect to the gNB-DU of another IAB-node or the IAB-donor and to connect to the gNB-CU on the IAB-donor. The DU of an IAB node DU is also referred to as IAB-DU. 
     As shown in  FIG.  3 A , IAB node  150 A may be connected to an upstream IAB node  150 B via MT  152 A. IAB node  150 A may be connected to UE  160 A via the Distributed Unit (DU)  151 A. IAB node  150 B may be connected to an upstream IAB node or IAB donor  140  via MT  152 B. IAB node  150 B may be connected to UE  160 B via DU  151 B. IAB node  150 B may be connected to downstream IAB node  150 A via DU  151 B. 
     A Central Unit (CU)  141  included in the IAB donor  140  controls the DUs of all IAB nodes (e.g., IAB node  150 A and IAB node  150 B) and the DU(s) (e.g., DU  142 ) resided in the IAB donor  140 . The DU(s) and the CU of an IAB donor may be co-located or may be located in different positions. The DU(s) and the CU of the IAB donor are connected via an F1 interface. In other words, the F1 interface provides a means for interconnecting the CU and the DU(s) of an IAB donor. The F1 Application Protocol (F1AP) supports the functions of the F1 interface by certain F1AP signaling procedures. 
     The wireless communication system  300 A is in a standalone (SA) mode, in which each IAB node has only one parent node. In some other embodiments of the present disclosure, a wireless communication system may be in a non-standalone (NSA) mode, in which one or more IAB nodes may have more than one parent node. 
       FIG.  3 B  illustrates a wireless communication system  300 B according to some embodiments of the present disclosure. Details described in all of the foregoing embodiments of the present disclosure are applicable for the embodiments shown in  FIG.  3 B . 
     Referring to  FIG.  3 B , the wireless communication system  300 B can include an IAB donor node (e.g., donor node  110 ), some IAB nodes (e.g., IAB node  120 A, IAB node  120 B, IAB node  120 C, and IAB node  120 D), and some UEs (e.g., UE  130 A and UE  130 B). Although merely, for simplicity, one donor node is illustrated in  FIG.  3 B , it is contemplated that the wireless communication system  300 B may include more donor node(s) in some other embodiments of the present disclosure. Similarly, although merely four IAB nodes and two UEs are illustrated in  FIG.  3 B  for simplicity, it is contemplated that the wireless communication system  300 B may include more or fewer IAB nodes and UEs in some other embodiments of the present disclosure. 
     IAB node  120 A is directly connected to donor node  110 . IAB node  120 B is directly connected to donor node  110 . IAB node  120 A can be connected to donor node(s) other than donor node  110  in accordance with some other embodiments of the present disclosure. IAB node  120 B can be connected to donor node(s) other than donor node  110  in accordance with some other embodiments of the present disclosure. 
     IAB node  120 C can reach donor node  110  via Dual-Connectivity (DC). For example, JAB node  120 C can reach donor node  110  via IAB node  120 A or IAB node  120 B. IAB node  120 A and IAB node  120 B are parent nodes of IAB node  120 C. In other words, IAB node  120 C is a child node of JAB node  120 A and IAB node  120 B. 
     In some embodiments of the present disclosure, IAB donor  110  may configure IAB  120 A and IAB  120 B as Master node (MN) and Secondary Node (SN), respectively, at IAB node  120 C. In other words, IAB donor  110  may configure IAB  120 A and IAB  120 B as the DU of an MN (hereinafter also referred to as “MN DU”) the DU of an SN (hereinafter also referred to as “SN DU”), respectively. The CU of the MN (hereinafter also referred to as “MN CU”) and the CU of the SN (hereinafter also referred to as “SN CU”) are configured at IAB donor  110 . That is, the MN and SN may share the same CU (e.g., the CU of IAB donor  110 ). 
     IAB node  120 D may be directly connected to IAB node  120 C, and is a child node of IAB node  120 C. IAB node  120 A, IAB node  120 B, and IAB node  120 C are upstream nodes of IAB node  120 D. IAB node  120 A and IAB node  120 B are upstream nodes of IAB node  120 C. IAB node  120 C and IAB node  120  are downstream nodes of IAB node  120 A and IAB node  120 B. Upstream node(s) of an IAB node includes parent node(s) of the IAB node, and downstream node(s) of an IAB node includes child node(s) of the IAB node. 
