KR101437693B1 - Method of Performing A Handover Procedure - Google Patents

Abstract

A method of performing a handover procedure includes receiving a handover command message from a source base station, transmitting a random access preamble to a target base station indicated by the handover command message, and determining whether a handover has failed in response to the random access preamble And receiving the random access response message including the type information indicating the type. The handover can be performed quickly without delay.

Description

[0001] The present invention relates to a method of performing a handover procedure,

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to wireless communication, and more particularly, to a method for performing a handover procedure in a wireless communication system.

A 3rd Generation Partnership Project (3GPP) mobile communication system based on WCDMA (Wideband Code Division Multiple Access) radio access technology is widely deployed all over the world. HSDPA (High Speed Downlink Packet Access), which can be defined as the first evolutionary phase of WCDMA, provides the 3GPP with highly competitive wireless access technology in the mid-term future. However, since the development of wireless access technology that continues to increase and compete with the requirements and expectations of users and operators continues, development of new technologies in 3GPP is required for future competitiveness. Reduced cost per bit, increased service availability, the use of flexible frequency bands, simple structure and open interfaces, and proper power consumption of terminals are becoming a requirement.

A wireless communication system is different from a wired communication system in that it has to provide continuous service to terminals having mobility. In other words, there must be support for cases where the terminal moves from one area to another. If the terminal is moving away from the currently connected base station and is approaching another base station at the same time, the network must perform the task of transferring the access point of the terminal to the new base station. A procedure for a previous base station to be referred to as a source base station, a new base station to be referred to as a target base station, and a terminal to move from a source base station to a target base station is referred to as a handover.

The handover process includes handover preparation, handover execution, and handover completion.

The handover preparation procedure includes a measurement report step for a source base station and a neighboring base station in which a signal is detected and a handover command step for informing a target base station to the terminal using the measurement report .

The handover procedure includes a step in which the UE transmits a random access preamble to the target base station and synchronizes with the target base station, and a step in which the target base station receiving the random access preamble transmits a message including the uplink radio resource and the time offset information in the random access response .

The handover completion process includes notifying the target base station of the change of the path to the upper network and transmitting a resource release message to the source base station.

After receiving the handover command, the terminal attempts to connect to the target base station. The connection may fail if the radio environment is bad, the base station is distributed for load balancing, or the channel parameters are changed during the handover process.

Generally, during the handover process, the terminal disconnects the connection to the source base station and can not transmit or receive data to or from the terminal while completing the connection to the target base station. Therefore, if the handover is delayed, data transmission and reception are interrupted and the service quality is degraded.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for a terminal to quickly perform a handover process.

A method for performing a handover according to an embodiment of the present invention includes receiving a handover command message from a source base station, transmitting a random access preamble to a target base station indicated by the handover command message, And receiving a random access response message including type information indicating whether or not the handover has failed in response.

According to another aspect of the present invention, there is provided a method for performing a handover procedure, comprising: receiving a handover request from a source base station; transmitting a response to the handover request; Receiving a random access preamble from the UE, and transmitting a random access response message including type information indicating a handover failure in response to the random access preamble received from the UE.

The present invention provides a method capable of fast handover even in the case where dispersion of a base station is required for load balancing as well as when channel parameters are changed during a handover process as well as when the wireless environment is poor.

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 an LTE (Long Term Evolution) 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.

A user equipment (UE) 10 may be fixed or mobile and may be referred to as another term such as a mobile station (MS), a user terminal (UT), a subscriber station (SS) The base station 20 generally refers to a fixed station that communicates with the terminal 10 and may be referred to by other terms such as an evolved-NodeB (eNB), a base transceiver system (BTS), an access point have. One base station 20 may have more than one cell. An interface for transmitting user traffic or control traffic may be used between the base stations 20. Hereinafter, downlink refers to communication from the base station 20 to the terminal 10, and uplink refers to communication from the terminal 10 to the base station 20.

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

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

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

The base station performs the following functions. (1) radio resource management such as radio bearer control, radio admission control, connection mobility control, and dynamic resource allocation to a terminal, (RRM) function, (2) Internet Protocol (IP) header compression and encryption of user data streams, (3) routing of user plane data to the S-GW, (4) Scheduling and transmission, (5) scheduling and transmission of broadcast information, and (6) measurement and measurement reporting setup for mobility and scheduling.

The MME performs the following functions. (2) security control, (3) idle state mobility control, (4) SAE bearer control, (5) NAS (Non-Access) Stratum signaling ciphering and integrity protection.

