FIG. 4.

Similar to the reestablishment procedure 300 in FIG. 3, the procedure 400 may occur, for example, in response to an RLF on a radio link between UE 401 and BS 402A (i.e., source BS). The UE 401 may select a target BS (e.g., BS 402B) to reestablish the radio link. The UE 401 may function as the UE 101a or UE 101b shown in FIG. 1, or UE 210 shown in FIG. 2. The BS 402A and BS 402B may function as the BS 102 shown in FIG. 1, or BS 220 or satellite 240 shown in FIG. 2.

Operations 411 and 413 in reestablishment procedure 400 correspond to operations 311 and 313 in reestablishment procedure 400 in FIG. 3. For example, in operation 411, the UE 401 may transmit an RRC reestablishment request message to the BS 402B. The UE 401 may start a timer (e.g., T301) associated with the RRC reestablishment request message in response to the transmission of the RRC reestablishment request message. In operation 413, the BS 402B may transmit a request to the BS 402A to retrieve the UE context of the UE 401 if the UE context is not available at the BS 402B.

The BS 402A may fail to find the UE context of the UE 401 based on the C-RNTI, PCI, and E-UTRAN cell identifier. In this case, the BS 402A may transmit, in operation 415, a retrieve UE context failure message to the BS 402B. The BS 402B may fail to the RRC connection resume procedure or reestablishment procedure. In operation 417, the BS 402B may transmit an RRC reestablishment failure message to the UE 401. The UE 401 may stop the timer (e.g., T301) associated with the RRC reestablishment request message in response to the RRC reestablishment failure 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.

It should also be appreciated by persons skilled in the art that the above reestablishment procedure as described above or as will be described below can be applied to various communication systems including, but not limited to, NR systems, NB-IoT systems, and NTN systems (e.g., wireless communication system 200).

In some embodiments of the present disclosure, a UE may enter an idle mode in response to a reestablishment failure. After the UE reconnects to the network (e.g., a BS), the UE may transmit a failure report (e.g., RLF report) to the BS. The RLF report may include information related to the reestablishment failure, and any other necessary information.

FIG. 5 illustrates a flow chart of an exemplary UE information procedure 500 according to some embodiments of the present disclosure.

In FIG. 5, UE 501 may function as the UE 101a or UE 101b shown in FIG. 1, or UE 210 shown in FIG. 2. The BS 502 may function as the BS 102 shown in FIG. 1, or BS 220 or satellite 230 shown in FIG. 2. Details described in all of the foregoing embodiments of the present disclosure are applicable for the embodiments shown in FIG. 5.

The procedure in FIG. 5 may be used by the network (e.g., a BS) to request a UE to report information. For example, in some embodiments of the present disclosure, a UE (e.g., UE 501 in FIG. 5) may report to its serving cell (e.g., BS 502 in FIG. 5) that RLF information is available. The BS 502 may then initiate a UE information procedure. In some embodiments of the present disclosure, the network (e.g., the BS 502) may initiate a UE information procedure only after successful security activation.

Referring to FIG. 5, the BS 502 may initiate the UE information procedure by transmitting a UE information request message to the UE 501 in operation 511. In response to the UE information request message, the UE 501 may respond a UE information response message to the BS 502. The UE information response message may include an RLF report.

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.

In some embodiments of the present disclosure, a radio link failure (RLF) indication procedure is introduced to enable mobility robustness and RLF recovery improvement in radio access networks (RANs) such as E-UTRAN or next-generation RAN (NG-RAN). For example, the procedure may include passing information about a failure event over an Xn (e.g., X2) interface.

More specifically, when, for example, a reestablishment request is received or a connection failure is reported after an RRC connection setup or an incoming successful handover, a currently serving BS (e.g., BS #2) of a UE may use the cell identifiers provided by the UE to identify the potential previous serving cell/BS (e.g., BS #1). BS #2 which has received the information about the failure (e.g., an RLF report) may transmit a failure indication (e.g., RLF indication) message to BS #1. The RLF indication message may include the RLF report. After receiving the RLF indication message, BS #1 may match the correct context, or use the information available in the RLF report (if included in the RLF indication message) to analyze the possible root cause of the failure. In some embodiments of the present disclosure, when BS #1 matches the correct context, it may trigger a handover preparation procedure towards BS #2 which initiates the RLF indication cause of the failure procedure. BS #1 may take into account a number of factors (e.g., the circuit switched fallback (CSFB) indicator in the UE context) to determine whether to trigger the handover preparation procedure.