     UE  130 A is directly connected to IAB node  120 C, and UE  130 B is directly connected to IAB node  120 D. In other words, UE  130 A and UE  130 B are served by IAB node  120 C and IAB node  120 D, respectively. Each of IAB node  120 A, IAB node  120 B, IAB node  120 C, and IAB node  120 D may be directly connected to one or more UEs in accordance with some other embodiments of the present disclosure. Each of IAB node  120 A, IAB node  120 B, IAB node  120 C, and IAB node  120 D may be directly connected to one or more IAB nodes in accordance with some other embodiments of the present disclosure. 
     As described above with respect to  FIGS.  1 ,  2 A and  2 B , a UE may initiate a procedure to report the SCG failure or a MCG recovery procedure under certain circumstances. An IAB node (e.g., IAB node  120 A) with multi-connectively (e.g., dual-connectivity) may also need to report the SCG failure or MCG failure to the MN under the above-mentioned circumstances. 
     Moreover, in the wireless communication system  300 B, which provides multi-hop relay, a wireless Backhaul (BH) link may fail due to, for example but is not limited to, blockage by moving object(s) (e.g., vehicle(s)), foliage (caused by seasonal changes), new building(s) (e.g., infrastructure changes). Such backhaul link failure may occur either on a physically stationary IAB node or a mobile IAB node. For example, a backhaul link failure may occur on the backhaul link between IAB node  120 A and IAB donor  110 , between IAB node  120 B and IAB donor  110 , between IAB node  120 A and IAB node  120 C, or between IAB node  120 B and IAB node  120 C. 
     When a backhaul RLF recovery failure is detected at IAB node  120 B (e.g., the MT of IAB node  120 B), for example, when a RLF occurs on the backhaul link between IAB node  120 B and IAB donor  110 , IAB node  120 B (which may act as a SN) may notify its child nodes (e.g., IAB node  120 C) of such failure. IAB node  120 C may need to report the SCG failure to inform the MN. 
     Similarly, when a backhaul RLF recovery failure is detected at IAB node  120 A (e.g., the MT of IAB node  120 A), for example, when a RLF occurs on the backhaul link between IAB node  120 A and IAB donor  110 , IAB node  120 A (which may act as a MN) may notify its child nodes (e.g., IAB node  120 C) of such failure. IAB node  120 C may need to report the MCG failure to inform the MN. 
     There is a need for handling the transmission and reception of failure information of the backhaul link in a wireless communication system deploying IAB nodes (also referred to as “an IAB system”). 
     Moreover, as mentioned above, a UE may be configured with split SRB1 or SRB3 to report MCG failures when a RLF on a MCG link happens. In an IAB system, split SRB may not be supported. In this scenario, the failure information message may be reported via SRB3. However, in an IAB system, it may be unnecessary to include the failure information message in, for example, the uplink information transfer message, since the MN and SN may share the same CU. There is a need to improve the transmission and reception of failure information via SRB3 in a wireless communication system deploying IAB nodes. 
     More details on the embodiments of the present disclosure will be illustrated in the following text in combination with the appended drawings. 
       FIG.  4    illustrates an exemplary MCG recovery procedure  400  according to some embodiments of the present disclosure. Details described in all of the foregoing embodiments of the present disclosure are applicable for the embodiments shown in  FIG.  4   . 
     In  FIG.  4   , access node  420 C is accessing a BS  410  via parent node  420 A and parent node  420 B, respectively. Parent node  420 A and parent node  420 A are configured as an MN DU and an SN DU, respectively. The MN CU and the SN CU are configured at BS  410 . Access node  420 C may connect to BS  410  through MN DU (e.g., parent node  420 A) and SN DU (e.g., parent node  420 B), respectively. 
     Access node  420 C may function as IAB node  120 C shown in  FIG.  3 B , parent node  420 A and parent node  420 B may function as IAB node  120 A and IAB node  120 B shown in  FIG.  3 B , and BS  410  may function as IAB donor  110  shown in  FIG.  3 B . In this case, the DUs of parent node  420 A and parent node  420 B may be configured as MN DU and SN DU, respectively. The CUs of MN and SN are configured at the CU of BS  410 . 
     In operation  435 , access node  420 C may determine a MCG failure. For example, access node  420 C may consider a radio link failure to be detected upon an expiry of a physical layer problem timer (e.g., T310), a random access problem, or reaching a maximum number of retransmission in response to the radio link failure (RLF) associated with the MCG, and the RLF in the MCG link may thus be declared. 
