KR20100032269A - Method of handover - Google Patents

Abstract

PURPOSE: A handover method is provided to prevent time delay of handover when a target cell provides a service by a serving cell and heterogeneous network. CONSTITUTION: Inter-RAT handover measurement information of an RAT(Radio Access Technology) of a serving cell and other RAT is transmitted to a terminal(S401). If the terminal measures other RATs according to the inter-RAT handover measurement information, a measurement result is reported from the terminal(S403). According to the measurement result, a handover is decided by selecting one target RAT among other RATs(S404). A handover command message with central frequency information of the target RAT is transmitted to the terminal according to the decision(S405).

Description

Handover method {METHOD OF HANDOVER}

The present invention relates to wireless communication, and more particularly, to a heterogeneous inter-network handover method in a wireless communication system.

The next generation multimedia wireless communication system, which is being actively researched recently, requires a system capable of processing and transmitting various information such as video, wireless data, etc., out of an initial voice-oriented service. The purpose of a wireless communication system is to enable a large number of users to communicate reliably regardless of location and mobility.

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 providing a service to a terminal is called a serving cell, and another cell adjacent to the serving cell is called a neighbor cell.

The difference between the wireless communication system and the wired communication system is that the terminal should provide seamless services to the terminals no matter what region they move to. When the terminal moves from the serving cell to the neighbor cell, it is necessary to change the moved neighbor cell to the serving cell to provide a seamless service to the terminal. As such, a procedure of changing the serving cell of the terminal due to the movement of the terminal is referred to as handover.

However, the adjacent cell may be a cell of a radio access technology (RAT) different from the serving cell. RAT is a type of technology used for radio access. For example, RAT includes Global System for Mobile communication (GSM) / General Packet Radio Service (GPRS), Universal Mobile Telecommunications System (UTTS) Terrestrial Radio Access Network (UTTS), and Evoloved-UMTS Terrestrial Radio Access Network (E-UTRAN). Etc. There are also various RATs.

GSM is a wireless technology developed as a system for standardizing wireless communication systems in Europe, and GPRS is intended to provide a packet switched data service in a circuit switched data service provided by GSM. Technology. GSM / GPRS is a system based on TDMA. GSM / GPRS may be referred to as a second generation wireless communication system.

UMTS may be referred to as a third generation wireless communication system based on wideband code division multiple access (WCDMA). E-UTRAN is a wireless communication system based on Orthogonal Frequency Division Multiple Access (OFDMA). E-UTRAN is also called Long Term Evolution (LTE). The GSM cell is a cell of which RAT is GSM / GPRS, the UTRAN cell is a cell of which RAT is UMTS, and the E-UTRAN cell is a cell of which RAT is E-UTRAN.

As various types of RATs emerge, interoperability between existing RATs such as GSM / GPRS and UMTS is problematic.

Even if a new RAT is introduced, compatibility with existing GSM / GPRS is convenient for the user, and the operator can also reuse the existing equipment. For interoperation between different RATs, the UE may support multi-RAT. The terminal supporting the multi-RAT may support not only GSM but also UMTS, E-UTRAN, and the like. For example, if the serving cell is a GSM cell, the terminal supporting the multi-RAT may handover to an adjacent E-UTRAN cell.

This handover between heterogeneous RATs is called Inter-RAT handover. In inter-RAT handover, in particular, the time required for cell reselection and synchronization between the target cell and the UE may be increased. There is a need for a method for quickly obtaining a system time required for synchronization in a target cell following Inter-RAT handover.

The present invention seeks to shorten the system time in heterogeneous network handover.

In particular, the present invention is to provide a terminal or a network system that can prevent the time required for handover is prolonged when the target cell is a cell that is serviced by the serving cell and the heterogeneous network.

According to an aspect of the present invention, the method comprises: receiving center frequency information of a RAT different from a serving cell from a network, and performing downlink synchronization with a target cell of the other RAT using the center frequency information; Provided is a handover method comprising a.

According to another aspect of the present invention, transmitting the Inter-RAT handover measurement information for the RAT and the other RAT of the serving cell to the terminal, the terminal to the Inter-RAT handover measurement information If the other RATs are measured, receiving a measurement result from the UE, selecting one target RAT among the other RATs according to the measurement result, and making a handover decision; according to the handover decision, the target RAT And transmitting the handover command message including the center frequency information to the terminal, wherein the terminal performs downlink synchronization with a target cell supporting the target RAT using the center frequency information. A handover method is provided.