FIG. 6 illustrates a flow chart of an exemplary RLF indication procedure 600 according to some embodiments of the present disclosure.

In FIG. 6, BS 602A and BS 602B may function as the BS 102 shown in FIG. 1, or BS 220 or satellite 230 shown in FIG. 2. Details described in all of the foregoing embodiments of the present disclosure are applicable for the embodiments shown in FIG. 6.

Referring to FIG. 6, in response to a reestablishment attempt or the reception of the UE information (e.g., an RLF report) from a UE, the BS 602B may initiate the procedure 600 when, for example, the BS 602B considers that the UE may have previously suffered from a connection failure at a cell controlled by the BS 602A. For example, in operation 611, the BS 602B may initiate the procedure 600 by transmitting a failure indication message to the BS 602A.

In some embodiments of the present disclosure, the failure indication message may include a container of an RLF report. The container of the RLF report may include an RLF report received from the UE at the BS 602B. In some examples, the failure indication message may include a NB-IoT RLF Report Container information element (IE) to indicate the container of the RLF report. The RLF report may enable the BS 602A to determine the nature of the failure. When the container of the RLF report is included in the failure indication message sent after a successful reestablishment, the failure indication message may further indicate an identity (e.g., E-UTRAN cell global identifier (ECGI)) of the cell where the reestablishment has succeeded. In some examples, the failure indication message may include a Re-establishment Cell ECGI IE to indicate the identity of the cell.

In some embodiments of the present disclosure, the failure indication message may include the following information:

- Message Type
- Failure Cell ID: PCI of the cell in which the UE was connected prior to the failure occurrence;
- Re-establishment Cell ECGI: ECGI of the cell where an RRC reestablishment attempt is made or the reestablishment has succeeded;
- C-RNTI: C-RNTI contained in the RRC reestablishment request message (specified in 3GPP specification TS 36.331);
- shortMAC-I (optional): ShortMAC-I contained in the RRC reestablishment request message (specified in 3GPP specification TS 36.331);
- UE RLF Report Container (optional): RLF-Report-r9 IE contained in the UE information response message (specified in 3GPP specification TS 36.331);
- RRC Conn Setup Indicator (optional): included when the RLF report within the UE RLF Report Container IE is retrieved after an RRC connection setup or an incoming successful handover;
- RRC Conn Reestab Indicator (optional): the reestablishment cause in an RRC connection reestablishment request message (specified in 3GPP specification TS 36.331);
- UE RLF Report Container for extended bands (optionally): RLF-Report-v9e0 IE contained in the UE information response message (specified in 3GPP specification TS 36.331);
- NB-IoT RLF Report Container (optionally): RLF-Report-NB-r16 IE contained in the UE information response NB message (specified in 3GPP specification TS 36.331).

A radio link can be established in the above-mentioned wireless communication systems to provide a service(s) (e.g., voice, data, or other(s)) to a UE. However, the radio link may fail due to various factors. When a link failure is detected, the UE can request radio link reestablishment. However, the reestablishment procedure can be time-consuming and may cause latency in a wireless communication system. To reduce reestablishment latency, embodiments of the present disclosure propose a conditional reestablishment procedure, which may be triggered before a radio link failure (RLF) happens. In addition, a command-based reestablishment procedure is proposed in the present disclosure. More details on the embodiments of the present disclosure will be illustrated in the following text in combination with the appended drawings.

FIG. 7 illustrates a flow chart of an exemplary reestablishment procedure 700 according to some embodiments of the present disclosure.

In FIG. 7, UE 701 may function as the UE 101a or UE 101b shown in FIG. 1, or UE 210 shown in FIG. 2. BS 702A and BS 702B may function as the BS 102 shown in FIG. 1, or BS 220 or satellite 270 shown in FIG. 2. Details described in all of the foregoing embodiments of the present disclosure are applicable for the embodiments shown in FIG. 7.