     After determining a MCG failure, access node  420 C may initiate a fast MCG link recovery procedure. For example, access node  420 C may operate as an IAB-MT, and may generate a failure information message (e.g., a MCG failure information message) to notify BS  410  of the MCG failure. The failure information message may include a failure type of a backhaul link (e.g., between access node  420 C and parent node  420 A). The failure type may be based on a failure type set including at least the following: an expiry of a physical layer problem timer, a random access problem, and reaching a maximum number of retransmission in response to the RLF. 
     For example, when access node  420 C determines an expiry of a physical layer problem timer (e.g., T310) for the MCG, the failure type may indicate a T310 expiry. Similarly, when access node  420 C determines a random access problem for the MCG, or reaching a maximum number of retransmission in response to the RLF associated with the MCG, the failure type may indicate a random access problem or reaching a maximum retransmission number. 
     In some embodiments of the present disclosure, access node  420 C may, in operation  433  (denoted by dotted arrow as an option), receive a signaling message from parent node  420 A. The signaling message may indicate a failure in a backhaul link (e.g., between parent node  420 A and BS  410 ). Access node  420 C may consider a radio link failure to be detected upon receiving the signaling message, and RLF in the MCG link may thus be declared. 
     In some embodiments of the present disclosure, the signaling message from parent node  420 A may indicate a cause value of the failure in the backhaul link (e.g., between parent node  420 A and BS  410 ). The cause value may indicate, for example, one of an expiry of a physical layer problem timer (e.g., T310), a random access problem, and reaching a maximum number of retransmission. 
     Access node  420 C may generate the failure information message based on the signaling message. For example, the failure type in the failure information message may be based on the cause value in the signaling message. For example, when the cause value indicates a T310 expiry for a backhaul link (e.g., between parent node  420 B and BS  410 ), the failure type may indicate a T310 expiry. Similarly, when the cause value indicates a random access problem for the backhaul link, or reaching a maximum number of retransmission in response to a RLF on the backhaul link, the failure type may indicate a random access problem or reaching a maximum retransmission number. 
     In some embodiments of the present disclosure, the failure type set may further include a backhaul RLF, besides an expiry of a physical layer problem timer, a random access problem, and reaching a maximum number of retransmission in response to a RLF, as mentioned above. After receiving the signaling message, access node  420 C may set the failure type in the failure information message to indicate a backhaul RLF. 
     In some embodiments of the present disclosure, the failure type set may be based on an enumerated list indicating at least the above-mentioned four failure types, i.e., an expiry of a physical layer problem timer, a random access problem, reaching a maximum number of retransmission in response to a RLF, and a backhaul RLF. The enumerated list may include one or more spare enumerated types reserved for future use. For example, the failure type set may be enumerated as {t310-Expiry, randomAccessProblem, rlc-MaxNumRetx, bh-rlf-r16, spare4, spare3, spare2, spare1}, wherein “t310-Expiry,” “randomAccessProblem,” “rlc-MaxNumRetx,” and “bh-rlf-r16” correspond to the above-mentioned four different failure types, and “spare4,” “spare3,” “spare2” and “spare1” are reserved for future use. 
     In some embodiments of the present disclosure, the failure type set may be based on two enumerated lists, one indicating at least the three failure types, i.e., an expiry of a physical layer problem timer, a random access problem, and reaching a maximum number of retransmission in response to a RLF, and the other indicating at least the fourth failure type, i.e., a backhaul RLF. Each or either one of the two enumerated lists may include one or more spare enumerated types reserved for future use. For example, one enumerated list may be {t310-Expiry, randomAccessProblem, rlc-MaxNumRetx, spare} and the other enumerated list may be {bh-rlf, spare3, spare2, spare1}, wherein “bh-rlf” corresponds to the above-mentioned fourth failure type, and “spare” is reserved for future use. 
     It should be understood that the above enumerated lists are only for illustrative purposes, and should not be construed as limiting the embodiments of the present disclosure. 
     In the above embodiments, an access node (e.g., access node  420 C operating as an IAB-MT or a UE) sets the failure type to indicate a backhaul RLF when the access node initiates the transmission of the failure information message (e.g., MCG failure information message) due to the reception of the signaling message (e.g., a backhaul RLF indication). 
     In some embodiments of the present disclosure, an access node may set the failure type to indicate a backhaul RLF when the access node initiates the transmission of the failure information message to provide backhaul RLF information for an MCG. 
     In some embodiments of the present disclosure, an access node may set the failure type to indicate a backhaul RLF when the access node initiates the transmission of the failure information message to provide the backhaul RLF indication from a Backhaul Adaptation Protocol (BAP) layer of an MCG. 
