KR20090111434A - Method for handling failure of handover - Google Patents

Abstract

PURPOSE: A handover failure processing method for improving the reliability of wireless communication is provided to reduce wireless resource for performing handover of the other target base station and reduce access time. CONSTITUTION: A handover failure processing method for improving the reliability of wireless communication is as follows. The handover command message is received from the source base station. A first random access preamble is transmitted to a target base station(S330). A second random access preamble is transmitted to the target base station since the response about the first random access preamble is unable to be received(S335). The handover failure timer transmits the second random access preamble. A cell selection is performed by starting the handover failure timer.

Description

How to handle handover failure {METHOD FOR HANDLING FAILURE OF HANDOVER}

The present invention relates to a wireless communication system, and more particularly, to a handover failure processing method.

3rd Generation Partnership Project (3GPP) mobile communication systems based on Wideband Code Division Multiple Access (WCDMA) radio access technology are widely deployed around the world. High Speed Downlink Packet Access (HSDPA), which can be defined as the first evolution of WCDMA, provides 3GPP with a highly competitive wireless access technology in the mid-term future. However, as the demands and expectations of users and operators continue to increase, and the development of competing wireless access technologies continues to progress, new technological evolution in 3GPP is required to be competitive in the future. Reduced cost per bit, increased service availability, the use of flexible frequency bands, simple structure and open interface, and adequate power consumption of the terminal are demanding requirements.

Meanwhile, the wireless communication system provides a communication service by dividing the service area into a plurality of cells in order to overcome the limitation of the service area and the user capacity. This is called a multi-cell environment. A cell is an area in which a base station provides a communication service, and one base station can provide a service for at least one cell. The UE belongs to one cell, and the cell to which the UE belongs is called a serving cell. Another cell adjacent to the serving cell is called a neighbor cell. The wireless communication system is different from the wired communication system in that it is necessary to provide endless services to mobile terminals. That is, when the terminal moves the position from the serving cell to the neighbor cell, it is necessary to change the moved neighbor cell to the serving cell to provide a continuous service to the terminal. As such, a procedure of changing a serving cell of the terminal due to the movement of the terminal is referred to as handover. In this case, the cell to which the terminal originally belongs is called a source cell, and a new cell to which the terminal moves is called a target cell. A base station providing a communication service to a source cell is called a source base station, and a base station providing a communication service to a target cell is called a target base station. In the handover process, the terminal must disconnect from the source base station and newly connect with the target base station.

However, since the wireless communication system is a time variant system, the radio condition may change with time. In addition, when the terminal moves at a high speed, the wireless environment between the target base station and the terminal may be rapidly deteriorated. Therefore, the handover may fail due to a sudden change in the wireless environment while the terminal attempts to handover to the target base station. If the handover fails, the UE needs handover failure processing such as accessing another base station or entering an idle mode. At this time, if the handover failure process is delayed, the terminal cannot perform reliable communication. In addition, unnecessary handover with the target base station wastes power of the terminal and wastes limited radio resources such as time and frequency. Therefore, there is a need for a method that can quickly and efficiently handle handover failure to ensure the reliability of wireless communications.

An object of the present invention is to provide a fast and efficient method for handling handover failure.

In one aspect, a method of handling handover failure is provided. The method includes receiving a handover command message from a source base station, transmitting a first random access preamble to a target base station indicated by the handover command message, and failing to receive a response to the first random access preamble. Transmitting a second random access preamble to a target base station, starting a handover failure timer according to transmitting the second random access preamble, performing a cell selection upon starting the handover failure timer, and the cell Transmitting the third random access preamble to the best base station selected by the selection.

In case a problem occurs in a wireless environment between the target base station and the terminal while the terminal attempts to access the target base station for handover, the terminal finds the best cell by measuring channel conditions of adjacent cells. When the best cell does not match the target cell, it stops performing handover to the target cell and attempts to connect to the best cell quickly. Therefore, when the channel state between the terminal and the target base station is not good, it is possible to reduce the time to continue to connect to the target base station unnecessarily, saving radio resources consumed to perform the handover with the target base station. In addition, the fast connection with the best cell can increase the reliability of wireless communication, which is efficient.