Referring to FIG. 7, the UE 701 may access the BS 702A (i.e., serving BS or source BS), and may be connected to a cell (i.e., a serving cell) of the BS 702A. The BS 702A may determine to apply a conditional reestablishment for the UE 701. In operation 711, the BS 702A may transmit conditional reestablishment configuration information to the UE 701. The conditional reestablishment configuration information may indicate a set of execution conditions for a set of candidate cells. Each candidate cell of the set of candidate cells may be associated with at least one execution condition. The set of candidate cells may include at least one cell of at least one potential target BS.

For example, the conditional reestablishment configuration information may indicate a candidate cell A and a candidate cell B, and at least one execution condition for each of the candidate cell A and candidate cell B. The execution conditions for the candidate cell A and candidate cell B may be the same or different. The candidate cell A and candidate cell B may be from the same or different BSs.

In some embodiments of the present disclosure, the procedure 700 may include operation 713 (denoted by dotted arrow as an option). In operation 713, the BS 702A may transmit a message including the UE context of the UE 701 to at least one candidate cell (e.g., BS 702B) of the set of candidate cells. The UE context may be transmitted to some or all of the candidate cells in the set of candidate cells. In some examples, the transmission of the UE context may occur in response to the transmission of the conditional reestablishment configuration information. In some other examples, the transmission of the UE context may occur prior to the transmission of the conditional reestablishment configuration information.

In operation 715, in response to the reception of the conditional reestablishment configuration information from the serving cell of the BS 702A, the UE 701 may evaluate the set of execution conditions based on the conditional reestablishment configuration information. For example, for each candidate cell, the UE may evaluate whether the associated condition(s) is fulfilled or not, and may determine whether a corresponding candidate cell satisfies a reestablishment condition based on the evaluation.

In some embodiments of the present disclosure, the set of candidate cells may include a candidate cell (e.g., candidate cell A) which is associated with at least two execution conditions. In some instances, the candidate cell A satisfies a reestablishment condition when all of the at least two execution conditions associated with the candidate cell A is fulfilled. In some other instances, the candidate cell A satisfies a reestablishment condition when one of the at least two execution conditions associated with the candidate cell is fulfilled. In some embodiments of the present disclosure, the set of candidate cells may include a candidate cell (e.g., candidate cell B) associated with one execution condition. The candidate cell B satisfies a reestablishment condition when the associated execution condition is fulfilled.

In response to a candidate cell (e.g., a cell of BS 702B, hereinafter referred to as “candidate cell C”) of the set of candidate cells satisfying a reestablishment condition, the UE 701 may go to operation 717. In operation 717, the UE 701 may initiate a reestablishment procedure and may stop a link establishment timer (e.g., timer T312) if the link establishment timer is running. In some embodiments of the present disclosure, the UE 701 may stop evaluating the set of execution conditions in response to the candidate cell C satisfying a reestablishment condition.

Since in the above scenario, the reestablishment procedure is initiated due to a satisfied reestablishment condition, such reestablishment procedure may be referred to as a “conditional reestablishment procedure” in the subject disclosure. However, in view of the context, persons skilled in the art would comprehend that a “reestablishment procedure” may refer to either a normal reestablishment procedure that is initiated due to, for example, an RLF (e.g., the one shown in FIG. 3), or a conditional reestablishment procedure.

In some examples, a UE may stop evaluating the set of execution conditions in response to one (e.g., candidate cell C) of the set of candidate cells satisfying the corresponding reestablishment condition. In some examples, a UE may stop evaluating the set of execution conditions in response to a number of candidate cells satisfying the corresponding reestablishment conditions.

In some embodiments of the present disclosure, in response to a reestablishment condition being satisfied or the initiation of a conditional reestablishment procedure, the UE 701 may inform its serving cell that it is leaving. This is beneficial because it could allow the serving cell to provide the UE context of the UE 701 to a target cell in advance and thus reduce reestablishment latency.

For example, referring to FIG. 7, in operation 719, the UE 701 may transmit an RRC message to BS 702A. The RRC message may include an indicator indicating that the UE 701 is leaving. The RRC message may further include an identity of the candidate cell C.

In some embodiments of the present disclosure, in the case that a reestablishment condition is satisfied immediately after the reception of the conditional reestablishment configuration information, the UE 701 may skip the transmission of the above RRC message. That is, in such scenario, operation 719 may be omitted.

In some embodiments of the present disclosure, in the case that a reestablishment condition is satisfied immediately after the reception of the conditional reestablishment configuration information, the UE 701 may transmit the above RRC message upon the satisfaction of the reestablishment condition.