     The failure type of “a backhaul RLF” may also be referred to as “receiving a backhaul RLF indication,” “backhaul link failure,” “backhaul link problem” or the like. 
     In some embodiments of the present disclosure, the failure information message may further include information indicating two nodes which terminate the backhaul link. For example, in the case that access node  420 C receives the signaling message from parent node  420 A indicating a failure in a backhaul link (e.g., between parent node  420 A and BS  410 ), the IDs of parent node  420 A (e.g., the DU ID of parent node  420 A) and BS  410  may be indicated in the failure information message. In the case that access node  420 C determines the failure in the backhaul link between parent node  420 A and access node  420 C, the IDs of parent node  420 A (e.g., the DU ID of parent node  420 A) and access node  420 C may be indicated in the failure information message. 
     In some embodiments of the present disclosure, access node  420 C may receive a configuration indicating for fast MCG link recovery from, for example, BS  410  (not shown in  FIG.  4   ). The configuration may indicate a value of a MCG link recovery timer (e.g., T316). Access node  420 C may start the MCG link recovery timer and may initiate the fast MCG link recovery procedure in response to a RLF (e.g., after a RLF is declared). If the MCG link is not recovered before the expiry of the MCG link recovery timer, access node  420 C may initiate a reestablishment procedure. 
     In some embodiments of the present disclosure, the value of the MCG link recovery timer may be set as infinite. In this case, access node  420 C is allowed to initiate the fast MCG link recovery procedure in response to a RLF in the MCG link, but the MCG link recovery timer will never expire. So the reestablishment procedure upon the expiry of the MCG link recovery timer will not be performed. In some examples, the possible value of the MCG link recovery timer may be enumerated as {50 ms, 100 ms, . . . , infinity}, wherein enumerated types “50 ms” and “100 ms” represent that the value of the timer is set as 50 ms and 100 ms, respectively, and enumerated type “infinity” represents that the value of the timer is set as infinite. 
     In some embodiments of the present disclosure, access node  420 C may not receive the above configuration information. In these embodiments, fast MCG link recovery procedure may not be allowed. In other word, access node  420 C may not be allowed to initiate the fast MCG link recovery procedure in response to a RLF in the MCG link. 
     In some embodiments of the present disclosure, the failure information message may be encapsulated in another RRC message (e.g., a UL information transfer message) for transmitting to parent node  420 B via SRB3. For example, after generating the failure information message as described above, access node  420 C may encapsulate the failure information message in a UL information transfer message. In operation  437 , access node  420 C (which may operate as an IAB-MT) may transmit the failure information message to parent node  420 B by transmitting the UL information transfer message on SRB3. In operation  439 , parent node  420 B may forward the UL information transfer message to BS  410 . In operation  441 , BS  410  may decode the failure information message encapsulated in the UL information transfer message, and may not transfer the UL information transfer message. 
     BS  410  may determine whether to keep, change, or release the failed MCG link. In the case that BS  410  determines to change or release the failed MCG link, BS  410  may encapsulate an RRC reconfiguration message or an RRC release message into another RRC message (e.g., a DL information transfer message). In operation  443  (denoted by dotted arrow as an option), BS  410  may transmit the DL information transfer message to parent node  420 B, which may forward the same to access node  420 C in operation  445  (denoted by dotted arrow as an option). Access node  420 C may decode the RRC reconfiguration message or the RRC release message encapsulated in the DL information transfer message, and may perform a corresponding procedure. In the case that BS  410  determines to keep the failed MCG link, operations  443  and  445  may be eliminated. 
     In some embodiments of the present disclosure, the failure information message may not be encapsulated in the UL information transfer message. For example, in operation  437 , access node  420 C (which may operate as an IAB-MT) may transmit the failure information message to parent node  420 B on SRB3. The failure information message is not encapsulated in the UL information transfer message. In operation  439 , parent node  420 B may forward the UL information transfer message to BS  410 . 
     In operation  441 , BS  410  may determine whether to keep, change, or release the failed MCG link. In the case that BS  410  determines to change or release the failed MCG link, in operation  443  (denoted by dotted arrow as an option), BS  410  may transmit an RRC reconfiguration message or an RRC release message to parent node  420 B. The RRC reconfiguration message or the RRC release message is not encapsulated in the DL information transfer message. In operation  445  (denoted by dotted arrow as an option), parent node  420 B may forward the RRC reconfiguration message or the RRC release message to access node  420 C. Access node  420 C may perform a corresponding procedure upon receiving the RRC reconfiguration message or the RRC release message. In the case that BS  410  determines to keep the failed MCG link, operations  443  and  445  may be eliminated. 