1 is a block diagram illustrating a wireless communication system. This may be a network structure of an Evolved-Universal Mobile Telecommunications System (E-UMTS). The E-UMTS system may be referred to as a Long Term Evolution (LTE) system. Wireless communication systems are widely deployed to provide various communication services such as voice, packet data, and the like.

Referring to FIG. 1, an Evolved-UMTS Terrestrial Radio Access Network (E-UTRAN) includes a base station (BS) 20 that provides a control plane and a user plane.

The UE 10 may be fixed or mobile and may be called by other terms such as a mobile station (MS), a user terminal (UT), a subscriber station (SS), a wireless device, and the like. The base station 20 generally refers to a fixed station communicating with the terminal 10, and may be referred to as other terms such as an evolved-NodeB (eNB), a base transceiver system (BTS), and an access point. have. One base station 20 may provide a service for at least one cell. The cell is an area where the base station 20 provides a communication service. An interface for transmitting user traffic or control traffic may be used between the base stations 20. Hereinafter, downlink means communication from the base station 20 to the terminal 10, and uplink means communication from the terminal 10 to the base station 20.

The base stations 20 may be connected to each other through an X2 interface. The base station 20 is connected to an Evolved Packet Core (EPC), more specifically, a Mobility Management Entity (MME) / Serving Gateway (S-GW) 30 through an S1 interface. The S1 interface supports a many-to-many-relation between base station 20 and MME / S-GW 30.

2 is a block diagram illustrating a functional split between an E-UTRAN and an EPC.

Referring to FIG. 2, the hatched block represents a radio protocol layer and the empty block represents a functional entity of the control plane.

The base station performs the following functions. (1) Radio Resource Management such as Radio Bearer Control, Radio Admission Control, Connection Mobility Control, Dynamic Resource Allocation to UE RRM), (2) Internet Protocol (IP) header compression and encryption of user data streams, (3) routing of user plane data to S-GW, and (4) paging messages. Scheduling and transmission, (5) scheduling and transmission of broadcast information, and (6) measurement and measurement report setup for mobility and scheduling.

The MME performs the following functions. (1) distribution of paging messages to base stations, (2) Security Control, (3) Idle State Mobility Control, (4) System Architecture Evolution (SAE) bearer control, (5) Ciphering and Integrity Protection of Non-Access Stratum (NAS) signaling.

S-GW performs the following functions. (1) termination of user plane packets for paging, and (2) user plane switching to support terminal mobility.

3 is a block diagram illustrating elements of a terminal. The terminal 50 includes a processor 51, a memory 52, an RF unit 53, a display unit 54, and a user interface unit 55. . The processor 51 is implemented with layers of the air interface protocol to provide a control plane and a user plane. The functions of each layer may be implemented through the processor 51. The memory 52 is connected to the processor 51 to store a terminal driving system, an application, and a general file. The RF unit 53 is connected to a processor and transmits and / or receives a radio signal. The display unit 54 displays various information of the terminal, and may use well-known elements such as liquid crystal display (LCD) and organic light emitting diodes (OLED). The user interface unit 55 may be a combination of a well-known user interface such as a keypad or a touch screen.

The layers of the radio interface protocol between the terminal and the network are based on the lower three layers of the Open System Interconnection (OSI) model, which are well known in communication systems. It may be divided into a second layer L2 and a third layer L3. Among these, a physical layer (PHY) belonging to the first layer provides an information transfer service using a physical channel, and radio resource control located in a third layer. (Hereinafter referred to as RRC) layer controls radio resources between the UE and the network. To this end, the RRC layer exchanges RRC messages between the UE and the network.

4 is a block diagram illustrating a radio protocol architecture for a user plane. 5 is a block diagram illustrating a radio protocol structure for a control plane. This shows the structure of the air interface protocol between the terminal and the E-UTRAN. The data plane is a protocol stack for user data transmission, and the control plane is a protocol stack for control signal transmission.

4 and 5, the physical layer, which is the first layer, provides an information transfer service to an upper layer using a physical channel. The physical layer is connected to the upper MAC layer through a transport channel, and data between the MAC layer and the physical layer moves through the transport channel. Data moves between physical layers between physical layers, that is, between physical layers of a transmitting side and a receiving side. The physical channel is modulated by an orthogonal frequency division multiplexing (OFDM) scheme, and may use time and frequency as radio resources.