According to still another aspect of the present invention, there is provided an RF unit for transmitting and receiving a radio signal and a processor connected to the RF unit, wherein the processor includes a center frequency of the target cell from a network for handover from a serving cell to a target cell. A handover performing terminal is provided which receives information and performs downlink synchronization with the target cell using the center frequency information, wherein the RATs of the serving cell and the target cell are different from each other.

According to an embodiment of the present invention, it is possible to shorten the time taken to access the target cell during heterogeneous manganese handover.

In addition, according to the embodiment of the present invention, service continuity can be guaranteed by making time synchronization with the target cell easier when performing handover.

1 shows a wireless communication system.

Referring to FIG. 1, the wireless communication system 10 includes at least one base station 11 (BS). Each base station 11 provides a communication service for a particular geographic area (generally called a cell) 15a, 15b, 15c. The cell may again be divided into multiple regions (referred to as sectors). The user equipment (UE) 12 may be fixed or mobile, and may include a mobile station (MS), a user terminal (UT), a subscriber station (SS), a wireless device, a personal digital assistant (PDA), It may be called other terms such as a wireless modem and a handheld device. The base station 11 generally refers to a fixed station communicating with the terminal 12 and may be called by other terms such as an evolved-NodeB (eNB), a base transceiver system (BTS), and an access point. Can be.

Hereinafter, downlink (DL) means communication from the base station to the terminal, and uplink (UL) means communication from the terminal to the base station. In downlink, a transmitter may be part of a base station, and a receiver may be part of a terminal. In uplink, a transmitter may be part of a terminal, and a receiver may be part of a base station.

The wireless communication system may support uplink and / or downlink Hybrid Automatic Repeat Request (HARQ). In addition, a channel quality indicator (CQI) may be used for link adaptation.

In order to perform the handover, the terminal receives a handover command message from the network through a downlink subframe.

2 is a diagram illustrating a situation in which Inter-RAT handover occurs.

The UE shown in FIG. 2 is moving out of the serving cell 201 of the first RAT (eg, LTE RAT) to the adjacent cell 202 of the second RAT (eg, CDMA RAT). In the process, the terminal is positioned over the boundary region 203 (hatched portion). The border zone means a zone where handover occurs. Although the area is included in the cell of the first RAT, a handover between heterogeneous RATs occurs due to, for example, a signal strength with a base station of a cell of another RAT.

This phenomenon occurs in the process of evolving from the existing network to the new network supporting the next generation communication service. If all the existing networks are changed to the new network at once, there is no such problem. However, since the cost and time are inefficient, and all the terminals owned by the user must be changed, the complete change to the new network is not possible at this time. Very difficult.

Accordingly, there exists a current communication environment in which cells having different RATs are mixed, and terminal manufacturers have developed a multi-band multi-mode (MB-MM) terminal to adapt to the communication environment. . Therefore, the terminal in the present invention means the MBMM terminal. Here, the MB-MM terminal refers to a terminal that can use a communication service of several RATs.

Referring to FIG. 2, there is a cell 201 of a first RAT and a cell 202 of a second RAT. The cell 201 of the first RAT is the serving cell 201, and the cell 202 of the second RAT is the target cell 202. In the serving cell 201, there is a border zone 203, and as described above, the terminal is currently moving from the serving cell 201 to the target cell 202. 203). In addition to the target cell 202, there may be other adjacent cells around the serving cell 201, but other adjacent cells are omitted in FIG. 2.

In the boundary area, the MM-MB terminal hands over the call mode from the first RAT to the second RAT. As a result, even when the serving cell 201 moves from the serving cell 201 to the target cell 202, that is, even if the RAT varies according to the movement of the terminal, the terminal can seamlessly communicate with the service provided at that time.

In this case, the continuity of services can be guaranteed by smooth Inter-RAT handover. In particular, the continuity of services depends on how much time is spent for Inter-RAT handover.

3 is a flowchart illustrating an operation in which a terminal performs cell selection or reselection. In general, handover occurs by cell reselection of the UE. The purpose of the terminal to perform cell selection is to register with the network to receive services from the base station. When the strength or quality of the signal between the terminal and the serving cell is reduced due to the movement of the terminal, the terminal reselects another cell to maintain the transmission quality of the data. Hereinafter, characteristic values of physical signals related to signal strength, signal-to-interference plus noise ratio (SINR), and the like are simply referred to as signal characteristic values. There is a method of selecting or reselecting a cell according to a signal characteristic value according to a wireless environment. In performing cell selection, according to the radio access technology (RAT) and frequency characteristics of a cell, There may be the same way.