     In some embodiments of the present disclosure, a BS (e.g., BS  410  in  FIG.  4   ) may indicate an access node (e.g., IAB node  420 C in  FIG.  4   ) whether to encapsulate the failure information message in the UL information transfer message. 
     For example, BS  410  in  FIG.  4    (e.g., CU of BS  410 ) may transmit a signaling message whether to encapsulate the failure information message in the UL information transfer message. When the signaling message indicates to encapsulate the failure information message in the UL information transfer message, access node  420 C may encapsulate the failure information message in a UL information transfer message, and may transmit the failure information message to parent node  420 B by transmitting the UL information transfer message on SRB3. 
     After receiving the UL information transfer message, BS  410  may, in operation  441 , decode the failure information message encapsulated in the UL information transfer message, and may not transfer the UL information transfer message. In the case that BS  410  determines to change or release the failed MCG link, BS  410  may encapsulate an RRC reconfiguration message or an RRC release message into a DL information transfer message, which may be transmitted to access node  420 C via parent node  420 B. Access node  420 C may decode the RRC reconfiguration message or the RRC release message encapsulated in the DL information transfer message, and may perform a corresponding procedure. 
     When the signaling message indicates not to encapsulate the failure information message in the UL information transfer message, access node  420 C may transmit the failure information message on SRB3 without encapsulating the failure information message in the UL information transfer message. 
     After receiving the failure information message, BS  410  may determine whether to keep, change, or release the failed MCG link. In the case that BS  410  determines to change or release the failed MCG link, in operations  443  and  445  (denoted by dotted arrow as an option), BS  410  may transmit an RRC reconfiguration message or an RRC release message to access node  420 C via parent node  420 B without encapsulating the RRC reconfiguration message or the RRC release message in a DL information transfer message. Access node  420 C may perform a corresponding procedure upon receiving the RRC reconfiguration message or the RRC release message. 
     It should be appreciated by persons skilled in the art that the sequence of the operations in exemplary procedure  400  may be changed and some of the operations in exemplary procedure  400  may be eliminated or modified, without departing from the spirit and scope of the disclosure. 
       FIG.  5    illustrates an exemplary procedure  400  of reporting a SCG failure according to some embodiments of the present disclosure. Details described in all of the foregoing embodiments of the present disclosure are applicable for the embodiments shown in  FIG.  5   . 
     In  FIG.  5   , access node  520 C is accessing a BS  510  via parent node  520 A and parent node  520 B. Parent node  520 A and parent node  520 B are configured as MN DU and SN DU, respectively. The MN CU and the SN CU are configured at BS  510 . Access node  520 C may connect to BS  510  through MN DU (e.g., parent node  520 A) and SN DU (e.g., parent node  520 B), respectively. 
     Access node  520 C may function as IAB node  120 C shown in  FIG.  3 B , parent node  520 A and parent node  520 B may function as IAB node  120 A and IAB node  120 B shown in  FIG.  3 B , and BS  510  may function as IAB donor  110  shown in  FIG.  3 B . In this case, the DUs of parent node  520 A and parent node  520 B may be configured as MN DU and SN DU, respectively. The CUs of MN and SN are configured at the CU of BS  510 . 
     In operation  535 , access node  520 C may determine a SCG failure. For example, access node  520 C may consider a radio link failure to be detected upon an expiry of a physical layer problem timer (e.g., T310), a random access problem, or reaching a maximum number of retransmission in response to the radio link failure (RLF) associated with the SCG, and the RLF in the SCG link may thus be declared. 
     After determining a SCG failure, access node  520 C may initiate a procedure to report the SCG failure. For example, access node  520 C may operate as an IAB-MT, and may generate a failure information message (e.g., a SCG failure information message) to notify BS  510  of the SCG failure. The failure information message may include a failure type of a backhaul link (e.g., between access node  520 C and parent node  520 B). The failure type may be based on a failure type set including at least the following: an expiry of a physical layer problem timer, a random access problem, and reaching a maximum number of retransmission in response to the RLF. 
     For example, when access node  520 C determines an expiry of a physical layer problem timer (e.g., T310) for the SCG, the failure type may indicate a T310 expiry. Similarly, when access node  520 C determines a random access problem for the SCG, or reaching a maximum number of retransmission in response to a RLF associated with the SCG, the failure type may indicate a random access problem or reaching a maximum retransmission number. 
     In operation  537 , access node  520 C (which may operate as an IAB-MT) may transmit the failure information message to parent node  520 A, which may forward the failure information message to BS  510  in operation  539 . 