The MAC layer of the second layer provides a service to a radio link control (RLC) layer, which is a higher layer, through a logical channel. The RLC layer of the second layer supports the transmission of reliable data. In the RLC layer, there are three operation modes according to a data transmission method: transparent mode (TM), unacknowledged mode (UM), and acknowledged mode (AM). The AM RLC provides a bidirectional data transmission service, and supports retransmission when an RLC Protocol Data Unit (PDU) fails to transmit.

The Packet Data Convergence Protocol (PDCP) layer of the second layer contains relatively large and unnecessary control information in order to efficiently transmit packets in a wireless bandwidth having a low bandwidth when transmitting an Internet Protocol (IP) packet such as IPv4 or IPv6. Perform header compression to reduce the IP packet header size.

The RRC layer of the third layer is defined only in the control plane. The RRC layer is responsible for controlling logical channels, transport channels, and physical channels in connection with configuration, re-configuration, and release of radio bearers (RBs). RB means a service provided by the second layer for data transmission between the UE and the E-UTRAN. If there is an RRC connection (RRC Connection) between the RRC of the terminal and the RRC of the network, the terminal is in the RRC Connected Mode, otherwise it is in the RRC Idle Mode.

The non-access stratum (NAS) layer located above the RRC layer performs functions such as session management and mobility management.

As a downlink transport channel for transmitting data from a network to a terminal, a BCH (Broadcast Channel) for transmitting system information, a DL-SCH (Downlink-Shared Channel) for transmitting user traffic or a control message, etc. There is this. Traffic or control messages of the downlink multicast or broadcast service may be transmitted through the DL-SCH, or may be transmitted through the downlink multicast channel (MCH). An uplink transport channel for transmitting data from a terminal to a network includes a random access channel (RAC) for transmitting an initial control message and an uplink-shared channel (UL-SCH) for transmitting user traffic or a control message.

As a downlink physical channel mapped to a downlink transport channel, a physical broadcast channel (PBCH) for transmitting BCH information, a physical multicast channel (PMCH) for transmitting MCH information, and a PDSCH for transmitting PCH and DL-SCH information (Physical Downlink shared Channel) and PDCCH (Physical Downlink Control Channel) for transmitting control information provided by the first and second layers, such as downlink or uplink radio resource allocation information (DL / UL Scheduling Grant). . The PDCCH is also called a downlink L1 / L2 control channel. As an uplink physical channel mapped to an uplink transport channel, a physical uplink shared channel (PUSCH) for transmitting UL-SCH information, a physical random access channel (PRACH) for transmitting RACH information, a HARQ ACK / NACK signal, and scheduling There is a Physical Uplink Control Channel (PUCCH) for transmitting control information provided by the first layer and the second layer, such as a scheduling request signal and a channel quality indicator (CQI).

Hereinafter, a successful handover process and a handover failure processing method will be described in detail.

6 is a flowchart illustrating a successful handover process.

Referring to FIG. 6, the UE transmits a measurement report to a source BS (S10). The source base station determines the handover using the received measurement report. When the source base station determines the handover to the neighbor cell, the neighbor cell becomes a target cell, and the base station belonging to the target cell becomes the target BS. The source base station transmits a handover preparation message to the target base station (S11). The target base station performs admission control to increase the likelihood of successful handover. The target base station transmits an acknowledgment (ACK) message to the source base station (S12). The handover preparation confirmation message may include a Cell-Radio Network Temporary Identifier (C-RNTI), a dedicated random access preamble, a connection parameter, and the like. C-RNTI is an identifier for distinguishing a terminal in a cell. The dedicated random access preamble and the access parameters are information necessary for the terminal to access the target base station. The source base station transmits a handover command message to the terminal (S13). The handover command message may be transmitted in the form of an RRC connection reconfiguration message. The handover command message may include a C-RNTI received from the target base station, a dedicated random access preamble, access parameters, and the like.

After receiving the handover command message from the source base station, the terminal synchronizes with the target base station and accesses the target base station. The terminal goes through a random access procedure to access the target base station. Hereinafter, the random access process will be described.