The 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 showing 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 implements layers of the air interface protocol to provide a control plane and a user plane. The functions of the respective layers can be implemented through the processor 51. [ The memory 52 is connected to the processor 51 and stores a terminal driving system, an application, and general files. The display unit 54 displays various information of the terminal and can use well known elements such as a liquid crystal display (LCD) and an organic light emitting diode (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 RF unit 53 is connected to the processor to transmit and / or receive a radio signal.

The layers of the radio interface protocol between the terminal and the network are classified into L1 (first layer), L2 (first layer), and L2 (third layer) based on the lower three layers of the Open System Interconnection (Second layer), and L3 (third layer). The physical layer belonging to the first layer provides an information transfer service using a physical channel, and a radio resource control (RRC) layer located at the third layer Controls the 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 wireless protocol structure for a control plane. This represents the structure of the radio interface protocol between the UE and the E-UTRAN. The data plane is a protocol stack for transmitting user data, and the control plane is a protocol stack for transmitting control signals.

Referring to FIGS. 4 and 5, a physical layer (PHY) layer of the first layer provides an information transfer service to an upper layer using a physical channel. The physical layer is connected to an upper layer MAC (Medium Access Control) layer through a transport channel, and data is transferred between the MAC layer and the physical layer through the transport channel. Data moves between physical layers between different physical layers, that is, between a transmitting side and a receiving physical layer. The physical channel is modulated by an Orthogonal Frequency Division Multiplexing (OFDM) scheme, and time and frequency can be utilized as radio resources.

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

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 radio section having a small bandwidth during transmission of an IP (Internet Protocol) packet such as IPv4 or IPv6 And performs header compression to reduce the size of the IP packet header.

The third layer of Radio Resource Control (RRC) layer is defined only in the control plane. The RRC layer is responsible for the control of logical channels, transport channels and physical channels in connection with the configuration, re-configuration and release of radio bearers (RBs). RB denotes a service provided by the second layer for data transmission between the UE and the E-UTRAN. If there is an RRC connection between the RRC of the UE and the RRC of the network, the UE is in the RRC Connected Mode, and if not, the UE is in the RRC Idle Mode.

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

Hereinafter, the handover process will be described. 6 is a flowchart illustrating a general handover process.

Referring to FIG. 6, a UE transmits a measurement report to a source base station to which the UE belongs (S10). The source base station determines whether the handover is performed using the received measurement report information (S11). The source base station transmits a handover request message to the target base station (S12). Upon receiving the handover request message, the target base station performs admission control and transmits an acknowledgment message for the handover request to the source base station (S13). The source base station transmits a handover command message to the terminal (S14).

After receiving the handover command message from the source base station, the mobile station synchronizes with the target base station in downlink synchronization and transmits a random access preamble through a Physical Random Access Channel (PRACH) (S15). Here, if a dedicated random access preamble is allocated in the handover command message, the UE can perform a contention free random access procedure. The target base station transmits a random access response message including uplink radio resource allocation information and timing advance information to the mobile station (S16). Here, the random access response message may be transmitted through a downlink-shared channel (DL-SCH). In addition, the random access response message may further include a temporary C-RNTI (Cell-Radio Network Temporary Identifier).

If the random access to the target BS of the MS is successful, the MS transmits a Handover Comfirm message including a UL Buffer Status Report message to the target BS (S17).

The target base station transmits a path switch request message to the MME to inform that the UE has changed the cell (S18). The MME transmits a user plane update request message to the serving gateway (S19). The serving gateway changes the downlink data path to the target base station (S20). The serving gateway transmits a user plane update response message to the MME (S21). The MME transmits a Path Switch Request Ack message to the target BS (S22).

The target base station transmits a Release Resource message to the source base station to notify the success of the handover (S23). The source base station releases resources related to the terminal (S24).

Hereinafter, a measurement report during handover will be described.

A terminal transmits a measurement report to a source base station when a specific event or a specific period occurs. The source base station selects a target base station using the information included in the received measurement report, And receives a handover request acknowledgment message from the target base station. Upon receiving the handover request acknowledgment message, the source base station transmits a Handover Command message having a form of an RRC Connection Reconfiguration message to the UE.

Here, for the measurement report, the base station transmits the Measurement Object in the form of SIB (System Information Block). The measurement target may include a parameter for a specific cell, which is an object to be measured by the terminal with respect to a neighboring cell list (Neighboring Cell List). The SIB includes center frequency and priority information for neighboring cells. The following table is an example of SIB for measurement report.