After receiving the RRC message indicating the leaving of the UE 701, the BS 702A may transmit the UE context of the UE 701 in operation 721. In operations 723-731, the UE 701 may perform a reestablishment procedure similar to the exemplary reestablishment procedure 300 as described above.

For example, in operation 723, the UE 701 may transmit a reestablishment request message to the BS 702B. The UE 701 may start a timer (e.g., T301) associated with the reestablishment request message in response to the transmission of the reestablishment request message.

After receiving the reestablishment request message, the BS 702B may determine whether to retrieve the UE context of the UE 701. In the case that the BS 702B determines that the UE context of the UE 701 is not locally available, the BS 702B may transmit a retrieve UE context request message to the BS 702A in operation 725, as described above with respect to FIG. 3. In response to the retrieve UE context request message, the BS 702A may transmit a retrieve UE context response message to the BS 702B, in operation 727, as described above with respect to FIG. 3. Operations 725 and 727 may be omitted when the UE context of the UE 701 is available at the BS 702B.

In response to the reestablishment request message, the BS 702B may transmit a reestablishment message to the UE 701 in operation 729. The UE 701 may stop the timer (e.g., T301) associated with the reestablishment request message in response to the reestablishment message. In operation 731, the UE 701 may transmit a reestablishment complete message to the BS 702B. The BS 702B may perform a path switch procedure with a CN entity (not shown in FIG. 7), as described above with respect to FIG. 3. The CN entity may be a MME or an AMF.

In some embodiments of the present disclosure, in response to successfully accessing the candidate cell C, the UE 701 may release the conditional reestablishment configuration information configured by the previous serving BS or cell (e.g., BS 702A) in operation 733.

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. For example, although operation 719 is performed after operation 717 in exemplary procedure 700, it is contempt that operation 719 may be performed prior to or at the same time as operation 717.

Although a successful conditional reestablishment procedure is shown in FIG. 7, in some embodiments of the present disclosure, similar to the exemplary procedure 400 in FIG. 4, a conditional reestablishment procedure may fail. For example, a conditional reestablishment procedure may fail in response to the expiry of the timer (e.g., T301) associated with the reestablishment request message. The UE 701 may enter an idle mode in response to the failure of the conditional reestablishment procedure.

In some embodiments of the present disclosure, the idle UE 701 may try to connect to a new cell from the idle mode. After the UE 701 connects to a new cell, the UE 701 may transmit an RLF report to the new cell to indicate the above reestablishment failure. This is beneficial because it may optimize the mobility robustness of the network.

FIG. 8 illustrates a flow chart of an exemplary wireless communication procedure 800 according to some embodiments of the present disclosure. The procedure 800 may occur after the UE 701 accesses a new cell.

Referring to FIG. 8, in operation 813, in response to connecting to a new cell (e.g., BS 702C), the UE 701 may report to the BS 702C that RLF information is available. The BS 702C may then initiate a UE information procedure. For example, the BS 702C may transmit a UE information request message to the UE 701 in operation 815. In response to the UE information request message, the UE 701 may transmit an RLF report to the BS 702C in operation 817. The RLF report may be included in a UE information response message.

In some embodiments of the present disclosure, an RLF report may include a failure cause indicating the type of a failure at a UE. For example, the failure cause may indicate whether the failure at the UE is due to an RLF, a handover (HO) failure, a conditional HO (CHO) failure, or a conditional reestablishment failure.

The RLF report transmitted by the UE 701 may include a failure cause indicating a conditional reestablishment failure. The RLF report may further indicate the identity of the candidate cell (e.g., candidate cell C as described with respect to FIG. 7) where the UE 701 fails to perform the conditional reestablishment. The RLF report may further indicate the identity of the original serving cell of the original BS (e.g., BS 702A in FIG. 7) which configures the conditional reestablishment for the UE 701. The identity of a cell may be an ECGI or PCI.

In response to the RLF report, the BS 702C may initiate an RLF indication procedure, as described above with respect to FIG. 6. For example, the BS 702C may transmit a failure indication message to the BS 702A in operation 819.

In some embodiments of the present disclosure, a failure indication message may include a container of an RLF report. For example, the failure indication message transmitted from the BS 702C to the BS 702A may include a container of an RLF report, which can include the RLF report received from the UE 701.