The terminal transmits a random access preamble to the target base station (S14). The random access preamble may be transmitted through a random access channel (RACH). If a plurality of terminals simultaneously transmit the same random access preamble to the target base station, collision occurs. If a collision occurs, the terminal may retransmit the random access preamble. This is called a contention-based random access process. In order to prevent a collision, the terminal may perform a contention free random access procedure. To this end, the terminal may transmit a dedicated random access preamble to the target base station. The dedicated random access preamble is assigned in the handover command message.

The target base station transmits a random access response message to the terminal (S15). The random access response message may be transmitted through a downlink-shared channel (DL-SCH). The random access response message may include uplink radio resource allocation information, timing advance information, temporary C-RNTI, and the like.

If the random access of the terminal to the target base station is successful, the terminal transmits a handover confirmation message (Handover Comfirm) to the target base station (S16). The handover confirmation message may include the C-RNTI. The handover confirmation message may be transmitted together with an uplink buffer status report (BSR).

The target base station transmits a path switch request message to the MME (S17). Through this, the target base station notifies the MME that the terminal has been handed over to the target cell. The MME transmits a user plane update request message to a serving gateway (S-GW) (S18). The serving gateway switches the downlink data path to the target base station (S19). The serving gateway transmits a user plane update response message to the MME (S20). The MME transmits a path switch request acknowledgment message to the target base station (S21). The target base station notifies the success of the handover by transmitting a release resource message to the source base station (S22). The source base station releases the resources associated with the terminal (S23).

7 is a flowchart illustrating an example of a handover failure processing method.

Referring to Figure 7, the terminal transmits a measurement report to the source base station (S100). The source base station determines the handover using the received measurement report. When the source base station determines the handover to the neighbor cell, the base station belonging to the neighbor cell becomes the target base station. The source base station transmits a handover preparation message to the target base station (S110). Here, the source base station may transmit a handover preparation message to a prepared base station (Prepared BS) in addition to the target base station. The target base station transmits a handover preparation confirmation (ACK) message to the source base station (S115). Here, the spare base station may also transmit a handover preparation confirmation message to the source base station. The source base station transmits a handover command message to the terminal (S120). The handover command message may be sent in the form of an RRC connection reset message.

Upon receiving the handover command message, the terminal starts a handover start timer. For example, the handover initiation timer may be started before, after or concurrent with the reception of the handover command message. After starting the handover start timer, the terminal attempts a random access to access the target base station (S130). If the random access of the terminal to the target base station is successful, the handover start timer is stopped. If the random access fails during the handover start timer period, the terminal retries the random access to the target base station. The handover start timer period is from the start of the handover start timer until the handover start timer expires (Expiry). Although the duration of the handover initiation timer is not limited, the duration of the handover initiation timer may take at least about 100 ms (milliseconds) or more considering the transmission / reception of a message for the target base station access during the handover initiation timer. will be.

If the terminal does not succeed in connecting to the target base station until the handover start timer expires, the terminal starts the handover return timer. After starting the handover return timer, the terminal attempts a random access to access the source base station (S140). The terminal attempts to reconfigure the RRC connection with the source base station (S150). If the terminal succeeds in reestablishing the RRC connection with the source base station, the handover return timer is stopped. If the terminal does not succeed in resetting the RRC connection with the source base station until the handover return timer expires, the terminal enters the RRC idle mode (Idel Mode).

As such, when the handover to the target base station fails, the terminal may handle the handover failure by returning to the source cell. However, the terminal must wait until the handover start timer expires to return to the source cell.

8 is a flowchart illustrating another example of a handover failure processing method.

Referring to Figure 8, the terminal transmits a measurement report to the source base station (S200). The source base station determines the handover using the received measurement report. When the source base station determines the handover to the neighbor cell, the base station belonging to the neighbor cell becomes the target base station. The source base station transmits a handover preparation message to the target base station (S210). Here, the source base station may transmit a handover preparation message to the spare base station in addition to the target base station. The target base station transmits a handover preparation confirmation (ACK) message to the source base station (S215). Here, the spare base station may also transmit a handover preparation confirmation message to the source base station. The source base station transmits a handover command message to the terminal (S220). The handover command message may be sent in the form of an RRC connection reset message.