(1) Intra-frequency cell reselection: Reselects a cell having the same center-frequency as the cell being used by the terminal

(2) Inter-frequency cell reselection: Reselects a cell having a center-frequency different from that of the UE

(3) Inter-RAT cell reselection: Reselects a cell using a different RAT from a cell used by the UE

Among these, when the UE reselects the Inter-RAT cell, the Inter-RAT handover occurs. Inter-RAT cell reselection does not necessarily mean that the RAT varies and the serving cell's center frequency is maintained. Inter-RAT cell reselection usually includes both cases where the center frequency is different and not.

Referring to FIG. 3, in step S310, the terminal selects a RAT for communicating with a Public Land Mobile Network (PLMN) to which the terminal desires to receive a service. The PLMN and RAT information may be selected by a user of the terminal or may be selected according to information stored in a universal subscriber identity module (USIM).

In step S320, the terminal selects a cell having the best signal characteristic value as an initial cell according to the measured signal characteristic value. Then, the system receives the system information periodically sent by the selected cell.

In step S330, the terminal determines whether to register by comparing the network information (eg, TAI (Tracking Area Identity)) received from the system information with the information of the network known to them. The terminal performs a registration procedure to the network when the information of the network received from the system information and the information of the network known to the terminal is different.

In step S340, the terminal registers its information (eg, IMSI) in order to receive a service from the network.

In step S350, if the signal characteristic value measured from the adjacent cell is better than the signal characteristic value measured from the serving cell, the terminal performs cell reselection. This is called cell reselection, which is distinguished from initial cell selection.

In step S360, if a new cell is selected, it is determined again from the registration status.

In step S390, the UE performs measurement periodically or irregularly for initial cell selection and cell reselection. As the measurement result value, RSRP (Reference Symbol Received Power), RSRQ (Reference Symbol Received Quality), RSSI (Received Signal Strength Indicator) can be used.

Cell reselection criteria such as UE capability, subscriber information, cell load balancing, and traffic load balancing in order for the UE to perform cell reselection according to the measurement result Is also needed.

Cell reselection according to the terminal capacity refers to performing cell reselection according to the selectable frequency band since the frequency band available for the terminal itself may be limited. Cell reselection based on subscriber information indicates that the terminal may be configured to select or not to select a cell according to subscriber information or operator policy. Cell reselection according to cell load balancing refers to allowing a small number of terminals to select a cell in use in order to reduce load due to data generated when terminals in idle mode are activated in one cell. Cell reselection according to traffic load balancing is to be a reference for changing a cell in terms of reducing the load caused by data generated in activated terminals.

The E-UTRAN system is likely to operate by extending the frequency band to the existing UTRAN for the purpose of installation, maintenance and repair. Therefore, in order to efficiently use radio resources and balance loads among cells, cell load balancing or traffic load balancing needs to be considered when reselecting cells.

4 is a diagram illustrating a process of performing an inter-RAT handover by a terminal according to an embodiment of the present invention.

The network first transmits Inter-RAT handover measurement information to the terminal for handover (S401). The inter-RAT handover measurement information is information about a neighboring RAT and is information required for RAT measurement for performing inter-RAT handover.

The terminal performs measurement on neighboring RATs using the Inter-RAT handover measurement information (S402). In order to perform inter-RAT handover, the terminal is an MB-MM terminal and should be able to search for different RATs or frequencies.

After the RAT measurement, the terminal reports the RAT measurement result to the serving cell (S403). The serving cell comparing the signal strength and the like from the RAT measurement result received from the UE selects the RAT to be handed over and makes a handover decision (S404). Of course, the RAT with the best signal strength may be selected. The RAT selected in the handover decision process of the network is referred to as a target RAT for convenience.

In operation S405, the UE transmits a handover command message to the target cell. The center frequency information of the target cell is included in the handover command message to the target cell.

Alternatively, the center frequency information may be transmitted separately without being included in the handover command message. For example, the center frequency information of the target RAT may be transmitted through a system information block.

Conventionally, when a handover occurs, the UE has to perform cell search for all frequencies of the target cell or the target RAT. Therefore, the sync channel search is performed in the form of a blind search for the entire frequency. However, as in the embodiment of the present invention, if the terminal uses the center frequency information included in the handover message received from the network, the synchronization channel search may be performed only for the frequency band according to the center frequency information. Therefore, the time required for performing synchronization can be shortened.