In some embodiments of the present disclosure, a failure indication message may further include a connection reestablishment indicator. For example, the connection reestablishment indicator may indicate whether the failure at the UE is due to a reconfiguration failure, a HO Failure, a conditional reestablishment failure, or other failures. The connection reestablishment indicator may be may be based on the failure cause in the RLF report. For example, the failure indication message transmitted from the BS 702C to the BS 702A may include a connection reestablishment indicator indicating a conditional reestablishment failure.

After receiving the failure indication message, the BS 702A may determine the cause of the failure at the UE 701, thereby achieving an MRO in the network.

In some embodiments of the present disclosure, the BS 702C may be a BS different from either the BS 702A or the BS 702B. In some embodiments of the present disclosure, the BS 702C may be the BS 702B. In some embodiments of the present disclosure, the BS 702C may be the BS 702A. In these embodiments, operation 819 may be omitted.

It should be appreciated by persons skilled in the art that the sequence of the operations in exemplary procedure 800 may be changed and some of the operations in exemplary procedure 800 may be eliminated or modified, without departing from the spirit and scope of the disclosure.

In some embodiments of the present disclosure, a UE may detect or declare an RLF before a reestablishment condition is satisfied at the UE. After the UE reestablishes a connection with a new cell, the UE may report an RLF report to the new cell to indicate that the conditional reestablishment configured at the UE is not triggered.

FIG. 9 illustrates a flow chart of an exemplary wireless communication procedure 900 according to some embodiments of the present disclosure.

In FIG. 9, UE 901 may function as the UE 101a or UE 101b shown in FIG. 1, or UE 210 shown in FIG. 2. BS 902A and BS 902C may function as the BS 102 shown in FIG. 1, or BS 220 or satellite 290 shown in FIG. 2. Details described in all of the foregoing embodiments of the present disclosure are applicable for the embodiments shown in FIG. 9.

Referring to FIG. 9, the UE 901 may access the BS 902A (i.e., serving BS or source BS), and may be connected to a cell (i.e., a serving cell) of the BS 902A. The BS 902A may determine to apply a conditional reestablishment for the UE 901. For example, similar to operation 711 as described with respect to FIG. 7, the BS 902A may transmit conditional reestablishment configuration information to the UE 901 in operation 911. The conditional reestablishment configuration information may indicate a set of execution conditions for a set of candidate cells.

In some embodiments of the present disclosure, similar to operation 713 as described with respect to FIG. 7, the procedure 900 may include operation 913 (denoted by dotted arrow as an option). In operation 913, the BS 902A may transmit a message including the UE context of the UE 901 to at least one candidate cell of the set of candidate cells.

Similar to operation 715 as described with respect to FIG. 7, in response to the reception of the conditional reestablishment configuration information, the UE 901 may evaluate the set of execution conditions based on the conditional reestablishment configuration information in operation 915.

In some embodiments of the present disclosure, before any candidate cell satisfies a reestablishment condition, the UE 901 may detect or declare an RLF in operation 917. In some embodiments of the present disclosure, the UE 901 may stop evaluating the set of execution conditions in response to the RLF. In some embodiments of the present disclosure, the UE 901 may release the conditional reestablishment configuration information in response to the RLF.

In some embodiments of the present disclosure, the UE 901 may initiate a reestablishment procedure in response to the RLF. For example, in operation 919, the UE 901 may perform a reestablishment procedure similar to the procedure 300 shown in FIG. 3 and may connect to the BS 902C. In some embodiments of the present disclosure, in response to connecting to the BS 902C, the UE 901 may initiate a procedure to report an RLF report to the BS 902C. In some examples, the UE 901 may initiate a procedure similar to the procedure 800 shown in FIG. 8.

For instance, in operation 921, the UE 901 may transmit an RLF report to the BS 902C in response to connecting to the BS 902C. The RLF report transmitted by the UE 901 may include a failure cause indicating an RLF. The RLF report may further include an indicator indicating that no conditional reestablishment is triggered at the UE 901. In some embodiments of the present disclosure, the RLF report may further include the conditional reestablishment configuration information configured for the UE 901. For example, the RLF report may include the set of execution conditions and the set of candidate cells. In some embodiments of the present disclosure, the RLF report may further include the identity of the original serving cell of the original BS (e.g., BS