Referring to Table 1, the SIB includes information on neighbor cells and frequencies for cell reselection. Cell reselection parameters common for a frequency as well as cell specific reselection parameters for the cell.

And re-selects the base station based on the priority included in the measurement report. For example, the UE receives information on a cell to be camped on in a idle state via a broadcast channel. That is, if the RRC connection re-establishment procedure is not performed at the time of the radio link failure (RLF), the UE temporarily camps on the Last Serving Cell, And selects a cell to camp on and receives a paging message.

Hereinafter, the types of the handover command and the complete message will be described. The MS receives the handover command message and transmits a handover complete message to the target BS. Here, the handover command message and the handover complete message are transmitted in the form of RRC Connection Reconfiguration and RRC Connection Reconfiguration Complete messages. Table 2 below shows an example of an RRC connection reset message.

Table 3 below shows an example of the RRC connection reset completion message.

Table 4 below shows an example of the Mobility Control Information Element included in the RRC connection re-establishment message.

7 is an example of a handover failure procedure.

Referring to FIG. 7, the terminal transmits a measurement report to the source base station (S100). The source base station selects a target base station based on the received measurement report and transmits a handover preparation message to the selected target base station (S110). Here, the source base station can transmit a handover preparation message to a base station (Prepared Base Station) other than the target base station. When the source base station receives the handover ready acknowledgment message from the target base station, the source base station transmits a handover command message having the form of an RRC connection re-establishment message to the mobile station (S120). The MS receiving the handover command message starts the timer T10 and attempts random access to the target BS in step S130. If the UE does not succeed in random access until the timer T10 expires, the UE starts a timer T11, retries the random access to the source base station (S140), and attempts to reset the RRC connection (S150) . If the RRC connection resetting is not successful until the timer T11 expires, the terminal enters the RRC idle mode (S160).

Therefore, the terminal has to wait until the timers T10 and T11 expire, so that a problem of connection delay occurs.

FIG. 8 shows another example of a procedure for failing in handover.

Referring to FIG. 8, the MS transmits a measurement report to a source BS (S200). The source base station selects the target base station based on the received measurement report, and transmits a handover request message to the target base station (S210). Here, the source base station can transmit a handover request message to a base station (Prepared Base Station) other than the target base station. Upon receiving the handover request acknowledgment message from the target base station, the source base station transmits a handover command message in the form of an RRC connection re-establishment message to the mobile station in step S220. The MS having received the handover command message starts the timer T10 and attempts random access to the target BS (S230). If the UE does not succeed in random access until the timer T10 expires, the UE starts a timer T311 and performs a cell selection (S240). The terminal attempts random access to the selected cell (S250) and attempts RRC connection (S260). If the RRC connection is not successful until the timer T311 expires, the terminal enters the RRC idle mode (S270).

Therefore, the terminal has to wait until the timers T10 and T311 expire, and thus there is a problem of connection delay.

Reasons for delaying the connection of the handover include the case where the radio environment is not good, the load balancing is required, and the channel parameter is changed. Thus, there is a need for an operational scheme for handover access delays for various reasons.

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

Referring to FIG. 9, when a signal strength of a source base station for a terminal drops below a threshold, the terminal transmits a measurement report to a source base station (S300). The source BS selects an appropriate target BS using the measurement report information received from the MS and transmits a handover request message to the selected target BS in step S310. And transmits an acknowledgment message for the handover request to the source base station (S315). Here, the source base station reserves radio resources for the UE through the handover request and acknowledgment procedure.

After receiving the acknowledgment for the handover request, the source base station transmits a handover command message to the terminal in step S320. The MS receiving the handover command message transmits a reception HARQ (Hybrid Automatic Repeat Request) to the source BS, and the source BS receiving the HARQ expects the MS to perform the handover process.

In step S330, the UE synchronizes with the target BS in downlink synchronization (DL sync) and transmits a random access preamble to the target BS. Here, the UE transmits the random access preamble using the random access parameter included in the handover command message received from the source base station.

The target base station can induce the terminal to connect to another base station when distribution of the base station is required for load balancing or when there is no currently allocable radio resource. That is, the target base station selects an appropriate best base station, transmits a handover request message to the best base station in step S340, and transmits the acknowledgment message for the handover request to the target base station in step S345. Here, the target BS can allocate radio resources for the UE and reserve the preamble through the handover request message transmission and receipt confirmation message reception procedure to the best BS. The best base station may be one selected from existing base stations.