     In some embodiments of the present disclosure, access node  520 C may, in operation  533  (denoted by dotted arrow as an option), receive a signaling message from parent node  520 B. The signaling message may indicate a failure in a backhaul link (e.g., between parent node  520 B and BS  510 ). Access node  520 C may consider a radio link failure to be detected upon receiving the signaling message, and may report the SCG failure. 
     In some embodiments of the present disclosure, the signaling message from parent node  520 B may indicate a cause value of the failure in the backhaul link (e.g., between parent node  520 B and BS  510 ). The cause value may indicate, for example, one of an expiry of a physical layer problem timer (e.g., T310), a random access problem, and reaching a maximum number of retransmission. 
     Access node  520 C may generate the failure information message based on the signaling message. For example, the failure type in the failure information message may be based on the cause value in the signaling message. For example, when the cause value indicates a T310 expiry for a backhaul link (e.g., between parent node  520 B and BS  510 ), the failure type may indicate a T310 expiry. Similarly, when the cause value indicates a random access problem for the backhaul link, or reaching a maximum number of retransmission in response to a RLF on the backhaul link, the failure type may indicate a random access problem or reaching a maximum retransmission number. 
     In some embodiments of the present disclosure, the failure type set may further include a backhaul RLF, besides an expiry of a physical layer problem timer, a random access problem, and reaching a maximum number of retransmission in response to a RLF, as mentioned above. After receiving the signaling message, access node  520 C may set the failure type in the failure information message to indicate a backhaul RLF. 
     In some embodiments of the present disclosure, the failure type set may be based on an enumerated list indicating at least the above-mentioned four failure types, i.e., an expiry of a physical layer problem timer, a random access problem, reaching a maximum number of retransmission in response to a RLF, and a backhaul RLF. The enumerated list may include one or more spare enumerated types reserved for future use. For example, the failure type set may be enumerated as {t310-Expiry, randomAccessProblem, rlc-MaxNumRetx, bh-rlf-r16, spare2, spare1}, wherein “t310-Expiry,” “randomAccessProblem,” “rlc-MaxNumRetx,” and “bh-rlf-r16” correspond to the above-mentioned four different failure types, and “spare2” and “spare1” are reserved for future use. 
     In some embodiments of the present disclosure, the failure type set may be based on two enumerated lists, one indicating at least the three failure types, i.e., an expiry of a physical layer problem timer, a random access problem, and reaching a maximum number of retransmission in response to a RLF, and the other indicating at least the fourth failure type, i.e., a backhaul RLF. Each or either one of the two enumerated lists may include one or more spare enumerated types reserved for future use. For example, one enumerated list may be {t310-Expiry, randomAccessProblem, rlc-MaxNumRetx, . . . } and the other enumerated list may be {bh-rlf, spare3, spare2, spare1}, wherein “bh-rlf” corresponds to the above-mentioned fourth failure type, and “spare3” is reserved for future use. 
     It should be understood that the above enumerated lists are only for illustrative purposes, and should not be construed as limiting the embodiments of the present disclosure. 
     In the above embodiments, an access node (e.g., access node  520 C operating as an IAB-MT or a UE) sets the failure type to indicate a backhaul RLF when the access node initiates the transmission of the failure information message due to the reception of the signaling message (e.g., a backhaul RLF indication). 
     In some embodiments of the present disclosure, an access node may set the failure type to indicate a backhaul RLF when the access node initiates the transmission of the failure information message to provide backhaul RLF information for an SCG. 
     In some embodiments of the present disclosure, an access node may set the failure type to indicate a backhaul RLF when the access node initiates the transmission of the failure information message to provide the backhaul RLF indication from a Backhaul Adaptation Protocol (BAP) layer of an SCG. 
     The failure type of “a backhaul RLF” may also be referred to as “receiving a backhaul RLF indication,” “backhaul link failure,” “backhaul link problem” or the like. 
     In some embodiments of the present disclosure, the failure information message may further include information indicating two nodes which terminate the backhaul link. For example, in the case that access node  520 C receives the signaling message from parent node  520 B indicating a failure in a backhaul link (e.g., between parent node  520 B and BS  510 ), the IDs of parent node  520 B (e.g., the DU ID of parent node  520 B) and BS  510  may be indicated in the failure information message. In the case that access node  520 C determines the failure in the backhaul link between parent node  520 B and access node  520 C, the IDs of parent node  520 B (e.g., the DU ID of parent node  520 B) and access node  520 C may be indicated in the failure information message. 