Upon receiving the handover command message, the terminal starts a handover start timer. The handover initiation timer may be started before, after or concurrent with the reception of the handover command message. After starting the handover start timer, the terminal attempts a random access to access the target base station (S230). If the random access to the target base station of the terminal is successful, the handover start timer is stopped. If the random access fails during the handover start timer period, the terminal retries the random access to the target base station.

If the terminal does not succeed in connecting to the target base station until the handover start timer expires, the terminal starts the handover rescan timer. As the handover rescanning timer starts, the terminal performs cell selection (S240). The terminal attempts random access to access the selected base station belonging to the selected cell (S250). The terminal attempts an RRC connection with the selected base station (S260). If the terminal succeeds in the RRC connection with the source base station, the handover rescan timer is stopped. If the terminal does not succeed in the RRC connection with the source base station until the handover rescan timer expires, the terminal enters the RRC idle mode (Idel Mode).

As such, when the handover to the target base station fails, the terminal finds a new target cell through cell selection. However, in order for the UE to select a cell, it must wait until the handover start timer expires.

In both cases of FIG. 7 and FIG. 8, it is necessary to keep trying to access the target base station until the handover start timer expires. However, due to the fast movement of the terminal before the handover start timer expires, the wireless environment between the target base station and the terminal may be rapidly deteriorated. If the handover fails due to a radio link failure (RLF) due to a sudden deterioration of the wireless environment, attempting to continuously access the target base station until the handover start timer expires delays the handover failure process.

Therefore, when the radio environment between the target base station and the terminal is rapidly deteriorated, there is a need for a method for quickly and efficiently handling handover failure without delaying the time until the handover initiation timer expires.

9 is a flowchart illustrating a handover failure processing method according to an embodiment of the present invention.

9, the terminal transmits a measurement report to the source base station (S300). The source base station determines the handover using the received measurement report. When the source base station determines the handover to the neighbor cell, the base station belonging to the neighbor cell becomes the target base station. The source base station transmits a handover preparation message to the target base station (S310). The target base station transmits a handover preparation confirmation message to the source base station (S315). The source base station transmits a handover command message to the terminal (S320). The handover command message may be sent in the form of an RRC connection reset message.

The terminal receiving the handover command message attempts random access to access the target base station. The terminal transmits a random access preamble to the target base station (S330). If a dedicated random access preamble is allocated in the handover command message, the terminal may perform a contention free random access procedure. If the terminal does not receive the random access response message, the terminal retransmits the random access preamble (S335). In this case, the retransmitted random access preamble may be a power ramped random access preamble. The power ramped random access preamble may be transmitted every power ramping period.

The UE starts the handover failure timer as the UE retransmits the random access preamble. The handover failure timer may be started before, after retransmission or concurrent with retransmission of the random access preamble retransmission. As the handover failure timer starts, the UE measures the channel state of the source cell and the neighbor cells. In this case, the measurement period may be a power ramping period. The terminal may measure channel states of neighbor cells according to a neighbor cell list transmitted by a source base station in the form of a system information block (SIB). The neighbor cell list may include a list of neighbor cells using the same carrier frequency (Intra-frequency), a list of neighbor cells using a different subcarrier frequency (Inter-frequency), a specific parameter of the neighbor cell, and the like. . The UE finds the best cell having the best channel state by using the measurement result.

If the best cell does not match the target cell, the UE performs cell selection with the best cell (S340). Here, the base station belonging to the best cell becomes the best base station (Best BS).

The terminal attempts random access by transmitting a random access preamble to access the best base station (S350). The terminal attempts an RRC connection with the best base station (S360). If the terminal succeeds in the RRC connection with the best base station, the handover failure timer is stopped. If the terminal does not succeed in the RRC connection with the best base station until the handover rescan timer expires, the terminal enters the RRC idle mode (Idel Mode).

In case a problem occurs in a wireless environment between the target base station and the terminal while the terminal attempts to access the target base station for handover, the terminal finds the best cell by measuring channel conditions of adjacent cells. When the best cell does not match the target cell, it stops performing handover to the target cell and attempts to connect to the best cell quickly. Therefore, when the channel state between the terminal and the target cell is not good, it is possible to reduce the time to continue to connect to the target base station unnecessarily, to save the radio resources consumed to perform the handover with the target base station. In addition, the fast connection with the best cell can increase the reliability of wireless communication, which is efficient.