If the target BS receives the acknowledgment message for the handover request from the best BS, the target BS transmits a random access response message for the random access preamble to the MS in step S350. The random access response message may include information indicating that handover has failed in the target base station. In addition, the random access response message may include a new random access preamble together with a cell identifier (ID) of a best base station.

If the random access response message received by the mobile station includes an identifier of a new cell that is not uplink radio resource allocation information, the terminal synchronizes the downlink with the best base station using the information included in the random access response message, The random access preamble is retransmitted to the base station (S360).

According to the conventional scheme, when the random access is started, the timer is started, and if the random access is not successfully performed until the timer expires, the RRC connection is retried to the source base station or the other base station, . However, if the target base station arbitrarily determines the best base station and notifies the terminal of the cell ID of the best base station, the handover can be performed quickly.

10 is a flowchart illustrating a handover recovery method according to another embodiment of the present invention.

Referring to FIG. 10, when a signal strength of a source base station for a terminal drops below a threshold, the terminal transmits a measurement report to a source base station (S400). The source BS selects an appropriate target BS using the measurement report information received from the MS and transmits a handover request message to the selected target BS in step S410. And transmits an acknowledgment message for the handover request to the source base station (S415). Here, the source base station reserves radio resources for the UE through the handover request and acknowledgment procedure.

After the source base station receives the acknowledgment for the handover request, it transmits a handover command message to the terminal in step S420. The handover command message may include a dedicated preamble allocated by the target BS. The MS receiving the handover command message transmits a reception HARQ (Hybrid Automatic Repeat Request) to the source BS, and the source BS receiving the HARQ expects the MS to perform the handover process.

In step S430, the UE synchronizes with the target BS in downlink synchronization (DL sync) and transmits the random access preamble to the target BS. Here, the UE transmits the random access preamble using the random access parameter included in the handover command message received from the source base station.

The target base station can induce the terminal to connect to another base station when distribution of the base station is required for load balancing or when there is no currently allocable radio resource. However, if the target base station can not find a suitable other base station to guide the access of the terminal, the target base station can force the terminal to camp on another base station.

The target base station transmits a random access response message in response to the random access preamble received from the terminal (S440). The random access response message may include information indicating that handover has failed in the target base station. In addition, the random access response message may include a new random access preamble together with a cell ID (Cell ID) of a base station for which the target base station has decided to force camping on. The MS receiving the random access response message camps on another base station using the information included in the message (S450).

According to the conventional method, when the target base station changes the load balancing or the priority between the base stations while the handover timer and the reset timer are in operation, the RRC connection is released after the timer expires and the camping on is performed using the received SIB information Try it. However, the time delay can be reduced by camping on the base station indicated by the target base station without waiting for the timer to expire.

11 is a flowchart illustrating a handover recovery method according to another embodiment of the present invention.

Referring to FIG. 11, when a signal strength of a source base station for a terminal drops below a threshold, the terminal transmits a measurement report to a source base station (S500). The source BS selects an appropriate target BS using the measurement report information received from the MS and transmits a handover request message to the selected target BS in step S510. And transmits an Acknowledgment message for the handover request to the source base station (S515). Here, the source base station reserves radio resources for the UE through the handover request and acknowledgment procedure.

After receiving the acknowledgment for the handover request, the source base station transmits a handover command message to the terminal in step S520. The handover command message may include a dedicated preamble allocated by the target BS. The MS receiving the handover command message transmits a reception HARQ (Hybrid Automatic Repeat Request) to the source BS, and the source BS receiving the HARQ expects the MS to perform the handover process.

The UE adjusts DL synchronization with the target BS and transmits a random access preamble to the target BS in step S530. Here, the UE transmits the random access preamble using the random access parameter included in the handover command message received from the source base station.

The target base station transmits a random access response message to the terminal in response to the random access preamble (S540). The random access response message may include information indicating that handover has failed in the target base station. In addition, the random access response message may include version information of the system information and a new random access preamble. The terminal compares the version information of the previous system information included in the handover command message with the version information of the current system information included in the random access response message (S550). If the version information of the previous system information differs from the version information of the current system information as a result of comparison, the system information is updated by receiving a new SIB (S512). The MS retransmits the new random access preamble using the information included in the random access response message and transmits a handover complete message to the target BS in step S570.

Generally, when a terminal fails in random access, it can be mistaken for a power ramping error or a contention. However, if the version information of the system information is included in the random access response message, it can be easily recognized that the random access failure is caused by the channel parameter change by comparing the version information with the version information of the system information included in the handover command message.