     In some embodiments of the present disclosure, the failure information message may further include information indicating child nodes (e.g., child IAB nodes) of access node  520 C. The information indicating the child nodes may include IDs of the child nodes. For example, in the case that EUTRA-NR dual connectivity is supported, the information can inform an eNB about the child IAB nodes. EUTRA is an abbreviation of Evolved Universal Terrestrial Radio Access. 
     Referring to  FIG.  5   , in operation  541 , BS  510  may determine whether to keep, change, or release the failed SCG link. In the case that BS  510  determines to change or release the failed SCG link, BS  510  may transmit, in operation  543  (denoted by dotted arrow as an option), an RRC reconfiguration message or an RRC release message to parent node  520 A, which may forward the RRC reconfiguration message or the RRC release message to access node  520 C in operation  545  (denoted by dotted arrow as an option). In the case that BS  510  determines to keep the failed SCG link, operations  543  and  545  may be eliminated. 
     It should be appreciated by persons skilled in the art that the sequence of the operations in exemplary procedure  500  may be changed and some of the operations in exemplary procedure  500  may be eliminated or modified, without departing from the spirit and scope of the disclosure. 
       FIG.  6    illustrates an exemplary procedure  600  of failure information transmission according to some embodiments of the present disclosure. Details described in all the foregoing embodiments of the present disclosure are applicable for the embodiments shown in  FIG.  6   . 
     The exemplary procedure  600  shows a procedure of an access node (e.g., access node  420 C in  FIG.  4    or access node  520 C in  FIG.  5   ) communicating with an MN (e.g., the MN DU is configured at parent node  420 A in  FIG.  4    or parent node  520 A in  FIG.  5   , and the MN CU is configured as BS  410  or BS  510 ) and an SN (e.g., the SN DU is configured at parent node  420 B in  FIG.  4    or parent node  520 B in  FIG.  5   , and the SN CU is configured as BS  410  or BS  510 ). 
     In some embodiments of the present disclosure, the access node may consider a radio link failure to be detected upon certain conditions as described above with respect to  FIGS.  4  and  5   . Then, the access node may, in operation  613 , transmitting a failure information message to the BS. The access node may generate the failure information message and may transmit the failure information message according to a method as described above with respect to  FIGS.  4  and  5   . 
     In some embodiments of the present disclosure, the access node may, in operation  615  (denoted by dotted block as an option), receive an RRC reconfiguration message or an RRC release message from the BS. The RRC reconfiguration message or an RRC release message may be received according to a method as described above with respect to  FIGS.  4  and  5   . 
     It should be appreciated by persons skilled in the art that the sequence of the operations in exemplary procedure  600  may be changed and some of the operations in exemplary procedure  600  may be eliminated or modified, without departing from the spirit and scope of the disclosure. 
       FIG.  7    illustrates an exemplary procedure  700  of failure information transmission according to some embodiments of the present disclosure. Details described in all the foregoing embodiments of the present disclosure are applicable for the embodiments shown in  FIG.  7   . 
     The exemplary procedure  700  shows a procedure of a BS (e.g., BS  410  in  FIG.  4    or BS  510  in  FIG.  5   ) communicating with an access node (e.g., access node  420 C in  FIG.  4    or access node  520 C in  FIG.  5   ) via a plurality of parent nodes (parent nodes  420 A and  420 B in  FIG.  4    or parent nodes  520 A and  520 B in  FIG.  4   ) of the access node. A parent node (e.g., parent node  420 A in  FIG.  4    or parent node  520 A in  FIG.  5   ) of the plurality of parent nodes may be configured an MN DU, and another parent node (e.g., parent node  420 B in  FIG.  4    or parent node  520 B in  FIG.  5   ) of the plurality of parent nodes may be configured an SN DU. 
     In some embodiments of the present disclosure, the BS may transmit a configuration for fast MCG link recovery to the access node (not shown in  FIG.  7   ). The configuration may indicate a value of a MCG link recovery timer, as described above with respect to  FIGS.  4  and  5   . 
     In operation  713 , the BS may receive a failure information message from the access node. The failure information message may be configured and transmitted from the access node to the BS according to a method as described above with respect to  FIGS.  4  and  5   . 
     In some embodiments of the present disclosure, in operation  711  (denoted by dotted block as an option), the BS may transmit to the access node a signaling message indicating whether to encapsulate the failure information message in a uplink (UL) information transfer message. 