Upon receiving the handover command message, the terminal stops transmitting and receiving data with the serving cell base station and starts searching for a synchronization channel transmitted from the base station of the target cell. The sync channel may also be referred to as a preamble or pilot channel. The reason why the UE searches for a synchronization channel is to synchronize time synchronization and frequency synchronization with the base station of the target cell.

A cell providing a wireless communication service to a terminal may be referred to as a serving cell and a serving cell's RAT as a serving RAT. In addition, a cell serving as a target of handover is called a target cell, and a RAT of the target cell is called a target RAT. Assuming that the period of the synchronization channel transmitted from the serving RAT and the target RAT is the same, the terminal should search for the synchronization channel for approximately 20 ms to receive the synchronization channel transmitted from the target RAT.

When the terminal finds a synchronization channel in the target RAT, synchronization between the base station and the terminal is performed in the target RAT (S406).

5 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 display unit 54 displays various information of the terminal and may use elements such as a 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 RF unit 53 is connected to a processor and transmits and / or receives a radio signal.

A processor of a terminal according to an embodiment of the present invention receives information about neighbor cells from a network. Information about neighbor cells is information necessary for cell measurement for performing inter-RAT handover. Hereinafter, this will be referred to as inter-RAT handover measurement information. In addition, neighbor cell measurement is performed using the inter-RAT handover measurement information, and the cell measurement result is transmitted to the network.

The network determines the target cell according to the cell measurement result, and the processor receives a handover command message. The handover command message includes center frequency information of the target cell to be reselected when the UE performs inter-RAT handover. If the center frequency information is not included in the handover command message, the terminal receives the center frequency information as a separate message.

Upon receiving the center frequency information together with the handover command message, the processor searches for a synchronization channel of the target cell and performs synchronization with the target cell.

The invention can 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 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.

As mentioned above, although preferred embodiments of the present invention have been described in detail, those skilled in the art to which the present invention pertains should have the present invention without departing from the spirit and scope of the present invention as defined in the appended claims. It will be appreciated that the present invention may be modified or modified in various ways. Accordingly, modifications to future embodiments of the present invention are included in the scope of the present invention.

1 illustrates a wireless communication system.

2 is a diagram illustrating a situation in which Inter-RAT handover occurs.

3 is a flowchart illustrating an operation of a cell performing cell selection or reselection in an idle mode.

4 is a diagram illustrating a process of performing an inter-RAT handover by a terminal according to an embodiment of the present invention.

5 is a block diagram illustrating elements of a terminal.

Claims (11)

Receiving center frequency information of a RAT different from that of a serving cell from a network; And Performing downlink synchronization with a target cell of the other RAT using the center frequency information. The method of claim 1, Before receiving the center frequency information, Receiving Inter-RAT handover measurement information from the serving cell; Measuring the other RATs using the Inter-RAT handover measurement information; And And transmitting a measurement result for the other RATs to the serving cell. The method of claim 2, Receiving from the serving cell a handover command message to the other RAT selected according to the measurement result, wherein the center frequency information is included in the handover command message. The method of claim 1, And performing downlink synchronization by searching for a synchronization channel within a frequency band according to the center frequency information. Transmitting Inter-RAT handover measurement information for the RAT different from the serving cell to the UE; Receiving the measurement result from the terminal when the terminal measures the other RATs according to the Inter-RAT handover measurement information; Determining a handover by selecting one target RAT among the other RATs according to the measurement result; And transmitting a handover command message including the center frequency information of the target RAT to the terminal according to the handover decision. The terminal performs downlink synchronization with a target cell supporting the target RAT using the center frequency information. The method of claim 5, The measurement result includes information on the signal strength of the other RAT, wherein the target RAT is characterized in that the signal strength is the strongest among the other RAT. The method of claim 5, The terminal is a multi-mode multi band terminal, characterized in that the handover method. The method of claim 5, The terminal performs the downlink synchronization by searching for the synchronization channel of the target RAT using the center frequency information. RF unit for transmitting and receiving a radio signal; And Including a processor connected to the RF unit, The processor receives center frequency information of the target cell from a network for handover from a serving cell to a target cell, and performs downlink synchronization with the target cell using the center frequency information, wherein the serving cell and the Handover performing terminal, characterized in that the RAT of the target cell is different from each other. 10. The method of claim 9, And the processor receives a handover command message from the network to perform the handover, and the center frequency information is included in the handover command message. 10. The method of claim 9, And the processor performs the downlink synchronization by searching for a synchronization channel within a frequency band according to the center frequency information.

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