Unlike attempting to make an RRC connection based on the current best radio condition within a reset timer if multiple attempts are made within the handover timer and the handover is not successful within the handover timer, load balancing, The access control or the channel parameter change is sent to the random access response message, thereby enabling the failure recovery to be performed quickly without causing the time delay.

12 illustrates an example of a random access response message format according to an embodiment of the present invention. Here, the random access response message may be a MAC message.

12, the random access response message includes Type Information, RA Preamble Identifier, TA (Timing Advance), UL Grant (Uplink Grant), C-RNTI, Version Information ), A cell ID (Cell ID), and a preamble identifier (Preamble Identifier). Here, E is an extension bit.

The type information is a bit for discriminating whether contention resolution or handover has failed. For example, 0 can be indicated when the random access response message is for contention resolution, and 1 when the handover failure is indicated. TA is time offset information, UL grant is UL acknowledgment, and C-RNTI is an identifier for distinguishing UEs in a cell. The version information indicates a version of system information, the cell identifier indicates a cell identifier for a base station indicated by the target base station, and the preamble identifier indicates an identifier for a random access preamble used for a new random access attempt.

Here, FIG. 12 is merely an example of a random access response message for handover. Therefore, some of the components constituting the message may be omitted, other components may be added, and the order may be freely modified.

The present invention may be implemented in hardware, software, or a combination thereof. (DSP), a programmable logic device (PLD), a field programmable gate array (FPGA), a processor, a controller, a microprocessor, and the like, which are designed to perform the above- , Other electronic units, or a combination thereof. In software implementation, it may be implemented as a module that performs the above-described functions. 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.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. It will be understood that various modifications and changes may be made without departing from the spirit and scope of the invention. Accordingly, modifications of the embodiments of the present invention will not depart from the scope of the present invention.

1 is a block diagram illustrating a wireless communication system.

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

3 is a block diagram showing elements of a terminal;

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

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

6 is a flowchart illustrating a general handover process.

7 is an example of a handover failure procedure.

FIG. 8 shows another example of a procedure for failing in handover.

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

10 is a flowchart illustrating a handover recovery method according to another embodiment of the present invention.

11 is a flowchart illustrating a handover recovery method according to another embodiment of the present invention.

12 illustrates an example of a random access response message format according to an embodiment of the present invention.

Claims (8)

In a handover method of a terminal, Receiving a handover command message from a source base station; The UE transmitting a first random access preamble to a first target BS indicated by the handover command message; The UE receives a random access response message from the first target BS, the random access response message including information indicating that handover to the first target BS has failed and a cell identifier of the second target BS, , step; And The handover to the second target base station by the terminal, Wherein the second target base station is a base station selected by the first target base station. The method according to claim 1, The step of the mobile station handing over to the second target base station The UE transmitting a second random access preamble to the second target base station, The second target BS receives a handover request message from the first target BS, transmits an acknowledgment message to the first target BS in response to the handover request message, Wherein the acknowledgment message indicates to permit the handover of the terminal to the second target base station, Wherein the first target BS allocates radio resources for the second random access preamble to the MS upon receiving the acknowledgment message. 3. The method of claim 2, Wherein the second target BS is selected when the available resource for handover of the MS does not exist in the first target BS. The method of claim 3, Wherein the random access response message further comprises system version information, Further comprising the step of the terminal receiving a system information block (SIB) from the first target base station if the system version information included in the random access response message is different from the current system information. A terminal performing handover, the terminal including a processor, The processor receives a handover command message from a source base station, transmits a first random access preamble to a first target base station indicated by the handover command message, Receiving a random access response message from the first target base station, A handover to a second target base station, Wherein the random access response message includes information indicating that the handover to the first target base station has failed and a cell identifier of the second target base station, And the second target base station is selected by the first target base station. 6. The method of claim 5, Wherein the processor is adapted to transmit a second random access preamble to the second target base station for handover to the second target base station, The second target BS receives a handover request message from the first target BS, transmits an acknowledgment message to the first target BS in response to the handover request message, Wherein the acknowledgment message indicates to permit the handover of the terminal to the second target base station, And the first target BS allocates a radio resource for the second random access preamble to the MS when receiving the acknowledgment message. The method according to claim 6, Wherein the second target BS is selected when the available resource for handover of the MS does not exist in the first target BS. 8. The method of claim 7, Wherein the random access response message further comprises system version information, Wherein the processor is configured to receive a system information block (SIB) from the first target base station if the system version information included in the random access response message is different from current system information.

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