     In some embodiments of the present disclosure, in operation  715  (denoted by dotted block as an option), the BS may transmit an RRC reconfiguration message or an RRC release message from the access node. The RRC reconfiguration message or an RRC release message may be transmitted according to a method as described above with respect to  FIGS.  4  and  5   . 
     It should be appreciated by persons skilled in the art that the sequence of the operations in exemplary procedure  700  may be changed and some of the operations in exemplary procedure  700  may be eliminated or modified, without departing from the spirit and scope of the disclosure. 
       FIG.  8    illustrates an example block diagram of an apparatus  800  according to some embodiments of the present disclosure. 
     As shown in  FIG.  8   , the apparatus  800  may include at least one non-transitory computer-readable medium (not illustrated in  FIG.  8   ), a receiving circuitry  802 , a transmitting circuitry  804 , and a processor  806  coupled to the non-transitory computer-readable medium (not illustrated in  FIG.  8   ), the receiving circuitry  802  and the transmitting circuitry  804 . The apparatus  800  may be a BS or an access node (e.g., an IAB node or a UE). 
     Although in this figure, elements such as processor  806 , transmitting circuitry  804 , and receiving circuitry  802  are described in the singular, the plural is contemplated unless limitation to the singular is explicitly stated. In some embodiments of the present disclosure, the receiving circuitry  802  and the transmitting circuitry  804  are combined into a single device, such as a transceiver. In certain embodiments of the present disclosure, the apparatus  800  may further include an input device, a memory, and/or other components. 
     In some embodiments of the present disclosure, the non-transitory computer-readable medium may have stored thereon computer-executable instructions to cause a processor to implement the method with respect to the access node or IAB node as described above. For example, the computer-executable instructions, when executed, cause the processor  806  interacting with receiving circuitry  802  and transmitting circuitry  804 , so as to perform the operations with respect to the access nodes or IAB nodes depicted in  FIGS.  1 - 7   . 
     In some examples, the transmitting circuitry  804  may transmit a failure information message. The failure information message may be generated according to one of the methods described above with respect to  FIGS.  1 - 7   . The receiving circuitry  802  may receive a signaling message indicating a failure in the backhaul link from a master node or a secondary node of the apparatus  800 . The signaling message may be configured according to one of the methods described above with respect to  FIGS.  1 - 7   . The receiving circuitry  802  may receive a signaling message indicating whether to encapsulate the failure information message in an uplink (UL) information transfer message. The processor  806  may perform correspond procedures based on the signaling message according to the descriptions with respect to  FIGS.  1 - 7   . The receiving circuitry  802  may receive a configuration for fast MCG link recovery. The receiving circuitry  802  may receive an RRC reconfiguration message or an RRC release message. 
     In some embodiments of the present disclosure, the non-transitory computer-readable medium may have stored thereon computer-executable instructions to cause a processor to implement the method with respect to the BS or IAB donor as described above. For example, the computer-executable instructions, when executed, cause the processor  806  interacting with receiving circuitry  802  and transmitting circuitry  804 , so as to perform the operations with respect to the IAB donors or base stations depicted in  FIGS.  1 - 7   . 
     In some examples, the receiving circuitry  802  may receive a failure information message. The failure information message may be configured according to one of the methods described above with respect to  FIGS.  1 - 7   . The transmitting circuitry  804  may transmit a signaling message indicating whether to encapsulate the failure information message in an uplink (UL) information transfer message. The transmitting circuitry  802  may transmit a configuration for fast MCG link recovery. The transmitting circuitry  802  may transmit an RRC reconfiguration message or an RRC release message. 
     Those having ordinary skill in the art would understand that the steps of a method described in connection with the aspects disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, a hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. Additionally, in some aspects, the steps of a method may reside as one or any combination or set of codes and/or instructions on a non-transitory computer-readable medium, which may be incorporated into a computer program product. 
     While this disclosure has been described with specific embodiments thereof, it is evident that many alternatives, modifications, and variations may be apparent to those skilled in the art. For example, various components of the embodiments may be interchanged, added, or substituted in the other embodiments. Also, all of the elements of each figure are not necessary for the operation of the disclosed embodiments. For example, one of ordinary skill in the art of the disclosed embodiments would be enabled to make and use the teachings of the disclosure by simply employing the elements of the independent claims. Accordingly, embodiments of the disclosure as set forth herein are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the disclosure. 
     In this document, the terms “includes”, “including”, or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that includes a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by “a”, “an”, or the like does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that includes the element. Also, the term “another” is defined as at least a second or more. The term “having” and the like, as used herein, are defined as “including.”