Hereinafter, the random access procedure (S350) and the RRC connection process (S360) with the best base station of the terminal will be described in detail.

10 is a flowchart illustrating a random access procedure and an RRC connection procedure.

Referring to FIG. 10, the terminal accesses the best base station through random access (S350). The terminal transmits a random access preamble to the best base station (S351). The random access preamble may be transmitted on the RACH. Here, since the terminal has not been allocated a dedicated random access preamble, the terminal performs a contention random access procedure. The best base station transmits a random access response message to the terminal (S352). The random access response message may be transmitted through a downlink-shared channel (DL-SCH). The random access response message may include uplink radio resource allocation information, timing advance information, temporary C-RNTI, and the like.

The terminal resets the RRC connection with the best base station (S360). The terminal transmits an RRC re-establishment request message to the best base station (S361). The RRC re-establishment request message may be an uplink message scheduled for the first time between the UE and the best base station. The RRC re-establishment request message may be generated by the RRC layer and transmitted through a common control channel (CCCH), which is a logical channel. The best base station transmits an RRC reestablishment message to the terminal (S362). The RRC reestablishment message is a message for resolving contention and establishing a signaling radio bearer (SRB). The RRC reestablishment message may be generated by the RRC layer and transmitted on the CCCH. The terminal transmits an RRC re-establishment complete message to the best base station (S363). The RRC reestablishment complete message may be transmitted through a dedicated control channel (DCCH), which is a logical channel. RLF (Radio Link Failure) is recovered by sending RRC re-establishment complete message. The best base station transmits an RRC reconfiguration message to the terminal (S361). The RRC reset message is for commanding establishment or modification of an RRC connection. The RRC reset message can be sent on the DCCH. The terminal transmits an RRC resetting completion message to the best base station (S362). The RRC reset complete message may be transmitted on the DCCH.

The invention can be implemented in hardware, software or a combination thereof. In hardware implementation, an application specific integrated circuit (ASIC), a digital signal processing (DSP), a programmable logic device (PLD), a field programmable gate array (FPGA), a processor, a controller, and a microprocessor are designed to perform the above functions. , Other electronic units, or a combination thereof. In the software implementation, the module may be implemented as a module that performs the above-described function. The software may be stored in a memory unit and executed by a processor. The memory unit or processor may employ various means well known to those skilled in the art.

Although the present invention has been described above with reference to the embodiments, it will be apparent to those skilled in the art that the present invention may be modified and changed in various ways without departing from the spirit and scope of the present invention. I can understand. Therefore, the present invention is not limited to the above-described embodiment, and the present invention will include all embodiments within the scope of the following claims.

1 is a block diagram illustrating a wireless communication system.

2 is a block diagram illustrating a functional split between an E-UTRAN and an EPC.

3 is a block diagram illustrating elements of a terminal.

4 is a block diagram illustrating a radio protocol structure for a user plane.

5 is a block diagram illustrating a radio protocol structure for a control plane.

6 is a flowchart illustrating a successful handover process.

7 is a flowchart illustrating an example of a handover failure processing method.

8 is a flowchart illustrating another example of a handover failure processing method.

9 is a flowchart illustrating a handover failure processing method according to an embodiment of the present invention.

10 is a flowchart illustrating a random access procedure and an RRC connection procedure.

Claims (5)

Receiving a handover command message from a source base station; Transmitting a first random access preamble to a target base station indicated by the handover command message; Transmitting a second random access preamble to the target base station without receiving a response to the first random access preamble; Starting a handover failure timer in response to transmitting the second random access preamble; Performing cell selection upon starting the handover failure timer; And And transmitting a third random access preamble to the best base station selected by the cell selection. The method of claim 1, wherein the second random access preamble is a power ramped random access preamble. The method of claim 1, further comprising performing a Radio Resource Control (RRC) connection with the best base station. 4. The method of claim 3, further comprising stopping the handover failure timer if the RRC connection is successful. 4. The method of claim 3, further comprising entering an RRC idle mode if the RRC connection fails until the handover failure timer expires.

Images