A self-optimizing network (SON) refers to a network performing self optimization in a certain manner/technique with no need of participation of an operation, administration and maintenance (OAM) entity.
Mobility robustness optimization (MRO) is SON means, with a main object being to detect and correct a connection failure of radio resource control (RRC) during a user moves resulted from improper setting of a network parameter.
Currently, connection failure referred in the MRO includes the following three types.
First Type: Too-Late Handover
The too-late handover refers to that as the handover is triggered too late, a connection failure has occurred when a handover command has not been successfully transmitted to user equipment (UE). After the failure occurs, the UE will initiate connection reestablishment to a reestablished cell (if the network side has finished handover preparation), or will initiate connection reestablishment to other non-source cells (if the network side has not performed handover preparation).
FIG. 1 is a schematic diagram of too-late handover. As shown in FIG. 1, as a parameter (such as a signal difference between a target cell signal and a source cell signal) is set improperly, that is, the signal difference Δ′ between the target cell signal and the source cell signal is set to be relatively large, which is larger than a signal difference Δ between the target cell signal and the source cell signal of an ideal handover point, thereby resulting in too-late handover.
Second Type: Too-Early Handover
The too-early handover refers to that as the handover is triggered too early, a connection failure occurs resulted from a handover is performed by UE when a signal of the source cell is still strong and a signal of the target cell is still weak. The connection failure may occur shortly after the UE is handed over to the target cell. And after the failure occurs, the UE initiates connection reestablishment to the source cell.
FIG. 2 is a schematic diagram of too-early handover. As shown in FIG. 2, as a parameter (such as a signal difference between the target cell signal and the source cell signal) is set improperly, that is, the signal difference Δ′ between the target cell signal and the source cell signal is set to be relatively small, which is smaller than a signal difference Δ between the target cell signal and the source cell signal of an ideal handover point, thereby resulting in too-early handover.
Third Type: Handover to Wrong Cell
The handover to wrong cell refers to that, due to improper setting of a handover parameter, UE which should be handed over to a cell A is handed over to a cell B; and at this moment, as signal quality of the cell B is not good, a connection failure is resulted. The connection failure may occur shortly after the UE is handed over to the cell B. And after the failure occurs, the UE initiates connection reestablishment to target cell A.
After the connection failure occurs when the UE is handing over, the UE will attempt to perform RRC connection reestablishment. If the connection reestablishment succeeds, the UE will keep the state of connection with the network side, and acquire services from the network side; and if the connection reestablishment fails, the UE will enter into an idle state, the connection with the network side will be interrupted, and the UE will possibly establish connection with the network side by initiating a new RRC connection reestablishment procedure.
The method for judging type of a handover failure in the MRO shall be described below regarding two cases where UE restores connection with the network side through an RRC reestablishment procedure or by initiating a new RRC connection establishment procedure.
First: After the UE Initiates RRC Connection Reestablishment
1. Too-Late Handover
A radio link failure (RLF) occurs when the UE is in a source cell A, and the UE initiates a connection reestablishment attempt to a cell B, and transmits a connection reestablishment request (RRC) message to the cell B, the request message may include identifier information of the UE and an identifier of a cell where the UE is present when the RLF occurs, that is, an identifier of the source cell A. After receiving the request message, the cell B may transmit an RLF indication message to the source cell A where the RLF occurs according to information in the request message, the indication message including information contained in the RRC connection reestablishment request message; and after receiving the RLF indication message, the source cell A may judge type of the handover failure according to the indication message and saved context information of the UE; if the UE does not receive a handover command according to the context information of the UE, the type of the handover failure may be judged as too-late handover.
2. Too-Early Handover
The UE is successfully handed over from the source cell A to the cell B, after a short period of time, an RLF occurs in the UE, and the UE initiates reestablishment connection to the cell A. During the procedure of the reestablishment connection, the UE transmits information, such as a user identifier and an identifier of the cell B where the UE is present when the link failure occurs to the cell A; the cell A transmits an RLF indication message to the cell B according to the obtained information; and if the cell B finds that a duration of the UE staying in the cell B is less than a parameter Tstore_UE_cntxt after receiving the message, it is judged that the UE turns back to the source cell A shortly after being connected to the cell B, and the cell B judges that too-early handover occurs in the UE.
3. Handover to Wrong Cell
The UE is successfully handed over from the source cell A to the cell B, but a connection failure occurs after a short period of time. The UE initiates a reestablishment connection to a cell C, and at this moment, the cell C transmits an RLF indication message to the cell B, information contained in the message being similar to that as described above; and if the cell B finds that a duration of the UE connected to the cell B is less than a parameter Tstore_UE_cntxt after receiving the message, it is judged that the UE is connected to the cell C shortly after being connected to the cell B, and the cell B judges that handover to wrong cell occurs in the UE.
Second: After the UE Initiates a New RRC Connection Reestablishment Procedure
1. Too-Late Handover
An RLF occurs when the UE is in the source cell A, and the UE initiates a connection reestablishment attempt to the cell B and the reestablishment fails. Then the UE initiates an RRC connection establishment procedure to the cell C (a cell selected via cell reselection, which may also be the cell A or B) via a cell reselection procedure, and reports an RLF report, the RLF report including an identifier of the cell where the UE is present when the RLF occurs, that is, an identifier of the source cell A, and sometimes further including a parameter timeConnFailure, which records a period of time experienced by the UE from receiving a handover command message for the last time to the occurrence of the link failure. The cell C transmits the RLF report reported by the UE via an RLF indication message to the source cell A where the RLF occurs. And if the source cell finds that the parameter timeConnFailure is not included in the RLF report or a value of the parameter timeConnFailure is greater than a preset value Tstore_UE_cntxt, the source cell A records this time of connection failure event as too-late handover.
2. Too-Early Handover
The UE is successfully handed over from the source cell A to the cell B, after a short period of time, an RLF occurs in the UE, and the UE initiates a connection reestablishment attempt to the source cell A and the reestablishment fails. Then the UE initiates an RRC connection establishment procedure to the cell C (a cell selected via cell reselection, which may also be the cell A or B) via a cell reselection procedure, and reports an RLF report, the RLF report including an identifier of the cell B where the UE is present when the RLF occurs and the parameter timeConnFailure. The cell C transmits the RLF report to the cell B via an RLF indication message. And after the cell B obtains the RLF report, if the parameter timeConnFailure is less than Tstore_UE_cntxt and the reestablished cell of the UE is a cell triggering the latest handover before the connection fails, it is judged that this time of connection failure event is too-early handover.
3. Handover to Wrong Cell
The UE is successfully handed over from the source cell A to the cell B, but an RLF occurs after a short period of time, and the UE initiates a connection reestablishment attempt to the cell C and the reestablishment fails. Then the UE initiates an RRC connection establishment procedure to a cell D via a cell reselection procedure, and reports an RLF report. The UE initiates reestablishment connection to the cell D, and at this moment, the cell D transmits an RLF indication message to the cell B, information contained in the message being similar to that as described above; and if the cell B, after receiving the message, finds that the parameter timeConnFailure is less than the parameter Tstore_UE_cntxt and the reestablished cell C of the UE is neither a source cell for the latest handover before the connection fails, nor a target cell, it is judged that this time of connection failure event is handover to wrong cell.
However, in the implementation of the present disclosure, the inventors found that a defect exists in the prior art is: for a heterogeneous network deployed with a relatively small cell (such as a pico, and an RRH, etc.), it is possible that judgment error will occur, thereby resulting in that setting parameters cannot be adjusted in time, and the network performance is affected.